Tracking an ICME from 1 AU to the Outer Heliosphere by Observations of the Associated Forbush Decreases
Abstract
A Forbush decrease (Forbush, 1938) is a rapid reduction in the Galactic cosmic ray (GCR) intensity followed by a gradual recovery associated with the passage of an interplanetary coronal mass ejection (ICME). A Forbush decrease may include two contributions, one associated with the sheath upstream of the ICME, usually attributed to increased particle scattering due to turbulence in the sheath, the other associated with passage of the ICME, which excludes GCRs. This may be due to the closed topology of the ICME, the relatively smooth magnetic fields in the ICME that inhibit particle scattering into the interior, or the exclusion of particles by enhanced magnetic fields, if present. Forbush decreases have been studied at Earth since the 1930's with ionization chambers and neutron monitors. They can also be detected when ICMEs pass spacecraft, providing a valuable tool to identify the arrival of ICMEs at spacecraft that lack full plasma and magnetic field instrumentation but carry particle instruments that can detect GCRs, often as a background in certain detectors. In particular, such observations have been exploited to identify the passage of ICMEs by planetary missions such as Mars Odyssey at Mars or Cassini when within Saturn's magnetosphere. A small number of Forbush decreases have even been identified far out in the Solar System, giving hints into the evolution of ICMEs with distance. In this presentation, the Forbush decreases created at different bodies in the Solar System by the passage of an ICME arising from a coronal mass ejection on 14 October 2014 will be discussed (Witasse et al., JGR, 2017). In particular, the ICME encountered Mars (at 1.4 AU), comet 67P/Churyumov Gerasimenko (3.1 AU), and Saturn (9.9 AU). The ICME did not pass Earth, but was observed by solar wind instruments on STEREO A when 169° west of Earth. This is the first time that a Forbush decrease due to the same ICME has been compared at three locations widely-separated in longitude and heliocentric distance. This comparison provides unique information on the global structure of the ICME (from the nose to the flanks), as well as on its expansion and deceleration with increasing distance from the Sun. This event will be considered in the context of other Forbush decreases observed in the solar wind and at Mercury, Earth, the Moon, Mars, comet 67P/Churyumov Gerasimenko, and Saturn. - Forbush, S. E. (1938), On world-wide changes in cosmic-ray intensity, Phys. Rev., 54, 12, 975-988. - Witasse, O., et al. (2017), Interplanetary coronal mass ejection observed at STEREO-A, Mars, comet67P/Churyumov-Gerasimenko, Saturn, and New Horizons en route to Pluto: Comparison of its Forbush decreases at 1.4, 3.1, and 9.9 AU, J. Geophys. Res. Space Physics, 122, 7865-7890, doi:10.1002/2017JA023884.
- Publication:
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43rd COSPAR Scientific Assembly. Held 28 January - 4 February
- Pub Date:
- January 2021
- Bibcode:
- 2021cosp...43E2369S