Magnetosheath Research Articles

Energetic ions in the magnetosheath: Statistical study of spectra and anistropy

Prognoz-10 energetic ion measurements (E > 15 keV) in the magnetosheath (MSH) covered wide range of longitudes and low as well as medium latitudes. From all available 4 energy channel measurements of DOK-1 in MSH the energy spectra in 15–950 keV is shown to be harder close to magnetopause (MP) than deep in MSH. The streaming of ions dawnward and duskward close to MP is not showing any clear pattern in (zy) GSE plane. This is consistent with suggestion of patchy structure of merging at MP and leakage of magnetospheric particles to MSH.

INTERBALL magnetotail boundary case studies

We present two examples of INTERBALL-1 data near both the high and low-latitude tail magnetopause (MP) under disturbed conditions. For the high-latitude case, MAGION-4 data determine the scales of the MP current sheets which are in the order of 100–500 km for the main ones, 50–200 km for Flux Transfer Events (FTEs) and a few km for the fine structures and ULF turbulence. The MP speed was 15–30 km/s. The energetic protons in the magnetosheath (MSH) provide evidence of reconnection upstream of the spacecraft (S/C). The tailward flows grow for the northward MSH magnetic field when the reconnection site is believed to be shifted tailward of the cusp. The inner boundary layer (BL) after the disturbance consists of tailward and earthward flowing plasma of MSH origin and cold mantle plasma flowing tailward The earthward flow is evidence of reconnection tailward of the S/C, which is regarded as a specific feature of the disturbed conditions. Local production of a plasma-sheet-like plasma at high latitudes is argued based on the inner BL plasma characteristics. The following features are observed in both cases: (a) FTEs for both northward and southward MSH fields; (b) waves in the current sheet vicinities over ten mV/m and 15 nT peak-to-peak; (c) electron fluxes with scales down to a few km with extra heating especially parallel to the magnetic field; (d) outer turbulent boundary layers with a deflected magnetic field; (e) ions with time-energy dispersion-like features and deflected ion fluxes. In the downstream dawn region at the transition between the low-latitude boundary layer and the plasma sheet (LLBL/PS), multiple MP encounters are observed. In the LLBL parallel electron intensifications correlate with ULF magnetic fluctuations.

The subsolar magnetosheath and magnetopause for high solar wind ram pressure: WIND observations

On a rapid inward pass through the subsolar magnetosheath (MSH) and magnetopause (MP), the WIND spacecraft initially encountered a moderately‐compressed low‐magnetic shear MP (at a radial distance of 8.6 RE), followed by multiple crossings of a high‐shear MP (at 8.2 RE). The large shear resulted from a southward turning of the external MSH field. Strong magnetic field pile‐up, a plasma depletion layer (PDL), and plasma flow acceleration and rotation to become more perpendicular to the local magnetic field were observed in the MSH on approach to the low‐shear MP. At the high‐shear MP, magnetic reconnection flows were detected, and there are some indications that plasma depletion effects were weak or absent in the adjacent MSH. We attribute the changes in the MP and MSH properties to the sudden rotation of the MSH field direction. In essence, the structure of the MP regions under the unusually high solar wind ram pressure condition in this case does not seem to be qualitatively different from that observed under more typical (less compressed) conditions. Also similar to previous observations, the mirror mode is marginally unstable in the MSH proper, but is stable in the PDL. In this region, the proton temperature anisotropy is inversely correlated with βp∥. Finally, the electron distributions are observed to be anisotropic (Te⟂/Te∥ ∼1.3) throughout the entire MSH.

Wave form analysis of the continuum radiation observed by GEOTAIL

The Plasma Wave Instrument (PWI) onboard the GEOTAIL spacecraft has frequently observed Continuum Radiation (CR) throughout the geomagnetic tail region, including the Magnetosheath (MS), the Lobe, the Plasma Sheet Boundary Layer (PSBL), and the Plasma Sheet (PS). In addition to the usual Escaping CR (from 10 kHz to 30 kHz) and the Trapped CR (from about 5 kHz to 10 kHz), CR with the frequency structure extending down to 1 kHz (the local plasma frequency) has often been observed in the Lobe region. Waveforms of the electric field in the frequency range less than 4 kHz, which is acquired by Wave Form Capture in the PWI, are used to analyze in detail the frequency‐time structure of such low‐frequency CR near the lower cutoff. Two distinct cutoff frequencies modulated by the antenna spinning indicates that the CR in the Lobe region propagates both in the O mode and in the X mode almost perpendicular to the earthward geomagnetic field line. The CR in the Lobe region, especially in the vicinity of the PSBL, is sometimes accompanied by intense electrostatic electron‐cyclotron‐harmonic (ECH) (n + 1/2) waves. These suggest that such a low‐frequency CR in the distant tail region is most likely to be generated from the ECH waves near the PSBL, and trapped within the Lobe between the PSBL and the MS.

Observations of magnetohydrodynamic modes in the Earth's magnetosheath at 0600 LT

We have performed an auto and cross spectral analysis of the electron density and magnetic field fluctuations recorded aboard ISEE 1 and 2 in the terrestrial magnetosheath far from the subsolar point. The data were obtained near 0600 local time and roughly equidistant from the magnetopause and the bow shock. The analysis shows the characteristic signatures of mirror type modes when the mean magnetic field is draping the magnetopause, and of Alfvén type wave modes at other times.

ELF plasma waves associated with plasma jets near the Earth magnetopause as observed by Prognoz-8

In this paper we present Prognoz-8 observations of plasma and low-frequency (2-105 Hz) waves near the high latitude nightside magnetopause. Dense magnetosheath (MS)-like plasma clouds apparently detached from the magnetopause were often encountered by Prognoz-8 in this region inside the magnetosphere. It is found that such MS-like plasma clouds are always associated with strong wave activity at frequencies Ωi < ω ≲ ωLH, where Ωi is the proton cyclotron frequency, ωLH is the lower hybrid (LH) frequency. We examined in detail 5 MS-like plasma clouds observed during 2 inbound satellite passes through the magnetopause and high latitude tail. In one case the bulk velocity in the apparently detached MS plasma cloud is higher than in the nearby MS. More frequently, high energy non-maxwellian tails or high energy cold ion beams are observed in the cloud or just outside it. Also, we have found that some characteristics of plasma, magnetic field and plasma waves in the reported phenomena are similar to those in the flux transfer events (FTE's) observed on the dayside. Simultaneous observations of FTE's with signatures of plasma acceleration (most probably being a result of magnetic field line reconnection) and intense LH waves is interpreted in the frame of theory predicting that the lower hybrid drift instability is the source of anomalous resistivity in reconnection.

Heos 2 plasma observations in the distant polar magnetosphere: The plasma mantle

Comprehensive plasma observations carried out on board the Heos 2 satellite have provided the first systematic description of plasmas in the distant polar magnetosphere. These observations have revealed the presence of a persistent layer of tailward-flowing magnetosheathlike plasma inside of and adjacent to the magnetopause. This region has been termed the ‘plasma mantle.’ The mantle has been found to extend over the entire surface of the magnetosphere tailward of the polar cusp and northward of the plasma sheet. Vela observations of a ‘magnetotail boundary layer’ obtained in the vicinity of the plasma sheet by Hones and coworkers refer to the same phenomenon. The salient features of the plasma mantle as provided by Heos measurements from February to December 1972 can be summarized as follows: (1) The mantle was found to be present in over 70% of the passes through the polar magnetosphere in the region described above. (2) Its thickness varies greatly, ranging up to ≳ 4 RE, and does not appear to depend significantly on position or the state of the magnetosphere as measured by Kp. (3) A tailward-directed bulk flow parallel to the local terrestrial magnetic field was nearly always distinctly measurable. It was found to lie usually between 100 and 200 km s−1 and was always less than the concurrent flow speed in the nearby magneto sheath. (4) The flow speed in the mantle is positively correlated with the flow speeds in the magnetosheath and solar wind but depends only very weakly, if at all, on distance from the polar cusp, i.e., on XGSM. (5) A narrow region of low density and/or low flow speed plasma, i.e., a ‘gap,’ 0.1–0.2 RE thick, is frequently observed between the plasma mantle and the magnetopause. (6) The mantle protons are normally significantly cooler along B than perpendicular, i.e., T∥ < T⊥. (7) The proton density, temperature, and bulk speed all tend to decrease gradually with depth inside the magnetopause, but this trend can at times be obscured by fluctuations and magnetopause motions. At the inner edge of the mantle the proton distribution is often very narrow in both energy and angle, i.e., relatively cold, before it finally disappears below the 100-eV threshold of the instrument. It is concluded, after an appraisal of several possible mechanisms, that the most probable cause for the formation of the mantle is the day side merging of terrestrial and interplanetary field lines.

Observation of solar-wind α-particles in the magnetosheath

Observation of solar-wind α-particles in the magnetosheath