Pages 939-948

IACG CAMPAIGN-1 SUMMARY REPORT: INITIAL RESULTS

T. Mukai1 and L. M. Zelenyi2

1
Institute of Space and Astronautical Science, Yoshinodai, Sagamihara, Kanagawa 229, Japan,
  E-mail: mukai@gtl.isas.ac.jp
2Space Research Institute, Russian Academy of Science, Moscow 117810, Russia

ABSTRACT

The IACG Campaign-1 multispacecraft coordinated observations were made in eleven time intervals during a period of October 18, 1995, through January 12, 1996, to study the global magnetotail structure and the magnetotail effects of the solar wind - magnetosphere interaction. The key spacecraft are GEOTAIL and INTERBALL with the WIND and/or IMP-8 spacecraft as a solar wind monitor. Eleven intervals cover various phases of the solar rotation, so that the multispacecraft as well as ground-based observations are expected to provide useful examples of data under various solar wind and IMF conditions. In this paper we present some initial results from selected events. They are the magnetic cloud on October 18-20, 1995, magnetotail activities under northward IMF on October 26, 1995, magnetotail effects of variations of the solar wind dynamic pressure and IMF-Bz polarity observed on November 27, 1995, and a substorm event on November 28, 1995.

INTRODUCTION

The Inter-Agency Consultative Group for Space Science (IACG) has defined four campaigns for multispacecraft mission coordination in Solar-Terrestrial Science (STS). The first campaign is on "Magnetotail Energy Flow and the Role of Nonlinear Dynamics". The scientific objectives are discussed in detail in the Airlie House Workshop report edited by Whipple and Lancaster [1992], focusing on two aspects; 1) to determine the structure of the global magnetotail system, and 2) to study the magnetotail effects of the global solar wind - magnetosphere interaction. The key spacecraft in this campaign are GEOTAIL, INTERBALL-Tail, WIND, and IMP-8. In September, 1995, one month after the successful launch of the INTERBALL-Tail satellite, IACG/WG-1 meeting was held at Sapporo, Japan, in which eleven campaign intervals were selected during a period of October 18, 1995, through January 12, 1996, based on orbit predictions. In the selected intervals, GEOTAIL and INTERBALL-Tail were located near their apogees in the mid-tail region, while WIND or IMP-8 monitored the solar wind in the upstream region. Two perigee passes of the WIND spacecraft were also included. Table 1 shows a list of the campaign intervals.

The campaign intervals covered a period of about three solar rotations, in which the solar wind and interplanetary magnetic field (IMF) conditions generally showed a recurrent pattern as is typical in quiet times of the solar activity. Eleven intervals cover various phases of the solar rotation, so that the multispacecraft as well as ground-based observations are expected to provide useful examples of data under various solar wind and IMF conditions toward achievement of the campaign goals. Especially interesting are the magnetotail activities under northward IMF. In terms of significant magnetospheric effects of the global solar wind - magnetosphere interaction, there occurred also a few special events, such as the magnetic cloud (Interval-1), and the extremely high dynamic pressure of the solar wind (November 27; Interval-6). A number of substorm events were also recorded. Initial results from several selected events are described in the following sections.

Table 1. List of IACG Campaign-1

Interval No. Dates of the Interval Events
Interval 1 Doy 291-294 (Oct. 18-21),1995 Magnetic cloud; Substorms
Interval 2 Doy 299-300 (Oct. 26-27),1995 Northward IMF
Interval 3 Doy 304-305 (Oct. 31-Nov. 1),1995 Substorms
Interval 4 Doy 315-316 (Nov. 11-12),1995 Substorms
Interval 5 Doy 321 (Nov. 17),1995 Northward IMF
Interval 6 Doy 331-334 (Nov. 27-30),1995 High Dynamics Pressure;
Substorms;WIND Perigee (DOY 333)
Interval 7 Doy 337-338 (Dec. 3-4),1995 Substorms
Interval 8 Doy 341 (Dec. 7),1995 Northward IMF;
GEOTAIL on the dayside
Interval 9 Doy 349 (Dec. 15),1995 Substorms
Interval 10 Doy 352-354 (Dec. 18-20),1995 Substorms
Interval 11 Doy 12 (Jan. 12),1996 WIND crossing MP;
Both GEOTAIL and INTERBALL in the tail;
IMP-8 in the upstream

MAGNETIC CLOUD EVENT IN INTERVAL-1

The Interval-1 started from October 18, 1995, on which a large magnetic cloud arrived at the WIND spacecraft at ~1900 UT in the solar wind upstream of the Earth at distance of 175 Re. Figure 1 shows a plot of the magnetic field and solar wind parameters from October 18 through 20. The WIND team made a preliminary analysis of the geometry of the magnetic cloud with a force-free model, and obtained the size of ~0.27 AU [Burlaga et al., 1995]. Figure 2 shows the GEOTAIL and INTERBALL orbits in the GSM coordinates, in which the positions are marked every three hours; cross for GEOTAIL and asterisk for INTERBALL. On October 18, the GEOTAIL satellite was located in the completely upstream region of the nominal bow shock, while the INTERBALL spacecraft traversed the northern lobe in the magnetotail. A shock wave was observed ahead of the magnetic cloud at ~1040 UT on October 18 by WIND, and about 45 minutes later at ~1124 UT by GEOTAIL (not shown). The magnetic cloud arrived at the GEOTAIL location (in the solar wind upstream of the Earth's bow shock) at 1945:30. The delay of about 45 minutes from WIND to GEOTAIL is consistent with the convection delay. It is noted that at 1949:30, about 4 minutes later after arrival of the magnetic cloud, GEOTAIL crossed the Earth's bow shock to the downstream magnetosheath. This is interpreted as movement of the bow shock to an unusually upstream location of the nominal position because of low Alfven Mach number of the magnetic cloud. The bow shock, however, was located close to the GEOTAIL satellite, so that GEOTAIL made a number of bow-shock crossings in the magnetic cloud. The structure of the bow shock is under study with the GEOTAIL data in the multiple crossings, which will be reported elsewhere. The arrivals of the shock wave and the magnetic cloud at the magnetosphere were also detected as a sudden impulse (SI) and a negative SI, respectively, in ground magnetograms as well as in the INTERBALL magnetic field data (not shown). Since the magnetic field in the cloud initially turned southward abruptly and then rotated gradually to a northward orientation (see Fig. 1), it is expected that the passage of the magnetic cloud produced systematic changes in the global convection pattern and other disturbances in the magnetosphere. With regard to this point, either of GEOTAIL or INTERBALL was not located properly, but it is noted that INTERBALL made multiple crossings of the tail magnetopause.

Fig.1. Magnetic field and plasma proton observations measured by WIND at ~175 Re upstream of the . Earth from October 18 through 20, 1995. The magnetic field is in GSM coordinates. [Burlaga et al., 1995]

Fig.2. GEOTAIL and INTERBALL orbits in the GSM coordinates.


MAGNETOTAIL ACTIVITIES UNDER NORTHWARD IMF

As noted in Table 1, there were a few intervals in which the IMF-Bz remained positive for a prolonged period. It is well known that a theta aurora and/or sun-aligned arcs appear in the polar cap under such IMF conditions, while the geomagnetic activities become quiescent at auroral and lower latitudes. However, the global magnetospheric structure associated with these low-altitude signatures is not yet resolved, since the structure and dynamics of the magnetotail under northward IMF have not well been established. The IACG Campaign-1 GEOTAIL and INTERBALL observations have provided unique examples of data from this point of view.

Figure 3 shows a plot of the magnetic field and plasma parameters during a period of 1200 to 2000 UT on October 26, 1995, observed by GEOTAIL. The IMF-Bz remained positive throughout this day, except for a few short periods of time (<30 min.) in which Bz became negative. On this day the GEOTAIL satellite traversed the magnetotail from dusk to dawn at ~20 Re downtail, while INTERBALL was located at high Zgsm coordinate (~15 Re) on the dawnside at downtail distances of 12 to 23 Re. Thus the GEOTAIL and INTERBALL data are quite interesting for studies of the magnetotail structure and dynamics under Northward IMF. The Kp indices were 0, 0+, 0, 1-, 0+, 1+, 1+, and 2 every three hours from the beginning of the day. Hence, the magnetosphere is expected to be quiet, yet it is remarkable that bursty bulk flows were frequently observed during the period of 1200 to 1500 UT near the neutral sheet at Xgsm ~ -21 Re, as shown in Fig. 3. They were accompanied by highly turbulent magnetic fields. This suggests that hot plasmas near the neutral sheet are not quiet but at times activated even under northward IMF for a prolonged period of time. The similar features are also observed in other cases near the neutral sheet around the midnight meridian under similar IMF conditions, and hence this phenomenon is not exceptional. They might represent an intrinsic instability of the current sheet. Further studies are needed to clarify the nature and mechanism(s) of this phenomenon. Another interesting phenomenon is the appearance of cool, dense plasmas during the time intervals of 1820 to 1845 UT and of 1920 to 1940 UT. The ion temperature is cooler than that of the ambient plasma sheet, but much higher (by an order of magnitude) than the typical value in the lobe. The boundary of their appearance is very close to the neutral sheet (Bx ~ 0). It is noteworthy that in association with this phenomenon, earthward flowing hot plasmas were observed by INTERBALL (not shown), which was located about 16 Re northward of GEOTAIL; the Xgsm and Ygsm coordinates of both satellites were nearly equal. They might be related to the sun-aligned arcs (or, theta aurora) in the polar cap. The implication of the phenomenon to the global structure and by dynamics is under study.

Fig. 3. Magnetic field and plasma (ion) parameters during a time period of 1200 to 2000 UT on October 26, 1995, measured by GEOTAIL. The magnetic field and velocity are in GSM coordinates.

NOVEMBER-27 EVENT: MAGNETOSPHERIC RESPONSE TO VARIATIONS OF THE SOLAR WIND DYNAMIC PRESSURE AND IMF-BZ POLARITY

On November 27 approximately one day before the bow shock crossing, the WIND spacecraft was approaching the magnetosphere and encountered a co-rotating interaction region. Figure 4 shows a daily plot of the interplanetary magnetic filed and solar wind parameters. The GSM coordinates of the WIND location were (20.6, 41.2, 5.5), (16.9, 39.3, -5.5), (13.1, 36.4, -9.2), (9.1, 34.9, 0.1), and (4.9, 31.6, 3.9), respectively, at the times every 6 hours from the beginning of the day. In the interaction region, the density increased up to ~ 100 cm-3 (139 cm-3 at the peak during a short interval of several min.) in the course of the velocity increase, so that the solar wind dynamic pressure increased by an order of magnitude. As shown in Figure 5, the GEOTAIL satellite was moving from dusk to midnight with increasing distance from 5 to 25 Re downtail, while the INTERBALL satellite was in the northern lobe in the midnight meridian with decreasing distance of 29 to 22 Re. In the following we discuss the magnetotail response to variations of the solar wind dynamic pressure and IMF-Bz polarity.

Fig.4. Magnetic field and plasma proton parameters measured by WIND on November 27, 1995.

Fig.5. GEOTAIL and INTERBALL orbits in the GSM coordinates.

Figure 6 shows a plot of the solar wind dynamic pressure, IMF-Bz, the total pressure at the GEOTAIL location, the magnetic pressure at the INTERBALL location, and the CANOPUS CU/CL indices during a time interval of 0800 to 1600 UT. Here, the GEOTAIL total pressure denotes the sum of the magnetic pressure and ion thermal pressure. Neglect of the electron pressure does not produce a significant error during this time interval, in which the spacecraft was located mostly in regions of plasma   1 and the electron pressure was ~15% of the ion contribution. The INTERBALL magnetic pressure also represents the total pressure, since the satellite stayed mostly in the lobe except for a few time periods, such as from 1230 to 1315 UT and from 1340 to 1405 UT, in which the INTERBALL satellite entered the plasma sheet. In Fig. 6, it is clearly seen that the tail pressure varied quickly in response to gradual as well as stepwise variations of the solar wind dynamic pressure, taking account of the convection delay of the solar wind from WIND to GEOTAIL and INTERBALL locations. For example, the tail pressure at GEOTAIL and INTERBALL increased sharply at ~0830 and ~0835 UT, corresponding to a stepwise increase of the solar wind dynamic pressure at ~0820 UT measured by WIND. The correspondence for an impulsive enhancement around 1100 UT is also clear. A possible effect of fast waves that might be generated by compression near the subsolar magnetopause was not clear in the tail. After the stepwise increase at ~0820 UT, the solar wind dynamic pressure gradually increased until ~0915 when it reached ~20 nPa. It is noted that this value of the solar wind dynamic pressure is greater by an order of magnitude than that at the beginning of the day (~2 nPa; not shown). Thereafter the dynamic pressure remained nearly constant around 20 nPa for a while (about one and half hours). However, the tail pressure continued to increase after the increase in the solar wind dynamic pressure had ceased, until the GEOTAIL pressure reached a peak of ~2 nPa at ~1000 UT, and the INTERBALL pressure got a peak of 1.2 nPa several minutes later. The continuing increase in the tail pressure can be interpreted in terms of strong southward IMF-Bz, under which the magnetic flux might be transported from the dayside merging region to the tail lobe and stored there. It is also noted that the Bz component of the tail magnetic field measured by GEOTAIL (see Figure 7) turned toward negative after ~0935 UT. This suggests that the stored flux in the lobe increased the flaring angle of the magnetotail as well as the tail pressure. The variation of the tail pressure after ~1000 UT is discussed later in terms of the substorm association. After ~1100 UT, the IMF changed to northward, and the solar wind dynamic pressure decreased gradually down to ~3 nPa at ~1145 UT. Correspondingly, the tail pressure also decreased.

Fig. 6. Solar wind dynamic pressure, IMF-Bz, total pressure at the GEOTAIL location, magnetic pressure at the INTERBALL location, and CANOPUS CU/CL indices during a time period of 0800 to 1600 UT on November 27, 1995.

Fig. 7. Magnetic field and plasma (ion) parameters during a time period of 0800 to 1600 UT on November 27, 1995, measured by GEOTAIL. The magnetic field and velocity are in GSM coordinates.

With regard to substorm activities, it seems curious that any large expansion onset did not occur in spite that the total pressure at the GEOTAIL location continued to increase up to ~2 nPa, by an order of magnitude higher than the average value at ~20 Re downtail, and IMF-Bz remained southward for ~2 hours. The CANOPUS CL index shows continuous electrojet activities with considerable intensity (a few hundreds of nT), which did not grow up to a clear expansion, however. At ~1010 UT, a big substorm expansion onset took place (see the CANOPUS/CL data), which might be related to impulsive northward turning of IMF at ~0955 UT. As shown in Figure 7, the magnetic Bz component measured by GEOTAIL changed the slope from negative to positive, corresponding to the substorm onset. This is a typical substorm signature in the near-Earth tail lobe. Before the big expansion onset, a number of fast flow bursts in the earthward direction were observed during the period of 0700 to 0920 UT by GEOTAIL, as shown in Figs. 7 and 8. They might be associated with many small substorms, or represent continuous dissipation of the tail energy but sporadically in space/time. The tail pressure continued to increase with decreasing plasma beta, and finally GEOTAIL exited to the lobe around the peak pressure at ~1000 UT. The plasma sheet might become very thin owing to the extremely high pressure in the lobe. The tail pressure began to decrease at ~1000 UT at the GEOTAIL location and several minutes later at INTERBALL. The decrease of the tail pressure does not correspond to a specific variation of the solar wind dynamic pressure or of the IMF-Bz polarity, but represents dissipation of the stored energy in the tail. Since the decrease of pressure at the INTERBALL location was delayed from the GEOTAIL signature, the region of energy dissipation might propagate tailward, while the substorm expansion onset took place further later (by several minutes) from the beginning of the pressure decrease at INTERBALL. This time sequence provides a hint for an important issue about the place and mechanism(s) triggering the substorm, but it is difficult to get a definite conclusion only with the presently available data. Further studies of the GEOTAIL and INTERBALL data in combination with other data, such as from geosynchronous satellites, will elucidate this problem more clearly.

Fig. 8. Energy-time spectrograms of electrons and ions in four azimuthal sectors observed by GEOTAIL during the same interval as shown in Fig. 7. The data are based on the key parameters with coarser time resolution (64 sec.) than the original time resolution (~12 sec.).

The substorm intensification took place twice at ~1048 and ~1100 UT, and the associated bursts of fast earthward flows were detected by GEOTAIL. After ~1200 UT, the tail pressure became nearly the average level but still larger. Thereafter, the tail pressure increased slightly and then began to decrease abruptly at ~1415 UT. Though the variation was not so prominent as for the big substorm case at ~1010 UT, this abrupt decrease of the tail pressure might be associated with substorm activities. Similar signatures were also observed at lower levels of the tail pressure in association with the substorm onset at ~1615 and ~1820 UT (not shown).

It is also noted that intense fluxes of cold ions were flowing tailward in the lobe, as seen in Figure 8. The cold ions are distributed at two discrete energies, the lowest energy (~30 eV/q) and several hundreds of eV/q; for example, see the ion spectra in the tailward azimuthal sector around 1020, 1130, 1500, and 1640 UT. Careful examination of their distribution functions have revealed that they are cold protons and singly charged oxygen ions flowing with nearly the same speeds. They are obviously of the ionospheric origin. The presence of cold ions in the lobe is also identified by INTERBALL (not shown). It is well known that the upflowing ions are enhanced in geomagnetically active periods. They might be transported toward the equatorial region in the near-Earth tail by enhanced convection during the active period.

In conclusion, this event clearly shows that the magnetotail pressure can quickly increase with increase of the solar wind dynamic pressure, while it also increases during southward IMF-Bz (possibly with time delay). However, the level of the tail pressure, or the duration of southward IMF is not a good measure for the threshold triggering a substorm expansion. The relation of fast flow events with substorms is not clear before ~1000 UT, but they obviously indicate the tail activities corresponding to continuous electrojet activities. Probably, the release of the stored energy was being consumed continuously as a whole but sporadically in space/time. The initiation of substorm expansion may need different (unknown) mechanism(s) leading to explosive instabilities in the neutral sheet. The northward turning is one of possible candidates for triggering, as has been discussed by many researchers. The tail pressure decreases clearly in association with the substorm expansion onset, which indicates the energy dissipation by substorms.

SUBSTORM EVENT

As noted in Table 1, a number of substorms also took place during several campaign intervals. In most cases, GEOTAIL observed the substorm-associated fast tailward/earthward flowing plasmas and/or plasmoids/flux ropes, while INTERBALL frequently observed a characteristic variation of the magnetic field intensity in the lobe as described in the previous section. However, there occurred a fortunate case in which both the INTERBALL and GEOTAIL satellite observed fast flows simultaneously near the neutral sheet at different downtail distances (-12 Re and -27 Re for INTERBALL and GEOTAIL, respectively) but at similar values of the Ygsm coordinate in association with a substorm around 1130 UT on November 28, 1995 (see Fig. 6 for the satellite locations). The details will be reported elsewhere, but it is noteworthy that INTERBALL observed energetic earthward flow bursts with positive Bz, while GEOTAIL observed tailward flowing plasmas associated with negative Bz. This event provides evidence for occurrence of magnetic reconnections in a region between both satellites associated with the substorm.

CONCLUDING REMARKS

The IACG Campaign-1 coordinated multispacecraft observations have provided a number of important data to study the global structure of the magnetotail and the magnetotail effects of the global solar wind - magnetosphere interaction. In this paper, we have presented some initial results from selected events. Further detailed data analysis in combination with theoretical and simulation studies will bring forth fruitful results for the campaign goals.

ACKNOWLEDGMENTS

We are greatly indebted to M.H. Acuna of NASA/GSFC, A. Nishida of ISAS, and A.A. Galeev of IKI for their efforts toward success of GGS/ISTP spacecraft including WIND, GEOTAIL, and INTERBALL.

A. Pedersen, the IACG/WG-1 chairperson, and J. L. Green, the lead coordinator are gratefully acknowledged for coordination of the IACG Campaign-1. The content of this report is based on discussions in a small workshop held in June at the Space Research Institute of Russian Academy of Science. Figures 2 and 5 (Plots of the GEOTAIL and orbital data in the GSM coordinates) were produced for the workshop by V. Prokhorenko of IKI. Participants and data providers are listed below.

GEOTAIL
ISAS A. Nishida, K. Tsuruda, T. Mukai, T. Yamamoto, H. Hayakawa,
M. Hoshino, Y. Saito, S. P. Petrinec
Univ. Tokyo T. Terasawa
Tokyo Inst. Tech. T. Nagai, M. Fujimoto
Nagoya Univ. S. Kokubun, K. Maezawa
Kyoto Univ. H. Matsumoto, T. Kojima, S. Machida



INTERBALL
IKI L. Zelenyi, N. Borodkova, E. Budnik, A. Fedorov, Y. Yermolaev,
G. Zastenker, S. Savin, M. Nozdrachev, A. Petrukovich,
V. Prokhorenko, V. Lutsenko
IRF/Sweden I. Sandahl
CNES/CESR J. -A. Sauvaud
Univ. St. Petersburg V. A. Sergeev



WIND, IMP-8 and Other Key Parameters
NASA/GSFC K. Ogilvie, R. P. Lepping, ISTP/SPOF
MIT A. J. Lazarus, J. T. Steinberg



CANOPUS
Univ. Alberta G. Rostoker


REFERENCES

Burlaga, L.F., R.P. Lepping, W. Mish, K.Ogilvie, A. Szabo, A.J. Lazarus, J.T. Steinberg, A magnetic cloud observed by WIND on October 18-20, 1995, URL homepage at http://iacg.org/iacg/campaign_1/paper/WC0CT95.html, 1995.

Toward a new era of global solar-terrestrial research "The first Inter-Agency Consultative Group (IACG) Campaign: magnetotail energy flow and non-linear dynamics", IACG Workshop Report edited by E.C. Whipple and H. Lancaster, October 1992.