JOURNAL OFGEOPHYSICAL RESEARCH, VOL. 98, NO. A1, PAGES 257-262, JANUARY 1, 1993 Ion Temperature Profilesin the Horns of the Plasma Sheet D. M. SUSZCYNSKY, J. T. GOSLING, AND M. F. THOMSEN Space Plasma Physics Group, Los Alamos National Laboratory, Los Alamos, NewMexico The plasma sheet horns are the low-altitude extensions of the plasma sheet that liepoleward ofthe plasmasphere and equatorward ofthe tail lobes. Within the horns, magnetic field lines ofincreasing geomagnetic latitudes map to increasing distances into the downtail plasma sheet. Plasma data from the fast plasma experiment on ISEE 2 have been analyzed for11 outbound crossings of the horns in the premidnight sector of the magnetosphere at typical altitudes of---2-4 R E.These crossings typically occurred on timescales of less than 1 hour, providing almost instantaneous snapshots of plasma gradients within the horns. Iontemperatures observed during these crossings generally decreased by a factor of 4-8 asmagnetic field lines of increasing geomagnetic latitude were traversed. If wemake the reasonable assumption thattheiontemperatures areconstant along the field lines within the plasma sheet, then this result implies that theiontemperatures in thedowntail plasma sheet also commonly decrease by this same factor over theradial range extending from theinner edge of the plasma sheet at the plasmapause boundary to the outer edge at a neutral linein the distant tail. 1. INTRODUCTION The plasma sheet (Figure 1) is a convective regionof the Earth'smagnetosphere that consists of hot (T i 1-10keV), moderately dense (n •- 0.1-1.0 cm -3) plasma on closed field lines. The plasmasheet stores energy from the solar wind and can regulate the precipitationof this energy into the ionosphere during magnetospheric sub- stoms [e.g., Rostokeret al., 1987]. Ion temperatures within the plasma sheet are related to the phasesof substorms [Akasofu et al., 1973; Lui et al., 1981;Huang and Frank, 1986; Baumjohannet al., 1989; Huang et al., 1989;Goertz and Smith, 1989; Baumjohann et al., 1991], are used to defineplasma sheet boundaries[Baumjohannet al., 1988], areimportant in determiningthe degree of adiabaticity in the plasma sheet [e.g., Huang et aI., 1989; Zhu, 1990; Goertz and Baumjohann, 1991], and can influence other types of convectiondynamics [e.g., Liu and Rostoker, 1991; Erick- son et al., 1991]. Plasmasheet ion temperatures are usually obtainedas profilesduring individual satellite trajectories through the downtail plasmasheetor as statistical profiles [Baumjohann et al., 1989] averaged over many plasma sheet traversals. Bothtypes of measurements must be interpreted with cau- tion. The measurement of ion temperature profiles during individual trajectories through the plasma sheet is compli- cated by the long time periods(often several hours) associ- atedwith the traversal of this region. During suchextended datacollection periods, geomagnetic activity and therefore plasma sheet properties can change substantially. Likewise, statistical measurements based on many traversals of the plasma sheet canpotentially exclude or average outimpor- tant plasma sheet properties [Goertz and Baumjohann, 1991]. We present here an indirect method to measure individual ion temperature profiles in the downtail plasma sheet on a nearly instantaneous basis by sampling the ion temperatures nearer the Earth in the horns of the plasma sheet, where the plasma sheet traversal can occur on a much shorter time Copyright 1993 by theAmerican Geophysical Union. Paper number 92JA01733. 0148-0227/93/92jA.01733502.00 scale. For the purposes of this paper, the "horns of the plasmasheet" wftl refer to the low-altitude extensions of the plasma sheet that lie poleward of the plasmasphere and equatorward of the tail lobes. The "downtail plasmasheet" will refer to all remaining regions of the plasmasheet. Within the horns, magnetic field lines of increasinggeomagnetic latitudes map to increasing distances in the downtaftplasma sheet(Figure 1). Therefore, if we make the assumption that the ion temperature is constant along field lines within the plasma sheet, then the ion temperature profile that is mea- suredin the horn region shouldbe representative of the ion temperature profile in the downtail plasma sheet region. This measurement technique is valuable in that it allows the ion temperature profile to be determined over a large region of the plasma sheet (several tens of Earth radii) on a nearly instantaneous basis (typically less than 1 hour). 2. OBSERVATIONS The ion temperatures were measured by the joint Los Alamos/Garching Fast PlasmaExperiment (FPE) aboard the ISEE 2 spacecraft [Bame et al., 1978]. The FPE consistsof three high-efficiency 90 ø spherical section electrostaticana- lyzers capable of producing two- and three-dimensional velocity distribution measurementsfor both ions (70 eV to 40 keV) and electrons (11 eV to 20 keV). Two-dimensional measurements of ions and electrons were made at 16 ener- giesand at 16 azimuthal look angles, integratedover _+55 øof elevationangierelative to the ecliptic in one spacecraft spin period (3 s). Ion temperatures were calculated from the secondvelocity moment of the ion distribution functions as collected during !1 outbound crossingsof the horns in the first half of 1980. The crossings typically occurred in the premidnightsector in local time at a geocentric distanceof 2-4 RE and lasted less than 1 hour. Table 1 summarizes the spacecraft locations at the plasma sheetentry and exit points for each of the 11 crossings. The locations are given in GSM (X, Y, Z) coordinates, along with the universal time (UT), local time (LT), magnetic latitude (MLAT), and radial dis- tance (R) in Earth radii. A typical outbound trajectory through the horn region is sketched in Figure 1. Plate 1 presentsa set of four color-codedenergy spectro- gramsas a function of UT, LT, R, and MLAT for a typical 257