Magnetic field depressions in the solar wind

Abstract

Abstract. 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.

On the origin of microscale magnetic holes in the solar wind

“…It is therefore unlikely that magnetic hole theories, such as described above, would be directly applicable in this case. This has been recently suggested by Fraenz et al [2000] on the basis of the size distribution of magnetic holes from Ulysses data. Burlaga [1968] discusses evidence that these larger events might be associated with reconnection processes, a notion that we will follow up on in a later study.…”

“…It was argued that these larger-scale magnetic field depletions were a signature of reconnection in the solar wind, resulting in directional discontinuities (D sheets). It was also pointed out that these discontinuities are most likely tangential in nature [Burlaga and Ness, 1968;Chisham et al, 2000 ' Fraenz et al, 2000]. The process responsible for the formation of D sheets can be illustrated using two adjacent flux tubes of antiparallel field.…”

EMHD theory and observations of electron solitary waves in magnetotail plasmas

“…Linear magnetic holes (LMHs), the magnetic field depression without field line direction variation, have been frequently observed in solar wind plasmas [Turner et al, 1977;Fränz et al, 2000;Tsurutani and Ho, 1999;Winterhalter et al, 1994Winterhalter et al, , 1995Winterhalter et al, , 2000Xiao et al, 2010;Zhang et al, 2008]. There are various explanations for mechanism of those LMHs, such as sheet-like equilibrium structures [Burlaga and Lemaire, 1978], solitons by the nonlinear Schrödinger equation (NLSE) in the Hall MHD model [Baumgärtel, 1999;Stasiewicz, 2004], and mirror instabilities [Winterhalter et al, 1994;Erdös and Balogh, 1996].…”

Phase‐steepened Alfvén waves, proton perpendicular energization and the creation of magnetic holes and magnetic decreases: The ponderomotive force

“…[2] Magnetic holes (MHs) [Turner et al, 1977;Winterhalter et al, 1994Winterhalter et al, , 2000Fränz et al, 2000] and magnetic decreases (MDs) [Tsurutani and Ho, 1999;Neugebauer et al, 2001] are decreases in the magnitude of the interplanetary magnetic field that can be relatively short (seconds) or can last as long as tens of minutes or even hours in duration. It has recently been shown by Tsurutani et al [2002] that MHs and MDs are presumably the same phenomenon (with MDs longer and with discontinuities bounding an edge), and that they often occur at the edges of phase steepened Alfvén waves.…”