Coronal mass ejections and magnetic flux ropes in interplanetary space

Abstract

Coronal mass ejections (CMEs) are important occasional sources of plasma and magnetic field in the solar wind at 1 AU. Formed in the corona by the ejection of solar material from closed field regions that were not previously participating in the solar wind expansion, CMEs at 1 AU generally have distinct plasma and field signatures by which they can be distinguished from the ordinary solar wind. Perhaps the most common of these signatures is a counterstreaming (along the magnetic field) flux of suprathermal electrons with energies ≳ 80 eV. This signature indicates that CMEs at 1 AU typically are closed field structures either rooted at both ends in the Sun or entirely disconnected from it. A subset, perhaps 30% of all CME events at 1 AU, exhibit the large and coherent internal field rotations characteristic of magnetic flux ropes. Often, but not always, these special flux rope events have the low proton temperatures and stronger than average fields characteristic of “magnetic clouds.” Although equilibrium flux rope models, in which the internal field pressure is balanced by the curvature stress of the field, generally are adequate for explaining the observed field rotations in these events, interplanetary flux ropes need not be equilibrium structures. We suggest that interplanetary magnetic flux ropes form as a result of reconnection within rising, previously sheared, coronal magnetic loops. Such reconnection probably occurs relatively rarely in CME events, since the majority of CME events in the solar wind at 1 AU do not appear to be flux ropes.