engleski

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

Forbush decreases (FDs), short-term drops in the flux of galactic cosmic rays (GCR), can be caused by the shielding from strong and/or turbulent magnetic structures in the solar wind, i.e. interplanetary coronal mass ejections (ICMEs) and their associated shocks as well as corotating interaction regions (CIRs). FDs are often used as a proxy for detecting the arrival of ICMEs or CIRs at locations where sufficient in situ solar wind measurements are not or not always available, such as at Mars. The Radiation Assessment Detector (RAD) onboard the Mars Science Laboratory (MSL) mission's Curiosity rover has been continuously measuring the GCR environment on the surface of Mars for more than 7 years since its landing in August 2012 and is thus an excellent source for measurements of FDs at Mars (see e.g. Guo et al. 2018, A&A). Based on the large catalog of FDs at Mars compiled by Papaioannou et al. (2019, Solar Physics) as well as results from our previous work (Freiherr von Forstner et al., 2019, Space Weather), we study the parameters of FDs at Mars and their relations, focusing on events produced by ICMEs. We then compare these data with catalogs of terrestrial FDs, investigating whether and to what extent the differences of certain FD characteristics between the two planets, at two different heliospheric distances, are related to the evolution of ICMEs between Earth and Mars. Our results show that there is a linear correlation between the FD amplitude (drop percentage) and the maximum hourly GCR decrease during the FD, which was already found at Earth by previous authors (Belov et al., 2008, Abunin et al., 2012). However, this correlation has a different proprtionality factor at Mars than at Earth, especially for ICME-induced events. As we do not find a clear dependence of this relationship on the observed GCR energy range, we suggest that this difference is probably caused by the expansion of the ICME sheath region as it propagates outward from 1 AU to ∼1.5 AU. The expansion factor derived from our analysis is in line with expansion factors of ICME sheaths within the inner heliosphere observed by <a href="https://doi.org/10.1029/2018JA025949>Janvier et al. (2019, JGR Space Physics).

coronal mass ejections, forbush decreases

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