A scale-free analysis of magnetic holes in the solar wind

Abstract

Magnetic holes are isolated intervals of depleted interplanetary magnetic field (IMF) strength on timescales of several seconds to several hours. These intervals have been seen as often as several times per day in the ecliptic and at high heliospheric latitudes from 1 AU to 5.4 AU. We present a scale-free statistical technique for identifying magnetic holes and evaluating their significance relative to turbulent fluctuations in the solar wind. We apply this technique in a comprehensive search for magnetic holes in the Wind and ACE magnetometer data streams through 2004. Using magnetic field and ion measurements on the Wind spacecraft, we present the first statistical study of magnetic hole plasma signatures on the kinetic scale and we present a comparison of magnetic holes as kinetic and microscale phenomena. Magnetic holes are shown to be pressure-balanced structures with similar properties on all scales. Temperature anisotropy measurements are combined with magnetic field measurements to give direct evidence that the mirror-mode instability in the solar wind is a likely source of magnetic holes. Two-satellite correlations between ACE and Wind indicate that magnetic holes are stationary with respect to the solar wind and elongated along the IMF.