Characterizing ICME-related Forbush Decreases at Mercury

Forbush decreases (Fds) are temporary decreases in the galactic cosmic ray (GCR) flux associated with large-scale solar wind transients. Such modulation of the GCR flux by interplanetary coronal mass ejections (ICMEs) and their magnetic substructures (regions of sheath and magnetic ejecta) present an indirect approach to study ICME evolution. Data obtained from the Neutron Spectrometer on board MESSENGER during its orbital phase around Mercury provides an opportunity to study such Fds related to the passage of ICMEs in the inner heliosphere and characterize their structure.

In this study, we identify 33 Fd events associated with ICMEs observed by MESSENGER, 73% of which possess a two-step Fd structure (24). We use a superposed epoch analysis to directly link the ICME magnetic substructure with the two-step Fd structure at Mercury. By using results from previous studies at Earth and Mars, we also address whether two-step Fds are more commonly observed closer to the Sun; we found that although likely, this is not conclusive when comparing to the wide range of results of previous studies conducted at Earth. Finally, we investigate how Fd profiles and properties change with increasing heliocentric distance, finding that on average, Fd profiles decrease in depth and steepness with increasing heliocentric distance. The strong relationship between the depth and maximum hourly steepness of the Fd is in agreement with previous studies, and follows trends relating to the expansion of ICMEs as they propagate through the heliosphere.