Comparing the Properties of ICME‐Induced Forbush Decreases at Earth and Mars

WIMMER?SCHWEINGRUBER, ROBERT F.; DÉMOULIN, PASCAL; PAPAIOANNOU, ATHANASIOS; ZEITLIN, CARY J.; GUO, JINGNAN; JANVIER, MIHO; TEMMER, MANUELA; HASSLER, DONALD M.; FREIHERR VON FORSTNER, JOHAN L.; DUMBOVI?, MATEJA; VERONIG, ASTRID; DASSO, SERGIO

Journal of Geophysical Research: Space Physics

AGU

Año: 2020 vol. 125

2169-9380

Forbush decreases (FDs), which are short-term drops in the flux of galactic cosmic rays, arecaused by the shielding from strong and/or turbulent magnetic structures in the solar wind, especiallyinterplanetary coronal mass ejections (ICMEs) and their associated shocks, as well as corotatinginteraction regions. Such events can be observed at Earth, for example, using neutron monitors, and also atmany other locations in the solar system, such as on the surface of Mars with the Radiation AssessmentDetector instrument onboard Mars Science Laboratory. They are often used as a proxy for detectingthe arrival of ICMEs or corotating interaction regions, especially when sufficient in situ solar windmeasurements are not available.We compare the properties of FDs observed at Earth and Mars, focusingon events produced by ICMEs.We find that FDs at both locations show a correlation between their totalamplitude and the maximum hourly decrease, but with different proportionality factors.We explainthis difference using theoretical modeling approaches and suggest that it is related to the size increase ofICMEs, and in particular their sheath regions, en route from Earth to Mars. From the FD data, we canderive the sheath broadening factor to be between about 1.5 and 1.9, agreeing with our theoreticalconsiderations. This factor is also in line with previous measurements of the sheath evolution closer tothe Sun.