Measurements of charged particles in the plasma sheet by the low energy proton and electron differential energy analyzer (LEPEDEA) and medium energy particle instrument (MEPI) on ISEE 1 are combined to obtain ion and electron differential energy spectra for use in studying eight plasma sheet temperature transitions, periods of low plasma bulk velocity typically ∼1 hour in length during which the plasma thermal energy either increases or decreases steadily. Over the entire kinetic energy range sampled (50 eV/e ≲ E ≲ 1 MeV), the plasma and energetic ion and electron populations respond collectively as a single unified particle population during these temperature transitions. In order to test the hypothesis that the energy spectra of plasma sheet ions and electrons can be represented by a single functional form, the observed particle energy spectra have been visually compared to three model distribution functions: the Maxwellian (, where ET is the thermal energy), the kappa (ƒ ∼ [1 + E/κET]−κ −1, where κ is a constant), and the velocity exponential (ƒ ∼ e−( E/ε)1/2, where ε is constant). The kappa and velocity exponential distributions both provide reasonable fits above ∼200 eV, with the kappa distribution being more successful at the highest energies but less successful at the lowest energies. The Maxwellian does not provide an adequate fit for the overall distributions observed in the temperature transitions. At high energies (E ≫ κET) the observed spectra are more often similar to the kappa than to the velocity exponential; that is, a roughly power law form (E−κ) is in evidence. Although the value of the index varies from event to event, the particle distributions maintain their overall shape throughout a transition, during which the spectral index at high energies stays roughly constant. This could indicate either that the relaxation time of the plasma is short with respect to the time scale of the temperature transitions or that the spatial regions being sampled were all maintaining a stationary state plasma population, or both. Both temporal and spatial effects are evident in the temperature transitions studied. An indication of temporal dependence during the transitions is that on the average, ET increases with geomagnetic activity as indicated by the AE index at low to moderate levels (∼30 to 600 nT). However, a spatial effect is evident as well, since temperature increases (decreases) occurred as ISEE 1 was traveling toward (away from) the geocentric solar magnetospheric equator.

We have determined the spectral characteristics of central plasma sheet ions and electrons observed during 71 hours when geomagnetic activity was at moderate to high levels (AE ≥ 100 nT). Particle data from the low‐energy proton and electron differential energy analyzer and the medium energy particle instrument on ISEE 1 are combined to obtain differential energy spectra (measured in units of particles/cm² s sr keV) in the kinetic energy range ∼30 eV/e to ∼1 MeV at geocentric radial distances >12 Re. Nearly isotropic central plasma sheet total ion and electron populations were chosen for analysis and were measured to be continuous particle distributions from our lowest to highest energies. During these high AE periods the >24 keV particle fluxes and the temperature of the entire particle distribution kT are significantly higher than during low AE periods (AE < 100 nT). The temperatures kTi and kTe are highly correlated during both quiet and disturbed periods. The active period spectral shape appears softer for ions and somewhat harder for electrons than during quiet periods. We find that the observed active period spectrum typically is complex and cannot be represented in general by a single functional form, as during quiet periods when it can be represented by the kappa distribution function. Although a power‐law shape is observed at higher energies, ion and electron spectral shapes deviate from a strictly kappalike form in different ways. In a limited energy range near the knee of the ion spectra (the knee is that portion of the spectrum at energies E ≳ Eo where the flux starts to decrease swiftly with increasing energy), the spectral shape can often be fit with a Maxwellian form, thus rolling over faster than the typical quiet time spectrum. At higher energies this shape merges into a harder nonthermal power‐law tail. Electron spectra also display this spectral characteristic, although at a lower occurrence frequency than for ions. The electron spectra are predominantly kappalike at energies near and above the knee. At energies below the knee, both ions and electrons often have an excess of flux with respect to the functional form that best fits the shape for energies at or above the knee, be it a kappa distribution or a Maxwellian distribution; the electron flux excess is significantly greater than the ion flux excess. We conclude that both ions and electrons participate in at least two separate acceleration mechanisms as geomagnetic activity evolves from low AE to high AE values. We suggest that both spectrum‐preserving and spectrum‐altering heating processes (possibly involving nonlocal betatron acceleration and crosstail current sheet acceleration, respectively) participate in overall particle energization during geomagnetic active periods. Observations are compared to model predictions.

We analyze 127 one‐hour average samples of central plasma sheet ions and electrons in order to determine spectral characteristics of these magnetotail particle populations during periods of low geomagnetic activity (AE<100 nT). Particle data from the low energy proton and electron differential energy analyzer (LEPEDEA) and medium energy particle instrument (MEPI) on ISEE 1 were combined to obtain differential energy spectra in the plasma sheet at geocentric radial distances R>12 RE. We find that, for even the longest periods sampled, the nearly isotropic central plasma sheet total ion and electron populations were measured to be continuous particle distributions from our lowest energy of tens of eV/e to a few hundred keV. The kappa distribution function (f ∼ [1 + E/κEo]−κ−1, where Eo, the energy of the peak differential number flux (measured in particles/cm² s sr keV), is related to the temperature through κ, a constant) most often reproduces the observed differential energy spectra. Spectra dominated by a single kappa functional form are observed during 83 (99) hours for ions (electrons). Spectra which are not dominated by a single kappa functional form can usually be closely approximated by superposed kappa functional forms. For both ions and electrons κ is typically in the range 4–8, with a most probable value between 5 and 6, so that the spectral shape is distinctly non‐Maxwellian. Eoi and Eoe are highly correlated, whereas κi and κe are not correlated; κi is roughly proportional to Eoi½, whereas κe is not correlated with Eoe. We statistically investigate the importance of flux and energy contributions from extramagnetospheric sources by separately analyzing intervals when simultaneously measured interplanetary particle fluxes are either enhanced or at low levels. A linear superposition of plasma sheet fluxes and interplanetary fluxes that have entered the magnetosphere is observed. The presence of interplanetary particles does not affect the average values of plasma sheet Eo or κ. We conclude that for AE<100 nT the nonthermal shape of plasma sheet particle distributions results from ongoing magnetospheric processes which are probably independent of geomagnetic activity as measured by AE.

[1] We have obtained a state-of-the-art picture of substorm-associated evolution of the near-Earth magnetotail and the inner magnetosphere for understanding the substorm triggering mechanism. We performed superposed epoch analysis of Geotail, Polar, and GOES data with 2-min resolution, utilizing a total of 3787 substorms for each of which auroral breakup was determined from Polar UVI or IMAGE FUV auroral imager data. The decrease of the north-south magnetic field associated with plasmoids and the initial total pressure decrease suggest that the magnetic reconnection first occurs in the premidnight tail, on average, at X $ À16 to À20 R E at least 2 min before auroral onset. The magnetic reconnection site is located near the tailward edge of a region of considerably taillike magnetic field lines and intense cross-tail current, which extends from X $ À5 to À20 R E in the premidnight sector. Then the plasmoid substantially evolves tailward of X $ À20 R E immediately after onset. Almost simultaneously with the magnetic reconnection, the dipolarization begins first at X $ À7 to À10 R E 2 min before onset. The dipolarization region then expands tailward as well as in the dawn-dusk directions and earthward. We find that the total pressure generally enhances in association with the dipolarization, with the contribution of high-energy particles. Also, energy release is more significant between the regions of the magnetic reconnection and the initial dipolarization. The present results will be helpful as a reference guide to developing the overall picture of magnetotail evolution and studying the causal relationship between the magnetic reconnection and the dipolarization as well as detailed mechanisms of each of the two processes on the basis of multispacecraft observations.

The Energetic Particles and Ion Composition (EPIC) instrument, flown onboard the GEOTAIL satellite, is designed to measure the characteristics ofparticle populations important to understanding the make-up and dynamics of the earth's geomagnetic tail. To do this, EPIC, a joint endeavor between the Technical University of Braunschweig (TUB), the University of Maryland (UM), and The Johns Hopkins University Applied Physics Laboratory (JHU/APL), is made up of five subassemblies: the SupraThermal Ion Composition Spectrometer (STICS) sensor, the STICS analog electronics, the Ion Composition System (ICS) sensor, the ICS analog electronics, and the Data Processing Unit (DPU). The STICS sensor provides -4ir angular coverage, composition and spectral observations, with charge state determination for all ions from 30 keV to 230 keV/e, and mass per charge measurements >_7.5 keV/e. The ICS sensor provides flux, composition, spectra, and angular distributions over two polar angles of the elemental species protons through iron from >_50 keV to 3 MeV along with angular distributions in one plane of electron fluxes >32 keV and >1 10 keV. The DPU provides the capability of numerous operating modes from which a small number will be selected to optimize data collection throughout the many phases of the GEOTAIL mission. To date the EPIC instrument performance has been very successful. In this paper we describe the instrument, its operation, and show some of our early results.

[1] We statistically examine changes in the composition of two different ion species, proton and oxygen ions, in the near-Earth plasma sheet (X = À16 R E $ À6 R E ) during substorm-associated dipolarization. We use 10 years of energetic (9-212 keV/e) ion data obtained by the suprathermal ion composition spectrometer (STICS) sensor of the energetic particles and ion composition (EPIC) instrument on board the Geotail spacecraft. The results are as follows: (1) Although the percentage increase in the energy density of O + ions before and after a dipolarization exceeds that of H + ions in the low-energy range (9-36 keV/e), this property is not evident in the high-energy range (56-212 keV/e); (2) the energy spectrum of H + and that of O + become harder after dipolarization in almost all events; and (3) in some events the energy spectrum of O + becomes harder than that of H + as reported by previous studies, and, importantly, in other events, the spectrum of H + becomes harder than that of O + . In order to investigate what mechanism causes these observational results, we focus on magnetic field fluctuations during dipolarization. It is found that the increase of the spectrum slope is positively correlated with the power of waves whose frequencies are close to the gyrofrequency of H + or O + , respectively (the correlation coefficient is 0.48 for H + and 0.68 for O + ). In conclusion, ions are nonadiabatically accelerated by the electric field induced by the magnetic field fluctuations whose frequencies are close to their gyrofrequencies.

[1] We studied dynamics of O + ions during the superstorm that occurred on 29-31 October 2003, using energetic (9-210 keV/e) ion flux data obtained by the energetic particle and ion composition (EPIC) instrument on board the Geotail satellite and neutral atom data in the energy range of 10 eV to a few keV acquired by the low-energy neutral atom (LENA) imager on board the Imager for Magnetopause-to-Aurora Global Exploration (IMAGE) satellite. Since the low-energy neutral atoms are created from the outflowing ionospheric ions by the charge exchange process, we could examine variations of ionospheric ion outflow with the IMAGE/LENA data. In the near-Earth plasma sheet of X GSM $ À6 R E to À8.5 R E , we found that the H + energy density showed no distinctive differences between the superstorm and quiet intervals (1-10 keV cm À3 ), while the O + energy density increased from 0.05-3 keV cm À3 during the quiet intervals to $100 keV cm À3 during the superstorm. The O + /H + energy density ratio reached 10-20 near the storm maximum, which is the largest ratio in the near-Earth plasma sheet ever observed by Geotail, indicating more than 90% of O + in the total energy density. We argued that such extreme increase of the O + /H + energy density ratio during the October 2003 superstorm was due to mass-dependent acceleration of ions by storm-time substorms as well as an additional supply of O + ions from the ionosphere to the plasma sheet. We compared the ion composition between the ring current and the near-Earth plasma sheet reported by previous studies and found that they are rather similar. On the basis of the similarity, we estimated that the ring current had

[1] The present study statistically examines the characteristics of energetic ions in the plasma sheet using the Geotail/Energetic Particle and Ion Composition data. An emphasis is placed on the O + ions, and the characteristics of the H + ions are used as references. The following is a summary of the results. (1) (5) The O + -to-H + ratios of number and energy densities increase toward Earth during all solar phases, but most clearly during solar maximum. These results suggest that the solar illumination enhances the ionospheric outflow more effectively with increasing geomagnetic activity and that a significant portion of the O + ions is transported directly from the ionosphere to the near-Earth region rather than through the distant tail.