Magnetopause crossings without a boundary layer

Abstract

The microstructure of pristine magnetopause crossings has been analyzed by using high‐resolution particle and field data obtained by the Active Magnetospheric Particle Tracer Explorers (AMPTE) Charge Composition Explorer (CCE) and ISEE 2 spacecraft. These crossings are pristine in the sense that they exhibit no adjoining magnetospheric boundary layer, or, at most, a low‐density plateau. The CCE crossings include the low‐latitude near‐noon region not typically sampled by ISEE 2, which covers all other local time sectors in a complementary way. Magnetopause crossings without a boundary layer are found to occur to all local times, and such crossings constitute about 10% of all magnetopause crossings. Total pressure balance across the magnetopause is observed to within experimental errors; however, electron data, full‐energy composition measurements, and occasionally field stress are needed to fully evaluate pressure balance. The microstructure of the magnetopause current layer is also found to depend on local time. Crossings within about 1 hour local time of the noon meridian often exhibit very sharp density gradients on scale lengths down to a few plasma skin depths. These gradients are reduced for crossings farther from local noon such that, for cases near the dawn‐dusk meridian, the scale length for density gradients and the magnetopause current are roughly comparable. Magnetopause crossings without a boundary layer impose severe constraints on various theories of boundary layer formation. Pristine magnetopause crossings may be direct cuts through the diffusion region for reconnection. With this interpretation our results are in qualitative agreement with recent simulations of the diffusion region and associated turbulence by Drake et al. [1994], who propose the current convective instability as the dominant process for current transport at the magnetopause.