Magnetopause modeling: Flux transfer events and magnetosheath quasi‐trapped distributions

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Abstract
From ISEE 1 magnetopause crossings on November 10, 1977, three‐dimensional distribution functions for energetic ions (24–120 keV) are studied in the magnetosphere, through the magnetopause, and in the magnetosheath. The particle distributions are particularly examined at and near the times that Russell and Elphic (1978) identified as flux transfer events (FTE). Using a simple, one‐dimensional, quasi‐static model, particle orbits are followed numerically, from the magnetosphere into the sheath. The inner, trapped, distribution initializes the distribution function. Liouville's theorem allows the inner distribution to be mapped into the sheath following the orbits. It is found that the modeled distribution function agrees quite well with that of the observed FTE's for an inward‐pointing, normal magnetic field component and the magnitude of any reconnection‐like tangential electric field must be less than about ½ mV/m. A tangential field up to about this limit may supplement gradient and curvature drifting, repopulating freshly ‘opened’ flux tubes. Electric fields associated with tangentially convecting sheath plasma are mapped along a connected flux tube, but do little to change the distribution function of the energetic ions. A quasi‐trapped population in the sheath usually seems to ‘sandwich’ the FTE distributions. These quasi‐trapped distributions are probably due to slow, large pitch angle, outward moving particles, left behind by the outward rush of the ions more field aligned at the time the flux tube was ‘opened.’ If this interpretation is correct, the spatial extent of ‘open’ field lines near the boundary is broader, not quite as localized as previously thought, at least for this particular data. The higher energy, outflowing particles with small to large pitch angles penetrate the magnetopause several thousand kilometers from the low energy particles. This result of the model, combined with 2 of the FTE observations, gives qualitative support to the suggestion that localized tangential electric fields above our upper limit may exist for this time period.