Reformation of an oblique shock observed by Cluster

TL;DR

This study observes the reformation process of an oblique shock at Earth's magnetosphere boundary, showing cyclic shock front development and providing real-time insights into shock dynamics similar to simulation predictions.

Contribution

First direct observation of shock reformation cycle at an oblique shock, linking spacecraft data with theoretical shock reformation models.

Findings

Shock front reformation occurs cyclically with a period of about 35Ω_p.

A pulsation upstream of the shock grows and steepens into a new shock front.

Remnants of older shock fronts are observed downstream, confirming cyclic reformation.

Abstract

On 16 March 2005, the Cluster spacecraft crossed a shock almost at the transition between the quasi-perpendicular and quasi-parallel regimes ($\theta_{Bn}=46^{\circ}$) preceded by an upstream low-frequency ($\sim$ 0.02 Hz in the spacecraft frame) wavetrain observed for more than 10 mn. The wave semi-cycle nearest to the shock was found to grow in time, steepen and reflect an increasing fraction of the incoming ions. This gives strong indication that this pulsation is becoming a new shock front, standing $\sim 5\lambda_p$ upstream of the main front and growing to shock-like amplitude on a time-scale of $\sim 35\Omega_p$. Downstream of the main shock transition, remnants of an older front are found indicating that the reformation is cyclic. This provides a unique example where the dynamics of shock reformation can be sequentially followed. The process shares many characteristics with simulations of reforming quasi-parallel shocks.