Evolution of a Long-Duration Coronal Mass Ejection and its Sheath Region Between Mercury and Earth on 2013 July 9-14

TL;DR

This study analyzes a long-duration coronal mass ejection from the Sun in July 2013, comparing its properties at Mercury and Earth, revealing minimal deceleration and insights into its expansion and sheath composition.

Contribution

It provides detailed in situ measurements of the CME's evolution between Mercury and Earth, highlighting the CME's size, expansion behavior, and sheath structure in a rare radial alignment scenario.

Findings

CME maintained similar speeds at Mercury and Earth, indicating little deceleration.

The large ejecta duration is due to the CME's inherent size, not rapid expansion.

The sheath contains both pre-shock compressed material and recently shocked material.

Abstract

Using in situ measurements and remote-sensing observations, we study a coronal mass ejection (CME) that left the Sun on 9 July 2013 and impacted both Mercury and Earth while the planets were in radial alignment (within $3^\circ$). The CME had an initial speed as measured by coronagraphs of 580 $\pm$ 20 km s$^{-1}$, an inferred speed at Mercury of 580 $\pm$ 30 km s$^{-1}$ and a measured maximum speed at Earth of 530 km s$^{-1}$, indicating that it did not decelerate substantially in the inner heliosphere. The magnetic field measurements made by MESSENGER and {\it Wind} reveal a very similar magnetic ejecta at both planets. We consider the CME expansion as measured by the ejecta duration and the decrease of the magnetic field strength between Mercury and Earth and the velocity profile measured {\it in situ} by {\it Wind}. The long-duration magnetic ejecta (20 and 42 hours at Mercury and Earth, respectively) is found to be associated with a relatively slowly expanding ejecta at 1 AU, revealing that the large size of the ejecta is due to the CME itself or its expansion in the corona or innermost heliosphere, and not due to a rapid expansion between Mercury at 0.45 AU and Earth at 1 AU. We also find evidence that the CME sheath is composed of compressed material accumulated before the shock formed, as well as more recently shocked material.