Journal of Geophysical Research: Space Physics

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1 RESEARCH ARTICLE Key Points: SAPS were first initiated around the dusk sector at the beginning of the storm main phase SAPS expanded toward the midnight, moved to low latitudes, and formed a wedge-shaped structure during the main phase The large-scale structure of SAPS corresponded well with the structures of ionospheric conductivity and region-2 field-aligned currents Large-Scale Structure of Subauroral Polarization Streams During the Main Phase of a Severe Geomagnetic Storm Fei He 1,2,3, Xiao-Xin Zhang 4, Wenbin Wang 5, Libo Liu 1,2,3, Zhi-Peng Ren 1,2,3, Xinan Yue 1,2,3, Lianhuan Hu 1,2,6, Weixing Wan 1,2,3, and Hui Wang 7 1 Key Laboratory of Earth and Planetary Physics, Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing, China, 2 Institutions of Earth Science, Chinese Academy of Sciences, Beijing, China, 3 College of Earth and Planetary Sciences, University of Chinese Academy of Sciences, Beijing, China, 4 Key Laboratory of Space Weather, National Center for Space Weather, China Meteorological Administration, Beijing, China, 5 High Altitude Observatory, National Center for Atmospheric Research, Boulder, CO, USA, 6 Beijing National Observatory of Space Environment, Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing, China, 7 Department of Space Physics, School of Electronic Information, Wuhan University, Wuhan, China Correspondence to: X.-X. Zhang, xxzhang@cma.gov.cn Citation: He, F., Zhang, X.-X., Wang, W., Liu, L., Ren, Z.-P., Yue, X., et al. (2018). Largescale structure of subauroral polarization streams during the main phase of a severe geomagnetic storm. Journal of Geophysical Research: Space Physics, Received 13 JAN 2018 Accepted 11 MAR 2018 Accepted article online 25 MAR American Geophysical Union. All Rights Reserved. Abstract In this study, we present multisatellite observations of the large-scale structures of subauroral polarization streams (SAPS) during the main phase of a severe geomagnetic storm that occurred on 31 March Observations by the Defense Meteorological Satellite Program F12 to F15 satellites indicate that the SAPS were first generated around the dusk sector at the beginning of the main phase. The SAPS channel then expanded toward the midnight sector and moved to lower latitudes as the main phase progressed. The peak velocity, latitudinal width, latitudinal alignment, and longitudinal span of the SAPS channel were highly dynamic during the storm main phase. The large westward velocities of the SAPS were located in the region of low electron densities, associated with low ionospheric conductivity. The large-scale structures of the SAPS also corresponded closely to those of the region-2 field-aligned currents, which were mainly determined by the azimuthal pressure gradient of the ring current. 1. Introduction During storms/substorms, the nightside midlatitude ionosphere is usually subjected to subauroral polarization streams (SAPS) (Foster & Burke, 2002), which refer to latitudinally confined and longitudinally elongated enhanced poleward electric fields or westward ion flow equatorward of the dusk convection cell and auroral electron precipitation. As a subset of the SAPS, the subauroral ion drifts (SAID) (Anderson et al., 2001; Spiro et al., 1979) are usually used to denote a localized intense westward ion flow (>1 km/s) within a SAPS channel and often appear as a spike in flow speed with a latitudinal width of 1 2 (Anderson et al., 2001; He et al., 2014). It is usually believed that SAID/SAPS are formed when the separation between the electron and ion inner boundaries of the ring current (RC) extends down to the subauroral region of low ionospheric conductivity and strong electric fields occur to ensure the current continuity. Many characteristics of SAID/SAPS have been determined from space-based measurements of electric fields and ion drifts in the ionosphere and/or the magnetosphere, and ground-based radar observations, such as the global occurrence based on the statistics of many individual events (e.g., Figueiredo et al., 2004; Foster & Vo, 2002; He et al., 2012, 2014; Karlsson et al., 1998), long-term variabilities (e.g., He et al., 2014), hemispheric asymmetries (e.g., Wang et al., 2008, 2011; Zhang et al., 2015), electromagnetic wave structures (e.g., Mishin et al., 2003), double-peak signatures (e.g., He et al., 2016), and evolution patterns during intense storms and quiet-time substorms (e.g., He et al., 2017). Due to the spatial/temporal limitations of in situ measurements from polar orbiting satellites or ionospheric scans by incoherent scatter radars, the global configuration and evolution of SAPS are mostly studied through averages of many individual events (e.g., Erickson et al., 2011; Foster & Vo, 2002; He et al., 2017; Karlsson et al., 1998), rather than instantaneous measurements. A larger longitudinal coverage of SAPS observations (e.g., Ebihara et al., 2009; Makarevich et al., 2009; Oksavik et al., 2006; Yeh et al., 1991) has been achieved using Super Dual Auroral Radar Network (SuperDARN) radars (Greenwald et al., 1995). Among these studies, Oksavik et al. (2006) presented the first two-dimensional observations of SAID variability within the SAPS channel from the SuperDARN Wallops radar, Ebihara et al. (2009) compared the SuperDARN Hokkaido radar measurements with the Comprehensive Ring Current Model, and demonstrated the HE ET AL. 1

2 importance of RC distribution to the variation of storm-time subauroral rapid flows. Later, Clausen et al. (2012) investigated for the first time the large-scale longitudinal velocity structure of SAPS using simultaneous measurements from six SuperDARN radars covering 3 hr of universal time (UT) and 6 hr of magnetic local time (MLT). Through a comparison with the modeling by the Rice Convection Model, Clausen et al. (2012) found a good agreement between the longitudinal distribution of the plasma pressure gradients in the RC and the SAPS variation with MLT. Other global simulation work (e.g., Goldstein et al., 2005; Wang, Lühr, & Ma, 2012; Yu et al., 2015; Zheng et al., 2008) also demonstrated that the subauroral plasma flow was a direct manifestation of the distributions of the RC and ionospheric conductivity, especially during magnetically disturbed periods. However, there are still some important questions that remained unanswered, such as what the latitudinal and longitudinal extents of a SAPS event are, when and where SAPS events first form, and how SAPS events evolve during their lifetime. Addressing these questions requires simultaneous global observations in the subauroral ionosphere. For over 30 years, the Defense Meteorological Satellite Program (DMSP) has been monitoring ionospheric plasma parameters with great success in the subauroral region in Sun-synchronous orbits at ~800 km altitude. Each DMSP satellite measures concurrently in situ plasma drifts, magnetic field perturbations (can be used to calculate field-aligned currents [FACs]), plasma density, and temperature that no other observations can do and, thus, by combining with other ground-based observations, can provide unique and comprehensive data sets to understand the evolution of SAPS. During some specific years, there are at least four DMSP satellites in orbits with a wide MLT coverage in the MLT sector, enabling us to investigate the generation and evolution of SAPS on a large scale. In this study we present multisatellite observations of the spatial-temporal variations of the SAPS during the main phase (MP) of a severe geomagnetic storm that occurred on 31 March 2001, with the values of the Dst index reaching 387 nt. Some interesting and new features of SAPS are revealed. 2. Observations The severe geomagnetic storm on 31 March 2001 was produced by a fast transient solar wind event with a strong southward interplanetary magnetic field (IMF) B z component. Many studies on the geospace responses during this storm have been reported in the literature (e.g., Skoug et al., 2003; Sojka et al., 2004). Skoug et al. (2003) found that the tail currents were the dominant contributors to the Dst index drop during the MP of this storm. Parameters of the solar wind, IMF, and geomagnetic indices during this storm are shown in Figures 1a and 1b. Due to the two periods of southward IMF, there were two MPs during this storm, namely, a primary MP that occurred from 03:00 to 09:00 UT (Dst decreased from 8nTto 387 nt) and a secondary MP that occurred from 16:00 to 22:00 UT (Dst decreased from 214 nt to 284 nt). In this investigation we will mainly focus on the primary MP that started at 03:00 UT (Skoug et al., 2003). Figures 1c 1f show the horizontal ion drift and ion density observations from DMSP F12 to F15 satellites during 03:00 and 09:00 UT. The corresponding FACs estimated from the z component (pointing toward the center of the Earth) of the perturbed geomagnetic field (e.g., He et al., 2014) are shown in Figure 2. Also shown at the bottom of Figure 1 are the Madrigal total electron content (TEC) maps in magnetic latitude (MLAT)-MLT grid. It is noted that although the DMSP satellites crossed the subauroral region at different MLT sectors in different hemispheres in Figures 1 and 2, the satellite crossings in the two hemispheres were treated as a whole based on the fact that the SAPS channels happen simultaneously in both hemispheres (e.g., Anderson et al., 2001). Before the beginning of the primary MP at ~03:00 UT, no SAPS channel was observed on the nightside, which was possibly because the prerequisite conditions for SAPS (e.g., Anderson et al., 1993; Rodger, 2008), namely, a midlatitude ionospheric trough (MIT) and a region-2 FAC (R2-FAC) in the subauroral region below 60 MLAT, were not satisfied (Figures 1c, 2g, 2h, and 2a). At ~03:42 UT, a SAPS channel was first observed by F13 at ~62.0 MLAT and ~1530 MLT in the Northern Hemisphere (NH) associated with a strong R2-FAC (Figure 2a). At ~04:08 UT, a SAPS channel was also observed by F12 at ~58.0 MLAT and ~1800 MLT in the NH with a larger drift velocity and stronger R2-FAC (Figure 2a). This indicates that the SAPS channel might first occur around the dusk sector between 03:30 UT and 04:00 UT when the Dst index began to drop quickly. The SAPS channels were then successively observed in the MLT sector between 1600 and 2000 MLT between 05:00 and 06:00 UT (Figure 1d) at relatively lower latitudes compared with those seen in the earlier UT interval HE ET AL. 2

3 Journal of Geophysical Research: Space Physics Figure 1. (a) The OMNI 5-min averaged interplanetary magnetic field (IMF) BY (blue), IMF BZ (red), and solar wind dynamic pressure, Pdyn (black). (b) The hourly Dst index (red) and 1-min AE index (black). (c f) Cross-track ion drift observations by Defense Meteorological Satellite Program (DMSP) F12 to F15 satellites during the 3 primary main phase. The ion densities (cm ) measured by the retarding potential analyzer (RPA) are logarithmically color coded along the trajectories of each satellite with the color bar shown at the center. Different colors of the drift velocity profiles represent observations from different satellite as shown by the strings on the left side. All the strings are in the form of HH:MM:SSXYY, with HH (hour), MM (minute), and SS (second) representing the UTs of each satellite crossing the equatorward boundary of auroral electron precipitation, which is marked by diamonds as extracted from DMSP SSJ/4 data, X representing crossings in different hemispheres (N/S for Northern/Southern Hemisphere), and YY representing the DMSP satellite ID, respectively. The drift velocity peaks of subauroral polarization streams channels are indicated by black solid arrows. (g x) The Madrigal total electron content (TEC) maps in MLAT-MLT grid with the locations of the subauroral polarization streams peaks indicated by the black dots. (Figure 1c). The peak of drift velocities of all the SAPS channels remained relatively small (<0.5 km/s), except for the one at ~1600 MLT observed by F13 at ~05:23 UT (>2 km/s). The locations of the SAPS channels were moved equatorward by ~8 (~4.0 /hr in MLAT). A comparison between Figures 1c and 1d indicates that the HE ET AL. 3

4 Figure 2. The same as Figures 1c 1f, but for the field-aligned currents (FAC) densities (μa/m 2 ) which are color coded along the trajectories of each satellite with the color bar shown at the center (positive values for downward region-2 FAC and negative for upward region-1 FAC). The peaks of the subauroral polarization streams channels are indicated by arrows, the same as those in Figures 1c 1f. The diamonds are the same as those in Figures 1c 1f, indicating the equatorward boundaries of auroral electron precipitation. SAPS channel was greatly strengthened in the predusk sector and slowly expanded toward the midnight at a speed of ~1 hr MLT per hour, which was approximately equal to the Earth s corotation speed. This is merely a rough estimation, because we have no data between 1800 and 2000 MLT in Figure 1c and between 2000 and 2200 MLT in Figure 1d. At the same time, strong R2-FACs (Figures 2a and 2b) and MITs (Figures 1c, 1d, and 1k 1o) were also observed within the SAPS channel. No SAPS channel was observed after 2100 MLT during this UT period, although the MIT occurrence was obvious (Figures 1k 1o). The absence of SAPS in later MLTs is probably related to the fact that there were no strong R2-FACs in the subauroral region after 2100 MLT (Figure 2b). Furthermore, the approximately simultaneous crossings of 05:07:40N15 in the NH and 04:49:54S13 in the Southern Hemisphere (SH) around 2000 MLT at ~ ± 51 MLAT indicate that the SAPS channels might be magnetically conjugated between the two hemispheres during this time interval. The SAPS channel was further strengthened and moved to lower latitudes after 06:00 UT (Figure 1e). The SAPS channel expanded to ~2200 MLT at a speed of ~2 hr MLT per hour, faster than the speed in the previous time interval. The difference in the westward drift velocity in just 1 hr MLT separation could be as large as 2.0 km/s as shown by 07:03:46S12 and 07:01:29S14 in the SH. The SAPS channel ceased to develop at around 2200 MLT, although a very significant MIT was shown in Figure 1s. This was possibly related to the fact that there was a reversal in the R2-FAC direction after 2200 MLT (Figure 2c). The reversal in the R2-FAC direction usually occurs in the Harang discontinuity region around midnight (He et al., 2012; Zheng et al., 2006). The SAPS channel exhibited a significant hemispheric asymmetry in velocity, width, and latitudinal location as shown by 06:50:17 N15, 07:03:46S12, and 07:01:29S14 in MLT sector in Figure 1e, with HE ET AL. 4

5 the SAPS in the SH ~6 more equatorward in MLAT than those in the NH and stronger. Possible reasons causing such an asymmetry are the following: 1. An effect of the MLT of observations given that there were differences of about 1 2 hr MLT between the NH and SH crossings, although the difference of SAPS in MLAT is generally less than 2 in a 1-hr MLT sector according to the statistical results of Foster and Vo (2002) and Zhang et al. (2015). 2. Hemispheric asymmetry of the auroral oval (see the diamonds on the 07:03:46S12 and 06:50:17 N15 orbits in Figure 2c). 3. Strong stretching of the geomagnetic field lines on the nightside caused by the large tail currents (Skoug et al., 2003) and the penetration of strong IMF B Y into the closed nightside inner magnetosphere (Reistad et al., 2013). 4. Hemispheric difference of qionospheric ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi Pedersen conductivity. The ionospheric total Pedersen conductivity (Σ P ) is expressed as Σ P ¼ Σ 2 PP þ Σ2 PS, where the precipitation-produced conductivity (Σ PP ) is calculated with Robinson s relationship (Robinson et al., 1987) based on the DMSP observations of particle precipitation and the solar-produced conductivity (Σ PS ) that is obtained with the Rich model (Zhang et al., 2005). The results indicate that the values of Σ P were mho for 06:50:17 N15, mho for 07:01:29S14, and mho for 07:03:46S12, respectively, in Figure 1e. Besides, estimations of Σ PS indicate that the conductivity in the SH is in general 0.25 mho larger than that in the NH at the conjugate points in the ionosphere on the nightside after 1900 MLT. Such a difference of conductivity at the conjugate points could cause hemispheric asymmetry of the SAPS channels. This seems to contradict our previous result (Wang et al., 2008) that the SAPS tend to occur more poleward for flux tubes of higher integrated conductivity. However, the above rough estimation with empirical models could not truly reflect the actual situation during the severe storm. Since no direct observations of the E region conductivity was available for the case studied in the paper, detailed simulation works might be necessary in future when models are available that can self-consistently calculate the ionospheric conductivity and its influence on SAPS evolution. 5. The latitudinal differences of R2-FAC in the two hemispheres as shown by the colored FAC density plot in Figure 2c. The R2-FAC in the SH was stronger, latitudinally narrower, and located at lower MLAT compared with that in the NH, making the strength and location of the SAPS channel in the SH different from that in the NH. Thus, it appears that a zonally elongated wedge-shaped SAPS channel was formed, which was characterized by decreasing MLAT, latitudinal width, and drift velocity with increasing MLT in the subauroral region. The SAPS channel had most likely reached its lowest latitude location at 39.8 MLAT as shown by 07:01:29S14 with a faster speed of ~10 /hr in latitude. Such a fast equatorward motion of the SAPS channel might be caused by the dipolarization of the geomagnetic fields at ~06:30 UT and the accompanying fast particle injection that went much deeper into the inner magnetosphere (Skoug et al., 2003). In the last UT interval, when the Dst index reached a minimum of 387 nt between 08:00 and 09:00 UT, the SAPS channel became more organized, with a gradual decrease in drift velocities from the dusk sector toward the midnight sector (Figure 1f), but the maximum drift velocities after 08:00 UT were smaller than those shown by 07:31:05 N12. The SAPS channel almost disappeared in the predusk sector as shown by 08:47:15 N13. The SAPS channels were overall moved toward higher latitudes. In the SAPS channel, strong R2-FAC and deep MIT can be seen in Figures 2d and 1f, respectively, and the westward drift velocity seemed to be proportional to the strength of R2-FAC. The decrease of the westward drift velocity from the dusk sector to the midnight sector in Figure 1f corresponded well with the decrease of the R2-FAC densities with increasing MLT in Figure 2d. Around and after 09:00 UT (data not shown here), the SAPS channels disappeared globally and the subauroral region returned to its quiet-time location but with a wider and deeper MIT. This disappearance of the SAPS coincided with a rapid northward turning in the IMF B Z component and a large positive B Y component. Under such IMF conditions, the decay of the RC and the decrease of the equatorial ion pressure gradients resulted in a significant decline or even a reversal of the R2-FAC in the subauroral region on the nightside (He et al., 2012; Ridley & Liemohn, 2002). This might be the main reason for the disappearance of SAPS. It can also be seen from Figures 1c 1f that SAPS channels were all located within the MIT except for 07:04:27N13 in Figure 1e. In the early stage of the primary MP, there was no obvious MIT on the nightside. HE ET AL. 5

6 As geomagnetic activity increased, a latitudinally wide and deep MIT formed throughout the dusk to midnight sector. Also demonstrated in Figure 1f is that the peak drift velocity of the SAPS channel corresponded to the minimum of the MIT but with a slight equatorward shift. 3. Discussion The statistical characteristics of SAPS (e.g., Foster & Vo, 2002; He et al., 2014; Wang et al., 2008; Zhang et al., 2015) and the spatial/temporal evolution of SAPS from radar observations (e.g., Clausen et al., 2012; Ebihara et al., 2009; Foster et al., 2004; Oksavik et al., 2006) have been reported previously. Our case study describes the large-scale evolution pattern of the SAPS channel over an extended local time sector during the primary MP of a severe geomagnetic storm based on in situ multisatellite observations of ion drift velocity, plasma density, and FAC crossing the flow channel. The main characteristics of the SAPS channel in this event are the following: 1. The SAPS channel developed first around the dusk sector at the beginning of the primary MP. The channel exhibited a SAPS structure with a wide latitudinal span (~8.0 ) and a relatively small westward drift velocity (<0.5 km/s). 2. After the formation of the SAPS, the SAPS channel was strengthened and widened in the predusk sector and expanded toward midnight at a speed of ~1 hr MLT per hour and the low latitudes at a speed of ~4.5 / hr, respectively. 3. As the storm MP persisted, the SAPS channel continued to expand toward midnight and low latitudes at faster speeds, with an azimuthal speed of ~2 hr MLT per hour and a latitudinal speed of ~10 /hr. At this stage, the westward flow channel appeared in the form of SAIDs (very large drift velocity and spike structure) in earlier MLTs and still as SAPS in later MLTs. This indicates a wedge-shaped configuration of the SAPS channel. Such longitudinal configuration might be related to the well-known fact that the separation between the inner edge of the electron and ion plasma sheets increases from midnight to dusk. 4. The SAPS channel began to be weakened at ~08:00 UT when the Dst index reached its minimum and IMF B Z began to turn northward quickly. The SAPS channel moved back to higher latitudes and maintained a wedge-shaped configuration which was not aligned with the L-shell. In the wedge-shaped channel, the peak drift velocities and their corresponding latitudes, and the latitudinal width of the channel decreased from dusk to midnight. No SAPS signature was observed between 09:00 and 10:00 UT in all the MLT sectors, indicating that plasma drifts in the nightside subauroral region had temporarily reverted to the quiet state. Based on the above mentioned signatures, the large-scale structures of the westward ion drift velocity in the SAPS channel are summarized in Figures 3a 3e in 1.5 hr intervals. The velocity distributions in Figure 3 are obtained by fitting the observed velocity profiles in the SAPS channel during each 1.5 hr bin to a Gaussian function:! ½ð V Fit ¼ A 0 exp λ A 1Þ=A 2 Š 2 (1) 2 where V Fit is the fitted westward drift velocity in km/s, λ denotes the MLAT in degrees, and A 0, A 1, and A 2 represent the peak drift velocity, MLAT of the peak, and the latitudinal width of the SAPS channel, respectively. Then, the values of A 0, A 1, and A 2 in the two hemispheres at different UTs are averaged and linearly interpolated to obtain the two-dimensional contours in Figure 3. Figure 3 clearly reveals for the first time the dynamic global-scale evolution pattern of the SAPS during the MP of a server storm. The evolution pattern is characterized by the generation of SAPS first around the dusk region in both hemispheres shortly after the MP; followed by strengthening, latitudinal widening, midnight-directed expansion, and north-to-south asymmetry during the MP; and then weakening and disappearing in the recovery phase. Such an evolution pattern, though with relatively low temporal resolution, cannot be revealed in the statistics of disconnected events. It is noted here that Figure 3 is just a schematic demonstration of the global-scale evolution of the SAPS channel at a specific temporal resolution during the MP of a severe storm. Different evolution patterns of SAPS might appear in different storms/substorms (e.g., He et al., 2017), and this requires a detailed investigation HE ET AL. 6

7 Figure 3. Schematic demonstration of (a e) the temporal evolution of the global configuration of the subauroral polarization streams channel based on multisatellite observations during the primary main phase of the storm in MLAT-MLT grids, and (f) the Dst index from 00:00 UT to 11:00 UT on 31 March The color bar indicating the magnitudes of the westward ion drift velocity is shown at the bottom middle. The gray vertical lines in (f) indicate the UTs for panels (a) (e). in the future. Besides, instabilities in the magnetosphere-ionosphere-thermosphere (M-I-T) coupling lead to large localized changes in the position and intensity of SAPS on the order of minutes (e.g., Foster et al., 2004). A detailed description of the global fine structure and evolution of the SAPS channel needs continuous spatial/temporal mapping of the SAPS channel, but this is beyond the current observation capability and the scope of this paper. According to Anderson et al. (2001), the FAC can be defined as J ¼ B mc 2B 2 e h B e ð PÞ r ð V Þ φ þ ð P Þ φ ð V where B m and B e are the magnetic field strengths at the ionospheric mirror point and the equator, respectively, V is the magnetic flux tube volume, P is the plasma pressure, and r and φ refer to the radial and azimuthal directions, respectively. During disturbed periods, the azimuthal pressure gradient dominates, allowing R2-FAC to be generated. At the beginning of the storm MP, the azimuthal pressure gradient of the RC peaks around the dusk sector, then the pressure gradient expands toward midnight (Fok et al., 2014; Zheng et al., 2006). This is confirmed by the R2-FAC estimations shown in Figure 2. Therefore, the Þ r i (2) HE ET AL. 7

8 variations in R2-FAC generated by the azimuthal pressure gradient of the RC might be the main driver of the evolution of the SAPS during the primary MP of the 31 March 2001 severe geomagnetic storm. Although both the R2-FAC (e.g., Ebihara et al., 2009) and MIT (e.g., Zheng et al., 2008) are the prerequisite conditions for the generation of SAPS, R2-FAC is clearly the most critical factor in this case. The changes in the speeds of the eastward (longitudinally) expansion and equatorward (latitudinally) motion of the SAPS might indicate changes of the radial and azimuthal transmission speeds of the pressure gradient of the RC (e.g., Clausen et al., 2012; Ebihara et al., 2009) and changes of the distribution of ionospheric conductivity (e.g., Zheng et al., 2008). The transmission speeds might become faster during the later stage of the MP (i.e., after 06:00 UT for this storm) possibly due to the magnetic field dipolarization at ~06:30 UT (Skoug et al., 2003). There may also be other factors of ionospheric origin that influenced the evolution of the SAPS. For example, fast deepening and widening of the MIT were seen on the nightside in the Madrigal TEC maps based on data from the ground-based Global Positioning Satellite receivers (Figures 1g 1x) after the magnetic field dipolarization. Most of the previous work used models of a specific part of the geospace, such as Rice Convection Model or Comprehensive Ring Current Model (e.g., Clausen et al., 2012; Ebihara et al., 2009), that are not coupled self-consistently with thermosphere and ionosphere, and/or the global magnetosphere magnetohydrodynamic simulations. Therefore, detailed simulation studies with fully coupled M-I-T models, such as the Comprehensive Inner Magnetosphere-Ionosphere model (Fok et al., 2014) and the Coupled Magnetosphere Ionosphere Thermosphere model (Wang et al., 2004; Wiltberger et al., 2004), are desirable to do in the future to further investigate the evolution of the RC and to reveal its impact on the subauroral ionosphere/thermosphere during the MP of this severe storm as well as during other storm events. Knowledge of the global evolution pattern of the SAPS channel can shed light on the dynamics of the M-I-T coupling. It is generally accepted that SAPS have significant influences on the dynamics of the plasmasphere (e.g., Goldstein et al., 2005; He et al., 2012; Zheng et al., 2008, and references therein) and the thermosphere (Wang et al., 2011; Wang, Lühr, & Ma, 2012; Wang, Lühr, Ritter, et al., 2012; Wang, Talaat, et al., 2012, and references therein). Once we know the dynamic global distribution of the subauroral electric fields (even with relatively low spatial resolution), we can more accurately simulate the effects of SAPS on the plasmasphere and the thermosphere-ionosphere system since current SAPS electric field models are all based on statistical results, and the dynamic features of SAPS during storms/substorms are not well represented in statistical averages. Acknowledgments The authors sincerely thank NOAA/NESDIS/National Geophysical Data Center for the provision of the DMSP IDM, RPA, SSJ/4, and SSM data ( NASA/GSFC OMNIWeb for providing the solar wind and IMF data ( and the Kyoto World Data Center for providing geomagnetic indices (Dst and AE) ( wdc.kugi.kyoto-u.ac.jp/). The Madrigal total electron content (TEC) maps were downloaded from the Madrigal Database at IGGCAS ( iggcas.ac.cn/madrigal/). This work was supported by National Natural Science Foundation of China ( , , and ) and Youth Innovation Promotion Association of the Chinese Academy of Sciences ( ). The National Center for Atmospheric Research is sponsored by the National Science Foundation. 4. Summary Based on the observations from DMSP F12 to F15 satellites, the large-scale structure of the SAPS channel during the primary MP of a severe geomagnetic storm that happened on 31 March 2001 has been observed and studied. The SAPS channel is highly dynamic during storms, including its peak velocity, latitudinal location and width, latitudinal alignment, and longitudinal/mlt extension. There is also a close relationship between SAPS and IMF. It is noted that the deduced large-scale evolution pattern of the SAPS during this severe storm is just an approximated illustration or a schematic demonstration due to the lack of continuous spatial/temporal mapping of the SAPS channel. 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