Structure of ICMEs and their driven shocks at 1 AU, and consequences on Forbush decreases

DASSO, S.; JANVIER, M.; DEMOULIN, P.; MASÍAS-MEZA, J.J.

Moscow

Congreso; 40th COSPAR Scientific Assembly; 2014

COSPAR

Solar wind structures, such as interplanetary (IP) shocks, can affect the transport of energetic particles in the heliosphere. In particular, the presence of IP shocks driven by interplanetary coronal mass ejections (ICMEs) is typically associated with a transient variation of the energetic particles flux (e.g., Forbush decreases, FDs). A FD can present two steps: one of them produced by a diffusive barrier associated with the turbulent region behind the shock, and the another one produced by the ICME itself. However, not every IP shock driven by an ICME is followed by a two-step FD, and it is under debate what are the properties of the solar wind for determining the presence of a two-step Forbush decrease, the presence of a single-step FD, or even the absence of a FD after the passage of the ICME. Magnetic clouds (MCs) are a subset of ICMEs, which present clear evidence in favor of the presence of an interplanetary flux rope in the solar wind. We recently found constraints to the geometrical shape of ICME shocks from a statistical study of shock orientations, and found constraints to the global shape of MCs from a statistical study of main axis orientation of a large sample of magnetic clouds, both at one astronomical unit from the Sun. The main aim of this study is to establish the link between Forbush decreases and the MC/shock properties, taking into account these geometrical shapes of MC axis and shocks surfaces. We present here a combined analysis of events MC-shock-FD, in order to better understand the effects of interplanetary structures on the propagation of energetic particles in the heliosphere.