The boundary layers as the primary transport regions of the Earth's magnetotail

Abstract

A comprehensive survey of ISEE and IMP LEPEDEA plasma measurements in the earth's magnetotail reveals that the magnetospheric boundary layer and the plasma sheet boundary layer are the primary transport regions there. These plasma measurements also distinguish various components of the plama sheet, including the central plasma sheet and plasma sheet boundary layer. A significant new result reported here is the existence of cold‐and hot‐plasma components that are spatially copresent within the central plasma sheet. Such plasma components cannot be explained merely by temporal variations in spectra involving the entire plasma sheet. Contributions to a low‐temperature component of the plasma sheet enter directly from the boundary layer located along the magnetotail flanks. Field‐aligned flows predominate within the plasma sheet boundary layer, which is almost always present and is located near the northern and southern border of the plasma sheet. The plasma sheet boundary layer comprises highly anisotropic ion distributions, including counterstreaming ion beams, that evolve into the hot, isotropic component of the plasma sheet. Tailward acceleration regions generate these ion beams with plasma input from the magnetospheric boundary layer. Antisunward flowing ion beams, at E/q < 1 kV and of ionospheric composition, are frequently observed in the plasma sheet boundary layer and in the tail lobes. These ion beams are likely accelerated at low altitude over the polar cap and especially along auroral field lines. An ionospheric component of the plasma sheet is supplied from this source via the plasma sheet boundary layer. On the basis of a survey of ∼2000 three‐dimensional velocity distribution functions sampled by the ISEE LEPEDEA in the magnetotail we find that interpretations based solely on moments parameters are inevitably inadequate for inferring plasma sources and acceleration processes. We find that earthward flowing ions or counterstreaming beams are often observed near expansive phase onset under conditions for which antisunward ion flow is predicted by the near‐earth neutral line model of magnetospheric substorms. Magnetotail plasma flows sampled out to 38 RE and reported as typical of onset and recovery of magnetospheric substorms can only be adequately understood by considering spatial and temporal variations of the boundary layers described above.

Mapping of the Precipitation Regions to the Plasma Sheet

“…Kovrazhkin et al (1987) and Zelenyi et al (1990) found within the Polar Diffuse Aurora] Zone specific VDIS-2 events (Velocity-Dispersed Ion Structures of the second type) from lowaltitude AUREOL-3 satellite in about 10% of passes. The VDIS-2 events were shown to be corresponding in width and velocity dispersion direction (energy increase with increasing latitude) to the projected to ionosphere velocity-dispersed ion beams of the Plasma Sheet Boundary Layer (PSBL) as measured in situ from ISEE orbits by Eastman et al (1985), , and Takahashi and Hones (1988). This inference was confirmed by Frank and Craven (1988) who traced a field line from the ISEE location inside the PSBL to the oval poleward boundary with a bright arc.…”

“…The spectacular discoveries of the Plasma Sheet Boundary Layer (PSBL) with high energy ion beams by Williams (1981) and Eastman et al (1984Eastman et al ( , 1985 revived the long sought hope to locate the Distant Neutral Line, DNL, in the tail. Many researches considered the role of the DNL in generation of discrete auroral forms of the oval starting from the pioneering work by Akasofu and Chapman (1961).…”

In situ Polar observation of transverse cold‐ion acceleration: Evidence that electric field generation is a hot‐ion finite gyroradii effect

“…These appear to be the same kind of flows that have been equated with a plasma sheet high-latitude ''boundary layer'' in the literature, the PSBL [e.g., Forbes et al, 1981;Eastman et al, 1985;Bosqued et al, 1993;Onsager and Mukai, 1995 and references therein]. The TIMAS observations, which were made at R(SM) $ 4-7 R E, revealed that when the 1-to 33-keV protons are distinctly dispersed in energy within the transition region between the plasma sheet and the (northern) lobe, dispersion has a downward slope as function of time, whether the satellite is moving poleward or equatorward, implying that those proton flows are in fact transient bursts, several of which usually appear in rapid succession over the course of tens of minutes at Polar and are accompanied by hot ($keV) electrons.…”

Multiple harmonic ULF waves in the plasma sheet boundary layer: Instability analysis

“…It is the primary region of mass, energy, and momentum transport in the magnetotail during quiet times [Eastman et al, 1985], although during more active periods bursty bulk flow events in the central plasma sheet are thought to dominate [Baumjohann et al, 1990;Angelopoulos et al, 1992Angelopoulos et al, , 1994. Observations of the PSBL from a single spacecraft are often of short duration because of the flapping and kink-like motions of the magnetotail that move it in the ±Z GSE and ±Y GSE directions, respectively [Grigorenko et al, 2007].…”