Structure of the magnetopause: Observations and implications for reconnection

Abstract

The Earth's magnetopause is the boundary between a hot tenuous plasma in the magnetosphere and a cooler denser plasma in the magnetosheath. Both of these plasmas contain magnetic fields whose directions are usually different but whose magnitudes are often comparable. Efforts to understand the structure of the magnetosphere have been hampered by the variability and complexity of this boundary. Waves on the magnetopause surface propagate toward the magnetotail and produce the multiple boundary crossings frequently seen by spacecraft. Boundary velocities are poorly known and range anywhere within an order of magnitude of 10 km s⁻¹. Typical thicknesses are probably on the order of a few hundred km which is a few times the gyroradius of a thermal proton. Although conclusive direct evidence for a field component, B _n , across the magnetopause has not been found, this lack of evidence may reflect the difficulty in determining B _n in the presence of magnetopause waves rather than the real absence of this component. Considerable indirect evidence exists for an open magnetosphere, but the importance of the reconnection process thought to produce open field lines has recently been questioned.