By fields are created in the inner plasma sheet boundary, and the total pressure is

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1 JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 105, NO. All, PAGES 25,291-25,303, NOVEMBER 1, 2000 Statistical visualization of Earth's magnetotail during substorms by means of multidimensional superposed epoch analysis with Geotail data S. Machida Department of Geophysics, Graduate School of Science, Kyoto University, Japan A. Ieda NASA Goddard Space Flight Center, Greenbelt, Maryland T. Mukai, Y. Saito, and A. Nishida 1 The Institute of Space and Astronautical Science, Sagamihara, Japan Abstract. A multidimensional superposed epoch analysis of plasm and magnetic field data from the Geotail spacecraft was used to visualize the time evolution of plasmoids in the Earth's magnetotail statistically. The substorm events were identified by Pi2 events at midlatitude ground stations when Geotail was in the downtail region from -30 Rz to-200 Rz. Plasma density, velocity, temperature, and magnetic field data were sorted and analyzed according to the Pi2 onsetime. We selected 156 substorm events that were either a single onset or the first onset of multiple onsets. In the statistical visualization, we found that plasma acceleration does not take place in a small region around X ~ -30 Rz, but occurs in a more widely spread region extending from X -30 Rz down to X -90 R. The spatial structure of the plasmoid is characterized by a bilatitudinm structure where the fast plasma flow in the plasma sheet boundary stretches from the equator and a relatively slow plasma sheet flow is encountered with preexisting plasma sheet populations. Finite By fields are created in the inner plasma sheet boundary, and the total pressure is enhanced inside the plasmoid. The evolution of the plasm and magnetic fields as well as the deduced parameters, such as the number flux of plasma or energy fluxes, agreed well with a magnetic reconnection model. 1. Introduction down to the interplanetary space and carries significant mass and energy away from the Earth's magnetosphere. It is generally accepted that magnetic field lines re- The existence of plasmoids was envisaged from the nearconnected with a terrestrial origin owing to a dayside re- Earth neutral line (NENL) model of substorms where connection are added to the lobe magnetic field compothe magnetic reconnection in the near-earth region is nent, increasing stored energy in the magnetotail in the regarded as a primary source of substorms; this is a form of a magnetic field. This stored energy is suddenly counterpart to earthward particle energization and disreleased, initiating and driving an expansion phase of a turbance [Hones, 1976]. substorm[russell and McPherron, 1973; Hones, 1976]. Insitu observation by a single spacecraft always faces Notable structural changes and severe disturbances are the difficulty of separating observed variations into spaproduced in the Earth's magnetosphere and ionosphere. tial and temporal components. Furthermore, the plas- One prominent feature is a hot and high-speed plasma ma density in the Earth's magnetotail is so low that cloud termed a "plasmoid", which is created and ejected it is not possible to directly observe the magnetotail with any imaging technique available today. In con- Now at Japan Society for the Promotion of Science, trast, imagery observations are very common in astron- Tokyo, Japan. omy, which is definitely a great advantage for evaluating data. We usually draw a schematic or cartoon picture Copyright 2000 by the American Geophysical Union. Paper number 2000JA /00/2000JA ,291 of the evolution of the magnetotail during a substorm on the basis of what we learn from insitu observations. However, if we can obtain an image of the magnetotail

2 25,292 MACHIDA ET AL- STUDY OF EARTH'S MAGNETOTAIL WITH GEOTAIL DATA Relati im min 0 ' hw.rd 'lows V Tailward l,w 600 km/s I tt I t 0 I 1-15, h I t i - tl t I II ! I Plate 1. Color-codedisplays for earthward and tailward flow velocities in X-Z(estimated) plane. Flows associated with plasmoid are shown in panels after time interval t min.

3 MACHIDA ET AL.: STUDY OF EARTH'S MAGNETOTAIL WITH GEOTAIL DATA 25,293 N Ti I cm ' ke x 600 km/ o lb I lb I nt No h I nt,uth ' R Plate 2. Magnetotail structure during t min, when plasmoid develops explosively (displayed parameters: ion number density, ion temperature, tailward flow velocity, earthward and duskward magnetic field intensities, and northward magnetic field).

4 25,294 MACHIDA ET AL.- STUDY OF EARTH'S MAGNETOTAIL WITH GEOTAIL DATA 'E elam

5 MACHIDA ET AL.: STUDY OF EARTH'S MAGNETOTAIL WITH GEOTAIL DATA 25,295 with minimal and reasonable assumptions, this would tail acquired suitable data for conducting research on enhance our knowledge of the physical processes in the the magnetotail properties, including dynamics, mass magnetotail. Hence we try to visualize the time evo- transport, energy transport, and particle accelerations. lution of the magnetotail during substorms, especially In this paper, we present the results of our statistical the plasmoid structure and its evolution. study on visualizing the dynamic evolution of the mag- In early studies of plasmoids, a plasmoid was thought netotail during substorms. This information is mainly to have a closed loop magnetic field structure that re- based on the data of the distant-tail phase with a spesults in a positive and then a negative change in the cial emphasis on the generation of plasmoids and their north-south magnetic field Bz for a single spacecraft evolution. when the plasmoid traverses the spacecraft with a fast tailward plasma flow [Hones et al., 1984]. Later, it was pointed out that there are considerably large dawndusk magnetic fields accompanying plasmoids. Thus the shape of a plasmoid was considered to be a flux rope, where each end of the magnetic field is traced back to either the ionosphere or solar wind rather than closing by itself [Hughes and $ibeck, 1987; Moldwin and Hughes, 1992]. It is now generally recognized that plasmolds have a flux rope shape. Henceforth, we use the term "plasmoid" in this context. An excellent review of the NENL model is given by Baker et al. [1996]. A distant tail exploration by ISEE 3 was highly successful, but a more detailed investigation of the thermal structure and formation process of plasmoids had to wait since the data on low-energy ions were not available and an intensive survey at medium distances in the magnetotail had not been conducted. Geotail was launched on July 24, 1992, to explore various distances in the Earth's magnetotail from X ~ -30 Re to X ~ -200 RE in an early distant-tail phase during the period of July 1992 to March 1994 and from X ~ -10 Re to X ~ -30 Re in a succeeding near-tail phase. We use the Geocentric Solar Magnetospheric (GSM) coordinate system throughouthis paper. Geo- 2. Results We used plasma moment and magnetic field data acquired with the Low Energy Particle/Energy Analyzer (LEP/EA) and Magnetic Field Instrument (MGF) onboard the Geotail spacecraft. The time resolution of both data sets is 12 s. A detailed description of the LEP and the MGF can be found in papers by Mukai et al. [1994] and Kokubun et al. [1994], respectively. Figure I shows the Geotail orbits from September 14, 1993, to February 28, 1995, during a distant-tail phase and an early near-tail phase. The data around X ~ -30 Re consist of observations obtained in January and February The spacecraft traversed the regions around X ~ -60 RE in September 1993 and February 1994; X ~ -90 Re in January 1994; X ~ -160 Re in October 1993 and October 1994; and X ~ -200 Re in A p ril Since we investigated the time evolution of the Earth's magnetotail relative to the onset time of substorms, the determination of the onset time is very important. We determined the onset time by Pi2 pulsation appearing in the H-component of the ground magnetic records at Kakioka located at (26.94 ø, ø) and at Wingst at ,1øo l i' i RN j i' N' i ' O t 4 i [- _ 2... : o l ll--.,, : : l---,---: l ::.:...,,?-- ---,, '...,:..+_.,:._ :... _.,.. _.., ,,..,.lttJ...'.:,..l.._.....:._...:.._.',,,r.,, a,-,j, ---' L_.J... J...L.....L... L..I L L..... ' ,.,,..,_..,..,...' GSM-X Figure 1. Geotail orbit projected on X-Y plane of GSM coordinates from September 14, 1993, to February 28,.1995 (coordinates are normalized by Earth radius).

6 25,296 MACHIDA ET AL.' STUDY OF EARTH'S MAGNETOTAIL WITH GEOTAIL DATA (54.15 ø, ø ) in the geomagneticoordinates. The local time of the stations was limited to the interval be- tween 2000 and 0400 UT, which corresponds to LT at Kakioka and LT at Wingst. We selected either a single Pi2 onset or the first onset of multiple Pi2 onsets with the requirement that the ground geomagnetic activity was quiet at least 30 min prior to the onset. Ambiguous events such as an extremely small Pi2 onset or a very gradual Pi2 onset were excluded. We analyzed 156 substorm events with such criteria. the structure in the north-south direction. Notice that, in most cases, the number of the data points cannot be divided evenly by 6, which is the number of bins. We assigned the data points to the center of the nearest bin (grid) after sorting them vertically with an equal separation similar to the nearest grid point (NGP) method to assign mass and charge to grids in the plasma particle simulations. Beyond X RE, we added an additional requirement that the plasma number density be < i cm -s to avoid contamination by the magnetosheath plasma. To achieve the visualization of the magnetotail, we We call this method a "multidimensional superposed separated data into six groups in terms of the distance epoch analysis". An advantage of this method comof the spacecraft from Earth, i.e., X - -30,-63,-91, pared to the other method, which directly takes the -131, -159, and -199 Re. For each group we assigned value of/ i as an ordinate, is that it can give us informadata such as number density, bulk flow velocity, or ion tion on the north-south scale length, i.e., the thickness temperature uniformly into six separated bins in the of the structure in the display. As an example, supcolumn placed in the north-south direction with the re- pose that the ion temperate increases as we approach quirement that the data point with a larger value of/ iz the center of the plasma sheet, and the number of data ( - nkti/(b,r2/21 o)) be located in the inner region of with energy higher than 4 kev occupies 1/2 of the tothe magnetotail, as shown in Figure 2, and obtained av- tal data assigned to a given column. The averaged ion eraged values of such parameters at each bin. We used energies in the inner three bins are evidently > 4 kev. / ix, which uses only the X component of the magnetic On the other hand, when the data with energy higher field, instead of the more conventional/ i because this than 4 kev occupies 1/6 of the total data, the averaged parameter more accurately distinguishes between the ion temperature is > 4 kev only in the innermost bin. plasma sheet and the magnetosheath. Our requirement Thus the number of bins for a particular range of any can be valid during quiet times. It is also satisfied in the parameters gives us some information on the vertical region near the neutral line as well as that upstream and scale length. Although we did not directly measure the downstream of the separatrix in the magnetic reconnec- distance from the plasma sheet center, we refer to the tion topology, so we used such a simple rule to obtain vertical axis as "Z (estimated)" since it provides a use- z j+l J B ix (j+l) < B ix O) /3 ix (j) =n kti/(bx0)2/2 it0) North-South Symmetry Qk = Ii Q(j)/AN Linear Interpolation z _:._- _: i I _:.: :' :- _: R x Figure 2. Illustration of how to sort data in the X and Z (estimated) plane.

7 MACHIDA ET AL.: STUDY OF EARTH'S MAGNETOTAIL WITH GEOTAIL DATA 25,297 ful measure of this parameter. By consulting previous ber density N, which is set in a logarithmic scale. The plasmoid and the magnetotail studies, we roughly estimate the maximum value of Z (estimated) at 15 Re. Of structure of N and the ion temperature T on the X-Z (estimated) plane show that relatively high density recourse, this number gives us only some idea of the scale gions exist in the mid tail and the very distant tail, but lengths, and further intensive study is necessary to obtain exact values of Z at each distance. Also, we should point out that the method of simply using Z of the spacecraft location in the GSM coordinates to obtain the magnetotail structure in the north-south direction will definitely mix the events in the lobe, the plasma the temperature of the distant-tail component is below I kev and relatively cold. There is a region of low density with T, i kev around X, -130 Rs between the tail parts. The temperature is, 6 kev inside the plasmoid, but the number density is relatively low and below 0.1 cm -3. High-density plasma of cm -3 sheet boundary layer, and the plasma sheet. Therefore appears in the top and bottom boundaries that correwe cannot use this method. spond to the lobe/mantle region around X The boundary between two regions, which is known as a plasma sheet boundary layer, might develop into the Plate i shows time sequential plots for the plasma bulk flow from the previously described method. We simply averaged V for 156 events, and the positive values corresponding to the earthward flows and negative values corresponding to the tailward flows are displayed separately to clarify the structure. The time interval between each panel is 5 min, and the time window is also 5 min. The vigorous acceleration takes place, 5 rain after the Pi2 onset. The fast flow can be seen in the region corresponding to the plasma sheet boundary with a characteristic structure we call a "bilatitudinal" structure. It is especially enhanced in the time inter- For magnetic fields an intense B field occupies a val of min. A similar bilatitudinal structure can large area in the region close to the Earth, as shown in be obtained when we simply replace either B or the the [B [panel. We found that the value of IB [is small ion plasma beta with to infer the north-south in the plasmoid; therefore, a region with a small structure. Such a structure is also found in a MHD that stretches around X Rs in the north-south simulation, which supports a realization of the bilatitu- direction gives us a good indication of the plasmoid; this dinal structure [K. Maezawa, personal communication, area actually moves downtail as time elapses. The 1998; Abe, 1999]. The leading edge of the fast flow prop- structure indicates that finite By fields can be created agates in the downfall region with a velocity of, 500 km/s and reaches the right boundary at X = -200 Re in the region from X Re to X Re. A further analysi showed that [B [ is maximized at the, 30 min after the Pi2 onset. The spatial dimension in the north-south direction seems to shrink when the plasmoids propagate to the distant tail region. Since substorms often have multiple onsets and the time interval between plasmoid release in a multiple onset substorm is typically between 5 and 10 min, it is possible for the plasmoid released first to be overtaken by one released later. This can result in a long structure extended in the Sun-Earth direction in our multidimen- sional superposed epoch analysis. Plate 2 shows the number density, temperature, tailward flow velocities of plasma, and magnetic field intensity IBzI, IB I, and Bz obtained at t min. The same panel for the tailward velocity when t min is included as a reference. We use the absolute slow-mode shock at this time [$aito et al., 1995]. Cold particles from the upstream region heat up and accelerate at this boundary and then supply the particles to the downstream region, i.e., the plasmoid. T shows a bilatitudinal structure in a similar manner to the tail- ward flow velocity -V. A very thin layer extending from X Rs to -200 Re is regarded as a current sheet, and it has not yet been affected by a plasmoid's arrival. region of - I (not shown here). The distribution of Bz shows a reversal of its sign from a positive to a negative value at X Re as one goes from the left boundary toward the right, i.e., toward the downtail region. This pattern strongly suggests an occurrence of magnetic reconnection. The location of the Bz field reversal was found to be slightly tailward compared to the actual case, the reason of which will be explained later. The structure explained above was obtained from the averaged values that were evaluated in each cell of the columns as mentioned. We were able to obtain a stan- dard deviation cr for each cell and obtain structures of various parameters, which deviate from the averaged values by -or or +. Therefore we evaluated a -1 sigma value and a + 1 sigma value in each cell, and ob- value for Bz and By since Bz can be regarded as antisymmetric in the northern and southern magnetotail, tained their structures on the X-Z (estimated) plane, and the intensification of B may considerably depend as shown in the left and right columns of Plate 3. The on an interplanetary B field whose polarity is variable. color bars at the right of the panels are set in a linear Therefore we regard an absolute value of the By field scale except for the number density N, which is set in as adequate for learning the generation of the nonzero a logarithmic scale similar to that in Plate 2, but the By field in the magnetotail as a first-order approxima- maximum scales are different from those in Plate 2 for tion. The color bars shown at the right of each panel the T, -V, [B [, and Bz plots. We can learn that the in Plate 2 are set in a linear scale except for the hum- number density approximately takes a value from 0.01

8 25,298 MACHIDA ET AL.- STUDY OF EARTH'S MAGNETOTAIL WITH GEOTAIL DATA to 0.5 cm -3, the ion temperature approximately ranges required that the Geotail be located in the region of from 3 to 10 kev, and the tailward velocity ranges from IYI ( 10 RE. We did not place any specific require- ~ 150 to over 1000 km/s inside the plasmoid. The dis- ments on the Z coordinate, but the spacecraft was in tribution of IB I does not change very much except for IZI ( 10 RE because oœ its orbit in nearly an eclipthe region of ~ 150 RE. The value of [By[ approxi- tic/equatorial plane. Using these definitions, we obmately ranges from 0 to 6 nt, and that of B ranges tained the earthward flow velocity Vx in the region of from about -4 nt to over 4 nt. When a plasmoid has a /3iz _ 0.05 ( i.e., the plasma sheet boundary layer and high density, high temperature, large tailward velocity, the plasma sheet) on a plane defined by the distance from the Earth X and relative time from the onset t. and large [By[ field, it should have structures like those shown in the right column. The distribution of [B [ should have the structure shown in the left panel since it has a steep spatial gradient in the outer side of the plasmoid and, suitably, can achieve a pressure balance with a high-pressure plasmoid. For B fields we have to use the structure with the large positive B region in the right panel to use it for the front part of a fast and high-pressure plasmoid. For the rear part of the plasmoid, the magnitude of B depends on the structure of the postplasmoid plasma sheet, and there may be many varieties. When this structure accompanies a large negative Bz, we should use the left panel. The top two panels in Plate 4 show an ion number flux toward the equator Fz (= - nvñz sign(bz)) and an ion number flux toward the downtail Fz (- - nv ), respectively. This demonstrates a route of mass transport when a plasmoid is formed. From these two panels it is evident that the plasmas are sufficiently supplied from the lobe region roughly from X RE to X RE with a maximum at X. -60 RE. The vector of the incoming flux is not directed exactly toward the center of the plasma sheet but tilts toward the downtail region. Thus the incoming plasma has a considerable amount of field-aligned tailward velocity, which has to be taken into account when we model the particle acceleration process in this region. As a result of the acceleration, a particle flux toward the downtail is created, which amounts to 1011 cm -2 s -1 in the construction of a plasmoid. The Poynting flux directing the equator Fpz (= - ( E x B // 0)z sign(b )), is dominant in the lobe region for the energy flux when the plasmoid is formed, as shown in the third panel in Plate 4. It is interesting that the Poynting flux takes a maximum at X. -30 Re in contrast to the equatorward ion number flux. When a plasmoid is formed, the thermal energy flux Ft (= - (5/2) nktiv ) is the most significant factor. The bulk energy flux Fb (= - (1/2) minv2vz) has a value less than a half of the thermal energy flux, where we assume that the adiabatic index for ions is ffi - 5/3. The tailward Poynting flux in the plasmoid is an order of magnitude smaller than the thermal energy flux and is not shown here. Both the thermal and bulk energy fluxes decrease as a plasmoid travels to the downtail region, but they become comparable in the region away from X ~ -150 RE [Ieda et al., 1998]. In this paper, we defined the lobe as a region of/ i < 0.05, the plasma sheet boundary layer as a region of 0.05 _ / iz < 1, and the plasma sheet as/ iz _ 1. We also Plate 5a shows this result. This diagram is useful for learning the temporal evolution and the propagation of structures to be investigated. It is apparent that the flow reversal region is located at roughly X RE before the onset, which corresponds to the distant neutral line (DNL) [e.g., Zwickl et al., 1984; Slavin et al., 1985; Nishida et al., 1996]. The region of flow reversal is relatively spread out. This may reflect the fact that the location of the flow reversal may scatter depending on solar wind conditions such as the orientation of the magnetic field, dynamic pressure, and time history. The activity of the DNL continues even after the onset of substorm. The other flow reversal appears around X RE just after the onset, which can be regarded as the formation of the NENL. The location of this flow reversal seems to shift slightly tailward soon after the onset of the substorm and stays almost stationary around X Rr. It should be noted that most of the events selected in this study consist of multiple onsets; therefore, a duration of more than 40 rain in Plate 5a does not necessarily reveal the real lifetime of a single NENL. There is another flow reversal that propagates with a velocity of roughly 700 km/s corresponding to a leading edge of the plasmoid. The region with negative V, i.e., the region tailward flow, spreads after t- 0. The northward magnetic field component Bz in the region of/ _ 0.05 is plotted in the same X-t plane, as shown in Plate 5b. There are abrupt changes in the flow reversal regions. If we simply follow the model of the magnetic reconnection, we can expect a positive B for the earthward flow and a negative B for the tailward flow. However, the regions of the tailward flow do not show clear negative B fields. This could be due to the fact that the intensity of the negative B field observed with the tailward flow is relatively small, nominally a few nt. In contrast, the intensity of the positive Bz with the earthward or stagnant flow is usually larger than that. There are cases when the Geotail did not encounter the plasmoid, and stayed outside the plasmoid observing the earthward or stagnant plasma flow with positive Bz. Therefore the averaged value of B inevitably has a positive offset if the data for the plasmoid encounter and those without plasmoid encounter are averaged together. Since the intensity of the dawn-dusk magnetic field component [By[ takes a maximum value in the region where/ ix ~ 1, we restricted the range of the/ ix value to 0.3 _ / i ( 3. This allows the event in a part of the

9 MACHIDA ET AL.- STUDY OF EARTH'S MAGNETOTAIL WITH GEOTAIL DATA 25,299 a port EIO/ms Fx 0-50!00 50 RF -00 North 8 10/m2s South Fp I1'" aris, ort F-5 W/m' b.e-5 W/m tx orth F.5 W/m' South 0 ; 0,[ I 0 00 Plate 4. Color coded displays showing mass transport (top two panels) and energy transport (bottom three panels).

10 25,$00 MACHIDA ET AL- STUDY OF EARTH'S MAGNETOTAIL WITH GEOTAIL DATA (a) Vx (b) Bz ( -B ix ) - 10 km/s :00 nt 2 " lo ß.: ' X (RE) X (RE) (c) ' gy (0.3 B ix < 3) (d) Pt (all B ix) - 10 nt -lo i o X (RE) X (RE) Plate 5. Time development of magnetotail: (a) earthward flow velocity, (b) northward magnetic field, (c) duskward magnetic field intensity, and (d) normalized total pressure.

11 MACHIDA ET AL.: STUDY OF EARTH'S MAGNETOTAIL WITH GEOTAIL DATA 25,301 plasma sheet as well as in a part of the plasma sheet boundary layer according to the aforementioned definitions, and a similar diagram for IBy[ is constructed, as in Plate 5c. The number of data during the period from -19 rain to 21 rain in the region around X ~ -130 Re is quite small and suffers from statistical limitations, so we did not use the data in that portion. Instead, we simply interpolated the By field data obtained at X, -91 Re and X, -159 Re linearly from t = -19 rain to t = 22 min. It is apparent that finite By fields are generated with the reconnection at the NENL. The region of finite By fields propagates down the tail with the motion of plasmoid, and the structure extends from the left boundary (X, -30 Re) to the right soon after the onset. This region overlaps with the time travel curve of Bz ~ 0 in the plasmoid and is embedded just behind the plasmoid front. Unfortunately, there is a data gap in the region X ~ -130 Re, and as a result, it the ground Pi2 onset [Machida et al., 1999; Miyashita is not certain whether the [By I enhancement originating et al., 2000]. Therefore it is quite possible that the refrom the near-earth region can propagate and connect connection starts a few minutes before the Pi2 onset, to the structure in the distant tail X, -200 Re at but the lateral extent of the NENL is limited, and the time t = 29 min. There are relatively large By fields chance of encountering plasmoids is relatively low. This in the region X ~ -200 Re, regardless of the geomag- may cause a - 5 min time lag in the plasmoid evolution netic activity, but no large By fields were observed in the region of X, -160 Re at t, 22 min. We cannot in our statistical analysis. We found a bilatitudinal structure in the tailward draw a definite conclusion on this subject. Plate 5d is a diagram for a total pressure Pt that is a flow velocity as well as in the ion temperature. Such a bilatitudinal structure can be created when a plasmoid sum of the magnetic pressure and the ion thermal pressure in the entire magnetotail, without specifying the range of the ion plasma beta. The contribution of the electrons is not included but is approximately % of the ion thermal pressure. The values of the total pressure at the locations X = -30,-63,-91,-131,-159, and -199 Re are normalized by their time averaged values and presented by color codes. As can be seen in Plate 5d, the enhancement of the total pressure occurs around 8 min after the Pi2 onset at around X = -60 Re region. This structure propagates to the downtail at ~ 650 km/s in the front part and 440 km/s in the rear part increasing its spatial scale length in the X-direction. If we compare this with Plate 5c, the location of positive-then-negative change of Bz is just behind the front of the total pressure enhancement. 3. Discussion As we have shown, considerable changes occur in the magnetotail during substorms. There is a flow reversal region around X ~ -130 Re for the growth phase, and the earthward flow originating from this region is enhanced in this phase. It is known that the tailward flow with a positive Bz is observed as well as that with a negative Bz [Nishida et al., 1994]. We believe that the positive Bz region in the downtail of the flow reversal is created in our statistical analysis because the magnitude of the positive Bz is usually greater than that of the negative Bz [Yamamoto et al., 1994]. The intense earthward flow definitely affects the condition of the near-earth tail and the triggering of substorms. The activity of the DNL continues even after the onset of substorms, and this earthward flow encounters the tailward flow owing to the reconnection at the NENL. This reconnection results in the bilatitudinal structure of the flow and the temperature in the mid-distant magnetotail. During the expansion phase the tailward flow is enhanced ~ 5 min after the ground Pi2 onset in our analysis. This may be due to the facts that the plasmoid evolution occurs not only in a localized region around the NENL but also in a much more extended region from X ~ -30 Re to X ~ -90 Re, and that there is a delay in the propagation time of the plasmoid reaching the distance of X Re starting from X Re. In reality, there are examples of a fast tailward flow at the distance of X Re, starting a few minutes before propagates in the downtail region and meets a stationary plasma sheet population. Thus, when the density of the stationary ions is high, a pronounced bilatitudinal structure is created. This follows from the momentum conservation law, which states that the mixing of the flowing plasmoid ions with the preexisting dense stationary ions lowers the flow velocity of the plasmoid as a whole, while it is less dense in the plasma sheet boundary layer and the plasmoid velocity is slowed less in this region. We can roughly assume the frozen-in relation of the magnetic field and plasma that inevitably creates a bilatitudinal structure. Notice that the flow pattern obtained in this analysis is for averaged values, and the absolute values of both the tailward and earthward flow velocities of an individual event can exceed 600 km/s. This speed is selected as the maximum value in the color bar in Plates I and 2 and sometimes reaches 1000 km/s. As for the flow velocity in the core part of plasmoid, a recent Geotail data analysis by Ieda et al. [1998] has shown that it maximizes around the 100 Re region and then decelerates as it travels farther down the tail, which is consistent with our results. Finite By fields are generated with the onset of a substorm. Such finite By fields seem to be produced from the transport of the dawn-to-dusk component of the magnetic fields from the lobe region of-30 Re X -90 Re across the separatrix layer and reflect a polarity of the interplanetary magnetic fields. Finite By fields can also be inherently generated by magnetic reconnection, as most of the electromagnetic hybrid simulations

12 25,302 MACHIDA ET AL.: STUDY OF EARTH'S MAGNETOTAIL WITH GEOTAIL DATA include the kinetic effects of ions [e.g., Nakabayashi and perposed epoch analysis to various parameters such as Machida, 1997] or Hall MHD simulations. There is a de- the number density, flow velocity, plasma temperature, crease in the By field intensity around X ~ -130 and three components of the magnetic fields on the X-Z Since the number of events available for the region of (estimated) plane. The same method was also applied X ~ -130 Re is relatively limited, as we mentioned to the particle number flux, and energy fluxes resulting earlier, it is quite possible that an effect of the inter- in a consistent structure for the magnetic reconnection planetary magnetic field occurs; thus the magnitude of model. By field is relatively small during that time interval. 4. Approximately 10 min after the onset, the earth- The structures of the flow and magnetic field, espe- ward flow originating from the distant tail region of cially Vx and Bz, as well as those of particle flux and X ~ -130 Rz disappeared, suggesting a quenching of energy flux, are all consistent with the model of mag- the reconnection at the D NL. netic reconnection. It is interesting that the particle 5. The time change of the magnetic fields obtained by supply from the lobe maximizes at X ~ -60 Rz, while the multidimensional superposed epoch analysis plotthe Poynting flux toward the center of the plasma sheet ted on the X-t plane showed a positive and negative Bz maximizes at the innermost boundary of the present change known as the bipolar signature of plasmoids. In analysis, i.e., X = -30 Re. This should be explained addition, finite By fields were created and carried with by future global simulations of the magnetotail in a the plasmoid. We also found a total pressure enhancequantitative manner. ment that contained the region of zero crossing of the During the recovery phase the location of NENL Bz field and that of the enhanced By field. gradually moves to the downtail region. By continu- 6. In a recovery phase a prolonged reconnection coning the analysis shown in Plate 5 until t - 90 min, we tinued in the region relatively close to the Earth, i.e. locate the neutral line at X ~ -80 Re. We do not think, the NENL, but the location of the NENL gradually the lifetime of one particular neutral line is 90 min, and shifted toward the downtail region in our statistical this motion may reflect a statistical location of the neu- analysis. tral line formed during the recovery phase. This should 7. A very active acceleration of lobe/mantle plasma be studied in the future. There are pronounced cold due to the magnetic reconnection occurred in the recov- ions surrounding a plasmoid. The transport and mixing of such cold ions into the plasmoid is not well understood, but the cooling and deceleration of the plasmoid in the distant-tail region suggesthe possibility of cold ion transport into the plasmoid [Machida et al., 1994; Ieda et al., 1998]. 4. Summary 1. Activation of the earthward flow from the region X ~ -130 Re where we could locate the DNL at least 20 rain prior to the onset, i.e., during the growth phase, was confirmed. 2. At t ~ -3 min, the NENL seemed to be formed around X ~ -30 Rz. 3. Approximately 5 rain after the Pi2 onset, vigorous particle acceleration took place in the region -30 Re X -90 Re and possibly occurred in both the central current sheet and the boundary layer of the plasma sheet. There were a significant number of cold ions in the lobe region at X Re. ery phase, which maintains the structure of the postplasmoid plasma sheet. 8. The features of plasmoids appeared to be considerably different when the spacecraft was located at different distances in the downtail. However, we were able to comprehend these various differences by using our results from the multidimensional superposed epoch analysis. The time evolution of a plasmoid, including its gen- Acknowledgments. We thank S. Kokubun and T. eration in the Earth's magnetotail, was analyzed. A Nagai for providing the MGF data. We also thank D. H. method of multidimensional superposed epoch analysis Fairfield, S. P. Christon, T. Araki and the members of the Division of Solar-Planetary Electromagnetism, Department was applied to 156 substorm events. The results are as of Geophysics, and World Data Center for Geomagnetism, follows. Kyoto University, for their valuable comments and discussions. Hiroshi Matsumoto thanks M. Nakamura and C. T. Russell for their assistance in evaluating this paper. References Abe, S. 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