First Observations of Irregular Surface of Interplanetary Shocks at Ion Scales by Cluster

We present the first observational evidence of the irregular surface of interplanetary (IP) shocks by using multispacecraft observations of the Cluster mission. In total we discuss observations of four IP shocks that exhibit moderate Alfvénic Mach numbers (MA ≤ 6.5). Three of them are high-β shocks with upstream β = 2.2–3.7. During the times when these shocks were observed, the Cluster spacecraft formed constellations with inter-spacecraft separations ranging from less than one upstream ion inertial length (di) up to 100 di. Expressed in kilometers, the distances ranged between 38 km and ∼104 km. We show that magnetic field profiles and the local shock normals of observed shocks are very similar when the spacecraft are of the order of one di apart, but are strikingly different when the distances increase to 10 or more di. We interpret these differences to be due to the irregular surface of IP shocks and discuss possible causes for such irregularity. We strengthen our interpretation by comparing observed shock profiles with profiles of simulated shocks. The latter had similar characteristics (MA, θBN, upstream ion β) as observed shocks and the profiles were obtained at separations across the simulation domain equivalent to the Cluster inter-spacecraft distances.