Shocks inside CMEs: A Survey of Properties from 1997 to 2006

TL;DR

This paper surveys 49 fast-mode shocks inside CMEs from 1997 to 2006, revealing their unique properties, upstream conditions, and impact on geomagnetic storms, with implications for shock evolution within CMEs.

Contribution

It provides the first comprehensive statistical analysis of shocks inside CMEs, highlighting their distinct upstream conditions and potential evolution during propagation.

Findings

Median upstream Alfvén speed is 85 km/s.

Median shock speed is 590 km/s.

Approximately half of the shocks may not remain fast-mode throughout the CME.

Abstract

We report on 49 fast-mode forward shocks propagating inside coronal mass ejections (CMEs) as measured by Wind and ACE at 1 AU from 1997 to 2006. Compared to typical CME-driven shocks, these shocks propagate in different upstream conditions, where the median upstream Alfv{\'e}n speed is 85 km s$^{-1}$, the proton $\beta = 0.08$ and the magnetic field strength is 8 nT. These shocks are fast with a median speed of 590 km s$^{-1}$ but weak with a median Alfv{\'e}nic Mach number of 1.9. They typically compress the magnetic field and density by a factor of 2-3. The most extreme upstream conditions found were a fast magnetosonic speed of 230 km s$^{-1}$, a plasma $\beta$ of 0.02, upstream solar wind speed of 740 km s$^{-1}$ and density of 0.5 cm$^{-3}$. Nineteen of these complex events were associated with an intense geomagnetic storm (peak Dst under $-100$ nT) within 12 hours of the shock detection at Wind, and fifteen were associated with a drop of the storm-time Dst index of more than 50 nT between 3 and 9 hours after shock detection. We also compare them to a sample of 45 shocks propagating in more typical upstream conditions. We show the average property of these shocks through a superposed epoch analysis, and we present some analytical considerations regarding the compression ratios of shocks in low $\beta$ regimes. As most of these shocks are measured in the back half of a CME, we conclude that about half the shocks may not remain fast-mode shocks as they propagate through an entire CME due to the large upstream and magnetosonic speeds.