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The Loading-Unloading Process in the Magnetotail During a Prolonged Steady Southward IMF Bz Period
THE L O A D I N G - U N L O A D I N G P R O C E S S IN THE M A G N E T O T A I L D U R I N G A P R O L O N G E D S T E A D Y S O U T H W A R D IMF Bz PERIOD T. Nagai ~, R. Nakamura 2, T. Hori 3, and S. Kokubun*
1Tokyo Inst#ute of Technology, Tokyo 152-8551, Japan :Space Research Institute, Austrian Academy of Sciences, Schmiedlstr. 6, A-8042, Graz, Austria 3TheJohns Hopkins UniversityAppliedPhysicalLaboratory, MD, U.S.A. 4The University of Tokyo, Tokyo 113-0033, Japan
ABSTRACT The interplanetary magnetic field (IMF) Bz was continuously southward for 16 hours on April 18, 2002. During this period, the spacecraft Geotail was in the northern tail lobe at radial distances of 24-28 RE, and it observed sequences of increasing and decreasing in the magnetic field intensity in association with substorms. The increases in the magnetic field took approximately 40 minutes, and the decreases took approximately 50 minutes. The time scale of the increase is consistent with the typical time scale of the substorm growth phase, which usually begins with a clear southward turn of the IMF Bz. The time scale of the decrease agrees well with the typical time scale of the substorm expansion phase, although the substorm expansion phase frequently starts with a northward turn of the IMF Bz. It is suggested that the magnetotail has an intrinsic time scale for the loading-unloading process for substorms irrespective of the IMF Bz conditions.
Introduction
It is known that the tail lobe magnetic field increases in the substorm growth phase and decreases in the substorm expansion phase (e.g., Caan et al., 1975, Fairfield et al., 1981, Kistler et al., 1993, Nagai et al., 1997). This process is interpreted as the loading and unloading of energy in the magnetotail for substorms (e.g., Baker et al., 1996). The loading process usually starts when the interplanetary magnetic field (IMF) Bz turns southward, and the major onset of a substorm occurs after the IMF Bz continues to be southward for 40-60 minutes. The unloading process usually corresponds to the expansion phase of the substorm, and it typically continues for 40 minutes on the ground. Since the major onset is frequently associated with a northward turn of the IMF Bz, the duration of 40 minutes for the unloading process is obtained for the northward IMF Bz period (e.g., Caan et al., 1975). On April 18, 2002, the IMF Bz was almost continuously southward, and substorms took place quasiperiodically. The spacecraft Geotail stayed almost continuously in the northern tail lobe at radial distances of 2428 RE. The observations provided a unique opportunity for examining the dynamics of the magnetotail for the steady southward IMF Bz conditions. We focus on the time scales of the loading-unloading process in this paper.
Observations
The upper panel of Figure 1 presents the IMF and the solar wind velocity observed with the spacecraft ACE for the period from 1800 UT on April 17, 2002, through 1200 UT on April 19, 2002. ACE was located near (+224, +22, -23 RE) in GSM coordinates. The data are shifted 50 minutes for the propagation time. The IMF Bz was continuously southward for the period of 0100-1800 UT on April 18, 2002. The spacecraft Wind observed almost identical behaviors in the solar wind. The lower panel of Figure 1 presents the magnetic field observation obtained with Geotail (Kokubun et al., 1994). Geotail entered the magnetotail near 2100 UT on April 17, 2002. According
-190-
to plasma measurements made with the low-energy plasma experiment (LEP) on Geotail (Mukai et al., 1994), Geotail stayed in the northern tail lobe after 0200 UT on April 18, 2002, except for a brief period in the tail lobe/plasma sheet boundary close to 1150 UT. For the period of 0400-1800 UT, there were five increase-decrease sequences in the total magnetic field, and each was associated with a southward turn of Bz in the tail lobe field. These magnetic field behaviors are known as traveling compression regions (TCRs) (Slavin et al., 1984; 1992).
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April 17-19, 2002
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XOSM -18.8 -20.6 -22.2 -23.6 -24.9 -26.1 -27.0 -27.9 -28.5 -29.0 -29.3 -29.5 -29.4 -29.2 -28.8 YCSM 16.7 14.7 13.8 14.0 14.3 14.0 12.9 10.9 8.3 5.9 4.6 3.9 3.4 2.4 0.9
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Fig. 1. The upper panel presents the IMF and the solar wind velocity observed with the spacecraft ACE for the period from 1800 UT on April 17, 2002, to 1200 UT on April 19, 2002. ACE is located near (+224, +22, -23 RE) in GSM coordinates. The lower panel presents the magnetic field observation from Geotail in GSM.
-191-
In order to find substorm activities, we examined ground magnetic field data from the mid-latitude stations Fredericksburg, Tucson, Honolulu, Kakioka, Urumqi, and Hermanus and from the auroral-zone stations Leirvogur, Narssarssuaq, Poste-de-la-Balaine, Fort Churchill, Yellowknife, College, Tixie Bay, and Kiruna. We also examined particle observations from the Los Alamos National Laboratory spacecraft and magnetic field observations from GOES 8 and GOES 10 at geosynchronous orbit. There were at least five major substorm onsets, at 0530, 0800, 1130, 1408, and 1632 UT for the period of 0300-1800 UT on April 18, 2002. These five onsets correspond well to the TCRs observed with Geotail in the magnetotail.
Fig. 2. The total pressure (magnetic pressure + plasma pressure)in the magnetotail observed with Geotail (thick curve) and the solar wind dynamic pressure observed with ACE (thin curve) on April 18, 2002.
Figure 2 shows the total pressure (magnetic pressure + plasma pressure) in the magnetotail observed with Geotail and the solar wind dynamic pressure observed with ACE on April 18, 2002. The solar wind data are shifted 50 minutes for the propagation time. The dynamic pressure changed significantly for the period of 00000300 UT, so we do not discuss substorm-associated pressure changes in the magnetotail. For the 0530, 0800, 1130, and 1408 UT substorms, since the total pressure in the magnetotail did not follow the solar wind dynamic pressure well for most intervals, variations in the total pressure are attributable to the loading-unloading processes in the magnetotail. For the 1632 UT substorm, the time scale of the decrease in total pressure is used. Just after each substorm onset, total pressure showed a significant peak, which was produced by a spike in Bx. These peaks are probably caused by large pressure pulses, which are produced by high-speed tailward flows inside the plasma sheet (e.g., Nakamura et al., 1999). Since the peaks are considered to be transient phenomena in TCRs, we neglect them in our analysis of the loading-unloading process. 8.0
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Time scales
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Event Event Event Event Event
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0.4
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42-48 40-52 30-50 68-80 42 - 48 60 28- 32 38 40 - 48
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Fig. 3. The lower panel presents the changes in total pressure relative to the substorm onset (the zero epoch corresponds to each substorm onset), and the upper panel presents solar wind electric field variations.
-192-
Figure 3 shows the changes in total pressure relative to substorm onset (the zero epoch corresponds to each substorm onset) as well as solar wind electric field variations. The solar wind electric field is calculated as Esw = Bs x V (Bs = 0 for the IMF Bz > 0, and Bs = -Bz for the IMF Bz < 0). The figure shows that the total pressure increased approximately 40 minutes before the substorm onset and reached to the lowest level approximately 50 minutes after the substorm onset. It is important to note that the solar wind electric field stayed at different levels for different events and did not show a correlation with any change in total pressure.
Discussion
The Geotail observations on April 18, 2002, provided an opportunity for examining substorm processes in the magnetotail for the prolonged southward IMF Bz period. In this paper, we study the time scales of the loadingunloading process in the magnetotail for substorms. The loading process has a time scale of 40 minutes, even for different solar wind electric field levels, and it seems to start without any significant changes in solar wind conditions. Irrespective of the IMF Bz sign, the unloading process has a time scale of 50 minutes. These results are somewhat unexpected. A definite start time for the unloading process is not expected for a prolonged southward IMF Bz. If there is any critical limit to the total pressure in the tail for substorm onsets, we would expect the time scale of the loading process to be short for the high solar wind electric field. Furthermore, we would expect the unloading process to continue for a long time for a prolonged southward IMF Bz. The present results suggest that the magnetotail has an intrinsic time scale for the loading-unloading process for substorms. These results are still highly preliminary; however, no other periods in the Geotail observations are good for testing them. ACKNOWLEDGMENTS CDAWeb was used for the ACE, Wind, Los Alamos National Laboratory spacecraft, and GOES data. We thank N. Ness and D. J. McComas for the ACE data. We thank T. Mukai for providing the LEP data from Geotail. The ground magnetic field data are provided by WDC-C2, Kyoto University. REFERENCES Baker, D. N., T. I. Pulkkinen, V. Angelopoulos, W. Baumjohann, and R. L. McPherron, Neutral line model of substorms: Past result and present view, J. Geophys. Res., 101, 12975-13010, 1996. Caan, M. N., R. L. McPherron, and C. T. Russell, Substorm and interplanetary magnetic field effects on the geomagnetic tail lobes,,/. Geophys. Res., 80, 191-194, 1975. Fairfield, D. H., R. P. Lepping, E. W. Hones, Jr., S. J. Bame, and J. R. Asbridge, Simultaneous measurements of magnetotail dynamics by IMP spacecraft, J. Geophys. Res., 86, 1396-1414, 1981. Kistler, L. M., W. Baumjohann, T. Nagai, and E. M6bius, Superposed epoch analysis of pressure and magnetic field configuration changes in the plasma sheet, J. Geophys. Res., 98, 9249-9258, 1993. Kokubun, S., T. Yamamoto, M. H. Acufia, et al., The GEOTAIL magnetic field experiment, J. Geomag. Geoelectr., 46, 7-21, 1994. Mukai, T., S. Machida, Y. Saito, et al., The low energy particle (LEP) experiment onboard the GEOTAIL satellite, .I. Geomag. Geoelectr., 46, 669-692, 1994. Nagai, T., T. Mukai, T. Yamamoto, A. Nishida, S. Kokubun, and R. P. Lepping, Plasma sheet pressure changes during the substorm growth phase, Geophys. Res. Lett., 24, 963, 1997. Nakamura, R., L. F. Bargatze, T. Mukai, et al., Response of the midtail electric field to enhanced solar wind energy input, J. Geophys. Res., 104, 17299-17310, 1999. Slavin, J. A., E. J. Smith, B. T. Tsurutani, et al., Substorm associated traveling compression regions in the distant tail: ISEE-3 geotail observations, Geophys. Res. Lett., 11,657-660, 1984. Slavin, J. A., M. F. Smith, E. L. Mazur, et al., ISEE 3 plasmoid and TCR observations during an extended interval of substorm activity, Geophys. Res. Lett., 19, 825-828, 1992.
E-mail address of [email protected]
-193-
1Tokyo Inst#ute of Technology, Tokyo 152-8551, Japan :Space Research Institute, Austrian Academy of Sciences, Schmiedlstr. 6, A-8042, Graz, Austria 3TheJohns Hopkins UniversityAppliedPhysicalLaboratory, MD, U.S.A. 4The University of Tokyo, Tokyo 113-0033, Japan
ABSTRACT The interplanetary magnetic field (IMF) Bz was continuously southward for 16 hours on April 18, 2002. During this period, the spacecraft Geotail was in the northern tail lobe at radial distances of 24-28 RE, and it observed sequences of increasing and decreasing in the magnetic field intensity in association with substorms. The increases in the magnetic field took approximately 40 minutes, and the decreases took approximately 50 minutes. The time scale of the increase is consistent with the typical time scale of the substorm growth phase, which usually begins with a clear southward turn of the IMF Bz. The time scale of the decrease agrees well with the typical time scale of the substorm expansion phase, although the substorm expansion phase frequently starts with a northward turn of the IMF Bz. It is suggested that the magnetotail has an intrinsic time scale for the loading-unloading process for substorms irrespective of the IMF Bz conditions.
Introduction
It is known that the tail lobe magnetic field increases in the substorm growth phase and decreases in the substorm expansion phase (e.g., Caan et al., 1975, Fairfield et al., 1981, Kistler et al., 1993, Nagai et al., 1997). This process is interpreted as the loading and unloading of energy in the magnetotail for substorms (e.g., Baker et al., 1996). The loading process usually starts when the interplanetary magnetic field (IMF) Bz turns southward, and the major onset of a substorm occurs after the IMF Bz continues to be southward for 40-60 minutes. The unloading process usually corresponds to the expansion phase of the substorm, and it typically continues for 40 minutes on the ground. Since the major onset is frequently associated with a northward turn of the IMF Bz, the duration of 40 minutes for the unloading process is obtained for the northward IMF Bz period (e.g., Caan et al., 1975). On April 18, 2002, the IMF Bz was almost continuously southward, and substorms took place quasiperiodically. The spacecraft Geotail stayed almost continuously in the northern tail lobe at radial distances of 2428 RE. The observations provided a unique opportunity for examining the dynamics of the magnetotail for the steady southward IMF Bz conditions. We focus on the time scales of the loading-unloading process in this paper.
Observations
The upper panel of Figure 1 presents the IMF and the solar wind velocity observed with the spacecraft ACE for the period from 1800 UT on April 17, 2002, through 1200 UT on April 19, 2002. ACE was located near (+224, +22, -23 RE) in GSM coordinates. The data are shifted 50 minutes for the propagation time. The IMF Bz was continuously southward for the period of 0100-1800 UT on April 18, 2002. The spacecraft Wind observed almost identical behaviors in the solar wind. The lower panel of Figure 1 presents the magnetic field observation obtained with Geotail (Kokubun et al., 1994). Geotail entered the magnetotail near 2100 UT on April 17, 2002. According
-190-
to plasma measurements made with the low-energy plasma experiment (LEP) on Geotail (Mukai et al., 1994), Geotail stayed in the northern tail lobe after 0200 UT on April 18, 2002, except for a brief period in the tail lobe/plasma sheet boundary close to 1150 UT. For the period of 0400-1800 UT, there were five increase-decrease sequences in the total magnetic field, and each was associated with a southward turn of Bz in the tail lobe field. These magnetic field behaviors are known as traveling compression regions (TCRs) (Slavin et al., 1984; 1992).
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April 17-19, 2002
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Fig. 1. The upper panel presents the IMF and the solar wind velocity observed with the spacecraft ACE for the period from 1800 UT on April 17, 2002, to 1200 UT on April 19, 2002. ACE is located near (+224, +22, -23 RE) in GSM coordinates. The lower panel presents the magnetic field observation from Geotail in GSM.
-191-
In order to find substorm activities, we examined ground magnetic field data from the mid-latitude stations Fredericksburg, Tucson, Honolulu, Kakioka, Urumqi, and Hermanus and from the auroral-zone stations Leirvogur, Narssarssuaq, Poste-de-la-Balaine, Fort Churchill, Yellowknife, College, Tixie Bay, and Kiruna. We also examined particle observations from the Los Alamos National Laboratory spacecraft and magnetic field observations from GOES 8 and GOES 10 at geosynchronous orbit. There were at least five major substorm onsets, at 0530, 0800, 1130, 1408, and 1632 UT for the period of 0300-1800 UT on April 18, 2002. These five onsets correspond well to the TCRs observed with Geotail in the magnetotail.
Fig. 2. The total pressure (magnetic pressure + plasma pressure)in the magnetotail observed with Geotail (thick curve) and the solar wind dynamic pressure observed with ACE (thin curve) on April 18, 2002.
Figure 2 shows the total pressure (magnetic pressure + plasma pressure) in the magnetotail observed with Geotail and the solar wind dynamic pressure observed with ACE on April 18, 2002. The solar wind data are shifted 50 minutes for the propagation time. The dynamic pressure changed significantly for the period of 00000300 UT, so we do not discuss substorm-associated pressure changes in the magnetotail. For the 0530, 0800, 1130, and 1408 UT substorms, since the total pressure in the magnetotail did not follow the solar wind dynamic pressure well for most intervals, variations in the total pressure are attributable to the loading-unloading processes in the magnetotail. For the 1632 UT substorm, the time scale of the decrease in total pressure is used. Just after each substorm onset, total pressure showed a significant peak, which was produced by a spike in Bx. These peaks are probably caused by large pressure pulses, which are produced by high-speed tailward flows inside the plasma sheet (e.g., Nakamura et al., 1999). Since the peaks are considered to be transient phenomena in TCRs, we neglect them in our analysis of the loading-unloading process. 8.0
Bs x'V in gW
'
'
Time scales
6.0
Increase Decrease
4.0 2.o o o -~
tP
I
I
ressure
Event Event Event Event Event
I
0.4
1 2 3 4 5
42-48 40-52 30-50 68-80 42 - 48 60 28- 32 38 40 - 48
1
0.3
Average 3 6 - 4 5
50-56min
0.2. 0.1
0"03
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1
-2
-1
0 Hours
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1
1
2
3
Fig. 3. The lower panel presents the changes in total pressure relative to the substorm onset (the zero epoch corresponds to each substorm onset), and the upper panel presents solar wind electric field variations.
-192-
Figure 3 shows the changes in total pressure relative to substorm onset (the zero epoch corresponds to each substorm onset) as well as solar wind electric field variations. The solar wind electric field is calculated as Esw = Bs x V (Bs = 0 for the IMF Bz > 0, and Bs = -Bz for the IMF Bz < 0). The figure shows that the total pressure increased approximately 40 minutes before the substorm onset and reached to the lowest level approximately 50 minutes after the substorm onset. It is important to note that the solar wind electric field stayed at different levels for different events and did not show a correlation with any change in total pressure.
Discussion
The Geotail observations on April 18, 2002, provided an opportunity for examining substorm processes in the magnetotail for the prolonged southward IMF Bz period. In this paper, we study the time scales of the loadingunloading process in the magnetotail for substorms. The loading process has a time scale of 40 minutes, even for different solar wind electric field levels, and it seems to start without any significant changes in solar wind conditions. Irrespective of the IMF Bz sign, the unloading process has a time scale of 50 minutes. These results are somewhat unexpected. A definite start time for the unloading process is not expected for a prolonged southward IMF Bz. If there is any critical limit to the total pressure in the tail for substorm onsets, we would expect the time scale of the loading process to be short for the high solar wind electric field. Furthermore, we would expect the unloading process to continue for a long time for a prolonged southward IMF Bz. The present results suggest that the magnetotail has an intrinsic time scale for the loading-unloading process for substorms. These results are still highly preliminary; however, no other periods in the Geotail observations are good for testing them. ACKNOWLEDGMENTS CDAWeb was used for the ACE, Wind, Los Alamos National Laboratory spacecraft, and GOES data. We thank N. Ness and D. J. McComas for the ACE data. We thank T. Mukai for providing the LEP data from Geotail. The ground magnetic field data are provided by WDC-C2, Kyoto University. REFERENCES Baker, D. N., T. I. Pulkkinen, V. Angelopoulos, W. Baumjohann, and R. L. McPherron, Neutral line model of substorms: Past result and present view, J. Geophys. Res., 101, 12975-13010, 1996. Caan, M. N., R. L. McPherron, and C. T. Russell, Substorm and interplanetary magnetic field effects on the geomagnetic tail lobes,,/. Geophys. Res., 80, 191-194, 1975. Fairfield, D. H., R. P. Lepping, E. W. Hones, Jr., S. J. Bame, and J. R. Asbridge, Simultaneous measurements of magnetotail dynamics by IMP spacecraft, J. Geophys. Res., 86, 1396-1414, 1981. Kistler, L. M., W. Baumjohann, T. Nagai, and E. M6bius, Superposed epoch analysis of pressure and magnetic field configuration changes in the plasma sheet, J. Geophys. Res., 98, 9249-9258, 1993. Kokubun, S., T. Yamamoto, M. H. Acufia, et al., The GEOTAIL magnetic field experiment, J. Geomag. Geoelectr., 46, 7-21, 1994. Mukai, T., S. Machida, Y. Saito, et al., The low energy particle (LEP) experiment onboard the GEOTAIL satellite, .I. Geomag. Geoelectr., 46, 669-692, 1994. Nagai, T., T. Mukai, T. Yamamoto, A. Nishida, S. Kokubun, and R. P. Lepping, Plasma sheet pressure changes during the substorm growth phase, Geophys. Res. Lett., 24, 963, 1997. Nakamura, R., L. F. Bargatze, T. Mukai, et al., Response of the midtail electric field to enhanced solar wind energy input, J. Geophys. Res., 104, 17299-17310, 1999. Slavin, J. A., E. J. Smith, B. T. Tsurutani, et al., Substorm associated traveling compression regions in the distant tail: ISEE-3 geotail observations, Geophys. Res. Lett., 11,657-660, 1984. Slavin, J. A., M. F. Smith, E. L. Mazur, et al., ISEE 3 plasmoid and TCR observations during an extended interval of substorm activity, Geophys. Res. Lett., 19, 825-828, 1992.
E-mail address of [email protected]
-193-