Measurements from the Lepedea plasma instruments and the flux gate magnetometers on ISEE 1 and 2 are used to examine the nature of the hydromagnetic waves associated with the various classes of ions backstreaming from the earth's bow shock. The reflected ions, which are confined to a narrow energy and angular range, are accompanied by small amplitude (≲½γ peak to peak) left‐handed waves at frequencies close to 1 Hz in the spacecraft frame. Diffuse backstreaming particles with a broad energy spectrum are associated with low frequency (∼ 30‐s period) large amplitude (∼5γ peak to peak) waves. Intermediate particles are associated with a mixture of these two wave types. Often the waves associated with the diffuse beams steepen as if they were mini shocks. The leading edge (trailing edge in the spacecraft frame) frequently appears to break up into a whistler mode wave packet. These discrete wave packets are right‐hand polarized and have frequencies from below the proton gyrofrequency to well above it in the plasma frame and are blown back towards the earth by the solar wind.

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.

Observations with the Los Alamos Scientific Laboratory (LASL) plasma probe and the Goddard Space Flight Center (GSFC) magnetometer on the IMP 6 satellite show that the magnetospheric boundary layer, first identified along the flanks of the magnetosphere, is also present at the magnetosphere's sunward surface. The magnetic field lines in this sunward sector of the boundary layer are closed, and the plasma flow has a component transverse to the field. These observations suggest that the boundary layer is a site of continual transfer of plasma, momentum and energy from the magnetosheath to the magnetosphere. These transfer processes supply plasma and magnetic field to the magnetotail. Also, they produce, indirectly, the dawn‐to‐dusk electric field across the polar cap, the field‐aligned currents that border the dayside polar cap, and the soft particle fluxes that characterize the cleft precipitation, including recently reported dawn‐dusk asymmetries of these fluxes. Magnetosheath plasma directly enters the outer few hundred to few thousand kilometers of the magnetosphere's surface to form the boundary layer. There it is enabled to flow across the magnetic field (and approximately parallel to the magnetosphere's surface) by becoming electrically polarized. Leakage of the polarization charge along magnetic field lines to the earth produces the dayside high latitude effects mentioned above. The polarizing current flowing across the boundary layer interacts with the magnetic field to oppose the boundary layer plasma flow, taking up its momentum. In this way the magnetic field lines are pulled downstream. The process described here is independent of the interplanetary magnetic field (IMF) and thus may constitute the principal transfer mechanism during prolonged periods of northward IMF when the magnetosphere is very quiet. It is not clear how the effects of southward IMF are superposed on this process.

We present a study of the low‐latitude boundary layer (LLBL) using ISEE 1 energetic particle, plasma, and magnetic field data obtained during numerous traversals of the LLBL that occurred on 66 ISEE 1 passes through the magnetospheric flank LLBL region. We use energetic particle distributions to determine dawn and dusk LLBL behavior and topology for varying orientations of the magnetosheath and/or interplanetary magnetic field (M/IMF), for different local times, and for changing levels of geomagnetic activity (Kp). This study corroborates and extends the earlier work of Williams et al. (1985) who presented a detailed study of two (dusk and dawn) ISEE 1 passes through the LLBL region for the case of northward M/IMF. We find that the dawn and dusk LLBL are on closed geomagnetic field lines for northward M/IMF but are on a combination of closed and open field lines for a southward M/IMF. The energetic particle distributions show that cases of reverse‐draped field lines in the LLBL are consistent with an open field line topology. In addition, we find that the LLBL is thicker (thinner) for northward (southward) M/IMF and becomes thicker with increasing distance from the subsolar point. LLBL electric fields nominally are in the few (3–5) millivolts per meter range and display an apparent maximum value of ∼10 mV/m. These electric fields capture magnetospherically drifting particles as they approach the LLBL and propel them tailward. In this way, the plasma sheet is the dominant source of energetic (≳10 keV) particles in the LLBL while the magnetosheath appears to be the dominant source for lower‐energy (≲10 keV) LLBL particles.

The plasma sheet boundary layer is a temporally variable transition region located between the magnetotail lobes and the central plasma sheet. We have made a survey of these regions by using particle spectra and three‐dimensional velocity‐space distributions sampled by the ISEE 1 LEPEDEA. Ion composition measurements obtained by the Lockheed ion mass spectrometers indicate that ionospheric ions play a crucial role in magnetotail dynamics. Eleven crossings from the lobes to the central plasma sheet taken at various local times and levels of geomagnetic activity are analyzed in detail. The average ratios of He+/H+, He++/H+, and O+/H+ are not significantly different between the plasma sheet boundary layer and central plasma sheet. Densities and temperatures intermediate between the central plasma sheet and lobes are observed in the plasma sheet boundary layer although bulk flow speeds there are typically enhanced. Counter‐streaming ion beams are often observed in the plasma sheet boundary layer at energies of ∼1 keV/q to >45 keV/q. Intense antisunward‐flowing beams of ionospheric origin at E/q of <1 kV are often seen in the tail lobes, the plasma sheet boundary layer, and, infrequently, in the central plasma sheet. Such beams are not commonly observed in the central plasma sheet, which is characterized by hotter and more isotropic ion and electron distributions. Our samples of ion distributions in the plasma sheet boundary layer frequently show an evolution of distribution functions from highly anisotropic single beams or counter‐streaming beams toward the more isotropic distributions typical of the hot component of the central plasma sheet. Provided that the acceleration process for these beams can be identified, we can then account for the transport and injection of hot plasma into the central plasma sheet. We conclude that the plasma sheet boundary layer is a primary transport region of the magnetotail.