Pages 961-971

SUBSTORMS, TAIL FLOWS, AND PLASMOIDS

T. Nagai1, R. Nakamura2, T. Mukai3, T. Yamamoto3, A. Nishida3, and S. Kokubun2

1 Earth & Planetary Sciences, Tokyo Institute of Technology, Meguro, Tokyo 152, Japan,
   E-mail: nagai@geo.titech.ac.jp
2 Solar-Terrestrial Environment Laboratory, Nagoya University, Toyokawa 446, Japan
3 Institute of Space and Astronautical Sciences, Sagamihara, Kanagawa 229, Japan


ABSTRACT

The GEOTAIL spacecraft has made comprehensive magnetic field and plasma observations in the Earth's magnetotail at radial distances of 10-210 Re. Data from the GEOTAIL spacecraft have led to the following conclusions regarding the relationship between substorms and field and plasma behaviors. Substorms produce systematic and repeatable signatures in magnetic field and plasmas in various radial distances in the magnetotail. A repeatable signature for substorms is tailward plasma flows with a southward magnetic field structure. These are often associated with plasmoids, as seen in the local magnetic field variations. Examples are presented of signatures seen at different distances from the Earth. From a statistical survey of the data, it is shown that magnetic reconnection takes place in the expansion phase at radial distances of 20-30 Re. Plasmoids are created there and travel tailward at high speeds. Earthward flows in the recovery phase appear only inside 100 Re. Plasmoids are dominant substorm signatures beyond 100 Re.

INTRODUCTION

An understanding of the global dynamics of the Earth's magnetotail during substorms is one of the major scientific objectives in space physics. The GEOTAIL mission was launched on July 24, 1992 as key space observation facilities in the magnetotail. Nagai et al. (1994) have performed initial survey studies of substorm signatures only using magnetic field data from GEOTAIL in the time interval from October 1992 to September 1993. Since September 1993, with comprehensive magnetic field and plasma measurements at radial distances of 10-210 Re, GEOTAIL has provided a systematic assemblage of data from the near-Earth magnetotail to the distant magnetotail.

Previous ISEE 3 studies have demonstrated that plasmoids are created and travel tailward in association with substorms (e.g., Hones, et al., 1984a; Baker et al., 1987; Slavin et al., 1992, 1993; Moldwin and Hughes, 1992; 1993). Plasmoids are usually identified with bipolar signatures in the northward component Bz of the magnetic field, because plasma measurements are limited only to electrons in ISEE 3.

Utilizing ion and electron plasma and magnetic field measurements with GEOTAIL, we have examined major variations in the plasma sheet as response of substorms. Tailward plasma flows with southward Bz fields are systematic and repeatable signatures for substorms in the plasma sheet beyond the radial distance of 30 Re. These tailward plasma flows, most of which are associated with magnetic field signatures called plasmoids, are studied in this paper. We focus on duration of flows and its relationship to substorm phases.

METHODOLOGY

We have selected 213 well-isolated substorms on the basis of ground magnetic field data and synchronous orbit particle data for the time interval when GEOTAIL is in the magnetotail. The selected substorms have well-defined positive bay signatures at mid-latitudes. We have examined 1-minute digital data from mid-latitude stations; Kakioka, Hermanus, San Juan, Fredericksburg, Boulder, and Honolulu. Substorm activities are also examined with magnetic field data from auroral zone stations; Poste-de-la-Baleine, Fort Churchill, Yellowknife, College, Kiruna, and Leirvogur and those from Russian auroral zone stations of 210° magnetic meridian (Yumoto et al., 1992). Furthermore, we examine energetic particle data from synchronous spacecraft; GMS-4 (at 140°E), GOES 6 (at 75°W) and GOES 7 (at 105°).

The onset time is determined mainly with the start time of a mid-latitude positive bay. When Pi 2 data are available, the onset time of Pi 2 activity is adopted. We define the expansion phase as the interval from the onset time to the peak time of a mid-latitude positive bay. We define the early recovery phase as the interval from the peak time to the end of a mid-latitude positive bay. In the auroral zone, electrojet activity usually continues after the early recovery phase in this definition. We estimate that the onset time can be determined with error of less than 5 minutes, however, the start time and the end time of the early recovery phase appear to have errors of 5-10 minutes. In these events, the expansion phase continues for 35 minutes and the early recovery phase continues for 40 minutes, respectively, on the average.

For the substorms determined with this procedure, we have examined GEOTAIL data for the period from September 1993 to February 1996. Magnetic field data are from the magnetic field experiment MGF (Kokubun et al., 1994) and ion and electron data are from the low energy particle experiment LEP (Mukai et al., 1994). We examine magnetic field values Bx, By, Bz, and Btotal in the GSM coordinates (the Geocentric Solar-Magnetospheric Coordinates) with 12-sec time resolution and plasma moment data; i.e., ion bulk flow Vx, Vy, and Vz, ion density, ion temperature, with 12-sec time resolution. Energy-time spectra for ions and electrons (E-t diagrams) in four directions (sunward, duskward, tailward, and dawnward) in the equatorial plane are also used to examine plasma characteristics. Three-dimensional ion and electron distribution functions are examined if available and necessary.

Figure 1 presents locations of GEOTAIL for these 213 substorms. The data beyond X = -50 Re were taken in 1994. In 1994, geomagnetic activity was high so that isolated substorms were less frequent. We classify major plasma flow characteristics seen after each onset into six classes: T tailward flow, E Earthward flow, R tailward and then Earthward flow reversal during the expansion phase, F fluctuating or oscillatory (Earthward and tailward) flow, P no significant change in flow in the plasma sheet, and L no plasma sheet flow because the spacecraft is in the tail lobe. Tailward flows associated with onsets are seen in the region beyond 15 Re, while Earthward flows associated with onsets are seen only in the region inside 30 Re. We focus on the tailward flows.

Fig. 1 GEOTAIL positions for 213 substorm events in the Xgsm-Ygsm plane. T indicates tailward flow event, see details in the text.


EVENT STUDIES

Here, we discuss four tailward flow events at different radial distances; Xgsm = -30 Re, -40 Re, -130 Re, and -200 Re. Each event is thought to be representative in each radial distance region of the magnetotail. Some events at Xgsm = -70, -90, and -140 Re are discussed in Mukai et al. (1996) and Machida et al. (1994).

The February 18, 1995 event near Xgsm = -30 Re

A well-isolated substorm starts at 0940 UT on February 18, 1995. An expansion phase is the period 0940-1015 UT and the early recovery phase is the period 1015-1115 UT. Before the onset time, magnetic activity is quiet in the auroral zone in the period 0730-0900 UT and a weak electrojet develops after 0900 UT. The IMF Bz becomes mostly southward after 0850 UT so that we can define the growth phase as the period 0850-0940 UT period.

The magnetic field and plasma data from GEOTAIL are presented in Figure 2. After 0915 UT, GEOTAIL traverses the neutral sheet from the northern hemisphere to the southern hemisphere. Near the neutral sheet plasma flows are almost stationary (Earthward velocity is less than 20 km/s). GEOTAIL continues to stay in the plasma sheet at least until 0940 UT, although plasma beta b becomes 1.0 at 0939 UT.

Fig. 2 GEOTAIL magnetic field and plasma observations for the period 0800-1200 UT on February 18, 1995. The lowest panel shows total pressure (thick line) and ion pressure (thin) line.


Tailward flows appear at 0940 UT with southward Bz. The tailward flows have a high speed that contrasts with a low speed observed in the growth phase. The Bz field shows only a southward turning at the front of the tailward flows in this case. This is characteristic of tailward flows in the near-Earth plasma sheet (Nagai et al., 1994). GEOTAIL is near the neutral sheet (Bx is near zero) until 0950 UT. Plasmas show tailward convections (flows perpendicular to the magnetic field) and tailward field-aligned flows for the period 0950-1000 UT. When GEOTAIL approaches the tail lobe (GEOTAIL enters the tail lobe briefly at 0957 UT), it observes mainly field-aligned flows. The tailward flows subside near 1000 UT, although GEOTAIL is still in the plasma sheet (plasma b is mostly higher than 1.0). The southward Bz field subsides in association with the tailward flows.

For the period 1000-1055 UT, plasmas are almost stationary. Plasma density is 8 cm-3 and plasma temperature is 0.6 keV. These values are taken near the neutral sheet at 1006 UT and 1035 UT. It is important to note that plasma density is 5 cm-3 and plasma temperature is 1.6 keV near the neutral sheet in the growth phase at 0920 UT. Therefore, cold and dense stationary plasmas appear at the peak of the substorm activity. The origin of these plasmas is not known. These plasmas have dawnward convection with speed of 50 km/s after 1000 UT. There is a possibility that these plasmas are transported from the duskside plasma sheet.

Brief Earthward flows appear near 1055 UT. After these Earthward flows, although plasmas become stationary, they show low density (0.015 cm-3) and high temperature (3 keV). In the recovery phase, Earthward flows usually appear, however, these Earthward flows are confined near the plasma sheet/tail lobe boundary region. Cold and dense plasmas almost motionless are frequently observed near the neutral sheet. In the previous studies, spacecraft usually enter the tail lobe after they observe tailward flows and then they observe Earthward flows with the expanding plasma sheet (e.g., Hones, 1979; Hones et al., 1984b). In these studies, it is assumed that the flows continue in the thinned (not observed) plasma sheet for the expansion phase and the early recovery phase. This event clearly demonstrates that the flow does not necessarily continue in the whole plasma sheet.

The December 11, 1994 Event near Xgsm = -40 Re

A multiple-onset substorm is clearly identified in the continuous activity. Three Pi 2 bursts are seen at 1755, 1803, and 1813 UT. A positive bay at Kakioka reaches its peak at 1900 UT and ends at 1940 UT. Figure 3 shows magnetic field and plasma data from GEOTAIL. There are Earthward flows near the plasma sheet /tail lobe boundary region prior to these Pi 2 activities. These Earthward flows are considered to be related to substorm activity starting at 1455 UT. Three tailward flow bursts are clearly seen. The first tailward flow burst starts at 1756:30 UT and ends at 1759 UT. This burst shows a bipolar Bz signature. The second burst starts 1803:30 UT and ends at 1806 UT. The third bursts starts at 1813:30 UT and ends at 1817 UT. The second and third bursts show bipolar Bz signatures with a northward component of 10 nT in high time resolution data. These suggest that reconnection takes place at least at three times. The flow bursts are a commonly observed signature inside 50 Re. After these tailward flows, GEOTAIL enters the tail lobe. The Bz field is northward after 1820 UT even in the expansion phase.

Fig. 3 GEOTAIL magnetic field and plasma observations for the period 1700-2100 UT on December 11, 1994.

Earthward flows appear at 1910 UT and continues until 2010 UT, although they are variable. These flows are in the early recovery phase. In this case, the flows exist near the neutral sheet as well as in the plasma sheet/tail lobe boundary. After these flows, stationary plasmas show high temperature near the neutral sheet. In the region from Xgsm = -40 Re to Xgsm = -100 Re, GEOTAIL usually exits from the plasma sheet during the expansion phase and stays in the tail lobe. It is likely that the plasma sheet becomes significantly thin in this region of the magnetotail. It is not evident whether flows exist in the thinned plasma sheet.

The July 21, 1994 Event near Xgsm = -136 Re

An isolated substorm starts at 1210 UT near the Alaska meridian. A positive bay reaches its peak at 1235 UT and ends at 1315 UT, although electrojet activity continues until 1430 UT. There is a Pi 2 burst at 1239 UT, indicating another onset. In the period 1300-1430 UT there is no evident Pi 2 activity. The IMF Bz is northward until 1100 UT and then it turns southward. Indeed, a weak eastward electrojet starts near 1110 UT in the Russian Sector (Tixie Bay). The growth phase of this substorm is the period 1100-1210 UT.

Figure 4 shows magnetic field and plasma data from GEOTAIL. In the growth phase, although GEOTAIL is mostly in the tail lobes, GEOTAIL makes several crossings of the plasma sheet. Near 1152 UT, GEOTAIL approaches the neutral sheet and observes isotropic plasmas with density of 0.5 cm-3 and temperature of 0.4 keV. Near the plasma sheet /tail lobe boundary region, plasmas show field-aligned Earthward flow with speed of 200 km/s (at 1151 and 1154 UT). Near the neutral sheet, plasmas show slow field-aligned flows (speeds of less than 100 km/s) and there is little convection motion in the Earthward or tailward direction. For the crossing of the plasma sheet at 1204 UT, GEOTAIL does not observe any Earthward flow. Plasmas in the plasma sheet shows tailward and duskward convection with speed of 170 km/s. Plasmas are isotropic with density of 0.4 cm-3 and temperature of 0.5 keV near the neutral sheet. There is another plasma sheet crossing at 1228 UT. Plasmas show field-aligned and convection motion with speed of 200 km/s and have density of 0.4 cm-3 and temperature of 0.4 keV. Thus, the plasma flow direction in the plasma sheet can be slow but highly variable. The thickness of the plasma sheet is estimated with plasma velocity measurements. The thickness is approximately 0.9 Re at 1204 UT and 1.1 Re at 1228 UT.

Fig. 4 GEOTAIL magnetic field and plasma observations for the period 1100-1500 UT on July 21, 1994.

Tailward flows with speed of higher than 500 km/s start at 1235 UT, although high energy ions ( > 10 keV) appear near 1231 UT. The tailward flows continue until 1357 UT. GEOTAIL enters the tail lobe and then encounters the magnetosheath near 1430 UT. MHD parameters of the tailward flowing plasmas are fairly constant in this event and differ significantly from those of the quiet time plasma sheet plasmas described above. Representative values are 750 km/s for tailward velocity, 0.15 cm-3 for density, and 2 keV for temperature. Thus, we can discriminate easily tailward flows associated with substorms from plasmas in quiet times even in the distant magnetotail. It is noted that the density in the tailward flows is smaller than that in the tail lobes.

The magnetic field shows a bipolar signature in Bz at 1238 UT and another bipolar signature in Bz at 1254 UT. These are likely associated with the 1210 UT onset and 1239 UT onset. These bipolar signatures show a "core" structure (e.g., Hughes and Sibeck, 1987; Slavin et al., 1995), although these have significant Bx fields. These "cores" produce spikes in total pressure. The total pressure in the tailward flow except the "core" region is almost the same as that in the tail lobes. Even after these bipolar signatures, the magnetic field shows fluctuations in Bz in the course of the tailward flow.

The April 24, 1994 Event near Xgsm = -200 Re

A well-defined positive bay starts at Kakioka with a Pi 2 burst at 1539 UT. This positive bay reaches its peak at 1605 UT and ends at 1700 UT. There is no Pi 2 activity at Kakioka for the period from 1600-1900 UT. Electrojet activity continues until 1730 UT in the auroral zone (Tixie Bay). Prior to this substorm, there is at least one substorm starting at 1412 UT and ending at 1500 UT.

Figure 5 shows magnetic field and plasma data from GEOTAIL. There are tailward flows until 1520 UT. These flows are effects of the previous substorms. There is a crossing of the plasma sheet near 1543 UT. Plasmas show tailward flows with speed of less than 100 km/s. Since this slow speed is a clear contrast with the fast speed seen before 1520 UT, the plasma sheet is thought to return to the no-substorm state. Plasma density is 0.3 cm-3 and plasma temperature is 0.3 keV near the neutral sheet. The thickness of the plasma sheet is estimated to be 1.6 Re. After 1543 UT, GEOTAIL stays in the outer part of the plasma sheet.

Fig. 5 GEOTAIL magnetic field and plasma observations for the period 1500-1900 UT on April 24, 1994.

Tailward flows appear at 1600 UT and continues until 1812 UT. Plasma density gradually increases while plasma temperature gradually decreases. There is a bipolar signature at the front of the tailward flows, however, the northward Bz excursion is less evident than the southward Bz excursion. There is a clear "core" structure with a large positive By field. For the "core", plasmas show high temperature and low density. There is a spike in the total pressure. The pressure spike is produced mainly by the magnetic field pressure. The total pressure in the course of the tailward flows is comparable to that in the tail lobe. This suggests that only a "core" can produce a pressure pulse observable as a traveling compression region (TCR) in the tail lobes (cf. Slavin et al., 1993). The magnetic field shows fluctuation in Bz in the course of the tailward flows.

STATISTICAL STUDIES

On the basis of individual event studies as described above and other event studies, we have concluded that fast tailward flows are major substorm-associated variations in the plasma sheet beyond the radial distance of 30 Re. We note that fast Earthward flows can follow fast tailward flows in some events. We discuss the fast tailward flows and the Earthward flows following the tailward flows statistically.

The start time of fast tailward flows can be easily determined with a sudden appearance of tailward flowing plasmas in the E-t diagrams as well as in the plasma moment data. Although high energy ions sometimes show time-dispersion, we use arrival of ions with energies less 1 keV. These ions appear to arrive to GEOTAIL simultaneously, with a time resolution of 1 minute. The end time of the fast tailward flows is determined as the disappearance time of the tailward flowing plasmas or the start time of the period when the 1-min average flow speed is zero. Tailward flows continue intermittently in some cases. In such cases, we define the period of fast tailward flows as long as possible. When tailward flows and Earthward flows appear alternately, we discard the event itself. We determine Earthward flows with the similar procedure.

Figure 6 presents the tailward flow intervals with respect to the substorm onset time as a function of the GEOTAIL position (solid lines). The start time of the tailward flow relative to the onset is delayed progressively as the satellite distance from the Earth increases. Time delay is typically 11 minutes at 100 Re and 25 minutes at 200 Re, respectively. A linear fit to data points in Figure 6 indicates that plasmoids start at Xgsm = -21 Re and have a tailward speed of 760 km/s.

Fig. 6 Tailward flow intervals relative to substorm onsets are presented with thick lines. Intervals for Earthward flows following the tailward flows are presented with dashed lines. Each event is presented at the GEOTAIL position in Xgsm.

Tailward flows continue typically for 14 minutes (median 9 min) inside Xgsm = -50 Re and for 30 minutes at X = -50 to -100 Re. Inside X = -50 Re, the tailward flows usually ends in the middle of the expansion phase, as seen in the February 18, 1995 event (Fig. 2) and the December 11, 1994 event (Fig. 3). In the region of Xgsm = -50 to -100 Re, the tailward flow duration is comparable to the expansion phase. Individual events support these findings. Beyond Xgsm = -100 Re, two classes exist for tailward flows: short-lived tailward flows continuing for 30 minutes and long-lived tailward flows continuing for more than 90 minutes.

Earthward flows are observed after the tailward flows in some cases, as seen in the February 18, 1995 event and the December 11, 1994 event. However, tailward flows are not necessarily followed by earthward flows ( 80% inside Xgsm = -50 Re and 40 % at Xgsm = -50 to -100 Re). The Earthward flow intervals are also presented in Figure 6 (dashed lines). A noteworthy point is that the substorm-related Earthward flows are generally observed only inside X = -100 Re. There are some Earthward flows beyond 100 Re; however, these are all seen after the early recovery phase and more than 120 minutes after the onset. It is difficult to associate these Earthward flows to the substorms.

The Earthward flows appear at various timings, mostly in the early recovery phase, inside Xgsm = -100 Re. As seen in the February 18, 1995 event, almost stationary plasmas exist in the plasma sheet between the tailward flow interval and the Earthward flow interval inside Xgsm = -60 Re. In the region Xgsm = -60 to -100 Re, GEOTAIL enters the tail lobe between the tailward flow interval and the

Earthward flow interval so that we cannot deduce the plasmas sheet behavior in the events studied here.

Earthward flows in the early recovery phase are assumed to be caused by tailward retreat of magnetic reconnection region (e.g., Hones, 1979). We cannot prove tailward retreat in the individual case studies of events.

Although the number of well-isolated substorms is small beyond X = -50 Re, there are a lot of substorm activities in 1994 when GEOTAIL is in the magnetotail beyond X = -50 Re. We have made other statistical analyses with plasma moment data to complement the studies described above. The data used here are those taken only from January 1994 to February 1995. We have selected 567 tailward flow events in the plasma sheet. Selection criteria are: 1. Tailward flow continues for at least 1 minute (five 12-sec moment samplings) with speed higher than 300 km/s. 2. Ion temperature is higher than 400 eV. Nishida et al. (1995) have reported that temperature in the plasma sheet is higher than 400 eV near 200 Re. Although this is not strict definition for the plasma sheet, visual inspection of E-t diagrams at various radial distances suggests that this definition works well irrespective of radial distance. We have examined E-t diagrams to exclude the solar wind data and the magnetosheath data. 3. One tailward flow event ends when one-minute averaged Vx value becomes higher that -100 km/s. 4. When there are more than two events in the 30-minute period, we adopt the first event. We examine electrojet activity when ground magnetic field data in the auroral zone near midnight are available. We have confirmed that most of events are observed when there is some electrojet activity in the auroral zone. Tailward flow positions selected with these criteria are presented in Figure 7.

Fig. 7 GEOTAIL positions for 567 tailward flows.

The occurrence frequency of intervals of tailward flows are examined in the six regions of the magnetotail in Figure 8. In the X = -10 to -50 Re region, tailward flows continue for less than 10 minutes. Tailward flows continues typically for 20-30 minutes (median is 16-18 min) beyond X = - 50 Re. The long-lived events are observed mainly beyond X = -100 Re, although their occurrence is low. These results support the previous studies with the limited numbers (although the events are all associated with well-defined substorms).

Fig. 8 Occurrence statistics for tailward flow intervals in the six regions of the magnetotail.

We have selected Earthward flows which follow the tailward flows. Here, Earthward flows have one-minute averaged Vx higher than +100 km/s. Figure 9 shows the start time of the Earthward flow relative to the start time of the associated tailward flow. When there is no Earthward flow within three hours (a typical substorm time scale), we judge that there is no Earthward flow. When a tailward flow is followed by another tailward flow within three hours, we judge that there is no Earthward flow. No Earthward flow events are presented in 180-minute bin in Figure 9. Inside X=-50 Re, most tailward flows (90%) are followed by Earthward flows, although the appearance time of the Earthward flows is highly variable. However, beyond X = -100 Re, Earthward flows are infrequent and only tailward flows appear in the substorm life time. These results are also consistent with the previous studies.

Fig. 9 Occurrence statistics for the start time of Earthward flow relative to the start time of tailward flow. The bin greater than 180 min indicates events in which no Earthward flow is observed within 180 min.

CONCLUSIONS

In this paper, we focus on the tailward flows associated with substorms. One of the important findings in our analyses is that the tailward flow interval is almost the same in the magnetotail beyond Xgsm = -50 Re. The interval is comparable to the expansion phase interval of substorms. This implies that magnetic reconnection takes place only inside Xgsm = -50 Re during the expansion phase of substorms. However, there are a number of points that should be studied further:

1. The tailward flows are short-lived inside Xgsm = -50 Re. This result is consistent with bursty nature of Earthward flows in the near-Earth magnetotail (Baumjohann et al., 1990; Angelopoulos et al., 1992; 1994). The February 18, 1995 event and other events indicate that the intense flows do not necessarily continue for the whole time interval of the expansion phase. However, it is highly unlikely that magnetic reconnection continues for only less than 10 minutes. The short life time is likely caused by the exit of the spacecraft from the outflow region of the active reconnection site.

2. Signatures for multiple-onset can easily found in the tailward flows, as seen in the December 11, 1994 event. However, these signatures can be found inside Xgsm = -50 Re. It is likely that several tailward flows are combined while they travel tailward with different speeds.

3. Tailward retreat of magnetic reconnection site is advocated for the recovery phase of substorms. The Earthward flows are observed after the tailward flows, however, these Earthward flows are seen only inside Xgsm = -100 Re. Furthermore, any continuous flow activity in the expansion-recovery phases have not been identified in this study. The present study supports that there are two sites preferable for magnetic reconnection : inside Xgsm = -30 Re and near Xgsm = -100 Re (Nishida et al., 1997), although relationship between these two reconnection sites is not clarified.

4. The long-lived ( more than 60 minutes) tailward flows seen beyond Xgsm = -100 Re cannot be attributable to a simple combination of several tailward flows. There is often one substorm activity on the ground. It is possible that magnetic reconnection inside Xgsm = -30 Re and magnetic reconnection near Xgsm = -100 Re together create long-lived tailward flows. In this case, it is hypothesized that passage of tailward flows created in the near-Earth tail activates reconnection near Xgsm = -100 Re.

ACKNOWLEDGMENTS

The magnetic field data from 210° magnetic meridian stations are supplied by K. Yumoto. The magnetic field data from Kakioka are supplied by Kakioka Magnetic Observatory. Other ground magnetic field data are supplied by WDC-C2, Kyoto University.

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