An intense substorm with AE exceeding 1000 nT took place on August 10, 2016, with two main positive bay onsets around 09:42 UT and 09:57 UT. Despite the intense electrojet at the auroral latitude, no Dst enhancement accompanied this event, which indicates that no pronounced enhancement in the ring current took place. The interplanetary magnetic field was weakly southward (−2 to −3 nT) for an interval of about 90 min before the onset of substorm expansion. During this substorm interval, the two Van Allen Probes and the Geostationary Operational Environmental Satellites (GOES) 13–15 spacecraft were located in the post-midnight sector, while the MMS, GEOTAIL, and Cluster were located between the pre-midnight and dusk sectors (Fig. 1a, b). A SCW with a maximum intensity of about 0.6 MA (Fig. 1c), deduced using the SCW model (Sergeev et al. 2011), was centered at post-midnight for onset at 09:42 UT and expanded toward the pre-midnight region for the 09:57 UT onset, and thus extended to local time sectors where the MMS was located (Fig. 1d).
Figure 2 shows the Bz (northward) component of the magnetic field from GOES 13, 14, and 15 (Singer et al. 1996), Van Allen Probe-A (Kletzing et al. 2013), MMS 3 (Russell et al. 2016), Geotail (Kokubun et al. 1994), and Cluster 1 (Balogh et al. 2001) and electron energy spectra from the MMS Energetic Ion Spectrometer (EIS) (Mauk et al. 2014) and the Fast Plasma Instruments (FPI) (Pollock et al. 2016) instrument together with the AU and AL indices. The electron energy spectra (panel f) are a combined data product from the MMS1 EIS instrument for energy >25 keV and from the MMS3 FPI instrument for energy <25 keV. To enhance the visibility of the high-energy portion, the EIS electron energy flux is increased by a factor of 2.75 in the panel. Here, we have used the solar magnetic coordinates (SM) for all the above-mentioned spacecraft except for Geotail, for which the geomagnetic solar magnetospheric coordinates (GSM) were used, because only Geotail was located behind the typical hinging distance in a more tail-like region (see Fig. 1e). For Van Allen Probe-A (VAA), which quickly traversed different L shells, the difference between observed field and the model magnetic field (T89) (Tsyganenko 1989) was plotted. As expected from the SCW distribution (Fig. 1d), the 09:42 UT onset is associated with the Bz disturbance, mainly in the post-midnight sector. The 09:57 UT onset, on the other hand, was first observed at GOES 15 along with some enhancement in the electron flux at MMS, followed by successive Bz disturbances detected at Geotail, MMS, GOES 13–15 and VAA, hence, expanding/propagating duskward and dawnward and covering a region of the SCW extending over the 9 MLT wide region (Fig. 1d). Bz disturbances also took place after 10:10 UT in all the spacecraft intermittently when the modeled SCW region covers the entire night-side local time.
Particle and field observations at MMS during the main dipolarization events (between 10:01 and 10:04 UT) are shown in Fig. 3. Panel a shows the energy spectra of electrons with the same format as Fig. 2f. Panel b shows ion energy spectra from MMS3 plotted using proton data from EIS for higher energy (>45 keV) and ion data from FPI for lower energy (<30 keV). For FPI ion data, background noise caused by energetic electrons was subtracted. To enhance the visibility of the high-energy part, the EIS proton energy flux was multiplied by 5. MMS was located in the outer plasma sheet until around 10:01:30 UT, but entered a hotter plasma sheet region during the dipolarization event. This transition from outer to center plasma sheet can also be seen in the magnetic field data (Fig. 3c–e) plotted in VDH coordinates, which better represent the magnetic disturbance at different local times in a dipolar configuration. Here, H is along the geomagnetic dipole axis and positive northward, and is therefore the same as the Z component in the SM coordinate system. D is perpendicular to the radial direction, R, and H, i.e., H × R, and is positive eastward. V denotes the radially outward direction, which corresponds approximately to the background field direction, and is closing the right-hand coordinate system. The overall changes from the tail-like configuration to a more dipolar configuration, as expected from the plasma profile, can well be identified based on the decrease in B
V
(panel c) as well as increase in B
H
(panel e).
The dipolarization (enhancement in B
H
) consists of multiple short-timescale enhancements. These enhancements are accompanied by large changes in B
D
, which indicate a crossing of the FAC sheets, as confirmed based on the FAC calculated using the curlometer method (Chanteur and Harvey 1998) and the four MMS spacecraft (black curve in panel f). Parallel currents calculated using particle data from the FPI onboard MMS3 (blue curve in panel f) coincide well with the curlometer current, which indicates that the spatial scales are sufficiently large relative to the inter-spacecraft distances, typically around 50 km. There are four main rapid crossings of intense FAC layers (B
D
change), all accompanied by dipolarization fronts (B
H
enhancements), as indicated by dashed lines. Each corresponds to the start time of a sharp B
D
change at: (1) 10:01:22, (2) 10:01:43, (3) 10:02:41, and (4) 10:03:01. The direction of the FAC for event (1) was anti-parallel to the field direction; that is, currents flowing into the ionosphere. The FACs for the other three events, (2)–(4), were directed parallel to the field direction, which means the currents flowed out from the ionosphere (upward FAC). Because the MMS is located in the dusk part of the current wedge, the event (1) was an R2-sense current, whereas the others were R1 sense currents.
Figure 3g–i shows plasma flows perpendicular to the magnetic field and E × B drifts, and Fig. 3j shows the electric field (Lindqvist et al. 2014). Here, the electric field (panel j and blue traces in panels g–i) and FPI ion velocity data (red traces in panels g–i) are averaged to 1-Hz resolution (i.e., about the ion-gyro frequency). Proton velocity data obtained by the Hot Plasma Composition Analyzer (HPCA) instrument (Young et al. 2014) are shown in 10-s resolution as black curves in panels g–i. The transient dipolarizations and the crossings of the current layers are associated with enhancements in the equatorward/Earthward plasma flows and in the dawn-to-dusk electric field (negative E
D
), as represented in Fig. 3g, i, j, except for the first part of event (2), when E
D
changed from dawnward to duskward and flow changed from outward/tailward to inward/Earthward. The intense flux extending to higher energy in the ion spectra suggests that the obtained ion velocity is likely underestimated, particularly after the plasma sheet proper is entered. Nonetheless, the overall traces show similar changes among the three velocity estimates. All four FAC events were accompanied by a strong dawn-to-dusk electric field (negative E
D
) with a magnitude exceeding several 10 s mV/m. This electric field strength is comparable or slightly stronger than the upper quartile value of the electric field of dipolarization flux bundle events in the CPS in the 6 R
E
region obtained from a statistical study (Liu et al. 2014). However, the largest changes take place in the east–west flows (i.e., dawn–dusk flows), V
D
, and the north–south electric field E
H
, except for in event (3). In particular, events (1) and (2) were associated with intense dawnward flow and reversal to duskward flows, respectively. One candidate to explain these strong dawn–dusk flows may be deflection of the BBFs in the flow-braking regions. However, in such cases, duskward flows such as that observed in event (2) are a more natural flow direction considering the local time of MMS relative to SCW rather than dawnward flows as observed in event (1).
The four MMS spacecraft were separated by about 50 km during this event and detected quite similar traces in the magnetic fields, except with some time shifts (Fig. 3c–e). Such a profile is ideal to infer the motion of the magnetic structures using the timing method. The obtained velocity of the magnetic structure, which we call “timing velocity,” V
tim, is plotted with horizontal bars in panels g–i. The timing velocity is inferred from the time difference between the spacecraft, which is determined by cross-correlating the B
D
components between different spacecraft during the time interval indicated as the horizontal length of the bars in panels g–i (horizontal black bars in Fig. 3). The relationships between V
tim (red), the average ion velocity perpendicular to the magnetic field, V
ion (green), and the electric field, E (blue), during the four events are given in the panels k–n in the H–D plane. It can be seen that V
tim during the FAC events (1) and (2) was directed southward/westward (~duskward), whereas during events (3) and (4), it was directed northward/dawnward. Note that these FACs (flowing mainly along V) were produced by magnetic disturbances predominantly along D, as indicated by the black arrow, ΔB, which is the difference in B vectors between the start and end time of each FAC-crossing event. Therefore, the timing velocity along H represents the motion of the current sheet. The outward (southward) motion of the current sheet in events (1) and (2) is likely associated with plasma sheet expansion. The motion of both the plasma (V
ion) and the boundary (V
tim) is equatorward for events (3) and (4), as is often the case for dipolarization fronts of enhanced Earthward fast flows. These different motions of the boundary change the polarity of the FAC direction with respect to the change in B
D
. Because the main magnetic disturbances are caused by the FAC, ΔB
D
/V
timH
, they will provide the polarity of FAC. This value is negative for event (1) and positive for the other three events, consistent with the curlometer current. For events (1)–(3), enhancements in the northward/equatorward motion of plasma were detected with respect to the boundary, as expected in the enhanced dawn-to-dusk convection electric field, whereas the FAC event (4) took place after the convection electric field enhancement. If we estimate the spatial scale of the four current sheets from V
timH
multiplied by the crossing times, we obtain the thicknesses of the current sheet for each event: (1) 1310 km, (2) 710 km, (3) 140, and (4) 170 km.
Figure 4a–h shows magnetic field and plasma data from the low-energy particle instrument (LEP) (Mukai et al. 1994) from Geotail observations between 09:50 and 10:30 UT. The MLT of Geotail was 22 MLT based on the spacecraft location (Fig. 1d), which corresponds to 21 MLT for its ionospheric foot point, taking into account the stretched tail-like configuration. Geotail was located west of the SCW during the 09:57 UT onset, but the westward expansion of the current wedge region then crossed the Geotail local time between 10:00 and 10:05 UT. At 9:59:50, there was a ~1 min long ~10% compression in the Bx centered on the inflection point of a slightly positive, then negative Bz variation with similar duration. This observation suggests a signature of a tailward-moving traveling compression region (TCR), which has been shown to be caused by the draping of the lobe magnetic field flux tubes about plasmoid-type flux ropes ejected down the tail by the plasma exhaust from x-lines in the plasma sheet (Slavin et al. 1984). Enhanced southward flows, corresponding to the dawn-to-dusk electric field of up to 2.5 mV/m, continued afterward, accompanied by a decrease in pressure, which indicates unloading and exiting to the lobe. Such signatures have been reported as the current sheet thinning associated with the enhanced flux transport rate due to activation of near-Earth reconnection tailward of the spacecraft (Sergeev et al. 2008). Therefore, Geotail was likely close to the reconnection region and then detected its tailward progression. Geotail then entered the plasma sheet, accompanied by enhancement in B
Z
from 10:14 UT. Positive/negative B
Y
and negative/positive V
Y
in the lobe/plasma sheet side were observed during the crossing. Note that the change in the dawnward to duskward motion of the plasma occurred mainly in the lower energy (~few keV) population, whereas the Earthward high-energy (~10 to 20 keV) ion beams were deflected duskward. This pattern is similar to the FAC events (1) and (2) observed by MMS, associated with outward motion of the current sheet. The FAC deduced from the particle spectra also shows a consistent pattern; that is, a downward current lobe-side and upward current at the plasma sheet side. The parallel current was calculated from the ion and electron moments, with the 12-s moment data averaged over five points to reduce noise. Note that in the plasma sheet region (after around 10:20 UT), when the amplitude of fluctuation was too large relative to the background field, determination of parallel current becomes less reliable.
Magnetic field data from GOES 14, 15, and MMS between 09:56 and 10:06 UT are shown in Fig. 4i–k. From the SCW location (Fig. 1d), it is expected that GOES 14 is located at the dawnward side of the current wedge center, whereas GOES 15 is near the duskside of the current wedge center. In such cases, the expected perturbation in B
D
at geosynchronous altitude in the northern hemisphere would be positive perturbation for GOES 15 and negative for GOES 14, assuming the SCW is located tailward of the spacecraft. However, the pattern was the opposite, as highlighted by the red and blue bars in Fig. 4j. During this onset, another peak in the D appeared in the pre-midnight, overlapped with a broader disturbance, which is visible in Additional file 1: Figure S1. This finding suggests an asymmetric disturbance caused by a localized pre-midnight SCW in addition to the broader disturbance modeled as one SCW (Fig. 1d; Additional file 1: S1). The high-latitude magnetograms (Additional file 1: Figure S2) also show strong westward electrojet, which was concentrated in the pre-midnight near the footpoint of MMS, and support this view. GOES 14 and 15 were therefore more likely located dawnside of this localized current wedge developed in the pre-midnight region. Furthermore, the relatively low-latitude location of the strong westward electrojet, as well as the possible X-line location near Earth, suggests that SCW was located near the geosynchronous orbit. The difference between GOES 14 and GOES 15 can then be interpreted as caused by the difference in the distance from the center of the current sheet. That is, GOES 14/15 observed the downward FAC, corresponding to the dawnside part of the localized intense SCW located equatorward/poleward of the spacecraft. The profile of the B
D
perturbations from GOES 14 therefore resembles that of MMS because of the opposite hemisphere and opposite local time relative to the current system compared with MMS.