Reformation of an oblique shock observed by Cluster

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.