Dynamics of low‐energy electrons (≥17 keV) and ions (≥80 keV) in the vicinity of the low‐latitude, duskside magnetopause: Helios 1 and 2 observations

Abstract

The space probes Helios 1 and 2 left the earth's magnetosphere to enter heliocentric orbits on the duskside, at nearly zero latitude, and in the dusk‐noon quadrant. During this period, electron and ion observations with Ee ≥ 17 keV and Ei ≥ 80 keV and with high time (13.5 s), energy (16 energy channels), and directional (16 sectors) resolutions were performed by the low‐energetic‐particle spectrometer of the Max‐Planck‐Institut für Aeronomie and magnetic field measurements by the flux‐gate magnetometer of the Institut für Geophysik und Meteorologie. At the magnetopause these particle and field observations indicate the following: (1) There is a ≥17‐keV electron and ≥80‐keV ion layer at the magnetopause. (2) In this layer the ion spectra are bipower law spectra with γi ≈ 7 for Ei < 200 keV and γi ≈ 3 for Ei ≥ 200 keV. For electrons of <70 keV we find power law spectra with γe ≈ 5. (3) The actually observed energy flows (ergs/s) associated with the ions are 1.1 × 1018−2.1 × 1019 (Ei ≥80 keV, Helios 1) and 1.2 × 1017 (Ei ≥ 87 keV, Helios 2), and those associated with the electrons 8 × 1015 (Ee ≥17 keV, Helios 1) and 1.8 × 1016 (Ee ≥ 20 keV, Helios 2), if a homogeneous layer over the entire magnetopause is assumed. An energy flow of 1018‐1019 ergs/s is required for a dayside reconnection in the earth's magnetosphere (Heikkila, 1975). (4) The number of ions (electrons) per second carrying 1018 ergs/s is 3(1) orders of magnitude smaller than the number of solar wind particles arriving at the magnetopause, as estimated by Heikkila (1975). Thus the existence of an ion magnetopause layer in the keV range of 1018 ergs/s cannot be proven or disproven by plasma observations in the hundreds of electron volts range alone. (5) Magnetic field and directional observations of ≥17‐keV electrons indicate that the magnetopause at least at these latitudes is near an X line or X‐type neutral line. (6) High time resolved directional electron and ion measurements indicate a two‐component structure of the layer of closed and open field lines, as proposed by Crooker (1977). On the open field lines the flows of both the electrons and the ions are highly anisotropic in the antisolar direction. (7) There are very distinct, highly anisotropic bursts of keV electrons and ions on both sides of the magnetopause layer: Within the outer magnetospheric region, i.e., the region directly inside the magnetopause, particles propagate predominantly toward the sun; in the magnetosheath they flow without exception into the antisolar direction. The spectra of the electron and ion components are much flatter for the magnetosheath bursts than for the magnetopause layer. This might reflect different acceleration mechanisms for these two sets of particles. For the ≥80‐keV ions we find more or less identical spectra and associated energy flows for both the outer magnetospheric burst and the magnetopause particles. This might reflect identical acceleration mechanisms, e.g., tailward and dayside magnetic merging.