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- Title
Comparing the Properties of ICME‐Induced Forbush Decreases at Earth and Mars.
- Authors
Freiherr von Forstner, Johan L.; Guo, Jingnan; Wimmer‐Schweingruber, Robert F.; Dumbović, Mateja; Janvier, Miho; Démoulin, Pascal; Veronig, Astrid; Temmer, Manuela; Papaioannou, Athanasios; Dasso, Sergio; Hassler, Donald M.; Zeitlin, Cary J.
- Abstract
Forbush decreases (FDs), which are short‐term drops in the flux of galactic cosmic rays, are caused by the shielding from strong and/or turbulent magnetic structures in the solar wind, especially interplanetary coronal mass ejections (ICMEs) and their associated shocks, as well as corotating interaction regions. Such events can be observed at Earth, for example, using neutron monitors, and also at many other locations in the solar system, such as on the surface of Mars with the Radiation Assessment Detector instrument onboard Mars Science Laboratory. They are often used as a proxy for detecting the arrival of ICMEs or corotating interaction regions, especially when sufficient in situ solar wind measurements are not available. We compare the properties of FDs observed at Earth and Mars, focusing on events produced by ICMEs. We find that FDs at both locations show a correlation between their total amplitude and the maximum hourly decrease, but with different proportionality factors. We explain this difference using theoretical modeling approaches and suggest that it is related to the size increase of ICMEs, and in particular their sheath regions, en route from Earth to Mars. From the FD data, we can derive the sheath broadening factor to be between about 1.5 and 1.9, agreeing with our theoretical considerations. This factor is also in line with previous measurements of the sheath evolution closer to the Sun. Plain Language Summary: When eruptions from the Sun propagate through the interplanetary space, their strong and turbulent magnetic field deflects background cosmic ray particles nearby. This causes a temporary decrease of the flux of cosmic rays observed at locations that were passed by the eruption, a so‐called Forbush decrease. These decreases can be measured on Earth, and also by space missions around the solar system, and are often used to detect the arrival of solar eruptions, especially when no other direct measurements are available. We look at catalogs of Forbush decreases observed at Earth and Mars, which is 50% farther away from the Sun than Earth, and compare their properties to investigate whether, in addition to the arrival time, it is possible to derive more information about the eruptions from the observed Forbush decreases. We find that the relation of characteristic parameters describing the Forbush decrease changes between the two planets and that this can be explained by the broadening of the interplanetary structure erupted from the Sun during its propagation. The magnitude of this broadening derived from our data agrees with theoretical expectations and is in line with previous measurements of the evolution of solar eruptions at locations closer to the Sun. Key Points: We examine a large number of Forbush decreases (FDs) at Mars observed by MSL/RAD, comparing their properties with those observed at EarthThe maximum hourly decrease of the FDs is correlated with their amplitude, but with different linear regression slopes at Earth and MarsThis effect may be due to the broadening of the coronal mass ejection's sheath region, which we motivate with theoretical calculations
- Subjects
FORBUSH decreases; COSMIC rays; MAGNETIC structure; SOLAR wind; CORONAL mass ejections
- Publication
Journal of Geophysical Research. Space Physics, 2020, Vol 125, Issue 3, p1
- ISSN
2169-9380
- Publication type
Article
- DOI
10.1029/2019JA027662