1. Institute of Geophysics, Chinese Academy of Sciences, Beijing, China
2. Institute of Geophysics and Planetary Physics, University of California, Los Angeles
3. Los Alamos National Laboratory, Los Alamos, New Mexico
4. Applied Physics Laboratory, Johns Hopkins University, Laurel, Maryland
Abstract. On April 22, 1979, from 0840 to 1018 UT, ISEE 1 and ISEE 2 stayed in the tail current sheet at 17 R_{E}, crossing the current sheet center several times while the initially northward interplanetary magnetic field Bz turned southward under conditions of constant solar wind velocity and dynamic pressure. An overview of the period from 0840 to 1050 UT, discussing the solar wind conditions, the tail magnetic field response at ISEE 1, ISEE 2 and IMP 8, and the ground electrojet activities has been presented in an accompanying paper. Herein under the assumption that the magnetic field is antisymmetric about the current sheet center, the hyperbolic tangent current sheet model of Harris has been used to calculate the current sheet thickness from these two close spacecraft measurements of the tail magnetic field. The thickness varied from 2.5 to 0.5 R_{E} during this period. The current sheet thinned just prior to a rapid poleward expansion of the auroral electrojet and just after the arrival of a southward turning of the IMF Bz and apparent tail field reconnection at ISEE 1 and 2. The plasma density and temperature distribution in the vertical direction appears to be close to adiabatic at this time and the total pressure is roughly constant through the lobe. In this case study, the polytropic index of the tail plasma is found to be 5/3 on the average. This implies that the plasma convection at 17 R_{E} in tail was steadily adiabatic over the period of about 100 min.
The plasma and current sheets in the geomagnetic tail are generally thought to play key roles in the dynamical processes in the magnetosphere. Whether one subscribes to the current disruption model or the nearEarth neutral point model of substorms, the properties and evolution of these layers are intimately involved. Under normal geomagnetic conditions the current sheet is relatively thick [Fairfield et al., 1981a], but under substorm conditions it can become very thin [McPherron et al., 1987]. We expect that this thickness is controlled by either interplanetary conditions or by the substorm activity itself. In the former case the development of this thin current sheet may be a prelude to intense magnetic reconnection and rapid reconfiguration as proposed for solar flares [Van Hoven et al., 1987]. This development may be an inherent part of the evolution of the magnetotail during substorm growth phases as proposed originally in the near Earth model of substorms [Russell and McPherron, 1973] and more recently by Wiegelmann and Schindler [1995], who postulate that the formation of a thin current is a consequence of magnetic flux transfer to the magnetotail. However, a quantitative determination of the temporal evolution of the current sheet thickness is difficult. The current sheet position varies with dipole tilt and solar wind direction [Russell and Brody, 1967; Hammond et al., 1994]. Also, the current sheet can twist about the SunEarth line in response to the By and Bz components of the interplanetary magnetic field (IMF) [Sibeck et al., 1985; Kaymaz et al., 1994; Zhou et al., 1997]. Authors have used several approaches to determining the thickness of the current sheet. McComas et al. [1986] used sudden swings of the tail caused by interplanetary shocks to obtain an instantaneous profile of the otherwise quiet current sheet. Tsyganenko [1989] developed an empirical magnetospheric magnetic field model from satellite observations. This model provides the statistical configuration of the entire neartail region and the current sheet thickness for different geomagnetic activity levels (Kp). Pulkkinen et al. [1992] analyzed the thinning and intensification of the crosstail current sheet during the CDAW6 substorm (March 22, 1979) growth phase. They found that the results obtained from the Tsyganenko model by varying several magnetospheric parameters are coincident with the results obtained from Harris current sheet model [McPherron et al., 1987]. An alternate approach to determining the structure of the current sheet is to adopt a model of the current sheet and derive best fit parameters of this model using spacecraft observations as a function of time. McPherron et al. [1987] and Sanny et al. [1994] adopted the Harris current sheet model, and ISEE 1 and ISEE 2 observations near the current sheet to monitor the timevarying thickness of the sheet during the CDAW6 substorm at 1054 UT on March 22, 1979. Their results demonstrate that the nearEarth current sheet undergoes a dramatic thinning late in the substorm growth phase.
In the accompanying paper [Zhou et al., 1997] we introduced a very interesting event occurring on April 22, 1979, from 0840 to 1050 UT. A global perspective of the event was given for an extended period of both southward and northward IMF conditions. During this event from 0840 to 1018 UT, ISEE 1 and ISEE 2 stayed in the tail current sheet and closely together or separately crossed the current sheet several times. This is a rare but ideal condition for the calculation of the current sheet thickness and the analysis of the current sheet structure. In this paper, we provide greater detail on the structure and variations of the current sheet and plasma during this period using the Harris current sheet model. The polytropic index which reflects the property of plasma convection in the tail is also discussed.
The crosstail current in the geomagnetic tail was first directly probed 3 decades ago by the magnetic field measurements of IMP 1 [Ness, 1965]. This current sheet is the region within the plasma sheet wherein the magnetic field rapidly changes from tailward to sunward orientation. If we assume the magnetic field is antisymmetric about the current sheet center, that there is little or no field crossing the current sheet, and that the hyperbolic tangent current sheet model of Harris [1962] is a good approximation to the profile of the current sheet, the thickness of the current sheet can be calculated from two vertically spaced measurements of the magnetic field with no assumption of time invariance. The Harris model is a simple analytical description of a onedimensional current sheet model and is selfconsistent using either MHD or kinetic theory. In a Harris current sheet, the magnetic field, plasma thermal pressure, and current density are given by
B_{x }= B_{o}tanh(z/h) (1)where B_{o}is the lobe field, z is the distance between the observation point and the current sheet center, and h is the current sheet half thickness.P_{T }= P_{o}sech^{2}(z/h) (2) j = (B_{o}/µ_{o}h)sech^{2}(z/h) (3)
An observation with one spacecraft in the current sheet is relatively easy to obtain. However, from formula (1), one can only obtain the ratio z/h instead of h. With observations from two vertically spaced spacecraft both in the current sheet, formula (1) can be rewritten
h = 2z/ln [(B_{o}^{2} +B_{o}B_{x}B_{x1}B_{x2})/(B_{o}^{2} B_{o}B_{x}B_{x1}B_{x2})] (4)
as shown in Figure 1, z=z_{1}z_{2}, z_{1} and z_{2} are the positions of the two spacecraft in Z direction of the GSM coordinates, B_{x}= B_{x1}B_{x2}, B_{x1} and B_{x2} are the B_{x} components observed by the two spacecraft separately.
Fig. 1. The Harris current sheet model, z=z_{1}z_{2}, z_{1} and z_{2} are the positions of the two spacecraft in the Z direction normal to the current sheet, and, B_{x}=B_{x1}B_{x2}, B_{x1} and B_{x2} are the Bx components in the same coordinates observed by the two spacecraft separately. 
B_{o}^{2}/ 2µ_{o}= (P_{T}+P_{B})_{cs} = NkT_{}+ B^{2}_{xy}/2µ_{o} (5)
where cs means in the current sheet, N is the plasma density in the current sheet, k is Boltzmann's constant, B^{2}_{xy }= B^{2}_{x}+B^{2}_{y} and T_{} is the temperature perpendicular to the magnetic field. Finally, the magnetic field in the lobe can be calculated by
B_{o}= (2µ_{o}NkT_{} + B^{2}_{xy})^{½ } (6)
When we use a coefficient of 2.15 to recalibrate the plasma density to get a constant total pressure through the plasma sheet from one lobe to the other, the lobe field B_{o}= 52.6 nT.
3.1. A Review of the Background Information
Fig. 2. (a) Spacecraft and (b) current sheet positions in GSM coordinates during the event period. The dashed lines represent the current sheet center. 
In this paper we use the observations of ISEE 1 and ISEE 2 to calculate the current sheet thickness and to analyze the current sheet structure. Thus a quick review of the requisite information about the two ISEE spacecraft is appropriate. For more detail one can refer to the accompanying paper. In Figure 2, the positions of the ISEE pair in the current sheet are shown in GSM coordinates. In the GSM XZ plane, the current sheet center at 0840 UT is 1 R_{E} above the equatorial plane, based on the Hammond formula [Hammond et al., 1994] for a dipole tilt angle of 6.2^{o} toward the Sun. Figure 2b is the current sheet viewed in the YZ plane. From 0840 to 1018 UT ISEE 1 and ISEE 2 stayed in the current sheet and crossed the center of the current sheet several times.
Fig. 3. Solar wind conditions measured by ISEE 3 at the forward libration point. From top to bottom are the interplanetary magnetic field in GSM coordinates. The time above the frame is the estimated time the solar wind arrived at the nose of the magnetopause. 
The interplanetary magnetic field data for this period is available from ISEE 3, 206 R_{E} upstream and 82 R_{E} to dawnside of the Earth, and is shown in Figure 3. The time between the two vertical lines is the IMF corresponding to the event period. From 0754 to 0932 UT the solar wind proton density was near 8 cm^{3}, the solar wind velocity was about 460 km/s, and the dynamic pressure was stable at 4 nPa. During this time, IMF Bz changed from northward to southward, while B total remained roughly constant. Since the IMF turned strongly southward in this period we expect a significant effect on the magnetotail, and in particular on the current sheet thickness and structure.
Fig. 4a. The magnetic variation in the tail from 0840 to 1018 UT measured by ISEE 1 (grey line) and ISEE 2 (black line) for both northward (before 0953 UT) and southward (after 0953 UT) IMF conditions.  
Fig. 4b. The plasma variation in the tail from 0840 to 1018 UT measured by ISEE 2 under the same IMF conditions with Figure 4a. (top) The black line is total velocity and the grey line is the Vx component. The plasma flows toward the Earth for positive Vx and away from Earth for negative Vx. (bottom) PB is the magnetic pressure, PT is the thermal pressure, and PB+PT is the total pressure in the current sheet. 
The magnetic field observed by ISEE 1 and ISEE 2, and the plasma data observed by ISEE 2 are shown in Figure 4a and 4b, respectively. Beginning at 0847 UT a dipolarization of the magnetic field occurred, accompanied by a sharply increasing tail Bz component at ISEE 1 and ISEE 2. A positive V_{x} which is almost the same as the total velocity indicates that this dipolarization was being carried directly toward the Earth in accord with the time delays seen between ISEE 1 and ISEE 2. Before 0920 UT the ISEE pair remained at the center of the current sheet, so that the total pressure was mainly due to the thermal pressure which is shown by the thin line in the bottom panel of Figure 4b. A rapid southward turning of the tail magnetic field occurred at 0953 UT corresponding to the expected arrival time of the IMF Bz southward turning seen at ISEE 3 at 0906 UT. This southward turning had many of the characteristics expected for a tail reconnection event but had significant interspacecraft differences in By. This apparent reconnection event then appeared to move down tail. At about 1000 UT ISEE 1 gradually crossed the current sheet center back and forth, while ISEE 2 was near the current sheet boundary as shown in Figure 4a. From Figure 2 and Table 1 of our accompanying paperthe separation of the two spacecraft in the Z direction was about 3000 km. Thus we can deduce that the current sheet here is very thin, only about 0.5 R_{E}. At the same time the large difference between total velocity and Vx as shown in Figure 4b (top) indicates that the plasma was flowing across the tail. We presume that this flow moved plasma from the center of the tail to the edges where it was lost to the magnetosheath. This crosstail flow was not accompanied by a plasma density decrease. Constant density during a current sheet thinning is expected since the pressure in the center of the current sheet where the field is weak must remain constant to balance the magnetic field pressure in the lobes. In the absence of cooling or heating the density must be constant even while the edges of the plasma sheet are eroded.
Table 1. Polytropic Index Corresponding to the IMF Conditions and Current Sheet Thickness
Time Period  Current Sheet Half Thickness 
IMF Bz  

08401018 UT  1.66±0.02     
08401000 UT  1.62±0.03  2.0 R_{E}  northward 
10001018 UT  1.57±0.05  0.5 R_{E}  southward 
3.2. Calculated Current Sheet Thickness
Fig. 5. (top) The Bx components of the tail field observed by ISEE 1 and ISEE 2. (middle) The current sheet center position in GSM coordinates. (bottom) The half current sheet thickness calculated by formula (4). 
Figure 5 gives the Bx components at ISEE 1 and ISEE 2, the current sheet center position z˜ in GSM and the current sheet half thickness. Up and down motion of the current sheet is very clear in the middle panel before 0938 UT. This agrees with the interpretation in our previous paper and is reasonable based on the Bx variations. Over the whole event the current sheet center was mainly above the equatorial plane, at an average height of about 1.1 R_{E}. As shown in the bottom panel of Figure 5, before 0920 UT the average thickness was about 2.5 R_{E}, but it varied very much. Between 0920 to 1000 UT the thickness was relatively smooth and the average value was about 1.5 R_{E}, but oscillated near the time of the reconnection event which corresponded to the IMF southward turning. After 1000 UT the current sheet monotonically thinned to an average thickness of 0.5 R_{E}. Thus the current sheet can be divided into three parts over this event period with three different average thickness of 2.5, 1.5, and 0.5 R_{E}.
3.3 Current Sheet Structure
Fig. 6a. The plasma density distribution in the vertical direction. The line is a fit to a Gaussian distribution, for which the correlation coefficient is 0.882.  
Fig. 6b. The spin average ion temperature distribution in the vertical direction. T_{ave }was calculated by (2T_{min}+T_{max})/3. The correlation coefficient of the Gaussian distribution is 0.915.  
Fig. 6c. The plasma pressure variation in the vertical direction. The thick line is the theoretical prediction of the formula (2). The thin line is a fit to a Gaussian distribution for which the correlation coefficient is 0.942.  
Fig. 6d.
The total pressure distribution in the vertical direction. The line is an average of the total pressure and it is 1.15 nPa. The deviation of the pressure is discussed in the text.

From the Harris current sheet model we also can analyze the structure of the plasma sheet along the vertical direction. We assume the current sheet magnetic structure is antisymmetric around the current sheet center and that the plasma properties as observed by ISEE 2 are symmetric about z˜, so we reflect the plasma data about z˜. Figure 6 shows the plasma distribution in the vertical direction. In Figure 6a the plasma density is high at the center and low at the edges, which is very close to a Gaussian distribution and also to the similar distribution for temperature in Figure 6b. Figure 6c is the thermal pressure distribution, where the thick line is the Harris model (2) and the thin line is a Gaussian distribution for which the correlation coefficient is 0.942. At the center of the current sheet the thermal pressure is lower than the theoretical expectations, while near the edge of the current sheet it is little bit higher. This is probably because in the analysis of the ISEE 2 plasma data, T_{} is assumed always equal to T_{min}. However, in the geomagnetotail sometimes T_{} is T_{min}, and sometimes T_{max}. So the calculated thermal pressure based on T_{} will cause some error when T_{}T_{min}. Another possible source of the error is that some plasma is outside of the energy range of the detector, so the measured plasma density does not include all the particles which should contribute to the thermal pressure. These errors also appear in the total pressure as shown in Figure 6d, which makes the total pressure oscillate around the average value shown by the vertical line.
From the fluid description of a plasma and the definition of the specific entropy we obtain
dS = c_{v}dln(P/N^{}) (7)
where S is the specific entropy, c_{v} is the specific heat capacity for constant volume, P is the pressure, and N is density. In an adiabatic flow dS/dt=0, and from equation (7), this implies
P = N^{} (8)
where is a constant of the motion. Under enormous simplification, the energy equation can be replaced by (8). The exponent is called the polytropic index [Siscoe, 1983]. The polytropic equation (8) is exact for steady state problems in which the heat flux is proportional to the convected flux of internal energy. So
= (5/3+) / (1+ ) (9)
where is a constant connecting the heat flux with the internal energy. If = 0, = 5/3 and if >, >1. These correspond to the adiabatic and isothermal limits.
Fig. 7a. The comparison of the theory (grey line) and the fitting (black line) to the plasma density and temperature for the period of 08401000 UT corresponding the IMF northward conditions and the thick current sheet. The slope of the black line is 0.62±0.03, and the intercept is 18.53±0.01. Thus the equation of the line is lnT=0.62lnN+18.53, for which the correlation coefficient is 0.760. The slope of the grey line is 1=2/3, and the intercept is 18.54.  
Fig. 7b. The comparison of the theory (grey line) and the fit (black line) to the plasma density and temperature for the period of 10001018 UT corresponding the IMF southward conditions and the thin current sheet. The slope of the black line is 0.57±0.05, and the intercept is 18.43±0.03. Thus the equation of the line is lnT=0.57lnN+18.43, and the correlation coefficient is 0.803. The grey line is the same as in Figure 7a.  
Fig. 7c.
The fit to the plasma density and temperature for the entire period of 08401018 UT. The slope of the fit is 0.66±0.02 and the intercept is 18.54±0.01. Thus the equation of the line is lnT=0.66lnN+18.54, and the correlation coefficient is 0.818. This is almost the same as the theoretical grey line.

Equation (8) may be written T = const(N^{1}). So for an adiabatic plasma, the slope of lnT versus lnN should be 2/3, and for an isothermal plasma, 0. This result can be used in analyzing the properties of the observed plasma if we assume that all the plasma observed originates in a uniform reservoir. Figure 7a shows plots of lnT versus lnN from 0840 to 1000 UT corresponding to the IMF northward conditions and the thick current sheet. The black line is the linear fit whose slope is 0.62±0.03. The slope of the grey line is 0.67 corresponding to an adiabatic plasma. In Figure 7b the data from 1000 to 1018 UT, corresponding to the southward IMF and a very thin current sheet are shown. The black and grey lines have the same meaning as in Figure 7a. The best fit slope is 0.57±0.05. The whole observation interval from 0840 to 1018 UT is shown in Figure 7c. The slope of the black line is 0.66±0.01 which is similar to the expected adiabatic slope, so that if we drew it, the two lines would overlap.
Fig.8. Plot of the counts of 300 keV ions versus the 50 keV ion counts seen at IMP 8. Clearly, there is a different population of ions present after ~1125 UT. 
We cannot perform such analysis at IMP 8 because we have only a single spacecraft. However, if we can make a simple assumption about the time stationarity of the particle fluxes and the external conditions, we can combine the magnetic field and energetic particle fluxes to give us a qualitative measure of the vertical structure of the plasma sheet. Figure 8 shows a plot of the flux of 300 keV ions versus the flux of 50 keV ions seen at IMP 8. Clearly, there are two different populations present, a cool population and a hot population. Examination of Figure 8d of our accompanying paper shows that the cool ions were present from 0800 to 1030 UT and the hot ions beginning about 1125 UT. If we restrict ourselves to the period from 0800 to 1030 UT we probably have a time stationary population of energetic particles whose flux varies with time principally due to the relative motion of the current sheet (including thinning and flapping) and the spacecraft. We also believe from the constancy of the total pressure in the plasma sheet and from the in situ solar wind data that the pressure on the tail and the magnetic field strength in the lobes remained constant from 0800 to 1030 UT. Thus the magnetic field strength or the component of the field in the X direction should be able to act (qualitatively) as a proxy for the distance from the center of the plasma sheet. The relationship of this "distance" to the actual distance in kilometers will vary as the neutral sheet thickens and thins, but it should remain roughly fixed in terms of scale lengths.
Fig.9. Vertical structure of the 30 keV electrons and ions seen by IMP 8 using the earthward directed component of the magnetic field as a proxy for the distance across the sheet. 
Figure 9 shows the flux of 30 keV electrons and ions plotted as a function of Bx. To a first approximation, these data seem to be consistent with a time stationary population of particles, with ions being somewhat more nonstationary than the electrons. There is one period in the electron data, however, where the current layer/plasma sheet seems to be nearly devoid of energetic particles. This period surrounds the twisted current sheet crossings at 0905 UT [Zhou et al., 1997]. With this one exception most of the variation in the plasma populations, thermal and energetic at ISEE and at IMP, appear to be spatial rather than temporal.
In this paper we have discussed the current sheet structure and the plasma properties over the period from 0840 to 1018 UT, April 22, 1979, at which time the IMF turned from northward to southward and the solar wind velocity and dynamic pressure were steady. ISEE 1 and ISEE 2 were down tail at 17 R_{E}, in the current sheet, and they crossed the current sheet center repeatedly in this period. The Harris model has been used for the calculation of the current sheet thickness and in turn the analysis of the structure. From the calculated results we find the Harris model to be a good approximation to the current sheet during this period. The calculated current sheet center position shows that the current sheet moved up and down as the tail flapped maintaining an average position is about 1.1 R_{E} above the equatorial plane. However, this model is sensitive to deviations from the idealized model caused by magnetic structure in the Bx component and by the fact that the two spacecraft are separated in X and Y as well as in Z. Thus as shown in the Figure 5 (bottom) when there are irregularities in the Bx components the derived thickness oscillates.
From the calculated result and Figure 5 it is very clear that the current sheet thinned gradually from 0840 to 1018 UT, especially after 1000 UT the half thickness is about 0.5 R_{E}. Current sheet thinning is a very significant aspect of the substorm growth phase [e.g., Fairfield et al., 1981b; McPherron et al., 1987; Mitchell et al., 1990; Sergeev et al., 1992, 1993; Pulkkinen et al., 1994]. In the study of Sanny et al. [1994] the current sheet thinning from 4 R_{E} to 500 km took about 2030 min. The corresponding southward IMF Bz reached to 20 nT, and the minimum of AL index was 1000 nT. While for our case, the current sheet thinning took less than 10 min corresponding to a 9 nT IMF Bz and a 510 nT minimum AL. Just before this thinning of the current sheet an apparent tail field reconnection occurred (at 0953 UT) which was thought to be the response to the IMF southward turning at ISEE 3 at 0906 UT. In our previous paper the observations of ground magnetometers (as shown in Figure 7 of that paper) exhibit an onset of intensified westward electrojet at about 0953 UT. This period was also identified by W. J. Hughes as a substorm onset in a computer based substorm classification study. (W. J. Hughes, personal communication, 1993). (Hughes defines a substorm onset by looking for a 100 nT decrease at AL index in any 10 min interval.) Thus it appears that the southward turning almost immediately resulted in an auroral electrojet intensification apparently accompanied by the tail field reconnection, and later followed by tail current sheet thinning, greater westward electrojet activity and poleward expansion, and more tail reconnection.
In section 4 we have examined the polytropic index implied by our data. The value of the polytropic index is not only important for the discussion whether steady state convection is possible or not but has also implications for the stability of the tail. For an assessment of the stability of the magnetotail it is important to know whether the plasma sheet behaves adiabatically or follows some other relation between density and pressure. Erickson and Wolf [1980] thought that steady, adiabatic convection probably cannot occur throughout a magnetotail. The significance of this proposal is profound, because if no quasistatic solution exists, substorms or analogous temporal variations would be driven by even rather slow convection in the magnetotail. Therefore Kivelson and Spence [1988] reexamined the arguments that led Erickson and Wolf to question the possibility of a tail stable to slow plasma convection and made the calculations based on using a more realistic magnetic field model and observed plasma pressures. They found for pertinent critical ratios of the potential drop across half the tail to the source temperature, of about 3, a Tsyganenko quiet time magnetic field model is consistent with steady convection. Adiabaticity of the tail plasma was assumed in these papers. Theoretically, Spence and Kivelson [1990] found from a simple model of twodimensional field structure and adiabatic inward convection of a uniform distant tail source that ranges between 5/3 and 1. The first observational study of the polytropic index was given by Lui et al. [1981] where was determined by looking into concurrent density and pressure changes. Another observational study [Huang et al., 1988] includes more cases and seems to yield values of less than 5/3. Baumjohann and Paschmann [1989] made a large statistical study to establish representative values of the polytropic index for different tail regions and disturbance conditions. They found, on average, =5/3. That means the plasma sheet ion population behaves adiabatically both in the central plasma sheet and the plasma sheet boundary layer. In our case study, over the whole range of plasma temperature versus density we obtained = 5/3 with an error range of ±0.02. Separately for a thick current sheet when IMF Bz was mainly northward = 1.62±0.03 and for the thin current sheet when IMF Bz southward =1.57±0.05. The values of polytropic index obtained from the fits are given in Table 1. In this period on average, the current sheet apparently behaves adiabatically and this behavior persists as the current sheet thins.
Finally, we conclude that the Harris model is a good firstorder approximation to the current sheet. Using this model to probe the thickness of the current sheet we find that the current sheet thinned just prior to a rapid poleward expansion of the auroral electrojet but just after the arrival of the IMF Bz southward turning and the apparent tail field reconnection at ISEE 1 and ISEE 2. The plasma convection at 17 R_{E} appeared to be steadily adiabatic over the entire event. While we cannot repeat this analysis at IMP 8, we can use the local magnetic field as a proxy measure of the distance from the center of the tail measured in scale heights. Using this proxy measure, we find that the plasma sheet density and temperature remained roughly constant as it thinned and flapped until about 1030 UT. Thus all the electrojet activity in the ionosphere and accelerated flows and southward magnetic fields seen in the tail did not lead to an overall change in the plasma sheet until after 1030 UT with the sole exception of the >250 keV electrons which were probably released from the magnetosphere proper when the southward IMF arrived at the Earth. The major change in the tail populations appeared to occur after 1100 UT well after the geomagnetic field had begun a typical substorm sequence.
The Editor thanks three referees for their assistance in evaluating this paper.
Baumjohann, W., and G. Paschmann, Determination of the polytropic index in the plasma sheet, Geophys. Res. Lett., 16, 295, 1989.
Erickson, G. M., and R. A. Wolf, Is steady state convection possible in the Earth's magnetotail? Geophys. Res. Lett., 7, 897, 1980.
Fairfield, D. H., R. P. Lepping, E. W. Hones Jr., S. J. Bame, and J. R. Asbridge, Simultaneous measurements of magnetotail dynamics by IMP spacecraft, J. Geophys. Res., 86, 1396, 1981a.
Fairfield, D. H., E. W. Hones., and C.I. Meng, Multiple crossings of a very thin current sheet in the Earth's magnetotail, J. Geophys. Res., 86, 11189, 1981b.
Hammond, C. M., M. G. Kivelson, and R. J. Walker, Imaging the effect of dipole tilt on magnetotail boundaries, J. Geophys. Res., 99, 6079, 1994.
Harris, E. G., On a plasma sheath separating regions of oppositely directed magnetic field, Nuovo Cimento, XXiii, 115, 1962.
Huang, C. Y., L. A. Frank, D. G. Mitchell, G. Rostoker, and J. F. Fennell, Substormassociated energization of the central plasma sheet, Eos Trans. AGU, 69(44), Fall Meet. Suppl., 1381, 1988.
Kaymaz, Z., G. L. Siscoe, N. A. Tsyganenko, and T. P. Lepping, Magnetotail views at Re: IMP 8 magnetometer observations, J. Geophys. Res., 99, 8705, 1994.
Kivelson, M. G., and H. E. Spence, On the possibility of quasistatic convection in the quiet magnetotail, Geophys. Res. Lett., 15, 1541, 1988.
Lui, A. T. Y., C.I. Meng, L. G. Frank, K. L. Ackerson, and S.I. Akasofu, Temperature variation of the plasma sheet during substorms, Planet. Space Sci., 29, 837, 1981.
McComas, D.J., C. T. Russell, R. C. Elphic, and S. J. Bame, The nearEarth crosstail current sheet: Detailed ISEE 1 and 2 case studies, J. Geophys. Res., 91, 4287, 1986.
McPherron, R. L., A. Nishida, and C. T. Russell, Is nearEarth current sheet thinning the cause of auroral substorm onset? paper presented at Conference on Quantitative Modeling of MagnetosphereIonosphere Coupling Processes, Kyoto Sangyo Univ., Kyoto, Japan, March 913, 1987.
Mitchell, D. G., D. J. Williams, C. Y. Huang, L. A. Frank, and C. T. Russell, Current carriers in the nearEarth cross tail current sheet during substorm growth phase, Geophys. Res. Lett., 17, 583, 1990.
Ness, N. G., The Earth's magnetic tail, J. Geophys. Res., 70, 2989, 1965.
Pulkkinen, T. I., D. N. Baker, D. G. Mitchell, R. L. McPherron, C. Y. Huang, and L. A. Frank, Global and local current sheet thickness estimated during the late growth phase, in Substorms 1, Eur. Space Agency Spec. Publ., ESA SP335, 131, 1992.
Pulkkinen, T. I., D. N. Baker, D. G. Mitchell, R. L. McPherron, C. Y. Huang, and L. A. Frank, Thin current sheet in the magnetotail during substorms: CDAW 6 revisited, J. Geophys. Res., 99, 5793, 1994.
Russell, C. T., and K. I. Brody, Some remarks on the position and shape of the neutral sheet, J. Geophys. Res., 72, 6104, 1967.
Russell, C. T., and R. L. McPherron, The magnetotail and substorms, Space Sci. Rev., 15, 205, 1973.
Sanny, J., R. L. McPherron, C. T. Russell, D. N. Baker, T. I. Pulkkinen, and A. Nishida, Growthphase thinning of the nearEarth current sheet during the CDAW 6 substorm, J. Geophys. Res., 99, 5805, 1994.
Sergeev, V. A., R. C. Elphic, F. S. Mozer, A. SaintMarc, and J. A. Sauvard, A twosatellite study of nightside flux transfer events in the plasma sheet, Planet. Space Sci., 40, 1551, 1992.
Sergeev, V. A., D. G. Mitchell, C. T. Russell, and D. J. Williams, Structure of the tail plasma/current sheet at ~11 Re and its changes in the course of a substorm, J. Geophys. Res., 98, 17345, 1993.
Sibeck, D. G., G. L. Siscoe, J. A. Slavin, E. J. Smith, B. T. Tsurutani, and R. P. Lepping, The distant magnetotail's response to a strong interplanetary magnetic field By: twisting, flattening, and field line bending, J. Geophys. Res., 90, 4011, 1985.
Siscoe, G. L., Solar system magnetohydrodynamics, in SolarTerrestrial Physics, edited by R. L. Carovillano and J. M Forbes, p.11, D. Reidel, Norwell, Mass., 1983.
Spence, H. E., and M. G. Kivelson, The variation of the plasma sheet polytropic index along the midnight meridian in a finite width magnetotail, Geophys. Res. Lett., 17, 591, 1990.
Tsyganenko, N. A., A magnetospheric magnetic field model with a warped tail current sheet, Planet. Space Sci., 37, 5, 1989.
Van Hoven, G., L. Sparks, and D. D. Schnack, Nonlinear radiative condensation in a sheared magnetic field, Astophys. J., 317, L91, 1987.
Wiegelmann, T., and K. Schindler, Formation of thin current sheets in a quasistatic magnetotail model, Geophys. Res. Lett., 22, 2057, 1995.
Zhou, X.Y., C. T. Russell, and D. Mitchell, Three spacecraft observations of the geomagnetic tail during moderately disturbed conditions: global perspective, J. Geophys. Res., this issue 1997.
X.Y. Zhou, Institute of Geophysics, Chinese Academy of Sciences, Beijing 100101, China. (email: xyzhou@cgeos15.cgeos.ac.cn)
C. T. Russell, Institute of Geophysics and Planetary Physics, 6877 Slichter Hall, University of California, Los Angeles, CA 900951567. (email: ctrussell@igpp.ucla.edu)
J. T. Gosling, Los Alamos National Laboratory, Los Alamos, NM 87545. (email: gosling@lanl.gov).
D. Mitchell, Applied Physics Laboratory, Johns Hopkins University, Johns Hopkins Road, Laurel, MD 20723. (email: apl::mitchell)
Received April 17, 1996; revised October 16, 1996;
accepted December 27, 1996.
Copyright 1997 by the American Geophysical Union.
Paper number 97JA00038.
01480227/97/97JA00038$98,00