Magnetic field depressions in the solar wind

Depressions in the interplanetary magnetic field strength occur on a wide range of temporal scales, starting with magnetic holes with a duration of several seconds and extending to larger-scale structures of more than 30 min duration. Using the magnetic field measurements of the Ulysses spacecraft, we quantify the statistical significance of the occurrence rate of depressions in the magnetic field compared to a lognormal distribution. On this basis we introduce measures for the length and depth of magnetic depressions. There is a weak indication for a change in the character of the length distribution at 20-40 s in the high-speed solar wind. An analysis of 115 depressions with a length > 2 min showed that (1) they are bounded by tangential discontinuities in 78% of all cases, (2) normals to the structure boundaries have no strong orientation with respect to background field or global geometry, and (3) an increased proton temperature anisotropy is the only bulk ion parameter correlating with the depressions.