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O + Escape During the Extreme Space Weather 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 Space Climate, EDP sciences, 2018, 16 (9), pp.1363-1376. �10.1029/2018SW001881�. �hal-02408153�
O
+escape during the extreme space weather event of
1
September 4–10, 2017
2
Audrey Schillings,1,2 Hans Nilsson,1,2 Rikard Slapak,3 Peter Wintoft,4
Masatoshi Yamauchi,1 Magnus Wik,4 Iannis Dandouras,5and Chris M. Carr,6
Corresponding author: Audrey Schillings, Swedish Institute of Space Physics, Kiruna, Sweden.
(audrey.schillings@irf.se)
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´e de Toulouse, CNRS, UPS, CNES, Toulouse, France.
6Imperial College, London, UK.
Abstract. We have investigated the consequences of extreme space weather
3
on ion outflow from the polar ionosphere by analyzing the solar storm that
4
occurred early September 2017, causing a severe geomagnetic storm. Sev-
5
eral X-flares and coronal mass ejections (CMEs) were observed between 4
6
and 10 September. The first shock – likely associated with a CME – hit the
7
Earth late on September 6, produced a storm sudden commencement (SSC)
8
and began the initial phase of the storm. It was followed by a second shock,
9
approximately 24h later, that initiated the main phase and simultaneously
10
the Dst index dropped to Dst = −142 nT and Kp index reached Kp = 8.
11
Using CODIF data onboard Cluster satellite 4, we estimated the ionospheric
12
O+ outflow before and after the second shock. We found an enhancement
13
in the polar cap by a factor of 3 for an unusually high ionospheric O+ out-
14
flow (mapped to an ionospheric reference altitude) of 1013 m−2s−1. We sug-
15
gest that this high ionospheric O+ outflow is due to a preheating of the iono-
16
sphere by the multiple X-flares. Finally, we briefly discuss the space weather
17
consequences on the magnetosphere as a whole and the enhanced O+ out-
18
flow in connection with enhanced satellite drag.
19
1. Introduction
Solar storms with associated flares, coronal mass ejections (CMEs), and radio bursts
20
may be hazardous to Earth, potentially affecting satellite operations through enhanced
21
satellite drag, causing electric power distribution failures due to extraordinary ground
22
induced currents (GIC), disturbing radio communications and causing radio blackouts due
23
to higher ionization rates in the lower ionosphere. Balch et al. [2004] report occurrence
24
rates of such events; extreme and severe radio blackouts occur approximately 1 day and 8
25
days per solar cycle respectively. Similarly, the planetary Kp index [Bartels et al., 1939;
26
Mayaud, 1980], widely used as a general indicator of geomagnetic disturbances for mid-
27
latitude regions and as a general geomagnetic alert and hazard scale, reaches extreme (Kp
28
= 9) and severe (Kp = 8) levels 4 and 100 days per solar cycle, respectively [Balch et al.,
29
2004].
30
In solar cycle 23, there were more than 100 X-flares, the strongest occurring in November
31
2003 (X28.0) and in April 2001 (X20.0). However for these events the solar active regions
32
were close to the solar limb, therefore no severe or extreme geomagnetic storms were ob-
33
served [Zhang et al., 2007]. At third strongest was an X17.2 flare detected on October
34
28, 2003 and due to a strong Interplanetary Coronal Mass Ejection (ICME) an extreme
35
geomagnetic storm – also known as the Halloween storm – took place [e.gYamauchi et al.,
36
2006; Rosenqvist et al., 2005; Gopalswamy et al., 2005]. It produced disturbances around
37
the world [Balch et al., 2004]. In the same solar cycle, on January 20, 2005 an outstanding
38
solar flare occurred (X7.1), followed by an ICME which was detected in the near-Earth
39
space on January 21. This ICME arrival resulted in an extreme magnetospheric com-
40
pression [Dandouras et al., 2009]. In solar cycle 24, in September 2017, several X-flares
41
occurred of which the most intense was an X9.0-flare (the largest observed since December
42
2006) which with strong CMEs produced an extreme geomagnetic storm. These extreme
43
events caused a strong response in the ionosphere and magnetosphere, for example vari-
44
ations in the total electron content (TEC) and enhanced magnetospheric convection. An
45
additional effect is the strongly enhanced ion outflow from the polar ionosphere [Schillings
46
et al., 2017].
47
Ion outflow from the ionosphere has been widely studied, i.e. by Shelley et al. [1982];
48
Chappell et al.[1987];Moore et al.[1997];Lennartsson et al.[2004];Kronberg et al.[2014];
49
Maes et al. [2015] and references therein. The most profound magnetospheric ion outflow
50
is usually observed in the open magnetic field line regions: the polar caps, cusps and
51
plasma mantle [Nilsson et al., 2012]. The mirror force plays a key role in the acceleration
52
of outflowing ions, with perpendicular energy converted into parallel energy as the ions
53
move into higher altitudes and weaker magnetic fields. Therefore, perpendicular heating
54
of ions causes subsequent outward acceleration. Wave particle-interaction can cause such
55
transverse ion heating, and these processes and their effects have been investigated at
56
different altitudes [e.g. Andre et al., 1990; Norqvist et al., 1996; Bouhram et al., 2005;
57
Nilsson et al., 2006; Moore and Horwitz, 2007; Slapak et al., 2011; Waara et al., 2011;
58
Nilsson et al., 2012]. If the heating is effective enough, the ions will gain sufficient velocities
59
to escape into the solar wind downstream in the tail [Nilsson et al., 2012] or even directly
60
from the cusp [Slapak et al., 2013].
61
Ion outflow has been studied for different solar wind and geomagnetic conditions. Cully
62
et al. [2003] identified four factors that influence the ion outflow; the solar radio flux at
63
10.7 cm (F10.7), the interplanetary magnetic field (IMF), the solar wind electric field and
64
the solar wind dynamic pressure. Using data in an altitude range of 1.3 to 2.0 RE, Yau
65
et al. [1988] found that ion outflow depends on the geomagnetic condition such that it
66
increases exponentially with Kp. Similarly, Slapak et al. [2017] made a statistical study
67
using high-altitude plasma mantle and magnetosheath Cluster data in order to quantify
68
the total O+ escape as a function of Kp. Complementary to that study, Schillings et al.
69
[2017] presented that the escape rate during extreme geomagnetic storms could be higher
70
than what a linear extrapolation of the results ofSlapak et al.[2017] would predict. As the
71
whole magnetosphere is affected by disturbed magnetospheric conditions, the O+ outflow
72
that does not escape into the solar wind is transported to the lobes through the polar cap.
73
Through reconnection in the plasma sheet, O+ coming from the lobe is heated and feeds
74
both the distant and near-Earth plasma sheet [Kistler et al., 2006;Mouikis et al., 2010].
75
The consequence of space weather on ion outflow has not been well studied. Therefore,
76
it is not clear what parameter is most influential with what time scale. In this paper,
77
we first investigate the space weather during 4 to 10 September 2017, from solar storm
78
to geomagnetic storm. Thereafter we examine the effects on magnetosphere-ionosphere
79
coupling, in particular ion outflow. The goal is primarily to see the space weather effects
80
on ion outflow for this well studied storm. Despite that ion outflow in itself not causing
81
hazardous to human activity, the enhanced ion outflow and resulting escape are mainly
82
observed in the cusp, which has an important role in some hazards as discussed in Section
83
5.3.
84
2. Instrumentation and data
2.1. Cluster mission
The Cluster mission [Escoubet et al., 2001] was launched in 2000 and consists of four
85
spacecraft flying in tetrahedral formation in an elliptical polar orbit. Only spacecraft 4
86
(SC4) was used in this study, because in 2017 the COmposition DIstribution Function
87
(CODIF) part of the Cluster Ion Spectrometer (CIS) [R`eme et al., 2001] onboard the other
88
spacecraft are no longer functional. The particle detection efficiency of CODIF onboard
89
spacecraft 4 has also been degraded [Kistler et al., 2013], this degradation is taken into
90
account by the instrument in-flight calibration files. The CODIF instrument measures
91
the 3D-distributions of H+, He2+, He+, and O+ using a time-of-flight technique. When
92
CODIF is subject to intense proton fluxes the heavier ion channels may be contaminated
93
with false counts. If so, the ratio of the O+ to H+ perpendicular bulk velocity will have
94
a local peak at 1/4, and these unreliable O+ data can be removed [Nilsson et al., 2006].
95
The magnetic field data are provided by the FluxGate Magnetometer (FGM) [Balogh
96
et al., 2001], which has a normal mode sample frequency of 22.4 Hz. In this study we are
97
interested in the background magnetic field and use the field averaged over the spacecraft
98
spin period of 4 seconds.
99
2.2. Solar and solar wind data
We utilized a range of observations and derived data from the Sun to the solar wind.
100
The Solar Dynamics Observatory (SDO) [Pesnell et al., 2012] is especially targeted at
101
producing observations of the solar drivers in order to understand and predict space
102
weather. Images from the photosphere and photospheric magnetic fields, and up through
103
the corona, are produced at high resolution and high cadence. Solar active regions (ARs)
104
are summarized and collected on a daily basis in the Solar Region Summary (SRS) by
105
NOAA/SWPC. SWPC also provides full-disk X-ray observations from the GOES space-
106
craft. CMEs are observed by the ESA/NASA Solar and Heliospheric Observatory (SOHO)
107
and the NASA Solar TErrestrial RElations Observatory (STEREO) (currently only one
108
spacecraft is operational). From coronal images, the Solar Influences Data Centre (SIDC)
109
automatically detects and estimates CME parameters, producing a catalog of CMEs. Fi-
110
nally, solar wind plasma and magnetic fields are measured by the DSCOVR spacecraft;
111
data are provided by NOAA/SWPC.
112
3. Method
This section describes how we analyzed the September 2017 storm from the solar point
113
of view to the O+ outflow in the polar regions at Earth. We study the complete chain
114
of events from the Sun, through the solar wind, geomagnetic indices, and ion outflow, in
115
particular the ionospheric O+ ions. The O+ ions mainly originate from the ionosphere
116
whereas the distinction between the ionospheric and solar origin of the H+ is more com-
117
plex. We first looked and analyzed the solar data such as X-flares and CMEs from the
118
beginning of September. Afterward, we checked the solar wind parameters from 4 to 10
119
September to identify the arrival time of the shocks at Earth and finally compare them
120
with Cluster observations.
121
At Earth, a geomagnetic storm is characterized by three phases – the initial, main and
122
recovery phase – which can be identified by the behavior of the Dst index. The Dst index is
123
a 1-hour index estimated from the deviations of the horizontal component of the magnetic
124
field at low-latitude magnetometer stations. In principle, it is a measure of the equatorial
125
ring current strength. The Kp index is a 3-hour index estimated from local disturbances
126
global scale. A combination of these two magnetic indices provides magnetospheric infor-
128
mation on the geomagnetic storm. The initial phase of a geomagnetic storm is typically
129
characterized by a positive disturbance in the Dst index, which could also be associated
130
with an SSC. When the positive perturbation drops drastically to negative values, the
131
main phase of the storm starts and lasts a few hours – typically 2 h to 10 h – and is
132
followed by the recovery phase that can last hours to a few days.
133
In order to estimate the O+ outflow during the geomagnetic storm, we first looked at
134
the O+and H+energy and pitch angle (PA) spectrograms and the magnetic field for SC4.
135
In Fig. 1, panel (a) shows that the local magnetic field varied slowly and was weaker
136
in the regions marked as polar cap (PC1 and PC2), while it was stronger and variable
137
in the cusp (Cusp 1 and Cusp 2). Panels (b), (c), (d) and (e) correspond to O+ and
138
H+ energy and pitch angle spectrograms respectively (for more detail see Schillings et al.
139
[2017]). The color bar represents counts per second (c/s), which is proportional to the
140
particle differential energy flux, and the dashed (pink) rectangles distinguish the polar
141
caps from the cusps. From Fig. 1, we visually identified the outflowing regions in the
142
northern hemisphere. In the polar cap, we observed an oxygen ion beam at lower energies
143
than in the cusp, see panel (b). Furthermore, the magnetic field in the polar cap is more
144
stable as opposed to the cusp, which is characterized by more variability. The pitch angle
145
spectrograms, displayed in panels (d) and (e), clearly show outflowing ions (PA ≃ 180◦,
146
northern hemisphere) for O+ and H+ in the polar cap regions. In the cusp, outflowing
147
ions (PA ≃180◦) are visible, but also ion populations with most energy perpendicular to
148
the magnetic field (PA ≃90◦).
149
The identified outflowing regions are confirmed from the observations by inspect-
150
ing the Cluster orbit. We first used the Orbit Visualization Tool (Khotyaintsev, Y.,
151
https://ovt.irfu.se) for a 3D view of the SC4 orbit with the location of the magnetopause
152
and the bow shock. Secondly, we plotted the O+ flux along the orbit for these time pe-
153
riods (Fig. 2). In Fig. 2, the outflowing regions are represented by the zoomed plots
154
(top for the north, bottom for the south) on the orbit planes XZGSM and XYGSM. The
155
color bars correspond to the O+ flux along the Cluster trajectory and the points mark
156
the identified regions. In the north, we observed polar cap and cusp regions while in the
157
south we identified polar cap and plasma mantle regions. During each time period, which
158
corresponds to either the polar cap, the cusp or the plasma mantle, we made histograms of
159
the scaled O+ outflow. The scaled O+ outflow is defined as the net outward flux scaled to
160
an ionospheric reference altitude and given by the formulaFmap = FOB+local∗Biono. We mapped
161
the fluxes (FO+) to an ionospheric reference altitude with a magnetic field strength of
162
Biono = 50 000 nT. The local outward flux can be mapped assuming conservation of par-
163
ticle flux along the corresponding magnetic flux tube (Blocal). This scaled net outward
164
flux is thus independent of the altitude and any magnetic compression giving an estimate
165
of the original ionospheric outflow.
166
4. Observations
This section presents the September storm from the solar storm to the ion escape
167
during the extreme geomagnetic storm. The solar regions where the flares and CMEs
168
were produced are described and we estimate the terrestrial magnetospheric O+ outflow
169
during the main phase of the geomagnetic storm.
170
4.1. Solar storm
Multiple X-type flares and Earth-directed CMEs were observed during 4–10 September
171
2017. On September 1, the solar Active Region (AR) 12673 was a small unipolar spot
172
just south of the solar equator (magnetic type α). At the same time AR12674 (Fig. 3,
173
panel (a), yellow dashed circle) on the northern hemisphere was much larger and more
174
complex (β −γ). However, during 3–4 September AR12673 grew dramatically both in
175
size (more than 10 times in area, see Fig. 3, panels (a) and (b), yellow solid circle) and
176
complexity (β−γ−δ) and thus became a highly interesting region. The region stayed
177
large and magnetically complex during its passage over the solar disk until it went around
178
the west limb during 10 September.
179
AR12673 produced numerous flares and CMEs from 4–10 September. Most notable was
180
a CME in the evening of 4 September and another on mid-day 6 September. The SIDC
181
identified speeds in both events close to 2000 km/s with onset times at 2017–09–04 19:12
182
UT and 2017–09–06 12:12 UT, respectively [Robbrecht et al., 2009]. Another fast CME
183
was launched on 10 September, but at that time AR12673 was on the western limb and
184
the CME was not Earth-directed.
185
Two X-flares occurred on 6 September, one on 7 September, and another on 10 Septem-
186
ber. The X-flares were observed by GOES-13 for short and long wavelengths with 1-minute
187
resolution X-ray flux, XS (0.05–0.4 nm) and XL (0.1–0.8 nm) as presented in Fig. 4, panel
188
(a). The horizontal solid blue line in panel (a) (Fig. 4) corresponds to the lower limit
189
for an X-type flare. The onset of the first CME on 4 September and the likely associated
190
shock in the solar wind are indicated by the vertical solid and dashed blue line, respec-
191
tively. The latter is clearly seen in the IMF (Fig. 4, panel (b)), density (panel (c)) and
192
velocity (panel (d)) 1-minute resolution data taken from DSCOVR. Finally, the vertical
193
solid and dashed red lines indicate the onset and the likely associated shock, respectively,
194
of the second CME. The two last panels (e) and (f) in Fig. 4 display the Dst and Kp
195
index values from Kyoto, WDC for Geomagnetism (blue line) and GFZ, Potsdam and
196
estimated by the Swedish Institute of Space Physics, Sweden (orange line) [Wintoft et al.,
197
2017].
198
4.2. Geomagnetic storm
Late on 6 September 2017, a shock associated with a CME was detected just before
199
midnight in the IMF, density and velocity. This first shock (Fig. 4, dashed blue line)
200
initiated the initial phase of the geomagnetic storm with a positive increase in the Dst
201
index (Fig. 4, panel (e)) and in the z component of the local magnetic field, causing an
202
SSC. After this abrupt change, the IMF turned and stayed northward for a few hours (Fig.
203
4, panel (b), Bz in orange) and with a compressed magnetopause and stronger currents
204
as a consequence. During the SSC, Cluster spacecraft 4 was located in the tail region
205
and therefore the O+ outflow cannot be estimated. This first shock probably caused a
206
small geomagnetic storm. However, approximately 24 hours later a second CME (Fig. 4,
207
dashed red line) reached the Earth. The IMF was strong and southward this time, and
208
with the earlier compression of the magnetosphere by the first CME (number 1 in panel
209
(e)), the second CME shock enhanced the incoming energy from the solar wind into the
210
magnetosphere and directly initiated the main phase (number 2 in panel (e)). The solar
211
wind density (panel (c)) was lower as compared to the first shock, the velocity (panel (d))
212
increased from approximately 200 km/s, the Kp index increased to Kp = 8 (panel (f))
213
and the Dst index dropped significantly to −142 nT. This negative drop is a signature
214
of the storm main phase, which continued for approximately 5 hours (20:00 – 01:00 UT).
215
During the main phase of the storm, Cluster was in the northern hemisphere (see Fig. 2,
216
and top panels for zooms).
217
The arrival of the second shock was coincident with an abrupt equatorward motion of
218
the cusp, at 23:00 UT on 7 September (Fig. 1, panels (b) and (c)) in the Cluster data.
219
When hitting the Earth, the shock again compressed the magnetopause, causing higher
220
convection in the magnetosphere. Simultaneously, the southward IMF opened up the
221
dayside geomagnetic field lines, such that the footprints of the cusp are at lower latitudes.
222
Newell and Meng [1994]; Newell et al. [1989] described how the polar cap and plasma
223
mantle move equatorward for disturbed magnetospheric conditions, and we observed such
224
a motion of the cusp. After the second shock, Cluster was located in the cusp for about
225
50 minutes, before the cusp moved equatorward such that Cluster was again located in
226
the polar cap for approximately 40 min (see Fig. 1). Afterward Cluster entered the cusp
227
a second time, where the spacecraft stayed for a few minutes before ending up in the
228
magnetosheath (see Fig. 2, top panels).
229
Following the main phase, the recovery phase (number 3 in panel (e)) was longer and
230
in the meantime Cluster was at the nightside in the southern hemisphere (8 September
231
around noon). In Fig. 2, the bottom panels show the encountered regions in the south.
232
We visually identified the polar cap and plasma mantle as Cluster was on the flank of
233
the magnetosphere and in the nightside. Cluster encountered 4 polar cap/plasma mantle
234
crossings.
235
4.3. O+ outflow during the geomagnetic storm 4.3.1. Northern hemisphere
236
We estimated the scaled O+ outflow in the different regions before and after the second
237
shock signature, from September 7, 22:11 UT to September 8, 00:33 UT, while Cluster
238
was located in the polar cap and in the cusp. Fig. 5 displays the distribution of the scaled
239
O+ outflow in the polar cap (panel (a)) and the cusp (panel (b)). Polar cap 1 and Cusp 1
240
(blue) correspond to the first Cluster passage of those regions and Polar cap 2 and Cusp
241
2 (red) to the second passage. The transitional color corresponds to the superposition
242
of the 2 distributions. The scaled O+ outflow, given in [m−2s−1] in logarithmic scale, is
243
shown on the x-axis, while the y-axis gives the relative probability from the number of
244
data points in each bin compared to the total data points from the distribution.
245
The scaled O+ outflow of the polar cap has clearly increased after the passage of the
246
second shock. The average scaled O+outflow in the Polar cap 1 and Cusp 1 are 3.5×1012
247
and 1.6×1013 m−2s−1, respectively, and increase by a factor of 3 (polar cap) and 2 (cusp)
248
after the shock signature. The highest value, with a minimum of 1% of the data in the
249
bin, are 6.3×1013 m−2s−1 for the polar cap and 1×1014 m−2s−1 for the cusp.
250
To determine if the scaled O+ outflow is escaping into the solar wind, we study the
251
O+ perpendicular and parallel temperatures, as well as the perpendicular and parallel
252
velocities. Table 1 gives the four parameters we took into account to estimate the fate of
253
the O+ ions. In the polar cap regions, the parallel to perpendicular temperature ratio is
254
high due to the high parallel component, therefore the transverse heating is not efficient,
255
this is as expected (significant transverse heating is usually observed only in the cusp due
256
to the wave-particle interactions [Strangeway et al., 2005; Slapak et al., 2013]). The ions
257
in the polar cap are mostly accelerated through the centrifugal acceleration [Nilsson et al.,
258
2012]. Furthermore, the parallel velocity is twice the perpendicular velocity (in the polar
259
cap), so that the ions follow the magnetic field upward with their fate in the plasma sheet
260
tailward of the X-line (neutral line in the magnetotail) at about 20 RE - 30 RE. Note
261
that the X-line can be pushed earthward around 10 RE for a severe geomagnetic storm
262
[Tsyganenko and Sitnov, 2005]. In the cusp region, the components of the temperature
263
are similar before the shock, whereas the perpendicular temperature is about twice the
264
parallel component. This indicates a transverse heating effect. However, the perpendicular
265
velocities are very high (> 40 km/s) compared to the average convection velocity of 7,5
266
km/s in the lobes [Haaland et al., 2012], and it leads to a strong convection in the cusp
267
region. The cusp field-lines will convect through the polar cap (lobes) before they reach
268
the plasma sheet and thus we should use the perpendicular velocity in the polar cap for
269
our calculations of where the ions reach the plasma sheet. A considerable amount of
270
flux is therefore transported tailward and towards the plasma sheet, despite this strong
271
convection the ions have enough energy to escape directly downtail.
272
Fig. 6 shows the relative probability of log10T⊥(O+) [eV] in the northern polar cap and
273
cusp before the shock signature (blue) and after (red). Note that the transitional color
274
corresponds to the superposition of the 2 distributions. The dashed blue (Cut 1) and red
275
(Cut 2) lines roughly mark the separation between the polar cap and the cusp. In the
276
cusp, we observed a higher perpendicular temperature than in the polar cap as expected
277
because the ions are more heated. The perpendicular energy is converted into parallel
278
energy by the mirror force for outflowing ions, thus accelerating the ions upward. Slapak
279
et al. [2011] showed that O+ is effectively heated transversely through wave particle-
280
interactions in the cusp. By the high perpendicular temperatures in the cusp combined
281
with a parallel velocity vk(O+)>30 km/s (not shown), we can conclude that the observed
282
O+ in the cusp will eventually escaping into the solar wind [Nilsson, 2011].
283
4.3.2. Southern hemisphere
284
Cluster was located in the southern hemisphere during the recovery phase (number 3 in
285
panel (e), Fig. 4) of the geomagnetic storm. Thus, we checked which regions it encountered
286
where O+ outflow might be observed. We identified 4 crossings from polar cap to plasma
287
mantle (see Fig. 2, bottom panels) where the scaled O+ outflow has been estimated. The
288
4 identified polar cap and plasma mantle observations are numbered from 1 to 4 according
289
to the passage on the Cluster orbit (color points on Fig. 2), i.e. number 1 equals to the first
290
polar cap/plasma mantle crossing in time along Cluster orbit. In plasma mantle regions
291
from 1 to 4, the scaled O+ outflow is roughly the same ∼3×1012 m−2s−1. Similarly to
292
the northern hemisphere, we looked at the perpendicular and parallel components of the
293
temperature and velocities in the plasma mantle regions. The parallel temperatures are
294
unexpectedly high (about twice the perpendicular component) for the 4 plasma mantle
295
passages, which means that a local heating is observed in these regions, otherwise the
296
velocity filter effect would have decreased the parallel component of the temperature.
297
Additionally, the perpendicular component of the velocity is not negligible, we thus have
298
a significant convection. This strong convection implies a considerable magnetic flux
299
transport that must be released through reconnection if it is not to accumulate in the
300
magnetotail. The reconnection rate must then be correspondingly high and the X-line
301
might move earthward. Thus, due to the high parallel component of the temperature and
302
the perpendicular velocity, the fate of the ions is less predictable than for the northern
303
hemisphere. They might end up in the plasma sheet and feed the ring current with O+
304
or tailward of the X-line and be lost in the magnetotail.
305
4.3.3. O+ flux in the plasma sheet
306
We also looked at the O+ flux in the first plasma sheet crossing after the storm. The
307
plasma sheet is identified around XGSM ≃-15 RE and the net O+ flux is small. Following
308
the method of Slapak et al. [2017] – dividing the flow between tailward and earthward
309
flux – we estimated an average O+ flux of ∼ 1.1×109 m−2s−1 in both directions. This
310
estimation is in agreement with the average O+flux calculated in the central plasma sheet
311
by Slapak et al. [2017]. In addition, Cluster was in the plasma sheet in the end of the
312
recovery phase (9 September, about 00:00 to 16:00 UT) and our estimation shows that
313
the average O+ return flux was the same as during quiet magnetospheric conditions. This
314
result is consistent with the results ofKistler et al.[2010], who showed that the O+ density
315
is reduced during the recovery phase.
316
5. Discussion
5.1. Solar drivers for O+ escape during geomagnetic storm
Extreme geomagnetic storms are caused by ICMEs, while smaller storms could be caused
317
by corotating interaction regions (CIRs). On September 4 and 6, 2017, two CMEs were
318
detected at the Sun and on September 6 and 7, three X-flares (X2.2; X9.3 (Sep 6) and
319
X1.3 (Sep 7)) were observed. The CMEs produced a severe geomagnetic storm recorded
320
with Kp = 8 and Dst ≃ –140 nT. The first CME, that arrived late 6 September had
321
northward IMF with 600 km/s velocity whereas the second that arrived late 7 September
322
had southward IMF with 800 km/s velocity. These features are most likely the causes
323
of higher scaled O+ outflow because both velocity and IMF are expected to influence
324
O+ outflow [Lennartsson et al., 2004; Yamauchi and Slapak, 2018]. Lennartsson et al.
325
[2004] showed that negative IMF Bz has a higher influence on the total O+ outflow by
326
approximately a factor 3. Considering also the timing of enhanced O+compared to arrival
327
timing of CMEs and enhanced EUV flux, the solar driver that influences O+outflow during
328
a geomagnetic storm is probably CMEs, despite that the X-flares with long wavelength
329
(0.1 - 0.08 nm) ionize the neutral atmosphere all the way down to approximately 95 km
330
[Rees, 1989; Tsurutani et al., 2005]. In comparison, the Halloween event that occurred
331
October 28–30, 2003, had stronger X-flares (X17.2 (Oct 28) and X10.0 (Oct 29)) and Kp
332
= 9+, andSchillings et al.[2017] estimated a higher polar cap O+outflow than is observed
333
for the September 2017 storm. Consequently, the ionization in the polar cap was higher
334
and the X-flares could have influenced the ionization process [Tsurutani et al., 2005] such
335
as a preheating of the ionosphere before the CME hit the Earth. Yamauchi et al.[2018]
336
wrote an overview of the ionospheric response to this storm with the EISCAT radars.
337
The authors showed that the electron density and temperature were enhanced after the
338
detection of the X-flares. They also suggested that the increased ion temperature due
339
to ion heating creates a precondition for O+ upflow (upflow means that the ions are still
340
gravitationally bound). This suggestion is strengthened by the increased F10.7 solar radio
341
flux, which can be taken as a proxy of EUV flux, before and during the magnetic storm.
342
In addition, Yau et al. [1988] found a clear correlation between O+ outflow and Kp and
343
F10.7. For low solar activity (F10.7 between 70 and 100 sfu) Yau et al. [1988] estimated
344
the O+ outflow to be a factor 4 lower than during high solar activity (F10.7 between
345
150 and 250 sfu). Cully et al. [2003] found a correlation between the total O+ outflow
346
and F10.7; higher O+ outflow is observed for increasing F10.7. Despite that we do not
347
have suitable Cluster data when the X-flares occurred, we conclude that the primary solar
348
driver for O+outflow is probably ICMEs due to the clear influence seen in the Cluster data
349
but X-flares might have a significant contribution in the heating process of the ionosphere
350
[Yamauchi et al., 2018] and in causing additional ionization.
351
5.2. 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
352
main phase of the storm. Fig. 5 shows the polar cap and cusps before and after the second
353
CME - shock (see section 4). We observed an enhancement of a factor 3 for the polar cap
354
and a factor 2 for the cusp after the shock signature. This weak enhancement is comparable
355
with the lowest relative scaled O+ outflow enhancement for intense geomagnetic storms
356
presented bySchillings et al.[2017], who found that the scaled O+outflow increases by 1 to
357
2 orders of magnitude during intense storm conditions compared to quiet magnetospheric
358
conditions. However, the absolute value of the scaled O+ outflow (∼ 1013 m−2s−1) is
359
quite high compared to similar storms studied in Schillings et al. [2017]. We suggest that
360
this unusual high scaled O+ outflow is due to the already compressed magnetosphere as a
361
consequence of the first shock approximately 24 hours earlier. Moreover, before the second
362
shock signature (September 7, 23:30 UT) the predicted Kp index was 3 and after the shock,
363
Kp = 8, thus according toSlapak et al. [2017] and their Fig. 5, the increase in the scaled
364
O+ should be around 1 order of magnitude. Our lower result could be explained by the
365
difference in the studied regions (polar cap compared to plasma mantle/magnetosheath for
366
Slapak et al.[2017]). This increase in the unusually high O+ outflow was observed within
367
roughly 40 minutes, which is fast for a magnetospheric response. The quick response and
368
lower relative enhancement in the O+outflow could be explained by the first shock, which
369
already initiated a geomagnetic storm.
370
Schillings et al. [2017] could not confirm that the enhanced scaled O+ outflow in the
371
polar cap during the storm was escaping and lost into the interplanetary space. How-
372
ever, for the September 2017 storm, we studied and could compare the scaled O+ outflow
373
in two regions, the polar cap and the cusp. For these two regions, we looked at the
374
perpendicular and parallel component of the temperature and the outflow parallel and
375
perpendicular bulk velocity. Fig. 6 shows that before the second CME-shock, the perpen-
376
dicular temperature in the polar cap 1 (blue, left side of Cut 1) is lower than after polar
377
cap 2 (red, left side of Cut 2). This means that the ions in the polar cap have been heated
378
and accelerated [Nilsson et al., 2006] after the shock passage. In both cusp regions, the
379
perpendicular temperature remains more or less the same, but is higher than in the polar
380
cap. Consequently, despite a significantly high perpendicular bulk velocity, we observed
381
ions with higher energy and parallel bulk velocity (not shown) in the cusp with sufficient
382
energy to escape into the solar wind. This result confirms what Schillings et al. [2017]
383
expected but could not claim.
384
In the southern hemisphere, the perpendicular temperature and the scaled O+outflow in
385
the 4 identified polar cap and plasma mantle regions (1-4) are constant, despite the parallel
386
velocity is decreasing as Cluster moves tailward. This result suggests that an enhancement
387
is still observed during the recovery phase, and in addition the local heating of O+ is still
388
significant as indicated by the unexpectedly high parallel temperature. Furthermore, the
389
perpendicular bulk velocity is not negligible and the strong convection might drive the ions
390
to the plasma sheet earthward of the X-line. In opposite, some ions experience enough
391
heating/acceleration to end up behind the X-line and therefore be lost in the tail. The
392
fate of these O+ ions is less predictable than in the northern hemisphere. Finally, during
393
the 3 first hours of the recovery phase, the Kp index was 5, hence these observations also
394
confirm the dependence of O+ outflow on Kp [Slapak et al., 2017].
395
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.
396
Space weather influences the occurrence of substorms, the life-time of the ring current,
397
the density of the plasma sheet and the occurrence of bursty bulk flows (BBFs) and finally
398
the energy conversion through mass loading in the cusp. Substorms do not always happen
399
during geomagnetic storms and therefore a storm does not necessarily include substorms
400
[Kamide et al., 1998;Hori, 2006]. At a substorm onset,Nos´e et al. [2005] found that the
401
O+/H+ energy density ratio increases. During substorm growth phase, energetic O+(tens
402
of keV), which mainly originate from the dayside polar region [Nakayama et al., 2017],
403
play an important role in feeding the ring current. The life-time of the ring current is
404
therefore prolonged by the addition of energy by dominant O+ under disturbed magne-
405
tospheric conditions, which have also lower charge-exchange cross-sections than the H+
406
ions, resulting in a slower loss [Hori, 2006; Denton et al., 2017].
407
Another aspect is the transport of O+ into the plasma sheet. Mouikis et al. [2010]
408
looked at the ion composition changes in the plasma sheet associated with geomagnetic
409
and solar activity and found a strong influence on O+ density. Under strong geomagnetic
410
activity, part of the O+ outflow is transported through the magnetotail and fed to the
411
plasma sheet [Kistler et al., 2010]. These ions can be seen in BBFs. However, Nilsson
412
et al.[2016] showed that O+is less accelerated during BBFs than the dominating protons.
413
Finally, high O+escape into the inflow solar wind plasma leads to a high mass-loading rate,
414
and high extraction rate of the solar wind kinetic energy to the ionosphere in a limited
415
and small region at high latitudes [Yamauchi and Slapak, 2018]. Therefore, we expect
416
ground induced currents at much higher latitudes than normal. Since human activity is
417
expanding to the polar region, such as arctic sea routes, this can become important.
418
The O+escape itself is not hazardous to human activity. However, enhanced O+ escape
419
rate can be observed at the same time as other space weather effects for example satellite
420
drag. The expansion of the neutral atmosphere in the thermosphere and extreme enhance-
421
ments of the ion escape can be linked because both occur when there is stronger energy
422
input into the atmosphere during extreme geomagnetic storms. When Kp and EUV flux
423
vary, the neutral temperature above the thermopause fluctuates [Chandra, 1967]. The
424
variation of EUV flux heats the upper atmosphere through ion-neutral collisions, which
425
tend to expand the thermosphere [Rees, 1989] and thus increase the atmospheric density
426
at higher altitudes. Therefore, high EUV flux (or the proxy F10.7 cm flux > 250 solar
427
units flux) and Kp (> 6) values result in extreme short-term increases in satellite drag,
428
particularly for satellites on low earth orbit (LEO). These parameters are used in satellite
429
drag models [Bowman et al., 2008;Storz et al., 2005]. Tsurutani et al. [2007] reported, for
430
the Halloween event, ion-neutral drag due to quick upward motion of O+ that causes up-
431
ward neutral oxygen (O) at equatorial latitudes. Similarly,Lakhina and Tsurutani [2017]
432
generalized the ion-neutral drag for oxygen for 1859-type (Carrington) superstorms. The
433
O+ outflow (higher latitudes) could have an impact on this ion-neutral drag.
434
The cusp plays a significant role in the ion outflow as well as for the satellite drag [L¨uhr
435
et al., 2004]. L¨uhr et al. [2004] studied the enhancement of the air density for moderate
436
geomagnetic activity (∼ Kp 3) over the cusp with the CHAMP satellite. They concluded
437
that these enhancements of air density are accompanied by small-scale structures such
438
as field-aligned currents (FAC) filaments and suggested the Joule heating as the primary
439
source for the upward movement of the air. This air up-welling over the cusp at low
440
altitude become mostly, due to several chemical reactions, O+ ions in the F region of
441
the ionosphere. Furthermore, Masutti [2017] studied the thermosphere during this storm
442
with the QB50 mission (CubeSats) and observed an orbit decay of roughly 2 km during
443
the September storm. Thus, the size or area, the current system or its equatorward
444
motion under extreme conditions is important for the accuracy of satellite drag models.
445
This speculative approach needs further investigations that were beyond the scope of this
446
study. However, we do believe that stronger O+ escape indicate an expansion of the
447
thermosphere, and could be strongly connected to increased satellite drag.
448
6. Conclusions
In this paper, we studied the consequences of the space weather on the O+ outflow
449
at Earth. We investigated the September 4–9, 2017, storm from the Sun down to the
450
ionosphere. At the beginning of September, several X-flares and two CMEs were detected,
451
which produced a severe geomagnetic storm (Kp ≃ 8 and Dst = -142 nT). The first
452
CME carried northward interplanetary magnetic field (IMF) and the associated shock
453
initiated a storm sudden commencement (SSC) at Earth followed by the initial phase of
454
a geomagnetic storm. A second shock, driven by a CME with southward IMF, hit the
455
magnetosphere 24 hours later and caused an equatorward motion of the cusp, which was
456
observed by Cluster satellite 4 in the northern hemisphere. The ionospheric O+ outflow
457
was estimated in the polar cap and cusp before and after the second shock, which initiated
458
the main phase of the storm. The ionospheric O+ outflow increased after the passage of
459
the second shock by a factor 3 in the polar cap and a factor 2 in the cusp. However, the
460
relative enhancement of the O+ outflow was lower than expected compared to Schillings
461
et al.[2017], nevertheless the initial value of O+ outflow (before the shock) for the regions
462
of observation was high ∼ 1013 m−2s−1. The unusually high ionospheric O+ outflow was
463
probably due to an enhancement after the first shock and a preheating of the ionosphere by
464
the EUV flux due to the X-flares in the previous days [Yamauchi et al., 2018]. Therefore,
465
the second shock did not increase O+ outflow during the main phase as much as previous
466
studies would have led us to believe [Schillings et al., 2017] but the heated O+had enough
467
velocity to escape directly into the solar wind. On September 8, the magnetosphere started
468
to recover from the massive injection of energy from the solar wind. During this phase,
469
Cluster was moving tailward in the southern hemisphere and we observed local heating of
470
the ionospheric O+ outflow indicated by high parallel temperature, and strong convection
471
as seen in the perpendicular component of the velocity. The fate of the O+ is therefore
472
less predictable. Furthermore, observations were made in the plasma sheet on September
473
9, the O+ flux estimated is consistent with the average flux calculated by Slapak et al.
474
[2017].
475
The observation of escaping O+ during geomagnetic storms caused by CMEs and X-
476
flares also shows that the neutral atmosphere could be ionized sufficiently to expand the
477
neutral thermosphere and lead to significant satellite drag effects. Satellite drag models
478
take into account the geomagnetic activity and the EUV flux [Bowman et al., 2008;Storz
479
et al., 2005] and the higher these parameters are, the more impact they have on satellite
480
drag. A similar Kp and EUV flux relation was also observed with O+ escape [Yau et al.,
481
1988; Cully et al., 2003; Slapak et al., 2017], and therefore we suggest that higher O+
482
outflow indirectly indicates stronger satellite drag. Our finding can be summarized as
483
followed:
484
1. In the polar cap and cusp, we observed an increase in the ionospheric O+ outflow
485
by a factor 3 and 2 respectively, after the passage of a shock associated with the second
486
CME. These ions will eventually escape into interplanetary space.
487
2. The increase in the O+ outflow is not extremely high, however, the magnetosphere
488
response to the second CME, is fast (∼ 40 minutes).
489
3. During the recovery phase, local heating and strong convection are observed in
490
the plasma mantle as seen, respectively, in extremely high parallel component of the
491
temperature and high perpendicular velocity.
492
4. The upper limit of the ionospheric O+ outflow (northern hemisphere) is 6.3×1013
493
and 1×1014 m−2s−1 in the polar cap and cusp respectively.
494
5. The solar drivers for O+ outflow seem to be mostly ICMEs whereas X-flares might
495
have a significant contribution in the earlier preheating [Yamauchi et al., 2018] and ion-
496
ization of the neutral atmosphere.
497
Acknowledgments. The authors want to thank the Swedish Institute of Space
498
Physics, the Graduate School of Space Technology in Lule˚a and the Swedish National
499
Space Board for financial support. We thank the CIS-CODIF team, especially Lynn
500
Kistler for providing calibrated oxygen ion data. All the Cluster data can be retrieved
501
online from the Cluster Science Archive (CSA) website https://csa.esac.esa.int/csa-web/.
502
The ”X-ray Flare” dataset was prepared and made available online by the NOAA National
503
Geophysical Data Center (NGDC), https://www.ngdc.noaa.gov/stp/space-weather/solar-
504
data/solar-features/solar-flares/x-rays/goes/xrs/. Solar wind DSCOVR data and xray
505
GOES data were provided by NOAA/SWPC. This paper also uses data from the CAC-
506
Tus CME catalog, generated and maintained by the SIDC at the Royal Observatory of
507
Belgium.
508
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