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

Abstract

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: (l) 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 R•, 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 -• and was always less than the concurrent flow speed in the nearby magnetosheath. (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 XOSM. (5) A narrow region of low density and/or low flow speed plasma, i.e., a 'gap,' 0.1-0.2 R• 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• < Tl. (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. now appears that this layer forms almost a complete cover of the magnetosphere with the exception of the day side region between the cusps and at low latitudes in the magnetotail adjacent to the plasma sheet, it will be referred to herein as the plasma mantle. INSTRUMENTATION The observations reported here were obtained with the MPI plasma analyzer flown on the Heos 2 spacecraft. Heos 2 was launched in January 1972 into a highly eccentric polar orbit with initial apogee and perigee heights of 240,194 km (38.7-R• geocentric distance) and 409 km, respectively. The inclination of the orbital plane was approximately 90 ø , and the ...

Initial observations of fine plasma structures at the flank magnetopause with the complex plasma analyzer SCA-1 onboard the Interball Tail Probe

“…The high-latitude portion of the boundary layer (plasma mantle) was first observed by Rosenbauer et al (1975) and studied in more detail by Gosling et al (1984) and Siscoe et al (1994). Magnetosheath plasma was found to enter directly into the tail lobe (Gosling et al, 1984;Mozer et al, 1994), in agreement with the slow-mode MHD expansion fan model of the plasma mantle (Siscoe and Sanchez, 1987).…”

Ion composition aspects of magnetotail plasma flows

“…That region, as is popularly known, bounds the tail lobes on the magnetopause side, does have persistent tailward flow [e.g., Rosenbauer et al, 1975], and can be reasonably assumed to contain solar origin plasma at all times. However, it is not known to border the plasma sheet at R < 23 R e to the extent implied by the tailward He + + drift samplings in the left panels of Figure 7.…”

Probing the boundary between antiparallel and component reconnection during southward interplanetary magnetic field conditions

“…For southward IMF conditions, newly opened field lines convect poleward under the joint action of magnetic tension and shocked solar wind flow, causing a time dispersion whereby particles of decreasing energy arrive at successively higher latitudes [e.g., Rosenbauer et al, 1975;Shelley et al, 1976]. This kinematic effect, which is the basis for calculating the distance to the reconnection site, gives rise to a distinctive ion energy-latitude dispersion shown in Figure 1 [e.g., Reiff et al, 1977;Smith and Lockwood, 1996].…”