B Duangta

Chambon

P.-M Bosc

Households' livelihood strategies facing market uncertainties: How did Thai farmers adapt to a rubber price drop?

Keywords: Family farming, Flexibility, Strategy, Price fluctuation, Rubber tree, Thailand

de niveau recherche, publiés ou non, émanant des établissements d'enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

Introduction

Agriculture is taking place in a changing environment. Changes in the economic environment include long term trends and short term shocks [START_REF] Killick | Flexibility and economic progress[END_REF]. In particular, agricultural products are subject to price volatility and cycles are key characteristics of the evolution of the commodity prices. This has important implications for the earnings of developing countries who are dependent on commodity exports and for the producers' income [START_REF] Collection A Savoir ; Cashin | Booms and slumps in world commodity prices[END_REF]. The impacts for producers of tree crops such as cocoa, coffee, or rubber are even more acute as they have to face price fluctuation in the short and medium terms when they are engaged in the long term with their perennial crops that usually require high investment to establish. As a result, price is a major concern for many tree crop producers and a major driver for production strategies and livelihood change [START_REF] Shively | Prices and tree planting on hillside farms in Palawan[END_REF][START_REF] Eakin | Adaptation in a multi-stressor environment: perceptions and responses to climatic and economic risks by coffee growers in Mesoamerica[END_REF]. An issue for tree crop-based producers, particularly when this crop is dedicated to the market, is to find ways to cope with the periods of low commodity prices.

Family agriculture is by far the main form of farm organization in the world [START_REF] Graeub | The state of family farms in the world[END_REF][START_REF] Lowder | The number, size, and distribution of farms, smallholder farms and family farms worldwide[END_REF]. It is characterized by: i) the organic and structural relationship between the productive assets and family patrimony; and ii) the predominance of family labor for the management and field operations (FAO, 2014;Belières et al., 2015). Tree crops are not an exception.

Smallholders represent 90% of the world rubber holdings and they contribute 85% of total natural rubber production, being mainly located in Southeast Asia (IRSG, 2017). Thailand alone produces more than one-third of world production and around 90% of the Thai rubber plantations belong to smallholders owning less than 8 hectares, with an average of 2 hectares (RRIT, 2013 cited by [START_REF] Somboonsuke | Agricultural system of natural Para Rubber smallholding sector in Thailand: system, technology, organization, economy, and policy implication[END_REF].

Rubber price has always fluctuated over time (Figure 1). Recently, rubber producers experienced a decreasing rubber price after a rubber boom during the second half of the first decade of the 2000's with a historical peak in 2011. The drop in the rubber price affected rubber farmers worldwide. In India, they reduced the labor involved in harvesting and the use of inputs with consequences on the productivity of rubber [START_REF] Pareed | Price volatility and its impact on rubber cultivation in India -An analysis of recent trends[END_REF], some farmers shifted to other crops and some had to reduce family expenses [START_REF] Karunakaran | Volatility in price of rubber crop in Kerala[END_REF]. In Indonesia, the severe reduction in rubber income led farmers to develop more of their own non-agricultural business activities and off-farm employment [START_REF] Kubitza | The economics behind an ecological crisis: livelihood effects of oil palm expansion in Sumatra, Indonesia[END_REF].

In Laos, many types of responses to the falling rubber price were observed. Some farmers stopped tapping, waiting for the rubber price to increase again while others replaced hired labor with family labor for latex harvesting. Some growers (mostly the poorest and the smallest) had no choice but to sell their land when others could lease land often resulting in conversion to other crops. Farmers also tried to develop collective action (cooperatives) to increase their marketing bargaining power and thus the rubber price [START_REF] Vongvisouk | Falling Rubber Prices in Northern Laos: Local Responses and Policy Options[END_REF]. In Thailand, the decrease in the rubber price since 2012 resulted in an important reduction in the income from rubber (Kumse, 2014). A recent study analyzed the coping mechanisms adopted by rubber farmers in South Thailand facing rubber price volatility but focusing only on diversification and community development [START_REF] Andriesse | Coping with the end of the commodities boom: rubber smallholders in Southern Thailand oscillating between near-poverty and middle-class status[END_REF]. The aim of the present article is to provide a comprehensive analysis of how farmers responded to the continuous decrease in the rubber price in the number one country for natural rubber production. The originality of this work was to combine a systemic agronomic perspective for decision making with the sustainable rural livelihoods framework for resource endowment and strategies to benefit from the complementarities of these two theoretical approaches.

Materials and methods

Scientific approach

We used an existing database providing a characterization of the structure of a sample of rubber households in 2011 (data collected in 2012). That year corresponded to the period when the rubber price was at its highest level. Interviews were conducted with a subsample of farmers from the 2011 database to characterize their farms' structures in 2016 (data collected in 2017). That year corresponded to a low rubber price which had been decreasing since 2012. Comparing the structure of each farm between 2011 and 2016 allowed identification of the changes that had happened during that period. This basis was used to analyze the strategies adopted by each household to face the decreasing rubber price.

In 2017, we used a mixed quantitative and qualitative method, with a sequential approach in two steps. Each household was interviewed twice. First, a quantitative approach was used to ask closed questions inducing answers that remained within the frame of the questionnaire. These data were used for the comparison between the two different temporal situations (2011 and 2016) and for the identification of the changes. Second, the qualitative approach with open questions allowed understanding of the mechanisms of change identified through the quantitative approach [START_REF] Creswell | Research design -Qualitative, Quantitative, and Mixed Methods Approaches[END_REF]. With the qualitative approach, focus was on the rationality of the households and the mechanisms of decision making.

Conceptual framework

For the quantitative approach, we used the Sustainable Rural Livelihoods (SRL) framework to provide a diagnosis of the rubber smallholders' characteristics in 2016. The first step was to identify the livelihood resources (i.e. the combination of natural, social, human, physical and economic or financial capitals) as well as capabilities (knowledge and know how) that the households could mobilize for their livelihood strategies when facing a commodity price decrease. There are several outcomes for the strategies adopted such as resilience or increased vulnerability that need to be highlighted [START_REF] Scoones | Sustainable rural livelihood: a framework for analysis[END_REF].

For the qualitative approach, we used the conceptual framework of the activity system [START_REF] Gasselin | The activity system[END_REF]). The farming system's concepts are not sufficient to understand the decisions of the farmers who are rarely mono active. An activity system is more appropriate since it includes all the productive activities of the farm and the family. Consequently, off-farm activities of family members are considered when analyzing the strategic objectives of the household and understanding the rationality of the farmer and his family, notably for family labor allocation. However, the agronomic concepts of a cropping system [START_REF] Sebillotte | Système de culture: un concept opératoire pour les agronomes[END_REF]) and a farming system [START_REF] Reboul | Mode de production et systèmes de culture et d'élevage. [Production mode and cropping and livestock systems[END_REF] were useful to analyze the farming subsystem (figure 2).

Figure 2: Activity system and its subsystems with a focus on agriculture

The SRL framework focuses on the available resources for households' livelihood and the result of their mobilization. The activity system framework focuses on the combination of activities, the flux of production factors and the decision making process; it added some dynamic dimension to the SRL [START_REF] Gasselin | Le système d'activité: retour sur un concept pour étudier l'agriculture en famille[END_REF]. The complementarities of these two approaches justify their combination to study family farms [START_REF] Sourisseau | Diversity in family farming: Theoretical and empirical approaches to its many forms[END_REF]. It is worth noting that these two conceptual frameworks are both quantitative and qualitative despite the fact that, for the fieldwork, one was mobilized for the quantitative step of the study and the other one for the qualitative step.

Last, we identified the adaptation strategies as a combination of several adaptation mechanisms adopted by the household to overcome the crisis. We mobilized the concept of flexibility defined as the capacity for a system to adjust its goals and resource allocations according to the changing constraints and opportunities [START_REF] Killick | Flexibility and economic progress[END_REF]. In other words, flexibility is "the ability of a system to maintain coherence within a changing environment" (Bathfield et al, 2013, p.357;Gasselin & Bathfield, 2013). This concept is useful to study the link (i) between short term decision making and long term strategies or (ii) between the need to adapt to uncertain context and the maintenance of some stability. Analyzing the farmers' response to the crisis through flexibility mechanisms reflects the mobilization of flexibility potential [START_REF] Bathfield | A flexibility framework to understand the adaptation of small coffee and honey producers facing market shocks[END_REF]. Each mechanism is characterized by i) a gradient of reversibility giving information on the temporal character of the adoption and ii) the resources mobilized.

Empirical data

Sampling

The study was conducted in three provinces of Thailand representing the different historical dynamics of rubber development and the different characteristics of the rubber smallholdings (Table 1). The objective was to cover the diversity of situations and environments which could affect decision-making by the farmers.

Respondents were selected from the sample of farmers interviewed in 2012 to characterize the farms in 2011. The 2012 study was conducted at the level of the country by one of the authors. To cover the diversity of situations in the different rubber producing areas, a zoning of the provinces was done considering the development of rubber plantations (historical zones versus new rubber producing areas) and the recent dynamics of rubber development for the five previous years (expansion, stagnation, regression). This was based on the analysis of the national statistics (Office of Agricultural Economics1 website) completed by individual interviews with staff from the Rubber Authority of Thailand (RAOT) in Bangkok and consultation of academic staff specialized in socioeconomy of the rubber farms in Thailand. From this zoning, 14 provinces were selected for the study.

In each province, individual interviews with key informants able to provide information on the local situation (Rubber Authority of Thailand in the provinces, rubber cooperatives, nurseries and traders) were conducted. This allowed the final validation of the provinces (10) and the selection of the districts to represent agronomic and socio-economic diversity. Random sampling of the farmers was not possible (no exhaustive list of the rubber farmers available) which is often the case for this kind of study. So the first contacts were often given by the Rubber Authority of Thailand, the nurseries or the traders. Then snowball-sampling method was used. The total sample size was 317. 9 These 317 households distributed in 10 provinces served as a basis for the sample in 2017. We selected three provinces. They were considered as representative of the dynamics of rubber development in Thailand and of the diversity of the rubber-based households. A purposive sample was done using five criteria: 1) presence of at least one mature plantation in 2012 sample; 2) typology of the rubber farms based on the kind of labor used (Belières et al, 2015): family farms using only family labor and family business farms employing at least one permanent hired labor usually for latex harvesting; 3) districts that could be discriminating in terms of opportunities for the rubber households; 4) diversity of the characteristics of the rubber-based households (age of the head of the farm, landholding, crops diversification, income diversification); 5) possibility to find the households (incomplete name registered, no phone number available). Finally, a total of 48 rubber farmers was interviewed.

Data collection

Data collected in 2012 concerned the socio-economic characteristics of the farm with a focus on land and labor, the description of the practices in the rubber plantations and the activity systems.

In 2017, all the 48 farmers were interviewed twice. For the first interview (quantitative phase), the questionnaire recorded the available resources: natural, human, physical and financial assets. Social capital, which is difficult to quantify, was included in the second interview (qualitative phase). Data to characterize the rubber cropping systems and the farming systems were also collected during this first phase. The questionnaire used for this phase can be viewed using this link: https://ee.kobotoolbox.org/x/#YX8a and a synthesis of the data collected in 2012 and 2017 is presented in supplementary material (Table 1).

The second interview (qualitative phase) was carried out after a preliminary analysis of the data collected during the first interview. Comparing the same rubber smallholders' characteristics over the six year interval (2011 and 2016) revealed points of change and no change between the initial and final states for each rubber smallholder. An interview guide was designed to collect information to explain the changes observed as well as options which were not used by the households. The interview guide was structured on the activity system and all its subsystems to get closer to smallholders' decision-making' consistency, understand their choices, and to identify what could and could not be linked to the rubber price crisis (Table 2, supplementary material).

Data analysis

Comparing the characteristics of each household in 2011 (available from 2012 study) and 2016 ( 2017interviews of quantitative phase), we were able to identify the changes (or non-changes) at the plots, farm and household levels. Analyzing farmers' discourses recorded during the qualitative phase of 2017 study, we could identify farmers' choices related to rubber price decrease and finally bring out the different flexibility mechanisms adopted to face falling prices. Each farmer could have adopted several flexibility mechanisms. In order to classify the smallholders according to the combinations of these mechanisms, we implemented a multiple correspondence analysis followed by a hierarchical classification using the open access R software. The multiple correspondence analysis allowed transforming nominal variables (the flexibility mechanisms adopted by each smallholder) into continuous numerical data that were used for the hierarchical classification analysis. The hierarchical cluster analysis using Ward's minimum variance algorithm allowed building a dendrogram. Some odd-distributed variables which influenced too much the results of the statistical analysis where not considered [START_REF] Escofier | Analyses factorielles simples et multiples [Simple and multiple factor analysis[END_REF]. The dendrogram was cut using a qualitative approach, at a level were resulting clusters could be characterized in accordance to the field reality. Within a cluster, the combination of flexibility mechanisms allowed to highlight the strategy adopted corresponding to the means used to reach a given objective here, to cope with the rubber price crisis.

We collected and analyzed some data to understand the economic logic behind the farmers' behaviors. However, we have chosen to simplify the presentation of the results and focus on the strategic mechanisms and the main dynamics of rubber households.

Results

Flexibility mechanisms

The 15 mechanisms identified were classified based on the levels where they applied: rubber cropping system, farming system and activity system. The number of farmers who adopted the mechanism is mentioned in brackets. This cannot be read as a frequency of occurrence since the sample was not representative of the rubber smallholders' population but was intended to cover the diversity of their situations. Thus, it only gives an idea of the mechanisms most adopted in our sample. One household could have adopted several flexibility mechanisms (and they usually did), these will be later synthetized through defining different types of strategic behaviors.

Six mechanisms, all reversible, were implemented at the rubber cropping system level.

1. Rubber fertilization management ( 24): some households reduced fertilization expenses. They were sensitive to the decline of the productivity of the capital invested and could rely on other resources to compensate for a possible decrease in rubber income. When reducing fertilization, smallholders expected a decrease in production but not proportional to the savings they made on fertilization expenses. This adjustment allowed households to adapt in the short term, without impacting on the farm structure.

2. Change in the total number of harvesting days (3): this mechanism was adopted in different ways.

In the first one (2a), two smallholders increased the total number of tapping days either by resuming tapping earlier than usual after the defoliation period or by increasing the harvesting frequency. This increase was temporary as both farmers realized that it did not increase the quantity of rubber produced and only increased the workload, therefore decreasing labor productivity. In the second one (2b), one farmer reduced the number of tapping days to reduce the workload for harvesting that has relative low labor productivity when the rubber price is low. Finally, this mechanism could respond to a search for income maximization through a more intensive exploitation of the rubber tree, or to a better valuation of the labor through a workload reduction. The change in harvesting had a quick economic effect on income. Even if this was not the main motivation for the farmers to change the number of tapping days, this also had a long-term effect on the tree capital.

3. Stop hiring labor for latex harvesting (2): this mechanism aimed at reducing the cost of production from rubber activity. Family labor was mobilized for latex harvesting previously done by hired labor.

Households did not have to reallocate resources since family labor was already available offering flexibility. Unlike the reduction in fertilization costs, the productivity of the rubber tree was not threatened by this adjustment. This reversible mechanism may not be restored to its original state when rubber price increases.

4. Change of the form of rubber sold ( 7): rubber can be sold in different forms and each form has a specific price, workload and collecting and sale frequencies. This mechanism was adopted with two different rationales. First (4a), below a certain price (not specified since each household set its threshold price based on its own workload capacity), some smallholders considered that the workload was not worth the additional remuneration it generated. They changed the form of rubber product to a less labor-consuming one which freed labor that could be better valorized with other activities. Second (4b), some smallholders were primarily motivated by a higher income frequency, low income being less viable when it is received at too long intervals. Therefore, some smallholders changed the form of product to a high selling-frequency one.

5. Change of the share-tapper contract's clauses ( 10): the share-tappers have an oral contract with the owners of the plantation usually including three parts: production sharing ratio, costs sharing ratio and side benefits. These smallholders changed the contract clauses (production and/or cost sharing ratio) to create sufficiently favorable conditions to discourage the tappers to seize other opportunities for higher paying jobs. Therefore changes were to the benefit of the hired tappers. The contracts were generally decided for the duration of a crop cycle and could be reviewed at the beginning of the new cycle if the rubber price changed. These adjustments had a cost for the owners but this was also in their own interest; if the tappers left the farm for other activities, the owners could no longer make full use of their rubber plantations, and could lose more than what they paid to convince their hired tappers to stay.

6. Lend money to the share-tappers (3): the owners lent money to their share-tappers without any interest or repayment term. These mechanisms aimed at improving the tappers' living conditions to encourage them to remain working on the farm which consequently limited the loss in the owner's income.

Five mechanisms, with different degrees of reversibility, were implemented at the farming system level.

7. Provide other work to the share-tappers (2): this mechanism follows the same logic of improving the hired tappers conditions to encourage them to continue working in the plantation. The owner gave the share-tappers the opportunity to work on other crops allowing them to get additional income. This could be combined with other mechanisms such as changing the form of rubber production to reduce the workload on rubber activity and free time for other remunerative activities.

This mechanism allowed the owner to exploit the maximum potential of the farm's plantation. 18): Some smallholders extended non-rubber crops and/or diversified the existing crops. Both approaches aimed at improving the distribution of risk on the various agricultural activities, and at strengthening self-consumption capacity and/or income from non-rubber crops. The extension was possible due to the available land that the smallholders owned and kept as a land reserve since such areas were usually far from the house. Another type of extension concerned small areas at the edge of the dwelling used for market gardening activity with high land productivity.

Develop other crops (

These small areas were previously unused and were conducive to intensive work because of their proximity to the house, and allowed an adjustment involving the family workforce and little financial means. Crop diversification sometimes resulted from a change in the areas allocated to non-rubber crops (developing crops with higher added value). However, the land use change never affected the rubber plantations.

9. Temporary agricultural land increase through a credit-land contract (3): "jamnong" is a loan of money with interest in the form of a right to use some land for a productive crop. The lender is assigned a right to use a productive plot, over a period attached to the duration of the contract itself dependent on the repayment time of the capital borrowed (maximum five years observed in our sample). The smallholders surveyed were always the lenders. This contract allowed them to invest their savings in a temporary agricultural extension, with, in principle, the assurance of recovering their initial investment at the end of the contract. In the cases found during the study, the extension did not induce any deep and definitive structural change; "jamnong" was just a temporary structural adjustment.

10. Renew rubber plantation [START_REF] Killick | Flexibility and economic progress[END_REF]: with this non-reversible mechanism, smallholders took the opportunity of the period of low rubber price to renew rubber plantations. Some farmers anticipated replanting by two or three years. Replanting during a period of low rubber price was a way to minimize the inevitable loss of income during the immature phase of a new plantation. In addition, the sale of timber often generated income allowing the household to regularize its situation if it had loans to refund, and put it back in a favorable economic position to cope with possible future disturbances.

11. Sell a mature or immature rubber plot (3): this non-reversible mechanism was implemented by smallholders who owned several plots of mature rubber plantations. They could sell a plot without sacrificing too much their rubber income, which remained the main source of income for the household. The sale of a productive plot generated an additional income which allowed the household to directly find a more comfortable financial situation. It also allowed investing in another long term activity such as off-farm business or new land for expansion and diversification of cash crops. Therefore, the transaction had two temporal levels: the response to a direct financial need, and a will to diversify the household sources of income in the long term: it was a decapitalization to reinvest in the household's capital.

Four mechanisms were implemented at the activity system level. One was easily reversible ( 14) and the three others could be reversed but they engaged the household in the medium or long term.

12. Hire share-tappers (3): some smallholders hired labor to harvest all or part of their plantations to relieve the family workload engaged in rubber activity. This allowed a reallocation of family labor to other activities. The loss of income generated by the sharing of production was at least compensated by a better valorization of family labor possible due to the time released.

13. Develop off-farm activities ( 13): some smallholders invested in off-farm activities. There was a huge diversity of types of activities. This mechanism required investment and was therefore often combined with other mechanisms that allowed a reallocation of production factors (time invested, workload, capital invested, frequency of rubber income), impacting the working capital of the new activity.

14. Decrease in expenditure related to household consumption ( 25): this adjustment did not concern the basic food basket and education expenditures. It related to expenditure on transport costs, new clothing, alcohol, ice cream and soft drinks, and expensive foods. It allowed the household to absorb part of the reduction of rubber income and for some, to preserve their savings used to invest in more profitable activities.

15. Use of loans to maintain the standard of living ( 6): households with the greatest financial difficulties did not sacrifice the schooling of their children and maintained a standard of living that met basic needs. For that, since the beginning of the crisis, they have contracted several loans. Credit was not only for income-generating activities, but also for repaying previous loans and for consumption and education expenditures. The smallholders without access to official financial products (no official land title) had to borrow money from usurers whose interest rates were around 15% per month when the interest rate from a bank was only 7% per year.

The 15 mechanisms could also be classified based on their objectives (Table 2). Only one mechanism fits in the combination of objective with level of implementation. The figures in brackets correspond to the occurrence of the flexibility mechanisms observed in our sample

Classification of coping strategies adopted by households

Analyzing the combination of the 15 flexibility mechanisms, eight types of farmers' strategies were identified.

A: Maintaining rubber income by keeping hired tappers in the farm

Smallholders were very dependent on rubber activity and on their hired tappers. This constituted rigidity in their system since they had to ensure a certain level of income for their tappers and absorb the fall in their own rubber income. The smallholders' financial margin allowed them to make some concessions to the hired tappers without impacting their household consumption expenses.

Improving the conditions of hired tappers could be achieved through: a change to the clauses in the share-tapping contract for production and/or cost-sharing percentages (mechanism 5), the shift from one form of rubber product to another one (mechanism 4), the possibility for hired tappers to work on other crops (mechanism 7) or getting loans from the owner with indefinite duration and no interest (mechanism 6). All these flexibility mechanisms allowed the hired tappers to: limit the loss of their income, reduce the workload and release time, diversify their farming activities and income sources, have the possibility to keep working and stay in the village without having running interests on debts. Keeping hired tappers on the farm allowed the owner to continue exploiting his plantation, and thus limited the decline in his rubber income. These adjustments were reversible; a rubber price increase would cause a return to the initial state. The farm's structure was not changed. This strategy has a temporal footprint limited to the crisis period.

B: Hiring new tappers to reallocate family labor to other activities

These farmers hired new tappers (mechanism 12) to release family labor that could be invested in on-farm diversification (mechanism 8) and/or in the development of off-farm activities (mechanism 13). The owners sacrificed part of their rubber income to invest family labor in an activity that would supplement their income and reduce the vulnerability to rubber price fluctuations. This strategy had a long term footprint because the off-farm activities were, according to the smallholders, opportunities that once mobilized would last. A rise in the rubber price would not cause a rollback.

The households who adopted this strategy did not reduce their consumption or rubber fertilization expenditures. These owners often had entrepreneurial profiles or had permanent non-farm employment. However, they depended on the rubber activity and could not afford to completely sacrifice this income, which was why they maintained the level of investment in fertilization.

C: Lender-tenants mobilizing household savings into temporary agricultural diversification

To absorb the decrease in income, these households reduced consumption expenditure by getting food from their own garden and by limiting everyday non-basic expenses (mechanism 14). However, they did not reduce their children's education, if any. While paying attention to the expenses, the household temporarily invested its savings in other crops, mostly rice, but also cassava and sometimes rubber. This temporary extension of other crops was possible through the credit-land contract locally called "jamnong" (mechanism 9). The financial lending position of these smallholders allowed them to quickly access an already productive crop, to take full advantage of it during the period of the loan, and at the end of the contract, to get their initial savings back. These smallholders therefore mobilized their social and financial capitals to access land capital, which until then was limited. The reversibility of this contract limited the risks of the investment in a new crop compared with a permanent investment inducing a deeper change in the farm structure. However, there is a risk that the owner who temporarily transferred land use right cannot pay back at the end of the contract, which would lead to a deep change in farm structure, since the lender would have to get full ownership on the land (and would not get his financial capital back).

Jamnong is a long-established form of contract. However, with the rubber price decrease, its implementation was more frequent perhaps accompanying a process mentioned by several 20 smallholders, namely the fall in the rubber price largely impacted the economy of rural areas and local markets. As not all individuals had access to formal financial services, some were encouraged to stimulate informal lending networks to borrow money.

D: Mobilizing household labor to minimize rubber costs and maintain rubber income

These smallholders had no off-farm activities or other crops. Their system was marked by rigidity at the level of remunerative activities but flexibility at the level of the resources in family manpower which was used to respond to the rubber price decrease. The smallholders first reduced household consumption expenditure (mechanism 14) and then stopped using hired labor for tapping (mechanism 3) or for rubber fertilization. The reduction in the cost of rubber activity was possible due to the available family manpower. The strategy adopted aimed at minimizing costs while limiting the loss of rubber income (fertilization was maintained). These smallholders tried to overcome the crisis without making major changes. The adaptation mechanisms were reversible but may not be restored to the original state with a rise in the rubber price.

E: Renewing part of the plantation and reallocating labor to other activities

These smallholders seized the opportunity of the low rubber price to renew part of their plantations (mechanism 10), diversify their crops (mechanism 8) and intensify or develop their non-agricultural activities (mechanism 13). Felling a rubber plantation decreased the workload for rubber activity and thus family labor could be invested in other activities, on-farm or off-farm.

This adaptation strategy was irreversible and had a long term footprint as the off-farm activities chosen were part of the household's long-term development and replanting rubber trees was a longterm investment.

F: Adopting technical and financial adjustments to maintain the systems

These smallholders had the same objective, namely to absorb the crisis within their systems, but used three different approaches.

The first was adopted by the poorest smallholders who tried to absorb the decline in rubber income by reducing their household consumption expenditure (mechanism 14) and by borrowing money to close the financial gap between their needs and their revenue (mechanism 15). Often, they were already over-indebted by having overlapping loans, with the most recent ones being used to repay the older ones. These smallholders did not reduce the investment in rubber fertilization. Indeed, rubber growing remained their main source of income. As they had no financial flexibility, they chose to secure their rubber income even if the capital invested could be better valorized through other activities.

The second situation corresponded to smallholders who reduced rubber fertilization expenses (mechanism 1). Half of them also reduced their household expenditure (mechanism 14), while the other half did not change anything. After reducing capital investment, some households relied on existing off-farm diversification and flexibility with household expenses; others simply relied on their pre-existing financial resources.

The third situation concerned smallholders who changed the form of rubber product sold (mechanism 4) to increase the frequency of payment facilitating the management of their daily expenses. Some smallholders also tried to increase the number of tapping days (mechanism 2) to offset the loss of income. In parallel, farmers absorbed household income loss through a reduction in consumption expenditure (mechanism 14).

These smallholders mobilized different mechanisms to absorb the economic disruption in a passive way; their strategy aimed at weathering the crisis without impacting their activity system.

G: Adopting technical adjustments to reallocate production factors and develop off-farm activities

The main technical adjustment of these smallholders was a change in the form of rubber sold (mechanism 4) to increase the income frequency. These smallholders implemented the same combination of coping mechanisms as the third case in strategy F; however, they were doing so for a different reason, as they reallocated production factors to strengthen a weakened system. The adjustments aimed at diversifying their income off-farm (mechanism 13).

These adjustments released time by reducing the workload and demand on capital by reducing household consumption expenditure. This time and capital, combined with a higher income frequency, allowed investment in off-farm business such as a street food stall, which required daily liquidity for working capital. The combination of the mechanisms mobilized allowed smallholders to better overcome the low rubber income and to redistribute production factors to orient the smallholders toward a more stable situation. This transformation might remain in time, even though the household could also return to its initial state.

H: Developing long term on-farm diversification

Moderately wealthy, these farmers reduced household consumption expenditure, without affecting basic needs or education (mechanism 14). They also developed other pre-existing crops (mechanism 8)-mainly cash crops such as sugar cane or cassava. These crops did not replace rubber plantations which were always renewed, but they provided additional income. This type of change was reversible since it only concerned short-term crops.

Having financial flexibility due to the other crops, these households reduced the expenses for rubber fertilization (mechanism 1) to optimize in the short term the capital investment and to balance the decreasing income of the activity. It was also a risk management mechanism; indeed, limiting the investment in fertilization allowed the smallholders to minimize the loss if they lacked the manpower to tap all the rubber trees, and therefore could not valorize the capital invested by exploiting the maximum potential of the plantation.

Three types of adaptation strategies, three temporal footprints

To simplify the description of farmers' responses to the decreasing rubber price, it is worthwhile considering the level of reversibility of the changes. Three major types of adaptation strategies were identified: (i) reversible adjustments at the level of activity system variables, to strengthen the capacity of disturbance absorption within the system, in order to maintain the smallholder systems and overcome the crisis; (ii) mobilization of available production factors invested in an activity that allows, in a longer time perspective, for compensation for the disturbances caused by the crisis. This type of adaptation was reversible, but its implementation expressed motivation in the medium-tolong term; (iii) reallocation of the production factors already mobilized at the level of the rubber cropping system toward other activities, to compensate the disturbances caused by the crisis. This latter type of adaptation induced a deeper transformation in the distribution of the production factors in the activity system and was anchored in a long-term vision.

These three main types of strategies corresponded to two main trends (Figure 3): absorption dynamics and the dynamics of compensation. The smallholders who based their strategy on the flexibility of the system in place and attempted to absorb the shock through adjustments in the variables of the rubber cropping system were defined as smallholders who tried to maintain themselves and overcome the crisis without transformation. The mechanisms mobilized were reversible and did not induce compensatory mechanisms for the crisis. This was the absorption dynamics observed in the three provinces.

The smallholders who mobilized available production factors to compensate for the fall in rubber income were involved in compensation dynamics. This was possible in Buriram and Rayong provinces and the mechanisms mobilized were reversible. This behavior reflected a desire to anchor the changes over a longer period and generated a transformation in the activity system. However, some smallholders made adjustments to reallocate the production factors within the activity system and so underwent a deeper transformation that was less reversible. This was observed in Rayong and

Phattalung provinces. This redistribution generated a certain locking of production factors at the activity level and was part of a longer time perspective. Therefore, sacrifices were made at the level of the rubber activity in favor of investment in other activities that allowed the smallholders to feed a dynamic of compensation for the crisis.

Economic status of the farmers in the different groups of strategy

Since the response of the rubber households to the rubber price decrease could be related to their economic situation, we looked at the total household income as well as the total area under rubber plantations and the area of mature rubber plantations. There was a high variability of the average (Figure 4). It is important to mention that it is difficult to assess precisely the income of the rubber smallholders by survey. Indeed, the rubber activity can generate income throughout the year with different frequencies depending on the form of the rubber product sold; it can be almost daily in the case of rubber farmers in Thailand. Moreover, the interviewed rubber smallholders generally did not record their sales information. So the results presented should be considered as approximates.

However, despite the approximation, these data allowed a comparison between groups and provided some insights to understand better farmers' responses to the price decrease.

Figure 4: Average total household income for the different coping strategies adopted for the year 2016

The area under rubber plantations (both total and mature) also presented important variability between the groups and within the groups (Figure 5).

Observing figures 4 and 5, we can gather four subsets of rubber households. Two groups (A and H) gathered well-to-do smallholders with large rubber areas (total and mature) and the highest average total income. Group B was medium-sized rubber area (total and mature) and had average total income. For four groups (D, E, F, G), the income was lower as well as the area of mature rubber plantations. Group C had small rubber area (total and mature), whereas it had a high average total income. This is to relate with its financial resources giving the ability of using the land credit contracts in order to temporaly extend their mature rubber areas. Finally, for all the groups except C, it seems that the level of income could be related to the area of mature rubber plantations, suggesting the importance of rubber in the income. The results showed a high diversity in the strategies adopted by rubber farmers when facing rubber price decreases. This diversity in response by long-term crop producers facing market shocks has been observed previously [START_REF] Vongvisouk | Falling Rubber Prices in Northern Laos: Local Responses and Policy Options[END_REF]. The strategies adopted by Thai rubber farmers present similarities with the strategies observed in other crisis contexts. For instance, in spite of the low coffee price, there was expansion and intensification of coffee areas and finally only a few farmers sold their land or reduced the area under coffee [START_REF] Eakin | Adaptation in a multi-stressor environment: perceptions and responses to climatic and economic risks by coffee growers in Mesoamerica[END_REF]. Low and variable coffee prices also induced economic diversification [START_REF] Padron | Diversification and labor market effects of the Mexican coffee crisis[END_REF]. However, the response by rubber farmers to a price decrease also showed some divergences. In several countries and for different crops [START_REF] Vimard | Sassandra: la fin d'une agriculture pionnière ou les enjeux d'une recomposition. [Sassandra: the end of a pioneer agriculture or the challenges of a recomposition[END_REF][START_REF] Samper | Costa Rica's response to the coffee crisis[END_REF], a decrease in the commodity price led to conversion to other crops. In Thailand, when the current study was conducted, the abandonment of rubber cropping appeared limited; some farmers even took the opportunity of this crisis to renew the production capital, especially in historical rubber-producing areas. This may have been encouraged by the long-term government support for rubber replanting and by the market for rubber wood. Perennial crops usually require high levels and long terms of investment. Once the plantation is there, reconversion is possible, but it has a cost and is an irreversible decision in the short term. Therefore, it is more difficult to change with a long-term crop than for a short-term crop.

This structural factor of crop longevity may explain time lags and can lead to incremental adaptations rather than transformative change [START_REF] Darnhofer | Resilience and why it matters for farm management[END_REF]. As a consequence, when the price decreases, perennial crops farmers first try to survive; reconversion is done only when the commodity price is so low that it does not allow the household to subsist. It seems that even with a low price (around USD 1.5/kg for RSS3 rubber grade in 2016), rubber remained the best crop opportunity. Our results are consistent with other work conducted in Southern Thailand where some farmers diversified their sources of income but even for farmers more dependent on rubber, this crop remained a good option for the future [START_REF] Andriesse | Coping with the end of the commodities boom: rubber smallholders in Southern Thailand oscillating between near-poverty and middle-class status[END_REF]. Almost 20 years ago, [START_REF] Penot | Rubber cushions the smallholder: no crisis, no windfall[END_REF] had already observed that throughout the history of rubber cultivation, in spite of the fluctuations, the rubber price had never dropped to the point where it would discourage farmers to grow rubber.

Another phenomenon associated with the commodity price decrease could be the sale of land (Ruf & Gerard, 2000;[START_REF] Bathfield | A flexibility framework to understand the adaptation of small coffee and honey producers facing market shocks[END_REF]. Nonetheless this also was limited in the Thai rubber sector.

Because of the adjustments at different levels in their systems and the jamnong credit-land contract, farmers were able to avoid this irreversible change that could have led them to increased poverty.

However, even if the situation was not evident in our study, jamnong could also lead to loss of land for the borrowers who may not be able to pay the credit back.

Despite the long-term character of the rubber crop, the technical adjustments provided some flexibility with limited risks in the short term. Technical change offers potential flexibility that is often used first by farmers facing a crisis [START_REF] Bathfield | A flexibility framework to understand the adaptation of small coffee and honey producers facing market shocks[END_REF]. These adjustments at the rubber cropping system level offer levers for action allowing reaction in the short term at different levels (workload, cost, income frequency) without compromising the profitability of the activity. The adjustments include the possible reduction of fertilization expenses and a change in the type of product. Sharetapping contracts also provided important flexibility to maintain labor for harvesting. This probably explains the persistence of this labor contract [START_REF] Kongmanee | Path dependence of agrarian change: an institutional economic analysis of the rubber economy in Southern Thailand[END_REF] and may make a difference with other countries using wage labor for latex harvesting. In such countries, low rubber prices have resulted in increased rubber farmer poverty as well as a shortage of labor for latex harvesting since tappers leave this sector for more remunerative ones [START_REF] Aidenvironment | Low prices drive natural rubber producers into poverty. An overview of sustainability issues and solutions in the rubber sector[END_REF]. The ability of farmers to accept remuneration for their factors of production, notably labor, below the market price is another well know flexibility and factor of resilience for family farms [START_REF] Lamarche | L'agriculture familiale. Tome 1: Une réalité polymorphe[END_REF].

In addition, in our study, different levels of resource endowment provided different levels of flexibility. Farmers employing hired labor for latex harvesting were usually those with the largest rubber plantation areas. They could adopt flexible mechanisms at the rubber cropping system level that were all reversible adjustments. In contrast, the farms using only family labor and with small landholdings were more vulnerable and needed to implement more profound and irreversible changes in their systems.

Finally, some rubber farms were able to mobilize their "buffer capability" to absorb the price decrease without any change in their structure or function and their "adaptive capability" allowed them to adjust and develop within the same trajectory (same goals and values), implementing reversible changes. However, so far, very few farms have mobilized their "transformative capability" which means they would implement radical changes [START_REF] Darnhofer | Resilience and why it matters for farm management[END_REF]. The fact that only a few of these radical changes were adopted by rubber farmers to face a rubber price decrease suggests that until 2017, rubber households had not yet been cast into poverty. Even if the drop in the rubber price was sudden and following a historically high price (Figure 1), most farmers considered that the situation remained viable and would finally return to the situation of the 1990s and 2000s. We found only two kinds of households in difficulty: the smallest farms that were highly dependent on rubber income and those who contracted debts during the period of high rubber prices and fell into over indebtedness when the rubber price dropped. However, overall, this study once again showed the flexibility of family forms of agriculture [START_REF] Chayanov | L'Organisation de l'économie paysanne[END_REF].

Return on methodological choices

The methodology adopted was based on the utilization of a survey previously conducted in 2012.

Using this survey as a basis presented some drawbacks. Due to the composition of the 2012 sample and the difficulties to meet again with the same farmers, it was not possible to have a balanced distribution of all the three types of farms based on the labor used and previously identified in our own unpublished work. Interviewing farmers in 2017 who were not in the initial sample could have corrected the bias. However, we found it more relevant to remain within the 2012 sample to satisfy our stated objective, and having an accurate description of the situation of the farms in 2011 appeared very valuable in the analysis of the changes. In addition, during the final restitutions organized in each province, exchanges with farmers and extension officers allowed the validation of the observations beyond the cases interviewed.

The combination of two approaches with complementarities provided some robustness to the analysis process and the results. The sustainable rural livelihood framework appeared very useful to quantify rubber household capital and its possible mobilization. This gave a robust starting point to analyze farmers' decisions using the activity system approach and to try to understand farmers' practices considering their poly-rationality in a systemic perspective [START_REF] Gasselin | Le système d'activité: retour sur un concept pour étudier l'agriculture en famille[END_REF].

Conclusion

This study focusing on the impact of the rubber price crisis on Thai farmers highly dependent on rubber income illustrates the adaptation of family agriculture when faced with a major economic shock. The strategies adopted showed that so far, the farmers could overcome the crisis. They themselves acknowledged that the changes were limited. The study was conducted after a period of 6 years of decreasing rubber prices, which was sudden and substantial. However, the period of low price was not as long as has been experienced, such as during the 1990s [START_REF] Penot | Rubber cushions the smallholder: no crisis, no windfall[END_REF]. We consider that the study was conducted at the beginning of a new rubber price crisis and family farmers with their well-known resilience and capacity of adaptation were able to cope with and adjust to this new situation. Nonetheless, things could change if there is a continued low price trend in the medium term. The rubber price is not expected to increase again before mid-2020 [START_REF] Jacob | Outlook on natural rubber during 2019 and beyond. Highlights of the talk given in the 11 th Sao Paulo lecture series on natural rubber[END_REF]. So, for how long can rubber households remain resilient? With a long-lasting crisis, the proportion of farmers implementing radical and irreversible changes may increase with important consequences for the long term of the rubber industry.

Several mechanisms applied at different levels from the rubber plot to the household were adopted to overcome the crisis. However, the present study showed that analyzing farmers' practices is a major key to understanding family agriculture. Indeed, practices and change in farmers' practices are parts of the farmers' strategies and therefore, they are very useful to describe and retrace the strategies (Yung & Zaslawsky, 1992). Setting up a tool for farms diagnosis and long term monitoring of the farm would allow studying practices in detail and following the evolution of the households in the medium and long terms. It illustrates the interest of empirical surveys documenting households' assets and performance, based on a selected sample of diverse farm types. This would provide policy makers with information to optimize rubber policy to support family farming and thus contribute to maintaining and/or increasing its resilience to shocks that are getting more and more unpredictable.

Figure 1 :

 1 Figure 1: Evolution of natural rubber current price from 1988 to 2017

Figure 3 :

 3 Figure 3: Dynamics, trends and strategies of adaptation to cope with rubber price decreases based on

Figure 5 :

 5 Figure 5: Average area of rubber plantations: total rubber plantations and mature plantations for the

Table 1 :

 1 Sample distribution and characteristics

	Region (province)	Northeast (Buriram)	Centre-east (Rayong)	South (Phattalung)
	Sample size	16	14	18
	Distribution of the sample in	Family farms: 7	Family farms: 8	Family farms: 18
	the typology based on labor*	Family business farms: 9	Family business farms: 6	Family business farms: 0
	Development of rubber	New area (since 1990s)	Historical area (more than a	Historical area (more than a
	plantations		century)	century)
	Ratio rubber / total farming
		6%	63%	72%
	area for the province
	Land	Available;	Still available but expensive;	High land pressure;
		relatively large landholdings	large landholdings	small landholdings
	Activities	Mainly farming, on-farm	On-farm diversification and	On-farm specialization on
		diversification	off-farm activities	rubber but off-farm
				diversification
	Environment	Strong institutions, cassava	Industrial zones (land	Government
		and sugar cane factories, no	pressure), urban	recommendations for on-

Table 2 :

 2 Classification of flexibility mechanisms based on level of application and main objective

		Rubber cropping system	Farming system	Activity system
	Level
	Objective
	Reduce expenses	Rubber fertilization		Decrease in expenditure
		management (24);
		Stop hiring labor for latex
		harvesting (2)

http://www.oae.go.th/view/1/Home/EN-US

Acknowledgments

This study was conducted under the framework of the Hevea Research Platform in Partnership (HRPP). It was supported and funded by the Thai International Cooperation Agency (TICA) and the French National Research Agency (ANR) under the HEVEADAPT project. We thank all the interviewees for their patience and kindness during the interviews as well as the staff from the Rubber Authority of Thailand (RAOT, ex ORRAF) for providing support in the field. This article is based on Timothée Nicod's master thesis. This study received the AREA 2018 Award (Association for Agro Economy Research), highlighting the quality of the work.


Shaosui Xu
email: shaosui.xu@ssl.berkeley.edu

Da L Mitchell

Jame P Mcfadd

Matthew O Fillingim

Laila Anders

David A Brain

Trista Weber

Nicholas M Schnei

Sonal Jain

Christop M Fowler

Robert Lillis

Christian Mazelle

Jared Espley

Inverted-V Electron Acceleration Events Concurring With Localized Auroral Observations at Mars by MAVEN

the Mars Atmosphere and Volatile EvolutioN (MAVEN) spacecraft repeatedly observed aurora near periapsis over Mars' southern strong crustal fields. During these orbits, the Solar Wind Electron Analyzer observed accelerated electrons at similar locations to where the auroras were observed, resembling the inverted-V structure observed near Earth's auroral region. In this study, we present a case study of such an acceleration event, where we estimate a field-aligned electrostatic potential drop of ∼440 V. We determine the field-aligned current from the observed magnetic perturbation reaches 1.1 μA/m 2 , agreeing reasonably well with the estimated net electron current carried by acceleration electrons with a maximum of 2.5 μA/m 2 . Similar to Earth, the potential drop develops when the ambient plasma cannot sustain the imposed field-aligned current. We also estimate the potential layer to be located above 750-to 850-km altitude and the associated electric field to be ∼0.6 V/m.

Introduction

Inverted-V electron acceleration events, characterized by an enhanced flux in a narrow energy band, have long been observed at Earth and are responsible for discrete auroral emissions (e.g., [START_REF] Birn | Particle acceleration in the magnetotail and aurora[END_REF][START_REF] Mcfadden | Field-aligned electron precipitation at the edge of an arc[END_REF]. These electrons are accelerated above the observational locations by a parallel electric field (E // ) within a narrow potential layer (typically a few kilometers in thickness) (e.g., [START_REF] Birn | Particle acceleration in the magnetotail and aurora[END_REF]Paschmann et al., 2002), attributed to one or more double layers in the auroral cavity (e.g., [START_REF] Ergun | Auroral particle acceleration by strong double layers: The upward current region[END_REF]. The energy shifts are interpreted as the spacecraft passing below a time stationary potential structure with a "U" shape (the simplest structure), an "S" shape, or more complicated shapes (see example schematics in our Figure 3 or in [START_REF] Marklund | Cluster observations of an auroral potential and associated field-aligned current reconfiguration during thinning of the plasma sheet boundary layer[END_REF]. The potentials associated with U shapes do not reach down to the atmosphere, while some or all of those associated with S shapes do. The inverted-V signature is a result of the spatial distribution of these potential structures and thus to be observed below these structures. Inverted-V electron signatures have been observed by a few spacecraft, for example, Freja (e.g., [START_REF] Haerendel | Inverted-V events simultaneously observed with the Freja satellite and from the ground[END_REF] and the Fast Auroral Snapshot (FAST) mission (e.g., [START_REF] Partamies | Statistical study of inverted-V structures in FAST data[END_REF], and the potential drop tends to be located above Freja's orbit at ∼1,700 km. However, the shape and altitude distribution of potential structures vary significantly depending on the boundary conditions. At Earth, the formation of this potential drop is associated with a shear in the magnetic field at the open-closed field line boundary or a twist in the magnetic field, indicating a field-aligned current. When this current cannot be sustained by ambient plasma, a parallel electric field is needed. At Jupiter, inverted-V electron acceleration events were also observed by Juno but were not the dominant source of the intense auroral emission, even though the acceleration is an order of magnitude more intense than at Earth [START_REF] Mauk | Discrete and broadband electron acceleration in Jupiter's powerful aurora[END_REF].

At Mars, the discovery of aurora was reported by [START_REF] Bertaux | Discovery of an aurora on Mars[END_REF] using images from the Spectroscopy for Investigation of Characteristics of the Atmosphere of Mars (SPICAM) onboard Mars Express (MEx). [START_REF] Leblanc | Origins of the Martian aurora observed by Spectroscopy for Investigation of Characteristics of the Atmosphere of Mars (SPICAM) on board Mars Express[END_REF] and [START_REF] Liemohn | Numerical modeling of the magnetic topology near Mars auroral observations[END_REF] suggest that low-energy electrons peaking at 20-30 eV (i.e., unaccelerated solar wind electrons or ionospheric photoelectrons) might be responsible for emissions observed by SPICAM. [START_REF] Brain | On the origin of aurora on Mars[END_REF] reported inverted-V electron acceleration events with measurements from the Mars Global Surveyor (MGS) spacecraft, where sharp spectral peaks were observed from 100 eV to 2.5 keV, and preferentially near the Martian crustal magnetic fields. For one orbit example in [START_REF] Brain | On the origin of aurora on Mars[END_REF], abrupt changes in magnetic field were observed at the edges of the inverted-V structure, indicative of field-aligned currents, which means the quasistatic electron acceleration was occurring similar to what often is associated with the inverted-V events observed at Earth. However, no UV observations that could have coincidently identified aurora were made. Later, simultaneous auroral emission and accelerated electrons were observed by MEx, as reported by [START_REF] Leblanc | Observations of aurorae by SPICAM ultraviolet spectrograph on board Mars Express: Simultaneous ASPERA-3 and MARSIS measurements[END_REF]. MEx also observed counterstreaming accelerated ions and electrons, indicative of an acceleration by a large-scale potential structure [START_REF] Lundin | Auroral plasma acceleration above Martian magnetic anomalies, The Mars plasma environment[END_REF]. However, the lack of magnetic field measurements made it difficult to determine the acceleration process. In addition, it is worth mentioning that other types of auroras, triggered by other particle sources, are observed at Mars, including auroras produced by the solar energetic particles (SEP) electrons [START_REF] Schneider | Discovery of diffuse aurora on Mars[END_REF][START_REF] Schneider | Global aurora on Mars during the September 2017 space weather event[END_REF] and auroras by solar wind proton precipitation [START_REF] Deighan | Discovery of a proton aurora at Mars[END_REF][START_REF] Ritter | Observations of the proton aurora on Mars with SPICAM on board Mars Express[END_REF].

During late February to early March 2019, the Imaging Ultraviolet Spectrograph (IUVS) instrument [START_REF] Mcclintock | The Imaging Ultraviolet Spectrograph (IUVS) for the MAVEN mission[END_REF] onboard MAVEN [START_REF] Jakosky | The Mars Atmosphere and Volatile Evolution (MAVEN) mission[END_REF] repeatedly observed localized auroral emissions near periapsis over the southern strong crustal field region at Mars. During these orbits, the Solar Wind Electron Analyzer (SWEA) instrument [START_REF] Mitchell | The MAVEN solar wind electron analyzer[END_REF] observed accelerated electrons at similar locations. In addition, magnetic vector field measurements from the magnetometer (MAG) instrument [START_REF] Connerney | The MAVEN magnetic field investigation[END_REF] identified disturbances in the magnetic field suggesting field-aligned currents associated with the accelerated electrons. In this study, we utilize data from SWEA and MAG onboard MAVEN to investigate the mechanism for the observed electron acceleration with an event on 26 February 2019.

MAVEN Observations

Observations from MAVEN on 26 February 2019 are displayed in Figure 1, when the spacecraft passed through a region of strong crustal magnetic fields (geographic latitude from -58 • to 15 • and longitude from 163 • to 189 • ) near periapsis and in the darkness (solar zenith angle 97-144 • ). Superthermal electron voids (electron fluxes close to the instrument background flux level) were observed for several time intervals in Figure 1a4, indicating closed crustal magnetic field loops, a common topology in this region (e.g., [START_REF] Steckiewicz | Altitude dependence of nightside Martian suprathermal electron depletions as revealed by MAVEN observations[END_REF][START_REF] Steckiewicz | Comparative study of the Martian suprathermal electron depletions based on Mars Global Surveyor, Mars Express, and Mars Atmosphere and Volatile EvolutioN mission observations[END_REF]. Between these voids, electron flux spikes are seen with one-sided loss cones (Figures 1a3 and1b5), generally interpreted as electron precipitation along open-field lines in the cusps (e.g., [START_REF] Brain | Electron pitch angle distributions as indicators of magnetic field topology near Mars[END_REF][START_REF] Lillis | Nightside electron precipitation at Mars: Geographic variability and dependence on solar wind conditions[END_REF][START_REF] Weber | Characterization of low-altitude nightside Martian magnetic topology using electron pitch angle distributions[END_REF][START_REF] Weber | The influence of solar wind pressure on Martian crustal magnetic field topology[END_REF][START_REF] Xu | Martian low-altitude magnetic topology deduced from MAVEN/SWEA observations[END_REF].

From 11:05:50 universal time (UT) to 11:06:50 UT, the pitch angle (PA)-averaged electron energy spectra from SWEA (Figure 1b6) exhibit an organized signature in time, high electron fluxes concentrated in a narrow energy range with decreasing peak energies from ∼500 to ∼40 eV and then increasing peak energies back to ∼500 eV. The descending and ascending peak energies resemble the inverted-V structures at Earth, and each spectral shape is similar to localized electron acceleration at Earth [START_REF] Evans | Precipitating electron fluxes formed by a magnetic field aligned potential difference[END_REF]. We therefore interpret these data as the spacecraft passing in and out of magnetic field lines that have a field-aligned potential structure above the spacecraft. A more detailed analysis is presented in section 3.

In Figure 1b2, we show the variation (ΔB) for each magnetic vector component by subtracting two adjacent 1-s magnetic vector measurements, that is, ΔB = B t -B t-1 for time-ordered data at a 1-s or 4-km cadence, which means we filter out variations less than 1 s. The smooth, systematic variation of the magnetic field over timescales of ∼1 min (∼240 km) results from passing through the large-scale magnetic field originating from crustal sources below the spacecraft. Small-scale and more abrupt perturbations in magnetic field are seen during 11:05:45-11:05:55 UT and 11:06:35-11:07:00 UT, right at the edges of the accelerated electron structure, indicative of a field-aligned current.

Field-Aligned Potential Estimation

To confirm that the observed energetic electrons are accelerated by field-aligned potentials, we examine whether the phase space densities are similar in shape to each other but shifted in energy, as illustrated in Figure 2. Here, phase space densities are obtained by averaging over PAs 0-60 • to account for downward electrons only, as the magnetic field points radially inward (Figure 1b1), which are mostly absorbed and form loss cone PAs 120-180 • (as shown in Figure 1b5). A few observations can be made. (1) The phase space density of the accelerated (blue) spectra remains roughly constant from a low energy (∼25-60 eV) to some higher energy (E 0 ) and then drops exponentially with increasing energy. This flat-top shape is interpreted XU ET AL. as electrons being accelerated by an electrostatic potential, with a magnitude comparable to E 0 , similar to electrons in the sheath being accelerated by the cross-shock potential (e.g., [START_REF] Feldman | Electron velocity distributions near the Earth's bow shock[END_REF][START_REF] Schwartz | Collisionless electron dynamics in the magnetosheath of Mars[END_REF]. Some departures from the flat-top shape may be caused by a cold plasma beam (probably the local plasma at the potential layer) also accelerated by this potential drop [START_REF] Akbari | Spectral analysis of accelerated electron populations at Mars[END_REF].

(2) The high fluxes at low energies (<∼ 25 eV) are probably from a combination of secondary electrons created by impact ionization as these energetic beams interact with the neutral atmosphere below the spacecraft and also secondary electrons produced within the instrument by these beams.

(3) The maximum acceleration is ∼300 eV higher than what is observed in the sheath (not shown) so that these electrons cannot be simply precipitating sheath electron, but an acceleration mechanism is required, implying a field-aligned potential.

We can infer field-aligned potentials from the shift in energy of these accelerated distribution functions (DFs) from the source electron spectrum. We assume the source electron spectrum to be the least accelerated spectrum (associated with magnetic flux tubes not connected to the potential layer), which we obtain by averaging over distributions within the time range marked by two magenta vertical lines in Figure 1b6. The power law decrease at high energies is similar for the blue spectra and the source spectrum, suggesting these electrons share the same source but are shifted by a range of potentials. The energy shift from the source spectrum to the most accelerated blue spectrum is ∼440 eV for both Period 1 (11:05:48-11:06:05 UT, XU ET AL. A schematic shows the geometry of the magnetic field, the field-aligned current, and the spacecraft motion in the local geographic frame, as well as the U-and/or S-shape potential structure and auroral emission by electron impact. The current sheet is perpendicular to the page with its projection the same as the magnetic field line, and the current within is antiparallel to the magnetic field. Δx = ΔL ns sin 𝜃 is the distance perpendicularly across the magnetic field, as the spacecraft moved horizontally from south to north (ΔL ns ), and 𝜃 is the magnetic elevation angle.

Figure 2a) and Period 2 (11:06:28-11:06:48 UT, Figure 2b). In other words, the field-aligned potentials can be up to ∼440 V, or more if the spacecraft did not pass through field lines that sample the entire potential layer. The magnitude of this potential drop is similar to results based on observations from other Mars orbiters (e.g., [START_REF] Brain | On the origin of aurora on Mars[END_REF][START_REF] Lillis | Field-aligned electrostatic potentials above the Martian exobase from MGS electron reflectometry: Structure and variability[END_REF][START_REF] Lundin | Auroral plasma acceleration above Martian magnetic anomalies, The Mars plasma environment[END_REF].

These accelerated blue DFs in Figures 2a and2b match up with the source electron spectrum down to 20 eV or higher energies but not very low energies. Correspondingly, the source electrons have an isotropic distribution for PAs 0-180 • below 20 eV but a loss cone above, as shown in Figure 2c. It is probable that this source electron spectrum experienced an acceleration around 20 V, and it is mainly secondary electrons below 20 eV.

Current Estimation

In this section, we evaluate whether the potential formation is similar to what occurs at Earth: Somewhere above the spacecraft, the ambient plasma cannot support an imposed field-aligned current, so that a potential forms to accelerate electrons to carry the current. To investigate this, we estimate field-aligned currents from magnetic field perturbations and then compare them to the net electron current.

To estimate currents from B, we take the time period of 11:05:45-11:05:55 UT as an example, as highlighted by the black bar in Figure 1b2, during which the perturbation in magnetic field is mainly in the local east-west direction (ΔB ew ) and has a clear baseline or background field of -1.7 nT.

We thus estimate the maximum magnetic disturbance of 4.2 nT in the east-west direction from a current layer by subtracting this background field of -1.7 nT. Over the same time interval, the spacecraft moved mainly horizontally from south to north, which gives a length scale in the north-south direction of ΔL ns ∼ -3.8 km with a negligible radial motion Δr ∼ -0.5 km. We can then simplify the geometry to what is shown in Figure 3 in the local geographic frame as it is a small-scale structure (∼50 km). In other words, assuming this current structure to be stable for this 10-s time period, the current sheet is perpendicular to the page with its projection the same as the magnetic field line, and the current within is antiparallel to the magnetic field. The spacecraft was crossing this current sheet at an angle equivalent to the magnetic elevation angle (𝜃) as the spacecraft moved horizontally from south to north instead of traversing the current sheet. This current sheet gave rise to a perturbation in magnetic field mostly in the east-west direction (ΔB ew ). Therefore,

j // = 1 𝜇 0 ΔB ew Δx = 1 𝜇 0 ΔB ew
ΔL ns sin 𝜃 , where Δx = ΔL ns sin 𝜃 is the distance perpendicularly across the magnetic field and 𝜃 =∼ -60 • for this time period. The calculated field-aligned current density is displayed in Figure 1b3 as the blue curve, with a maximum magnitude of max(|𝑗 ∕∕ |) = 1.1 μA/m 2 , which is of the same order as terrestrial auroral currents. The field-aligned current j // is negative with respect to the radially inward magnetic field (Figure 1b1), and thus upward, consistent with downward traveling electrons being the main current carrier.

Similarly, for Period 2, the perturbation in magnetic field is mainly in the north-south direction but spreads over a larger spatial distance, which means the field-aligned current density is smaller than Period 1. These differences between Periods 1 and 2 might be explained by the spacecraft passing through different parts of the same potential/current structure or temporal variation of these structures. It is worth noticing that the background magnetic field is less clear for this period, and therefore, we carry out our quantitative calculation for Period 1 only.

These downward field-aligned currents are carried predominantly by electrons because of their high mobility, mainly precipitating superthermal (solar wind origin) electrons above the potential layer. We estimate the net current density carried by superthermal electrons (the red line in Figure 1b4) by integrating the XU ET AL. The ratio of the presumed magnetic field strength and the magnetic field strength at the spacecraft (Bsc) as a function of altitude. The black and blue lines assume the magnetic field amplitude vary as a function of r -3 and r -2 , respectively, where r is the distance from the dipole source and effectively the distance from the surface here. The green line is the profile based on magnetic field line tracing from a crustal field model [START_REF] Morschhauser | A spherical harmonic model of the lithospheric magnetic field of Mars[END_REF] starting at the spacecraft location at around 11:05:50 UT. The vertical red dashed line marks the critical ratio of ∼0.08.

DF for PA 0-60 • (downgoing) subtracted from that for PA 120-180 • (upgoing). The current density of these accelerated electrons has a maximum value of max(|𝑗 se |) = 2.5 μA/m 2 for Period 1, agreeing reasonably well with the maximum current density derived from the magnetic perturbation (max(|𝑗 ∕∕ |) = 1.1 μA/m 2 ). The factor of two difference is probably because the spacecraft crossed the current sheet at an angle (i.e., the current sheet is rotated in the north-south direction from the east-west direction in the horizontal plane) or did not cross the center of the current sheet so that we underestimate the maximum field-aligned current density. Similarly, for Period 2, the maximum electron current density reaches 0.5 μA/m 2 , corresponding to a smaller current density from magnetic disturbances. In other words, the net current carried by the acceleration superthermal electrons agrees with the field-aligned current inferred from the observed magnetic perturbation.

Moreover, source electrons (Figure 2c) can be identified from SWEA observations, which give a net electron current < 1 × 10 -2 μA/m 2 (Figure 1b4), much smaller than the field-aligned current from ΔB. This suggests that the source electrons cannot carry the imposed current unless they are accelerated by an electrostatic potential.

Discussion of the Potential Layer

We can estimate the lower altitude limit of the potential layer with PA distributions of the source electrons and accelerated electrons. As shown in Figure 2c, the precipitating source electrons are observed to be isotropic for PA 0-90 • . At high altitudes, isotropic source electrons become more field-aligned immediately after acceleration by the potential layer. As these now more beamed electrons travel to lower altitudes, the crustal magnetic field gets stronger and electron distributions broaden again in PA to conserve the first adiabatic invariant. These accelerated electrons are observed to be nearly isotropic again at the spacecraft for PA 0-90 • for energies above 100 eV (e.g., P-1 and P-2 in Figure 1b5), which establishes a minimum ratio of the magnetic field strength at the potential layer to that at the spacecraft.

Assume that, just above the potential layer, electrons have an energy 𝜀 0 and an isotropic distribution for PA 0-90 • and a local magnetic field strength B 0 . After an acceleration of 𝜙 by the potential layer, these electrons have a width of PA of 𝜃 = sin -1 ( √ 𝜀 0 ∕(𝜀 0 + e𝜙)) immediately below the potential layer. At a critical altitude with a magnetic field strength of B C , the PA width becomes 90 • , which means B 0 ∕B C = sin 2 𝜃 = 𝜀 0 ∕(𝜀 0 + e𝜙). This critical altitude has to be above the spacecraft so that B SC > B C , which gives B 0 ∕B SC < 𝜀 0 ∕(𝜀 0 + e𝜙), where B SC is the magnetic field strength at the spacecraft. To estimate the lower altitude limit of the potential layer, given 𝜙 = 440 V, we take the minimum 𝜀 0 to be the temperature of the source electrons. By fitting the source distribution with a Gaussian distribution for 40-100 eV (as shown in supporting information Figure S1), we obtain T e = 37 eV and n e = 0.44 cm -3 , which gives B 0 ∕B SC < 0.08. Figure 4 shows the ratio of the presumed magnetic field strength and B SC against altitude for different approximations of the magnetic field configuration, B ∼ r -3 for a magnetic dipole (black), B ∼ r -2 for a line dipole (blue), and field tracing results from a crustal field model (Morschhauser et al., 2014) (green). The distance of the spacecraft from the crustal magnetic sources (∼200 km) is small compared with the east-west extent of the crustal sources (>2,000 km), so the latter two are better approximations. With the vertical red line marking the ratio of 0.08, we estimate the lower altitude limit for the potential layer to be around 750-850 km.

We can also estimate the magnitude of the associated field-aligned electric field. Assuming that the acceleration of these electrons comes from one double layer, then one can estimate the maximum parallel electric field associated with the acceleration. The thickness of a double layer at Earth is often ∼10 Debye lengths [START_REF] Andersson | Characteristics of parallel electric fields in the downward current region of the aurora[END_REF]. As thermal plasma from below does not have sufficient energy to diffuse past the potential layer, we estimate the Debye length (𝜆 D ) at the potential layer with the hot population, that is, the source electrons, which is ∼70 m. Therefore, assuming the thickness to be 10𝜆 D , given a total potential drop of 440 V, we can obtain a parallel electric field of ∼0.6 V/m, similar to typical values at Earth. The location of the potential layer can also be inferred from a statistical analysis of many electron acceleration events. Generally, field-aligned accelerated electrons are observed below the potential layer so that the XU ET AL.

10.1029/2020GL087414 altitude distribution of these accelerated electrons can establish a lower altitude limit for the potential layer. [START_REF] Brain | On the origin of aurora on Mars[END_REF] reported tens of thousands of peaked spectra from MGS observations, which were made at a fixed altitude of ∼400 km and fixed local times of 2 a.m./p.m. This suggests that many of the potential layers, assuming these are what caused the peaked spectra, occurred above 400-km altitude. Alternatively, field-aligned potentials can be inferred from the energy dependence of loss cone sizes, as well as accelerated ion beams, above the potential layer (e.g., [START_REF] Lundin | Plasma energization on auroral field lines as observed by the Viking spacecraft[END_REF][START_REF] Lundin | Auroral plasma acceleration above Martian magnetic anomalies, The Mars plasma environment[END_REF][START_REF] Mcfadden | Spatial structure and gradients of ion beams observed by FAST[END_REF]. It is reasonable to assume some of large field-aligned potentials derived from loss cone shapes with MGS data by [START_REF] Lillis | Field-aligned electrostatic potentials above the Martian exobase from MGS electron reflectometry: Structure and variability[END_REF] can be explained by the mechanism discussed here, which means potential layers might occur below 400 km as well.

Concluding Remarks

In this study, we analyze an electron acceleration event on 26 February 2019, closely associated in time and space with localized auroral emissions detected by the IUVS instrument. We infer the field-aligned potentials to be up to ∼440 V from electron observations. At the edges of the most intense electron acceleration, significant magnetic perturbations were observed, which suggest the existence of a field-aligned current sheet. We estimate a maximum upward current density of max(|𝑗 ∕∕ |) ∼ 1.1 μA/m 2 . Meanwhile, the electron current carried by the identified source electrons is much smaller than max(|𝑗 ∕∕ |), but that carried by the accelerated electrons can be up to 2.5 μA/m 2 in locations where magnetic perturbations were observed. This set of observations suggests a mechanism very similar to what occurs at Earth: There exists a shear in magnetic fields at the open-closed field line boundary or a twist in magnetic fields, indicating a field-aligned current; when this current cannot be sustained by ambient plasma, a parallel electric field is needed. The potential acceleration gives the inverted-V electron signature.

We estimate that the lower altitude limit of the potential layer is around 750-850 km from the PA distributions of the source electrons and accelerated electrons. As the thickness of the potential layer is typically a few Debye lengths, we obtain 𝜆 D = 70 m, and the associated parallel electric field is therefore estimated to be ∼0.6 V/m. Finally, we have manually searched for and identified inverted-V electron events from late February to March 2019 for when auroras were observed and found that these events mainly occurred near the terminator and over the strong crustal magnetic fields. This is consistent with MGS observations (e.g., [START_REF] Brain | On the origin of aurora on Mars[END_REF] of a clustering of inverted-V events near the strong crustal regions. This might suggest the importance of a significant intrinsic field, like Earth, for such a mechanism to operate. A more systematic statistical analysis is, however, needed to characterize the general behavior of these events.

  a field-aligned potential drop of 440 V during an inverted-V electron acceleration event concurring with auroral observations • The current density estimated from the observed magnetic perturbation reaches 1.1 μA/m2, agreeing with estimated net electron current • Similar to Earth, this potential drop develops when the ambient plasma is unable to support the necessary field-aligned current 10.1029/2020GL087414

Figure 1 .

 1 Figure1. Time series on 26 February 2019 of (a1) MAVEN altitudes, (a2) observed (MAG, blue) and modeled[START_REF] Morschhauser | A spherical harmonic model of the lithospheric magnetic field of Mars[END_REF] (red) magnetic field magnitudes, normalized pitch angle distribution for 111-140 eV (a3) and averaged electron energy spectra (a4) measured by SWEA. Panels (b1)-(b6) show the zoom-in orbit: (b1) magnetic vectors (nT) relative to the local geographic plane (blue for the local east direction, green for the local north direction, and red for the radial direction), (b2) ΔB (nT) for each magnetic vector component (i.e., ΔB = B t -B t-1 for time-ordered data at a 1-s or 4-km cadence), (b3) field-aligned current density (A/m 2 ) estimated from magnetic field measurements, (b4) the absolute parallel current density from ΔB (blue) and the net electron current carried by superthermal electrons (red), (b5) normalized pitch angle distribution for 111-140 eV, and (b6) pitch angle-averaged electron energy spectra measured by SWEA. Electron spectra within vertical dashed lines in panel (b6) are plotted in Figure2. The gray vertical stripes below 30 eV in (a4), for example, 10:50-11:00 and 11:25-11:40 UT, identify data affected by sporadic low-energy electron suppression, an instrumental effect that occurred from January to April 2019 and is well understood.

Figure 2 .

 2 Figure 2. Electron distribution functions (DF) averaging over pitch angles 0-60 • (downward traveling) for the two time periods marked by vertical black lines in Figure 1b6, (a) for Period 1 (11:05:48-11:06:05 UT) and (b) for Period 2 (11:06:28-11:06:48 UT), colored in light (earlier times) to dark (later times) blue. The solid magenta lines are the assumed source electron DF to be compared with, averaging over a time range marked by the two vertical magenta lines in Figure 1b6. The dashed magenta lines are the source electron DF (only >20 eV used) shifted by some potentials (phi, labeled in the upper left corners). (c) The normalized pitch angle distribution for each energy, resampled into 128 × 1.4 • bins, for source electrons.

Figure 3 .

 3 Figure3. A schematic shows the geometry of the magnetic field, the field-aligned current, and the spacecraft motion in the local geographic frame, as well as the U-and/or S-shape potential structure and auroral emission by electron impact. The current sheet is perpendicular to the page with its projection the same as the magnetic field line, and the current within is antiparallel to the magnetic field. Δx = ΔL ns sin 𝜃 is the distance perpendicularly across the magnetic field, as the spacecraft moved horizontally from south to north (ΔL ns ), and 𝜃 is the magnetic elevation angle.

Figure 4 .

 4 Figure 4. The ratio of the presumed magnetic field strength and the magnetic field strength at the spacecraft (Bsc) as a function of altitude. The black and blue lines assume the magnetic field amplitude vary as a function of r -3 and r -2 , respectively, where r is the distance from the dipole source and effectively the distance from the surface here. The green line is the profile based on magnetic field line tracing from a crustal field model[START_REF] Morschhauser | A spherical harmonic model of the lithospheric magnetic field of Mars[END_REF] starting at the spacecraft location at around 11:05:50 UT. The vertical red dashed line marks the critical ratio of∼0.08.

Acknowledgments

This work was supported by the National Aeronautics and Space