Structure on Interplanetary Shock Fronts: Type II Radio Burst Source Regions

TL;DR

This study uses Wind spacecraft data to analyze interplanetary shock front structures and their role in type II radio burst sources, revealing foreshock regions and electron acceleration mechanisms similar to terrestrial shocks.

Contribution

It provides the first measurement of the lateral scale size of interplanetary foreshock regions and links shock front structure to radio burst fine structures.

Findings

Foreshock regions exist on IP shock fronts.

Electron acceleration occurs via a fast Fermi process.

First measurement of lateral scale size of IP foreshock regions.

Abstract

We present \emph{in situ} observations of the source regions of interplanetary (IP) type II radio bursts, using data from the Wind spacecraft during the period 1996-2002. We show the results of this survey as well as in-depth analysis of several individual events. Each event analyzed in detail is associated with an interplanetary coronal mass ejection (ICME) and an IP shock driven by the ICME. Immediately prior to the arrival of each shock, electron beams along the interplanetary magnetic field (IMF) and associated Langmuir waves are detected, implying magnetic connection to a quasiperpendicular shock front acceleration site. These observations are analogous to those made in the terrestrial foreshock region, indicating that a similar foreshock region exists on IP shock fronts. The analogy suggests that the electron acceleration process is a fast Fermi process, and this suggestion is borne out by loss cone features in the electron distribution functions. The presence of a foreshock region requires nonplanar structure on the shock front. Using Wind burst mode data, the foreshock electrons are analyzed to estimate the dimensions of the curved region. We present the first measurement of the lateral, shock-parallel scale size of IP foreshock regions. The presence of these regions on IP shock fronts can explain the fine structure often seen in the spectra of type II bursts.