engleski

Forbush decreases as signatures of Interplanetary Coronal Mass Ejections (ICMEs)

ICMEs are often associated with short-term reduction in galactic cosmic ray (GCR) flux, so-called Forbush decreases (FDs), which in turn can be regarded as one of the signatures of an ICME passage. Therefore, they can provide insight about ICMEs where/when in situ plasma and magnetic field measurements are not available. However, in order to use FDs as ICME signatures efficiently, it is important to understand the interaction of ICMEs with GCRs. We theoretically regard a textbook-example ICME which consists of the shock/sheath region and a magnetic ejecta producing a so-called two-step FD, where the first step is caused by the CME-driven shock and the second one by the magnetic ejecta. The shock/sheath region is magnetically connected to the ambient interplanetary space and characterized by disturbed plasma conditions and highly fluctuating magnetic field. We assume that the corresponding decrease is mainly caused by the magnetic mirror effect at the shock, where the shock thickness is much smaller than the GCR gyroradius, and the guiding- centre approximation and the magnetic-moment conservation are not applicable (Vrsnak et al., 2018, In prep.). On the other hand, the magnetic structure itself is not magnetically connected to the ambient plasma and is characterised by smooth magnetic field. Therefore, we assume an initially empty, closed magnetic structure which fills up slowly with GCRs as it propagates and expands in the interplanetary space (Dumbovic et al., 2018, ApJ). In both cases an analytical solution is obtained, and the corresponding FD can be related to a number of ICME properties, such as central magnetic field strength, expansion factor, shock strength and thickness. We analyze these theoretical considerations and apply them to selected events, where initial CME conditions are constrained using remote CME observation and 3D CME reconstruction. In addition, forward modelling and in situ measurements are used to analyse and constrain ICME evolution, utilizing a number of spacecraft and planetary observation, including those by the Radiation Assessment Detector aboard the Mars Rover Curiosity. This research has received funding from the European Union’s Horizon 2020 research and innovation programme under the Marie Skłodowska-Curie grant agreement No 745782, and has been supported in part by Croatian Science Foundation under the project 6212 „Solar and Stellar Variability“.

coronal mass ejections, forbush decreases

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