Traveling foreshocks and transient foreshock phenomena

TL;DR

This paper identifies and characterizes two types of foreshocks using multi-spacecraft data, revealing their properties, boundaries, and related phenomena like cavities and hot flow anomalies.

Contribution

It distinguishes between global and traveling foreshocks, introduces the concept of foreshock cavities, and provides statistical analysis of related transient phenomena.

Findings

Traveling foreshocks are bounded by rotational discontinuities and propagate along the bow-shock.

Foreshock cavities are a subset of traveling foreshocks with minimal wave development.

Spontaneous hot flow anomalies are highly depleted in magnetic field and plasma density.

Abstract

We use the multi-spacecraft capabilities of the Cluster and THEMIS missions to show that two types of foreshock may be detected in spacecraft data. One is the global foreshock that appears upstream of the Earth's quasi-parallel bow-shock under steady or variable interplanetary magnetic field. Another type is a traveling foreshock that is bounded by two rotational discontinuities in the interplanetary magnetic field and propagates along the bow-shock. Foreshock compressional boundaries are found at the edges of both types of foreshock. We show that isolated foreshock cavities are a subset of the traveling foreshock that form when two bounding rotational discontinuities are so close that the ultra-low frequency waves do not develop in the region between them. We also report observations of a spontaneous hot flow anomaly inside a traveling foreshock. This means that other phenomena, such as foreshock cavitons, may also exist inside this type of foreshock. In the second part of this work we present statistical properties of phenomena related to the foreshock, namely foreshock cavities, cavitons, spontaneous hot flow anomalies and foreshock compressional boundaries. We show that spontaneous hot flow anomalies are the most depleted transient structures in terms of the B-field and plasma density inside them and that the foreshock compressional boundaries and foreshock cavities are closely related structures.