# First observations of irregular surface of interplanetary shocks at ion scales by Cluster

## TL;DR

This study provides the first observational evidence of the irregular surface of interplanetary shocks at ion scales, using multi-spacecraft data from the Cluster mission to analyze shock surface irregularities and compare with simulations.

## Contribution

It presents novel multi-spacecraft observations of IP shock surface irregularities at ion scales and compares these with simulation profiles to understand shock surface structure.

## Key findings

- Shock surface irregularities increase with spacecraft separation.
- Magnetic field profiles vary significantly at separations >10 d_i.
- Simulated shock profiles match observed irregularities.

## Abstract

We present the first observational evidence of the irregular surface of interplanetary (IP) shocks by using multi-spacecraft observations of the Cluster mission. In total we discuss observations of four IP shocks that exhibit moderate Alfv\'enic Mach numbers (M$_A\leq$6.5). Three of them are high-$\beta$ shocks with upstream $\beta$ = 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 (d$_i$) up to 100~d$_i$. Expressed in kilometers, the distances ranged between 38~km and $\sim$10$^4$~km. We show that magnetic field profiles and the local shock normals of observed shocks are very similar when the spacecraft are of the order of one d$_i$ apart, but are strikingly different when the distances increase to ten or more d$_i$. 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 (M$_A$, $\theta_{BN}$, upstream ion $\beta$) as observed shocks and the profiles were obtained at separations across the simulation domain equivalent to the Cluster inter-spacecraft distances.