Interplanetary field enhancements travel at the solar wind speed

Abstract

Interplanetary Field Enhancements (IFEs) are cusp‐shaped increases and decreases in the interplanetary field, accompanied by a thin current sheet. They can last many hours and even the strongest have no leading or trailing shocks. These rare structures are of possible significance because their occurrence has been associated with potential sources of charged dust. On December 24, 2006, four well‐separated spacecraft observed an IFE close to the Earth. From these four spacecraft measurements, we find that this IFE was traveling at the solar wind speed. This observation is consistent with previous encounters with fewer multiple satellites but is the first definitive measure of the speed of an IFE. This high speed is consistent with the lack of observation of shocks in association with these structures. At least on this occasion, the IFE was a pressure‐balanced structure in which the thermal plasma pressure gradients balanced the magnetic pressure gradients.