Central plasma sheet ion properties as inferred from ionospheric observations

Transcription

1 JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 103, NO. A4, PAGES , APRIL 1, 1998 Central plasma sheet ion properties as inferred from ionospheric observations Simon Wing and Patrick T. Newell The Johns Hopkins University Applied Physics Laboratory, Laurel, Maryland Abstract. A method of inferring central plasma sheet (CPS) temperature, density, and pressure from ionospheric observations is developed. The advantage of this method over in situ measurements is that the CPS can be studied in its entirety, rather than only in fragments. As a result, for the first time, comprehensive two-dimensional equatorial maps of CPS pressure, density, and temperature within the isotropic plasma sheet are produced. These particle properties are calculated from data taken by the Special Sensor for Precipitating Particles, version 4 (SSJ4) particle instruments onboard DMSP F8, F9, F10, and Fll satellites during the entire year of Ion spectra occurring in conjunction with electron acceleration events are specifically excluded. Because of the variability of magnetotail stretching, the mapping to the plasma sheet is done using a modified Tsyganenko [1989] magnetic field model (T89) adjusted to agree with the actual magnetotail stretch at observation time. The latter is inferred with a high degree of accuracy (correlation coefficient -0.9) from the latitude of the DMSP b2i boundary (equivalento the ion isotropy boundary). The results show that temperature, pressure, and density all exhibit dawn-dusk asymmetries unresolved with previous measurements. The ion temperature peaks near the midnight meridian. This peak, which has been associated with bursty bulk flow events, w.idens in the Y direction with increased activity. The temperature is higher at dusk than at dawn, and this asymmetry increases with decreasing distance from the Earth. In contrast, the density is higher at dawn than at dusk, and there appears to be a density enhancement in the low-latitude boundary layer regions which increases with decreasing magnetic activity. In the near-earth regions, the pressure is higher at dusk than at dawn, but this asymmetry weakens with increasing distance from the Earth and may even reverse so that at distances X < --10 to -12 Rœ, depending on magnetic activity, the dawn sector has slightly higher pressure. The temperature and density asymmetries in the near-earth region are consistent with the ion westward gradient/curvature drift as the ions ExB convect earthward. When the solar wind dynamic pressure increases, CPS density and pressure appear to increase, but the temperature remains relatively constant. Comparison with previously published work indicates good agreement between the inferred pressure, temperature, and density and those obtained from in situ data. This new method should provide a continuous mechanism to monitor the pressure, temperature, and density in the magnetotail with unprecedented comprehensiveness. 1. Introduction hundreds of cubic Earth radii (R ). However, low-altitude polar-orbiting satellites such as DMSP, which have been in The dynamics of many ionospheric and magnetospheric phecontinuous deployment for the last 14 years, take a single cut nomena are driven by physical processes which occur in the through the entire plasma sheet twice per orbit. Isotropic magnetotail central plasma sheet (CPS). Many of these proplasma temperature, density, and pressure remain constants on cesses directly alter the plasma temperature, density, and pressure, and consequently these plasma properties provide clues to the same field line [e.g., Spence et al., 1989; Goertz and the underlying mechanisms at work. For the last 3 decades, Baumjohann, 1991]. Hence, from these ionospheric particle prethere have been a number of studies on the plasma properties cipitation measurements, the temperature, density, and pressure in this region [e.g., Meng and Mihalov, 1972; Baumjohann et of the magnetot.ail can be inferred tailward of the ion isotropy al., 1989; Spence et al., 1989; Kistler et al., 1992, 1993; Lui boundary. and Hamilton, 1992; Angelopoulos et al., 1993; Huang and Ion isotropy is maintained by pitch angle scattering when- Frank, 1986, 1994]. All of these studies have used in situ mea- ever the ion gyroradius is > -1/8 the radius of curvature of the surements taken by satellites which sampled only relatively field line [Sergeev and Gvozdevsky, 1995]. Therefore ion scatsmall portions of the magnetotail. As a result, these studies tering and precipitatio n are energy-dependent. Newell et al. typically have to average over large areas that are in tens to [1996b] developed an algorithm for the automated identification in the DMSP particle data of a good proxy ("b2i") for the isotropy boundary for ions between 3 and 30 kev. More recently, Copyright 1998 by the American Geophysical Union. Newell et al. [1998] have demonstrated that the DMSP b2i Paper number 97JA boundary agrees well with the National Oceanic and Atmo /98/97JA spheric Administration (NOAA) ion isotropy boundary and cor- 6785

2 6786 WING AND NEWELL: CENTRAL PLASMA SHEET ION PROPERTIES relates very well ( ) with the observed magnetotail stretching at geosynchronous orbit. The sharp termination of precipitation equatorward of b2i of course does not represent a lack of ions but only that the ions form a trapped population this far earthward. There is also a corresponding particle precipitation cutoff at the poleward edge of the main auroral oval. The ion precipitation typically drops by an order of magnitude in a few tenths of a degree of magnetic latitude (MLAT) at this boundary, termed b5i by Newell et al. [1996b]. The observations presented within this paper therefore all lie poleward of b2i and equatorward of b5i, that is, within the main isotropic plasma sheet. Discrete auroral arcs represent electron acceleration, which occurs between about 1 and 3 Rœ, and do not represent the original magnetotail population. Since the same process which accelerates the electrons inhibits the ions, we have used another automated algorithm, that of Newell et al. [1996a], to identify and exclude ion spectra associated with electron acceleration events. the entire year of 1992 DMSP data, we create for the first time two-dimensional equatorial maps of plasma sheet temperature, density, and pressure in unprecedented spatial coverage and resolution and compare them with the in situ measurements of previous studies. Previous statistical studies of CPS pressure only show pressure variation radially on the midnight plane [e.g., Spence et al., 1989' Kistler et al., 1992] or a one-dimensional cut across the magnetotail from dawn to dusk centered at some distance x down the tail [e.g., Huang and Frank, 1994; Angelopoulos et al., 1993]. As stated earlier, owing to the limited spatial coverage of high-altitude satellites, their data had to be averaged over areas of tens to hundreds of cubic Earth radii on the equatorial plane. Furthermore, with a database orders of magnitude larger and with the concurrent smaller bin size, we are able to separate the data by their magnetic activity levels to show how the temperature, density, and pressure respond to magnetic activities at different locations. In order to relate particle precipitation to its source in the magnetotail, we need to trace particles along a magnetic field line. However, the magnetotail configuration is profoundly altered by the state of magnetic activity. There are many magnetic indices, such as Kp, AE, Dst, etc., which in some ways indicate the severity of the disturbances or perhaps more precisely the strength of the magnetosphericurrents. For example, Dst is a measure of ring current strength, AE/A U/AL is a measure of the auroral electrojet strength when the electrojet is near an appropriate magnetic observatory, and Kp is a 3-hour average of magnetic observations at middle to high latitudes. These indices, although useful in many applications, do not indicate the instantaneous magnetotail magnetic field configuration very well, that is, the stretching of the field lines, which is 2. Data In our study, we use the data obtained from the Special Sensor for Precipitating Particles, version 4 (SSJ4) instrument onboard DMSP satellites for the entire year of DMSP are Sun-synchronous satellites in nearly circular polar orbit at an altitude of roughly 835 km and a period of approximately 101 min per orbit. The SSJ4 instrument package included on all recent DMSP flights uses curved plate electrostatic analyzers to measure ions and electrons from 32 ev to 30 kev in logarithmically spaced steps [Hardy et al., 1984]. One complete 19- point electron and ion spectrum is obtained each second, during which time the satellites move 7.5 km. The satellites are threeaxis stabilized, and the detector apertures always point toward local zenith. This means that at the latitudes of interest herein crucial for field line mapping [e.g., Fairfield, 1991' only highly field-aligned particles well within the atmospheric Tsyganenko, 1990; Sergeev et al., 1993]. loss cone are observed. We picked the 1992 data set because Sergeev and Gvozdevsky [1995] demonstrated that the latitude of the equatorward isotropic boundary has a high correladuring 1992 there were at least three DMSP satellites in operation, and for 1 month, March, there were four DMSP satellites, tion, r , with the magnetic field inclination, that is, the namely F8, F9, F10, and Fll (the other months had F8, F10, degree of stretching, measured simultaneously at the geomagnetic equator (approximately the same correlation holds true for b2i as determined from DMSP by the algorithm of Newell et al. [1996b]). Sergeev and Gvozdevsky proposed a new magnetic index (MT) which is based on the invariant latitude of the isotropic boundary. Magnetic field models parameterized by Kp and either F9 or F11 in operation). This allowed an optimal spatial coverage as each satellite covered different local times. Additionally, the satellites drift slowly in local time, which also helped improve the spatial coverage. Fll and especially F10 drifted at significantly faster than typical "Sun-synchronous orbits," reaching all local times, fortuitous for this study. or the interplanetary magnetic field (IMF) do not compute the stretching of the field lines very accurately, especially during active time [Fairfield, 1991' Tsyganenko, 1990; Peredo et al., 1993; Sergeev et al., 1993; Pulkkinen and Tsyganenko, 1996]. Using the MT index, Sergeev and Gvozdevsky [1995] introduced the technique of modifying the Tsyganenko [1989] model (T89) such that the magnetic field inclination angle in the near- Earth tail agrees with the observations. We have adopted this technique for the present study. In this paper, we examine the feasibility of using polarorbiting satellites such as DMSP to investigate the ion properties of the equatorial regions of the central plasma sheet (CPS) at distances approximately between 8 and 50 Rœ. Ions in this region are isotropic irrespective of activity levels as indicated from the previous in situ measurements or from the ionospheric particle precipitation data [Kistler et al., 1992; Spence et al., 1989; Newell et al., 1996b' Huang and Frank, 1994]. We investigate how well b2i latitude can be used to organize the pressure, temperature, and density profiles of the CPS. Using 3. Technique and Analytical Approach The procedure to obtain pressure in the CPS essentially involves two separate problems: (1) computing the pressure from DMSP data in the ionosphere and (2) mapping the pressure from the ionosphere to the CPS in the tail. The ions dominate the plasma pressure, with the electrons contributing only about 15% of the total pressure [Baumjohann et al., 1989, Spence et al., 1989]. Therefore electron contribution to the pressure is ignored in this study. The density and temperature are obtained from the same spectral fits used to calculate pressure. First, it is necessary to restrict the spectra observed to those suitable for determining plasma sheet properties. This involves (1) restricting the range to the isotropic plasma sheet and (2) eliminating electron acceleration events. Within the isotropic region, the pressure, temperature, and density are constant along the magnetic field line [Spence et al., 1989; Goertz and Baumjohann, 1991]. Within this region,

3 WING AND NEWELL: CENTRAL PLASMA SHEET ION PROPERTIES 6787 the existence of a cross-tail current bends magnetic field lines enough that pitch angle scattering in crossing the current sheet maintains ion isotropy [Lyons and Speiser, 1982; $ergeev et al., 1993]. Fortunately, the isotropic region can be identified from the precipitating particles themselves, and in the terminology of Newell et al. [1996b] lies between b2i and bsi, for which sophisticated automated identification algorithms exist. Sometimes within the isotropic region, electron acceleration events (i.e., monoenergetic electron peaks) are found. Such events are characterized by electron spectra which show a sharp drop in the differential energy flux at energies higher than the spectral flux peak. There have been several explanations suggested in the literature including a quasi-static potential drop [e.g., Kletzing et al., 1983; Ternerin et al., 1981], stochastic wave heating [Bryant et al., 1991], etc. These events are eliminated from our database because the ion spectra are generally retarded in such events [e.g., see Newell et al., 1996b, Plates 3 and 4] Computing Plasma Pressure From DMSP Data The plasma pressure can be obtained from the measured differential energy flux, dj(e, )/de P = 5- ae aaae where p is pressure, E is energy, m is mass, and f is a solid angle. The CPS ions have been observed to have energies from a few ev up to a few hundred kev or higher [e.g., Kistler et al., 1992; Christon et al., 1988, 1989; Lennartsson and Shelley, 1986; Meng et al., 1981]. However, the DMSP SSJ4 particle detectors have an energy range from 32 ev to 30 kev only. In order to take into account particles with energies outside the instrument energy range, the data are fitted to a distribution function. Previous magnetotail plasma observations have been fitted to a Maxwellian distribution [e.g., Montgomery, 1968; Vasyliunas, 1971; Lennartsson and Shelley, 1986] with phase space density function and to a : distribution [e.g., Vasyliunas, 1971' Christon et al., 1988, 1989] 3 3 (2) E )-K-1 + (3) where n is density, k is the Boltzmann constant, and T is temperature. The magnetotail plasma has also been observed to have two or more components which may indicate the presence of more than one type of species or the same species with different temperatures, for example, in the region where there is a mixture of ionospheric and magnetospheric particles or a mixture of solar wind and terrestrial particles [e.g., Peterson et al., 1981; Lennartsson and Shelley, 1986]. In order to take all of this into account, for each spectrum, a nonlinear least squares fit [Berington, 1969] to the differential energy flux dj 2E 2 - = f(e) m (4) is performed where f is one of the three distribution functions: (1) Maxwellian distribution and f = fro, (2) K distribution and f = f,, and (3) two-component Maxwellian and f = fm + fro2, where the subscripts 1 and 2 denote the phase space density of the first and second component, respectively. From these three fits, the best fit is selected according to reduced X2 and F x tests which take into account the number of free parameters used [Bevington, 1969]. Although each ion spectrum has 19 points corresponding to the 19 energy channels, after the bad points and the channels with zero counts are removed, there may typically be 8 to 14 good points left. The lowest three energy channels, which have an energy range from 32 to 68 ev, are not used because these channels are often susceptible to spacecraft charging and other noise sources. When eight or less points with significance exist, a fit with more than four free parameters would not satisfy statistical goodness-of-fit criteria. Therefore we do not attempt to fit more than two-component Maxwellian or more than onecomponent K distribution. It should be noted that, with the instrument energy range, distribution with > -10 may not be easily distinguishable from Maxwellian distribution. As a result, the particles may be fitted with Maxwellian distribution when they could have distribution. In reality, this does not pose a serious problem as can be noted from (1), since for large values or Maxwellian distribution, the majority of the particles are within the instrument energy range. In this circumstance, any function which approximates the observed dj/de will give a good estimate of pressure [e.g., Lennartsson and Shelley, 1986]. The DMSP SSJ4 cannot distinguish different kinds of ions species, and in our analysis we have assumed that all the ions are H +. Previous results show that most of the ions in the magnetotail are H +, and therefore if there is only one component found, then it is likely to be H + [Lennartsson and Shelley, 1986' Kistler et al., 1993]. However, when there are two constituents present in the plasma and if the second constituent is not H+, then the density and pressure due to the second con- stituent are underestimated by a factor of 4- as can be easily seen from (1)-(4). However, it turns out that the problem is not as bad as it seems for pressure because the density and temperature associated with the second component are typically only a fraction of the main component, up to 25% and 30% of the main component, respectively. Also, previous observations indicate that the likely candidates for the second nonproton constituent, if there is a second constituent, are helium (amu = 4) or oxygen (amu = 16) [Lennartsson and Shelley, 1986; Kistler et al., 1993]. Therefore, in the worst case, if the second constituent is not H +, the total density may be underestimated up to 38%, but the total pressure would only be underestimated up to 18% on the average. Kistler et al. [1993], using in situ data, determined that the nonproton contribution to ion pressure during postsubstorm injection is roughly 15%. The temperature is not affected by much because the temperature is very much determined by the peak of the energy of the dj/de (the characteristic energy or E0), for example, for Maxwellian the peak energy, E0 = kt, and for K, kt = E0 /( : - 3/2) [Christon et al., 1989]. In a simulation designed to check the accuracy of the fitting algorithm, p spectra with Maxwellian distribution having 16 points per spectrum are generated. Then, each point within the spectrum is perturbed up to Ax%. We did this q

4 6788 WING AND NEWELL: CENTRAL PLASMA SHEET ION PROPERTIES times and found that when Ax = 5%, the fitted values of n and T have standard deviations of 2% and 0.6%, respectively, and when Ax = 10%, n and T have standard deviations of 4% and 2%, respectively. The values for p and q are set to 100. As expected, the simulation confirms that the temperature can be obtained more accurately from the fit because it can be obtained simply from the peak of the spectrum. Figure 1 shows an example of Maxwellian, K, and two-component Maxwellian fits to DMSP spectra Mapping From Ionosphere to Central Plasma Sheet For mgpping the field line from the ionosphere to the CPS, b2i has another significance. Using simultaneous GOES and DMSP F Day 011 Time 02:09: '5 Q. 2.0'1 04 '5 1.5'104 g 1.0,10 4 E 5.0,!03 m 0, Ko Figure 2. Histogram showing the data distribution in (a) b2i and (b) Kp ' Energy DMSP F Day 011 Time 01:03: ::3 1= e-. = x., i Energy DMSP F Day 001 Time 01:48:13... i...! -...!... c 1 = 4.016e-01 NOAA satellites observations, Sergeev et al. [1993] demonstrated that the latitude of the ion isotropy boundary has a high correlation, r- 0.9, with the magnetic field inclination, that is, the degree of stretching, measured simultaneously at the geomagnetic equator. They suggested that the ion isotropy latitude can be used as an index of magnetic activity at least in the near-earth magnetotail. As discussed in section 1, the presently available indices such as AE, Kp, Dst, IMF, solar wind dynamic pressure, and E, etc., are known to be rather poor indicators of the instantaneous magnetospheri configuration or at least do not determine magnetotail stretching, which is crucial for mapping regions from the ionosphere to the tail [Fairfield, 1991; Tsyganenko, 1990; Sergeev et al,, 1993]. Sergeev and Gvozdevsky [1995] noted that increasing tail current and/or thinning of the tail current sheet moves the isotropic boundary earthward. Also, as the field is stretched tailward, the projec- tion of the tail equatorward point moves to a lower latitude in the ionosphere. Both of these processes cause b2i to move equatorwarduring a more active time and poleward during a quieter time. The b2i latitude depends on the magnetic local time (MLT) as well, and therefore in order to use the isotropic boundary as an index, b2i latitude needs to be converted or normalized to its equivalent value at the midnight index which is taken to be the invariant index [Newell et al., 1998]. The empirical formula for normalized b2i is normalized b2i = lb2 i MLAT I o lo '103 Normalized b2i is analogous to MT [Sergeev and Gvozdevsky, 1995] or MTd [Newell et al., 1998]. For brevity, hereafter the normalized b2i is referred to as simply b2i. Figure 2 shows b2i and Kp distribution for the data used in this study. Figure 2a shows that b2i has a smooth Gaussian-like distribution centered around 65, rather preferable to the distribution of Kp values. This study uses the T89 model because not only is it one of õ the most widely used and tested models but also because the Energy errors in the field line mapping from the ionosphere to the Figure 1. An example of differential energy flux with (a) equatorial plane are generally small within the auroral zone single-component Maxwellian, (b) two-component Maxwellian, (where b2i and b5i are located). Pulkkinen and Tsyganenko and (c) distribution. [1996] found that the mapping errors at latitudes <69 ø (SM)

5 WING AND NEWELL: CENTRAL PLASMA SHEET ION PROPERTIES Plasma Pressure Profile Plasma Pressure Profile -30[... I''''''''' t''' ''''''1'''' I... ''1-20,,,,... -' oj IlJll 1.0 II I" ,. 0.1 npa 2O. I 0.1 npa O -50 b2i b2i Plasma Pressure Profile -30 I;... ' ""... "' '... '... s... ' npa ß I 3O b2i Plate 1. Two-dimensional maps of plasma pressure for (a) high magnetic activities, b2i = 62-64; (b) moderate activities, b2i = 64-66; and (c) low activities, b2i = Each point is averaged over 1 x 1 Re 2 regions. Some regions are left blank because of insufficient data points. Notice the color bar is in log scale. are less than 1 Rœ for Kp < 3. For more active conditions, as a result of the excessive tailward stretching of the T89 field lines, the errors may increase up to a few Earth radii near the midnight sector [Pulkkinen and Tsyganenko, 1996; Peredo et al., 1993]. In order to improve the mapping, Sergeev and Gvozdevsky [1995] change the contribution from the tail current by a factor f such that the total contribution of the external field is given by the following equation: Bex t = Bcf + B r + f x B t (6) where Bex t is the total external field B, Bcf is the B contribution from Chapman-Ferraro currents, B r is the B contribution from ring currents, B t is the B contribution from cross-tail currents, and f is the free parameter. The free parameter f is a function of b2i and Kp and is determined empirically so that the model fits the GOES observations with resulting correlation coefficient r = 0.89 [Sergeev and Gvozdevsky, 1995]. The values of f have been carefully kept near 1 in order to prevent abnormal effects in the magnetic field model. The values of f for some of the b2i and Kp pairs are given by Sergeev and Gvozdevsky [1995, Table 3]. This Table 3, however, does not give all the combinations of b2i and Kp pairs and so whenever a value of f is missing from the table, the nearest value of f is used.

6 ß, WING AND NEWELL: CENTRAL PLASMA SHEET ION PROPERTIES Plasma Pressure Profile -30J: ""' I... ' j -lo 3O b2i' ii I -5O , Plate 2. Smoothed version of Plate lb. The degree of smoothness can be controlled by adjusting the filter parameters. npa pressure is dominated by plasma pressure. Hence, the plasma pressure in the CPS is expected to decrease as the distance down the tail increases. The pressure exhibits a dawn-dusk 3.3. Data Processing: Technique Summary asymmetry which is described next, where comparisons with in situ measurements are presented. All ion spectra from all DMSP satellites (F8, F9, F10, and Since the method used for inferring CPS plasma pressure F11) during 1992 were analyzed whenever the satellites were from ionospheric observations is new and has not been used within the isotropic plasma region and outside electron accelelsewhere in the literature, it is worthwhile and important to eration events. There are 3,258,343 spectra in our database that examine in detail how the results compared with the previously satisfy these conditions, out of which slightly less than one reported in situ measurements. However, exact numerical comthird are discarded because of poor statistical significance as parisons among various studies are not easy because all the determined by X 2. For the remaining data (2,201,173 points), studies with in situ data use larger temporal and spatial resoluthe pressure of four consecutive seconds are sorted, the highest and lowest are thrown out, and the middle two are averaged. Then the points are mapped to the neutral sheet using the modified T89 model as described in section 3.2. The geometric averages of the pressure are computed in 1 Rœ x 1 Rœ bins on the X and Y plane, where X is - (XosM 2 + ZosM2) ø'5 and Y is YGs, and in 2 ø b2i steps. Here X has the same direction as XGs, and it is used instead of Xos because it organizes the data better [see, e.g., Spence et al., 1989, Figure 2]. Likewise, a geometric average is chosen over an arithmetic average for the same consideration [see, e.g., Baumjohann et al., 1990, Figures 3 and 4]. 4. Plasma Pressure in the Central Plasma Sheet The two-dimensional plasma pressure profiles in the neutral sheet are presented in Plate 1 for various magnetic activity levels. Owing to a lack of data points during extremely active (b2i < 62) and extremely quiet (b2i > 68) magnetic conditions, only data during high (b2i = 62-64), moderate (b2i = 64-66), and low (b2i = 66-68) activity levels are presented in this paper. Plate 1 show that b2i organizes the pressure data well. The pressure increases with b2i which increases with magnetic activity. This pressure increase has previously been observed and discussed in terms of merging and southward turning IMF. During periods of southward IMF, flux is added to the tail which increases the flaring angle and hence the pressure [Kistler et al., 1992]. Plate 1 shows that the plasma pressure decreases with X. The pressure exerted by the solar wind on the magnetopause decreases with increasing distance down the tail as the magnetopause flaring angle decreases. This implies that from the pressure balance considerations among the lobe, the PSBL (plasma sheet boundary layer), and the CPS [e.g., Baumjohann et al., 1990; Kistler et al., 1992], the CPS total pressure also decreases with increasing distance down the tail. In the neutral sheet, the magnetic field is small, and the total tions and/or measurements taken from different years than our present study. Consequently, more emphasis will be given to the general features or trends rather than the actual numerical results. Figure 3a shows the radial plasma pressure profile in the midnight meridian; Figure 3b shows the dawn-dusk profile at constant X = Re. The error bars in these plots and subsequent plots in this paper indicate the errors of the standard deviation of the samples. Figures 3a and 3b are slices from the quiet time, b2i = 66-68, of the equatorial pressure map shown in Plate l c. In order to facilitate the comparisons more easily, whenever we compare figures throughouthis paper, we attempt to match as much as possible the resolutions The DMSP SSJ4 instrument on each satellite has been cali- and the conditions under which the previously published figures brated prior to launch. However, the instrument sensitivity has been known to degrade over time. It would be very difficult to were made. The radial pressure profile shown in Figure 3a is averaged over regions of -5 Re < Y < 5 Re, and it is to be independently calibrate the instrument in space. In this study, compared with, for example, Figure 4 of Angelopoulos et al. each instrument on the four satellites is cross-calibrated against [1993] (1978, 1979, and 1985), Figures 6 and 8 of Spence et one another based on the statistically computed averaged pressure. We believe that out of the four instruments the calibration al. [1989] ( ), or Figure 12 of Huang and Frank [1994] (1979). The numbers in parentheses indicate the years for the Fll SSJ4 is the most accurate because it has been in which the data were taken. All of the figures are broadly launched the most recently and its sensitivity is not too different from F10. Therefore we normalize the sensitivity of the other three instruments to that of F11. If the sensitivity had similar but with significant individual differences. These latter may have resulted from the different years for which these averages were computed. Kistler et al. [1992, 1993] showed that been normalized to that of F8 instead of F11, then the reported the variations in solar wind dynamic pressure (Pa) during the density and pressure would have to be multiplied by roughly a factor of 0.6. The temperature calculation should not have been affected because the location of the peak energy remains the same. solar cycle may affect the statistical average of the pressure in the magnetotail in a manner consistent with the pressure balance argument: higher average Pa results in higher average total pressure in the magnetotail. Figure 3a shows slightly higher

7 ß,. WING AND NEWELL: CENTRAL PLASMA SHEET ION PROPERTIES Temperature Profile -30 I' Temperature Profile,,,,,,,,i,,,,,,,,,i,,,,,,,,,t,,,,,,,,,i,,,,,, I m vv"( m. lo 8 m% m m 107 K mm mm mm lo 7 30 f b2i' lo -2O b2i' Temperature Profile -30 '" I... ' l m ß -20 -lo lo ß L 20 m K 30 o b2i ß Plate 3. Same as Plate 1, except for temperature. Maps of plasma temperature are for (a) b2i = 62-64, (b) b2i = 64-66, (c) b2i = Notice that the color bar is in log scale. If there are two components, then the temperature of the component with the highest density is used to compute the average. averaged pressure than those in the previous studies. However, this is the expected result since our study is conducted for the year 1992 which has higher P, than that in the periods used in the previous studies [e.g., Richardson et al., 1996, Figure l a]. In fact, the average plasma pressure of-0.35 npa at 20 RE (see, for example, Figure 3a) and P, of npa [Richardson et al., 1996, Figure l a] appear to be consistent with the rest of the points shown in Figure 3 of Kistler et al. [1993] which plots the average magnetotail pressure at 20 RE and the corresponding solar wind pressure of previous studies. In addition to the solar cycle and P,, the differences among the studies include the definitions of quiet time and spatial domain from which the averages were computed. Given all these variations, the pressure calculations shown in Figure 3a are consistent with previous in situ work, or at least as consistent as any of the previous studies are with one another. Figure 3b is averaged over regions -19 RE< X < -16 RE which is the resolution used in the corresponding Figure 4 of Angelopoulos [1993]. In general, the two figures are very similar. Figure 3b shows that the pressure is slightly higher at the midnight sector than at the dawn or dusk flank. In the region beyond --12 RE, the pressure at dawn may be slightly higher

8 6792 WING AND NEWELL: CENTRAL PLASMA SHEET ION PROPERTIES Density Profile i , Density Profile ''''"'"''1... i '''''1''"''... I''" =''1= i 1.o 1.0 lo o. 1 cm cm-3 o -lo -2o -3o -40-5o b2i b2i' Density Profile -30 I ', i... "'"'' ' '"'! -2O -10 II ß cm b2i' Plate 4. Same as Plate 1, except for density. Maps of plasma density are for (a) b2i = 62-64, (b) b2i = 64-66, (c) b2i = than at dusk, but the error bars are larger than the differences ward gradient/curvature drift as the ions ExB convect earthbetween the two sectors. Huang and Frank [1994] reported that ward [Lyons and Samson, 1992; Spence and Kivelson, 1993]. the neutral sheet plasma pressure is highest at dawn and lowest The ExB drift is relatively energy-independent but the gradient/ at dusk, with the pressure at the midnight region between the curvature drift is energy-dependent, resulting in energytwo [Huang and Frank, 1994, Figure 11 and Table 1]. How- dependent ion distribution in the tail. Spence and Kivelson ever, their results were averaged over mach larger regions, [ 1993] noted that at large distances from the Earth, the gradi- 5 x 15 x 2 Rœ 3 (bin width in GSM X, Y, Z) and over all AEs. ent/curvature drift affects preferentially the high-energy ions As shown in Plate 1, the pressure has finer spatial structures which contribute little to the pressure. In contrast, in the nearand dependencies on magnetic activity. In fact, in the regions Earth region, the westward gradient/curvature drift ions domicloser than -9 to 12 Re, depending son magnetic activity, the nate the pressure. Spence and Kivelson [1993], using the finite plasma pressure appears to be higher in the dusk-midnight sec- width magnetotail convection model which takes into account tor than the midnight-dawn sector. This higher pressure in the tail and low-latitude boundary layer (LLBL) sources, showed dusk than dawn has been predicted and attributed to the west- that the gradient/curvature and ExB drift can explain, at least

9 WING AND NEWELL: CENTRAL PLASMA SHEET ION PROPERTIES 6793 (a) 1.0' 0.1 (b) -10 Y(Re) from the T89 model is slightly lower than in situ measurements. In fact, Peredo et al. [1993] and Pulkkinen and Tsyganenko [1996] evaluated the T89 model and found that neutral sheet <Bz> is underestimated in this model. In order to check how the pressure varies as a function of magnetic activity and how well b2i separates data by their activity levels, we present Figure 5, which shows total pressure as a function of b2i for different distances down the tail near the midnight meridian, -12 Rœ < Y < 12 Rœ. Figures, 5a and 5b are similar to Figure 5 of Baumjohann et al. [1990] which shows the total pressure as a function of AE for years With the exception of the plots in our Figure 5 being smoother, both figures show that pressure increases monotonically with magnetic activity. In Plate 1 there has been no attempt to smooth the data, and those bins with insufficient data are left blank. However, with standard image processing techniques, the map can be smoothed, and some of the empty bins can be filled in. An example is shown in Plate 2 which is a smoothed version of Plate lb. The smoothing applied is a version of a median filter and a neighbor-averaging filter. The degree of smoothness can be controlled by adjusting the filter parameters. Figure 3. One-dimensional slices of the quiet time (b2i = Temperature 68) plasma pressure (a) radially along the midnight meridian, with Y = 0 Re and (b) along the line of X = Re. In or- Plate 3 shows CPS temperature as a function of b2i and loder to facilitate comparisons with previous in situ observations, cation. This plate shows that temperature increases with mag- Figure 3a data are averaged over the region -5 Re < Y < 5 Re netic activity throughout the CPS. This result is consistent with and 1 Re bin width in X and Figure 3b data are averaged over the previously observed heating in the CPS during substorms, a -19 Re< X < -16 Re and 1 Re bin width in Y. These slices process which is often associated with magnetic field are taken from Plate l c. reconfiguration and the unloading of magnetotail energy [e.g., Baumjohann eta!., 1989; Kistler et al., 1992], wave energy absorption [e.g., Smith et al., 1986], current sheet energization partly, the dawn-dusk asymmetry as a function of the distance [e.g., Mitchell et al., 1990], higher frequency of BBFs down tail [see Spence and Kivelson, 1993, Figure 2]. However, [Angelopoulos, 1996], etc. Figures 6a-6f show temperature the location of the asymmetry in their model results (their Figslices along the midnight meridian and along the line of conure 2) is different than that in Plate 1 which may indicate that stant X at -20 Rœ in the order of decreasing magnetic activity. perhaps their model needs to take into account other processes, Figure 6 is comparable to Figure 4 of Baumjohann et al. some of which were discussed in their paper. Although their [1989] which shows temperature as a function of AE. Figures model was intended only for quiet time, it appears that the 6a, 6c, and 6e show that the temperature increases monotonidawn-dusk asymmetry in the near-earth region persists at all activity levels. Absent from the model result are the pressure peaks near midnight in the region beyond -12 Rœ, which may have resulted from the lower magnetic field strength in this ret ' i... I... i... I... gion and pressure balance. As will be shown in section 5, the higher pressure may also be a consequence of a higher temperature near midnight which could be associated with bursty bulk flows (BBFs) which occur more frequently near midnight [Angelopoulos, 1996]. This pressure peak diminishes with decreasing activity which i s consistent with BBF behavior [e.g., Angelopoulos et al., 1993]. Figure 4 shows the radial profile of the total pressure along the midnight meridian. The total pressure is the sum of the 0.1 magnetic pressure, B2/21.to, and the plasma pressure. Here the magnetic pressure of the T89 model is used. In general, Figure 4 shows trends similar to those in Figure 7a of Kistler et al., I,, I,,, I... I [1992] and Figure 2 of Kistler et al. [1993]. The latter two fig- ures show the average total pressure for the years Since the plasma pressure comparisonshow our plasma pres- Figure 4. One-dimensional slice of quiet time (b2i = 66-68) sure to be comparable if not slightly higher than the previous total pressure along the midnight meridian. The total pressure results, we would expect the total pressure comparisons to be is the sum of the magnetic and plasma pressures. The data are just the same. However, our total pressure is slightly lower averaged over the region -5 Re < Y < 5 Re and 1 R bin width than theirs. This suggests that the magnetic pressure obtained in X.

10 6794 WING AND NEWELL: CENTRAL PLASMA SHEET ION PROPERTIES (a) (-6.0 x 107 K) and the lowestemperature the dawn flank 1.0 (4.3 x 107 K). Their midnight peak is much smaller than that in this study and that given by Angelopoulos [1996, Figure 4], which may be attributed to the fact that their temperature values are averaged over all AEs and over regions of 15 x 5 x RE 3 (in GSM X, Y, and Z, respectively). The present study also 70 finds similar dawn-dusk variation, but this asymmetry has de- B2i pendencies on magnetic activities and locations in the neutral (b) sheet. In the region where the radial distance R > -12 Re, the 1.0 dawn-dusk asymmetry is much weaker than previously reported. For example, for b2i = 64-66, averaging the temperature over the same area in the X-Y plane as in the Huang and Frank [1994] study, the dawn, midnight, and dusk temperatures are 3.0 x 107 K, 4.2 x 107 K, and 3.2 x 107 K, respectively. The B2i Figure 5. Total pressure as a function of magnetic activity (b2i) in regions (a) - 15 Re < X < - 11 Re and - 12 Re < ¾ < 12 Re and (b) - 19 Re < X < - 15 Re and - 12 Re < Y < 12 Re. The O ratio of dawn to dusk temperature in our study is 0.93, whereas in theirs it is A very pronounced effect, as shown in Plate 3, is that the dawn-dusk asymmetry is much more significant closer than -10 to 12 Re (depending on magnetic activity) to the Earth. This asymmetry, with temperature being total pressure is the sum of the magnetic and plasma pressures. higher at dusk than at dawn, is also consistent with the pre- Figure 5 is similar to Figure 5 of Baumjohann et al. [1990]. dicted result from the ion gradient/curvature and ExB drifts. Plate 3 shows, at least qualitatively, similar features shown in the temperature contour map calculated from the Spence and cally with magnetic activity at all X ranges, -50 Re < X < -8 Re, from -3 x 107 K to -5 x 107 K as b2i increases from 62 to 68. Baumjohann et al. [1989], using AMPTE/IRM data that are closer to Earth (from -9 to -20 Re ), show that the tempera- Kivelson [1993, Figure 5] magnetotail convection model. 6. Density The analysis done above is repeated for density. Plate 4 ture increases with AE from 4.5 to 5.5 x 107 K, except for one shows density maps for high, moderate, and low magnetic acdata point (-9 Re < X < -14 Re and AE > 500 nt), which is perhaps due to lack of statistics. Figure 6c which is for moderate magnetic activity (b2i = 64-66) can also be compared with the second panel of Figure 12 of Huang and Frank [1994] which shows the radial midnight temperature profile averaged over all AEs and Iz Z1 < 10 Re where ZkZ is the distance to the model neutral sheet defined in Gosling et al. [1986]. Although their temperature seems to have a roughly similar range, 4 to tivities, respectively. Figures 7a-7f show the density slices along the midnight meridian and along a line of constant X at -20 Re as in Figures 6a-6f. The density decreases with increasing distance from the Earth, but during active time (b2i = 62-64), the density gradient is much weaker, as can be seen in Figures 7a, 7c, and 7e. The density seems higher in the near- Earth regions and the regions close to the magnetopause along the dawn and dusk flanks regardless of magnetic activity. This 5 x 107 K, their temperature plot seems to have irregular fluc- density enhancement appears stronger at the dawn flank than tuations whereas Figure 6c shows a much smoother variation with X. Figures 6a, 6c, and 6e show that near the midnight meridian the.temperature decreases with increasing geocentric distance up to -15 Re and thereafter remains more or less constant or perhaps decreases only slightly. the dusk flank, but the enhancement becomes more pronounced and azimuthally broader with decreasing activity at both flanks. This density enhancement region seems to coincide with the LLBL which, along with the plasma mantle, has long been considered a region where magnetosheath particles can enter Ther have been fewer studies on how the temperature var- the magnetosphere through diffusion and other processes [e.g., ies azimutha!ly from dawn to dusk. Plate 3 shows that the tem- Eastman et al., 1985]. Density also appears to increase slightly perature is higher in the midnight sector than at the dawn or dusk flank regardless of magnetic activity. This can be seen also in Figures 6b, 6d, and 6f which show the temperature with increased activity particularly in the near-earth and midnight-dawn sectors. Our quiet time density map, Plate 4c, shows qualitative slice along X =-20 Re. Angelopoulos [1996] observed a simi- agreement with the model result of Spence and Kivelson [1993, lar peak near midnight in ISEE and AMPTE/IRM 1985 data sets [see Angelopoulos, 1996, Figure 4] which apparently is associated with BBFs which peak in occurrence near Figure 4]. Their model shows that during quiet time the colder LLBL ions from the dawn flank contribute significantly to the number density in the dawn flank and the near-earth region. the midnight meridian. The peak could perhaps be associated The bulk of LLBL ions ExB drift sunward at large distances, with the higher pressure near the midnight meridian as described in section 4. Plate 3 shows that the width of the peak but in the near-earth region the duskward gradient/curvature drift is comparable to the sunward ExB drift. It tums out that, seems to widen in the Y direction with increased activity from IYI < 8 Re during quiet time to IYI < 15 Re during active time. If the peak could be attributed to BBFs, then this would suggest that BBF events widen in the Y direction with increased activity. Huang and Frank [1994] showed that temperature varies nonlinearly from dawn to dusk with the highest temperature near midnight (-6.1 x 107 K), followed by the dusk flank from the analysis of the spectra as described in section 3.1,.the probability of finding particles having two-component Maxwellian distribution is higher in the near-earth region and in both flanks than anywhere else in the CPS. The second component typically has a lower density of to 0.2 cm -3 and a colder temperature of -5 to 6 x 106 K. In this paper, we have not investigated the contributions from the ionospheric plasma sources.

11 ß ß WING AND NEWELL: CENTRAL PLASMA SHEET ION PROPERTIES o 107 6o b -2O O Y (Re) 6.10? ,..., d , ,...,..., ol ol !... i... i... i i i I i i ii -4o o Y (Re) Figure 6. One-dimensional Slices of temperature along (a) the midnight meridian Y = 0 Re and (b) X = -20 Re during high magnetic activities (b2i ). (c) Y - 0 Re and (d)x Re during moderate magnetic activities (b2i = 64-66). (e) Y - 0 Re and (f)x Re during low magnetic activities (b2i ). In Figures 6a, 6c, and 6e the dat are averaged overegions -10 Re < Y < 10 Re and 1 Re bin width in X, and in Figures 6b, 6d, and 6f the data are averaged over regions -30 Re < X < -10 Re and 1 Re bin width in Y. f tioned earlier in section 5, Huang and Frank obtained densities Just as with temperature, there is also a dawn-dusk asymmetry in the density maps, but the asymmetd, is stronger and of 0.5, 0.29, 0.23 cm -3 for dawn, midnight, and dusk regions, in the opposite sense to that of temperature. The density is respectively. Using the same bin size that they used we obhigher in the dawn sector than in the dusk sector. The dawn- tained higher average densities of 1.0, 0.93, and 0.76 cm -3 dusk asymmetry in the density profiles has been observed pre- for dawn, midnight, and dusk, respectively. Figure 12 of Hhang viously with in situ measurements [Huang and Frank, 1994]. and Frank [1994] also shows that the density declines with in- Using ISEE 1 data, all activity levels, and the bin size men- creasing distance down the tail in the midnight meridian, but

12 _ WING AND NEWELL: CENTRAL PLASMA SHEET ION PROPERTIES 2.Oil I1'... I... I... I... i.., b [,, O Y (Re) !... i... i...!...!... d.5 ; 1.5 'i 1.o - v 0.5 ' Y (Re) toe ' Y (Re) Figure 7. The same as Figure 6, excepthat the plots show density. (a) Y = 0 Re and (b) X = -20 Re during high magnetic activities (b2i = 62-64). (c) Y = 0 Re and (d)x = -20 Re during moderate magnetic activities (b2i = 64-66). (e) Y = 0 Re and (f)x = -20 Re during low magnetic activities (b2i = 66-68). In Figures 7a, 7c, and 7e the data are averaged over regions -10 Re < Y< 10 Re and 1 Re bin width in X, and in Figures 7b, 7d, and 7f the data are averaged over regions -30 Re< X < -10 Re and 1 Re bin width in Y. their averagedensities, ranging from 0.3 to 0.5 cm -3, are ner CPS measurements of 0.15 to 0.45 cm -3. Their densities lower than the 0.5 to 1.4 cm -3 range in our moderate magnetic appear to be lower than ours and those of Huang and Frank activity data (b2i = 64-66). [1994], which partly could be due to the fact that their mea- We have also compared our results with those of surements were taken mostly on the dusk sector, which as dis- Baurnjohann et al. [1989]. Their Figure 4 shows that inner CPS cussed earlier, has lower density than the dawn sector. Even if density increases with AE, except for the region -9 Re < this is taken into consideration, our density calculation still ap- Xas t <- 14 Re. Our densities also increase slightly with mag- pear higher than theirs and those of Huang and Frank [1994]. netic activity, with the greatest increase on the dusk side of Lennartsson and Shelley [1986] and Lennartsson [1992] remidnight in addition to the near-earth region which ported that the plasma sheet average ion density ranges from Baurnjohann et al. [1989] also found. However, our averaged 0.4 to 1 cm -3 which is closer to ours but still somewhat lower. densities of 0.5 to 1.4 cm -3 are higher than their averaged in- However, Christon et al. [1991], using instruments with an en-

13 WING AND NEWELL: CENTRAL PLASMA SHEET ION PROPERTIES 6797 ergy range of.--30 ev to ~1 MeV, found that the density of ions -< 16 kev (the upper limit of the energy range of the instrument used in the two studies) only represents 73% of the total density. If this calculation was correct, then the density values reported by Lennartsson and Shelley [1986] and Lennartsson [1992], corrected for the missing higher energy, would be right in same range as the density values reported here. Angelopoulos [1996] showed that the density is lowest near the midnight meridian which is corroborated in Figure 7f. However, the minimum near midnight disappears with increasing activity as shown in Figures 7b and 7d. His density range of 0.2 to 0.4 cm -3 obtained from data taken in 1985, 1978, and 1979 is lower than the density obtained here. The density comparisons have been summarized in Table 1 which shows that our average densities are consistently higher than the previously reported values, except perhaps those reported by Lennartsson and Shelley [1986] and Lennartsson [1992]. These differences could be attributed to several factors. First, the average Pa is higher for 1992 than other periods used in the other studies. As discussed above, the magnetotail response to higher Pa is higher total pressure which implies higher plasma pressure in the CPS. Higher Pa also causes the magnetosphere to have smaller volume [Roelof and Sibeck, 1993] which may increase the density inside the magnetosphere, but this alone may not account for all the increase in the density. Higher density may also be caused by higher solar wind flux entering the magnetosphere during a period of higher Pa. An increase in density is consistent with the observed increase in pressure resulting from higher Pa which was discussed in section 4. Second, our method for calculating density accounts for the particles outside our instrument energy range of 32 ev to 30 kev and for < and two-component Maxwellian distribution, whereas other studies included only the particles within their instruments' energy range, but this alone is not sufficient to explain all the differences [cf. Christon et al. [1991]. Third, as demonstrated in the discussion above, it seems that the density fluctuations are very high as indicated by the error bars, and numerical agreements are hard to produce even between two studies using in situ data from the same satellite and almost the same period but different instruments [e.g., Lennartsson and Shelley, 1986; Huang and Frank, 1994]. Out of the three parameters, density is more susceptible to errors, as discussed in section Summary and Conclusion Two-dimensional equatorial maps of the CPS pressure, temperature, and density during high, moderate, and low magnetic activity levels have been produced for the first time by using a method which infers the plasma properties from the ionosphere. This method overcomes the limitations of in situ measurements. Inescapably, a high-altitude satellite can sample only a minuscule portion of the plasma sheet over any reasonable length of Table 1. Comparison of CPS Ion Parameters With Previous In Situ Studies Lennartsson and Shelley [ 1986] and Lennartsson [ 1992] Baumjohann et al. Huang and [ 1989, 1990] Frank [ 1994] This Study Instrument range, kev a Years covered Resolution, GSMR e -15x30x20-5x15 x5-2x5x5 -lxl -2x 10x5-15x5x5 b Spatial range, GSM R e 10 < R < < R < 25 X<0, -20<X<-9, -25 <X<-10, -50<X<-6, c IYI < 20, 0 < Y < 15, IYI < 20, IYI < 30, -7 <Z< 14-6 <Zns< -1-7 <Z< 14 Zns=O Solar wind pressure, npa d Density, cm e Temperature, 107 K (2.3)-5.5 f Total Pressure, npag N/A h Owing to space limitation, not all the previous studies are listed here. See text for comparisons with other studies. These studies were chosen because they used rather large data sets to survey multiple parameters in CPS. Comparisons need to take into acounthat each study uses data from different years, spatial resolutions, magnetic activity resolutions, instrument energy ranges, etc. N/A indicates not available. adata are fitted to account for ions outside the instrument range. bmultiple spatial resolutions for various plots are used. CFor comparison with other values listed in Table 1, the range is restricted to - 10 < X < -25. adata are from Figure 1 a of Richardson et al. [ 1996]. echriston et al. [ 1991 ] calculated that ions < 16 kev represent 73 % of total density. ftemperature below AE = 50nT was not computed but could be as low as 2.3 x 107 K [see Baumjohann et al., 1989, Figure 9]. gfigure 3 of Kistler et al. [1993] shows additional pressure values from other studies not listed here. hfull width half maximum (FWHM) of Figure 3 of Baumjohann et al. [1990].

14 6798 WING AND NEWELL: CENTRAL PLASMA SHEET ION PROPERTIES time. Statistical studies with such data sets have had to average over areas that are tens to hundreds of cubic Earth radii. Even Finally, it is necessary to map the field lines from the ionosphere to the neutral sheet. This also requires special care. Magnetic field configuration, that is, the stretching of the field line, in the tail varies greatly with magnetic activity. Many magnetic indices such as Kp, AE, Dst, etc., do not state the magnetic field configuration very accurately. It tums out that the equatorward boundary of isotropic region b2i correlates very well with the stretching of the corresponding field line in the tail, at least as determined from GOES (geosynchronous) observations. Increases in tail stretching increase b2i and vice versa. Therefore b2i can be used to slightly modify the T89 magnetic model so the stretching of its field lines agrees with observations [Sergeev and Gvozdevsky, 1995]. Using this method and the relative comprehensiveness of DMSP observations, for the first time pressure, temperature, closer to Earth, at radial distances <-10 to --12 Re, depending on magnetic activity. Beyond -12 R e the dawn-dusk asymmetry is much weaker. This asymmetry is required for some substorm models, which therefore can now be seen to be possible only within roughly about 10 to 12 R e. The ion temperature peaks near midnight at all activity levels, which appears clearly in the region beyond -12 Re. The width of the peak appears to widen in the Y direction with increased magnetic activity from 114 < 8 Re during quiet times to 114 < 15 Rœ during active times, multispacecraft missions with dozens of satellites could not achieve comprehensive coverage. Plasma pressure, temperature, and density remain constant along magnetic field lines tailward (poleward) of the ion isotropy boundary. Hence low-altitude satellites can be used to infer plasma properties in the CPS, which has been observed to be isotropic at distances between 8 and 50 Re. DMSP series satellites are ideal for this task because there are usually two or more in operation simultaneously. In fact, during 1992, It has previously been suggested that magnetotail pressure is the period of our study, there were three to four functioning higher in the dawn sector than the dusk. At the finer resolution simultaneously. of the present study, it tums out that the dawn-dusk asymmetry A series of careful steps is required to infer CPS properties is real, although weak, at distances >-12 Re. However, at disfrom ionospheric observations with reasonable confidence. tances <-10 to -12 R e the dawn-dusk asymmetry is stronger First, the latitudinal range corresponding to the isotropic plasma but reversed, so that the pressure is higher in the dusk sector. sheet must be determined; for the DMSP data set this lies be- The pressure maps show that the downtail pressure is slightly tween b2i and b5i as determined by the algorithms given by higher in the midnight meridian than along either the dawn or Newell et al. [1996b]. Next, electron acceleration events must dusk flanks, which may be the result of weaker magnetic presbe excluded, since these retard ions (and since they occur at sure in the midnight sector. Just as with temperature, the peak low altitude and not in the equatorial plasma sheet). These becomes more prominent with increasing activity. As previevents are identified as in the procedure given by Newell et al. ously reported and as is consistent with pressure balance, the [1996 a]. CPS pressure increases with increasing magnetic activity and It is next necessary to fit the ion temperature, density, and decreases with increasing distance down the t,ail. pressure for the remaining points. A fit to dj/de is done with Ion density exhibits a dawn-dusk asymmetry with the denthree distribution functions: (1) one-component Maxwellian (2) sity higher in the dawn sector. The density appears higher near two-component Maxwellian, and (c) K. The distribution func- Earth and along the dawn and dusk flanks of the magnetotail, tion with the best X2 statistical significance is then used (or the presumably the LLBL. The density enhancement in the dawn spectrum is discarded if no fit is statistically valid). As the last flank appears stronger than that in the dusk flank. This density step in data selection, the pressure values are considered four enhancement is more prominent with decreasing activity. Durconsecutive spectra at a time, and high and low values are dis- ing quiet times, the density has a minimum near the midnight carded. meridian but the minimum disappears with increasing activity. Exact numerical comparisons with in situ studies are difficult because the data were taken from different years. Kistler et al. [1992, 1993] showed that variation in Pa affects magnetotail pressure, as is predicted from pressure balance. This implies that the temperature or/and the density has to increase as well. The average density and pressure in the present study appear higher than those in previous studies. Pa is higher in 1992 than in the years used in other previous studies. In contrast, the observed temperature agrees well with earlier work. These pressure, temperature, and density observations suggesthat an increase in density is mostly responsible for the increase in pressure in the plasma sheet during periods of high Pa. Increases in Pa reduce the size or volume of the magnetosphere [Roelof and Sibeck, 1993]. It is not clear whether the smaller magnetosphere alone is enough to account for the denand density can be presented as two-dimensional maps in the sity increase. Higher Pa may have resulted also from higher soneutral sheet equatorial plane. The results indicate that b2i or- lar wind density, which may imply higher flux entering the ganize these parameters very well (specifically, better than Kp magnetosphere. did). These maps confirm some of the previous results from in The asymmetries in the temperature and pressure maps situ data and in some cases reveal new features which previ- show qualitative agreement with predictions of the finite width ously were obscured because of averaging over large regions magnetotail convection model [Spence and Kivelson, 1993]. Aland/or all magnetic activity. For example, the dawn-dusk though the location and degree of the asymmetries of these paasymmetry, which previously has been observed with data av- rameters in their model results do not quite match those oberaged over all AEs and 15 x 5 x 2 RE 3 (in GSM X, Y, and Z, tained in the present paper, their model, nonetheless, demonrespectively) [Huang and Frank, 1994], has a clear dependence strates that these asymmetries can be explained, at least partly, on the magnetic activity and distance down the tail. by the westward gradient/curvature drift as the ions ExB con- It has previously been reported that the temperature is vect earthward. At large distances from the Earth, the ExB higher in the dusk sector than in the dawn sector at the resolu- earthward drift is dominant for the bulk of the ions, but in the tion mentioned above. The present high-resolution study reveals near-earth region the gradient/curvature drift is large relative to that asymmetry varies with location. The asymmetry is stronger ExB drift. Although the model was developed for quiet time,

15 WING AND NEWELL: CENTRAL PLASMA SHEET ION PROPERTIES 6799 the near-earth region asymmetries appear to persist for more disturbed times as well. The model also shows that the density enhancement in the near-earth region and dawn flank can be, at least partly, attributed to the colder LLBL ions from the dawn flank. The dusk LLBL ions contribute little to the near- nidnight CPS density because of their gradient/ curvature drift. Apart from the variabilities due to solar cycle, there is good agreement between the plasma properties inferred here and those obtained from in situ measurements. Since DMSP satel- lites have been continuously operational for over a solar cycle and are expected to continue into the future, the method developed in this study provides a new mechanism to monitor the pressure, temperature, and density of the magnetotail with unprecedented comprehensiveness. In essence, the magnetotail can be "imaged" as a single entity, rather than sparsely sampled. 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