Galactic Cosmic Ray Density Variations in Magnetic Clouds

Abstract

We investigate the characteristics of Galactic cosmic rays in events associated with magnetic clouds that reach Earth. A mathematical model, capable of describing the distribution of the cosmic-ray density in a magnetic cloud is considered. We show that in most cases the behavior of the cosmic-ray density within magnetic clouds at 1 AU can be described accurately by a parabolic function of the distance to the center of the magnetic cloud measured in gyroradii. As expected, the majority of magnetic clouds modulate cosmic rays, resulting in a reduction of their density. However, there is a group of events (about one fifth of the total sample) in which the density of cosmic rays in a magnetic cloud increases. Furthermore, the extremum (a minimum or a maximum) of the cosmic-ray density is found closer to the cloud center and not at its edges. We consider a number of the factors contributing to the model and estimate the effect of each factor.

Superposed epoch study of ICME sub-structures near Earth and their effects on Galactic cosmic rays

“…Furthermore, our model also reproduces a local n GCR minima inside the MC for the sub-group of fast events. This feature is consistent with Belov et al (2015), in which they find that strong field (>18 nT) is associated with local minima of n GCR within MCs.…”

“…This second minimum is associated with the shielding of a strong magnetic field within the flux rope configuration (with closed field lines still connected to the Sun). This result is consistent with Belov et al (2015), in which they report that for a strong-field (>18 nT), there is a local minimum of n GCR inside MCs.…”

“…Finally, our study is related to that of Belov et al (2015) since they model the response of GCRs inside MCs as a function of the interplanetary conditions (plasma speed and magnetic field), and geomagnetic properties (Dst index). In particular, they report the presence of a local minimum of GCR inside MCs for strong-field events.…”

Image of Forbush Decrease in a Magnetic Cloud by Three Moments of Cosmic Ray Distribution Function

“…It can be noted that the size of the FD region (interval 45–54 hr) is smaller than the MC transverse dimension. However, in real events, FDs are observed throughout or even longer than the entire passage of MCs (Belov et al, ; Masías‐Meza et al, ). In our model, there is no FD during 41–45 and 55–61 hr because: (1) There are CRs that have recently entered the MC and have not experienced its effect.…”

“…Abunin et al () and Belov et al () investigated FD characteristics in the events associated with MCs that reach the Earth. The analysis is based on the catalog of ICMEs that covers the time from 1996 to 2010 (Richardson & Cane, ).…”