O + Escape During the Extreme Event of 4-10 September 2017 Audrey Schillings, Hans Nilsson, Rikard Slapak, Peter Wintoft, Masatoshi Yamauchi, Magnus Wik, Iannis Dandouras, Chris Carr

To cite this version:

Audrey Schillings, Hans Nilsson, Rikard Slapak, Peter Wintoft, Masatoshi Yamauchi, et al.. O + Es- cape During the Extreme Space Weather Event of 4-10 September 2017. Journal of Space Weather and , EDP sciences, 2018, 16 (9), pp.1363-1376. ￿10.1029/2018SW001881￿. ￿hal-02408153￿

HAL Id: hal-02408153 https://hal.archives-ouvertes.fr/hal-02408153 Submitted on 12 Dec 2019

HAL is a multi-disciplinary open access L’archive ouverte pluridisciplinaire HAL, est archive for the deposit and dissemination of sci- destinée au dépôt et à la diffusion de documents entific research documents, whether they are pub- scientifiques de niveau recherche, publiés ou non, lished or not. The documents may come from émanant des établissements d’enseignement et de teaching and research institutions in France or recherche français ou étrangers, des laboratoires abroad, or from public or private research centers. publics ou privés. SPACE WEATHER, VOL. ???, XXXX, DOI:10.1002/,

+ 1 O escape during the extreme space weather event of

2 September 4–10, 2017 Audrey Schillings,1,2 Hans Nilsson,1,2 Rikard Slapak,3 Peter Wintoft,4

Masatoshi Yamauchi,1 Magnus Wik,4 Iannis Dandouras,5 and Chris M. Carr,6

Corresponding author: Audrey Schillings, Swedish Institute of Space Physics, Kiruna, Sweden.

([email protected])

1Swedish Institute of Space Physics,

Kiruna, Sweden.

2Division of Space Technology, Lule˚a

University of Technology, Kiruna, Sweden.

3EISCAT Scientific Association, Kiruna,

Sweden.

4Swedish Institute of Space Physics,

Lund, Sweden.

5IRAP, Universit´ede Toulouse, CNRS,

UPS, CNES, Toulouse, France.

6Imperial College, London, UK.

D R A F T July 3, 2018, 7:30am D R A F T X-2 SCHILLINGS ET AL.: SEPTEMBER 2017 STORM

3 Abstract. We have investigated the consequences of extreme space weather

4 on ion outflow from the by analyzing the solar storm that

5 occurred early September 2017, causing a severe . Sev-

6 eral X-flares and coronal mass ejections (CMEs) were observed between 4

7 and 10 September. The first shock – likely associated with a CME – hit the

8 Earth late on September 6, produced a storm sudden commencement (SSC)

9 and began the initial phase of the storm. It was followed by a second shock,

10 approximately 24h later, that initiated the main phase and simultaneously

11 the Dst index dropped to Dst = −142 nT and Kp index reached Kp = 8.

12 Using CODIF data onboard Cluster satellite 4, we estimated the ionospheric

+ 13 O outflow before and after the second shock. We found an enhancement

+ 14 in the polar cap by a factor of 3 for an unusually high ionospheric O out-

13 −2 −1 15 flow (mapped to an ionospheric reference altitude) of 10 m s . We sug-

+ 16 gest that this high ionospheric O outflow is due to a preheating of the iono-

17 sphere by the multiple X-flares. Finally, we briefly discuss the space weather

+ 18 consequences on the as a whole and the enhanced O out-

19 flow in connection with enhanced satellite drag.

D R A F T July 3, 2018, 7:30am D R A F T SCHILLINGS ET AL.: SEPTEMBER 2017 STORM X-3

1. Introduction

20 Solar storms with associated flares, coronal mass ejections (CMEs), and radio bursts

21 may be hazardous to Earth, potentially affecting satellite operations through enhanced

22 satellite drag, causing electric power distribution failures due to extraordinary ground

23 induced currents (GIC), disturbing radio communications and causing radio blackouts due

24 to higher ionization rates in the lower ionosphere. Balch et al. [2004] report occurrence

25 rates of such events; extreme and severe radio blackouts occur approximately 1 day and 8

26 days per solar cycle respectively. Similarly, the planetary Kp index [Bartels et al., 1939;

27 Mayaud, 1980], widely used as a general indicator of geomagnetic disturbances for mid-

28 latitude regions and as a general geomagnetic alert and hazard scale, reaches extreme (Kp

29 = 9) and severe (Kp = 8) levels 4 and 100 days per solar cycle, respectively [Balch et al.,

30 2004].

31 In solar cycle 23, there were more than 100 X-flares, the strongest occurring in November

32 2003 (X28.0) and in April 2001 (X20.0). However for these events the solar active regions

33 were close to the solar limb, therefore no severe or extreme geomagnetic storms were ob-

34 served [Zhang et al., 2007]. At third strongest was an X17.2 flare detected on October

35 28, 2003 and due to a strong Interplanetary (ICME) an extreme

36 geomagnetic storm – also known as the Halloween storm – took place [e.g Yamauchi et al.,

37 2006; Rosenqvist et al., 2005; Gopalswamy et al., 2005]. It produced disturbances around

38 the world [Balch et al., 2004]. In the same solar cycle, on January 20, 2005 an outstanding

39 solar flare occurred (X7.1), followed by an ICME which was detected in the near-Earth

40 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 X-4 SCHILLINGS ET AL.: SEPTEMBER 2017 STORM

41 pression [Dandouras et al., 2009]. In solar cycle 24, in September 2017, several X-flares

42 occurred of which the most intense was an X9.0-flare (the largest observed since December

43 2006) which with strong CMEs produced an extreme geomagnetic storm. These extreme

44 events caused a strong response in the ionosphere and magnetosphere, for example vari-

45 ations in the total electron content (TEC) and enhanced magnetospheric convection. An

46 additional effect is the strongly enhanced ion outflow from the polar ionosphere [Schillings

47 et al., 2017].

48 Ion outflow from the ionosphere has been widely studied, i.e. by Shelley et al. [1982];

49 Chappell et al. [1987]; Moore et al. [1997]; Lennartsson et al. [2004]; Kronberg et al. [2014];

50 Maes et al. [2015] and references therein. The most profound magnetospheric ion outflow

51 is usually observed in the open magnetic field line regions: the polar caps, cusps and

52 mantle [Nilsson et al., 2012]. The mirror force plays a key role in the acceleration

53 of outflowing ions, with perpendicular energy converted into parallel energy as the ions

54 move into higher altitudes and weaker magnetic fields. Therefore, perpendicular heating

55 of ions causes subsequent outward acceleration. Wave particle-interaction can cause such

56 transverse ion heating, and these processes and their effects have been investigated at

57 different altitudes [e.g. Andre et al., 1990; Norqvist et al., 1996; Bouhram et al., 2005;

58 Nilsson et al., 2006; Moore and Horwitz, 2007; Slapak et al., 2011; Waara et al., 2011;

59 Nilsson et al., 2012]. If the heating is effective enough, the ions will gain sufficient velocities

60 to escape into the solar downstream in the tail [Nilsson et al., 2012] or even directly

61 from the cusp [Slapak et al., 2013].

62 Ion outflow has been studied for different and geomagnetic conditions. Cully

63 et al. [2003] 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 SCHILLINGS ET AL.: SEPTEMBER 2017 STORM X-5

64 10.7 cm (F10.7), the interplanetary magnetic field (IMF), the solar wind electric field and

65 the solar wind dynamic pressure. Using data in an altitude range of 1.3 to 2.0 RE, Yau

66 et al. [1988] found that ion outflow depends on the geomagnetic condition such that it

67 increases exponentially with Kp. Similarly, Slapak et al. [2017] made a statistical study

68 using high-altitude plasma mantle and Cluster data in order to quantify

+ 69 the total O escape as a function of Kp. Complementary to that study, Schillings et al.

70 [2017] presented that the escape rate during extreme geomagnetic storms could be higher

71 than what a linear extrapolation of the results of Slapak et al. [2017] would predict. As the

+ 72 whole magnetosphere is affected by disturbed magnetospheric conditions, the O outflow

73 that does not escape into the solar wind is transported to the lobes through the polar cap.

+ 74 Through reconnection in the , O coming from the lobe is heated and feeds

75 both the distant and near-Earth plasma sheet [Kistler et al., 2006; Mouikis et al., 2010].

76 The consequence of space weather on ion outflow has not been well studied. Therefore,

77 it is not clear what parameter is most influential with what time scale. In this paper,

78 we first investigate the space weather during 4 to 10 September 2017, from solar storm

79 to geomagnetic storm. Thereafter we examine the effects on magnetosphere-ionosphere

80 coupling, in particular ion outflow. The goal is primarily to see the space weather effects

81 on ion outflow for this well studied storm. Despite that ion outflow in itself not causing

82 hazardous to human activity, the enhanced ion outflow and resulting escape are mainly

83 observed in the cusp, which has an important role in some hazards as discussed in Section

84 5.3.

2. Instrumentation and data

D R A F T July 3, 2018, 7:30am D R A F T X-6 SCHILLINGS ET AL.: SEPTEMBER 2017 STORM

2.1. Cluster mission

85 The Cluster mission [Escoubet et al., 2001] was launched in 2000 and consists of four

86 spacecraft flying in tetrahedral formation in an elliptical polar orbit. Only spacecraft 4

87 (SC4) was used in this study, because in 2017 the COmposition DIstribution Function

88 (CODIF) part of the Cluster Ion Spectrometer (CIS) [R`eme et al., 2001] onboard the other

89 spacecraft are no longer functional. The particle detection efficiency of CODIF onboard

90 spacecraft 4 has also been degraded [Kistler et al., 2013], this degradation is taken into

91 account by the instrument in-flight calibration files. The CODIF instrument measures

+ 2+ + + 92 the 3D-distributions of H , He , He , and O using a time-of-flight technique. When

93 CODIF is subject to intense proton fluxes the heavier ion channels may be contaminated

+ + 94 with false counts. If so, the ratio of the O to H perpendicular bulk velocity will have

+ 95 a local peak at 1/4, and these unreliable O data can be removed [Nilsson et al., 2006].

96 The magnetic field data are provided by the FluxGate Magnetometer (FGM) [Balogh

97 et al., 2001], which has a normal mode sample frequency of 22.4 Hz. In this study we are

98 interested in the background magnetic field and use the field averaged over the spacecraft

99 spin period of 4 seconds.

2.2. Solar and solar wind data

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

101 The Solar Dynamics Observatory (SDO) [Pesnell et al., 2012] is especially targeted at

102 producing observations of the solar drivers in order to understand and predict space

103 weather. Images from the photosphere and photospheric magnetic fields, and up through

104 the corona, are produced at high resolution and high cadence. Solar active regions (ARs)

105 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 SCHILLINGS ET AL.: SEPTEMBER 2017 STORM X-7

106 NOAA/SWPC. SWPC also provides full-disk X-ray observations from the GOES space-

107 craft. CMEs are observed by the ESA/NASA Solar and Heliospheric Observatory (SOHO)

108 and the NASA Solar TErrestrial RElations Observatory (STEREO) (currently only one

109 spacecraft is operational). From coronal images, the Solar Influences Data Centre (SIDC)

110 automatically detects and estimates CME parameters, producing a catalog of CMEs. Fi-

111 nally, solar wind plasma and magnetic fields are measured by the DSCOVR spacecraft;

112 data are provided by NOAA/SWPC.

3. Method

113 This section describes how we analyzed the September 2017 storm from the solar point

+ 114 of view to the O outflow in the polar regions at Earth. We study the complete chain

115 of events from the Sun, through the solar wind, geomagnetic indices, and ion outflow, in

+ + 116 particular the ionospheric O ions. The O ions mainly originate from the ionosphere

+ 117 whereas the distinction between the ionospheric and solar origin of the H is more com-

118 plex. We first looked and analyzed the solar data such as X-flares and CMEs from the

119 beginning of September. Afterward, we checked the solar wind parameters from 4 to 10

120 September to identify the arrival time of the shocks at Earth and finally compare them

121 with Cluster observations.

122 At Earth, a geomagnetic storm is characterized by three phases – the initial, main and

123 recovery phase – which can be identified by the behavior of the Dst index. The Dst index is

124 a 1-hour index estimated from the deviations of the horizontal component of the magnetic

125 field at low-latitude magnetometer stations. In principle, it is a measure of the equatorial

126 strength. The Kp index is a 3-hour index estimated from local disturbances

127 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 X-8 SCHILLINGS ET AL.: SEPTEMBER 2017 STORM

128 global scale. A combination of these two magnetic indices provides magnetospheric infor-

129 mation on the geomagnetic storm. The initial phase of a geomagnetic storm is typically

130 characterized by a positive disturbance in the Dst index, which could also be associated

131 with an SSC. When the positive perturbation drops drastically to negative values, the

132 main phase of the storm starts and lasts a few hours – typically 2 h to 10 h – and is

133 followed by the recovery phase that can last hours to a few days.

+ 134 In order to estimate the O outflow during the geomagnetic storm, we first looked at

+ + 135 the O and H energy and pitch angle (PA) spectrograms and the magnetic field for SC4.

136 In Fig. 1, panel (a) shows that the local magnetic field varied slowly and was weaker

137 in the regions marked as polar cap (PC1 and PC2), while it was stronger and variable

+ 138 in the cusp (Cusp 1 and Cusp 2). Panels (b), (c), (d) and (e) correspond to O and

+ 139 H energy and pitch angle spectrograms respectively (for more detail see Schillings et al.

140 [2017]). The color bar represents counts per second (c/s), which is proportional to the

141 particle differential energy flux, and the dashed (pink) rectangles distinguish the polar

142 caps from the cusps. From Fig. 1, we visually identified the outflowing regions in the

143 northern hemisphere. In the polar cap, we observed an oxygen ion beam at lower energies

144 than in the cusp, see panel (b). Furthermore, the magnetic field in the polar cap is more

145 stable as opposed to the cusp, which is characterized by more variability. The pitch angle

◦ 146 spectrograms, displayed in panels (d) and (e), clearly show outflowing ions (PA ≃ 180 ,

+ + 147 northern hemisphere) for O and H in the polar cap regions. In the cusp, outflowing

◦ 148 ions (PA ≃ 180 ) are visible, but also ion populations with most energy perpendicular to

◦ 149 the magnetic field (PA ≃ 90 ).

D R A F T July 3, 2018, 7:30am D R A F T SCHILLINGS ET AL.: SEPTEMBER 2017 STORM X-9

150 The identified outflowing regions are confirmed from the observations by inspect-

151 ing the Cluster orbit. We first used the Orbit Visualization Tool (Khotyaintsev, Y.,

152 https://ovt.irfu.se) for a 3D view of the SC4 orbit with the location of the

+ 153 and the . Secondly, we plotted the O flux along the orbit for these time pe-

154 riods (Fig. 2). In Fig. 2, the outflowing regions are represented by the zoomed plots

155 (top for the north, bottom for the south) on the orbit planes XZGSM and XYGSM . The

+ 156 color bars correspond to the O flux along the Cluster trajectory and the points mark

157 the identified regions. In the north, we observed polar cap and cusp regions while in the

158 south we identified polar cap and plasma mantle regions. During each time period, which

159 corresponds to either the polar cap, the cusp or the plasma mantle, we made histograms of

+ + 160 the scaled O outflow. The scaled O outflow is defined as the net outward flux scaled to

FO+∗Biono 161 an ionospheric reference altitude and given by the formula Fmap = Blocal . We mapped

162 the fluxes (FO+ ) to an ionospheric reference altitude with a magnetic field strength of

163 Biono = 50 000 nT. The local outward flux can be mapped assuming conservation of par-

164 ticle flux along the corresponding magnetic flux tube (Blocal). This scaled net outward

165 flux is thus independent of the altitude and any magnetic compression giving an estimate

166 of the original ionospheric outflow.

4. Observations

167 This section presents the September storm from the solar storm to the ion escape

168 during the extreme geomagnetic storm. The solar regions where the flares and CMEs

+ 169 were produced are described and we estimate the terrestrial magnetospheric O outflow

170 during the main phase of the geomagnetic storm.

D R A F T July 3, 2018, 7:30am D R A F T X - 10 SCHILLINGS ET AL.: SEPTEMBER 2017 STORM

4.1. Solar storm

171 Multiple X-type flares and Earth-directed CMEs were observed during 4–10 September

172 2017. On September 1, the solar Active Region (AR) 12673 was a small unipolar spot

173 just south of the solar equator (magnetic type α). At the same time AR12674 (Fig. 3,

174 panel (a), yellow dashed circle) on the northern hemisphere was much larger and more

175 complex (β − γ). However, during 3–4 September AR12673 grew dramatically both in

176 size (more than 10 times in area, see Fig. 3, panels (a) and (b), yellow solid circle) and

177 complexity (β − γ − δ) and thus became a highly interesting region. The region stayed

178 large and magnetically complex during its passage over the solar disk until it went around

179 the west limb during 10 September.

180 AR12673 produced numerous flares and CMEs from 4–10 September. Most notable was

181 a CME in the evening of 4 September and another on mid-day 6 September. The SIDC

182 identified speeds in both events close to 2000 km/s with onset times at 2017–09–04 19:12

183 UT and 2017–09–06 12:12 UT, respectively [Robbrecht et al., 2009]. Another fast CME

184 was launched on 10 September, but at that time AR12673 was on the western limb and

185 the CME was not Earth-directed.

186 Two X-flares occurred on 6 September, one on 7 September, and another on 10 Septem-

187 ber. The X-flares were observed by GOES-13 for short and long wavelengths with 1-minute

188 resolution X-ray flux, XS (0.05–0.4 nm) and XL (0.1–0.8 nm) as presented in Fig. 4, panel

189 (a). The horizontal solid blue line in panel (a) (Fig. 4) corresponds to the lower limit

190 for an X-type flare. The onset of the first CME on 4 September and the likely associated

191 shock in the solar wind are indicated by the vertical solid and dashed blue line, respec-

192 tively. 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 SCHILLINGS ET AL.: SEPTEMBER 2017 STORM X - 11

193 velocity (panel (d)) 1-minute resolution data taken from DSCOVR. Finally, the vertical

194 solid and dashed red lines indicate the onset and the likely associated shock, respectively,

195 of the second CME. The two last panels (e) and (f) in Fig. 4 display the Dst and Kp

196 index values from Kyoto, WDC for Geomagnetism (blue line) and GFZ, Potsdam and

197 estimated by the Swedish Institute of Space Physics, Sweden (orange line) [Wintoft et al.,

198 2017].

4.2. Geomagnetic storm

199 Late on 6 September 2017, a shock associated with a CME was detected just before

200 midnight in the IMF, density and velocity. This first shock (Fig. 4, dashed blue line)

201 initiated the initial phase of the geomagnetic storm with a positive increase in the Dst

202 index (Fig. 4, panel (e)) and in the z component of the local magnetic field, causing an

203 SSC. After this abrupt change, the IMF turned and stayed northward for a few hours (Fig.

204 4, panel (b), Bz in orange) and with a compressed magnetopause and stronger currents

205 as a consequence. During the SSC, Cluster spacecraft 4 was located in the tail region

+ 206 and therefore the O outflow cannot be estimated. This first shock probably caused a

207 small geomagnetic storm. However, approximately 24 hours later a second CME (Fig. 4,

208 dashed red line) reached the Earth. The IMF was strong and southward this time, and

209 with the earlier compression of the magnetosphere by the first CME (number 1 in panel

210 (e)), the second CME shock enhanced the incoming energy from the solar wind into the

211 magnetosphere and directly initiated the main phase (number 2 in panel (e)). The solar

212 wind density (panel (c)) was lower as compared to the first shock, the velocity (panel (d))

213 increased from approximately 200 km/s, the Kp index increased to Kp = 8 (panel (f))

214 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 X - 12 SCHILLINGS ET AL.: SEPTEMBER 2017 STORM

215 of the storm main phase, which continued for approximately 5 hours (20:00 – 01:00 UT).

216 During the main phase of the storm, Cluster was in the northern hemisphere (see Fig. 2,

217 and top panels for zooms).

218 The arrival of the second shock was coincident with an abrupt equatorward motion of

219 the cusp, at 23:00 UT on 7 September (Fig. 1, panels (b) and (c)) in the Cluster data.

220 When hitting the Earth, the shock again compressed the magnetopause, causing higher

221 convection in the magnetosphere. Simultaneously, the southward IMF opened up the

222 dayside geomagnetic field lines, such that the footprints of the cusp are at lower latitudes.

223 Newell and Meng [1994]; Newell et al. [1989] described how the polar cap and plasma

224 mantle move equatorward for disturbed magnetospheric conditions, and we observed such

225 a motion of the cusp. After the second shock, Cluster was located in the cusp for about

226 50 minutes, before the cusp moved equatorward such that Cluster was again located in

227 the polar cap for approximately 40 min (see Fig. 1). Afterward Cluster entered the cusp

228 a second time, where the spacecraft stayed for a few minutes before ending up in the

229 magnetosheath (see Fig. 2, top panels).

230 Following the main phase, the recovery phase (number 3 in panel (e)) was longer and

231 in the meantime Cluster was at the nightside in the southern hemisphere (8 September

232 around noon). In Fig. 2, the bottom panels show the encountered regions in the south.

233 We visually identified the polar cap and plasma mantle as Cluster was on the flank of

234 the magnetosphere and in the nightside. Cluster encountered 4 polar cap/plasma mantle

235 crossings.

4.3. O+ outflow during the geomagnetic storm

236 4.3.1. Northern hemisphere

D R A F T July 3, 2018, 7:30am D R A F T SCHILLINGS ET AL.: SEPTEMBER 2017 STORM X - 13

+ 237 We estimated the scaled O outflow in the different regions before and after the second

238 shock signature, from September 7, 22:11 UT to September 8, 00:33 UT, while Cluster

239 was located in the polar cap and in the cusp. Fig. 5 displays the distribution of the scaled

+ 240 O outflow in the polar cap (panel (a)) and the cusp (panel (b)). Polar cap 1 and Cusp 1

241 (blue) correspond to the first Cluster passage of those regions and Polar cap 2 and Cusp

242 2 (red) to the second passage. The transitional color corresponds to the superposition

+ −2 −1 243 of the 2 distributions. The scaled O outflow, given in [m s ] in logarithmic scale, is

244 shown on the x-axis, while the y-axis gives the relative probability from the number of

245 data points in each bin compared to the total data points from the distribution.

+ 246 The scaled O outflow of the polar cap has clearly increased after the passage of the

+ 12 247 second shock. The average scaled O outflow in the Polar cap 1 and Cusp 1 are 3.5 × 10

13 −2 −1 248 and 1.6 × 10 m s , respectively, and increase by a factor of 3 (polar cap) and 2 (cusp)

249 after the shock signature. The highest value, with a minimum of 1% of the data in the

13 −2 −1 14 −2 −1 250 bin, are 6.3 × 10 m s for the polar cap and 1 × 10 m s for the cusp.

+ 251 To determine if the scaled O outflow is escaping into the solar wind, we study the

+ 252 O perpendicular and parallel temperatures, as well as the perpendicular and parallel

253 velocities. Table 1 gives the four parameters we took into account to estimate the fate of

+ 254 the O ions. In the polar cap regions, the parallel to perpendicular temperature ratio is

255 high due to the high parallel component, therefore the transverse heating is not efficient,

256 this is as expected (significant transverse heating is usually observed only in the cusp due

257 to the wave-particle interactions [Strangeway et al., 2005; Slapak et al., 2013]). The ions

258 in the polar cap are mostly accelerated through the centrifugal acceleration [Nilsson et al.,

259 2012]. 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 X - 14 SCHILLINGS ET AL.: SEPTEMBER 2017 STORM

260 cap), so that the ions follow the magnetic field upward with their fate in the plasma sheet

261 tailward of the X-line (neutral line in the magnetotail) at about 20 RE - 30 RE. Note

262 that the X-line can be pushed earthward around 10 RE for a severe geomagnetic storm

263 [Tsyganenko and Sitnov, 2005]. In the cusp region, the components of the temperature

264 are similar before the shock, whereas the perpendicular temperature is about twice the

265 parallel component. This indicates a transverse heating effect. However, the perpendicular

266 velocities are very high (> 40 km/s) compared to the average convection velocity of 7,5

267 km/s in the lobes [Haaland et al., 2012], and it leads to a strong convection in the cusp

268 region. The cusp field-lines will convect through the polar cap (lobes) before they reach

269 the plasma sheet and thus we should use the perpendicular velocity in the polar cap for

270 our calculations of where the ions reach the plasma sheet. A considerable amount of

271 flux is therefore transported tailward and towards the plasma sheet, despite this strong

272 convection the ions have enough energy to escape directly downtail.

+ 273 Fig. 6 shows the relative probability of log10 T⊥(O ) [eV] in the northern polar cap and

274 cusp before the shock signature (blue) and after (red). Note that the transitional color

275 corresponds to the superposition of the 2 distributions. The dashed blue (Cut 1) and red

276 (Cut 2) lines roughly mark the separation between the polar cap and the cusp. In the

277 cusp, we observed a higher perpendicular temperature than in the polar cap as expected

278 because the ions are more heated. The perpendicular energy is converted into parallel

279 energy by the mirror force for outflowing ions, thus accelerating the ions upward. Slapak

+ 280 et al. [2011] showed that O is effectively heated transversely through wave particle-

281 interactions 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 SCHILLINGS ET AL.: SEPTEMBER 2017 STORM X - 15

+ 282 with a parallel velocity vk(O ) > 30 km/s (not shown), we can conclude that the observed

+ 283 O in the cusp will eventually escaping into the solar wind [Nilsson, 2011].

284 4.3.2. Southern hemisphere

285 Cluster was located in the southern hemisphere during the recovery phase (number 3 in

286 panel (e), Fig. 4) of the geomagnetic storm. Thus, we checked which regions it encountered

+ 287 where O outflow might be observed. We identified 4 crossings from polar cap to plasma

+ 288 mantle (see Fig. 2, bottom panels) where the scaled O outflow has been estimated. The

289 4 identified polar cap and plasma mantle observations are numbered from 1 to 4 according

290 to the passage on the Cluster orbit (color points on Fig. 2), i.e. number 1 equals to the first

291 polar cap/plasma mantle crossing in time along Cluster orbit. In plasma mantle regions

+ 12 −2 −1 292 from 1 to 4, the scaled O outflow is roughly the same ∼ 3 × 10 m s . Similarly to

293 the northern hemisphere, we looked at the perpendicular and parallel components of the

294 temperature and velocities in the plasma mantle regions. The parallel temperatures are

295 unexpectedly high (about twice the perpendicular component) for the 4 plasma mantle

296 passages, which means that a local heating is observed in these regions, otherwise the

297 velocity filter effect would have decreased the parallel component of the temperature.

298 Additionally, the perpendicular component of the velocity is not negligible, we thus have

299 a significant convection. This strong convection implies a considerable magnetic flux

300 transport that must be released through reconnection if it is not to accumulate in the

301 magnetotail. The reconnection rate must then be correspondingly high and the X-line

302 might move earthward. Thus, due to the high parallel component of the temperature and

303 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 X - 16 SCHILLINGS ET AL.: SEPTEMBER 2017 STORM

+ 304 hemisphere. They might end up in the plasma sheet and feed the ring current with O

305 or tailward of the X-line and be lost in the magnetotail.

+ 306 4.3.3. O flux in the plasma sheet

+ 307 We also looked at the O flux in the first plasma sheet crossing after the storm. The

+ 308 plasma sheet is identified around XGSM ≃ -15 RE and the net O flux is small. Following

309 the method of Slapak et al. [2017] – dividing the flow between tailward and earthward

+ 9 −2 −1 310 flux – we estimated an average O flux of ∼ 1.1 × 10 m s in both directions. This

+ 311 estimation is in agreement with the average O flux calculated in the central plasma sheet

312 by Slapak et al. [2017]. In addition, Cluster was in the plasma sheet in the end of the

313 recovery phase (9 September, about 00:00 to 16:00 UT) and our estimation shows that

+ 314 the average O return flux was the same as during quiet magnetospheric conditions. This

+ 315 result is consistent with the results of Kistler et al. [2010], who showed that the O density

316 is reduced during the recovery phase.

5. Discussion

5.1. Solar drivers for O+ escape during geomagnetic storm

317 Extreme geomagnetic storms are caused by ICMEs, while smaller storms could be caused

318 by corotating interaction regions (CIRs). On September 4 and 6, 2017, two CMEs were

319 detected at the Sun and on September 6 and 7, three X-flares (X2.2; X9.3 (Sep 6) and

320 X1.3 (Sep 7)) were observed. The CMEs produced a severe geomagnetic storm recorded

321 with Kp = 8 and Dst ≃ –140 nT. The first CME, that arrived late 6 September had

322 northward IMF with 600 km/s velocity whereas the second that arrived late 7 September

323 had southward IMF with 800 km/s velocity. These features are most likely the causes

+ 324 of higher scaled O outflow because both velocity and IMF are expected to influence

D R A F T July 3, 2018, 7:30am D R A F T SCHILLINGS ET AL.: SEPTEMBER 2017 STORM X - 17

+ 325 O outflow [Lennartsson et al., 2004; Yamauchi and Slapak, 2018]. Lennartsson et al.

+ 326 [2004] showed that negative IMF Bz has a higher influence on the total O outflow by

+ 327 approximately a factor 3. Considering also the timing of enhanced O compared to arrival

+ 328 timing of CMEs and enhanced EUV flux, the solar driver that influencesO outflow during

329 a geomagnetic storm is probably CMEs, despite that the X-flares with long wavelength

330 (0.1 - 0.08 nm) ionize the neutral atmosphere all the way down to approximately 95 km

331 [Rees, 1989; Tsurutani et al., 2005]. In comparison, the Halloween event that occurred

332 October 28–30, 2003, had stronger X-flares (X17.2 (Oct 28) and X10.0 (Oct 29)) and Kp

+ 333 =9+, and Schillings et al. [2017] estimated a higher polar cap O outflow than is observed

334 for the September 2017 storm. Consequently, the ionization in the polar cap was higher

335 and the X-flares could have influenced the ionization process [Tsurutani et al., 2005] such

336 as a preheating of the ionosphere before the CME hit the Earth. Yamauchi et al. [2018]

337 wrote an overview of the ionospheric response to this storm with the EISCAT radars.

338 The authors showed that the electron density and temperature were enhanced after the

339 detection of the X-flares. They also suggested that the increased ion temperature due

+ 340 to ion heating creates a precondition for O upflow (upflow means that the ions are still

341 gravitationally bound). This suggestion is strengthened by the increased F10.7 solar radio

342 flux, which can be taken as a proxy of EUV flux, before and during the magnetic storm.

+ 343 In addition, Yau et al. [1988] found a clear correlation between O outflow and Kp and

344 F10.7. For low solar activity (F10.7 between 70 and 100 sfu) Yau et al. [1988] estimated

+ 345 the O outflow to be a factor 4 lower than during high solar activity (F10.7 between

+ 346 150 and 250 sfu). Cully et al. [2003] found a correlation between the total O outflow

+ 347 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 X - 18 SCHILLINGS ET AL.: SEPTEMBER 2017 STORM

348 have suitable Cluster data when the X-flares occurred, we conclude that the primary solar

+ 349 driver for O outflow is probably ICMEs due to the clear influence seen in the Cluster data

350 but X-flares might have a significant contribution in the heating process of the ionosphere

351 [Yamauchi et al., 2018] and in causing additional ionization.

5.2. Escaping O+ during the September 7 – 8, 2017 geomagnetic storm

+ 352 We estimated the scaled O outflow in the northern polar cap and cusp region during the

353 main phase of the storm. Fig. 5 shows the polar cap and cusps before and after the second

354 CME - shock (see section 4). We observed an enhancement of a factor 3 for the polar cap

355 and a factor 2 for the cusp after the shock signature. This weak enhancement is comparable

+ 356 with the lowest relative scaled O outflow enhancement for intense geomagnetic storms

+ 357 presented by Schillings et al. [2017], who found that the scaled O outflow increases by 1 to

358 2 orders of magnitude during intense storm conditions compared to quiet magnetospheric

+ 13 −2 −1 359 conditions. However, the absolute value of the scaled O outflow (∼ 10 m s ) is

360 quite high compared to similar storms studied in Schillings et al. [2017]. We suggest that

+ 361 this unusual high scaled O outflow is due to the already compressed magnetosphere as a

362 consequence of the first shock approximately 24 hours earlier. Moreover, before the second

363 shock signature (September 7, 23:30 UT) the predicted Kp index was 3 and after the shock,

364 Kp = 8, thus according to Slapak et al. [2017] and their Fig. 5, the increase in the scaled

+ 365 O should be around 1 order of magnitude. Our lower result could be explained by the

366 difference in the studied regions (polar cap compared to plasma mantle/magnetosheath for

+ 367 Slapak et al. [2017]). This increase in the unusually high O outflow was observed within

368 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 SCHILLINGS ET AL.: SEPTEMBER 2017 STORM X - 19

+ 369 lower relative enhancement in the O outflow could be explained by the first shock, which

370 already initiated a geomagnetic storm.

+ 371 Schillings et al. [2017] could not confirm that the enhanced scaled O outflow in the

372 polar cap during the storm was escaping and lost into the interplanetary space. How-

+ 373 ever, for the September 2017 storm, we studied and could compare the scaled O outflow

374 in two regions, the polar cap and the cusp. For these two regions, we looked at the

375 perpendicular and parallel component of the temperature and the outflow parallel and

376 perpendicular bulk velocity. Fig. 6 shows that before the second CME-shock, the perpen-

377 dicular temperature in the polar cap 1 (blue, left side of Cut 1) is lower than after polar

378 cap 2 (red, left side of Cut 2). This means that the ions in the polar cap have been heated

379 and accelerated [Nilsson et al., 2006] after the shock passage. In both cusp regions, the

380 perpendicular temperature remains more or less the same, but is higher than in the polar

381 cap. Consequently, despite a significantly high perpendicular bulk velocity, we observed

382 ions with higher energy and parallel bulk velocity (not shown) in the cusp with sufficient

383 energy to escape into the solar wind. This result confirms what Schillings et al. [2017]

384 expected but could not claim.

+ 385 In the southern hemisphere, the perpendicular temperature and the scaled O outflow in

386 the 4 identified polar cap and plasma mantle regions (1-4) are constant, despite the parallel

387 velocity is decreasing as Cluster moves tailward. This result suggests that an enhancement

+ 388 is still observed during the recovery phase, and in addition the local heating of O is still

389 significant as indicated by the unexpectedly high parallel temperature. Furthermore, the

390 perpendicular bulk velocity is not negligible and the strong convection might drive the ions

391 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 X - 20 SCHILLINGS ET AL.: SEPTEMBER 2017 STORM

392 heating/acceleration to end up behind the X-line and therefore be lost in the tail. The

+ 393 fate of these O ions is less predictable than in the northern hemisphere. Finally, during

394 the 3 first hours of the recovery phase, the Kp index was 5, hence these observations also

+ 395 confirm the dependence of O outflow on Kp [Slapak et al., 2017].

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

+ 396 The space weather affects not only the O outflow, but the magnetosphere as a whole.

397 Space weather influences the occurrence of substorms, the life-time of the ring current,

398 the density of the plasma sheet and the occurrence of bursty bulk flows (BBFs) and finally

399 the energy conversion through mass loading in the cusp. Substorms do not always happen

400 during geomagnetic storms and therefore a storm does not necessarily include substorms

401 [Kamide et al., 1998; Hori, 2006]. At a substorm onset, Nos´eet al. [2005] found that the

+ + + 402 O /H energy density ratio increases. During substorm growth phase, energetic O (tens

403 of keV), which mainly originate from the dayside polar region [Nakayama et al., 2017],

404 play an important role in feeding the ring current. The life-time of the ring current is

+ 405 therefore prolonged by the addition of energy by dominant O under disturbed magne-

+ 406 tospheric conditions, which have also lower charge-exchange cross-sections than the H

407 ions, resulting in a slower loss [Hori, 2006; Denton et al., 2017].

+ 408 Another aspect is the transport of O into the plasma sheet. Mouikis et al. [2010]

409 looked at the ion composition changes in the plasma sheet associated with geomagnetic

+ 410 and solar activity and found a strong influence on O density. Under strong geomagnetic

+ 411 activity, part of the O outflow is transported through the magnetotail and fed to the

412 plasma sheet [Kistler et al., 2010]. These ions can be seen in BBFs. However, Nilsson

+ 413 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 SCHILLINGS ET AL.: SEPTEMBER 2017 STORM X - 21

+ 414 Finally, high O escape into the inflow solar wind plasma leads to a high mass-loading rate,

415 and high extraction rate of the solar wind kinetic energy to the ionosphere in a limited

416 and small region at high latitudes [Yamauchi and Slapak, 2018]. Therefore, we expect

417 ground induced currents at much higher latitudes than normal. Since human activity is

418 expanding to the polar region, such as arctic sea routes, this can become important.

+ + 419 The O escape itself is not hazardous to human activity. However, enhanced O escape

420 rate can be observed at the same time as other space weather effects for example satellite

421 drag. The expansion of the neutral atmosphere in the thermosphere and extreme enhance-

422 ments of the ion escape can be linked because both occur when there is stronger energy

423 input into the atmosphere during extreme geomagnetic storms. When Kp and EUV flux

424 vary, the neutral temperature above the thermopause fluctuates [Chandra, 1967]. The

425 variation of EUV flux heats the upper atmosphere through ion-neutral collisions, which

426 tend to expand the thermosphere [Rees, 1989] and thus increase the atmospheric density

427 at higher altitudes. Therefore, high EUV flux (or the proxy F10.7 cm flux > 250 solar

428 units flux) and Kp (> 6) values result in extreme short-term increases in satellite drag,

429 particularly for satellites on low earth orbit (LEO). These parameters are used in satellite

430 drag models [Bowman et al., 2008; Storz et al., 2005]. Tsurutani et al. [2007] reported, for

+ 431 the Halloween event, ion-neutral drag due to quick upward motion ofO that causes up-

432 ward neutral oxygen (O) at equatorial latitudes. Similarly, Lakhina and Tsurutani [2017]

433 generalized the ion-neutral drag for oxygen for 1859-type (Carrington) superstorms. The

+ 434 O outflow (higher latitudes) could have an impact on this ion-neutral drag.

435 The cusp plays a significant role in the ion outflow as well as for the satellite drag [L¨uhr

436 et al., 2004]. L¨uhr 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 X - 22 SCHILLINGS ET AL.: SEPTEMBER 2017 STORM

437 geomagnetic activity (∼ Kp 3) over the cusp with the CHAMP satellite. They concluded

438 that these enhancements of air density are accompanied by small-scale structures such

439 as field-aligned currents (FAC) filaments and suggested the Joule heating as the primary

440 source for the upward movement of the air. This air up-welling over the cusp at low

+ 441 altitude become mostly, due to several chemical reactions, O ions in the F region of

442 the ionosphere. Furthermore, Masutti [2017] studied the thermosphere during this storm

443 with the QB50 mission (CubeSats) and observed an orbit decay of roughly 2 km during

444 the September storm. Thus, the size or area, the current system or its equatorward

445 motion under extreme conditions is important for the accuracy of satellite drag models.

446 This speculative approach needs further investigations that were beyond the scope of this

+ 447 study. However, we do believe that stronger O escape indicate an expansion of the

448 thermosphere, and could be strongly connected to increased satellite drag.

6. Conclusions

+ 449 In this paper, we studied the consequences of the space weather on the O outflow

450 at Earth. We investigated the September 4–9, 2017, storm from the Sun down to the

451 ionosphere. At the beginning of September, several X-flares and two CMEs were detected,

452 which produced a severe geomagnetic storm (Kp ≃ 8 and Dst = -142 nT). The first

453 CME carried northward interplanetary magnetic field (IMF) and the associated shock

454 initiated a storm sudden commencement (SSC) at Earth followed by the initial phase of

455 a geomagnetic storm. A second shock, driven by a CME with southward IMF, hit the

456 magnetosphere 24 hours later and caused an equatorward motion of the cusp, which was

+ 457 observed by Cluster satellite 4 in the northern hemisphere. The ionospheric O outflow

458 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 SCHILLINGS ET AL.: SEPTEMBER 2017 STORM X - 23

+ 459 the main phase of the storm. The ionospheric O outflow increased after the passage of

460 the second shock by a factor 3 in the polar cap and a factor 2 in the cusp. However, the

+ 461 relative enhancement of the O outflow was lower than expected compared to Schillings

+ 462 et al. [2017], nevertheless the initial value of O outflow (before the shock) for the regions

13 −2 −1 + 463 of observation was high ∼ 10 m s . The unusually high ionospheric O outflow was

464 probably due to an enhancement after the first shock and a preheating of the ionosphere by

465 the EUV flux due to the X-flares in the previous days [Yamauchi et al., 2018]. Therefore,

+ 466 the second shock did not increase O outflow during the main phase as much as previous

+ 467 studies would have led us to believe [Schillings et al., 2017] but the heated O had enough

468 velocity to escape directly into the solar wind. On September 8, the magnetosphere started

469 to recover from the massive injection of energy from the solar wind. During this phase,

470 Cluster was moving tailward in the southern hemisphere and we observed local heating of

+ 471 the ionospheric O outflow indicated by high parallel temperature, and strong convection

+ 472 as seen in the perpendicular component of the velocity. The fate of the O is therefore

473 less predictable. Furthermore, observations were made in the plasma sheet on September

+ 474 9, the O flux estimated is consistent with the average flux calculated by Slapak et al.

475 [2017].

+ 476 The observation of escaping O during geomagnetic storms caused by CMEs and X-

477 flares also shows that the neutral atmosphere could be ionized sufficiently to expand the

478 neutral thermosphere and lead to significant satellite drag effects. Satellite drag models

479 take into account the geomagnetic activity and the EUV flux [Bowman et al., 2008; Storz

480 et al., 2005] and the higher these parameters are, the more impact they have on satellite

+ 481 drag. A similar Kp and EUV flux relation was also observed with O escape [Yau et al.,

D R A F T July 3, 2018, 7:30am D R A F T X - 24 SCHILLINGS ET AL.: SEPTEMBER 2017 STORM

+ 482 1988; Cully et al., 2003; Slapak et al., 2017], and therefore we suggest that higher O

483 outflow indirectly indicates stronger satellite drag. Our finding can be summarized as

484 followed:

+ 485 1. In the polar cap and cusp, we observed an increase in the ionospheric O outflow

486 by a factor 3 and 2 respectively, after the passage of a shock associated with the second

487 CME. These ions will eventually escape into interplanetary space.

+ 488 2. The increase in the O outflow is not extremely high, however, the magnetosphere

489 response to the second CME, is fast (∼ 40 minutes).

490 3. During the recovery phase, local heating and strong convection are observed in

491 the plasma mantle as seen, respectively, in extremely high parallel component of the

492 temperature and high perpendicular velocity.

+ 13 493 4. The upper limit of the ionospheric O outflow (northern hemisphere) is 6.3 × 10

14 −2 −1 494 and 1 × 10 m s in the polar cap and cusp respectively.

+ 495 5. The solar drivers for O outflow seem to be mostly ICMEs whereas X-flares might

496 have a significant contribution in the earlier preheating [Yamauchi et al., 2018] and ion-

497 ization of the neutral atmosphere.

498 Acknowledgments. The authors want to thank the Swedish Institute of Space

499 Physics, the Graduate School of Space Technology in Lule˚aand the Swedish National

500 Space Board for financial support. We thank the CIS-CODIF team, especially Lynn

501 Kistler for providing calibrated oxygen ion data. All the Cluster data can be retrieved

502 online from the Cluster Science Archive (CSA) website https://csa.esac.esa.int/csa-web/.

503 The ”X-ray Flare” dataset was prepared and made available online by the NOAA National

504 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 SCHILLINGS ET AL.: SEPTEMBER 2017 STORM X - 25

505 data/solar-features/solar-flares/x-rays/goes/xrs/. Solar wind DSCOVR data and xray

506 GOES data were provided by NOAA/SWPC. This paper also uses data from the CAC-

507 Tus CME catalog, generated and maintained by the SIDC at the Royal Observatory of

508 Belgium.

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D R A F T July 3, 2018, 7:30am D R A F T SCHILLINGS ET AL.: SEPTEMBER 2017 STORM X - 35

Figure 1. September 7-8, 2017. Panel (a) shows the local magnetic field in the northern hemisphere for the polar caps (PC1, PC2) and the cusps (cusp1, cusp2). Panels (b), (c), (d) and

(e) are the energy and pitch angle (PA) spectrograms for the O+ and H+ respectively.

D R A F T July 3, 2018, 7:30am D R A F T X - 36 SCHILLINGS ET AL.: SEPTEMBER 2017 STORM

+ −2 −1 Figure 2. Log10 O flux [m s ] (colorbar) along Cluster orbit in the planes XZGSM and

XYGSM for September 7-9, 2017. The top and bottom panels correspond to the zoom of the northern and southern hemispheres respectively where the outflowing regions were identified.

PC=polar cap, PM=plasma mantle.

D R A F T July 3, 2018, 7:30am D R A F T SCHILLINGS ET AL.: SEPTEMBER 2017 STORM X - 37

Figure 3. Continuum images from the Sun taken by the HMI onboard SDO. Panel (a) shows the solar Active Regions (AR) 12673 in yellow dashed circle and the AR12674 in yellow solid circle in the morning of September 3. Similarly, panel (b) shows the same regions in the evening of September 3.

D R A F T July 3, 2018, 7:30am D R A F T X - 38 SCHILLINGS ET AL.: SEPTEMBER 2017 STORM

Figure 4. Solar wind parameters for September 4 to 12 2017. Panel (a) shows the short (xs,

0.05 - 0.4 nm) and long wavelength (xl, 0.1-0.8 nm) of the solar flux from SXI/SDO, panel (b), D R A F T July 3, 2018, 7:30am D R A F T (c) and (d) IMF, density and velocity respectively and panels (e) and (f) the Dst and Kp indices respectively. SCHILLINGS ET AL.: SEPTEMBER 2017 STORM X - 39

+ −2 −1 Figure 5. Distribution of the scaled O outflow [log10m s ] in the polar cap (PC 1, Sept 7

22:11-23:00 UT; PC2, Sep 7-8 23:35-00:24 UT) and in the cusp (Cusp 1, Sept 7 23:00-23:35 UT;

PC2, Sep 8 00:24-00:33 UT) in the northern hemisphere.

D R A F T July 3, 2018, 7:30am D R A F T X - 40 SCHILLINGS ET AL.: SEPTEMBER 2017 STORM

+ Figure 6. Distribution of the perpendicular temperature for the scaled O outflow [log10 m−2s−1] in the polar and cusp regions in the northern hemisphere for Sep 7-8 22:11-00:33 UT.

D R A F T July 3, 2018, 7:30am D R A F T SCHILLINGS ET AL.: SEPTEMBER 2017 STORM X - 41

Table 1. Parameters for escaping O+ ions

Tperp [eV] Tpar [eV] vperp [km/s] vpar [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

D R A F T July 3, 2018, 7:30am D R A F T Figure 6.

Figure 4. a

b

c

d

e 1 3

SSC 2 f

Figure 1.

Figure 2.

Figure 3.

a b

AR12674

AR12673 AR12673 Figure 5.