# Unusual plasma and particle signatures at Mars and STEREO-A related to CME-CME interaction

## TL;DR

This study analyzes a complex CME-CME interaction event from July 2017, linking in situ plasma, magnetic field, and particle signatures at Mars and STEREO-A, revealing how CME interactions influence space weather effects.

## Contribution

It provides a detailed multi-spacecraft analysis and modeling of CME interactions on the Sun's far side, highlighting their impact on interplanetary disturbances and particle signatures at Mars and STEREO-A.

## Key findings

- CME interaction inhibited expansion in STEREO-A direction
- Multiple sub-structures caused by CME interaction led to a large Forbush decrease at Mars
- Event complexity results from CME interactions and ambient solar wind effects

## Abstract

On July 25 2017 a multi-step Forbush decrease (FD) with the remarkable total amplitude of more than 15\% was observed by MSL/RAD at Mars. We find that these particle signatures are related to very pronounced plasma and magnetic field signatures detected in situ by STEREO-A on July 24 2017, with a higher than average total magnetic field strength reaching more than 60 nT. In the observed time period STEREO-A was at a relatively small longitudinal separation (46 degrees) to Mars and both were located at the back side of the Sun as viewed from Earth. We analyse a number of multi-spacecraft and multi-instrument (both in situ and remote-sensing) observations, and employ modelling to understand these signatures. We find that the solar sources are two CMEs which erupted on July 23 2017 from the same source region on the back side of the Sun as viewed from Earth. Moreover, we find that the two CMEs interact non-uniformly, inhibiting the expansion of one of the CMEs in STEREO-A direction, whereas allowing it to expand more freely in the Mars direction. The interaction of the two CMEs with the ambient solar wind adds up to the complexity of the event, resulting in a long, sub-structured interplanetary disturbance at Mars, where different sub-structures correspond to different steps of the FD, adding-up to a globally large-amplitude FD.