Plasma in the Earth's polar magnetosphere

Abstract

First observations of the plasmas in the dayside polar magnetosphere were obtained with the earth-satellite Imp 5 during July-August 1969. Several of the more important observational results are the following. (1) The polar neutral ‘points’ that appear at the high-latitude magnetopause in mathematical models for the shape of the geomagnetic cavity formed by the interaction of the solar wind with the geomagnetic field are observationally ‘bands’ with width ∼1 RE across the dayside high-latitude magnetopause (one band in the northern hemisphere and presumably a second in the southern hemisphere). (2) These two bands, or regions of the magnetopause through which the magnetosheath plasma has direct access to the magnetosphere, and the corresponding extension of these bands from magnetopause to auroral altitudes have been designated herein as the ‘polar cusps.’ (3) At all other positions of the dayside magnetopause, the magnetopause appears to be an effective barrier against the direct entry of magnetosheath plasma. (4) During periods of relative magnetic quiescence the intersection of the dayside polar cusp with the auroral zone is positioned at invariant latitude Λ=79° (±1°) and its latitudinal width is 20 to 400 km projected onto the auroral zone. (5) During periods of the relative magnetic disturbance the position of the polar cusp moves equatorward by several degrees in invariant latitude without a large increase in its latitudinal width, i.e., by factors ≲2. (6) The high-latitude termination of energetic trapped electron (E>45 kev) intensities in the high-latitude dayside outer radiation zone occurs coincident with the polar cusp, albeit these intensities are small and of irregular profile with radial distance in this region. (7) No measurable intensities of energetic electrons (E>40 kev), magnetosheath protons and electrons, and ring-current protons were observed at latitudes above the polar cusp, i.e., in the polar cap region. (8) The proton and electron differential energy spectrums as viewed in the solar direction in the distant polar cusp (within several earth radii of the magnetopause) are identical to those observed within the magnetosheath to within observational accuracy. (9) The bulk velocity of protons in the distant polar cusp as deduced from the angular distributions appears to be lower than that of the magnetosheath plasma near the magnetopause by factors ∼2 or 3. (10) In the midaltitude polar cusp at ∼4 to 5 RE geocentric radial distances, the proton spectrum differs from that at the magnetosheath in that protons with energies ≲500 ev are severely less than those observed in the magnetosheath. (11) The proton spectrums in the midaltitude polar cusp are similar to those in the distant plasma sheet with the exception that the proton number densities in the polar cusp are typically larger by factors ∼20 to 200. (12) The angular distributions of proton intensities in the midaltitude polar cusp are strongly peaked along the local magnetic field (i.e., down into the auroral zone); the dimensions of the atmospheric loss cone at these altitudes appear to be insufficiently large to account for the observed anisotropy; and (13) the magnetosheath plasma in the midaltitude polar cusp is observed to be separated into two thin sheets, one of magnetosheath proton intensities and the other populated with magnetosheath electrons; these sheets are immediately adjacent to each other with the electron sheet equatorward of the proton sheet; their latitudinal widths as projected into the auroral zone are roughly equal, ∼10 to 200 km. These observations, along with recent measurements from other earth satellites, have been interpreted in terms of a proposed magnetospheric model with several new features, among which are the following. (1) Plasma sheet protons gain access to the magnetospheric field lines via the dayside polar cusps. (2) All magnetic field lines threading the distant plasma sheet beyond ∼20 or 30 RE were convected from the polar cusps. (3) Magnetic field lines in the polar cap region of the magnetotail do not merge or pass through the plasma sheet.