Audrey Schillings
email: audrey.schillings@irf.se

Hans Nilsson

Rikard Slapak

Peter Wintoft

Masatoshi Yamauchi

Magnus Wik

Iannis Dandouras

Chris M Carr

D C Dandouras

E E Delcourt

Grigorenko

H Rème

C Aoustin

J M Bosqued

B Lavraud

J A Sauvaud

A Barthe

J Bouyssou

T Camus

O Coeur-Joly

A Cros

J Cuvilo

F Ducay

Y Garbarowitz

J L Medale

E Penou

H Perrier

D Romefort

J Rouzaud

C Vallat

D Alcaydé

C Jacquey

C Mazelle

C D'uston

E Möbius

L M Kistler

K Crocker

M Granoff

C Mouikis

M Popecki

M Vosbury

B Klecker

D Hovestadt

H Kucharek

E Kuen- Neth

G Paschmann

M Scholer

N Sckopke

E Seidenschwang

C W Carlson

D W Curtis

C Ingraham

R P Lin

J P Mcfadden

G K Parks

T Phan

V Formisano

E Amata

M B Bavassano-Cattaneo

P Baldetti

R Bruno

G Chionchio

A Di Lel- Lis

M F Marcucci

G Pallocchia

A Korth

P W Daly

B Graeve

H Rosenbauer

V Vasyliunas

M Mccarthy

M Wilber

L Eliasson

R Lundin

S Olsen

E G Shelley

S Fuselier

A G Ghielmetti

W Lennartsson

C P Escoubet

H Balsiger

R Friedel

O + Escape During the Extreme Space Weather Event of 4-10 September 2017

published or not. The documents may come

Introduction

Solar storms with associated flares, coronal mass ejections (CMEs), and radio bursts may be hazardous to Earth, potentially affecting satellite operations through enhanced satellite drag, causing electric power distribution failures due to extraordinary ground induced currents (GIC), disturbing radio communications and causing radio blackouts due to higher ionization rates in the lower ionosphere. [START_REF] Balch | Service assessment: Intense space weather storms october 19november 07[END_REF] report occurrence rates of such events; extreme and severe radio blackouts occur approximately 1 day and 8 days per solar cycle respectively. Similarly, the planetary Kp index [START_REF] Bartels | The three-hour-range index measuring geomagnetic activity[END_REF][START_REF] Mayaud | Derivation, Meaning, and Use of Geomagnetic Indices[END_REF], widely used as a general indicator of geomagnetic disturbances for midlatitude regions and as a general geomagnetic alert and hazard scale, reaches extreme (Kp = 9) and severe (Kp = 8) levels 4 and 100 days per solar cycle, respectively [START_REF] Balch | Service assessment: Intense space weather storms october 19november 07[END_REF].

In solar cycle 23, there were more than 100 X-flares, the strongest occurring in November 2003 (X28.0) and in April 2001 (X20.0). However for these events the solar active regions were close to the solar limb, therefore no severe or extreme geomagnetic storms were observed [START_REF] Zhang | Solar and interplanetary sources of major geomagnetic storms (Dst ≤ -100 nT) during 1996-2005[END_REF]. At third strongest was an X17.2 flare detected on October 28, 2003 and due to a strong Interplanetary Coronal Mass Ejection (ICME) an extreme geomagnetic storm -also known as the Halloween storm -took place [e.g [START_REF] Yamauchi | Unusually quick development of a 4000 nt substorm during the initial 10 min of the 29 october 2003 magnetic storm[END_REF][START_REF] Rosenqvist | Extreme solar-terrestrial events of October 2003: High-latitude and Cluster observations of the large geomagnetic disturbances on 30 October[END_REF][START_REF] Gopalswamy | Introduction to violent Sun-Earth connection events of October-November 2003[END_REF]. It produced disturbances around the world [START_REF] Balch | Service assessment: Intense space weather storms october 19november 07[END_REF]. In the same solar cycle, on January 20, 2005 an outstanding solar flare occurred (X7.1), followed by an ICME which was detected in the near-Earth space on January 21. This ICME arrival resulted in an extreme magnetospheric com-D R A F T July 3, 2018, 7:30am D R A F T pression [START_REF] Dandouras | Magnetosphere response to the 2005 and 2006 extreme solar events as observed by the cluster and double star spacecraft[END_REF]. In solar cycle 24, in September 2017, several X-flares occurred of which the most intense was an X9.0-flare (the largest observed since December 2006) which with strong CMEs produced an extreme geomagnetic storm. These extreme events caused a strong response in the ionosphere and magnetosphere, for example variations in the total electron content (TEC) and enhanced magnetospheric convection. An additional effect is the strongly enhanced ion outflow from the polar ionosphere [START_REF] Schillings | Relative outflow enhancements during major geomagnetic storms -cluster observations[END_REF].

Ion outflow from the ionosphere has been widely studied, i.e. by [START_REF] Shelley | The polar ionosphere as a source of energetic magnetospheric plasma[END_REF]; [START_REF] Chappell | The ionosphere as a fully adequate source of plasma for the earth's magnetosphere[END_REF]; [START_REF] Moore | High-altitude observations of the polar wind[END_REF]; Lennartsson et al. [2004]; Kronberg et al. [2014];

Maes et al. [2015] and references therein. The most profound magnetospheric ion outflow is usually observed in the open magnetic field line regions: the polar caps, cusps and plasma mantle [START_REF] Haaland | Estimating the capture and loss of cold plasma from ionospheric outflow[END_REF]. The mirror force plays a key role in the acceleration of outflowing ions, with perpendicular energy converted into parallel energy as the ions move into higher altitudes and weaker magnetic fields. Therefore, perpendicular heating of ions causes subsequent outward acceleration. Wave particle-interaction can cause such transverse ion heating, and these processes and their effects have been investigated at different altitudes [e.g. [START_REF] Andre | Ion heating by broadband low-frequency waves in the cusp/cleft[END_REF]Norqvist et al., 1996;[START_REF] Bouhram | Survey of energetic O + ions near the dayside mid-latitude magnetopause with Cluster[END_REF]Nilsson et al., 2006;[START_REF] Moore | Stellar ablation of planetary atmospheres[END_REF][START_REF] Slapak | O + heating associated with strong wave activity in the high altitude cusp and mantle[END_REF][START_REF] Slapak | O + heating associated with strong wave activity in the high altitude cusp and mantle[END_REF][START_REF] Haaland | Estimating the capture and loss of cold plasma from ionospheric outflow[END_REF]. If the heating is effective enough, the ions will gain sufficient velocities to escape into the solar wind downstream in the tail [START_REF] Haaland | Estimating the capture and loss of cold plasma from ionospheric outflow[END_REF] or even directly from the cusp [START_REF] Slapak | A statistical study on O + flux in the dayside magnetosheath[END_REF].

Ion outflow has been studied for different solar wind and geomagnetic conditions. [START_REF] Cully | Akebono/Suprathermal Mass Spectrometer observations of low-energy ion outflow: Dependence on magnetic activity and solar wind conditions[END_REF] identified four factors that influence the ion outflow; the solar radio flux at D R A F T July 3, 2018, 7:30am D R A F T 10.7 cm (F10.7), the interplanetary magnetic field (IMF), the solar wind electric field and the solar wind dynamic pressure. Using data in an altitude range of 1.3 to 2.0 R E , [START_REF] Yau | Quantitative parametrization of energetic ionospheric ion outflow[END_REF] found that ion outflow depends on the geomagnetic condition such that it increases exponentially with Kp. Similarly, Slapak et al. [2017] made a statistical study using high-altitude plasma mantle and magnetosheath Cluster data in order to quantify the total O + escape as a function of Kp. Complementary to that study, Schillings et al.

[2017] presented that the escape rate during extreme geomagnetic storms could be higher than what a linear extrapolation of the results of Slapak et al. [2017] would predict. As the whole magnetosphere is affected by disturbed magnetospheric conditions, the O + outflow that does not escape into the solar wind is transported to the lobes through the polar cap.

Through reconnection in the plasma sheet, O + coming from the lobe is heated and feeds both the distant and near-Earth plasma sheet [START_REF] Kistler | Ion composition and pressure changes in storm time and nonstorm substorms in the vicinity of the near-Earth neutral line[END_REF][START_REF] Mouikis | H + and O + content of the plasma sheet at 15-19 Re as a function of geomagnetic and solar activity[END_REF].

The consequence of space weather on ion outflow has not been well studied. Therefore, it is not clear what parameter is most influential with what time scale. In this paper, we first investigate the space weather during 4 to 10 September 2017, from solar storm to geomagnetic storm. Thereafter we examine the effects on magnetosphere-ionosphere coupling, in particular ion outflow. The goal is primarily to see the space weather effects on ion outflow for this well studied storm. Despite that ion outflow in itself not causing hazardous to human activity, the enhanced ion outflow and resulting escape are mainly observed in the cusp, which has an important role in some hazards as discussed in Section 5.3.

Instrumentation and data

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Cluster mission

The Cluster mission [START_REF] Escoubet | IntroductionThe Cluster mission[END_REF] was launched in 2000 and consists of four spacecraft flying in tetrahedral formation in an elliptical polar orbit. Only spacecraft 4 (SC4) was used in this study, because in 2017 the COmposition DIstribution Function (CODIF) part of the Cluster Ion Spectrometer (CIS) [Rème et al., 2001] onboard the other spacecraft are no longer functional. The particle detection efficiency of CODIF onboard spacecraft 4 has also been degraded [Kistler et al., 2013], this degradation is taken into account by the instrument in-flight calibration files. The CODIF instrument measures the 3D-distributions of H + , He 2+ , He + , and O + using a time-of-flight technique. When CODIF is subject to intense proton fluxes the heavier ion channels may be contaminated with false counts. If so, the ratio of the O + to H + perpendicular bulk velocity will have a local peak at 1/4, and these unreliable O + data can be removed [Nilsson et al., 2006].

The magnetic field data are provided by the FluxGate Magnetometer (FGM) [START_REF] Balogh | The Cluster Magnetic Field Investigation: overview of in-flight performance and initial results[END_REF], which has a normal mode sample frequency of 22.4 Hz. In this study we are interested in the background magnetic field and use the field averaged over the spacecraft spin period of 4 seconds.

Solar and solar wind data

We utilized a range of observations and derived data from the Sun to the solar wind.

The Solar Dynamics Observatory (SDO) [Pesnell et al., 2012] is especially targeted at producing observations of the solar drivers in order to understand and predict space weather. Images from the photosphere and photospheric magnetic fields, and up through the corona, are produced at high resolution and high cadence. Solar active regions (ARs) are summarized and collected on a daily basis in the Solar Region Summary (SRS) by

D R A F T July 3, 2018, 7:30am D R A F T X -7
NOAA/SWPC. SWPC also provides full-disk X-ray observations from the GOES spacecraft. CMEs are observed by the ESA/NASA Solar and Heliospheric Observatory (SOHO)

and the NASA Solar TErrestrial RElations Observatory (STEREO) (currently only one spacecraft is operational). From coronal images, the Solar Influences Data Centre (SIDC) automatically detects and estimates CME parameters, producing a catalog of CMEs. Finally, solar wind plasma and magnetic fields are measured by the DSCOVR spacecraft;

data are provided by NOAA/SWPC.

Method

This section describes how we analyzed the September 2017 storm from the solar point of view to the O + outflow in the polar regions at Earth. We study the complete chain of events from the Sun, through the solar wind, geomagnetic indices, and ion outflow, in particular the ionospheric O + ions. The O + ions mainly originate from the ionosphere whereas the distinction between the ionospheric and solar origin of the H + is more complex. We first looked and analyzed the solar data such as X-flares and CMEs from the beginning of September. Afterward, we checked the solar wind parameters from 4 to 10 September to identify the arrival time of the shocks at Earth and finally compare them with Cluster observations. At Earth, a geomagnetic storm is characterized by three phases -the initial, main and recovery phase -which can be identified by the behavior of the Dst index. The Dst index is a 1-hour index estimated from the deviations of the horizontal component of the magnetic field at low-latitude magnetometer stations. In principle, it is a measure of the equatorial ring current strength. The Kp index is a 3-hour index estimated from local disturbances in the horizontal magnetic field component and relates to geomagnetic activity over a D R A F T July 3, 2018, 7:30am D R A F T global scale. A combination of these two magnetic indices provides magnetospheric information on the geomagnetic storm. The initial phase of a geomagnetic storm is typically characterized by a positive disturbance in the Dst index, which could also be associated with an SSC. When the positive perturbation drops drastically to negative values, the main phase of the storm starts and lasts a few hours -typically 2 h to 10 h -and is followed by the recovery phase that can last hours to a few days.

In order to estimate the O + outflow during the geomagnetic storm, we first looked at the O + and H + energy and pitch angle (PA) spectrograms and the magnetic field for SC4.

In Fig. 1, panel (a) shows that the local magnetic field varied slowly and was weaker in the regions marked as polar cap (PC1 and PC2), while it was stronger and variable in the cusp (Cusp 1 and Cusp 2). Panels (b), (c), (d) and (e) correspond to O + and H + energy and pitch angle spectrograms respectively (for more detail see Schillings et al.

[2017]). The color bar represents counts per second (c/s), which is proportional to the particle differential energy flux, and the dashed (pink) rectangles distinguish the polar caps from the cusps. From Fig. 1, we visually identified the outflowing regions in the northern hemisphere. In the polar cap, we observed an oxygen ion beam at lower energies than in the cusp, see panel (b). Furthermore, the magnetic field in the polar cap is more stable as opposed to the cusp, which is characterized by more variability.

Observations

This section presents the September storm from the solar storm to the ion escape during the extreme geomagnetic storm. The solar regions where the flares and CMEs were produced are described and we estimate the terrestrial magnetospheric O + outflow during the main phase of the geomagnetic storm.

D R A F T July 3, 2018, 7:30am D R A F T 4.1. Solar storm
Multiple X-type flares and Earth-directed CMEs were observed during 4-10 September 2017. On September 1, the solar Active Region (AR) 12673 was a small unipolar spot just south of the solar equator (magnetic type α). At the same time AR12674 (Fig. 3, panel (a), yellow dashed circle) on the northern hemisphere was much larger and more complex (βγ). However, during 3-4 September AR12673 grew dramatically both in size (more than 10 times in area, see Fig. 3, panels (a) and (b), yellow solid circle) and complexity (βγδ) and thus became a highly interesting region. The region stayed large and magnetically complex during its passage over the solar disk until it went around the west limb during 10 September.

AR12673 produced numerous flares and CMEs from 4-10 September. Most notable was a CME in the evening of 4 September and another on mid-day 6 September. The SIDC identified speeds in both events close to 2000 km/s with onset times at 2017-09-04 19:12

UT and 2017-09-06 12:12 UT, respectively [Robbrecht et al., 2009]. Another fast CME was launched on 10 September, but at that time AR12673 was on the western limb and the CME was not Earth-directed.

Two X-flares occurred on 6 September, one on 7 September, and another on 10 September. The X-flares were observed by GOES-13 for short and long wavelengths with 1-minute resolution X-ray flux, XS (0.05-0.4 nm) and XL (0.1-0.8 nm) as presented in Fig. 4, panel and the Dst index dropped significantly to -142 nT. This negative drop is a signature

D R A F T July 3, 2018, 7:30am D R A F T
of the storm main phase, which continued for approximately 5 hours (20:00 -01:00 UT).

During the main phase of the storm, Cluster was in the northern hemisphere (see Fig. 2, and top panels for zooms).

The arrival of the second shock was coincident with an abrupt equatorward motion of the cusp, at 23:00 UT on 7 September (Fig. 1, panels (b) and (c)) in the Cluster data.

When hitting the Earth, the shock again compressed the magnetopause, causing higher convection in the magnetosphere. Simultaneously, the southward IMF opened up the dayside geomagnetic field lines, such that the footprints of the cusp are at lower latitudes. [1994]; Newell et al. [1989] described how the polar cap and plasma mantle move equatorward for disturbed magnetospheric conditions, and we observed such a motion of the cusp. After the second shock, Cluster was located in the cusp for about 50 minutes, before the cusp moved equatorward such that Cluster was again located in the polar cap for approximately 40 min (see Fig. 1). Afterward Cluster entered the cusp a second time, where the spacecraft stayed for a few minutes before ending up in the magnetosheath (see Fig. 2, top panels).

Newell and Meng

Following the main phase, the recovery phase (number 3 in panel (e)) was longer and in the meantime Cluster was at the nightside in the southern hemisphere (8 September around noon). In Fig. 2, the bottom panels show the encountered regions in the south.

We visually identified the polar cap and plasma mantle as Cluster was on the flank of the magnetosphere and in the nightside. Cluster encountered 4 polar cap/plasma mantle crossings.

4.3. O + outflow during the geomagnetic storm 4.3.1. Northern hemisphere

D R A F T July 3, 2018, 7:30am D R A F T X -13
We estimated the scaled O + outflow in the different regions before and after the second shock signature, from September 7, 22:11 UT to September 8, 00:33 UT, while Cluster was located in the polar cap and in the cusp. and 1.6 × 10 13 m -2 s -1 , respectively, and increase by a factor of 3 (polar cap) and 2 (cusp)

after the shock signature. The highest value, with a minimum of 1% of the data in the bin, are 6.3 × 10 13 m -2 s -1 for the polar cap and 1 × 10 14 m -2 s -1 for the cusp.

To determine if the scaled O + outflow is escaping into the solar wind, we study the O + perpendicular and parallel temperatures, as well as the perpendicular and parallel velocities. Table 1 gives the four parameters we took into account to estimate the fate of the O + ions. In the polar cap regions, the parallel to perpendicular temperature ratio is high due to the high parallel component, therefore the transverse heating is not efficient, this is as expected (significant transverse heating is usually observed only in the cusp due to the wave-particle interactions [Strangeway et al., 2005;[START_REF] Slapak | A statistical study on O + flux in the dayside magnetosheath[END_REF]). The ions in the polar cap are mostly accelerated through the centrifugal acceleration [START_REF] Haaland | Estimating the capture and loss of cold plasma from ionospheric outflow[END_REF]. Furthermore, the parallel velocity is twice the perpendicular velocity (in the polar

D R A F T July 3, 2018, 7:30am D R A F T cap)
, so that the ions follow the magnetic field upward with their fate in the plasma sheet tailward of the X-line (neutral line in the magnetotail) at about 20 R E -30 R E . Note that the X-line can be pushed earthward around 10 R E for a severe geomagnetic storm [Tsyganenko and Sitnov , 2005]. In the cusp region, the components of the temperature are similar before the shock, whereas the perpendicular temperature is about twice the parallel component. This indicates a transverse heating effect. However, the perpendicular velocities are very high (> 40 km/s) compared to the average convection velocity of 7,5 km/s in the lobes [START_REF] Haaland | Estimating the capture and loss of cold plasma from ionospheric outflow[END_REF], and it leads to a strong convection in the cusp region. The cusp field-lines will convect through the polar cap (lobes) before they reach the plasma sheet and thus we should use the perpendicular velocity in the polar cap for our calculations of where the ions reach the plasma sheet. A considerable amount of flux is therefore transported tailward and towards the plasma sheet, despite this strong convection the ions have enough energy to escape directly downtail.

Fig. 6 shows the relative probability of log 10 T ⊥ (O + ) [eV] in the northern polar cap and cusp before the shock signature (blue) and after (red). Note that the transitional color corresponds to the superposition of the 2 distributions. The dashed blue (Cut 1) and red (Cut 2) lines roughly mark the separation between the polar cap and the cusp. In the cusp, we observed a higher perpendicular temperature than in the polar cap as expected because the ions are more heated. The perpendicular energy is converted into parallel energy by the mirror force for outflowing ions, thus accelerating the ions upward. [START_REF] Slapak | O + heating associated with strong wave activity in the high altitude cusp and mantle[END_REF] showed that O + is effectively heated transversely through wave particleinteractions in the cusp. By the high perpendicular temperatures in the cusp combined

D R A F T July 3, 2018, 7:30am D R A F T X -15
with a parallel velocity v (O + ) > 30 km/s (not shown), we can conclude that the observed O + in the cusp will eventually escaping into the solar wind [START_REF] Slapak | O + heating associated with strong wave activity in the high altitude cusp and mantle[END_REF].

Southern hemisphere

Cluster was located in the southern hemisphere during the recovery phase (number 3 in panel (e), Fig. 4) of the geomagnetic storm. Thus, we checked which regions it encountered where O + outflow might be observed. We identified 4 crossings from polar cap to plasma mantle (see Fig. 2, bottom panels) where the scaled O + outflow has been estimated. The 4 identified polar cap and plasma mantle observations are numbered from 1 to 4 according to the passage on the Cluster orbit (color points on Fig. 2), i.e. number 1 equals to the first polar cap/plasma mantle crossing in time along Cluster orbit. In plasma mantle regions from 1 to 4, the scaled O + outflow is roughly the same ∼ 3 × 10 12 m -2 s -1 . Similarly to the northern hemisphere, we looked at the perpendicular and parallel components of the temperature and velocities in the plasma mantle regions. The parallel temperatures are unexpectedly high (about twice the perpendicular component) for the 4 plasma mantle passages, which means that a local heating is observed in these regions, otherwise the velocity filter effect would have decreased the parallel component of the temperature.

Additionally, the perpendicular component of the velocity is not negligible, we thus have a significant convection. This strong convection implies a considerable magnetic flux transport that must be released through reconnection if it is not to accumulate in the magnetotail. The reconnection rate must then be correspondingly high and the X-line might move earthward. Thus, due to the high parallel component of the temperature and the perpendicular velocity, the fate of the ions is less predictable than for the northern D R A F T July 3, 2018, 7:30am D R A F T hemisphere. They might end up in the plasma sheet and feed the ring current with O + or tailward of the X-line and be lost in the magnetotail.

O + flux in the plasma sheet

We also looked at the O + flux in the first plasma sheet crossing after the storm. The plasma sheet is identified around X GSM ≃ -15 R E and the net O + flux is small. Following the method of Slapak et al. [2017] -dividing the flow between tailward and earthward flux -we estimated an average O + flux of ∼ 1.1 × 10 9 m -2 s -1 in both directions. This estimation is in agreement with the average O + flux calculated in the central plasma sheet by Slapak et al. [2017]. In addition, Cluster was in the plasma sheet in the end of the recovery phase (9 September, about 00:00 to 16:00 UT) and our estimation shows that the average O + return flux was the same as during quiet magnetospheric conditions. This result is consistent with the results of [START_REF] Mouikis | H + and O + content of the plasma sheet at 15-19 Re as a function of geomagnetic and solar activity[END_REF], who showed that the O + density is reduced during the recovery phase.

Discussion

Solar drivers for O + escape during geomagnetic storm

Extreme geomagnetic storms are caused by ICMEs, while smaller storms could be caused by corotating interaction regions (CIRs). On September 4 and 6, 2017, two CMEs were detected at the Sun and on September 6 and 7, three X-flares (X2.2; X9. [START_REF] Balch | Service assessment: Intense space weather storms october 19november 07[END_REF]Yamauchi and Slapak , 2018]. Lennartsson et al.

[2004] showed that negative IMF Bz has a higher influence on the total O + outflow by approximately a factor 3. Considering also the timing of enhanced O + compared to arrival timing of CMEs and enhanced EUV flux, the solar driver that influences O + outflow during a geomagnetic storm is probably CMEs, despite that the X-flares with long wavelength (0.1 -0.08 nm) ionize the neutral atmosphere all the way down to approximately 95 km [Rees, 1989;Tsurutani et al., 2005]. In comparison, the Halloween event that occurred October 28-30, 2003, had stronger X-flares (X17.2 (Oct 28) and X10.0 (Oct 29)) and Kp = 9 + , and Schillings et al.

[2017] estimated a higher polar cap O + outflow than is observed for the September 2017 storm. Consequently, the ionization in the polar cap was higher and the X-flares could have influenced the ionization process [Tsurutani et al., 2005] such as a preheating of the ionosphere before the CME hit the Earth. Yamauchi et al. [2018] wrote an overview of the ionospheric response to this storm with the EISCAT radars.

The authors showed that the electron density and temperature were enhanced after the detection of the X-flares. They also suggested that the increased ion temperature due to ion heating creates a precondition for O + upflow (upflow means that the ions are still gravitationally bound). This suggestion is strengthened by the increased F10.7 solar radio flux, which can be taken as a proxy of EUV flux, before and during the magnetic storm.

In addition, [START_REF] Yau | Quantitative parametrization of energetic ionospheric ion outflow[END_REF] found a clear correlation between O + outflow and Kp and F10.7. For low solar activity (F10.7 between 70 and 100 sfu) [START_REF] Yau | Quantitative parametrization of energetic ionospheric ion outflow[END_REF] estimated the O + outflow to be a factor 4 lower than during high solar activity (F10.7 between 150 and 250 sfu). [START_REF] Cully | Akebono/Suprathermal Mass Spectrometer observations of low-energy ion outflow: Dependence on magnetic activity and solar wind conditions[END_REF] found a correlation between the total O + outflow and F10.7; higher O + outflow is observed for increasing F10.7. Despite that we do not

D R A F T July 3, 2018, 7:30am D R A F T
have suitable Cluster data when the X-flares occurred, we conclude that the primary solar driver for O + outflow is probably ICMEs due to the clear influence seen in the Cluster data but X-flares might have a significant contribution in the heating process of the ionosphere [Yamauchi et al., 2018] and in causing additional ionization.

Escaping O

+ during the September 7 -8, 2017 geomagnetic storm

We estimated the scaled O + outflow in the northern polar cap and cusp region during the main phase of the storm. Fig. 5 shows the polar cap and cusps before and after the second CME -shock (see section 4). We observed an enhancement of a factor 3 for the polar cap and a factor 2 for the cusp after the shock signature. This weak enhancement is comparable with the lowest relative scaled O + outflow enhancement for intense geomagnetic storms presented by [START_REF] Schillings | Relative outflow enhancements during major geomagnetic storms -cluster observations[END_REF], who found that the scaled O + outflow increases by 1 to 2 orders of magnitude during intense storm conditions compared to quiet magnetospheric conditions. However, the absolute value of the scaled O + outflow (∼ 10 13 m -2 s -1 ) is quite high compared to similar storms studied in [START_REF] Schillings | Relative outflow enhancements during major geomagnetic storms -cluster observations[END_REF]. We suggest that this unusual high scaled O + outflow is due to the already compressed magnetosphere as a consequence of the first shock approximately 24 hours earlier. Moreover, before the second shock signature (September 7, 23:30 UT) the predicted Kp index was 3 and after the shock, Kp = 8, thus according to Slapak et al. [2017] and their Fig. 5, the increase in the scaled O + should be around 1 order of magnitude. Our lower result could be explained by the difference in the studied regions (polar cap compared to plasma mantle/magnetosheath for Slapak et al. [2017]). This increase in the unusually high O + outflow was observed within roughly 40 minutes, which is fast for a magnetospheric response. The quick response and D R A F T July 3, 2018, 7:30am D R A F T lower relative enhancement in the O + outflow could be explained by the first shock, which already initiated a geomagnetic storm. [START_REF] Schillings | Relative outflow enhancements during major geomagnetic storms -cluster observations[END_REF] could not confirm that the enhanced scaled O + outflow in the polar cap during the storm was escaping and lost into the interplanetary space. However, for the September 2017 storm, we studied and could compare the scaled O + outflow in two regions, the polar cap and the cusp. For these two regions, we looked at the perpendicular and parallel component of the temperature and the outflow parallel and perpendicular bulk velocity. Fig. 6 shows that before the second CME-shock, the perpendicular temperature in the polar cap 1 (blue, left side of Cut 1) is lower than after polar cap 2 (red, left side of Cut 2). This means that the ions in the polar cap have been heated

and accelerated [Nilsson et al., 2006] after the shock passage. In both cusp regions, the perpendicular temperature remains more or less the same, but is higher than in the polar cap. Consequently, despite a significantly high perpendicular bulk velocity, we observed ions with higher energy and parallel bulk velocity (not shown) in the cusp with sufficient energy to escape into the solar wind. This result confirms what [START_REF] Schillings | Relative outflow enhancements during major geomagnetic storms -cluster observations[END_REF] expected but could not claim.

In the southern hemisphere, the perpendicular temperature and the scaled O + outflow in the 4 identified polar cap and plasma mantle regions (1-4) are constant, despite the parallel velocity is decreasing as Cluster moves tailward. This result suggests that an enhancement is still observed during the recovery phase, and in addition the local heating of O + is still significant as indicated by the unexpectedly high parallel temperature. Furthermore, the perpendicular bulk velocity is not negligible and the strong convection might drive the ions to the plasma sheet earthward of the X-line. In opposite, some ions experience enough

D R A F T July 3, 2018, 7:30am D R A F T
heating/acceleration to end up behind the X-line and therefore be lost in the tail. The fate of these O + ions is less predictable than in the northern hemisphere. Finally, during the 3 first hours of the recovery phase, the Kp index was 5, hence these observations also confirm the dependence of O + outflow on Kp [Slapak et al., 2017].

5.3. How does outflowing O + relate to space weather effects?

The space weather affects not only the O + outflow, but the magnetosphere as a whole.

Space weather influences the occurrence of substorms, the life-time of the ring current, the density of the plasma sheet and the occurrence of bursty bulk flows (BBFs) and finally the energy conversion through mass loading in the cusp. Substorms do not always happen during geomagnetic storms and therefore a storm does not necessarily include substorms [START_REF] Kamide | Current understanding of magnetic storms: Storm-substorm relationships[END_REF][START_REF] Hori | Storm-Substorm Relationship: Controversies and Recent Development[END_REF]. At a substorm onset, Nosé et al. [2005] found that the O + /H + energy density ratio increases. During substorm growth phase, energetic O + (tens of keV), which mainly originate from the dayside polar region [START_REF] Nakayama | Impact of substorm time O + outflow on ring current enhancement[END_REF], play an important role in feeding the ring current. The life-time of the ring current is therefore prolonged by the addition of energy by dominant O + under disturbed magnetospheric conditions, which have also lower charge-exchange cross-sections than the H + ions, resulting in a slower loss [START_REF] Hori | Storm-Substorm Relationship: Controversies and Recent Development[END_REF][START_REF] Denton | The evolution of the plasma sheet ion composi-D R A F T July 3, 2018, 7:30am D R A F T tion: Storms and recoveries[END_REF].

Another aspect is the transport of O + into the plasma sheet. [START_REF] Mouikis | H + and O + content of the plasma sheet at 15-19 Re as a function of geomagnetic and solar activity[END_REF] looked at the ion composition changes in the plasma sheet associated with geomagnetic and solar activity and found a strong influence on O + density. Under strong geomagnetic activity, part of the O + outflow is transported through the magnetotail and fed to the plasma sheet [START_REF] Mouikis | H + and O + content of the plasma sheet at 15-19 Re as a function of geomagnetic and solar activity[END_REF]. These ions can be seen in BBFs. However, Nilsson et al. [2016] showed that O + is less accelerated during BBFs than the dominating protons.

D R A F T July 3, 2018, 7:30am D R A F T Finally, high O + escape into the inflow solar wind plasma leads to a high mass-loading rate, and high extraction rate of the solar wind kinetic energy to the ionosphere in a limited and small region at high latitudes [Yamauchi and Slapak , 2018]. Therefore, we expect ground induced currents at much higher latitudes than normal. Since human activity is expanding to the polar region, such as arctic sea routes, this can become important.

The O + escape itself is not hazardous to human activity. However, enhanced O + escape rate can be observed at the same time as other space weather effects for example satellite drag. The expansion of the neutral atmosphere in the thermosphere and extreme enhancements of the ion escape can be linked because both occur when there is stronger energy input into the atmosphere during extreme geomagnetic storms. When Kp and EUV flux vary, the neutral temperature above the thermopause fluctuates [START_REF] Chandra | The response of the upper atmospheric temperature to changes in solar euv radiation and geomagnetic activity[END_REF]. The variation of EUV flux heats the upper atmosphere through ion-neutral collisions, which tend to expand the thermosphere [Rees, 1989] and thus increase the atmospheric density at higher altitudes. Therefore, high EUV flux (or the proxy F10.7 cm flux > 250 solar units flux) and Kp (> 6) values result in extreme short-term increases in satellite drag, particularly for satellites on low earth orbit (LEO). These parameters are used in satellite drag models [START_REF] Bowman | A New Empirical Thermospheric Density Model JB2008 Using New Solar and Geomagnetic Indices[END_REF]Storz et al., 2005]. Tsurutani et al. [2007] reported, for the Halloween event, ion-neutral drag due to quick upward motion of O + that causes upward neutral oxygen (O) at equatorial latitudes. Similarly, Lakhina and Tsurutani [2017] generalized the ion-neutral drag for oxygen for 1859-type (Carrington) superstorms. The O + outflow (higher latitudes) could have an impact on this ion-neutral drag.

The cusp plays a significant role in the ion outflow as well as for the satellite drag [Lühr et al., 2004]. Lühr et al. [2004] studied the enhancement of the air density for moderate D R A F T July 3, 2018, 7:30am D R A F T geomagnetic activity (∼ Kp 3) over the cusp with the CHAMP satellite. They concluded that these enhancements of air density are accompanied by small-scale structures such as field-aligned currents (FAC) filaments and suggested the Joule heating as the primary source for the upward movement of the air. This air up-welling over the cusp at low altitude become mostly, due to several chemical reactions, O + ions in the F region of the ionosphere. Furthermore, [START_REF] Masutti | Lessons learned from the first educational cubesat constellation[END_REF] studied the thermosphere during this storm with the QB50 mission (CubeSats) and observed an orbit decay of roughly 2 km during the September storm. Thus, the size or area, the current system or its equatorward motion under extreme conditions is important for the accuracy of satellite drag models.

This speculative approach needs further investigations that were beyond the scope of this study. However, we do believe that stronger O + escape indicate an expansion of the thermosphere, and could be strongly connected to increased satellite drag.

Conclusions

In this paper, we studied the consequences of the space weather on the O + outflow at Earth. We investigated the September 4-9, 2017, storm from the Sun down to the ionosphere. At the beginning of September, several X-flares and two CMEs were detected, which produced a severe geomagnetic storm (Kp ≃ 8 and Dst = -142 nT). The first CME carried northward interplanetary magnetic field (IMF) and the associated shock initiated a storm sudden commencement (SSC) at Earth followed by the initial phase of a geomagnetic storm. A second shock, driven by a CME with southward IMF, hit the magnetosphere 24 hours later and caused an equatorward motion of the cusp, which was observed by Cluster satellite 4 in the northern hemisphere. The ionospheric O + outflow was estimated in the polar cap and cusp before and after the second shock, which initiated

D R A F T July 3, 2018, 7:30am D R A F T X -23
the main phase of the storm. The ionospheric O + outflow increased after the passage of the second shock by a factor 3 in the polar cap and a factor 2 in the cusp. However, the relative enhancement of the O + outflow was lower than expected compared to [START_REF] Schillings | Relative outflow enhancements during major geomagnetic storms -cluster observations[END_REF], nevertheless the initial value of O + outflow (before the shock) for the regions of observation was high ∼ 10 13 m -2 s -1 . The unusually high ionospheric O + outflow was probably due to an enhancement after the first shock and a preheating of the ionosphere by the EUV flux due to the X-flares in the previous days [Yamauchi et al., 2018]. Therefore, the second shock did not increase O + outflow during the main phase as much as previous studies would have led us to believe [START_REF] Schillings | Relative outflow enhancements during major geomagnetic storms -cluster observations[END_REF] but the heated O + had enough velocity to escape directly into the solar wind. On September 8, the magnetosphere started to recover from the massive injection of energy from the solar wind. During this phase,

Cluster was moving tailward in the southern hemisphere and we observed local heating of the ionospheric O + outflow indicated by high parallel temperature, and strong convection as seen in the perpendicular component of the velocity. The fate of the O + is therefore less predictable. Furthermore, observations were made in the plasma sheet on September 9, the O + flux estimated is consistent with the average flux calculated by Slapak et al.

[2017].

The observation of escaping O + during geomagnetic storms caused by CMEs and Xflares also shows that the neutral atmosphere could be ionized sufficiently to expand the neutral thermosphere and lead to significant satellite drag effects. Satellite drag models take into account the geomagnetic activity and the EUV flux [START_REF] Bowman | A New Empirical Thermospheric Density Model JB2008 Using New Solar and Geomagnetic Indices[END_REF]Storz et al., 2005] and the higher these parameters are, the more impact they have on satellite [START_REF] Yau | Quantitative parametrization of energetic ionospheric ion outflow[END_REF][START_REF] Cully | Akebono/Suprathermal Mass Spectrometer observations of low-energy ion outflow: Dependence on magnetic activity and solar wind conditions[END_REF]Slapak et al., 2017], and therefore we suggest that higher O + outflow indirectly indicates stronger satellite drag. Our finding can be summarized as followed:

1. In the polar cap and cusp, we observed an increase in the ionospheric O + outflow by a factor 3 and 2 respectively, after the passage of a shock associated with the second CME. These ions will eventually escape into interplanetary space.

2. The increase in the O + outflow is not extremely high, however, the magnetosphere response to the second CME, is fast (∼ 40 minutes).

3. During the recovery phase, local heating and strong convection are observed in the plasma mantle as seen, respectively, in extremely high parallel component of the temperature and high perpendicular velocity.

4. The upper limit of the ionospheric O + outflow (northern hemisphere) is 6.3 × 10 13 and 1 × 10 14 m -2 s -1 in the polar cap and cusp respectively.

5. The solar drivers for O + outflow seem to be mostly ICMEs whereas X-flares might have a significant contribution in the earlier preheating [Yamauchi et al., 2018]

  The pitch angle spectrograms, displayed in panels (d) and (e), clearly show outflowing ions (PA ≃ 180 • , northern hemisphere) for O + and H + in the polar cap regions. In the cusp, outflowing ions (PA ≃ 180 • ) are visible, but also ion populations with most energy perpendicular to the magnetic field (PA ≃ 90 • ).The identified outflowing regions are confirmed from the observations by inspecting the Cluster orbit. We first used the Orbit Visualization Tool (Khotyaintsev, Y., https://ovt.irfu.se) for a 3D view of the SC4 orbit with the location of the magnetopause and the bow shock. Secondly, we plotted the O + flux along the orbit for these time periods (Fig.2). In Fig.2, the outflowing regions are represented by the zoomed plots (top for the north, bottom for the south) on the orbit planes XZ GSM and XY GSM . The color bars correspond to the O + flux along the Cluster trajectory and the points mark the identified regions. In the north, we observed polar cap and cusp regions while in the south we identified polar cap and plasma mantle regions. During each time period, which corresponds to either the polar cap, the cusp or the plasma mantle, we made histograms of the scaled O + outflow. The scaled O + outflow is defined as the net outward flux scaled to an ionospheric reference altitude and given by the formula F map = F O + * B iono B local . We mapped the fluxes (F O + ) to an ionospheric reference altitude with a magnetic field strength of B iono = 50 000 nT. The local outward flux can be mapped assuming conservation of particle flux along the corresponding magnetic flux tube (B local ). This scaled net outward flux is thus independent of the altitude and any magnetic compression giving an estimate of the original ionospheric outflow.

  (a). The horizontal solid blue line in panel (a) (Fig. 4) corresponds to the lower limit for an X-type flare. The onset of the first CME on 4 September and the likely associated shock in the solar wind are indicated by the vertical solid and dashed blue line, respectively. The latter is clearly seen in the IMF (Fig. 4, panel (b)), density (panel (c)) and D R A F T July 3, 2018, 7:30am D R A F T velocity (panel (d)) 1-minute resolution data taken from DSCOVR. Finally, the vertical solid and dashed red lines indicate the onset and the likely associated shock, respectively, of the second CME. The two last panels (e) and (f) in Fig. 4 display the Dst and Kp index values from Kyoto, WDC for Geomagnetism (blue line) and GFZ, Potsdam and estimated by the Swedish Institute of Space Physics, Sweden (orange line) [Wintoft et al., 2017]. 4.2. Geomagnetic storm Late on 6 September 2017, a shock associated with a CME was detected just before midnight in the IMF, density and velocity. This first shock (Fig. 4, dashed blue line) initiated the initial phase of the geomagnetic storm with a positive increase in the Dst index (Fig. 4, panel (e)) and in the z component of the local magnetic field, causing an SSC. After this abrupt change, the IMF turned and stayed northward for a few hours (Fig. 4, panel (b), Bz in orange) and with a compressed magnetopause and stronger currents as a consequence. During the SSC, Cluster spacecraft 4 was located in the tail region and therefore the O + outflow cannot be estimated. This first shock probably caused a small geomagnetic storm. However, approximately 24 hours later a second CME (Fig. 4, dashed red line) reached the Earth. The IMF was strong and southward this time, and with the earlier compression of the magnetosphere by the first CME (number 1 in panel (e)), the second CME shock enhanced the incoming energy from the solar wind into the magnetosphere and directly initiated the main phase (number 2 in panel (e)). The solar wind density (panel (c)) was lower as compared to the first shock, the velocity (panel (d)) increased from approximately 200 km/s, the Kp index increased to Kp = 8 (panel (f))

  Fig. 5 displays the distribution of the scaled O + outflow in the polar cap (panel (a)) and the cusp (panel (b)). Polar cap 1 and Cusp 1 (blue) correspond to the first Cluster passage of those regions and Polar cap 2 and Cusp 2 (red) to the second passage. The transitional color corresponds to the superposition of the 2 distributions. The scaled O + outflow, given in [m -2 s -1 ] in logarithmic scale, is shown on the x-axis, while the y-axis gives the relative probability from the number of data points in each bin compared to the total data points from the distribution. The scaled O + outflow of the polar cap has clearly increased after the passage of the second shock. The average scaled O + outflow in the Polar cap 1 and Cusp 1 are 3.5 × 10 12

  drag. A similar Kp and EUV flux relation was also observed with O + escape[Yau et al.,

(Figure 2 .Figure 3 .Figure 4 .Figure 5 .Figure 6 .

 23456 Figure 1. September 7-8, 2017. Panel (a) shows the local magnetic field in the northern

  Figure 6.

  Figure 4.

  Figure 2.

  Storz, M. F., B. R. Bowman, M. J. I. Branson, S. J. Casali, and W. K. Tobiska (2005), High accuracy satellite drag model (hasdm), Advances in Space Research, 36 (12), 2497 -2505, doi:https://doi.org/10.1016/j.asr.2004.02.020, space Weather. Strangeway, R. J., R. E. Ergun, Y. Su, C. W. Carlson, and R. C. Elphic (2005), Factors controlling ionospheric outflows as observed at intermediate altitudes, Journal of Geophysical Research: Space Physics, 110 (A3), doi:10.1029/2004JA010829. Tsurutani, B. T., D. L. Judge, F. L. Guarnieri, P. Gangopadhyay, A. R. Jones, J. Nuttall, G. A. Zambon, L. Didkovsky, A. J. Mannucci, B. Iijima, R. R. Meier, T. J. Immel, T. N. Woods, S. Prasad, L. Floyd, J. Huba, S. C. Solomon, P. Straus, and R. Viereck

	sheet, Annales Geophysicae, 35 (4), 869-877, doi:10.5194/angeo-35-869-2017.
	and ion-
	ization of the neutral atmosphere.

Table 1 .

 1 Parameters for escaping O+ ions T perp [eV] T par [eV] v perp [km/s] v par [km/s]

	PC 1	11.02	86.97	14.51	37.26
	Cusp 1 1546.7	1553.8	46.19	46.31
	PC 2	38.07	94.63	20.63	50.76
	Cusp 2 3115.6	1755.1	47.01	40.2

Acknowledgments. The authors want to thank the Swedish Institute of Space Physics, the Graduate School of Space Technology in Luleå and the Swedish National Space Board for financial support. We thank the CIS-CODIF team, especially Lynn Kistler for providing calibrated oxygen ion data. All the Cluster data can be retrieved online from the Cluster Science Archive (CSA) website https://csa.esac.esa.int/csa-web/.

The "X-ray Flare" dataset was prepared and made available online by the NOAA National Geophysical Data Center (NGDC), https://www.ngdc.noaa.gov/stp/space-weather/solar-D R A F T July 3, 2018, 7:30am D R A F T data/solar-features/solar-flares/x-rays/goes/xrs/. Solar wind DSCOVR data and xray GOES data were provided by NOAA/SWPC. This paper also uses data from the CAC-Tus CME catalog, generated and maintained by the SIDC at the Royal Observatory of Belgium.