Deborah Baker

Lidia Van Driel-Gesztelyi

David H Brooks

Pascal D Émoulin

Gherardo Valori

David M Long

J Martin Laming

Andy S H To

Typeset using L A T E X twocolumn style in AASTeX62 Can Sub-photospheric Magnetic Reconnection Change the Elemental Composition in the Solar Corona?

Keywords: Sun, abundances -Sun, corona -Sun, magnetic fields

Within the coronae of stars, abundances of those elements with low first ionization potential (FIP) often differ from their photospheric values. The coronae of the Sun and solar-type stars mostly show enhancements of low-FIP elements (the FIP effect) while more active stars such as M-dwarfs have coronae generally characterized by the inverse-FIP effect (I-FIP). Here we observe patches of I-FIP effect solar plasma in AR 12673, a highly complex βγδ active region. We argue that the umbrae of coalescing sunspots and more specifically strong light bridges within the umbrae, are preferential locations for observing I-FIP effect plasma. Furthermore, the magnetic complexity of the active region and major episodes of fast flux emergence also lead to repetitive and intense flares. The induced evaporation of the chromospheric plasma in flare ribbons crossing umbrae enables the observation of four localized patches of I-FIP effect plasma in the corona of AR 12673. These observations can be interpreted in the context of the ponderomotive force fractionation model which predicts that plasma with I-FIP effect composition is created by the refraction of waves coming from below the chromosphere. We propose that the waves generating the I-FIP effect plasma in solar active regions are generated by sub-photospheric reconnection of coalescing flux systems. Although we only glimpse signatures of I-FIP effect fractionation produced by this interaction in patches on the Sun, on highly active M-stars it may be the dominant process.

INTRODUCTION

Recent spectroscopic observations of the Sun have shown how the elemental composition of the solar atmosphere varies in space and time. The variability of composition appears to be intrinsically linked to the distribution and evolution of the Sun's magnetic field on all scales (e.g. Brooks et al. 2017;Baker et al. 2018). More generally, the overall composition of a star's corona depends on the first ionization potential (FIP) of the main elements comprising the corona. In solar-type stars, low-FIP elements are observed to be overabundant relative to their photospheric abundances, while high-FIP elements maintain their photospheric abundances (the FIP effect; e.g. Laming et al. 1995;Wood & Linsky 2010;Testa et al. 2015;Laming 2015;Brooks et al. 2017). In cooler, more active stars such as M-dwarfs, low-FIP/high-FIP elements are under-/over-abundant in the corona compared to the photosphere (the inverse FIP or I-FIP effect; Laming 2015; Brooks 2018). Wood & Linsky (2010) and Wood et al. (2018) first established an almost linear relationship between stellar composition and spectral type F to M in the X-ray spectra of moderately active stars (X-ray luminosity <10 29 ergs s -1 ). In addition, a dependence has been found of coronal composition on stellar magnetic activity, with high-activity stars having I-FIP-effect coronae and low-activity stars like our Sun having FIP effect dominated coronae (Audard et al. 2003;[START_REF] García-Alvarez | Cambridge Workshop on Cool Stars, Stellar Systems, and the Sun[END_REF]Testa et al. 2015).

One plasma fractionation model that is able to account for the FIP and I-FIP effects observed in stars is the ponderomotive force model (Laming 2015). The model invokes the ponderomotive force exerted by Alfvén waves giving rise to ion-neutral separation in the chromosphere of the Sun and other stars. Fractionation takes place in the chromosphere where the temperature and density gradients are high and temperatures are such that low-FIP elements are mainly ionized and high-FIP elements remain neutral. The direction of the ponderomotive force determines whether low-FIP elements become enhanced or depleted in the corona. Downward propagating Alfvén waves generated by magnetic reconnection in the corona are mostly reflected back into the corona at the high density gradient in the chromosphere, creating an upward-directed ponderomotive force acting on the ions (Laming 2017). When the plasma is observed in the corona, the low-FIP elements are over-abundant relative to the photosphere. An inverse FIP effect is created when upward-traveling waves undergoing reflection or refraction back downwards produce a downward-directed ponderomotive force acting on the ions in the chromosphere. Such upward-propagating waves can be generated, for instance, by magneto-acoustic waves originating from below the chromosphere that are mode converted to fast mode waves at the plasma β = 1 boundary. As a consequence of the downward ponderomotive force, the plasma is then depleted of low-FIP elements (Laming 2015;Brooks 2018). In this article, we present observations that suggest a more specific origin for the waves producing the I-FIP effect, namely a subphotospheric one.

Though the Sun's corona is dominated by the FIP effect (Laming et al. 1995;Brooks et al. 2015Brooks et al. , 2017)), recent spectral scans taken by the EUV Imaging Spectrometer (EIS) (Culhane et al. 2007) on board the Hinode spacecraft (Kosugi et al. 2007) have shown highly localized regions of I-FIP effect plasma near sunspots in flare spectra (Doschek et al. 2015;Doschek & Warren 2016, 2017). Doschek & Warren (2017) proposed that the FIP effect 'shuts down' near sunspots where p-modes are suppressed, leading to areas of photospheric, weak I-FIP, or I-FIP effect plasma, while the rest of the active region has coronal composition.

At low spatial resolution, solar coronal abundances in flares have been previously analyzed and shown to be very different from quiescent conditions (e.g. Sylwester et al. 1984;Feldman & Widing 1990;Warren 2014), having a tendency to be close to photospheric values. Composition studies of stellar flares provide strong indications of a similar effect, i.e. that the elemental abundances tend to approach photospheric values both on FIP-effect and I-FIP-effect dominated stars (e.g. Nordon & Behar 2008;Laming & Hwang 2009). Recently, Katsuda et al. (2020) reported the I-FIP effect in four large X-class flares (X17.0, X5.4, X6.2 from AR 10808 and X9.0 from AR 10930) derived from Earth albedo X-ray spectra from the imaging spectrometer on board the Suzaku astronomical satellite. Baker et al. (2019) analyzed in detail Hinode/EIS observations of plasma composition during the decay phase of an M-class flare in AR 11429. Patches of I-FIP effect plasma appeared in a highly sheared emerging flux region above sunspot umbrae 10 min after the flare peak and disappeared 40 min later. The authors proposed that sub-chromospheric reconnection of highly sheared coalescing strands of the same polarity magnetic field played a key role in the creation of I-FIP effect plasma within the highly complex magnetic field of the active region. During episodes of strong emergence, flux approached and interacted with pre-existing magnetic field, forcing the coalescence of the smaller flux fragments into growing, coherent umbrae surrounded by common penumbrae in two of the sunspots. The convergence of magnetic field in the location of the coalescing umbrae is highly suggestive of the presence of sub-chromospheric magnetic reconnection. Reconnection that occurs below the region of plasma fractionation in the chromosphere generates a fast-mode wave flux in the direction required by the ponderomotive force fractionation model to produce I-FIP effect plasma (Laming 2015). The fractionated plasma is then evaporated into the corona by flaring and observed by Hinode/EIS when flare ribbons cross the coalescing umbra.

In this paper, magnetic field and continuum observations of the most unusual and complex magnetic field of AR 12673 are combined with Hinode/EIS scans to investigate the presence of I-FIP effect plasma. We confirm the findings of Baker et al. (2019) and provide evidence in support of the generation of I-FIP effect plasma by sub-photospheric reconnection in coalescing sunspot umbrae with strong light bridges (LBs). The paper is organized as follows: Section 2 describes the magnetic field evolution, Section 3 details the Hinode/EIS observations, Section 4 provides our analysis and interpretation of the observations of anomalous plasma composition in the context of the ponderomotive force fractionation model, and in Section 5 we present our conclusions.

MAGNETIC FIELD EVOLUTION OF AR 12673

AR 12673 was visible on the Sun from 2017 August 28 to September 10. Its magnetic field evolved into one of the most complex structures observed during solar cycle 24 [START_REF] Van Driel-Gesztelyi | SoPh[END_REF]. The sunspot group's Mount Wilson magnetic classification was βγδ, the total unsigned flux exceeded 5×10 22 Mx and the projected whole spot area peaked on September 6 at ∼2,100 MSH (millionths of a solar hemisphere; from the Debrecen Photoheliographic Database: http://fenyi.solarobs.csfk.mta.hu/DPD/ 2017/index.html). Overall activity included 4 X-, 27 M-, and 55 C-class flares, making AR 12673 the most flare produc-tive of solar cycle 24. Several significant coronal mass ejections (CMEs) were launched during its disk passage (Redmon et al. 2018). Furthermore, this 'monster' active region exhibited highly unusual characteristics and extreme behavior including the fastest flux emergence ever recorded (Sun & Norton 2017), the strongest transverse (Wang et al. 2018) and coronal magnetic fields (Anfinogentov et al. 2019).

Figure 1 shows selected co-temporal radial magnetic field and continuum images from the Helioseismic and Magnetic Imager (HMI; Scherrer et al. 2012) on board the Solar Dynamics Observatory (SDO; Scherrer et al. 2012). The data are from the Spaceweather HMI Active Region Patch (SHARP; Bobra et al. 2014) pipeline. The enclosed movie of the figure is entitled Fig1 movie.mp4. A full account of the structure and evolution of the magnetic field leading to the first direct evidence of sub-photospheric reconnection is provided in [START_REF] Van Driel-Gesztelyi | SoPh[END_REF]. The main features are summarized here using the same notation.

AR 12673 was formed from the emergence and subsequent interaction of three flux systems labeled in Figure 1: a longlived, stable positive spot P 0 ; an arcade system formed of n 1 -p 1 and n 2 -p 2 ; and the main flux system composed of N 3 -P 3 and N 4 -P 4 (the arcade/main flux systems are labeled in gray/black in Figure 1). Early on September 2, bipole n 1 -p 1 emerged to the south-east of pre-existing P 0 . Bipole n 2 -p 2 followed one day later and emerged to the north-east of P 0 . Very quickly the two bipoles formed a coherent arcade system with a C-shaped magnetic polarity inversion line (PIL), as the fast moving emerging flux wrapped around the wellanchored P 0 . On September 3, the main field comprising bipoles N 3 -P 3 and N 4 -P 4 emerged in a dominantly N-S direction underneath the E-W oriented arcade system represented by n 1 -p 1 and n 2 -p 2 . At the location where the main system emerged into the arcade system lying above it, an Mshaped flux tube was present. It was formed by a concave-up U-loop (van Driel-Gesztelyi et al. 2000) between N 4 and P 3 and two lobes of concave-down Ω loops, P 3 -N 3 in the north and P 4 -N 4 in the south (Fan et al. 2003;Toriumi et al. 2014).

Key aspects of the active region's magnetic field evolution occurred in the time period of September 4-6 leading up to the times of the X2.2 and X9.3 flares. First, during the ongoing emergence of the constituent bipoles of the three flux systems, strands of same polarity magnetic field coalesced to form coherent sunspot umbrae in the active region core to the east of P 0 (at p 2 and N 4 ) and at its periphery to the north and south-west at n 2 and p 1 , respectively. These were also the locations where significant LBs formed between merging umbral flux strands. Coalescence was driven by especially strong and fast episodes of flux emergence on September 4-6 (see Fig1 movie.mp4). Second, early on September 6, the north-western part of N 4 detached and began moving northward. This detached polarity penetrated the 'wall' of opposite polarity field along P 3 -p 2 (Figure 1, middle panels), then it broke in two pieces. One piece, N 4u , mostly rotated counter-clockwise around P 3 indicating that it was still bound by the U-loop anchorage (Figure 1, bottom panels).

The other piece, N 4f , moved rapidly northward, then westward in the direction of N 3 . On September 7, the main flux system -at least temporarily-ceased its emergence into the E-W arcade flux system thereby removing the major driver of the unusual and extreme magnetic field evolution in AR 12673.

OBSERVATIONS OF AR 12673

In this analysis, we focus on the three day period encompassing the largest flare of solar cycle 24, the X9.3 flare that peaked at ∼11:53 UT on September 6. Figure 2 shows the GOES 1-8 Å soft X-ray light curve from 00:00 UT on September 5 -00:00 UT on September 8 when Hinode/EIS was observing AR 12673 employing a multitude of studies and scanning modes. However, not all studies contain suitable emission lines from high-and low-FIP elements for measuring plasma composition. The basic details of the studies used here are provided in Table 1. The times of the Hinode/EIS composition observations are indicated by the dashed red lines plotted with the GOES soft X-ray curve in Figure 2.

The studies listed in Table 1 contain the high-FIP Ar XIV 194.40 Å (FIP = 15.76 eV) and low-FIP Ca XIV 193.87 Å (FIP = 6.11 eV) emission lines. The two ions have similar contribution functions therefore their intensity ratio is suitable to determine (I)FIP levels within the AR (Feldman et al. 2009). In line with Doschek et al. (2015), Doschek & Warren (2016, 2017) and Baker et al. (2019), we use the following log 10 abundance values relative to log 10 H = 12: Middle and lower panels correspond to the times of when Hinode/EIS observed I-FIP effect plasma (Figures 3,4,5). Major components of the flux systems are labeled in each image: arcade field n1-p1 and n2-p2 (labels in gray); main field N3-P3 and N4-P4 (labels in black), and pre-existing, well anchored spot P0. The U-loop is formed in between N4-P3 as well as N4u-P3 (bottom panels). Near the end of the emergence phase, polarities N4u and N 4f detached from the north part of N4. Note that n and N notations are used for negative, p and P notations for positive polarity umbrae. Red/blue corresponds to positive/negative Br in the magnetograms. See the included movie Fig1 movie.mp4 of this figure. For ease of comparison between figures, black arrows are used to show the distances from the center of the major polarities. At 16:46 UT on September 6, the vertical arrow is ∼50 in length between the centers of P0 and P4. The horizontal arrow is ∼30 in length from the centers of P0 and N4. The center of P0 is X = 590 , Y = -242 . This convention is repeated for all figures with maps. Ca = 6.93 (Feldman 1992) and 6.33 (Caffau et al. 2011) for the corona and photosphere, respectively; Ar = 6.5 (Lodders 2008) is the same for the corona and the photosphere. Calculations of the contribution functions using these abundances were performed with the software included in the CHIANTI Atomic Database, version 8.0 (Dere et al. 1997;Del Zanna et al. 2015). Based on the intensity ratio of the contribution functions of high-FIP Ar XIV and low-FIP Ca XIV, values >1 indicate the I-FIP effect, = 1 is photospheric plasma, and <1 is FIP effect or typical solar coronal plasma. The estimated uncertainty of the ratio is ±0.28 assuming an intensity error of 20%. Considering the uncertainties, in the text and figures we refer to ratio values ≥ 1.3 as I-FIP effect plasma.

Though Ar XIV and Ca XIV have similar contribution functions, it is not immediately apparent what, if any, temperature effects flaring might have on the uncertainties of the I-FIP effect measurements as large flares can have much higher densities (N) at high temperatures (T) (e.g. Watanabe et al. 2010;Graham et al. 2011;Simões et al. 2015) compared to non-flaring active regions (Doschek et al. 2007;Tripathi et al. 2008). The Ar XIV/Ca XIV ratio can vary by a factor ∼4 in the log 10 T = 6.6-6.9 range (with T in K, see e.g. Doschek & Warren 2017;Baker et al. 2019).

We derived the mean temperatures and densities for all strong I-FIP effect regions corresponding to Table 2 where the appropriate diagnostic line pairs were included in the EIS studies (e.g. Ar XIV 187.9/194.4 Å and Fe XIII 203.8/202.0 Å). For raster times from 19:29-20:14 UT, we were only able to obtain temperature measurements. Log 10 N were in the range 10.2-11.1 (with N in cm -3 ) and the log 10 T were 6.6-6.7. These compare to values in FIP effect regions of log 10 N = 8.9-9.9 and log 10 T = 6.2-6.4, respectively.

Higher density in the I-FIP effect regions lowers the theoretical ratio, so high observed I-FIP effect values of Ar XIV/Ca XIV become more extreme in the abundance differences that they imply. Furthermore, the temperatures fall within the range that is considered to be plausible for the formation temperatures of Ar XIV and Ca XIV (Doschek & Warren 2017). It is very difficult to disentangle the density and temperature effects caused by the flaring but the increase in density combined with the fact that the temperature does not exceed log 10 T = 6.7 suggests that the density and temperature effects are unlikely to substantially affect the estimated uncertainty of ±0.28. The results may be very different for measurements made during the peak phase of large flares but this is not the case here as the rasters were timed during the decay phases of the flares on September 6. Spectroscopic data were reduced using standard routines available in the Hinode/EIS section of Solar Software (SSW; Freeland & Handy 1998). Three Gaussians were fit to the Ar XIV 194.40 Å emission line to remove two unidentified lines in its blue wing. The Ca XIV 193.87 Å line was also fit with three Gaussians to separate the line from nearby Fe X 193.72 Å, Ni XVI 194.05 Å, and 194.10 Å lines (Brown et al. 2008;Doschek et al. 2015;Baker et al. 2019).

All Hinode/EIS maps and SDO/HMI line-of-sight magnetograms were co-registered in a two step process. First, the aia prep.pro routine located in the SDO branch of SSW was used to align HMI and AIA data. The routine takes into account differences in the plate scales and roll angles between the two instruments. Second, either the Fe XII 195.12 Å or 186.88 Å EIS intensity maps were aligned by eye with the cotemporal AIA 193 Å passband images. The EIS Fe XII emission lines and AIA 193 Å broadband imager sample plasma at similar temperatures so that the same coronal structures are identifiable in each image making alignment straightforward between Hinode/EIS and SDO. To help the reader to track different features in the figures, we have added arrows to indicate the central positions of major polarities and the distances between them. A vertical arrow shows the positions and distance of the P 0 -P 4 centers (∼50 ) and a horizontal arrow shows the same for P 0 -N 4 (∼30 ).

Figure 2, lower panel, shows a sample Ar XIV/Ca XIV ratio map with/without SDO/HMI contours of ±500 G (white/magenta for positive/negative polarities) at 19:56 UT on September 6. Four regions of interest based on the magnetic flux systems described in Section 2 are labeled as follows: n 2 in the spatially extended magnetic field to the north; N 4u which detached northward from N 4 ; p 2 located on the east side of the large positive sunspot P 0 ; and p 1 along the extended positive polarity to the south. Similar ratio maps with the corresponding Ar XIV 194.40 Å and Ca XIV 193.87 Å intensity maps are displayed in Figures 3 and4.

Hinode/EIS observed AR 12673 at 02:35 UT on September 6 during a relatively quiet period (Figure 3, top panel). The core of the active region is predominantly composed of typical coronal plasma. FIP effect levels range from ∼0.3 to 0.5 using the high-FIP Ar XIV/low-FIP Ca XIV ratio, which is equivalent to the conventional solar low-FIP/high-FIP ratio of 2.0-3.3. Localized regions at p 2 and N 4 show weak indications of I-FIP effect plasma. Both patches of I-FIP effect plasma evolve to photospheric composition within ∼25 min (at 02:59 UT) and the patch associated with N 4 returns to coronal plasma within ∼50 min (at 03:23 UT) when Hinode/EIS composition observations ended.

EIS composition observations resumed at 16:13 UT during the extended decay phase of the X9.3 flare, just after the peak of an M2.5 flare at 15:56 UT (see the GOES soft X-ray light curve in Figure 2). At this time plasma of photospheric composition was present in the southern part of n 2 and at N 4u . Thirty minutes later, the plasma at N 4u had evolved to I-FIP effect plasma while weak I-FIP patches appeared at p 2 (Figure 3, bottom panel). I-FIP effect plasma was observed at these locations until the Hinode/EIS observing sequence ended at 17:07 UT The data shown in Figure 4 were obtained when Hinode/EIS was operating in its autonomous observing mode. A flare response study was triggered by an M1.4 flare at 19:29 UT on September 6. The large FOV is centered on the active region and extends to the flare loops which is not the case in Figure 3 where the smaller FOV covers only the core of the active region (see the FOVs shown in white in Figure 4). All regions defined in Figure 2 2. In general, I-FIP effect plasma persisted in the n 2 and p 1 regions and at N 4u , but less so within p 2 .

On September 7 at 08:01 UT, photospheric plasma was present within the flare loops associated with p 2 and at both pieces of the well-separated breakaway negative polarity N 4u and N 4f in the top panel of Figure 5. This single Hinode/EIS observation was acquired during the decay phase of a C8.2 flare that peaked at 06:42 UT. The plasma within the p 1 re- In this study, we have analyzed the evolution of plasma composition in AR 12673 on 2017 September 5-7. Hinode/EIS detected highly localized patches of I-FIP effect plasma embedded within the characteristic FIP effect plasma of the active region core (e.g. Del Zanna & Mason 2014; Baker et al. 2015). I-FIP effect plasma was present ∼8 hours before the X9.3 class flare at 12:02 UT on September 6 and also during the long decay phase extending to ∼20:00 UT. The plasma composition of AR 12673 had returned to FIP effect when Hinode/EIS observed it ∼13 hr later.

I-FIP Effect Plasma Observed at the Locations of Sub-photospheric Reconnection

A notable feature of the evolution of the highly complex magnetic field of AR 12673 is the coalescence of same polarity magnetic field strands in a number of locations throughout the active region. From the time of its emergence on September 3, the positive polarity strands of the E-W ori- ented arcade system merged together east of P 0 until a coherent umbra was evident at p 2 in the continuum images early on September 4 (see Fig1 movie.mp4). A train of positive umbrae approaching from the north continued to coalesce with the already coherent p 2 umbra for the next ∼2 days before the structure began to fragment on September 7.

The main N-S oriented flux system emerged after the arcade system later on September 3 when the opposite polarities rapidly separated, N 4 moving in a northward direction and P 4 progressing to the south-southwest. Right from the start, there was a continuous influx of new strands of negative polarity coalescing at N 4 for the same time period as with p 2 . Co-temporal emergence of P 4 with N 4 forced positive field in the vicinity of p 1 to amalgamate into the sunspot in the southwest of the active region. The process of sunspot coalescence driven by strong flux emergence of same polarity field was repeated in n 2 when N 4u broke away from the main N 4 umbra. It pushed into the negative field at n 2 and then rotated around P 3 . I-FIP effect plasma was observed within the coalescing umbrae at p 2 , N 4 , P 4 -p 1 , and N 4u -n 2 . The patches can be located more precisely within the umbrae where strong LBs formed during the merging and coalescence of the sunspots. So-called strong LBs appear between magnetic field strands of the same polarity during the early stages of the AR's development rather than during sunspot decay which is more commonly associated with faint LBs (e.g. Felipe et al. 2016, and references therein). Red arrows indicate the locations of strong LBs within N 4 , p 2 , and p 1 in the HMI continuum images from September 6 in Figure 6 (top panels). Contours of I-FIP effect plasma are overplotted on zoomed images of the LBs. The I-FIP effect plasma is precisely located at the LBs within the coalescing umbrae of the respective regions.

Once major flux emergence paused in the core of the AR late on September 6, the LBs were either no longer visible (N 4 ) or they became part of the fragmentation process in the sunspot during its decay phase (p 2 ). On the southern edge of the AR, the LB at p 1 remained prominent for at least another day as flux emergence was still ongoing, driving P 4 into p 1 and forcing coalescence of the sunspot. The evolution of the LBs can be viewed in the HMI continuum images of Fig1 movie.mp4.

Coalescing sunspot umbrae are preferential sites for subchromospheric reconnection to take place especially during the early development of highly complex active regions. Major flux emergence episodes push flux tubes of the same polarity against each others. From their different sub-photospheric evolution, these flux tubes are expected to typically have a finite angle between them. Then, component magnetic reconnection is expected to be present between the coalescing flux tubes (see Figure 7a). There is a plausible alternative scenario of internal reconnection within an individual flux tube in the case of N 4u . The flux tube may have been deformed as it rotated around P 3 , creating magnetic shear, beginning at around the time of the X2.2 flare and continuing during the early decay phase of the X9.3 flare. This evolution is shown from early on September 6 in the movie linked to Figure 1. Significant deformation of the N 4u flux tube may generate internal current sheets where sub-photospheric reconnection could take place. Either scenario or a combination of them is plausible at N 4u , however internal reconnection does not appear to be a likely cause of reconnection at p 2 , N 4 , and p 1 which showed little evidence of magnetic shear in their formation.

Revealing I-FIP effect plasma with Flare Energy Input

Successive episodes of major flux emergence and its interaction with a pre-existing, well-anchored sunspot resulted in the formation of the highly complex magnetic structure of AR 12673. Continual interaction gave rise to significant flare productivity (e.g. Yang et al. 2017;Verma 2018;Romano et al. 2018Romano et al. , 2019;;Toriumi & Wang 2019), indicating the occurrence of magnetic reconnection in the solar corona.

Table 3. Flare class, flare start, peak, and end times from 2017 September 6 with the locations of flare ribbons corresponding to Figures 1 and2. Flare details are from LMSAL SolarSoft Latest Events: http://www.lmsal.com/solarsoft/latest events archive/. * The X9.3 flare loops are still present well after this time so that the decay phase of the X9.3 flare is mixed with the following flares. This is also the case for other M and X flares. Evidence of significant localized heating generated by flaring is provided in Table 3 which lists the details for all flares > C1.0 on September 6: flare class, timings (start/peak/end), and locations of flare ribbons. The locations of the flare ribbons were identified either explicitly as ribbons in the SDO/Atmospheric Imaging Assembly (AIA) (Lemen et al. 2012) flare loops, the coronal counterpart of the ribbons. Major flaring occurred in the vicinity of the U-loop configuration introduced in Section 2 (Yang et al. 2017;Yan et al. 2018).

During the rise phase of the X9.3 flare, two ribbons began to develop on either side of the C-shaped PIL within the active region core involving mainly P 3 -p 2 -P 4 and N 4u -N 4 . The ribbons then extended into N 4f and N 3 soon after the eruption of the magnetic flux rope/CME coinciding with the flare peak at 12:02 UT (Mitra et al. 2018). The overlying post-flare loop arcade rooted in the two flare ribbons remained visible in the SDO/AIA coronal passbands throughout the extended decay phase of the flare until the early hours of September 7.

Ribbons associated with the M2.5 and M1.4 flares (peaking at 15:56 and 19:30 UT, respectively) also appeared along the PIL, tracing parts of the X9.3 ribbon 'tracks' at p 2 -P 3 and N 3 -N 4u -N 4f . The ribbons are clearly evident in SDO/AIA 1600 Å images at 15:55 and 19:25 UT in the bottom panel of Figure 6. Blue contours of I-FIP effect plasma (ratio ≥ 1.3) are overplotted on the images and have been derotated from the times of the Hinode/EIS raster times given in the figure. Contours located at p 2 and N 4u lie directly along the ribbons of both flares as does the n 2 contour associated with the M2.5 flare. There are no obvious flare ribbons extending to P 4 in the AIA 1600 Å image at 15:55 UT, however, the I-FIP effect contours correspond to the footpoints of the post-flare arcade at 16:43 UT (not shown).

Comparison of ARs 11429 and 12673

The Hinode/EIS observations of plasma composition in AR 12673 are globally comparable to those of AR 11429 (Baker et al. 2019). Distinct patches of I-FIP effect plasma were located at coalescing sunspot umbrae during flaring activity. The patches were observed when and where flare ribbons crossed the umbrae.

In the case of AR 11429, two compact patches of I-FIP effect plasma appeared and disappeared within one hour during the decay phase of a single, isolated M-class flare. Though the scenarios are very similar, the scales are different for AR 12673 in that four patches were present for time scales of hours not minutes over the merging umbrae of four instead of two sunspots. Furthermore, soft X-ray emission was significantly elevated for hours leading up to the first X-class flare and continued at very high levels for over 16 hrs after the X9.3 flare peak (Figure 2) compared to less than two hours during the M-class flare in AR 11429. The two M-class flares that occurred during the extended decay phase of the larger of the two X flares added additional heating sources. This does not necessarily mean that an increase in flaring activity directly produces more I-FIP effect plasma since more flaring would only allow us to observe more I-FIP effect plasma regions via chromospheric evaporation if they are present.

However, more globally, an increase of magnetic complexity can induce more reconnection below the fractionation region (in the chromosphere) and in the corona implying both the creation of more I-FIP effect plasma and more flares. Then, we expect a generic correlation between the complex-ity and emergence rate of the magnetic field, the amount of I-FIP effect plasma, and the flaring activity level.

Another common feature to both active regions is the occurrence of patches of I-FIP effect plasma at LBs within the coalescing umbrae. After noticing the precise location of the patches at the strong LBs during the formation stages of the same polarity sunspot umbrae, we re-examined the continuum movie of AR 11429 and indeed, patches of I-FIP effect plasma were also found under similar circumstances at strong LBs.

4.4. Why is I-FIP effect plasma observed at light bridges? Baker et al. (2019) proposed that transient patches of I-FIP effect plasma observed in AR 11429 were created by increased fast mode wave flux that was generated by sub-chromospheric/-photospheric reconnection of coalescing umbrae (Figure 7b). Fast mode waves coming from below the fractionation region of the chromosphere and undergoing a total internal reflection mean that the ponderomotive force is directed downward so that low-FIP elements are depleted from the chromospheric plasma (Laming 2015). Sunspot umbrae are preferential locations to observe I-FIP effect plasma. They are where the magnetic field is strongest therefore the plasma β = 1 layer is lower in the photosphere (Avrett et al. 2015) so that the fast mode wave flux from below is enhanced. The vertical field of the umbrae is also where the downward-directed ponderomotive acceleration is likely to be the strongest as the plasma upflow must be along the magnetic field, making the field-aligned ponderomotive force most relevant.

EIS observed I-FIP effect plasma patches in four regions where sub-chromospheric or even sub-photospheric reconnection was likely to be an ongoing process. In all cases, significant flux emergence was the main driver of sunspot coalescence. This is consistent with the findings of Baker et al. (2019). The classical scenario is forced magnetic reconnection between two independent flux tubes where there is some angle between the same polarity strands that are pushed together by convective motions (Figure 7a), leading to the formation of MHD waves in the process. AR 12673 is an extreme case of the forced reconnection scenario due to the amount and complexity of the emerging magnetic field. There is essentially a rescaling due to energy input. Finally, I-FIP effect plasma was not observed within the AR after major flux emergence of the main flux system ceased.

Previously, we have argued in Baker et al. (2019) that fast mode waves cause the I-FIP effect, mainly because of the ease, relative to Alfvén waves, of achieving the required degree of reflection to produce I-FIP rather than FIP effect fractionation. Fast mode waves (sound waves) produced below the β = 1 layer by reconnection mode convert where β = 1 to continue as fast modes (now magneto-acoustic waves) in the β < 1 chromosphere, where they refract/reflect exerting a downward ponderomotive force on the ions (Figure 7b). The mode conversion is essential, and increases with increasing angle between the magnetic field direction and the wave vector. While this might produce some selectivity in where I-FIP can be produced, upward propagating sound waves are less likely to mode convert and produce I-FIP in the vertical magnetic field, and it is still less obvious why I-FIP should be restricted to the LBs, as observed in AR 11429 and AR 12673.

We propose in the following a slightly more refined scenario, which consists of two interlinked parts: the first ingredient is the geometry of the sub-photospheric reconnection that creates the train of waves responsible for the I-FIP effect fractionation; the second one is the nature of waves such that they remain associated with a specific bundle of field lines, emerging at a very specific location, and only there.

LBs are very particular areas within an active region where pockets of relatively unmagnetized plasma are trapped within two merging flux systems of the same strong polarity, forming a sunspot. The sub-photospheric forces that merge the two flux systems may eventually be able to squeeze out the unmagnetized plasma and lead to a structure that looks like a single-polarity spot in white light images and line of sight magnetograms. Therefore, LBs are the photospheric trace of the merging plane between two flux systems that, generally, extend down in the convection zone, at least deeper than the vertical extension of the unmagnetized plasma volume that supports the LB. At the sub-photospheric location where the two flux systems are pushed together by the highβ plasma it is conceivable that component field reconnection between nearly parallel flux bundles of the same sign occurs (Figure 7a). This is the main assumption of this scenario. If this is true, then the numerical simulations by Kigure et al. (2010) provide crucial information, namely that in a highβ plasma environment, the reconnection between nearlyparallel flux systems would mostly generate incompressible (shear) Alfvén waves. Applying this result in our hypothetical scenario (Figure 7c), the sub-photospheric reconnection process would then launch incompressible Alfvén waves along the field lines of the flanks of the reconnecting flux systems that surrounds the LB. Since Alfvén waves must propagate along field lines, as they travel upward, these waves would then eventually emerge at the sides of the LB, irrespective of how deep the LB extends below the photosphere. Furthermore, the magnetic field right above a LB was observed recently by Felipe et al. (2016) using high resolution measurements from the GREGOR telescope (Schmidt et al. 2012). The authors conclude that the field at the side of the LB stretches up along its two sides and converges above the LB, forming a cusp-shape across it, with the mainly horizontal LB field being confined below the cusp. Following the cusp lines, the shear Alfvén waves of sub-photospheric origin would reach the chromosphere, where they could provide the energy, and the correct direction of travel, for the ponderomotive I-FIP effect fractionation in a very localized volume, namely right above the LB. Hence, if the above scenario is correct, the observations of I-FIP effect fractionated plasma should be located between the merging sunspot's fragments, i.e. at the LB, which is indeed what is observed.

The above scenario is highly speculative. Getting sufficient reflection of Alfvén waves to cause I-FIP rather than FIP effect fractionation probably requires more wave interaction physics in the chromospheric model, as Alfvén waves likely reflect off sound waves, shocks or other density inhomogeneities. This can be modeled analytically, but will require both numerical testing and observational confirmation. The numerical testing is needed to confirm that upwards traveling shear Alfvén waves from below the photosphere can reflect efficiently and drive the I-FIP effect fractionation. The observational verification entails a multi-line spectropolarimetric study of LBs to verify the presence of such waves at the sides of the LB. On the other hand, such a scenario assumes sub-photospheric reconnection between the merging flux systems that drives the formation of an observed LB and therefore determines the observed localization of I-FIP effect plasma on the basis of the properties of reconnection between nearly-parallel flux systems. As mentioned above, such a localization at LBs is more difficult to explain with compressible waves which can travel across field lines.

CONCLUSIONS

I-FIP effect plasma has been detected in only eight active regions (Doschek et al. 2015;Doschek & Warren 2016, 2017), but it is not yet clear how widespread the phenomenon is. Observational constraints are likely to play a role in limiting the number of I-FIP effect plasma detections. It requires the right composition lines in the employed study, the correct target selection, and pointing at the specific flaring locations within the target active region during the time of flaring activity.

A significant fraction of the I-FIP effect plasma observations were obtained while Hinode/EIS was operating in its flare trigger mode. This is an observing strategy to catch flares, and the response study is initiated only at or near the flare peak time. It remains to be seen whether a different observing strategy, dedicated to observing I-FIP effect events, would be more successful at capturing the evolution of plasma composition before and during the flare rise phase.

Notwithstanding these limitations, observations of AR 11429 and AR 12673 show that they share common characteristics in their magnetic configuration which distinguish them as potential candidates for the detection of I-FIP effect plasma. Both active regions were large (> 500 MSH) and magnetically complex (βγδ) with ongoing major flux emergence of sheared/twisted field which violently interacted with pre-existing field.

Our interpretation of the Hinode/EIS observations of I-FIP effect plasma in AR 12673 is consistent with the ponderomotive fractionation model. Ongoing sub-chromospheric or sub-photospheric reconnection at multiple sunspot umbrae is likely to increase the wave flux from below the chromosphere thereby providing favorable circumstances for the accumulation of I-FIP effect plasma in more locations and on longer time scales than for AR 11429.

Flaring and evaporation of chromospheric material are essential parts of this scenario. Along flare ribbons at the footpoints of newly formed loops, which rarely traverse coalescing umbrae, the expected chromospheric composition is close to photospheric. The Sun-as-a-star (Warren 2014) and stellar (Audard et al. 2003;[START_REF] García-Alvarez | Cambridge Workshop on Cool Stars, Stellar Systems, and the Sun[END_REF]Testa et al. 2015) observations during flares are consistent with this since they indicate that the overall elemental composition is getting close to photospheric value. The presence of bright I-FIP patches shifts the overall composition of FIP-effect dominated coronae even more towards photospheric or indeed to I-FIP effect values (e.g. Katsuda et al. 2020).

Do we need 'monster' active regions to produce I-FIP effect coronal composition? So far the evidence is pointing that way, but the picture is clouded by observational constraints. What is likely to be a requirement is a strong wave flux from below the chromospheric fractionation region. There is a considerable fraction, ∼30%, of the total released energy of Alfvén waves generated in high plasma-β reconnection events (Kigure et al. 2010), so that I-FIP effect is expected to occur more often than what has been observed so far if sub-photospheric reconnection occurs frequently between the multiple thin flux tubes present in emerging ARs. Based on the observations of AR 12673, we propose a scenario where shear Alfvén waves originating below the photosphere and traveling upward can localize the I-FIP effect plasma at strong light bridges at coalescing umbrae.

Active region complexity is highly correlated with activity (see e.g. Toriumi & Wang 2019, and references therein) so there is a temptation to associate the presence of localized regions of I-FIP effect plasma in so-called monster active regions on the Sun with more active M-dwarf stars containing very large/strong starspots (e.g. Berdyugina 2005; Reiners 2012) and whose coronae are dominated by I-FIP effect plasma. It is expected to be a matter of the scale of waves generated by magnetic reconnection in sub-fractionation layers of stars. X-ray flux is a function of overall heating in a star's atmosphere (e.g. Wood et al. 2018) which in turn governs the elements that are ionized on that star. The presence of strong and highly complex magnetic field provides additional heating and MHD waves that determine the direction and strength of the ponderomotive force and therefore the degree and direction of plasma fractionation, whether I-FIP or FIP effect, on stars of spectral types F-M.

The filling factor of spot umbrae on the photosphere of a star is likely to be a significant factor in determining the extent to which I-FIP effect plasma fills the star's corona. On the Sun, strong magnetic fields are found in the small localized areas of sunspots covering a small fraction of the surface (< 0.5%, Hathaway 2015) hence we may observe very small patches of I-FIP effect plasma. On the more active M-dwarfs, starspots can cover up to 2 orders of magnitude more of a star's photosphere (e.g. Jackson & Jeffries 2013; Tregloan-Reed & Unda-Sanzana 2019), which may explain why their coronae are dominated by I-FIP effect plasma.

The recognition that stellar coronal composition has a dependence on magnetic activity (Audard et al. 2003;[START_REF] García-Alvarez | Cambridge Workshop on Cool Stars, Stellar Systems, and the Sun[END_REF]Testa et al. 2015) is in line with our results of resolved I-FIP generation on the Sun in a FIP-effect dominated low-activity star. The I-FIP patches are closely related to strong magnetic field concentrations and made observable in the corona by flaring.

Figure 1 .

 1 Figure1. SDO/HMI SHARP radial magnetic field, Br, and 6173 Å continuum images(Bobra et al. 2014) during the evolution in AR 12673. Middle and lower panels correspond to the times of when Hinode/EIS observed I-FIP effect plasma(Figures 3, 4, 5). Major components of the flux systems are labeled in each image: arcade field n1-p1 and n2-p2 (labels in gray); main field N3-P3 and N4-P4 (labels in black), and pre-existing, well anchored spot P0. The U-loop is formed in between N4-P3 as well as N4u-P3 (bottom panels). Near the end of the emergence phase, polarities N4u and N 4f detached from the north part of N4. Note that n and N notations are used for negative, p and P notations for positive polarity umbrae. Red/blue corresponds to positive/negative Br in the magnetograms. See the included movie Fig1 movie.mp4 of this figure. For ease of comparison between figures, black arrows are used to show the distances from the center of the major polarities. At 16:46 UT on September 6, the vertical arrow is ∼50 in length between the centers of P0 and P4. The horizontal arrow is ∼30 in length from the centers of P0 and N4. The center of P0 is X = 590 , Y = -242 . This convention is repeated for all figures with maps.

Figure 2 .

 2 Figure 2. Top panel: GOES 1-8 Å soft X-ray light curve from 00:00 UT on 2017 September 5 to 00:00 UT on 2017 September 8. Red dashed lines indicate the times of Hinode/EIS rasters used in this study. The thicker lines represent a series of nearby observations. Bottom panels: Hinode/EIS Ar XIV/Ca XIV ratio map at 19:56 UT on September 6 overplotted with boxes defining the regions used in Table 2 and throughout the text (left) and SDO/HMI contours of ± 500 G of white/magenta for positive/negative polarities (right). The color bar scale shows the FIP effect as blue/green, photospheric composition as orange, I-FIP effect as yellow. (P0 is located at X = 611 , Y = -241 at 19:56 UT).

Figure 3 .

 3 Figure 3. Left to right: Hinode/EIS Ar XIV 194.40 Å and Ca XIV 193.87 Å intensity maps, Ar XIV/Ca XIV ratio maps without and with SDO/HMI contours of ±500 (white/magenta for positive/negative polarities) before (top section) and during the decay phase of the X9.3 flare (bottom section) on 2017 September 6. The color bar scale shows the FIP effect as blue/green, photospheric composition as orange, I-FIP effect as yellow. All Hinode/EIS maps are co-aligned to SDO/HMI maps at the times shown. (P0 is located at X = 492 , Y = -248 at 03:23 UT and X = 590 , Y = -242 at 16:43 UT).

  have patches of I-FIP effect plasma from 19:29-20:14 UT except for p 2 where the plasma showing weak indications of the I-FIP effect evolved to photospheric composition from 20:05 to 20:14 UT. The mean I-FIP effect ratios for pixels with values ≥ 1.3 for each of the regions defined in the lower left panel of Figure 2 are given for Hinode/EIS rasters on September 6 in Table

Figure 4 .

 4 Figure 4. Left to right: Hinode/EIS Ar XIV 194.40 Å and Ca XIV 193.87 Å intensity maps, Ar XIV/Ca XIV ratio maps without and with SDO/HMI contours of ±500 (white/magenta for positive/negative polarities). Observations are from 19:29 UT to 20:14 UT on 2017 September 6. The color bar scale shows the FIP effect as blue/green, photospheric composition as orange, I-FIP effect as yellow. All Hinode/EIS maps are co-aligned to SDO/HMI maps at the times shown. (P0 is located at X = 614 , Y = -241 at 20:14 UT).Table 2. Mean I-FIP effect ratio values on September 6 in regions defined in Figure 2 with the selection of pixels having ratio values ≥ 1.3. Raster Times (UT) n2 N4u p2 p 04:11 -1.5 --16:19 1.5 1.4 --16:25 -1.4 --16:31 -1.5 --16:37 -1.4 --16:43 -1.3 -1.4 16:49 -1.3 -1.4 16:55 -1.3 -1.3 17:01 1.3 1.4 --19:29 1.8 --1.3 19:38 1.3 -1.5 1.8 19:47 1.4 -1.3 1.3 19:56 1.6 1.4 1.3 1.4 20:05 1.4 1.4 -1.5 20:14 1.4 1.3 -1.5

Figure 5 .

 5 Figure 5. Top panel: Hinode/EIS Ar XIV intensity and Ar XIV/Ca XIV ratio maps without and with SDO/HMI contours of ±500 (white/magenta for positive/negative polarities) on 2017 September 7 at 08:01 UT. Bottom panel: Hinode/EIS Ar XIV 194.40 Å and Ca XIV 193.87 Å intensity maps, Ar XIV/Ca XIV ratio maps without and with SDO/HMI contours on 2017 September 7 at 21:35 UT. (P0 is located at X = 692 , Y = -232 at 08:01 UT and at X = 766 , Y = -222 at 21:35 UT).

Figure 6 .

 6 Figure 6. Top panel (left to right): SDO/HMI continuum images at 04:11, 16:49, and 19:38 UT on 2017 September 6. Red arrows indicate locations of strong light bridges at N4, p1, and p2 regions defined in Figure 2. Insets are zoomed images of the light bridges overplotted with red contours of I-FIP effect = 1.3. The I-FIP effect plasma is precisely at the locations of the strong light bridges. Bottom panel: SDO/AIA 1600 Å images overplotted with blue I-FIP effect plasma contours of ≥ 1.3. Contours have been derotated from the times of the Hinode/EIS rasters. (P0 is located at X = 610 , Y = -241 at 19:38 UT and at X = 608 , Y = -241 at 19:25 UT).

Figure 7 .

 7 photosphere

Table 1 .

 1 Hinode/EIS study details.

	Study Name	HIC2 SCAN 201×512
	Composition	Ca XIV 193.87 Å and Ar XIV 194.40	Å
	Field of view	210 × 512
	Rastering	2 slit, 21 positions, 10 coarse steps
	Exposure Time	15 s
	Study name	Atlas 60
	Composition Lines Full spectral atlas including
		Ca XIV 193.87 Å and Ar XIV 194.40	Å
	Field of view	120 × 160
	Rastering	2 slit, 60 positions, 2 steps
	Exposure Time	60 s
	Study name	FlareResponse01
	Composition Lines Ca XIV 193.87 Å and Ar XIV 194.40	Å
	Field of view	240 × 304
	Rastering	2 slit, 80 positions, 3 coarse steps
	Exposure Time	5 s

We thank the referee for their helpful suggestions to improve the clarity of the manuscript. The authors are very grateful to Konkoly Observatory, Budapest, Hungary, for hosting two workshops on Elemental Composition in Solar and Stellar Atmospheres (IFIPWS-1, 13-15 Feb, 2017 and IFIPWS-2, 27 Feb-1 Mar, 2018) and acknowledge the finan-cial support from the Hungarian Academy of Sciences under grant NKSZ 2018 2. The workshops have fostered collaboration by exploiting synergies in solar and stellar magnetic activity studies and exchanging experience and knowledge in both research fields. Hinode is a Japanese mission developed and launched by ISAS/JAXA, collaborating with NAOJ as a domestic partner, and NASA and STFC (UK) as international partners. Scientific operation of Hinode is performed by the Hinode science team organized at ISAS/JAXA. This team mainly consists of scientists from institutes in the partner countries. Support for the post-launch operation is provided by JAXA and NAOJ (Japan), STFC (UK), NASA, ESA, and NSC (Norway). SDO data were obtained courtesy of NASA/SDO and the AIA and HMI science teams. D.B. is funded under STFC consolidated grant number ST/S000240/1 and L.v.D.G. is partially funded under the same grant. The work of D.H.B. was performed under contract to the Naval Research Laboratory and was funded by the NASA Hinode program. GV acknowledges the support from the European Union's Horizon 2020 research and innovation programme under grant agreement No 824135 and of the STFC grant number ST/T000317/1 J.M.L. was supported by the NASA HGI (80HQTR19T0029), NASA HSR (NNH16AC391), and LARS (NNH17AE601) programs, the Chandra GO program and by Basic Research Funds of the Chief of Naval Research. DML is grateful to the Science Technology and Facilities Council for the award of an Ernest Rutherford Fellowship (ST/R003246/1).


Luiz H G Tizei

Vahagn Mkhitaryan

Hugo Lourenço-Martins

Leonardo Scarabelli

Kenji Watanabe

Takashi Taniguchi

Marcel Tencé

Jean-Denis Blazit

Xiaoyan Li

Alexandre Gloter

Alberto Zobelli

Luis Liz-Marzán

F Javier García De Abajo

Odile Stéphan

Mathieu Kociak
email: mathieu.kociak@u-psud.fr

Tailored nanoscale plasmon-enhanced vibrational electron spectroscopy

Vibrational optical spectroscopies can be enhanced by surface plasmons to reach molecular-sized limits of detection and characterization. The level of enhancement strongly depends on microscopic details of the sample that are generally missed by macroscopic techniques. Here we investigate phonons in h-BN by coupling them to silver-nanowire plasmons, whose energy is tuned by modifying the nanowire length. Specifically, we use electron beam milling to accurately and iteratively change the nanowire length, followed by electron energy-loss spectroscopy to reveal the plasmon-enhanced vibrational features of h-BN. This allows us to investigate otherwise hidden bulk phonons and observe strong plasmon-phonon coupling. The new milling-and-spectroscopy technique holds great potential for resolving vibrational features in material nanostructures.

Vibrational spectroscopy is key in a wide range of research areas and technological applications, from molecular fingerprinting to fundamental solid-state physics [START_REF] Zenobi | Introduction: Vibrational nanoscopy[END_REF][START_REF] Verma | [END_REF] . The discovery that plasmonic structures can increase the measured vibrational signal has driven the development of ultra-sensitive analytical techniques capable of reaching single-molecule detection, such as in surfaceenhanced Raman spectroscopy 3,4 (SERS) and surfaceenhanced infrared absorption 5 (SEIRA). Typically, plasmonic structures are designed in advance and molecules are randomly dispersed over them, leading to strongly enhanced signals associated with those sitting on the socalled hotspots 4 . Alternatively, a metallic tip can be scanned over a sample to induce SERS locally, leading to chemical mapping at sub-molecular scales, a technique known as tip-enhanced Raman scattering 6 (TERS).

Vibrational electron energy-loss spectroscopy (EELS) has been performed for decades mainly as a surface technique 7 using wide beams with poor spatial resolution. The recent development of a new family of electron monochromators [START_REF] Krivanek | [END_REF] has allowed vibration mode measurements to be performed based on EELS [9][10][11] down to atomic spatial resolution in bulk materials 12 . Unfortunately, the signal-to-noise ratio of vibrational EELS is low for materials that are sensitive to the electron beam, thus imposing a lower bound on the volume necessary for analysis. This jeopardizes the high resolution mapping of fragile materials, such as organic molecules 13 . Recently, theoretical studies [14][15][16] have proposed the use of infrared plasmonic fields to develop a new form of enhanced vibrational EELS which would overcome these limitations by making the molecules interact with the beam at a dis-tance mediated by plasmons extended in a nanoparticle.

Here, we demonstrate plasmon-enhanced vibrational electron spectroscopy (PEVES) through the tailored coupling of plasmon resonances in metallic nanowires to phonon modes in h-BN thin flakes. Coupling is achieved by continuously shifting the energies of the plasmon modes of micrometer-long metallic nanowires (Fig. S1) using electron-beam controlled milling to bring them into resonance with specific h-BN vibrational modes (Fig. S2). We reveal three new effects when a plasmon-phonon resonance is encountered: 1) strong coupling between surface phonons and plasmons; 2) enhancement of the bulk vibrational EELS signal; and 3) emergence of previously geometry-forbidden dark phonon modes.

EELS measures the distribution of energy losses experienced by free electrons when interacting with a target [START_REF] Egerton | Electron Energy-Loss Spectroscopy in the Electron Microscope[END_REF] . The energy resolution is determined by the energy spread of the electron source, typically ∼250 meV for a cold field emitter source. This resolution can be improved down to a few meV using an electron monochromator (Fig. S3). To achieve subnanometer spatial resolution, the electron monochromator can be coupled to an electron microscope (Fig. S3). Finally, an electron spectrometer is used to acquire the spectrum of the electron beam after interaction with the sample. Here, we use such a setup implemented on a NION Hermes scanning transmission microscope (STEM), see Methods.

The plasmonic silver metallic nanowires were synthesized by chemical seeded growth as detailed elsewhere [START_REF] Mayer | [END_REF] (Fig. 1A) and subsequently deposited, either entirely (configuration (1)) or partially (configuration (2)), on an h-BN substrate 19 . In theory, extended h-BN possesses a variety of infrared-active either transverse optical (TO) or longitudinal optical (LO) phonons. Due to the h-BN anisotropy, TO and LO phonons may possess different energies depending on whether they are polarized out-or in-plane 20 . Bulk EELS is therefore very dependent on the orientation of the electron to the h-BN crystallographic orientation. Indeed, when the electron propagates along the [0001] h-BN direction, mainly the in-plane LO bulk mode is excited, while the out-of-plane modes can only be excited in a tilted configuration. Thin h-BN substrates sustain additionally two surface phonon modes: the charge-symmetric (FKS) and charge-antisymmetric (FKA) Fuchs-Kliewer modes, whose energies disperse as a function of thickness. A qualitative sketch of the energy hierarchy of the different modes discussed in the text is presented in Fig. 1B. For very thin substrates, the former (FKS) arises at the in-plane TO phonon energy (around 169 meV) and the later (FKA) at the outof-plane LO phonon energy (around 198 meV). In EELS experiments, the FKS (resp. FKA) energy varies between the TO (resp. LO) energy and that of the h-BN interface phonon (around 195 meV). Also, the FKA intensity is usually much smaller than that of the FKS 21,22 , see (Fig. 1C, green curves). The FK modes usually overshadow the bulk modes, at least for the thickness range probed in this study (50-100 nm). Nanowire plasmons of welldefined energy are produced due to confinement, leading to modes that can be classified as dipolar, quadrupolar, etc., depending on the number of nodes in the inducedcharge along the nanowire length. In the nanowire crosssection, the charge distribution is homogeneous. EELS is extremely sensitive to surface plasmon modes and reveals their nearly uniform energy spacing (Fig. 1C). Their energies are roughly proportional to the inverse of the nanowire length, which thus becomes a perfect parameter to tune the nanowire plasmon energies. Finally, we note that both surface plasmons and surface phonons (FK modes) can also be excited in a geometry where the beam does not directly hit the sample ("aloof geometry") 9,10,23 .

Plasmon-phonon interaction was probed initially using a sample configuration ("1" in Fig. 1A) where only surface phonons interact with surface plasmons, to avoid any potential overlap between surface and bulk effects. In this geometry, long silver nanowires with only one tip placed on different volumes of h-BN were modified in length by electron milling in order to bring their dipolar plasmon into and then out of resonance with respect to the h-BN FK modes. 1A) as a function of nanowire length. By changing through milling the length of the nanowire, its dipolar plasmon mode is brought in and out strong coupling with the h-BN phonon modes at around 180 meV. The strong coupling is confirmed by the anti-crossing of the two modes, indicated by the two dashed curves (added as guides to the eye) on the left of (A). (B) Theoretical calculations for the energies of the coupled modes (dashed red curve) and the energy position measured in experiments (black and orange crosses for the dipolar and quadrupolar modes). The light line is marked in blue. The quadrupolar plasmon mode is marked by orange crosses.

When their energies differ by more than ∼ 100 meV, phonons and surface plasmons do not interact (Figs. 1D, blue curve, and 2A, orange curves). As the nanowire length is shortened, the dipolar plasmon energy approaches that of the FKS mode. Close to resonance, a second peak is observed at the nanowire tip in vacuum, away from h-BN, at an energy close to 180 meV (Figs. 1D and2A, purple curves). The two peaks do not cross each other as the nanowire length is varied, exhibiting a minimum splitting of 37 meV (average splitting in the 1.7-2.9 µm nanowire length range), comparable to the full width at half maximum of the two peaks (between 30 and 40 meV). The energy of the first peak continues to increase for nanowire lengths shorter than 2.9 µm. However, it does not disperse proportionally to the inverse of the nanowire length 1/L, as expected for plasmons on long metallic nanowires. This is in contrast with the higher order plasmon modes that indeed disperse as expected as 1/L (Fig. S4). This behavior is nicely reproduced by numerical calculations (Fig. 2B). Peak-splitting was observed to vary between nanowires (in the 0-52 meV range in Figs. S5-8).

These observations indicate strong coupling between the dipolar surface plasmon and the FKS mode. For small detuning, the two peaks cannot be assigned as either a plasmon or a phonon, but rather as hybrid plasmon-phonon (PP) modes, either charge-symmetric (low-energy, PPS) or charge-antisymmetric (high-energy, PPA), see Fig. 1B. After the anticrossing, the nature of the modes is inverted, as expected for strong coupling: the lower (higher) energy peak changes continuously from the plasmon (phonon) to the phonon (plasmon) mode. Strong coupling evidences a coherent field across the whole nanowire, as its signature is observed at a region of the nanowire where no h-BN is present and the aloof signal from h-BN without nanowire is not experimentally detectable. In fact, the coupled peaks are only observed where the dipolar mode is measurable, close to the nanowire tips (Fig. S9). This can be more clearly observed in a line profile of the hyperspectral image going from the nanowire tip in vacuum to the region supported on thin h-BN (Fig. S9). As the electron beam is moved away from the nanowire tip, the two coupled modes lose intensity. When it approaches h-BN, the lower energy FKS mode is observed at an intermediate energy between the two coupled modes. Finally, with the electron beam on h-BN, both FK modes are revealed.

The energy splitting due to coupling depends on the amount of h-BN contained inside the modal volume of the dipole surface plasmon 15 . For small quantities, no splitting is observed (Fig. S6). For intermediate amounts, a 17 meV splitting is observed in some measurements (Fig. S7). The largest observed splitting in this study is 52 meV Fig. S8. These results support the hypothesis that the volume of h-BN within the dipole surface plasmon modal volume determines the splitting of the two strongly-coupled peaks, starting from a large volume of h-BN, where the splitting continuously decreases from 52 meV down to 12 meV (Fig. S8). We note that a removal of h-BN performed only within 500 nm of the nanowire tip did modify the splitting, while removal farther away did not affect the splitting, again indicating that the coupling is mediated by the overlap of the plasmon field lines and h-BN.

The above-described behavior is observed at the nanowire tip lying on h-BN, but with the complicating factor that h-BN losses are also present. To understand the signal when h-BN is present, a second sample geometry ("2" in Fig. 1A), in which the nanowire is lying completely on an homogeneous piece of h-BN, was used. The presence of plasmons with energies away from the phonon energies (Fig. 1E, orange curve) does not affect the phonon spectra. Close to a plasmon-phonon resonance, the spectra change substantially (Fig. 1E, purple curve and S10), with the appearance of multiple peaks at 100 meV, 161 meV, 177 meV, 197 meV, and 217 meV (Fig. . 3A,B and marked 1 through 5, respectively in C). This multiple peak structure is reproduced by theoretical calculations (dashed curves in Fig. 3A). The 161 meV and 217 meV modes do disperse with nanowire length, and can be safely attributed to the PPS and PPA modes (their dispersion is confirmed by numerical calculations shown in Fig. S11). Three peaks are left to be identified (100, 177, and 197 meV). For the 2.1 µm long nanowire, peak energies do not change as a function of the distance away from the nanowire tip at which they are probed (Fig. 3B-C andS12), but their intensities do.

The 177 meV mode intensity increases away from the nanowire surface, while the four others vanish (Fig. 3B-C). The 177 meV mode can be interpreted as the FKS, excited in an aloof-like configuration when the beam is close to the nanowire tip. The probability of exciting it obviously decreases as the PPA and PPS, which are hybrid plasmon-FKS modes, are more intensely excited close to the nanowire tip.

Away from the influence area of the dipolar mode, the vanishing of the PPA and PPS signals is expected, as they are formed where the plasmon field lines are intense enough. The 100 and 197 meV peaks show similar spatial variation, indicating that they have a common origin. The first of these energies matches that of the ZO normal mode 24 (a longitudinal optical phonon polarized along the anisotropy axis), and the second one corresponds well with that of the LO mode polarized perpendicular to the anisotropy axis (see a schematics on Fig. 1B). As these modes should not be observed in the sample geometry here considered, their observation can only be explained by their coupling to the plasmon mode, which enhances them. The enhancement is maximal within 30 nm of the nanowire surface, which strengthens the link to the surface plasmon.

In summary, two main effects are observed: the possibility to detect a bulk phonon mode (the ZO mode), whose excitation, in the absence of a nanowire, is geometry-forbidden, and an extreme enhancement of the LO and ZO modes. The magnitude of the enhancement cannot be quantified, as in the absence of nanowire both modes are not experimentally detectable.

Interestingly, the enhancement is not homogeneous across the whole nanowire: both ZO and LO modes are enhanced at the tips, while only the ZO mode is enhanced at the center (Fig. 3D,E). We attribute this effect to the dependence of the enhancement on the direction of the plasmon electric field, which changes along the nanowire length: close to the tips the field has substantial projections on all directions, while at the nanowire center it is mostly oriented parallel to the metal surface. Some phonon modes are only observed when the electric field has a component out of the h-BN plane. The PPS and PPA peaks are visible close to the nanowire tips (left and right of Fig. 3D), while others appear along the entire nanowire (Fig. 3D, ZO peak). In this respect, the symmetry of the plasmon electric field mode provides additional information about the phonons with which it interacts.

Controlled coupling between plasmon and phonon modes with precision below 10 meV opens the way to a new form of plasmon-enhanced vibrational spectroscopy (PEVES). Plasmon modes are highly localized in space, leading to stronger enhancement effects in volumes of the order of 10 3 nm 314 ; only a technique with sub-10 nm spatial resolution can exploit this high localization. The plasmonic enhancement has the additional benefit that smaller sample volumes can be probed than with regular EELS. More advanced sample designs should allow fingerprinting of molecular vibration modes. For example, one could produce dedicated slots in metallic structures, each of them containing different analytes; or a movable metallic tip could be added to the microscope holder (as demonstrated in previous work 25 ) to selectively enhance the vibrational modes along a molecule. Sequencing of DNA strands also appears as a possible application of PEVES by scanning both the tip and the electron beam. Experiments were performed on a modified Nion Hermes200 (known as ChromaTEM) equipped with an electron monochromator, an optimized electron spectrometer (Nion Iris) and a cold field emission electron gun. The NION Iris spectrometer was fitted with a Princeton Instrument Kuros camera. The electron beam used had a kinetic energy of 60 keV, an energy resolution (defined as the full width at half maximum (FWHM) of the zero-loss peak) between 6 and 10 meV (this value was optimized and controlled after each acquisition), a current intensity between 5 and 20 pA, a convergence half-angle of 10 mrad, and a FWHM in real space of about 1 nm. The spectrometer was set to have a diffraction pattern at its entrance (image coupling mode) and a 1 mm entrance aperture was used to collect electrons up to 15 mrad (half-angle). All data were acquired as hyperspectral images, with two spatial dimensions (electron beam scan in real space) and one energy loss dimension. All peak positions have been reported with an error of ± 5 meV (which include errors due to the background subtraction).

B. Sample preparation

The synthesis follows the protocol reported by Mayer et al. in 2015 [START_REF] Mayer | [END_REF] . In order to proceed with the growth of AgAuAg bimetallic nanowires, it is necessary to synthetize Penta-twinned gold nanorods to be used as core for the epitaxial deposition of silver. The synthesis of this material can be found in the literature 26 .

Materials. Benzyldimethylhexadecylammonium chloride (BDAC), silver nitrate (AgNO3, ≥ 99.9%), L-ascorbic acid (AA, ≥ 99%), were purchased from Aldrich. All chemicals were used as received. Milli-Q water (resistivity 18.2 MΩcm at 25 • C) was used in all experiments. All glassware was washed with aqua regia, rinsed with water, and dried before use.

AgAuAg bimetallica nanowires synthesis. 20 mL of the purified Penta-Twinned Au Nanorod solution was heated to 60 • C using an oil bath. A AgNO3 solution (0.004 M in water) and an AA solution (0.016 M in 20 mM BDAC) were prepared and loaded into separated syringes. Using a syringe pump, both AgNO3 and AA were added to the Penta-Twinned Au Nanorod solution at a rate of 300 µL/h (0.24 mol of Ag(I) per mol of Au(0) per hour) under slow stirring keeping the temperature constant at 60 • C. The growth of the nanowires was monitored through UV-vis-NIR spectroscopy on a small sample (1 mL) periodically withdrawn from the growing solution. To obtain AgAuAg nanowires above 3 µm in length (average 3.4 ± 0.6 µm for the specific sample used in this manuscript), the growth was continued for 72 hours.

C. AgAuAg nanowire and h-BN milling

Metallic nanowire and h-BN milling was performed on a JEOL 2010 electron microscope equipped with a thermionic electron gun (LaB 6 crystal). This kind of microscope was chosen to ensure the maximum total current available in the focused electron beam (in contrast to the maximum current density, which would be achieved with cold field emission gun). Most efficient milling was achieved with 200 keV kinetic energy electrons and a beam extraction current between 10 and 15 µA.

Milling of the two materials was achieved through different mechanisms. Ag atoms are heavy and energy conservation prevents efficient atoms removal by momentum transfer. For this reason, large currents were used to heat the material and diffuse atoms away from the target area. Because of this, milling was quite dependent on the underlying substrate. The highest precision was achieved with nanowires parts on vacuum or on h-BN. On the carbon support film milling was less favorable, probably due to good heat conductivity of the film. B and N atoms are light and are easily removed by direct momentum transfer from the electron beam.

D. Data analysis

Data acquisition and initial check was performed using Nion Swift and in-house Kuros driver. Data analysis was performed with Python libraries: Hyperspy, Numpy, Matplotlib and Scipy. Spectra were fitted with multiple Gaussians unless otherwise noted.

E. Sample preparation

Samples were prepared by drop casting sequentially solutions containing the metallic nanowires and h-BN flakes (chemically exfoliated in IPA). Nanowires where chosen by inspection in an electron microscope considering their length and the quantity of h-BN underlaying them.

F. Purcell factor calculations

In order to understand the experimental measurements more quantitatively, we present numerical simulations of the optical response of our system (Ag nanowires on top of h-BN thin films) carried out by using a finite-difference in the time domain (FDTD) method. In these simulations, plasmon modes are excited by an electric dipole source placed near the end of the nanowire, at a position away from high-symmetry points in order to guarantee a nonzero electric field of the plasmon standing wave, and therefore, a high excitation efficiently. We exploit the proportionality between the EELS probability and the local density of electromagnetic states (LDOS) [1]

Γ EELS = -2πe 2 hω ρ z (R 0 , q v , ω) (1)
where e is the elementary charge, ω is the frequency of the electromagnetic field, q v = ω/v , and v is the electron velocity. Here, ρ z (R 0 , q v , ω) is the projected LDOS (along the direction of the electron trajectory, which is taken along the z axis) at the in-plane position R 0 (x 0 , y 0 ) of the electron trajectory and evaluated in wave vector space along the z direction. Instead of this quantity, we compute the LDOS in real space along z, which is in turn proportional to the so-called the Purcell factor (i.e., the enhancement of the decay rate of a dipole emitter placed at the position of interest, compared with the free space decay rate γ 0 ). More precisely [2],

P = γ γ 0 = 2πc 2 ω 2 ρ z (R 0 , z, ω) (2)
Although the LDOS forms in two expressions above are not equivalent, we argue that both plasmons and polaritons are extremely localized in space, so the main contribution to EELS will be roughly described by the second form, which is more direct to compute in our method.

G. Strong coupling

Experimental data shows the anti-crossing behavior apparent to the strong coupling regime between plasmon and phonon modes. This can be modeled using a classical model of strongly coupled harmonic oscillators whose dispersion is give by a simple expression

1 2 (ω pl + ω ph ) ± ( 1 4 (ω pl -ω ph ) 2 + g 2 ) 1 2 (3)
where ω pl is the plasmon resonant frequency, ω ph is the phonon resonant frequency and g is the coupling strength between plasmons and the phonons. The later served as a fitting parameter for us to fit the experimental data using the analytically calculated plasmon ω pl (q) dispersion of the infinite nanowire and the phonon dispersion ω ph (q) curves for the h-BN thin film. The infinite nanowire plasmon dispersion can be shown to be given with the expression 27

mq 2 a 2 1 k 2 2⊥ - 1 k 2 1⊥ 2 = 1 k 2⊥ J m (k 2⊥ a) J m (k 2⊥ a) - 1 k 1⊥ H (1) m (k 1⊥ a) H (1) m (k 1⊥ a) x k 2 2 k 2⊥ J m (k 2⊥ a) J m (k 2⊥ a) - k 2 1 k 1⊥ H (1) m (k 1⊥ a) H (1) m (k 1⊥ a) (4)
where a is the wire radius, m is the azimuthal order of the plasmon (m = 0 in our case), k j = j k 0 ,k 0 = ω/c , and k j⊥ = k 2 j -q 2 is the transverse wave vector in medium j. The phonon-polariton modes of the h-BN film are calculated from Fresnel reflection coefficient. In particular, for Fig. S13A we use the imaginary part of the reflection coefficient (r p )for the p polarized fields , which readily gives the hyperbolic phonon-polaritonic bands (KF modes). In Fig. S13B we compare the coupling strength with the plasmon and phonon resonance widths that are again extracted from simulations. It is clear that the coupling strength is larger than the plasmon and phonon widths for small values of q, therefore indicating that the system undergoes strong coupling

EELS spectrum

Sample

Scan optics

FIG. 1 .

 1 FIG. 1. (A) Samples consisting of silver nanowires deposited on h-BN are investigated with two different configurations: (1) nanowires partially supported by h-BN with one of its ends in vacuum; and (2) nanowires fully supported on h-BN. High-current (∼ 10 µA at 200 keV, orange) and low-current (∼ 10 pA at 60 keV, purple) electron beams are used for milling and probing, respectively. (B) Sketches of the different phonons, plasmons, and hybrid excitations discussed in the text. (C) Spectra of a 3.3 µm long metallic nanowire in vacuum (blue) and h-BN (green). The in-vacuum nanowire exhibits equally spaced spectral modes labeled P. h-BN spectra are shown for the electron beam placed outside the material (aloof configuration) and on either thin or thick h-BN, where the symmetric (FKS) or antisymmetric (FKA) Fuchs-Kliever modes are observed. (D) Spectrum of a metallic nanowire in sample configuration (1) measured on the tip in vacuum (upper curve). The dipolar mode at ∼ 200 meV (black arrow) is split into two peaks compared with the nanowire in vacuum (lower curve). (E) Spectra for the same metallic nanowire in configuration (2) under off-(orange) and on-resonance (purple) conditions.

FIG. 2 .

 2 FIG.2. Plasmon-phonon strong coupling (A) EELS spectra measured at the tip in vacuum of a metallic nanowire (under sample configuration (1) in Fig.1A) as a function of nanowire length. By changing through milling the length of the nanowire, its dipolar plasmon mode is brought in and out strong coupling with the h-BN phonon modes at around 180 meV. The strong coupling is confirmed by the anti-crossing of the two modes, indicated by the two dashed curves (added as guides to the eye) on the left of (A). (B) Theoretical calculations for the energies of the coupled modes (dashed red curve) and the energy position measured in experiments (black and orange crosses for the dipolar and quadrupolar modes). The light line is marked in blue. The quadrupolar plasmon mode is marked by orange crosses.

FIG. 3 . 1 Supplemental

 31 FIG.3. Plasmon enhanced phonon losses (A) Experimental (full curves) and calculated (dashed curves) EELS spectra at one of the tips of a metallic nanowire fully supported on h-BN (configuration 2 in Fig.1A) for different lengths of the nanowire: 1.6 , 2.0, 2.1, and 2.9 µm. When the dipolar plasmon mode is close to or on resonance with h-BN phonon modes, the latter are enhanced, leading to the appearance of multiple peaks.(B-C) Spatial evolution of the EELS spectra along a direction perpendicular to the nanowire close to the bottom tip (red arrow marked in (D)). The ZO, PPS, FKS, LO, and PPA modes are marked by 1, 2, 3, 4, and 5, respectively in (C) (D) STEM image of the nanowire on h-BN. (E) Spatial evolution of the EELS spectra intensity as a function of energy loss and position along the nanowire length (blue arrow in (D)).

FIG. 1 .FIG. 2 .

 12 FIG. 1. Characterization of AgAuAg bimetallic nanowires. (A) UV-vis-NIR spectra of the nanowire colloidal suspension used in this study, taken in deuterated water. Asterisks indicate residual water peaks. (B-D) TEM images taken at different filed of views. In panel D the gold cores are clearly visible.

FIG. 3 .FIG. 4 .

 34 FIG. 3. General description of the experimental configuration. (A)An electron monochromator is used to reduce the energy width of a free electron beam down to 5-10 meV. (B) An electron microscope focuses the monochromated electron beam down to subnanometer sizes. The focused beam is scanned in real space to construct energy-resolved maps of the samples.

FIG. 5 .

 5 FIG.5. Spectra measured on tip 1 (on h-BN) of the nanowire presented in Fig.2of the main text.

FIG. 6 .FIG. 7 .FIG. 8 .FIG. 9 . 3 FIG. 10 .FIG. 11 .

 678931011 FIG.6. Nanowire with small amount of h-BN and no coupling. (A) and (B) show EELS spectra measured at tips 1 (dark blue; h-BN close by) and 2 (light blue, no h-BN close by) of the same nanowire with different lengths. The plasmon mode at around 180 meV disperses as a function of length and does not show peak splitting (the two peaks observed are the expected ones for h-BN, superimposed on the nanowire dipolar plasmon peak), the behavior expected for the uncoupled system.

3 FIG. 12 .

 312 FIG.12. Spectral evolution away from the surface of the nanowire for different nanowire length. The left panel shows a 2D map of EELS intensity as a function of energy loss and distance to the nanowire (the nanowire is positioned at the dark bands in the center of each panel. The center and left panel shows the spectral evolution as a function of distance for selected distances with and without an offset (given by the distance), respectively.

ACKNOWLEDGMENTS

This project has received funding from the european unions horizon 2020 research and innovation programme under grant agreement no 823717.This work has received support from the National Agency for Research under the program of future investment TEMPOS-CHROMATEM with the Reference No. ANR-10-EQPX-50. We acknowledge support from Spanish MINECO (MAT2017-88492-R and SEV2015-0522), ERC (Advanced Grants 789104-eNANO and 4DBIOSERS, AdG 787510), the Catalan CERCA Program, and Fundació Privada Cellex. The authors acknowledge Christoph Hanske for help in the nanowire sample preparation.