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Geotail spacecraft observations of plasma flow rotations at magnetic field dipolarization in near-tail during substorm expansion
Journal of Atmospheric and Solar-Terrestrial Physics 63 (2001) 1739–1752
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Geotail spacecraft observations of plasma *ow rotations at magnetic ,eld dipolarization in near-tail during substorm expansion D.-Y. Leea;∗ , J.H. Seonb , K.W. Mina , S.H. Baec a Physics
Department, Korea Advanced Institute of Science and Technology, 373-1 Kuesong-dong, Yuseong-ku, Taejon, 305-701, South Korea b SaTrecI, Taejon, South Korea c Radio Research Laboratory, Seoul, South Korea Received 31 October 2000; received in revised form 15 May 2001; accepted 6 June 2001
Abstract The magnetic ,eld dipolarization at near-tail is one of the key elements in the substorm phenomena, and it often accompanies the plasma *ow in tail in some pattern. In this paper, we present 4 events of dipolarization observed from the Geotail satellite, in which the plasma *ow rotates in X –Y plane. We found them out of 30 events, identi,ed for 8 months period while Geotail was in the near-tail plasma sheet region de,ned by XGSE ¿ − 15RE and |YGSE | ¡ 10RE . In some cases of those 4 events we ,nd *ow rotations that begin nearly simultaneously with the dipolarization initiation and continues afterward for some time. In the other cases, they even precede the initiation of dipolarization by some minutes and continue throughout the maximum interval of the remaining dipolarization period. These *ow rotations reveal no features of bursty bulk *ows (BBF) and the speed of *ow components is moderate, less than ∼300 km=s. Also, *ow rotations presented here are distinguished from the previously reported ones which appear only after, and thus presumably caused by, the passage of the earthward BBF (thus seen only well after the initiation of the dipolarization). Overall, we ,nd diAculties and inconsistencies in relating the observed *ow rotations at dipolarization to BBF as a possible cause. Based on intuitive ground, instead we suggest an alternative, though c 2001 tentative, possibility that they may be a manifestation of some near-tail instability during the substorm expansion. Published by Elsevier Science Ltd. Keywords: Substorm; Magnetotail; Plasma sheet; Magnetosphere; MHD instabilities
1. Introduction The magnetic ,eld dipolarization is an essential element in the magnetospheric substorm event. The magnetic ,eld at near-earth tail becomes stretched much like a tail during the substorm growth phase. Then at the expansion phase, it returns to a more dipolar-like ,eld normally within a very short time interval often with large amplitude *uctuations. The issue of what causes the dipolarization process and its ∗
Corresponding author. Fax: +82-42-869-5525. E-mail address: [email protected] (D.-Y. Lee).
subsequent current wedge system is an active area of present day research. While there have been several proposed scenarios, the dipolarization process in many of them often involves the issue of the plasma *ow in one way or another. Most notably, the near-earth neutral line model (e.g., Baker et al., 1996) suggests that the dipolarization and substorm current wedge would be formed as the earthward *ow of the closed *ux tubes from the midtail reconnection site arrives at the inner edge of the plasma sheet. In fact, Geotail satellite observations, by Baumjohann et al. (1999) and Machida et al. (1999), for example, often indicate fast
c 2001 Published by Elsevier Science Ltd. 1364-6826/01/$ - see front matter PII: S 1 3 6 4 - 6 8 2 6 ( 0 1 ) 0 0 0 5 0 - 5
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earthward *ows in the near tail of X ¿ (−20 ∼ −25)RE , and tailward *ows for X ¡ (−20 ∼ −25)RE , prior to the substorm onset time. Many of such earthward *ows have been identi,ed as the so-called bursty bulk *ows (BBF) accompanying the dipolarization (Angelopoulos et al., 1996; Sergeev et al., 2000, and references therein). Furthermore, several researchers (Birn et al., 1999, and references therein) have suggested that this earthward-directed BBF and its associated braking at near-tail could be responsible for the dipolarization and buildup of current wedge system. It is, however, also true from other observational and theoretical reports that the dipolarization in many cases is not necessarily always related to the earthward BBF events. For example, recently, Lui et al. (1998, 1999) based on both case and statistical studies report the substorm dipolarization events that occur with tailward *ows or even without significant *ows, namely, the dipolarization-associated *ows that are not necessarily “bursty” or fast, and not even necessarily earthward, unlike in BBF scenario. Indeed, various types of *ows at near-tail can be generated as a consequence of some type of internal instabilities such as ballooning instability or Kelvin–Helmholtz instability (Hurricane et al., 1999; Roux et al., 1991; Voronkov et al., 1997), which are not necessarily related to the magnetic reconnection at midtail. They constitute another substorm scenario, for example, the current disruption model (Lui, 1996), distinguished from the near-earth neutral line model. Therefore, in order to resolve the controversial issues among diKerent scenarios, it is important to understand the *ow patterns in association with the dipolarization at near-tail. It is the main purpose of this paper to investigate the characteristics of the *ow patterns and the associated dipolarization in the near-earth nightside region using the observed data, and to place the events into the observational context. Most of the previous works tend to focus on the issue of whether the *ow direction is earthward or tailward. Other possible situations like rotating or vortical motions which necessarily involve the dawn–dusk-directed *ow components have been given relatively less attention. Here, in this paper, we are interested in one such situation, i.e., the plasma *ow rotation in association with the magnetic ,eld dipolarization in the near-earth inner plasma sheet. We report the observations of the tight temporal relationship between the two processes. We note that there are previous reports on the magnetospheric *ow rotations or vortices observed by ISEE spacecraft (Hones et al., 1981; Saunders et al., 1983a,b) and by Geotail spacecraft observations (Saito et al., 1994). However, they are not directly associated with the substorm dynamics. Also, another Geotail observation revealed vortical *ows in association with substorm dipolarization, and they were considered to be direct by-products resulting from fast earthward *ows (Fair,eld et al., 1998; Nagai et al., 2000). We will show in the following sections that our observations on *ow rotation here are different from those in the previous reports. We ,rst present
the observations in Section 2, and provide discussions in Section 3. 2. Observations We searched for substorm expansion events by examining the ground magnetic data for the intervals, 07=1995 – 09=1995, 06=1996 – 08=1996, and 05=1997– 06=1997. In this search for 8 months interval, however we have strictly limited our scan only to the speci,c intervals when the Geotail spacecraft was located in the near-earth tail region de,ned by XGSE ¿ − 15RE and |YGSE | ¡ 10RE , and well within the plasma sheet identi,ed by checking the plasma density, beta (), and magnetic ,eld Bx values. We used the MGF magnetic ,eld and LEP plasma data of Geotail in the Geocentric Solar Ecliptic coordinates, at 3 and 12 s time resolutions, respectively (Kokubun et al., 1994; Mukai et al., 1994). We identi,ed 30 events of magnetic ,eld dipolarization at near-tail plasma sheet during substorm expansion. Then, for these 30 events, we examined and classi,ed the plasma *ow pattern by interpreting the data from the Geotail LEP experiment. First, for 6 events out of 30, there existed serious contaminations in the plasma data due to energetic particles beyond the LEP energy limit (∼40 keV). For these cases, no de,nite classi,cation of *ows for the purpose of our study could be made. For other 12 events out of 30, the dipolarizations were found to be timely correlated with BBF in the conventional way as reported previously (e.g., Fair,eld et al., 1998). In this classi,cation of BBF here, we set the conditions of |Vx | ¿ 400 km=s and being earthward as BBF de,nition. For another 4 events out of 30, *ows were found to be rotating at and around magnetic dipolarization process, and this is the main, new feature that we will describe in detail, in this paper. Lastly, we found the remaining 8 events out of 30 to be neither BBF nor rotating types. These correspond to very weak or even no *ow, tailward *ow, and other irregular types of *ow at and around the dipolarization. In our statistical search, the BBF events are therefore most frequent (40%). The prevailing view with the BBF these days is that it can support the reconnection scenario for the substorm via the *ow-braking scenario (Birn et al., 1999). On the other hand, Lui et al. (1999) recently found an event which is inconsistent with possible variations of fast *ow braking scenario from reconnection. They suggest that their observations can be more convincingly explained by the current disruption scenario. In line with Lui et al.’s suggestion, the 8 events of our statistical search above, which exhibit neither BBF nor rotations may likely ,t in the current disruption scenario. In the present paper, on the other hand, we pay attention to the 4 events characterized by *ow rotations for which we ,nd remarkable temporal correlation with dipolarization, but with no apparent detection of BBF. In the following, we present details of those 4 events: One event on June 18, 1997, two events on Sept. 2, 1995, another one on July 5, 1995.
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Fig. 1. Geomagnetic X -component variations from selected stations in Canadian sector on June 18, 1997.
Fig. 2. Ground stations in Canadian sector and ground footprints of Geotail at 0310 UT on June 18, 1997 (solid circle with number 1), at 1035 UT on Sept. 2, 1995 (solid circle with number 2), and 0836 UT on July 5, 1995 (solid circle with number 3), respectively.
2.1. June 18, 1997, event The geomagnetic variations at several selected ground stations in the Canadian site are displayed in Fig. 1 for an interval [00–06] UT on this day. Fig. 2 shows the locations of
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Canadian ground stations as well as the footpoints of Geotail (solid circles) obtained by Tsyganenko model (Tsyganenko, 1989): The three numbers near solid circles refer to footpoints at diKerent times and dates; number 1 representing the footpoint at 0310 UT on June 18, 1997, while the other two being the cases that will be discussed in the subsequent sections. In Fig. 1, we de,ne onset timing of this substorm to be 0310 UT when the ,rst major negative X-bay is seen at FCHU. This disturbance is not too steep, but sharp negative bays follow at ESKI and FCHU at later times. Longitudinal expansion of the magnetic bays is not signi,cant. The Geotail sector lies eastward, away from the ground onset sector, which we de,ne here as the FCHU sector. The POLAR satellite around the ground onset time was near perigee over the southern hemisphere moving fast, and the viewing angle was very slanted. Therefore, it does not seem possible to accurately determine the onset site and time from the images available (note shown here), nevertheless, the intense auroral brightening at about the time of the major ground and tail disturbances was clearly observed by the UVI experiment on POLAR. After somewhat long time delay with respect to the ground onset apparently due to slow expansion between onset sector and Geotail sector (see Fig. 2 for Geotail footpoint), Geotail at near-tail (XGSE ∼ −10RE ) observed interesting *ow features in close association with magnetic ,eld dipolarization. The data from Geotail are displayed in Fig. 3 for the interval [0300–0500] UT. Geotail was located near neutral sheet and crossed the neutral sheet at ∼0350 UT. First, the magnetic ,eld components exhibit weak-to-moderate amplitude disturbances starting at ∼0315 UT. The ,rst major dipolarization at Geotail with large *uctuations ( Bz ∼ 6–7 nT) begins at ∼0335 UT, and then the second one at ∼0353 UT. The overall *ow speed during the interval of major magnetic *uctuation (see plots (b) and (c)) is weak to moderate, its maximum being less than 250 km=s, and the *ow does not belong to a type of the bursty bulk *ow according to the de,nition by Angelopoulos et al. (1996). Also, both Vx and Vy components are oscillating, and interestingly, Vy magnitude is comparable to and in some interval even larger than Vx magnitude. Most importantly, the two time zones marked by vertical lines and horizontal arrows in (b) are characterized by rotating *ows in X –Y plane. The two major dipolarizations occur mostly within these time zones. The hodograms in Fig. 4 show the rotations in the *ow more clearly. Plot (a) in Fig. 4 corresponds to the ,rst time zone in Fig. 3. Initially, the *ow is partially rotating in the counter-clockwise sense though it does not form a complete rotation (number 1–17, the numbers representing time sequence in 12 s separation). It is noted that the ,rst dipolarization at ∼0335 UT begins nearly simultaneously with the initiation of this partial rotation of plasma. The subsequent motions from number 18 constitute a complete, still counter-clockwise, rotation, Likewise, plot (b) in Fig. 4 corresponds to the second time zone in Fig. 3. The *ow patterns in this zone
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Fig. 3. The magnetic and electric ,eld, and plasma data from Geotail on June 18, 1997. In (c), V perp x (dotted line) is the x-component of the *ow perpendicular to the magnetic ,eld. Notice the shorter scale in vertical axis in (c) than in (b), for clarity. In (e), Ex (upper dotted line) and Ey (lower thick solid line) are the measured electric ,elds from the Geotail EFD experiment, while Ex VB (lower dotted line) and Ey VB (lower thin solid line) are x and y components of −(V × B), respectively.
exhibit two clockwise rotations. The ,rst one occurs from number 1–18, here 18 corresponding to 0353:38 UT is roughly the time of the ,rst Bz peak at the beginning of the second dipolarization period indicated by thick arrow
in Fig. 3(a). Namely, the dipolarization in the second time zone begins right after one *ow rotation is completed. Then the next rotation follows throughout the subsequent ,eld *uctuations. In each of the two times zones,
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Fig. 4. Hodogram in Vx –Vy space for time intervals on June 18, 1997 as indicated by vertical lines in Fig. 3. Numbers in each plot indicate the time sequences in 12 s separation, 1 representing the starting point and the biggest number denoting the end point in each plot which are also indicated by thick arrows for clarity. Notice the diKerent plot scales between (a) and (b).
roughly two rotations were observed, each one taking ∼3–4 min. During major dipolarization periods, the ion temperature Ti shows increasing tendency, while the number density N decreases slightly overall. Plot (c) in Fig. 3 shows that Vx is nearly identical to V perp x which is the x-component of the perpendicular *ow. This point and the electric ,eld data (plot (e)) from Geotail EFD experiment (Tsuruda et al., 1994) will be further discussed in Section 3. 2.2. Two events on September 2, 1995 For an interval [07–13] UT on this day, the geomagnetic X or H -component variations from several stations are dis-
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Fig. 5. Geomagnetic X - or H -component variations from selected stations in Canadian sector on Sept. 2, 1995. Hollow circle refers to onset site at 1040 UT and smaller solid circle is the Geotail location at about the same time. The data are plotted from the eastmost station (FCHU) at the top to the westmost station (BRW) at the bottom.
played in Fig. 5. The data are plotted from the eastmost station (FCHU) at the top to the westmost station (BRW) at the bottom. There is an onset of the substorm negative bays at ∼0750 UT that spreads over several stations as marked by the ,rst vertical line. However, for this, there is a data gap in Geotail measurements, and we skip this event. The other two onsets marked by next two vertical lines are of our interest here. Small *uctuations make it diAcult to determine precise onset timing and site, but it seems most likely that the next negative bay initiated near YKC at ∼0850 UT, which then spread over to other stations. Now, starting at ∼1030 UT, rather longer-term (much less steep) negative bays are seen at stations FCHU through FSIM. During these bays, those stations that lie in the postmidnight sector, and
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the interplanetary magnetic ,eld (IMF) Bz remain southward as observed by WIND spacecraft at XGSE ∼103RE (not shown here). The negative bays at these stations are likely due to the ionospheric Hall current *owing opposite to the convection driven by the reconnected IMF. On the other hand, the steep negative bays starting at ∼1040 UT at BRW (hollow circle in Fig. 5), being followed by a weaker one at FYU, seem to be the substorm electrojet-induced bays, for which the corresponding near-tail disturbances were observed by Geotail. Around this time, Geotail’s footpoint was
located between FSIM and DAWS (closer to DAWS) longitudinally as shown by a solid circle with number 2 in Fig. 2 and solid circle in Fig. 5. The spacecraft was therefore located eastward from the sector of BRW which we assume to be the substorm onset sector. Corresponding to the latter two onsets (at 0850 UT and 1040 UT, respectively) at ground, there occurred two major events at the Geotail location (XGSE ∼ −11:6 ∼ −12:7RE ). Both events at Geotail reveal the typical dipolarization, and exhibit increasing (decreasing) tendency of ion temperature
Fig. 6. The magnetic ,eld and plasma data from Geotail for the period [0830 –1130] UT on Sept. 2, 1995. In (d), V perp x (dotted line) is the x-component of the *ow perpendicular to the magnetic ,eld.
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when the Bx magnitude overall starts to decrease with some *uctuations although the changes in Bz are much smaller. Until this point, the *ow Vx shows mostly oscillations between earthward and tailward directions. Vy is persistently duskward, but its magnitude also exhibits overall oscillations. Then the major largest magnetic *uctuation ( Bx ∼ 15 nT) begins at ∼1040–1041 UT as indicated by the ,rst dotted line, and continues further. Most interestingly, around the time of the initiation of this largest magnetic *uctuation, the *ow pattern begins to rotate in X –Y plane. This is more clearly seen in Fig. 8 which represents the hodogram in Vx –Vy space. Plots (a) and (b) in Fig. 8 show the rotating motions for the time zone indicated by dotted vertical lines in Fig. 7, and plot (c) in Fig. 8 shows similar rotations right after that. The rotation is initially counter-clockwise (plot (a)), then becomes clockwise (plot(b)). This clockwise sense is still retained through another rotation (plot(c)); then ,nally the rotation returns to counter-clockwise sense (from 14 to 37 in plot (c)). On an average, each rotation takes 4 –5 min to complete. In brief, the key point here is that the start of dipolarization at ∼1041 UT is roughly consistent with the initiation of *ow rotation which then continues throughout the dipolarization process. Also, later in Section 3, we will discuss V perp x (Fig. 6(d)) as well as the electric ,eld (Fig. 7(c) and (d)). Similar rotating motions also exist in the ,rst event at ∼[0840 – 0930] UT in Fig. 6. Although they are not as fully evident as in the second event, Geotail observes the rotating motions for the intervals ∼[0900 – 0904] UT and ∼[0907– 0909] UT which are very much a part of the major magnetic *uctuation period. In this event, Geotail was initially somewhat away from the neutral sheet, but seems still well inside the plasma sheet region. It suggests that the *ow rotation is not necessarily always con,ned to the neutral sheet (Bx = 0) plane. 2.3. July 5, 1995, event Fig. 7. The magnetic and electric ,eld, and velocity from Geotail at [0940 –1130] UT on Sept. 2, 1995. In (c) and (d), Ex and Ey are the measured electric ,elds from the Geotail EFD experiment, while Ex VB and Ey VB are x and y components of −(V × B), respectively.
Ti (number density N ), the data of which are shown in Fig. 6. We ,rst focus on the second one in more detail. At Geotail, Vx and Vy are weak to moderate throughout the maximum interval (its maximum being less than 150 km=s), and Vz speed is also small overall. Again they do not belong to the bursty bulk *ow. In an expanded view for the interval [0940–1130] UT in Fig. 7, one can see that the magnetic ,eld initially undergoes small amplitude disturbances, but the Bx (Bz ) magnitude increases (decreases) overall, implying a ,eld line stretching prior to dipolarization initiation. The ,rst sign of dipolarization appears at ∼1025–1026 UT
The ground magnetic signatures on this day are shown in Fig. 9 for an interval [06–10] UT. The locations of ground stations are shown in Fig. 2. The geomagnetic X -component variations shown indicate two onsets of magnetic bays, one at ∼0730 UT and the other at ∼0828 UT. The ,rst one seems to have initiated near FCHU and GILL, then somewhat spreads westward. However, the disturbance is overall not very strong compared to the second one. This ,rst event seems to resemble a pseudo-breakup. The corresponding disturbance at Geotail was observed about 2 min later. As the tail *ow pattern for this case is a BBF, we defer further discussion on this event to Section 3. The second bays at ∼0828 UT also initiated near FCHU and GILL (as indicated by hollow circles in Fig. 9), and propagated longitudinally westward. The Geotail footpoint was near FSMI at this onset time (see solid circle with number 3 in Fig. 2 and solid circle in Fig. 9), and observed tail disturbance with much time delay of ∼8 min, main *uctuation beginning at
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Fig. 9. Geomagnetic X -component variations from selected stations in Canadian sector on July 5, 1995. Hollow circles refer to onset sites at ∼0828 UT and smaller solid circle is the Geotail location at around the same time.
Fig. 8. Hodogram in Vx –Vy space for time intervals on Sept. 2, 1995. Plots (a) and (b) correspond to time interval indicated by vertical lines in Fig. 7, and plot (c) represents an interval right after that.
∼0836 UT as discussed below. The spacecraft location in tail seems to correspond to the upward ,eld-aligned current sector of the conventional current wedge as we checked By
at Geotail and the Y -component variations at ground stations. Also, the center of the current wedge is likely located in between FCHU and RABB, but closer to FCHU. The Geotail must have been westward from the onset sector. For the second event, the Geotail data is displayed in Fig. 10. In the ,gure V perp x (plot (c)) and the electric ,eld (plots (e) and (f)) will be discussed later in Section 3. From plot (b), for the initial 8:5 min until ∼0833 UT, the plasma *ow has an oscillating feature between earthward and tailward directions. This Vx speed is moderate and the oscillation period is slightly over 2 min on average. The *ow also has a dawn–dusk component Vy with its maximum speed of 113 km=s and exhibits a similar oscillation. However, Vy is always duskward in this time interval. The magnetic ,eld in this interval exhibits only weak variations: B ∼ 1–4 nT. Next to this interval, starting at 0833:48 UT until 0836:40 UT (the interval I in Fig. 10), a clear rotation of plasma in Vx Vy -space is observed as shown in plot (a) of Fig. 11. One complete clockwise rotation is executed for about 2:5 min with its amplitude of ∼50–114 km=s. The magnetic ,eld variations are still weak, with ∼3–4 nT amplitude, in this interval.
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Fig. 10. The magnetic and electric ,eld, and plasma data from Geotail for a period on July 5, 1995. In (c), V perp x is the x-component of the perpendicular *ow. Notice the shorter scale in vertical axis in (c) than in (b) for clarity. In (e) and (f ), Ex and Ey are the measured electric ,elds from the Geotail EFD experiment, while Ex VB and Ey VB are x and y components of −(V × B), respectively.
The major *uctuation in the magnetic ,eld starts at ∼0837 UT. The *uctuation continues during the interval II, which basically leads to the dipolarization. Bx varies between −20 and +1:4 nT, and Bz value reaches up to ∼33 nT in its maximum with a very short interval of 2:8 nT in its minimum. The most remarkable point to note here is that this major ,eld *uctuation begins roughly at the time when the plasma *ow completes the ,rst rotation in the interval I, namely, at ∼0836–0837 UT.
Then the rotation continues further with larger amplitudes as the dipolarization proceeds in the interval II. Two more clockwise rotations are done for ∼4:5 min as shown in Fig. 11(b): One from 1 to ∼15, and another smaller one from 16 to 22. The *ow speed is higher but still moderate, ranging −290 km=s (in dawnward direction) to 310 km=s (in earthward direction). The *ow rotations still persist to exist even after interval II, though with smaller speeds, as shown in Fig. 11(c): One in counter-clockwise direction
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During the intervals of these *ow rotations, there are some tendencies of slightly decreasing particle number density N and increasing ion temperature Ti (plot (d) in Fig. 10), although they are not unambiguously de,ned. 3. Summary and discussions
Fig. 11. Hodogram in Vx –Vy space for time intervals on July 5, 1995. Plots (a) and (b) correspond to the time intervals I and II, respectively, in Fig. 10, and (c) represents an interval right after that.
(1–8), then another one in clockwise direction, average rotation period being ∼3 min.
To summarize our observations, we have presented several events from Geotail observations, where the plasma *ow in near-earth tail is rotating in association with magnetic dipolarization. In some cases presented here (the ,rst dipolarization in June 18, 1997 event, and Sept. 2, 1995 events), the *ow rotation begins to occur almost simultaneously with the initiation of the magnetic ,eld dipolarization, then it continues further with the entire dipolarization process. In other cases (the second dipolarization in June 18, 1997 event, and the July 5, 1995 event), it even precedes the dipolarization onset timing. Namely, the *ow rotation appears just prior to the dipolarization initiation, then it further continues as the dipolarization proceeds. In all events of both cases, rotations have typical periods of 2–5 min on average, and 2–3 rotations are normally done throughout the entire dipolarization interval in either counter-clockwise or clockwise sense. De,nitely, such *ow rotations do not belong to the conventional BBF type. Also, while BBF, by its de,nition, refers to high-speed *ows, our rotating *ow speed is mostly weak to moderate, |Vx | ¡ ∼300 km=s. The *ow rotations for all events presented here were seen away from the ground onset sector determined by ground magnetic signatures. Thus, they are most likely a manifestation of longitudinally expanded stage of initial substorm-triggering process at the onset sector, or may be a separate one of multiple activities occurring at diKerent longitudinal sectors. We should remark that the *ow rotations in this work are presented in X –Y plane rather than in the “perpendicular plane” (perpendicular to the magnetic ,eld). However, the *ow rotations are measured near or not much away from the neutral sheet, and as shown in Figs. 3(c), 6(d), and 10(c), Vx is nearly same as V perp x which is the x-component of the perpendicular *ow. Therefore, the X –Y plane should not be very diKerent from the perpendicular plane. In order to check if the *ow rotation observed is a strict E × B *ow based on MHD frozen-in condition, we looked at the measured electric ,eld in comparison with the values of −(V × B) (see Figs. 3(e), 7(c), (d), 10(e) and (f)): The oKset values of a few mV=m in Ex need to be taken into account, in particular, in the events of June 18, 1997 and July 5, 1995. Overall, the agreement in phase between the measured electric ,eld and the expected values from −(V×B) is reasonably good, but the deviations in detail are not negligible at the same time. Therefore, we expect that the (rotating) *ow deviates to some extent from the perfect E × B *ow due to slippage between plasma and magnetic ,eld (nevertheless, the magnetic ,eld should still be twisted by some amount as the *ow rotates).
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What caused this *ow rotation in close temporal association with dipolarization is an interesting question to consider. In the following, we will discuss two possible causes of the observed *ow rotations. First, we consider a possibility of connection between the observed *ow rotations and a BBF. One natural expectation is that the Kelvin–Helmholtz instability may be excited due to the *ow shear at the sides of BBF, which may then generate *ow rotations aside the BBF. In such a case, the Geotail satellite adjacent to BBF might have a chance to observe such a rotation without observing direct appearance of BBF. However, there are some diAculties in applying this view at least to our observed events. The earthward BBF would generate the clockwise (counter-clockwise) rotating *ows at the east (west) side of the BBF, although the sense of the rotation could become reversed (through a winding-then-unwinding process) by restoring the tendency of the twisted ,eld lines after one or more rotations were done. However, in all our events, initial rotating senses are opposite to the ones expected from the assumption of an adjacent BBF passage provided that the BBF would be in the ground onset sector: For example, in the July 5, 1995, event, the Geotail was westward from the ground onset sector, but it observed clockwise rotations initially. Besides, Lui et al. (1999) recently suggested that the clockwise (counter-clockwise) *ow vortices at the east (west) side of the BBF would generate the ,eld-aligned current system of region II type, being inconsistent with the conventional current wedge system (region I type). This suggestion was further supported by a more recent three-dimensional particle simulation by Pritchett and Coroniti (2000) which is in contrast to the earlier MHD result by Birn et al. (1999), where the consistency between the *ow braking and the region I type current wedge had been found. The BBF could generate *ow rotations or vortices not only beside it, but also behind it. In fact, the previously observed *ow rotations or vortices in direct relation with the BBF events reported by Fair,eld et al. (1998) and Nagai et al. (2000), for example, are those generated behind the BBF passage. We emphasize that our present rotating *ows at and around the dipolarization initiation are diKerent from such previous ,ndings. In those previous reports, the appearance of earthward BBF roughly coincides with the dipolarization initiation, but the *ow rotation appears only well after the magnetic ,eld is already dipolarized considerably. Below, we give such an example, selected out of our 12 BBF events. Fig. 12 displays the data of that example, the event at [0730-0750] UT on July 5, 1995. This event corresponds to the ground activity initiated at ∼0730 UT that was shown earlier in Fig. 9. In Fig. 12, at ∼0732 UT, the high-speed earthward *ow with a maximum of 484 km=s is observed which lasts for 6 6 min until ∼0738 UT. The dipolarizing magnetic ,eld *uctuation is accompanied by initiating the high-speed *ow appearance. Then *ow rotation starts to appear at ∼0739 UT continuing until ∼0746 UT as indicated by vertical lines in Fig. 12. The hodogram for that interval
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is shown in Fig. 13, displaying ,rst counter-clockwise, then clockwise rotations over ∼7 min, while the magnetic ,eld continues its *uctuation. This is well after the major dipolarization has already proceeded signi,cantly with bursty bulk *ows. This event contrasts with those presented in Section 2, where the *ow rotations appear nearly simultaneously with or even somewhat prior to the ,eld dipolarization initiation and continue throughout the dipolarization, where no BBF type *ow is seen. Considering the diAculties and inconsistencies in relating our observed *ow rotations (all those presented in Section 2) to the earthward BBF, we are motivated to seek an alternative possibility that the observed *ow rotations at dipolarization may be caused by some near-tail instability developed at and near the observation point, being irrelevant to BBF. One candidate for generation of local *ow rotations or vortices is perhaps the Kelvin–Helmholtz (KH) instability which would grow at the presence of suAcient *ow shear. Besides, many previous works on KH in the general literature, and a recent simulation by Voronkov et al. (1997) showed that KH (and its coupled mode with Rayleigh–Taylor instability) became unstable and the instability generated vortices in the nightside magnetospheric con,guration when the *ow shear of the type vy = vy (x) was present. The *ow shear is a prerequisite for *ow rotation generation through KH instability. Such a *ow shear could exist in equilibrium state from the ,rst, or it may be even generated as a consequence of another instability such as the ballooning instability. There are a number of papers in space literature that suggest the excitation of linear ballooning instability in the magnetotail (For example, see Lee, 1999 and references therein). Here, we further expect that the nonlinear growth of the ballooning could provide either the vortex directly or the sheared *ow which may in turn drive a KH mode unstable in order to generate a vortex. We illustrate this possibility on intuitive ground using cartoons in Fig. 14. First, the initial ballooning mode may look like Fig. 14(a) in the equatorial plane. If it can grow nonlinearly, then some possible patterns of nonlinearly-growing *ows may be something like the sketches (b) – (d) of Fig. 14. Cartoon (b) shows two oppositely growing ,nger-shaped *ows, while more than two ,ngers of growing *ows are sketched in (c). In these cases, the *ow shear between ,nger-shaped *ows could excite the KH instability that would accompany the *ow vortex. Hurricane et al.’s (1999) nonlinear calculation shows a ,nger-shaped *ow growth in earth-tail direction, which is similar to Fig. 14(a). Now the cartoon (d), on the other hand, shows a vortex that may be generated directly at the nonlinear stage of the ballooning itself or as a further consequence (through a coupling to KH instability) of the ballooning *ow shear of types (b) and (c). It should be appreciated that the generation of *ow vortex by Rayleigh–Taylor or ballooning instability and the coupling between KH and ballooning instabilities have long been recognized in some previous works (e.g., Drake et al., 1992; Finn, 1993). It will be worthwhile to perform
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Fig. 12. The magnetic ,eld and plasma data from Geotail for the period [07– 08] UT on July 5, 1995.
in future the nonlinear ballooning mode simulation for the substorm-related plasma sheet con,guration. There are at least two limitations in the present work. As was already mentioned earlier in this section, the observed *ow rotations and magnetic dipolarizations were seen away from the ground onset sector. Thus, although we suggested the near-tail instability as a responsible mechanism for the generation of the *ow rotation above, it is not clear from the present data set whether the same instability was also activated at the ground onset sector to give similar *ow ro-
tations. Also, the fact that the *ow rotation appears around the dipolarization initiation brings up the question of causality between *ow rotation and dipolarization. Unfortunately, it is uncertain from the present data set whether the *ow rotation is a cause of the dipolarization or a consequence of it. As ,nal remarks, we do not preclude the possibility that the braking of BBF at near-tail could cause ballooning-type modes which propagate away from the BBF. This may generate the *ow vortex if the mode is unstable. However, to the authors’ best knowledge, no theoretical stability analysis
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Fig. 13. Hodogram in Vx –Vy space for the time interval on July 5, 1995, corresponding to the interval indicated by vertical lines in Fig. 12.
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distance during the observed rotation period, on the other hand.) Another interesting point to consider in a future work is the relationship between the observed *ow pattern and the Doppler-shift eKect from the travelling ballooning mode at the ion-diamagnetic drift speed. In conclusion, we have reported 4 events, where the near-tail *ow rotation occurs at and around the magnetic ,eld dipolarization during substorm expansion. We ,nd it diAcult to relate the observed rotations to BBF as a possible cause. Instead, we have suggested, based on intuitive ground, that they could be a manifestation of some near-tail instability development such as Kelvin–Helmholtz and ballooning instabilities during the substorm expansion. The computational veri,cation of the suggestion will form our future project. Statistically, the 4 *ow rotation events were seen out of the total 30 events of dipolarization that we have identi,ed for 8 months period at near-tail. For other 12 out of 30 events, the dipolarizations are associated with the standard BBF type *ows. Another 8 events out of 30 are found to be neither BBF nor rotating types. Acknowledgements
Fig. 14. Sketch of the ballooning evolution in the equatorial plane: (a) initial wave, (b) – (d) possible nonlinear developments.
has been done for this special situation where the near-tail is locally compressed by the braking of the narrow (a few RE in Y extent) *ow channel. Also, more sophisticated work is required to check the possibility of travelling vortex. This might aKect the determination of the rotating sense, if it travels fast, depending on how it travels across the satellite. (The Geotail is found to transverse over quite short
Geotail magnetic ,eld and plasma data were provided by S. Kokubun and T. Mukai through DARTS at the Institute of Space and Astronautical Science in Japan, and the Geotail electric ,eld data were provided by H. Hayakawa. D.-Y. Lee is grateful to T. Mukai for helpful comments on the Geotail data. The CANOPUS data were kindly provided by T. Hughes. The CANOPUS instrument array was constructed and is maintained and operated by the Canadian Space Agency for the Canadian scienti,c community. We are grateful to L. Newitt of Geological Survey of Canada for supplying the data at Canadian INTERMAGNET magnetic obervatories. Data at some stations in Canadian sector were also obtained from NOAA National Geophysical Data Center. We also thank G.K. Parks and M. Fillingim for the POLAR UVI images of June 18, 1997, event. We are grateful to both referees for their interesting suggestions, in particular, the comments on the possible ballooning excitation by BBF braking and on the relationship between the *ow pattern and Doppler-shift eKect from the travelling ballooning mode. The work at Korea Advanced Institute of Science and Technology has been supported by the Brain Korea 21 project of Korea Ministry of Education.
References Angelopoulos, V., Coroniti, F.V., Kennel, C.F., Kivelson, M.G., Walker, R.J., Russell, C.T., McPherron, R.L., Sanchez, E., Meng, C.-I., Baumjohann, W., Reeves, G.D., Belian, R.D., Sato, N., Friis-Christensen, E., SutcliKe, P.R., Yumoto, K., Harris, T., 1996. Multipoint analysis of a bursty bulk *ow event on April 11, 1985. Journal of Geophysical Research A101, 4967.
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Baker, D.A., Pulkkinen, T.I., Angelopoulos, V., Baumjohann, W., McPherron, R.L., 1996. The neutral line model of substorms: past results and present view. Journal of Geophysical Research A101, 12,975. Baumjohann, W., Hesse, M., Kokubun, S., Mukai, T., Nagai, T., Petrukovich, A.A., 1999. Substorm dipolarization and recovery. Journal of Geophysical Research A104, 24,995. Birn, J., Hesse, M., Haerendel, G., Baumjohann, W., Shiokawa, K., 1999. Flow braking and the substorm current wedge. Journal of Geophysical Research A104, 19,895. Drake, J.F., Finn, J.M., Guzdar, P., Shapiro, V., Shevchenko, V., Waelbroek, F., Hassam, A.B., Liu, C.S., Sagdeev, R., 1992. Peeling of convection cells and generation of sheared *ow. Physics of Fluids B4, 488. Fair,eld, D.H., Mukai, T., Lui, A.T.Y., Cattell, C.A., Reeves, G.D., Nagai, T., Rostoker, G., Singer, H.J., Kaiser, M.L., Kokubun, S., Lazarus, A.J., Lepping, R.P., Nakamura, M., Steinberg, J.T., Tsuruda, K., Williams, D.J., Yamamoto, T., 1998. Geotail observations of substorm onset in the inner magnetotail. Journal of Geophysical Research A103, 103. Finn, J.M., 1993. Nonlinear interaction of Rayleigh–Taylor and shear instabilities. Physics of Fluids B5, 415. Hones Jr., E.W., Birn, J., Bame, S.J., Asbridge, J.R., Paschmann, G., Scopke, N., Haerendel, G., 1981. Further determination of the characteristics of magnetospheric plasma vortices with ISEE 1 and 2. Journal of Geophysical Research A86, 814. Hurricane, O.A., Fong, B.H., Cowley, S.C., Coroniti, F.V., Kennel, C.F., Pellat, R., 1999. Substorm detonation. Journal of Geophysical Research A104, 10,221. Kokubun, S., Yamamoto, T., Acuna, M.H., Hayashi, K., Shiyokawa, K., Kawano, H., 1994. The GEOTAIL magnetic ,eld experiment. Journal of Geomagnetism Geoelectricity 46, 7. Lee, D.-Y., 1999. EKect of plasma compression on plasma sheet stability. Geophysical Research Letters 26, 2705. Lui, A.T.Y., 1996. Current disruption in the Earth’s magnetosphere: observations and models. Journal of Geophysical Research A101, 13,067. Lui, A.T.Y., Liou, K., Newell, P.T., Meng, C.-I., Ohtani, S.-I., Ogino, T., Kokubun, S., Brittnacher, Parks, G.K., 1998. Plasma and magnetic *ux transport associated with auroral breakups. Geophysical Research Letters 25, 4059. Lui, A.T.Y., Liou, K., Nose, M., Ohtani, S., Williams, D.J., Mukai, T., Tsurda, K., Kokubun, S., 1999. Near-Earth dipolarization: evidence for a non-MHD process. Geophysical Research Letters 26, 2905.
Machida, S., Miyashita, Y., Ieda, A., Nishida, A., Mukai, T., Saito, Y., Kokubun, S., 1999. Geotail observations of *ow velocity and north-south magnetic ,eld variations in the near and mid-distance tail associated with substorm onsets. Geophysical Research Letters 26, 635. Mukai, T., Machida, S., Saito, Y., Hirahara, M., Terasawa, T., Kaya, N., Obara, T., Ejiri, M., Nishida, A., 1994. The low energy particle (LEP) experiment onboard the GEOTAIL satellite. Journal of Geomagnetism and Geoelectricity 46, 669. Nagai, T., Singer, H.J., Mukai, T., Yamamoto, T., Kokubun, S., 2000. Development of substorms in the near-earth tail. Advances in Space Research 7=8, 1651. Pritchett, P.L., Coroniti, F.V., 2000. Localized convection *ows and ,eld-aligned current generation in a kinetic model of the near-earth plasma sheet. Geophysical Research Letters 27, 3161. Roux, A., Perraut, S., Robert, P., Morane, A., Pedersen, A., Korth, A., Kremser, G., Aparicio, B., Rodgers, D., Pellinen, R., 1991. Plasma sheet instability related to the westward traveling surge. Journal of Geophysical Research A96, 17,697. Saito, Y., Mukai, T., Machida, S., Hirahara, M., Nishida, A., Terasawa, T., 1994. Rapid variations of the plasma bulk *ow direction observed in the plasma sheet at XGSE ∼ −60RE . Geophysical Research Letters 21, 393. Saunders, M.A., Southwood, D.J., Fritz, T.A., Hones Jr., E.W., 1983a. Hydromagnetic vortices—1. The 11 December 1997 event. Planetary and Space Sciences 31, 1,009. Saunders, M.A., Southwood, D.J., Hones Jr., E.W., 1983b. Hydromagnetic vortices—2. Further dawnside events. Planetary and Space Sciences 31, 1,117. Sergeev, V.A., Sauvaud, J.-A., Popescu, D., Kovrazhkin, R.A., Liou, K., Newell, P.T., Brittnacher, M., Parks, G., Nakamura, R., Mukai, T., Reeves, G.D., 2000. Multiple-spacecraft observation of a narrow transient plasma jet in the earth’s plasma sheet. Geophysical Research Letters 27, 851. Tsuruda, K., Hayakawa, H., Nakamura, M., Okada, T., Matsuoka, A., Mozer, F.S., Schmidt, R., 1994. Electric ,eld measurements on the Geotail satellite. Journal of Geomagnetism and Geoelectricity 46, 693. Tsyganenko, N.A., 1989. A magnetospheric magnetic ,eld model with a warped tail current sheet. Planetary and Space Sciences 37, 5. Voronkov, I., Rankin, R., Frycz, P., Tikhonchuk, V.T., Samson, J.C., 1997. Coupling of shear *ow and pressure gradient instabilities. Journal of Geophysical Research A102, 9639.
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Geotail spacecraft observations of plasma *ow rotations at magnetic ,eld dipolarization in near-tail during substorm expansion D.-Y. Leea;∗ , J.H. Seonb , K.W. Mina , S.H. Baec a Physics
Department, Korea Advanced Institute of Science and Technology, 373-1 Kuesong-dong, Yuseong-ku, Taejon, 305-701, South Korea b SaTrecI, Taejon, South Korea c Radio Research Laboratory, Seoul, South Korea Received 31 October 2000; received in revised form 15 May 2001; accepted 6 June 2001
Abstract The magnetic ,eld dipolarization at near-tail is one of the key elements in the substorm phenomena, and it often accompanies the plasma *ow in tail in some pattern. In this paper, we present 4 events of dipolarization observed from the Geotail satellite, in which the plasma *ow rotates in X –Y plane. We found them out of 30 events, identi,ed for 8 months period while Geotail was in the near-tail plasma sheet region de,ned by XGSE ¿ − 15RE and |YGSE | ¡ 10RE . In some cases of those 4 events we ,nd *ow rotations that begin nearly simultaneously with the dipolarization initiation and continues afterward for some time. In the other cases, they even precede the initiation of dipolarization by some minutes and continue throughout the maximum interval of the remaining dipolarization period. These *ow rotations reveal no features of bursty bulk *ows (BBF) and the speed of *ow components is moderate, less than ∼300 km=s. Also, *ow rotations presented here are distinguished from the previously reported ones which appear only after, and thus presumably caused by, the passage of the earthward BBF (thus seen only well after the initiation of the dipolarization). Overall, we ,nd diAculties and inconsistencies in relating the observed *ow rotations at dipolarization to BBF as a possible cause. Based on intuitive ground, instead we suggest an alternative, though c 2001 tentative, possibility that they may be a manifestation of some near-tail instability during the substorm expansion. Published by Elsevier Science Ltd. Keywords: Substorm; Magnetotail; Plasma sheet; Magnetosphere; MHD instabilities
1. Introduction The magnetic ,eld dipolarization is an essential element in the magnetospheric substorm event. The magnetic ,eld at near-earth tail becomes stretched much like a tail during the substorm growth phase. Then at the expansion phase, it returns to a more dipolar-like ,eld normally within a very short time interval often with large amplitude *uctuations. The issue of what causes the dipolarization process and its ∗
Corresponding author. Fax: +82-42-869-5525. E-mail address: [email protected] (D.-Y. Lee).
subsequent current wedge system is an active area of present day research. While there have been several proposed scenarios, the dipolarization process in many of them often involves the issue of the plasma *ow in one way or another. Most notably, the near-earth neutral line model (e.g., Baker et al., 1996) suggests that the dipolarization and substorm current wedge would be formed as the earthward *ow of the closed *ux tubes from the midtail reconnection site arrives at the inner edge of the plasma sheet. In fact, Geotail satellite observations, by Baumjohann et al. (1999) and Machida et al. (1999), for example, often indicate fast
c 2001 Published by Elsevier Science Ltd. 1364-6826/01/$ - see front matter PII: S 1 3 6 4 - 6 8 2 6 ( 0 1 ) 0 0 0 5 0 - 5
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earthward *ows in the near tail of X ¿ (−20 ∼ −25)RE , and tailward *ows for X ¡ (−20 ∼ −25)RE , prior to the substorm onset time. Many of such earthward *ows have been identi,ed as the so-called bursty bulk *ows (BBF) accompanying the dipolarization (Angelopoulos et al., 1996; Sergeev et al., 2000, and references therein). Furthermore, several researchers (Birn et al., 1999, and references therein) have suggested that this earthward-directed BBF and its associated braking at near-tail could be responsible for the dipolarization and buildup of current wedge system. It is, however, also true from other observational and theoretical reports that the dipolarization in many cases is not necessarily always related to the earthward BBF events. For example, recently, Lui et al. (1998, 1999) based on both case and statistical studies report the substorm dipolarization events that occur with tailward *ows or even without significant *ows, namely, the dipolarization-associated *ows that are not necessarily “bursty” or fast, and not even necessarily earthward, unlike in BBF scenario. Indeed, various types of *ows at near-tail can be generated as a consequence of some type of internal instabilities such as ballooning instability or Kelvin–Helmholtz instability (Hurricane et al., 1999; Roux et al., 1991; Voronkov et al., 1997), which are not necessarily related to the magnetic reconnection at midtail. They constitute another substorm scenario, for example, the current disruption model (Lui, 1996), distinguished from the near-earth neutral line model. Therefore, in order to resolve the controversial issues among diKerent scenarios, it is important to understand the *ow patterns in association with the dipolarization at near-tail. It is the main purpose of this paper to investigate the characteristics of the *ow patterns and the associated dipolarization in the near-earth nightside region using the observed data, and to place the events into the observational context. Most of the previous works tend to focus on the issue of whether the *ow direction is earthward or tailward. Other possible situations like rotating or vortical motions which necessarily involve the dawn–dusk-directed *ow components have been given relatively less attention. Here, in this paper, we are interested in one such situation, i.e., the plasma *ow rotation in association with the magnetic ,eld dipolarization in the near-earth inner plasma sheet. We report the observations of the tight temporal relationship between the two processes. We note that there are previous reports on the magnetospheric *ow rotations or vortices observed by ISEE spacecraft (Hones et al., 1981; Saunders et al., 1983a,b) and by Geotail spacecraft observations (Saito et al., 1994). However, they are not directly associated with the substorm dynamics. Also, another Geotail observation revealed vortical *ows in association with substorm dipolarization, and they were considered to be direct by-products resulting from fast earthward *ows (Fair,eld et al., 1998; Nagai et al., 2000). We will show in the following sections that our observations on *ow rotation here are different from those in the previous reports. We ,rst present
the observations in Section 2, and provide discussions in Section 3. 2. Observations We searched for substorm expansion events by examining the ground magnetic data for the intervals, 07=1995 – 09=1995, 06=1996 – 08=1996, and 05=1997– 06=1997. In this search for 8 months interval, however we have strictly limited our scan only to the speci,c intervals when the Geotail spacecraft was located in the near-earth tail region de,ned by XGSE ¿ − 15RE and |YGSE | ¡ 10RE , and well within the plasma sheet identi,ed by checking the plasma density, beta (), and magnetic ,eld Bx values. We used the MGF magnetic ,eld and LEP plasma data of Geotail in the Geocentric Solar Ecliptic coordinates, at 3 and 12 s time resolutions, respectively (Kokubun et al., 1994; Mukai et al., 1994). We identi,ed 30 events of magnetic ,eld dipolarization at near-tail plasma sheet during substorm expansion. Then, for these 30 events, we examined and classi,ed the plasma *ow pattern by interpreting the data from the Geotail LEP experiment. First, for 6 events out of 30, there existed serious contaminations in the plasma data due to energetic particles beyond the LEP energy limit (∼40 keV). For these cases, no de,nite classi,cation of *ows for the purpose of our study could be made. For other 12 events out of 30, the dipolarizations were found to be timely correlated with BBF in the conventional way as reported previously (e.g., Fair,eld et al., 1998). In this classi,cation of BBF here, we set the conditions of |Vx | ¿ 400 km=s and being earthward as BBF de,nition. For another 4 events out of 30, *ows were found to be rotating at and around magnetic dipolarization process, and this is the main, new feature that we will describe in detail, in this paper. Lastly, we found the remaining 8 events out of 30 to be neither BBF nor rotating types. These correspond to very weak or even no *ow, tailward *ow, and other irregular types of *ow at and around the dipolarization. In our statistical search, the BBF events are therefore most frequent (40%). The prevailing view with the BBF these days is that it can support the reconnection scenario for the substorm via the *ow-braking scenario (Birn et al., 1999). On the other hand, Lui et al. (1999) recently found an event which is inconsistent with possible variations of fast *ow braking scenario from reconnection. They suggest that their observations can be more convincingly explained by the current disruption scenario. In line with Lui et al.’s suggestion, the 8 events of our statistical search above, which exhibit neither BBF nor rotations may likely ,t in the current disruption scenario. In the present paper, on the other hand, we pay attention to the 4 events characterized by *ow rotations for which we ,nd remarkable temporal correlation with dipolarization, but with no apparent detection of BBF. In the following, we present details of those 4 events: One event on June 18, 1997, two events on Sept. 2, 1995, another one on July 5, 1995.
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Fig. 1. Geomagnetic X -component variations from selected stations in Canadian sector on June 18, 1997.
Fig. 2. Ground stations in Canadian sector and ground footprints of Geotail at 0310 UT on June 18, 1997 (solid circle with number 1), at 1035 UT on Sept. 2, 1995 (solid circle with number 2), and 0836 UT on July 5, 1995 (solid circle with number 3), respectively.
2.1. June 18, 1997, event The geomagnetic variations at several selected ground stations in the Canadian site are displayed in Fig. 1 for an interval [00–06] UT on this day. Fig. 2 shows the locations of
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Canadian ground stations as well as the footpoints of Geotail (solid circles) obtained by Tsyganenko model (Tsyganenko, 1989): The three numbers near solid circles refer to footpoints at diKerent times and dates; number 1 representing the footpoint at 0310 UT on June 18, 1997, while the other two being the cases that will be discussed in the subsequent sections. In Fig. 1, we de,ne onset timing of this substorm to be 0310 UT when the ,rst major negative X-bay is seen at FCHU. This disturbance is not too steep, but sharp negative bays follow at ESKI and FCHU at later times. Longitudinal expansion of the magnetic bays is not signi,cant. The Geotail sector lies eastward, away from the ground onset sector, which we de,ne here as the FCHU sector. The POLAR satellite around the ground onset time was near perigee over the southern hemisphere moving fast, and the viewing angle was very slanted. Therefore, it does not seem possible to accurately determine the onset site and time from the images available (note shown here), nevertheless, the intense auroral brightening at about the time of the major ground and tail disturbances was clearly observed by the UVI experiment on POLAR. After somewhat long time delay with respect to the ground onset apparently due to slow expansion between onset sector and Geotail sector (see Fig. 2 for Geotail footpoint), Geotail at near-tail (XGSE ∼ −10RE ) observed interesting *ow features in close association with magnetic ,eld dipolarization. The data from Geotail are displayed in Fig. 3 for the interval [0300–0500] UT. Geotail was located near neutral sheet and crossed the neutral sheet at ∼0350 UT. First, the magnetic ,eld components exhibit weak-to-moderate amplitude disturbances starting at ∼0315 UT. The ,rst major dipolarization at Geotail with large *uctuations ( Bz ∼ 6–7 nT) begins at ∼0335 UT, and then the second one at ∼0353 UT. The overall *ow speed during the interval of major magnetic *uctuation (see plots (b) and (c)) is weak to moderate, its maximum being less than 250 km=s, and the *ow does not belong to a type of the bursty bulk *ow according to the de,nition by Angelopoulos et al. (1996). Also, both Vx and Vy components are oscillating, and interestingly, Vy magnitude is comparable to and in some interval even larger than Vx magnitude. Most importantly, the two time zones marked by vertical lines and horizontal arrows in (b) are characterized by rotating *ows in X –Y plane. The two major dipolarizations occur mostly within these time zones. The hodograms in Fig. 4 show the rotations in the *ow more clearly. Plot (a) in Fig. 4 corresponds to the ,rst time zone in Fig. 3. Initially, the *ow is partially rotating in the counter-clockwise sense though it does not form a complete rotation (number 1–17, the numbers representing time sequence in 12 s separation). It is noted that the ,rst dipolarization at ∼0335 UT begins nearly simultaneously with the initiation of this partial rotation of plasma. The subsequent motions from number 18 constitute a complete, still counter-clockwise, rotation, Likewise, plot (b) in Fig. 4 corresponds to the second time zone in Fig. 3. The *ow patterns in this zone
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Fig. 3. The magnetic and electric ,eld, and plasma data from Geotail on June 18, 1997. In (c), V perp x (dotted line) is the x-component of the *ow perpendicular to the magnetic ,eld. Notice the shorter scale in vertical axis in (c) than in (b), for clarity. In (e), Ex (upper dotted line) and Ey (lower thick solid line) are the measured electric ,elds from the Geotail EFD experiment, while Ex VB (lower dotted line) and Ey VB (lower thin solid line) are x and y components of −(V × B), respectively.
exhibit two clockwise rotations. The ,rst one occurs from number 1–18, here 18 corresponding to 0353:38 UT is roughly the time of the ,rst Bz peak at the beginning of the second dipolarization period indicated by thick arrow
in Fig. 3(a). Namely, the dipolarization in the second time zone begins right after one *ow rotation is completed. Then the next rotation follows throughout the subsequent ,eld *uctuations. In each of the two times zones,
D.-Y. Lee et al. / Journal of Atmospheric and Solar-Terrestrial Physics 63 (2001) 1739–1752
Fig. 4. Hodogram in Vx –Vy space for time intervals on June 18, 1997 as indicated by vertical lines in Fig. 3. Numbers in each plot indicate the time sequences in 12 s separation, 1 representing the starting point and the biggest number denoting the end point in each plot which are also indicated by thick arrows for clarity. Notice the diKerent plot scales between (a) and (b).
roughly two rotations were observed, each one taking ∼3–4 min. During major dipolarization periods, the ion temperature Ti shows increasing tendency, while the number density N decreases slightly overall. Plot (c) in Fig. 3 shows that Vx is nearly identical to V perp x which is the x-component of the perpendicular *ow. This point and the electric ,eld data (plot (e)) from Geotail EFD experiment (Tsuruda et al., 1994) will be further discussed in Section 3. 2.2. Two events on September 2, 1995 For an interval [07–13] UT on this day, the geomagnetic X or H -component variations from several stations are dis-
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Fig. 5. Geomagnetic X - or H -component variations from selected stations in Canadian sector on Sept. 2, 1995. Hollow circle refers to onset site at 1040 UT and smaller solid circle is the Geotail location at about the same time. The data are plotted from the eastmost station (FCHU) at the top to the westmost station (BRW) at the bottom.
played in Fig. 5. The data are plotted from the eastmost station (FCHU) at the top to the westmost station (BRW) at the bottom. There is an onset of the substorm negative bays at ∼0750 UT that spreads over several stations as marked by the ,rst vertical line. However, for this, there is a data gap in Geotail measurements, and we skip this event. The other two onsets marked by next two vertical lines are of our interest here. Small *uctuations make it diAcult to determine precise onset timing and site, but it seems most likely that the next negative bay initiated near YKC at ∼0850 UT, which then spread over to other stations. Now, starting at ∼1030 UT, rather longer-term (much less steep) negative bays are seen at stations FCHU through FSIM. During these bays, those stations that lie in the postmidnight sector, and
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the interplanetary magnetic ,eld (IMF) Bz remain southward as observed by WIND spacecraft at XGSE ∼103RE (not shown here). The negative bays at these stations are likely due to the ionospheric Hall current *owing opposite to the convection driven by the reconnected IMF. On the other hand, the steep negative bays starting at ∼1040 UT at BRW (hollow circle in Fig. 5), being followed by a weaker one at FYU, seem to be the substorm electrojet-induced bays, for which the corresponding near-tail disturbances were observed by Geotail. Around this time, Geotail’s footpoint was
located between FSIM and DAWS (closer to DAWS) longitudinally as shown by a solid circle with number 2 in Fig. 2 and solid circle in Fig. 5. The spacecraft was therefore located eastward from the sector of BRW which we assume to be the substorm onset sector. Corresponding to the latter two onsets (at 0850 UT and 1040 UT, respectively) at ground, there occurred two major events at the Geotail location (XGSE ∼ −11:6 ∼ −12:7RE ). Both events at Geotail reveal the typical dipolarization, and exhibit increasing (decreasing) tendency of ion temperature
Fig. 6. The magnetic ,eld and plasma data from Geotail for the period [0830 –1130] UT on Sept. 2, 1995. In (d), V perp x (dotted line) is the x-component of the *ow perpendicular to the magnetic ,eld.
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when the Bx magnitude overall starts to decrease with some *uctuations although the changes in Bz are much smaller. Until this point, the *ow Vx shows mostly oscillations between earthward and tailward directions. Vy is persistently duskward, but its magnitude also exhibits overall oscillations. Then the major largest magnetic *uctuation ( Bx ∼ 15 nT) begins at ∼1040–1041 UT as indicated by the ,rst dotted line, and continues further. Most interestingly, around the time of the initiation of this largest magnetic *uctuation, the *ow pattern begins to rotate in X –Y plane. This is more clearly seen in Fig. 8 which represents the hodogram in Vx –Vy space. Plots (a) and (b) in Fig. 8 show the rotating motions for the time zone indicated by dotted vertical lines in Fig. 7, and plot (c) in Fig. 8 shows similar rotations right after that. The rotation is initially counter-clockwise (plot (a)), then becomes clockwise (plot(b)). This clockwise sense is still retained through another rotation (plot(c)); then ,nally the rotation returns to counter-clockwise sense (from 14 to 37 in plot (c)). On an average, each rotation takes 4 –5 min to complete. In brief, the key point here is that the start of dipolarization at ∼1041 UT is roughly consistent with the initiation of *ow rotation which then continues throughout the dipolarization process. Also, later in Section 3, we will discuss V perp x (Fig. 6(d)) as well as the electric ,eld (Fig. 7(c) and (d)). Similar rotating motions also exist in the ,rst event at ∼[0840 – 0930] UT in Fig. 6. Although they are not as fully evident as in the second event, Geotail observes the rotating motions for the intervals ∼[0900 – 0904] UT and ∼[0907– 0909] UT which are very much a part of the major magnetic *uctuation period. In this event, Geotail was initially somewhat away from the neutral sheet, but seems still well inside the plasma sheet region. It suggests that the *ow rotation is not necessarily always con,ned to the neutral sheet (Bx = 0) plane. 2.3. July 5, 1995, event Fig. 7. The magnetic and electric ,eld, and velocity from Geotail at [0940 –1130] UT on Sept. 2, 1995. In (c) and (d), Ex and Ey are the measured electric ,elds from the Geotail EFD experiment, while Ex VB and Ey VB are x and y components of −(V × B), respectively.
Ti (number density N ), the data of which are shown in Fig. 6. We ,rst focus on the second one in more detail. At Geotail, Vx and Vy are weak to moderate throughout the maximum interval (its maximum being less than 150 km=s), and Vz speed is also small overall. Again they do not belong to the bursty bulk *ow. In an expanded view for the interval [0940–1130] UT in Fig. 7, one can see that the magnetic ,eld initially undergoes small amplitude disturbances, but the Bx (Bz ) magnitude increases (decreases) overall, implying a ,eld line stretching prior to dipolarization initiation. The ,rst sign of dipolarization appears at ∼1025–1026 UT
The ground magnetic signatures on this day are shown in Fig. 9 for an interval [06–10] UT. The locations of ground stations are shown in Fig. 2. The geomagnetic X -component variations shown indicate two onsets of magnetic bays, one at ∼0730 UT and the other at ∼0828 UT. The ,rst one seems to have initiated near FCHU and GILL, then somewhat spreads westward. However, the disturbance is overall not very strong compared to the second one. This ,rst event seems to resemble a pseudo-breakup. The corresponding disturbance at Geotail was observed about 2 min later. As the tail *ow pattern for this case is a BBF, we defer further discussion on this event to Section 3. The second bays at ∼0828 UT also initiated near FCHU and GILL (as indicated by hollow circles in Fig. 9), and propagated longitudinally westward. The Geotail footpoint was near FSMI at this onset time (see solid circle with number 3 in Fig. 2 and solid circle in Fig. 9), and observed tail disturbance with much time delay of ∼8 min, main *uctuation beginning at
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Fig. 9. Geomagnetic X -component variations from selected stations in Canadian sector on July 5, 1995. Hollow circles refer to onset sites at ∼0828 UT and smaller solid circle is the Geotail location at around the same time.
Fig. 8. Hodogram in Vx –Vy space for time intervals on Sept. 2, 1995. Plots (a) and (b) correspond to time interval indicated by vertical lines in Fig. 7, and plot (c) represents an interval right after that.
∼0836 UT as discussed below. The spacecraft location in tail seems to correspond to the upward ,eld-aligned current sector of the conventional current wedge as we checked By
at Geotail and the Y -component variations at ground stations. Also, the center of the current wedge is likely located in between FCHU and RABB, but closer to FCHU. The Geotail must have been westward from the onset sector. For the second event, the Geotail data is displayed in Fig. 10. In the ,gure V perp x (plot (c)) and the electric ,eld (plots (e) and (f)) will be discussed later in Section 3. From plot (b), for the initial 8:5 min until ∼0833 UT, the plasma *ow has an oscillating feature between earthward and tailward directions. This Vx speed is moderate and the oscillation period is slightly over 2 min on average. The *ow also has a dawn–dusk component Vy with its maximum speed of 113 km=s and exhibits a similar oscillation. However, Vy is always duskward in this time interval. The magnetic ,eld in this interval exhibits only weak variations: B ∼ 1–4 nT. Next to this interval, starting at 0833:48 UT until 0836:40 UT (the interval I in Fig. 10), a clear rotation of plasma in Vx Vy -space is observed as shown in plot (a) of Fig. 11. One complete clockwise rotation is executed for about 2:5 min with its amplitude of ∼50–114 km=s. The magnetic ,eld variations are still weak, with ∼3–4 nT amplitude, in this interval.
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Fig. 10. The magnetic and electric ,eld, and plasma data from Geotail for a period on July 5, 1995. In (c), V perp x is the x-component of the perpendicular *ow. Notice the shorter scale in vertical axis in (c) than in (b) for clarity. In (e) and (f ), Ex and Ey are the measured electric ,elds from the Geotail EFD experiment, while Ex VB and Ey VB are x and y components of −(V × B), respectively.
The major *uctuation in the magnetic ,eld starts at ∼0837 UT. The *uctuation continues during the interval II, which basically leads to the dipolarization. Bx varies between −20 and +1:4 nT, and Bz value reaches up to ∼33 nT in its maximum with a very short interval of 2:8 nT in its minimum. The most remarkable point to note here is that this major ,eld *uctuation begins roughly at the time when the plasma *ow completes the ,rst rotation in the interval I, namely, at ∼0836–0837 UT.
Then the rotation continues further with larger amplitudes as the dipolarization proceeds in the interval II. Two more clockwise rotations are done for ∼4:5 min as shown in Fig. 11(b): One from 1 to ∼15, and another smaller one from 16 to 22. The *ow speed is higher but still moderate, ranging −290 km=s (in dawnward direction) to 310 km=s (in earthward direction). The *ow rotations still persist to exist even after interval II, though with smaller speeds, as shown in Fig. 11(c): One in counter-clockwise direction
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During the intervals of these *ow rotations, there are some tendencies of slightly decreasing particle number density N and increasing ion temperature Ti (plot (d) in Fig. 10), although they are not unambiguously de,ned. 3. Summary and discussions
Fig. 11. Hodogram in Vx –Vy space for time intervals on July 5, 1995. Plots (a) and (b) correspond to the time intervals I and II, respectively, in Fig. 10, and (c) represents an interval right after that.
(1–8), then another one in clockwise direction, average rotation period being ∼3 min.
To summarize our observations, we have presented several events from Geotail observations, where the plasma *ow in near-earth tail is rotating in association with magnetic dipolarization. In some cases presented here (the ,rst dipolarization in June 18, 1997 event, and Sept. 2, 1995 events), the *ow rotation begins to occur almost simultaneously with the initiation of the magnetic ,eld dipolarization, then it continues further with the entire dipolarization process. In other cases (the second dipolarization in June 18, 1997 event, and the July 5, 1995 event), it even precedes the dipolarization onset timing. Namely, the *ow rotation appears just prior to the dipolarization initiation, then it further continues as the dipolarization proceeds. In all events of both cases, rotations have typical periods of 2–5 min on average, and 2–3 rotations are normally done throughout the entire dipolarization interval in either counter-clockwise or clockwise sense. De,nitely, such *ow rotations do not belong to the conventional BBF type. Also, while BBF, by its de,nition, refers to high-speed *ows, our rotating *ow speed is mostly weak to moderate, |Vx | ¡ ∼300 km=s. The *ow rotations for all events presented here were seen away from the ground onset sector determined by ground magnetic signatures. Thus, they are most likely a manifestation of longitudinally expanded stage of initial substorm-triggering process at the onset sector, or may be a separate one of multiple activities occurring at diKerent longitudinal sectors. We should remark that the *ow rotations in this work are presented in X –Y plane rather than in the “perpendicular plane” (perpendicular to the magnetic ,eld). However, the *ow rotations are measured near or not much away from the neutral sheet, and as shown in Figs. 3(c), 6(d), and 10(c), Vx is nearly same as V perp x which is the x-component of the perpendicular *ow. Therefore, the X –Y plane should not be very diKerent from the perpendicular plane. In order to check if the *ow rotation observed is a strict E × B *ow based on MHD frozen-in condition, we looked at the measured electric ,eld in comparison with the values of −(V × B) (see Figs. 3(e), 7(c), (d), 10(e) and (f)): The oKset values of a few mV=m in Ex need to be taken into account, in particular, in the events of June 18, 1997 and July 5, 1995. Overall, the agreement in phase between the measured electric ,eld and the expected values from −(V×B) is reasonably good, but the deviations in detail are not negligible at the same time. Therefore, we expect that the (rotating) *ow deviates to some extent from the perfect E × B *ow due to slippage between plasma and magnetic ,eld (nevertheless, the magnetic ,eld should still be twisted by some amount as the *ow rotates).
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What caused this *ow rotation in close temporal association with dipolarization is an interesting question to consider. In the following, we will discuss two possible causes of the observed *ow rotations. First, we consider a possibility of connection between the observed *ow rotations and a BBF. One natural expectation is that the Kelvin–Helmholtz instability may be excited due to the *ow shear at the sides of BBF, which may then generate *ow rotations aside the BBF. In such a case, the Geotail satellite adjacent to BBF might have a chance to observe such a rotation without observing direct appearance of BBF. However, there are some diAculties in applying this view at least to our observed events. The earthward BBF would generate the clockwise (counter-clockwise) rotating *ows at the east (west) side of the BBF, although the sense of the rotation could become reversed (through a winding-then-unwinding process) by restoring the tendency of the twisted ,eld lines after one or more rotations were done. However, in all our events, initial rotating senses are opposite to the ones expected from the assumption of an adjacent BBF passage provided that the BBF would be in the ground onset sector: For example, in the July 5, 1995, event, the Geotail was westward from the ground onset sector, but it observed clockwise rotations initially. Besides, Lui et al. (1999) recently suggested that the clockwise (counter-clockwise) *ow vortices at the east (west) side of the BBF would generate the ,eld-aligned current system of region II type, being inconsistent with the conventional current wedge system (region I type). This suggestion was further supported by a more recent three-dimensional particle simulation by Pritchett and Coroniti (2000) which is in contrast to the earlier MHD result by Birn et al. (1999), where the consistency between the *ow braking and the region I type current wedge had been found. The BBF could generate *ow rotations or vortices not only beside it, but also behind it. In fact, the previously observed *ow rotations or vortices in direct relation with the BBF events reported by Fair,eld et al. (1998) and Nagai et al. (2000), for example, are those generated behind the BBF passage. We emphasize that our present rotating *ows at and around the dipolarization initiation are diKerent from such previous ,ndings. In those previous reports, the appearance of earthward BBF roughly coincides with the dipolarization initiation, but the *ow rotation appears only well after the magnetic ,eld is already dipolarized considerably. Below, we give such an example, selected out of our 12 BBF events. Fig. 12 displays the data of that example, the event at [0730-0750] UT on July 5, 1995. This event corresponds to the ground activity initiated at ∼0730 UT that was shown earlier in Fig. 9. In Fig. 12, at ∼0732 UT, the high-speed earthward *ow with a maximum of 484 km=s is observed which lasts for 6 6 min until ∼0738 UT. The dipolarizing magnetic ,eld *uctuation is accompanied by initiating the high-speed *ow appearance. Then *ow rotation starts to appear at ∼0739 UT continuing until ∼0746 UT as indicated by vertical lines in Fig. 12. The hodogram for that interval
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is shown in Fig. 13, displaying ,rst counter-clockwise, then clockwise rotations over ∼7 min, while the magnetic ,eld continues its *uctuation. This is well after the major dipolarization has already proceeded signi,cantly with bursty bulk *ows. This event contrasts with those presented in Section 2, where the *ow rotations appear nearly simultaneously with or even somewhat prior to the ,eld dipolarization initiation and continue throughout the dipolarization, where no BBF type *ow is seen. Considering the diAculties and inconsistencies in relating our observed *ow rotations (all those presented in Section 2) to the earthward BBF, we are motivated to seek an alternative possibility that the observed *ow rotations at dipolarization may be caused by some near-tail instability developed at and near the observation point, being irrelevant to BBF. One candidate for generation of local *ow rotations or vortices is perhaps the Kelvin–Helmholtz (KH) instability which would grow at the presence of suAcient *ow shear. Besides, many previous works on KH in the general literature, and a recent simulation by Voronkov et al. (1997) showed that KH (and its coupled mode with Rayleigh–Taylor instability) became unstable and the instability generated vortices in the nightside magnetospheric con,guration when the *ow shear of the type vy = vy (x) was present. The *ow shear is a prerequisite for *ow rotation generation through KH instability. Such a *ow shear could exist in equilibrium state from the ,rst, or it may be even generated as a consequence of another instability such as the ballooning instability. There are a number of papers in space literature that suggest the excitation of linear ballooning instability in the magnetotail (For example, see Lee, 1999 and references therein). Here, we further expect that the nonlinear growth of the ballooning could provide either the vortex directly or the sheared *ow which may in turn drive a KH mode unstable in order to generate a vortex. We illustrate this possibility on intuitive ground using cartoons in Fig. 14. First, the initial ballooning mode may look like Fig. 14(a) in the equatorial plane. If it can grow nonlinearly, then some possible patterns of nonlinearly-growing *ows may be something like the sketches (b) – (d) of Fig. 14. Cartoon (b) shows two oppositely growing ,nger-shaped *ows, while more than two ,ngers of growing *ows are sketched in (c). In these cases, the *ow shear between ,nger-shaped *ows could excite the KH instability that would accompany the *ow vortex. Hurricane et al.’s (1999) nonlinear calculation shows a ,nger-shaped *ow growth in earth-tail direction, which is similar to Fig. 14(a). Now the cartoon (d), on the other hand, shows a vortex that may be generated directly at the nonlinear stage of the ballooning itself or as a further consequence (through a coupling to KH instability) of the ballooning *ow shear of types (b) and (c). It should be appreciated that the generation of *ow vortex by Rayleigh–Taylor or ballooning instability and the coupling between KH and ballooning instabilities have long been recognized in some previous works (e.g., Drake et al., 1992; Finn, 1993). It will be worthwhile to perform
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Fig. 12. The magnetic ,eld and plasma data from Geotail for the period [07– 08] UT on July 5, 1995.
in future the nonlinear ballooning mode simulation for the substorm-related plasma sheet con,guration. There are at least two limitations in the present work. As was already mentioned earlier in this section, the observed *ow rotations and magnetic dipolarizations were seen away from the ground onset sector. Thus, although we suggested the near-tail instability as a responsible mechanism for the generation of the *ow rotation above, it is not clear from the present data set whether the same instability was also activated at the ground onset sector to give similar *ow ro-
tations. Also, the fact that the *ow rotation appears around the dipolarization initiation brings up the question of causality between *ow rotation and dipolarization. Unfortunately, it is uncertain from the present data set whether the *ow rotation is a cause of the dipolarization or a consequence of it. As ,nal remarks, we do not preclude the possibility that the braking of BBF at near-tail could cause ballooning-type modes which propagate away from the BBF. This may generate the *ow vortex if the mode is unstable. However, to the authors’ best knowledge, no theoretical stability analysis
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Fig. 13. Hodogram in Vx –Vy space for the time interval on July 5, 1995, corresponding to the interval indicated by vertical lines in Fig. 12.
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distance during the observed rotation period, on the other hand.) Another interesting point to consider in a future work is the relationship between the observed *ow pattern and the Doppler-shift eKect from the travelling ballooning mode at the ion-diamagnetic drift speed. In conclusion, we have reported 4 events, where the near-tail *ow rotation occurs at and around the magnetic ,eld dipolarization during substorm expansion. We ,nd it diAcult to relate the observed rotations to BBF as a possible cause. Instead, we have suggested, based on intuitive ground, that they could be a manifestation of some near-tail instability development such as Kelvin–Helmholtz and ballooning instabilities during the substorm expansion. The computational veri,cation of the suggestion will form our future project. Statistically, the 4 *ow rotation events were seen out of the total 30 events of dipolarization that we have identi,ed for 8 months period at near-tail. For other 12 out of 30 events, the dipolarizations are associated with the standard BBF type *ows. Another 8 events out of 30 are found to be neither BBF nor rotating types. Acknowledgements
Fig. 14. Sketch of the ballooning evolution in the equatorial plane: (a) initial wave, (b) – (d) possible nonlinear developments.
has been done for this special situation where the near-tail is locally compressed by the braking of the narrow (a few RE in Y extent) *ow channel. Also, more sophisticated work is required to check the possibility of travelling vortex. This might aKect the determination of the rotating sense, if it travels fast, depending on how it travels across the satellite. (The Geotail is found to transverse over quite short
Geotail magnetic ,eld and plasma data were provided by S. Kokubun and T. Mukai through DARTS at the Institute of Space and Astronautical Science in Japan, and the Geotail electric ,eld data were provided by H. Hayakawa. D.-Y. Lee is grateful to T. Mukai for helpful comments on the Geotail data. The CANOPUS data were kindly provided by T. Hughes. The CANOPUS instrument array was constructed and is maintained and operated by the Canadian Space Agency for the Canadian scienti,c community. We are grateful to L. Newitt of Geological Survey of Canada for supplying the data at Canadian INTERMAGNET magnetic obervatories. Data at some stations in Canadian sector were also obtained from NOAA National Geophysical Data Center. We also thank G.K. Parks and M. Fillingim for the POLAR UVI images of June 18, 1997, event. We are grateful to both referees for their interesting suggestions, in particular, the comments on the possible ballooning excitation by BBF braking and on the relationship between the *ow pattern and Doppler-shift eKect from the travelling ballooning mode. The work at Korea Advanced Institute of Science and Technology has been supported by the Brain Korea 21 project of Korea Ministry of Education.
References Angelopoulos, V., Coroniti, F.V., Kennel, C.F., Kivelson, M.G., Walker, R.J., Russell, C.T., McPherron, R.L., Sanchez, E., Meng, C.-I., Baumjohann, W., Reeves, G.D., Belian, R.D., Sato, N., Friis-Christensen, E., SutcliKe, P.R., Yumoto, K., Harris, T., 1996. Multipoint analysis of a bursty bulk *ow event on April 11, 1985. Journal of Geophysical Research A101, 4967.
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Baker, D.A., Pulkkinen, T.I., Angelopoulos, V., Baumjohann, W., McPherron, R.L., 1996. The neutral line model of substorms: past results and present view. Journal of Geophysical Research A101, 12,975. Baumjohann, W., Hesse, M., Kokubun, S., Mukai, T., Nagai, T., Petrukovich, A.A., 1999. Substorm dipolarization and recovery. Journal of Geophysical Research A104, 24,995. Birn, J., Hesse, M., Haerendel, G., Baumjohann, W., Shiokawa, K., 1999. Flow braking and the substorm current wedge. Journal of Geophysical Research A104, 19,895. Drake, J.F., Finn, J.M., Guzdar, P., Shapiro, V., Shevchenko, V., Waelbroek, F., Hassam, A.B., Liu, C.S., Sagdeev, R., 1992. Peeling of convection cells and generation of sheared *ow. Physics of Fluids B4, 488. Fair,eld, D.H., Mukai, T., Lui, A.T.Y., Cattell, C.A., Reeves, G.D., Nagai, T., Rostoker, G., Singer, H.J., Kaiser, M.L., Kokubun, S., Lazarus, A.J., Lepping, R.P., Nakamura, M., Steinberg, J.T., Tsuruda, K., Williams, D.J., Yamamoto, T., 1998. Geotail observations of substorm onset in the inner magnetotail. Journal of Geophysical Research A103, 103. Finn, J.M., 1993. Nonlinear interaction of Rayleigh–Taylor and shear instabilities. Physics of Fluids B5, 415. Hones Jr., E.W., Birn, J., Bame, S.J., Asbridge, J.R., Paschmann, G., Scopke, N., Haerendel, G., 1981. Further determination of the characteristics of magnetospheric plasma vortices with ISEE 1 and 2. Journal of Geophysical Research A86, 814. Hurricane, O.A., Fong, B.H., Cowley, S.C., Coroniti, F.V., Kennel, C.F., Pellat, R., 1999. Substorm detonation. Journal of Geophysical Research A104, 10,221. Kokubun, S., Yamamoto, T., Acuna, M.H., Hayashi, K., Shiyokawa, K., Kawano, H., 1994. The GEOTAIL magnetic ,eld experiment. Journal of Geomagnetism Geoelectricity 46, 7. Lee, D.-Y., 1999. EKect of plasma compression on plasma sheet stability. Geophysical Research Letters 26, 2705. Lui, A.T.Y., 1996. Current disruption in the Earth’s magnetosphere: observations and models. Journal of Geophysical Research A101, 13,067. Lui, A.T.Y., Liou, K., Newell, P.T., Meng, C.-I., Ohtani, S.-I., Ogino, T., Kokubun, S., Brittnacher, Parks, G.K., 1998. Plasma and magnetic *ux transport associated with auroral breakups. Geophysical Research Letters 25, 4059. Lui, A.T.Y., Liou, K., Nose, M., Ohtani, S., Williams, D.J., Mukai, T., Tsurda, K., Kokubun, S., 1999. Near-Earth dipolarization: evidence for a non-MHD process. Geophysical Research Letters 26, 2905.
Machida, S., Miyashita, Y., Ieda, A., Nishida, A., Mukai, T., Saito, Y., Kokubun, S., 1999. Geotail observations of *ow velocity and north-south magnetic ,eld variations in the near and mid-distance tail associated with substorm onsets. Geophysical Research Letters 26, 635. Mukai, T., Machida, S., Saito, Y., Hirahara, M., Terasawa, T., Kaya, N., Obara, T., Ejiri, M., Nishida, A., 1994. The low energy particle (LEP) experiment onboard the GEOTAIL satellite. Journal of Geomagnetism and Geoelectricity 46, 669. Nagai, T., Singer, H.J., Mukai, T., Yamamoto, T., Kokubun, S., 2000. Development of substorms in the near-earth tail. Advances in Space Research 7=8, 1651. Pritchett, P.L., Coroniti, F.V., 2000. Localized convection *ows and ,eld-aligned current generation in a kinetic model of the near-earth plasma sheet. Geophysical Research Letters 27, 3161. Roux, A., Perraut, S., Robert, P., Morane, A., Pedersen, A., Korth, A., Kremser, G., Aparicio, B., Rodgers, D., Pellinen, R., 1991. Plasma sheet instability related to the westward traveling surge. Journal of Geophysical Research A96, 17,697. Saito, Y., Mukai, T., Machida, S., Hirahara, M., Nishida, A., Terasawa, T., 1994. Rapid variations of the plasma bulk *ow direction observed in the plasma sheet at XGSE ∼ −60RE . Geophysical Research Letters 21, 393. Saunders, M.A., Southwood, D.J., Fritz, T.A., Hones Jr., E.W., 1983a. Hydromagnetic vortices—1. The 11 December 1997 event. Planetary and Space Sciences 31, 1,009. Saunders, M.A., Southwood, D.J., Hones Jr., E.W., 1983b. Hydromagnetic vortices—2. Further dawnside events. Planetary and Space Sciences 31, 1,117. Sergeev, V.A., Sauvaud, J.-A., Popescu, D., Kovrazhkin, R.A., Liou, K., Newell, P.T., Brittnacher, M., Parks, G., Nakamura, R., Mukai, T., Reeves, G.D., 2000. Multiple-spacecraft observation of a narrow transient plasma jet in the earth’s plasma sheet. Geophysical Research Letters 27, 851. Tsuruda, K., Hayakawa, H., Nakamura, M., Okada, T., Matsuoka, A., Mozer, F.S., Schmidt, R., 1994. Electric ,eld measurements on the Geotail satellite. Journal of Geomagnetism and Geoelectricity 46, 693. Tsyganenko, N.A., 1989. A magnetospheric magnetic ,eld model with a warped tail current sheet. Planetary and Space Sciences 37, 5. Voronkov, I., Rankin, R., Frycz, P., Tikhonchuk, V.T., Samson, J.C., 1997. Coupling of shear *ow and pressure gradient instabilities. Journal of Geophysical Research A102, 9639.