engleski

Unusual Plasma and Particle Signatures at Mars and STEREO-A Related to CME─CME Interaction

On 2017 July 25 a multistep Forbush decrease (FD) with a remarkable total amplitude of more than 15% was observed by Mars Science Laboratory/Radiation Assessment Detector 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 2017 July 24, 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°) to Mars, and both were located at the back side of the Sun as viewed from Earth. We analyze a number of multispacecraft and multi-instrument (both in situ and remote-sensing) observations and employ modeling to understand these signatures. We find that the solar sources are two coronal mass ejections (CMEs) that erupted on 2017 July 23 from the same source region on the back side of the Sun as viewed from Earth. Moreover, we find that the two CMEs interact nonuniformly, inhibiting the expansion of one of the CMEs in the 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, substructured interplanetary disturbance at Mars, where different substructures correspond to different steps of the FD, adding up to a globally large- amplitude FD.

solar─terrestrial relations ; Sun: coronal mass ejections: CMEs ; Astrophysics - Solar and Stellar Astrophysics ; Physics - Space Physics

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