Estimated relations at a shock driven by a coronal mass ejection

TL;DR

This study analyzes SOHO/LASCO data to identify and characterize shock waves ahead of CMEs, estimating their velocities, Mach numbers, and effective adiabatic indices, supporting their classification as collisionless shocks.

Contribution

It provides the first detailed analysis of shock wave properties ahead of CMEs using SOHO data, estimating key parameters and comparing them with Earth's bow shocks.

Findings

Shock velocities range from 800 to 2500 km/s.

Effective adiabatic index is between 2 and 5/3.

Discontinuities are consistent with collisionless shock waves.

Abstract

Analysis of SOHO/LASCO C3 data reveals a discontinuity, interpreted as a shock wave, in plasma density radial profiles in a restricted region ahead of each of ten selected coronal mass ejections (CME) along their travel directions. In various events, shock wave velocity $V\approx$ 800-2500 km s$^{-1}$. Comparing the dependence of Alfv\'{e}n Mach number $M_A$ on shock wave strength $\rho_2/\rho_1$, measured at $R > 10R_\odot$ from the center of the Sun, to ideal MHD calculations suggests that the effective adiabatic index $\gamma$, characterizing the processes inside the shock front, is largely between 2 and 5/3. This corresponds to the effective number of degrees of freedom of motion 2 to 3. A similar dependence, $M_A(\rho_2/\rho_1)$, was derived for the Earth's bow shock and interplanetary collisionless shock waves. All this supports the assumption that the discontinuities in front of CMEs are collisionless shock waves.