3 − 1. Selection of Pc5 PMAA pulsation event
To identify “Pc5 PMAA pulsation” events in this study, we first examined a summary plot of the THEMIS data (for example, that of the March 2, 2011, <http://themis.igpp.ucla.edu/summary.php?year=2011&month=03&day=02&hour=0024&sumType=asi&type=keograms>). Our selection method is based on qualitative visual inspection of the images. Pc5 PMAA pulsation-like events can be identified in the keogram because most of them display a “repeated poleward moving” structure. Subsequently, we analyzed the auroral shapes using a sequence of ASI images to verify whether the shapes of the auroras resembled an auroral arc. Finally, we selected 119 events during a five-year period from January 2007 that exhibited the specific characteristics of Pc5 PMAA pulsations.
3 − 2. Ground-space coordinated observations of Pc5 PMAA pulsations
We examine the fortunate event that occurred when the footprints of the THEMIS spacecraft were located very close to the Pc5 PMAA pulsations region observed from the ground during the period 1210–1240 UT on 2nd March 2011.
3-2-1. Ground-based observations of Pc5 PMAA pulsations
The upper panel (a) of "Figure 1" shows the keogram observed at Fort Smith (FSMI). The geographic and geomagnetic coordinates and magnetic midnight at FSMI are, respectively, 60.0° N, 248.1° E, 67.3°, 307.1°, and 8:05 UT. Panels (b) and (c) show the relative intensity of the luminosity at the lines of ~ 70.0° magnetic latitude (MLAT) and ~ 68.2° MLAT, respectively. Lower panels (d) and (e) show the magnetic and electric field variations with a bandpass filter of 60–600 s observed onboard TH-A. As can be seen from the keogram, Pc5 PMAA pulsations were often observed in association with the enhancement of the magnetic and electric field oscillations, for example, at ~ 1015–1050 UT, ~ 1105–1125 UT, ~ 1135–1200 UT, 1215–1240 UT, and ~ 1245–1300 UT. In this study, we focused on the time interval of 1210–1240 UT where the most outstanding Pc5 PMAA pulsations were observed, and the footprints of the THEMIS satellites were located in the fields of view of the FSMI all-sky imager, as shown below.
"Figure 2" shows the optical characteristics of the Pc5 PMMA pulsations observed at Fort Smith (FSMI) and Fort Simpson (FSIM) in Canada and the magnetogram observed at Yellowknife (YKNF) and FSIM. The magnetogram at FSMI was out of data during the event. The geographic and geomagnetic coordinates and magnetic midnight at FSIM are, respectively, 61.8° N, 238.8° E, 67.2°, 294.4°, and 8:55 UT; those at YKNF are, respectively, 62.5° N, 245.7° E, 69.3°, 302.7°, and 8:22 UT. It is found from the keogram at FSMI (upper panel) that the Pc5 PMMA pulsations occurred from ~ 1221 UT and, following, there were a clear three-cycle oscillations with a period of approximately 4.5 min, increasing their luminosity with time. The keogram observed at FSIM (panel (b) of “Figure 2”) also shows the Pc5 PMMA pulsations with three-cycles oscillations, but their luminosity decreased with time. The main occurrence region drifted equatorward with time during the three cycle oscillations. As can be seen from the combined all-sky snapshot images at both observatories (right panel), which show the spatial structure of the auroral forms giving rise to the keogram behavior, the auroral arc elongated approximately along the east-west direction from horizon to horizon, for more than 2000 km, though inclined slightly towards north-south. The north-south width of the auroral arc was narrow; it could be less than 20 km at the zenith of FSIM. FSIM is located at almost the same geomagnetic latitude as FSMI. When the event occurred at 1220 UT, the magnetic local time at FSMI and FSIM were 0325 MLT and 0235 MLT, respectively. Therefore, the auroral arc observed at FSIM appeared at a lower latitude than that at FSMI, which may indicate that the arc appeared approximately along the auroral oval, as the averaged auroral oval is located at the lowest latitude in the midnight region. YKNF is located approximately 2° higher in MLAT and 5° westward in MLON from FSMI. The Pc5 PMAA pulsations were observed over the zenith of YKNF using keogram and ASI. The magnetogram located at YKNF, to which a bandpass filter of 60–600 s was applied, demonstrated that an increase in the H-component started at ~ 1214 UT and reached its maximum amplitude at ~ 1218 UT. Subsequently, it decreased and reached its negative peak at ~ 1222 UT, which was followed by weak two-cycle oscillations with positive peaks at ~ 1225 UT and ~ 1229 UT. The oscillation amplitude of the H component was much larger than that of the D and Z components. Specifically, the maximum peak-to-peak (pp) amplitude of the H component was ~ 80 nT. The first negative peak at ~ 1222 UT indicated by the dotted line coincides with the occurrence of the first PMAA observed at FSMI. However, it is difficult to find clear coincidence with the H component of magnetic oscillations at the two subsequent PMAAs, especially at the third PMAA.
The magnetogram at FSIM shown in the panel (d) of "Figure 2", which is located at a lower latitude than YKNF, reveals almost the same signature of the magnetic pulsation as that seen at YKNF, although the amplitude of the magnetic pulsation is smaller than that observed at YKNF. For the relationship to the PMAA pulsations observed at FSIM, the magnetogram also revealed a similar signature as that seen at YKNF and FSMI.
Herein, we examine the characteristics of the Pc5 PMAA in detail. The top panel of "Figure 3a" repeats the keogram observed at FSMI. Hereinafter, we refer to the three sequences of PMAA as PMAA-1, PMAA-2, and PMAA-3. The bottom panel of "Figure 3a" depicts the expanded keogram of PMAA-2 in both the time and latitude at the dotted square, as marked in the top panel. "Figure 3b" shows the luminosity distribution over magnetic latitude for the time intervals (a), (b), and (c), which are depicted by vertical lines in the bottom panel of "Figure 3a" during the PMAA-2. The time intervals (a), (b), and (c) correspond to the growing, maximum, and declining phases of the activity of PMAA-2, respectively.
We define a half-width of luminosity as W(half) = W((Ip+Ig)/2), where Ip is the peak luminosity and Ig is the background luminosity. W(half) corresponds to the latitudinal width where the luminosity reduces to half of the peak luminosity. In the case of (b), Ip is ~ 6000 and Ig is ~ 4200; so, (Ip+Ig)/2 = ~ 5100. W(half) becomes ~ 0.6°. It is found that the W(half) was approximately the same at (a), (b), and (c). That is, the half-width of luminosity did not depend on the growing, maximum, and declining phases.
The upper panel of "Figure 4" shows the latitudinal distribution of the maximum auroral luminosity at each sampling time on the keogram for PMAA-1, PMAA-2, and PMAA-3. That is, each figure shows the maximum luminosity of aurora considering its latitude during the one-cycle of PMAA. It was found that the luminosity at PMAA-1 during the growing and maximum phases was lower than that at PMAA-2 and 3. Moreover, the luminosity at PMAA-2 was higher during the maximum and declining phases than that at PMAA-1 and 3 Furthermore, it was found that the luminosity at PMAA-3 was somewhat equal to that at PMAA-2 during the growing phase until ~ 69.1°, after which it formed a broad peak at approximately 69.1°- 69.4°. After the peak, the luminosity dropped sharply during the declining phase from ~ 69.4°. This demonstrates that the main region of PMAA-3 drifted further to the lower latitude side than that of PMAA-1 and PMAA-2. The latitude of the luminosity maximum during one cycle may correspond to the central region on the FLRs. The signatures shown above demonstrate the dynamic overall activity of PMAA during this event.
The lower panel of "Figure 4" shows the relative phase on the luminosity oscillation of PMAA versus magnetic latitude. In this figure, phase "zero" is the latitude where the luminosity was maximum in PMAA-2, at ~ 69.4° MLAT. The rate of the latitudinal phase variation was found to be ~ 75°/deg. This phase-latitude diagram demonstrates that the PMAA pulsations could be explained by the FLR model as demonstrated by previous studies, for example, Milan et al. (2001) and Samson et al. (2003), wherein it is demonstrated that this rate is ~ 165°/deg and 110°/deg, respectively.
To examine the PMAA feature in relation to the FLR model, it may be important to investigate a latitudinal dependency of the repetition period of PMAA pulsations between PMAA-1, PMAA-2, and PMAA-3. "Figure 5" depicts the luminosity plots with time for 10 luminosity lines, including line-190 (~ 68.7° MLAT) to line-223 (~ 70.5° MLAT). To visualize the periodicity at each line more easily, the plots were arranged as follows: the timing of the luminosity maximum at each line during the PMAA-1 was fixed at the same moment by shifting the start time. From this figure, the periodicity between PMAA-1 and PMAA-2 and that between PMAA-2 and PMAA-3 at each line can be easily found. An interesting feature is that the period between PMAA-1 and PMAA-2 on the lower latitude side between the line-190 (~ 68.7° MLAT) and line-218 (~ 70.0° MLA) was almost constant giving a period of ~ 265 s. On the other hand, the period at the higher latitude side between the line-218 and line-223 (~ 70.5° MLAT) increased from ~ 265 s to ~ 285 s. Moreover, during the interval between PMAA-2 and PMAA-3, the period on the lower latitude side between the line-190 and line-218 was constant, ~ 300 sec, whereas on the higher latitude side, it is difficult to find a periodicity because no luminosity peak exists during the PMAA-3. Such a characteristic of low latitude portion as the constant period without latitudinal dependence, e. g., monochromatic oscillation, fits to the FLR model proposed by Chen and Hasegawa (1974) and Southwood (1974), though the period between PMAA-2 and 3 is longer than that between PMAA-1 and 2. On the other hand, the period at higher latitude side of PMAA, between PMAA-1 and 2, lengthens with increasing latitude. This characteristic suggests that the period is related to the local filed line length.
3-2-2. Field-line resonance signals observed in space
"Figure 6" shows auroral snapshot images observed at FSMI during the time period from the growing phase of the auroral arc pulsation at (a) 1226:00 UT to the declining phase at (c) 1228:09 UT thorough the maximum phase at (b) 1226:57 UT plotted with the footprints of TH-A, TH-D, and TH-E spacecraft at a 110 km altitude based on the TS05 model. The footprints were calculated by the TS05 model using following parameters: Dst, -28.0 nT; solar wind dynamic pressure, 2.0 nPa; interplanetary magnetic field (IMF) By, 0.5 nT; and Bz, as -1.5 nT. In each snapshot image, the footprints were shown with the triangle symbol under the half-hour trajectory during 1210–1240 UT (see the spatially expanded plot in "Figure 6"). Although TH-D was located in the most westward side and TH-A was located at the highest latitude side, the footprints of the three satellites were located very close to each other, as shown in the plot figures of "Figure 6". The model-calculated footprint was well within the fields of view of the ASIs where the auroral arc was detected, and the footprints were located on the poleward side from the PMAA during most of time in this study. The snapshot image at 1226:00 UT ((a) in "Figure 6") demonstrates that the auroral arc with higher luminosity at the eastern part was located a few tens of kilometers poleward from the zenith. At 1226:57 UT ((b) in "Figure 6"), the luminosity enhancement region elongated westward and moved in the poleward direction. Hence, the footprints of the spacecraft were located more equatorward relative to (a). At 1228:09 UT ((c) in "Figure 6"), the portion of the auroral arc moved more poleward with decreasing luminosity, so the footprints of the spacecraft were located more equatorward than in (a) and (b), and were located at just poleward boundary of the PMAA. A faint PMAA-3 appeared at the lower latitude side of the PMAA-2.
"Figure 7" shows the spacecraft orbit configuration projected onto three different planes (X-Y, X-Z, and Y-Z) in the geocentric solar magnetospheric (GSM) coordinate system during 1210–1240 UT on 2nd March 2011. The details of the orbit of TH-A, TH-D, and TH-E at 1210 UT on the X, Y, and Z planes with GSM coordinates are the following: X is ~ -6.2 Re, ~ -6.0 Re, and ~ -6.0 Re; Y is ~ -9.8 Re, ~ -9.5 Re, and ~ -9.8 Re; and Z is ~ 1.4 Re, ~ 1.0 Re, and ~ 1.0 Re. These orbits demonstrate that, at ~ 0410 MLT, the spacecraft were located near the northern hemisphere’ s magnetic equatorial region in the post-midnight sector. Regarding the relative orbital location among the three satellites, it is worth noting that TH-D was located at almost the same position as TH-E in the X and Z coordinates, but they were separated by ~ 0.2 Re in the Y direction, and the position of TH-A was located at almost the same position as TH-E in the Y plane, but was separated by 0.4 Re northward in the Z plane from TH-D and TH-E.
In the following analysis, general mean-field-aligned (MFA) coordinates were used. In this system, the Z component is parallel to the average direction of the ambient magnetic field, the Y component is azimuthally perpendicular to the magnetic meridian (westward), and the X component is roughly in the radial direction. Since the magnetic field near the equatorial plane was stretched, deviating from a dipole-like topology. We found that, when the data were plotted on GSM coordinates, the X and Y components of the magnetic field intensity were comparable to the total magnetic field intensity (not shown here the figure). This signal suggests that the field-line topology was extremely stretched to tail-ward. Therefore, the MFA coordinate system can display more physically understandable wave signals than the GSM coordinate system. The average magnetic field was calculated as sliding averages of 600 points (30 min) of observed magnetic field variations.
The panel (a) of "Figure 8" depicts auroral keogram observed at FSMI. The vertical dotted line at ~ 1221 UT indicates the start time of the luminosity enhancement of the PMAA. The panel (b) of "Figure 8” demonstrates the intensity of the luminosity at line ~ 70.2° MLAT near the footprint location of TH-D and TH-E, as presented later in Table-1, with a lowpass filter of 60 s. The panel (c) of "Figure 8" graphs the X component of the electric field (Ex) displayed with a lowpass filter of 60 s. The positive oscillation started at ~ 1218 UT, as indicated by a vertical solid line, simultaneously onboard the TH-A, TH-D, and TH-E spacecraft. It was found that the oscillation of the Ex started ~ 3 minutes earlier than that of the luminosity enhancement of the Pc5 PMAA. Subsequently, the signal displayed three-cycle oscillations and a weak increase at the fourth cycle oscillation, which could correspond to the fourth cycle oscillation with weak luminosity, as shown in the panel (a) of "Figure 8". Furthermore, it was observed that the oscillation amplitude attained a maximum at the second cycle, which may correspond to the occurrence of PMAA-2, whose luminosity was highest among three cycles of PMAA, as demonstrated in "Figure 4". It was noticed that after the start of the oscillation at ~ 1218 UT, the oscillation at TH-A exhibited a phase lag with respect to that of TH-D and TH-E. Moreover, this relative phase lag between TH-A and TH-D lengthened during the following oscillations. This implies that the period of the Ex oscillation in TH-A was longer than that in TH-D and TH-E. Such signatures are more clearly found in the Y component of magnetic field (By) variations as seen in the panel (d) of "Figure 8" This signature is the same as the periodicity of PMAA observed at the higher latitude side (~ 70.0°-70.5° MLAT), as depicted in "Figure 5" where the period between PMAA-1 and PMAA-2 lengthened with increasing latitude. This signature suggests that the footprint of TH-A could be located in the region of the higher latitude side of the PMAA. Accordingly, this characteristic suggests that the period relates to the local filed line length. The oscillation phase lag at the field line of the poleward location suggests that these waves were generated by FLR. However, the FLR model (Chen and Hasegawa, 1974; Southwood, 1974) postulates that waves are monochromatic, that is, the wave period is the same everywhere, as demonstrated in "Figure 5." Therefore, the features observed at TH-A did not agree with the monochromatic frequency FLR model, instead they indicated that each field line oscillates at a different eigen-frequency. This is discussed further in Session 4. Furthermore, before the oscillations started, the base-line intensities at TH-A, TH-D, and TH-E were ~ − 6 mV/m, − 9 mV/m, and − 3 mV/m, respectively. The peak-to-peak amplitudes of TH-D and TH-E were almost the same during the entire oscillation period, whereas the amplitude at TH-A was ~ 20–30% larger than that at TH-D and TH-E during the first cycle, and became comparable to that at TH-D and TH-E during the second and third cycles. It is worth noting that the relative intensity difference between TH-D and TH-E was rather high in comparison to the By as seen in the panel (d) of "Figure 8". It was found that the oscillation amplitude at TH-A was larger than that at TH-D and TH-E, whereas the amplitudes of TH-D and TH-E were approximately equal. Moreover, the oscillation amplitude attained a maximum at the second cycle, which may correspond to the PMAA-2, in the same manner as demonstrated in the Ex behavior. The fundamental-mode Alfven waves have the property that the magnetic node is located at the equatorial region; therefore, it is reasonable that the amplitude at TH-A was larger than at TH-D and TH-E, because TH-A was located at ~ + 1.4 Re in the Z coordinate and TH-D and TH-E were located more equatorward at ~ + 1.0 Re in the Z coordinate, as shown in "Figure 7." It was also observed that the oscillation of TH-A exhibited a phase lag with respect to that of TH-D and TH-E. The TH-A lag time from TH-E at the positive first, second, third, and fourth peaks were ~ 20 s, ~ 30 s, ~ 80 s, and ~ 90 s, respectively. Moreover, the relative phase lag between TH-D and TH-E was less than ~ 20 s in all four peaks.
The compressional mode wave at the Z component of magnetic filed (Bz) oscillations is plotted in the panel (e) of "Figure 8." This field-aligned component of the magnetic field demonstrated that both the baseline and oscillation amplitude were larger at TH-A than at TH-D and TH-E. The relative amplitudes at TH-D and TH-E were approximately equal. Such characteristics are reasonably explained by the relative location of the spacecraft. That is, TH-A was located at the higher latitude side (in the northern hemisphere) from the magnetic equator, than TH-D and TH-E. Furthermore, TH-A data demonstrated that a negative oscillation started from ~ 1218 UT and reached a negative peak at ~ 1220 UT, after which it formed a positive peak at ~ 1222 UT, whereas TH-D and TH-E presented weaker negative and positive peaks almost simultaneously with TH-A. Subsequently, the waveforms at TH-A displayed a phase lag from TH-D and TH-E signals in the same manner as documented above.
The panel (f) of "Figure 8" presents the variation in ion pressure based on Electrostatic Analyzer observation made by three satellites. It was found that the pressure was gradually increased with time until ~ 1218 UT at the location of all three satellites, after which the pressure decreased with time and started the long-period oscillations. The pressure at TH-E was higher than that at TH-D and TH-A during the entire event. This suggests that TH-E would be located closer to the central plasma sheet than the TH-D and TH-A because the ion pressure escalates at that location. It is interesting to note that the decrease in ion pressure started at ~ 1218 UT, which is when the Ex and By oscillations started. The luminosity peaks, which are marked with vertical long and short dash lines in the panel (b) of “Figure 8”, correspond to the pressure peaks observed at TH-D and TH-E.
To compare the magnetic field pulsations observed on the ground and in the magnetosphere, the H component of the magnetogram observed at YKNF is repeated in the panel (g) of "Figure 8" with a band pass filter of 60–600 s. It can be seen that the H component of the magnetic field increased with time until ~ 1218.30 UT, after which it decreased with time until ~ 1222.00 UT. It is important to note that the signature of the magnetic variations observed on the ground does not indicate a clear correlation with that observed on the magnetospheric By magnetic oscillations, as shown in the panel (d) of "Figure 8". On the other hand, the signatures observed on the ground magnetometer demonstrate a rather similar behavior to that observed in case of ion pressure variation in the magnetosphere as shown in the panel (f) of "Figure 8", especially during the interval of ~ 1214–1226 U.
3-2-3. Cross-correlation analysis between FLR oscillations and optical luminosity pulsations
To examine the relationship between the magnetic and electric field oscillations in the magnetosphere and the optical luminosity pulsations observed on the ground, we performed a cross-correlation analysis among the luminosity lines of 223, 220, 210, and 200, which correspond to the MLAT of ~ 70.5°, ~ 70.2°, ~ 69.5°, and ~ 69.0°, and the magnetic field and electric field variations observed onboard the TH-A, TH-D, and TH-E spacecraft with a bandpass filter of 60–600 s.
"Figure 9" shows an example of the result of the cross-correlation analysis. The left two panels show the power spectra of the By oscillations at TH-D (upper panel) and the luminosity pulsations at line 220 (lower panel). Meanwhile, the right two panels of "Figure 9" show the coherence (upper panel) and the phase difference (lower panel) between the two data. It can be seen that the coherence had a high value of more than 0.9 at the period of ~ 200–300 s, and the phase difference was ~ 150°. The power spectrum did not show a clear peak power at the specified period in this cross-correlation analysis.
"Table 1" shows the summary of the coherence around the period of ~ 200–300 s between the luminosity pulsations at lines 223, 220, 210, and 200 observed on the ground, and the X, Y, and Z components of the magnetic and the X and Y components of the electric field oscillations observed onboard the TH-A, TH-D, and TH-E satellites. Higher coherence columns, > 0.8 are shown in different colors. From this, the magnetic field (Bx, By, Bz) and electric field (Ex, Ey) variations and the luminosity pulsation at line 220 showed the highest correlation when compared with the other luminosity lines for the TH-D and E satellites, and that at line 223 for the TH-A. Meanwhile, the coherence with the luminosity at line 200 had the lowest value for the TH-A, D, and E satellites. From these results, we can draw an important conclusion that the luminosity pulsations around line 220 and 223 correlated well with the magnetic and electric oscillations observed in the magnetosphere. This indicates that the real footprints of the TH-A, TH-D, and TH-E satellites may be located near the field line of lines 223 and 220. Moreover, the footprints calculated by the TS05 model shown in "Figure 6" were located at the poleward side on the PMAA, which showed good agreement with the results shown in "Table 1". For the relation between the Z component of the magnetic field (Bz) variations and the luminosity pulsations, a high coherence (> 0.9) was found to occur at lines 220 and 223.
The detailed comparison of the periods of the Pc5 PMAA pulsations and the magnetospheric magnetic and electric field oscillations is very interesting and important. The panel (a) of "Figure 10" shows the auroral keogram observed at FSMI. The panel (b) of "Figure 10" shows the relative luminosity of the auroral keogram at line 210 (~ 69.5° MLAT) (green), 215 (~ 69.8° MLAT) (black), and 220 (~ 70.2° MLAT) (light blue) with a low pass filter of 30 s. The time at each positive peak was delayed at the higher latitude lines (poleward moving signal). Furthermore, the periods between the first and second peaks at lines 210, 215, and 220 were estimated to be ~ 265 s, ~ 265 s, and ~ 275 s, respectively. The periods between the second and third peaks at lines 210 and 215 were ~ 300 s and ~ 293 s. The lines 210 and 215 are located in the monochromatic frequency FLR region where the period is constant and independent of latitude. Moreover, line 220 is located in the multi-frequency FLR region where the period dilates with increasing latitude, as plotted in "Figure 5." The panel (c) of "Figure 10" shows the Y component of the magnetic field observed at TH-A, TH-D, and TH-E. The periods at TH-D and TH-E are approximately equal, whereas the period at TH-A is longer than that at TH-D and TH-E. This signature suggests that the TH-D and TH-E were located in the monochromatic frequency FLR region, but the TH-A was located at a multi-frequency FLR region. It is worth noting that the repetition period of the magnetic oscillation was ~ 10–25% longer than that of the Pc5 PMAA pulsations.
3–3. Relation between FLR and Solar wind parameters
The source mechanism of the FLR that has received the most attention is surface instabilities at the magnetopause (such as Kelvin–Helmholtz instability (KHI) and pressure impulse in the solar wind plasma) [Rostoker and Sullivan, 1987; Engebretson et al., 1998; Kivelson and Southwood,1985]. If the ground-based Pc5 PMAA pulsations are the result of this instability, it is likely that their characteristics depend on solar wind conditions. Therefore, we now examine this relationship. The panels (a) and (b) of "Figure 13" show the Y component of the magnetic field and the Y component of flow velocity, respectively, observed onboard the TH-A. The panels (c) and (d) of "Figure 11" show the Y and Z components, respectively, of interplanetary magnetic field (IMF), the panels (e) and (f) of "Figure 11" show the solar wind flow speed and pressure, and the panel (g) of "Figure 11" shows the AE index. The solar wind and IMF parameters were taken from the time-shifted OMNI-1 min data. The highlighted region indicates the time interval we examined. It is found from (a) and (b) that few FLR cycle signals were sporadically observed before and after our observations. Corresponding to these FLR oscillations, the Pc5 PMAA pulsations were also observed on the ground as shown in "Figure 1". By looking at IMF Bz, it is found that the FLR phenomena in this study were observed during the period of positive Bz after a recovery of about 1.5 hours of negative Bz, which started at ~ 1045 UT. It is worth noting that a positive Bz (~ 2 nT) was observed at ~ 1220 UT when the FLR phenomena were observed. The most distinct characteristic is that the solar wind speed was very high, approximately 650 km/s, during the observations. We will take a look on solar wind conditions in more detail for this event using Geotail data.
Geotail satellite was located at the upstream region near the dayside magnetopause. The orbit at 1200 UT on the X, Y, and Z planes with GSM coordinates was 19.6 Re, ~ -22.0 Re, and ~ -6.1 Re. The panel (a) of "Figure 12" shows the X, Y, and Z components of the magnetic field observed at TH-A. The Pc5 oscillations were observed from ~ 1218 UT at the vertical line in this figure, as shown in "Figure 8." The IMF magnetic field and the solar wind speed data obtained by Geotail was significantly different from the data obtained from OMNI, as depicted in "Figure 11". Sudden changes were observed at ~ 1219 UT on IMF By (from ~ -3 nT to ~ 2 nT) and Bz (from ~ 2 nT to ~ -2 nT). Similar sudden changes were also found in the solar wind velocity at Vy (from ~ 20 km/s to ~ 50 km/s) and Vz (from ~ 30 km/s to ~ -10 km/s). Even if we assumed that these drastic changes in the IMF and solar wind speed could trigger the FLR oscillations, their timing would be inconsistent. Specifically, because the Vx is ~ 660 km/s, it takes approximately 4–5 minutes when the rapid change of the solar wind observed at Geotail reaches TH-A. Therefore, under the general/standard solar wind condition, we have to exclude the possibility that the magnetospheric magnetic and electric field oscillations observed at ~ 1218 UT onboard TH-A were affected by the solar wind discontinuity observed at 1219 UT onboard Geotail. However, when we checked the polar cap Super Dual Auroral Radar Network (SuperDARN) (Greenwald et al., 1995;
Chisham et al., 2007; Nishitani et al., 2019) data for the scan plots of the line of site velocity obtained at Rankin Inlet (Geo. lat. 62.8°, lon. − 92.1°; Mag. lat. 71.5°, lon. − 21.7°), as depicted in the panel (a) and the map potential plot data in the panel (b) of "Figure 13," we found that the polar cap convection enhanced to more than ~ 800 m/s from ~ 1218 UT. These signatures obtained from the SuperDARN data indicate a typical convection pattern under the sudden negative changes in IMF Bz (Ruohoniemi and Greenwald, 1998; Shepherd et al., 1999). To explain why the discontinuity arrived at Earth's magnetopause earlier than Geotail, we can speculate that the surface discontinuity of the solar wind had large tilt in the Vx direction. Under this condition, the sudden negative changes in the IMF Bz may cause the sudden enhancement of magnetospheric convection, which could have affected the generation of the FLR oscillations, and caused the thinning of plasma sheet, as shown in "Figure 8." The negative Bz also caused the expansion of the auroral oval, after which, the primary occurrence region of PMAA drifted equatorward with time, as shown in "Figures 2, 4, and 8."