The ULF wave foreshock boundary: Cluster observations

TL;DR

This study uses Cluster satellite data to precisely identify and analyze the ULF wave foreshock boundary, revealing its properties, relation to magnetic field variations, and connection to ion emission mechanisms at the bow shock.

Contribution

A new, accurate criterion based on IMF rotation is introduced for identifying the ULF wave foreshock boundary, improving upon previous methods.

Findings

The ULF wave foreshock boundary's properties are characterized in detail.

The boundary's location is sensitive to the IMF cone angle.

Connections between boundary features and ion emission mechanisms are explored.

Abstract

The interaction of backstreaming ions with the incoming solar wind in the upstream region of the bow shock gives rise to a number of plasma instabilities from which ultra-low frequency (ULF) waves can grow. Because of their finite growth rate, the ULF waves are spatially localized in the foreshock region. Previous studies have reported observational evidences of the existence of a ULF wave foreshock boundary, which geometrical characteristics are very sensitive to the interplanetary magnetic field (IMF) cone angle. The statistical properties of the ULF wave foreshock boundary is examined in detail using Cluster data. A new identification of the ULF wave foreshock boundary is presented using specific and accurate criterion for a precises determination of boundary crossings. The criterion is based on the degree of IMF rotation as Cluster crosses the boundary. The obtained ULF wave foreshock boundary is compared with previous results reported in the literature as well as with theoretical predictions. Also, we examined the possible connexion between the foreshock boundary properties and the ion emission mechanisms at the bow shock.