Solar activity relations in energetic electron events measured by the MESSENGER mission

TL;DR

This study analyzes the relationship between solar energetic electron events measured by MESSENGER and their associated solar phenomena, revealing asymmetries and identifying CME shock speed as a key factor in electron acceleration.

Contribution

It provides new insights into the correlation between CME shock speed and electron event intensities at near-Sun distances, highlighting the roles of flare and shock processes.

Findings

Asymmetry in connection angles affects peak intensities.

CME shock speed correlates strongly with electron peak intensities.

Both flare and shock processes contribute to electron acceleration.

Abstract

Aims. We perform a statistical study of the relations between the properties of solar energetic electron (SEE) events measured by the MESSENGER mission from 2010 to 2015 and the parameters of the respective parent solar activity phenomena to identify the potential correlations between them. During the time of analysis MESSENGER heliocentric distance varied between 0.31 and 0.47 au. Results. There is an asymmetry to the east in the range of connection angles (CAs) for which the SEE events present the highest peak intensities, where the CA is the longitudinal separation between the footpoint of the magnetic field connecting to the spacecraft and the flare location. Based on this asymmetry, we define the subsample of well-connected events as when -65$^{\circ}\leq$ CA $\leq+33^{\circ}$. Conclusions. Based on the comparison of the correlation coefficients presented in this study using near 0.4 au data, (1) both flare and shock-related processes may contribute to the acceleration of near relativistic electrons in large SEE events, in agreement with previous studies based on near 1 au data; and (2) the maximum speed of the CME-driven shock is a better parameter to investigate particle acceleration related mechanisms than the average CME speed, as suggested by the stronger correlation with the SEE peak intensities.