On The Possible Mechanism Of Energy Dissipation In Shock-Wave Fronts Driven Ahead Of Coronal Mass Ejections

TL;DR

This paper investigates the energy dissipation mechanisms in CME-driven shock fronts, revealing a transition from collisional to collisionless shocks as the distance from the Sun increases, based on coronagraph data analysis.

Contribution

It provides observational evidence of a transition from collisional to collisionless shock fronts in CME events at different solar distances.

Findings

Shock front thickness is comparable to proton mean free path near the Sun.

A new, thinner discontinuity appears at larger distances, likely indicating a transition to collisionless shocks.

The discontinuity's relative amplitude increases with distance, suggesting evolving shock properties.

Abstract

Analysis of Mark 4 and LASCO C2, C3 coronagraph data shows that, at the distance $R \leq 6$ R$_\odot$ from the center of the Sun, the thickness of a CME-generated shock-wave front ($\delta_F$) may be of order of the proton mean free path. This means that the energy dissipation mechanism in the shock front at these distances is collisional. A new discontinuity (thickness $\delta_F^* \ll \delta_F$) is observed to appear in the anterior part of the front at $R \geq 10$ R$_\odot$. Within the limits of experimental error, the thickness $\delta_F^* \approx$ 0.1-0.2 R$_\odot$ does not vary with distance and is determined by the spatial resolution of the LASCO C3 instrument. At the initial stage of formation, the discontinuity on the scale of $\delta_F^*$ has rather small amplitude and exists simultaneously with the front having thickness $\delta_F$. The relative amplitude of the discontinuity gradually increases with distance, and the brightness profile behind it becomes even. Such transformations may be associated with the transition from a collisional shock wave to a collisionless one.