Frontiers in Magnetospheric Plasma Physics, Volume 16 (Cospar)

TEN YEARS OF GEOTAIL AND ITS CURRENT STATUS: A BRIEF SUMMARY T. Mukai Institute of Space and Astronautical Science/JAXA, Yoshinodai, Sagamihara 229-8510, Japan

ABSTRACT Since its launch in 1992, GEOTAIL has extensively surveyed the magnetotail with a full set of plasma and field instruments over a wide range of distances from 9 Re to 220 Re away from the Earth by means of a sophisticated orbit strategy. In the first two years, the orbit was optimized to explore the distant tail, and thereafter was changed to study substorm processes in the near-Earth tail region. The near-tail orbit has also facilitated exploration of the dayside outer magnetosphere, the magnetopause, the magnetosheath, and the bow shock, as well as the upstream solar wind. GEOTAIL observations have revealed a number of new phenomena in these regions, and as of the end of 2003, about 600 papers have been published in refereed journals. Recent GEOTAIL studies have significantly advanced our understanding of the structure and formation of thin current sheets in the mid-tail plasma sheet during substorms, and have elucidated new kinetic aspects of magnetic reconnection. GEOTAIL has operated far beyond the designed mission life of three and half years. Most of the onboard instruments are still functioning well, and it is expected that GEOTAIL will continue to generate scientifically useful data.

GEOTAIL MISSION The GEOTAIL spacecraft was launched on 24 July 1992 by a Delta-II launch vehicle from Cape Canaveral, Florida, U. S. A. This is a joint program of the Institute of Space and Astronautical Science (ISAS) of Japan and the National Aeronautics and Space Administration (NASA) of U. S. A. ISAS developed the spacecraft and provided about two thirds of the science instruments, while NASA provided the launch and about one third of the science instruments. The spacecraft is operated from ISAS, but the data are acquired by both agencies. The prime objective is to investigate the structure and dynamics of the Earth's magnetotail with a comprehensive set of scientific instruments [Nishida, 1994]. As shown in Figure 1, in the first two years, the apogee was maintained on the nightside of the Earth by means of the lunar double swingby maneuvers, ranging from 80 to 220 Re in order to explore the distant tail. Later, from November 1994 the apogee was reduced first to 50 Re and then to 30 Re in order to study substorm processes in the near-Earth tail region. The perigee was set at 10 Re, and in June 1997, slightly reduced to about 9 Re. The inclination was set at -7° with respect to the ecliptic plane, so that the spacecraft has most frequently traversed the neutral sheet at the apogee near the December solstice. This orbit strategy has worked highly satisfactorily for the mission objectives, and also enabled exploration of the dayside outer magnetosphere, the magnetopause, the magnetosheath, the bow shock, and the upstream solar wind.

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Figure 2 shows a schematic view of the GEOTAIL spacecraft. The spacecraft weighed 1008kg initially, but now it is about 660 kg, since most of hydrazine propellant has been consumed for the orbit maneuvers. It has a cylindrical shape with diameter of 2.2 m and height of 1.6 m. Two masts which are 6-m long are deployed symmetrically to separate the magnetometers from the main body, and four 50-m antennas are deployed to measure the electric field from DC to 800 kHz. The spacecraft is spin-stabilized with the spin axis being nearly perpendicular to the solar ecliptic plane. The spin rate is 20 rpm.

Figure 2 A schematic view of the GEOTAIL spacecraft on orbit.

Figure 1 GEOTAIL orbit in the Geocentric Solar Magnetospheric (GSM) coordinates. The upper and lower panels show the distant-tail orbit (from September 1, 1992 to November 10, 1994) and the near-tail orbit (from November 10, 1994 to February 5, 1996), respectively. The present one is nearly the same as that shown in the lower panel.

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BRIEF SUMMARY OF RESULTS FROM GEOTAIL The initial GEOTAIL results were summarized in a special section of Geophysical Research Letters [December, 1994], and some highlights were compiled in Journal of Geomagnetism and Geoelectricity [Vol. 48, Nos. 5,6, 1996], a special section of Journal of Geophysical Research [March, 1998] and the AGU Geophysical Monograph 105 "New Perspectives on the Earth's Magnetotail" [1998]. Nishida [2000] reviewed the dynamic magnetotail based on the GEOTAIL observations. As of the end of 2003, about 600 papers have been published in refereed journals, as shown in Figure 3.

Figure 3

Number of GEOTAIL papers

Several new discoveries have resulted from the GEOTAIL observations, not only due to the orbit strategy optimized to the first extensive survey of the magnetotail over a wide range of distances, but also as a result of sophisticated measurement techniques. For example, the discovery of electrostatic solitary waves (ESW) in the plasma sheet boundary layer was due to the waveform capture (WFC) technique of the Plasma Wave Instrument [Matsumoto et al., 1994]. The WFC method has produced a number of new findings in wave properties in the magnetotail, the dayside magnetosphere, the magnetosheath, the bow shock and the upstream solar wind. Another important contribution to the success of GEOTAIL was the high time resolution measurement of 3-D distribution functions of ions and electrons with the Low Energy Particle (LEP) experiment [Mukai et al., 1994]. In particular, the LEP ion measurements with high sensitivity (large geometrical factors) have been quite effective for measurements of tenuous plasmas in the magnetotail; for example, the discovery of cold oxygen ion beams in the distant lobe/mantle regions [e.g., Seki et al., 1998]. The LEP ion measurements have also provided 24-hour continuous data of the ion fluid parameters (velocity moments; density, velocity and temperature), which have

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been very useful for statistical analysis of plasma properties. Combined with the magnetic field data, these plasma properties have been used to investigate the magnetotail structure and dynamics, which are well expressed in terms of the ideal MHD. Magnetic reconnection is one of the most important processes in space plasmas. Its in-depth understanding was the main target of GEOTAIL in terms of space plasma physics, and in the following we briefly summarize some important results on magnetic reconnection. Firstly, GEOTAIL has established that that the structure and dynamics of the magnetotail are basically determined by magnetic reconnection under both southward and northward IMF conditions, except possibly when IMF is almost due northward [Nishida et al., 1998]. A further important finding is that the neutral sheet in the distant tail is twisted under the influence of the By component of IMF, the twisting being more severe when the IMF is northward. GEOTAIL observations have clearly demonstrated the importance of magnetic reconnection for the magnetospheric substorms. Statistical results of plasma flow properties suggest that near-Earth reconnection starts before the expansion phase starts on the ground, and that this neutral line is initially formed at the distance of 22-30 Re in the midnight-premidnight region, coinciding with the local time range where the auroral and geomagnetic signatures of the expansion phase onset take place [Nagai, et al., 1998]. Studies on evolution of plasmoids have revealed that plasmoids, after initial formation in association with the near-Earth neutral line, expand toward the flank sides (dawnwad and duskward) during tailward propagation until the mid-tail region (-70 Re), and they have a full width of the magnetotail beyond this distance down the tail [Ieda et al., 1998]. The plasmoids are accelerated tailward until -100 Re, whereas the tailward speed is reduced beyond this distance, probably due to an interaction with pre-existing plasmas. In addition to the above MHD features, GEOTAIL observations of electron and ion velocity distribution functions have produced a new kinetic understanding of plasmoids and flux ropes [Mukai et al., 1998]. Characteristic signatures of the ion distribution functions are identified during the passage of plasmoids, and by comparison with computer simulations, they are qualitatively and quantitatively understood in terms of temporal/spatial evolution of the particle acceleration and heating by the magnetic reconnection [Hoshino et al., 1998]. Suprathermal electron acceleration in magnetic reconnection is also revealed by comparison between GEOTAIL observations and computer simulations [Hoshino et al., 2001]. Recent studies have significantly advanced our understanding of the evolution of thin current sheets and the associated occurrence of magnetic reconnection during the course of substorms [e.g., Mukai et al., 2000; Asano et al., 2003]. For these studies, the availability of reliable electron data has been crucial, and the structure of an extremely thin current sheet has been revealed with the current density estimated from the electron and ion velocity moments. One of the most important GEOTAIL discoveries is that the cross-tail current sheet in the late growth phase and the early expansion phase temporally forms a bifurcated structure in which the current density becomes the largest away from the neutral sheet [Asano et al., 2003]. The intense current sheet is thinner closer to the X-line, and the thickness becomes less than the ion inertial length. In the thin current sheet, the current is mainly carried by electrons, contrary to the ion-dominated diamagnetic current with the higher ion temperature than the electron one. The Hall electric field toward the neutral sheet causes the electron-dominated current sheet by enhancing the dawnward drift for both ions and electrons. GEOTAIL observations have also revealed the structure of the Hall current system and characteristic features of the associated electron distribution functions carrying the field-aligned current for magnetic reconnection [e.g., Nagai et al., 2003; Asano et al., 2004]. CURRENT STATUS Two data bases, namely the Key Parameters and the Science Data Base, have been produced from experiments onboard GEOTAIL. The Key Parameters are produced by the NASA Goddard Space Flight Center from the 24-hour continuous data dumped over the NASA/DSN, and are made available to the worldwide science community for event search and preliminary studies via the NASA/CDAWeb system. The calibrated plasma and

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magnetic field data with high time resolution are archived in the Science Data Base, and are also made available to the worldwide science community via DARTS (Data ARchive and Transmission System at the Center for Planning and Information Systems, ISAS/JAXA) at . The plasma wave data and energetic particle data are also available via web-pages at the home institutions of the principal investigators, and , respectively. GEOTAIL has operated beyond the designed mission life of three and half years. The occurrence of eclipses with duration longer than the designed length (2 hours) is unavoidable, but the spacecraft could be operated without any serious problem for an eclipse lasting for 269 minutes in February 2000, the longest period in the past. Most of the onboard instruments are still functioning well, and it is expected that GEOTAIL will continue to generate scientifically useful data. ACKNOWLEDGMENTS The author thanks Professor Emeritus A. Nishida and Professor T. K. Uesugi of ISAS/JAXA, Dr. Mario Acuna of NASA Goddard Space Flight Center, and all Pi's and their teams for the success of GEOTAIL and production of excellent quality data. REFERENCES Asano, Y., T. Mukai, M. Hoshino, Y. Saito, H. Hayakawa, and T. Nagai; Evolution of the thin current sheet in a substorm observed by Geotail, J. Geophys. Res., 108, 1189, doi:l0.1029/2002JA009785, 2003. Asano, Y., T. Mukai, M. Hoshino, Y. Saito, H. Hayakawa, and T. Nagai; Current sheet structure around the nearEarth neutral line observed by Geotail, J. Geophys. Res., 109, A02212, doi:1029/2003JA010114, 2004. Hoshino, M., T. Mukai, T. Yamamoto, and S. Kokubun; Ion dynamics in magnetic reconnection: Comaprison between numerical simulation and Geotail observation, J. Geophys. Res., 103, 4509-4530, 1998. Hoshino, M., T. Mukai, T. Terasawa, and I. Shinohara; Suprathermal electron acceleration in magnetic reconnection,/. Geophys. Res., 106, 25,979-25,997, 2001. Ieda, A., S. Machida, T. Mukai, Y. Saito, T. Yamamoto, A. Nishida, T. Terasawa, and S. Kokubun; Statistical anaysis of the plasmoid evolution with Geotail observations, J. Geophys. Res., 103, 4453-4465, 1998. Matsumoto, H., H. Kojima, T. Miyatake, Y. Omura, M. Okada, I. Nagano, and M. Tsutsui; Electrostatic solitary waves (ESW) in the magnetotail: BEN wave forms observed by GEOTAIL, Geophys. Res. Letter, 21, 29152918, 1994. Mukai, T., S. Machida, Y. Saito, M. Hirahara, T. Terasawa, N. Kaya, T. Obara, M. Ejiri, and A. Nishida, The Low Energy Particle (LEP) experiment onboard the GEOTAIL satellite,./. Geomag. Geoelectr., 46, 669-692, 1994. Mukai, T., S. Machida, and T. Yamamoto; Dynamics and kinetic properties of plasmoids and flux ropes: GEOTAIL observations, "New Perspectives on the Earth 's Magnetotail" ed. A. Nishida, D. N. Baker and S. W. H. Cowley, Geophys. Mono., 105, 117-137, 1998. Mukai, T., T. Nagai, M. Hoshino, Y. Saito, I. Shinohara, T. Yamamoto, and S. Kokubun; GEOTAIL observations of magnetic reconnection in the near-Earth magnetotail, Adv. Space Res., 25 (No. 7/8), 1679-1684, 2000. Nagai, T., and S. Machida; Magnetic reconnection in the near-earth magnetotail, "New Perspectives on the Earth's Magnetotail" ed. A. Nishida, D. N. Baker and S. W. H. Cowley, Geophys. Mono., 105, 211-224, 1998. Nagai, T., I. Shinohara, M. Fujimoto, S. Machida, R. Nakamura, Y. Saito, and T. Mukai; The structure of the Hall current system in the vicinity of the magnetic reconnection site, J. Geophys. Res., 108, A10, 1357, doi: 10.1029/2003JA009900, 2003. Nishida, A.; The Geotail mission, Geophys. Res. Letters, 25, 2871-2873, 1994. Nishida, A., T. Mukai, T. Yamamoto, S. Kokubun, and K. Maezawa; A unified model of the magnetotail convection in geomagnetically quiet and active times, J. Geophys. Res., 103, 4409-4418, 1998. Nishida, A.; The Earth's dynamic magnetotail, Space Scie. Rev., 91, 507-577, 2000. Seki, K., M. Hirahara, T. Terasawa, T. Mukai, Y. Saito, S. Machida, T. Yamamoto, and S. Kokubun; Statistical properties and possible supply mechanisms of tailward cold O + beams in the lobe/mantle regions,./. Geophys. Res., 103, 4419-44Z9, 1998.

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SECTION 1: Magneto spheric Dynamics

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CLUSTER: NEW VIEW ON THE BOUNDARIES OF THE MAGNETOSPHERE C. P. Escoubet and M. Fehringer ESA/ESTEC, SCI-SH, Keplerlaan 1, 2200 AG Noordwijk, The Netherlands

ABSTRACT After 1.5 years of operations, the Cluster mission is fulfilling successfully his scientific objectives. The main goal of the Cluster mission is to study, in three dimensions, the small-scale plasma stuctures in the key plasma regions in the Earth environment: solar wind and bow shock, magnetopause, polar cusps, magnetotail, and auroral zone. The relative distance between the four spacecraft is varied, according to the scientific region, between 100 and 18000 km during the course of the mission. During the first phase of the mission, the four spacecraft crossed the exterior cusp (Feb. 2001) with an inter-spacecraft distance of 600 km, during the second phase they were in the magnetotail (Aug. 2001) with 2000 km inter-spacecraft distance. Since January 2002, the four spacecraft are again in the cusp/solar wind with the smallest inter-spacecraft distance of 100 km and in June 2002, in the tail, this distance will be increased to 4000 km.. A few results obtained during the first 1.5 years of operation are presented as well as the access to data through the Cluster Science data System and the future operations for the extended mission.

INTRODUCTION Cluster is one of the two missions - the other being the Solar and Heliospheric Observatory (SOHO) constituting the Solar Terrestrial Science Programme (STSP), the first 'Cornerstone' of ESA's Horizon 2000 Programme. The Cluster mission was first proposed in November 1982 in response to an ESA call for proposals for the next series of scientific missions. After the tragic accident of Ariane 5 on 4 June 1996 and the destruction of the four original Cluster spacecraft, the Cluster scientists convinced the ESA Science Programme Committee (SPC) that it was essential for the European scientific community to rebuild the spacecraft. This was agreed by the SPC in April 1997. In the meantime, SOHO, launched in December 1995, had begun to make some exciting discoveries about the Sun and its environment. Now, with the successful launch of the rebuilt Cluster satellites, the STSP Cornerstone is complete and it is possible to combine these two missions in order to study the full chain of processes from the Sun's interior to the Earth. When the first Soyuz blasted off from Baikonur Cosmodrome on 16 July 2000, we knew that Cluster was well on the way to recovery from the previous launch setback. However, it was not until the second launch on 9 August 2000 and the proper injection of the second pair of spacecraft into orbit that we knew that the Cluster mission was back on track. In fact, the experimenters said that they knew they had a mission only after switching on their last instruments on the fourth spacecraft. During the first 1.5 years of operations, Cluster visited the bow shock, the polar cusp, the magnetopause, the plasmasphere, the auroral region and the magnetotail.. In a first section, the Cluster instrumentation will be briefly described, then a few examples of Cluster observations will be presented and finally the data distribution through the Cluster Science Data System will be presented.

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MISSION The scientific objectives of the Cluster mission are to study in three dimensions the plasma structures observed at the bow shock magnetopause polar cusp and magnetotail In addition the temporal variations of structures observed in the auroral zone mid-altitude polar cusp plasmasphere can be studied for the first time as the spacecraft are aligned as a "string of pearls" near perigee. To perform these objectives, the Cluster spacecraft have been placed on a 4x19.6 RE polar orbit (Figure 1). The spacecraft have a slightly different orbit to form a perfect tetrahedron in key regions of space like the Northern polar cusp, Southern magnetopause and plasmasheet (Figure 1)

Fig. 1. Regions of the magnetosphere crossed by the Cluster spacecraft. The left panel shows the orbit in February and the right panel in August. Table 1. Spacecraft separation distances Year 2001 2001 2002 2002 2003 2003 2004 2004 2005 2005

Phase Cusp Tail Cusp Tail Cusp Tail Cusp Tail Cusp Tail

Separation (km) 600 2000 100 4000 5000 100-700 100-700 10000 10000-20000 20000

Fig. 2. Separation distances during the course of the mission

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The separation distance is changed approximately every 6 months to study a particular physical process (Table 1 and Figure 2). It was decided to start with small distances (down to 100 km) and then to increase it toward the end of the mission (up to 20000 km). All measurements at small distances have to be done first since after 20000 km, the remaining fuel will not allow to substantially change the separation distance any more. The Cluster mission has been extended an additional 3 years from January 2003 to December 2005 to cover more separation distances and spend more time at a particular distance. After early 2003, the constellation manoeuvres will be done once a year that means that a tetrahedron will be formed in the tail and 6 months later in the Northern cusp without any manoeuvres in between. This was possible with the innovative manoeuvre method used by the Flight Dynamics Team at the European Space Operations Centre (ESOC). The small distances in the magnetotail in Aug. 2003 were not in the initial planning of the Cluster mission. But were recommended afterwards by the International Space Science Institute substorm working group. The small scales in the tail are necessary to investigate the processes that produce geomagnetic substorms. There are two competing models: magnetic reconnection and current disruption. The existence of a small " diffusion " region where the plasma is rapidly accelerated is expected in the first model, while a disruption of cross-tail current is expected in the second model. Both phenomena have a scale size of approximately 500 km, which will require a spacecraft separation distance of a few hundred kilometres to be studied. This separation distance will be achieved in the first year of extension. In addition, the mission data return has also been augmented by adding a second receiving ground-station. Due to the very large amount of data produced by Cluster, the baseline data return was limited to 50% of the orbit. After a few months of operations, it was however realised that many explosive events were missed due to their unpredictable behavior (sudden storm commencement, substorms, storms) and large scientific regions (like magnetotail and North and South cusp) could not be fully observed. In February 2002, the ESA SPC agreed to extend both the data coverage and the mission. The 100% coverage started in June 2002.

INSTRUMENTATION Each Cluster spacecraft contains a complete suite of instruments to measure magnetic fields, electric field, electromagnetic waves, and particles (Table 2). In addition a potential control instrument keep the spacecraft potential close to a few Volts positive in very tenuous plasma. More details on the payload can be found in Escoubetetal. (2001). Table 2: The 11 instruments on each of the four Cluster spacecraft. Instrument

Principal Investigator

ASPOC (Spacecraft potential control)

K. Torkar (IWF, A)

CIS (Ion composition, 0<E — 0.3Xgsm) plotted separately. One can see that the typical density in this region is 0.5 cm" 3 on both sides. With smaller probabilities, higher density > 1 cm" 3 samples appear preferentially on danwside. By noting that some of them have temperatures above 2 keV, we can tell that HDIs are present at R < 15 Re on dawnside. Let us now focus on the HDIs. The procedure for selecting HDI intervals is the same as CDPS in the previous section except that the criteria for the ion temperature is now T > 2 keV (instead of < 2 keV). We plot in Figure 4 the spatial distribution of the HDI intervals by plus symbols. We plot only those detected consecutively for more than 3 hours. Excluding one exceptional event, all the long duration HDI intervals are on dawnside at the radial distances of ~ 12Re. Since the region closer than 10i?e to the Earth is not surveyed by Geotail, the inner limit of HDI cannot be determined by the present dataset.

Fig. 5. IMF Bz for HDIs under nominal solar wind condition. T=0 are the start times of the events. Data for T=—6 to +6 hours are shown. Tendency for northward IMF from 3 hours prior to and during the events can be seen.

Inspection of the concurrent solar wind condition (WIND KP data, courtesy, A. J. Lazarus) shows that some of the long-duration HDI events plotted in Figure 4 are associated with elevated solar wind density -23-

and/or dynamic pressure. Since these cases are more or less trivial, we exclude the HDI events that are associated with solar wind density exceeding 20 cm~3 or dynamic pressure exceeding 5 nPa. We re-plot by solid squares in Figure 4 the long duration HDI events detected under normal solar wind condition. It can be seen that the exceptional case on duskside is due to the elevated solar wind density and long duration HDIs under normal solar wind condition are distributed only on dawnside. Figure 5 shows the IMF condition (WIND KP data, courtesy, R. Lepping) for normal solar wind density HDI events. T=0 in this plot corresponds to the start time of the HDI events and the IMF Bz component from T=—6 to +6 hours interval are shown. Time lags are calculated by Xgsmtw/Vsw {Xgsm^y. GSM X of WIND, Vsw: concurrent solar wind speed) and are taken into account in plotting Figure 5. The plot suggests a tendency for these long duration HDIs to appear during extended northward IMF. It should be noted that CDPS also tends to appear during northward IMF (e.g., Terasawa et al., 1997). Indeed, this study on HDI is initiated by the idea that a category of CDIs in the plasma sheet might transform to HDIs in the inner magnetosphere while they experience more heating in the way. The fact that both CDPS and HDIs favor northward IMF implies that both appear under northward IMF with HDI being the inner-magnetosphere extension of CDPS.

"SNAPSHOTS" TAKEN UNDER STEADY NORTHWARD IMF The above statistical analyses reveal the structure of the plasma sheet under northward IMF as follows: CDIs appear on both flanks. CDIs on the dawnside are heated as they approach the inner-magnetosphere. The temperature of CDIs on duskside stays below 1 keV, suggesting no heating and thus no approach to the inner-magnetosphere is in action on this side of the plasma sheet. This asymmetric transport leads to the asymmetric distribution of the HDIs in the inner-magnetosphere. More than 60 extended CDI and 9 extended HDI intervals obtained in 1995-99 set the basis for this conclusion.

Fig. 6. A schematic figure to show the appropriate orbits from which snapshots of the plasma sheet structure under northward IMF can be taken, (a) Skimming the plasma sheet inner-edge, mostly symmetric about midnight, (b) From the dawnside plasma sheet inner-edge to deeper into the dawnside flank. If data are taken under steady nominal solar wind and northward IMF conditons, they should snapshot (a) dawn-dusk asymmetry of the HDIs and (b) CDI-HDI connection on dawnside, respectively.

The above picture will be clearer if we inspect data from appropriate orbits under stable solar wind/IMF condition that snapshot the spatial structures described above. In the followings, we show two examples -24-

that indeed show (1) the dawn-dusk asymmetry of HDIs and (2) the connection of CDIs to HDIs at the plasma sheet inner-edge on dawnside. The two orbits are schematically shown in Figure 6. To show the dawn-dusk asymmetry of HDIs, we look for HDIs obtained along the orbits that skim the inner-edge of the plasma sheet nearly symmetric about midnight. We then check if there is any temporal variation in the solar wind/IMF condition during the traversals of the magnetotail. If not, that would imply spatial dawn-dusk asymmetry. Figure 7a shows the data from a traversal selected in this manner. Shown are data obtained in 24 hours during which the spacecraft moved all the way from duskside LLBL slightly tailward of the terminator (0000 UT), across the midnight meridian at ~ 10 Re in the tail (1200 UT), and finally to the dawnside terminator magnetopause (2300 UT). The data in 0400-0700 UT show the duskside low-latitude (small \BX\, 3rd panel) plasma sheet at the radial distances of 13 Re and MLT=19-21. The density (1st panel) remains small (< 1 cm~3) and the temperature (2nd, in keV unit) stays high (> 4 keV) in this region. As such, this duskside plasma sheet inner-edge shows the typical status. The density is elevated to above 1 cm" 3 at 1600 UT when the spacecraft is on dawnside (MLT=2.5). The temperature keeps > 2 keV and thus the criteria for HDIs are satisfied. The interval that strictly satisfies the HDI criteria described before spans 1600 - 2300 UT. It is evident that the Bx (3rd) and By (4th) components have only moderate magnitudes indicating that the HDIs are situated at low-latitude part of the plasma sheet inner-edge on dawnside. As already described, it is the low-latitude part of the plasma sheet inner-edge on both sides that are surveyed by the spacecraft. The striking contrast between dusk and dawn seen in Figure 7a is not due to difference in latitudes. To confirm the connection between CDIs and HDIs on dawnside, we look for HDIs on the orbits that is at ~ 10 Re from the Earth on the midnight meridian and then move tailward into the dawnside plasma sheet. In Figure 7b, which shows the data obtained on such an orbit under stable solar wind/IMF condition. HDIs are identified in 0300-0600 UT when the spacecraft is on the dawnside plasma sheet inner-edge. Then, as the spacecraft moves deeper tailward into the dawnside plasma sheet, CDIs are detected at 0600-1000 UT. This finding of HDIs and CDIs being situated adjacent to each other strongly supports the idea that HDIs are the inner extension of the CDIs.

Fig. 7. Data from the orbits described in Figure 6. (a) HDIs on dawnside (1600-2300 UT) but no such a counterpart on duskside. (b) CDIs sitting adjacent to HDIs on dawnside.

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DISCUSSION In this paper, we have investigated the structure of the plasma sheet under northward IMF. CDPS (n > 1 cm~3 and T < 2 keV) is well known to appear under northward IMF. We have shown that they appear mostly in the region more than 10 Re off the tail axis to the flanks. By inspecting the ion energy spectrum features, we have found that the CDIs can be classified into three categories. One of the categories that preferentially appears on dawnside is indicative of a heating process on low energy (< 1 keV) ions. A part of these showing temperatures > 1.5 keV are located at the dawnside inner-edge of the plasma sheet. On the earthward side of this region, present under northward IMF are HDIs (n > 1 cm~3 and T > 2 keV). CDIs in the plasma sheet are transformed to HDIs as they are heated more in the course of earthward convection. The fact that there are no HDIs on duskside implies that such supply of plasma from the duskside plasma sheet is not present under northward IMF. Indeed, duskside CDIs are characterized by a spectrum feature that indicates absence of heating on the cold component. The dawn CDPS-HDI connection can be explained by the test particle transport calculations by Spence and Kivelson (1994) and Wang et al. (2001). Assuming two sources for the plasma sheet ions, one in the distant tail that produces the normal energetic component and the other at the dawnside flank, these calculations follow the adiabatic motions of test ions in model magnetic and electric fields. An ion's thermal energy is calculated from the adiabatic relation. For quite intervals, the effects of the convection electric fields do not dominate and the cold component drifting duskward from the dawn-flank can substantially contribute in populating the plasma sheet. This produces CDPS close to the dawn-flanks and, since they are heated adiabatically as they move inward and duskward, produces HDI in the dawnside inner-magnetosphere as well. In the model, a cold ion source at the dusk flank is not situated. Even if it is situated, the dawn-todusk directed motion will make it not effective in supplying ions over a wide area in the plasma sheet. Thus the duskside plasma sheet inner-edge is dominated by the ions from the distant tail source and shows the normal hot and tenuous status. While the agreement between the observations and the model calculations may seem simple, it points to the two items that do not seem to be widely accepted in the magnetospheric community: (1) The dawn flank is a substantial source for the plasma sheet during quiet times. (2) The effects of non-E x B drifts can be significant in shaping the spatial distribution of (not only the energetic but also) the thermal population during quiet times. These two points have been theorized by Spence and Kivelson (1994) but are now supported by a data-based study. An inferred two-dimensional structure of the plasma sheet by mapping DMSP data seems to point to the same conclusion (Wing and Newell, 1998). One question that is not answered by these models are, of course, the formation mechanism of duskside CDPS. The above agreement with the adiabatic model calculations makes us expect gradual change from CDIs to HDIs. The example in Fig. 7b, however, is puzzling in this aspect. One can see that the transition at 0600 UT is rather sharp, not in accordance with the slow adiabatic picture. Furthermore, the magnitude of the magnetic field is not larger on the HDI side of the boundary. There seems more to be considered on the heating mechanism. Let us note on the fate of the HDIs. If they drift duskward, we should be detecting HDIs on duskside as well. Since we do not, this is not the case. It could be that, in the dawn-tail inner magnetosphere, the weak electric field convection under northward IMF that tries to move the particles dawn-sunward and the weak grad-B/curvature drift of lower energy ions that is directed dusk-tailward are mostly balanced. Then the lower energy ions that carry most of the high density may be stagnant in the dawn-tail part. If not stagnant, because they are absent on duskside, they should be moving dawn-sunward. In this case, we would expect the dawn-dusk asymmetry to be seen in the dayside as well. ACKNOWLEDGMENTS M. F. acknowledges fruitful discussion with T. Phan, H. Hasegawa, and M.N. Nishino. The key parameter data from WIND spacecraft were provided by the NASA/GSFC data processing team. REFERENCES Baumjohann, W., G. Paschmann, and C. A. Cattell, Average plasma properties in the central plasma sheet, J. Geophys. Res., 94, 6597, 1989.

Borovsky, J. E., M. F. Thomsen, and D. J. McComas, The superdense plasma sheet: Plasmasperic origin, solar wind origin, or ionospheric origin?, J. Geophys. Res., 102, 22,089, 1997. -26-

Fujimoto, M., et al, Plasma entry from the flanks of the near-Earth magnetotail: Geotail observations J. Geophys. Res., 103, 4391, 1998. Fujimoto, M., T. Mukai, and S. Kokubun, The cold-dense plasma sheet and the hot-dense ions in the inner magnetosphere, Adv. Space Res., 30 (10), 2279, 2002. Fuselier, S. A., et al, Composition measurements in the dusk flank magnetosphere, J. Geophys. Res., 104, 4515, 1999. Lennartsson, W. and E. G. Shelley, Survey of 0.1 to 16 keV/e plasma sheet ion composition, J. Geophys. Res., 91, 3061, 1986. Phan, T. D., et al., WIND observations of the halo/cold plasma sheet, Substorms-4, Eds. S. Kokubun and Y. Kamide, Terra Sci. Pub., Tokyo, 1998. Phan, T. D., et al., WIND observations of mixed magnetosheath-plasma sheet ions deep inside the magnetosphere, J. Geophys. Res., 105, 5497, 2000. Spence, H. E., and M. G. Kivelson, Contributions of the low-latitude boundary layer to the finite width magnetotail convection model, J. Geophys. Res., 98, 15,487, 1993. Terasawa, T., et al., Solar wind control of density and temperature in the near-Earth plasma sheet: WINDGEOTAIL collaboration, Geophys. Res. Lett, 24, 935, 1997. Wang, C. P., et al., Modeling the quiet time inner plasma sheet protons, J. Geophys. Res., 106, 6161, 2001. Wing, S. and P. T. Newell, Central plasma sheet ion properties as inferred from ionospheric observations, J. Geophys. Res., 103, 6785, 1998. Wing, S. and P. T. Newell, 2D plasma sheet ion density and temperature profiles for northward and southward IMF, Goephys. Res. Lett, 29, 10.1029/2001GL013950, 2002.

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GEOTAIL OBSERVATIONS OF THE COLD PLASMA SHEET ON THE DUSKSIDE MAGNETOTAIL M. N. Nishino1, T. Terasawa1, and M. Hoshino1 1

University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo, 113-0033, JAPAN ABSTRACT

We have studied signatures of the cold plasma sheet in the duskside near-earth magnetotail with the Geotail data. In the duskside plasma sheet, cold plasmas are found 3-4 hours after the northward turning of the interplanetary magnetic field. The cold plasma sheet with two-temperature ions is often found on the duskside, while cold plasma sheet with one-temperature are also found on the duskside when northward interplanetary magnetic field continues for a very long interval (more than several hours). We discuss the development and evolution of the cold plasma sheet on the duskside.

INTRODUCTION The origin of the plasma particles in the near-earth plasma sheet has not been fully understood. For southward interplanetary magnetic field (IMF) condition, it is widely accepted that magnetic reconnection plays an important role in plasma entry from the magnetosheath into the plasma sheet and plasma heating there. On the other hand, two mechanisms have been proposed to explain plasma transport during the northward IMF periods; that is, magnetic reconnection at high-latitude magnetopause and diffusive process at the low-latitude boundary layer (LLBL). However, what is really going on at the magnetopause and in the plasma sheet under the northward IMF conditions is still an open question. Several authors (Fairfield et al., 1981; Lennartsson and Shelly, 1986; Baumjohann et al., 1989; Lennartsson, 1992) reported that the near-earth plasma sheet becomes cold and dense during geomagnetically quiet periods. In recent years several studies have suggested that plasma particles which are of magnetosheath origin enter directly into the plasma sheet via near-earth tail flanks (e.g. Fujimoto et al., 1997; Terasawa et al., 1997; Borovsky et al., 1997). Nishino et al. (2002) reported that the total plasma content in the near-earth plasma sheet increases during northward IMF periods, and they concluded that the effect of the plasma transport during the northward IMF intervals is not negligible. These studies have shown that the cold plasma sheet (Cold-PS) is formed in the tail flanks during the northward IMF periods. Fujimoto et al. (1997) concluded that cold ions of magnetosheath origin directly enter into the flank plasma sheet via the magnetopause to form the Cold-PS with a two-temperature velocity distribution function. This two-temperature state shows that hot ions of magnetospheric origin and cold ions of magnetosheath origin co-exist in the flank plasma sheet. Recently, Takashima (2002) reported that the Cold-PS with twotemperature ions exists only on the duskside flank, and that not only IMF-Bz but also IMF-By plays an important role in formation of a Cold-PS with two-temperature ions. In this report we focus on the development and evolution of the Cold-PS on the duskside near-earth magnetotail flank and its dependence on the northward IMF. INSTRUMENTATION For the plasma sheet data, we use 12 s averaged magnetic field data from the magnetic field experiment (MGF) (Kokubun, et al., 1994) and 12 s averaged plasma moment data (density, velocity, and temperature) from the low energy particle experiment (LEP) (Mukai et al., 1994) on board Geotail. For the solar wind -28-

Table 1. Event list of cold plasma sheet in the dusk flank magnetotail. From left, date, T\Ag (see text for its definition), three components of the IMF, solar wind density (Nsw), GEOTAIL position, and ion velocity distribution (one-/two-temperature) in Cold-PS are shown. No. (1) (2) (3) (4) (5) (6) (7) (8) (9)

date yyyy/ m m /dd 1995/02/24 1995/03/07 1995/03/24 1995/04/04 1995/05/01 1996/02/22 1996/04/01 1997/02/12 1997/02/24

Tlag

IMF Bx

hour

nT

4.3 5.8 8 30 7.8 7.5 2.6 9.3 13

-0.8 ±1.3 -0.2 ±1.6 1.4 ±1.5 -2.8 ±3.9 -2.2 ±1.3 3.9 ±0.7

3.1 ±0.7 -1.0±0.9 2.2 ±3.4

IMF By nT 2.2 ±1.9 2.8 ±2.4 -2.6 ±2.6 2.9 ±2.8 2.4 ±1.2 0.9 ±4.3 0.1 ±1.2 0.7 ±1.2 1.2 ±2.5

IMFBz nT

Nsw /cc

1.9±1.0 19.0 ±0.8 4.6 ±1.5 4.8 ±0.4 9.2 ±1.4 16.6 ±3.0 6.8 ±2.2 4.5 ±1.0 2.2 ±1.3 6.0 ± 0.6 2.1 ±1.9 5.1 ±2.0 2.4 ±0.6 20.8 ±2.2 2.5 ±0.8 7.2 ±1.6 2.5 ±2.0 5.1 ±1.2

GEOTAIL

Ions

(X,Y,Z)RE (-26.1,20.1,7.4) (-18.0,18.2,8.2) (-16.1,16.3,1.8) (-10.8,19.7,9.9) (-7.7,17.4,4.5) (-18.8,23.3,4.0) (-16.5,18.5,3.2) (-17.3,20.3,8.2) (-21.7,20.5,4.7)

2-Temp. 2-Temp. 2-Temp. 1-Temp. 2-Temp. 1-Temp. 2-Temp. 1-Temp. 1-Temp.

data, 92 s averaged data from WIND/SWE and MFI provided via CDAWeb are used. We use the geocentric solar magnetosphere (GSM) coordinate throughout this report. OBSERVATION We have surveyed the Geotail data during January 1994 - December 1997 to find Cold-PS in the nearearth magnetotail (0 > X > —50 RE)- The criteria for Cold-PS are Tj(ion temperature)< 1 keV and /3(ratio of magnetic and plasma pressure) > 1. We do not impose a condition on the ion density (N\), because the ion density in the plasma sheet depends on the solar wind density as well as the distance from the earth. To avoid contaminations of the low-latitude boundary layer (LLBL) and the magnetosheath, the events with tailward flowing ions (Vx < —100 km/s) in the flanks are excluded. In our data set we have selected 9 typical Cold-PS events near the dusk magnetopause (Table 1). To study the effect of the length of northward IMF duration on formation of Cold-PS, we introduce T]ag which represents the time delay between the northward turning of the IMF and the time when Cold-PS was first seen. In defining the beginning of a northward IMF interval we require 30 minutes of southward field and then we select the following northward field interval that is closest to the Cold-PS observation. We have neglected short (interval of < 30 min) and small (time integration of < —30 nT-min) southward excursions of the IMF during prolonged northward IMF intervals. We have checked the ion velocity distribution function of these Cold-PS events and divided them into two groups; that is, two-temperature Cold-PS (Figure 1 (a)) and one-temperature Cold-PS (Figure 1 (b)). The IMF-Bz for each event is shown in Figure 1 (convection delay included), where the intervals of T]ag are shown as hatched regions. It seems that the duskside plasma sheet is filled with two-temperature ions if the northward IMF continues for 3-4 hours, and that the ions there become one-temperature if the northward IMF holds for more than several hours. The dependence of these Cold-PS emergence on IMF-By j if any, can not be discussed with our data set, because IMF-By was positive in all cases except one. Here we focus on an event of Cold-PS with one-temperature ions. Figure 2 shows the Wind and Geotail observations on April 2-4, 1995 (Event No. 4 in Table 1). The left panels (a) show the Wind data between 0 UT of April 2 and 8 UT of April 4. From the top, magnitude of magnetic field (\B\) and its Z-component (Bz), the latitudinal angle of magnetic field (