Forbush decrease spectrum in a magnetic cloud in the 2004 July 27 event

Abstract

Magnetic clouds affect the intensity of galactic cosmic rays. The diffusion mechanism is usually used as the formation mechanism for Forbush decrease (FD) in a magnetic cloud (MC). An FD is an observed decrease in the cosmic ray intensity. There is a new theory of FD formation, in which the mechanism is the loss of particle energy in the electromagnetic field of a magnetic cloud. The shape of the FD spectrum is calculated for a wide range of particle energies in the 2004 July 27 event. According to the measurements of ground-based neutron monitors and muon telescopes, synchronous changes in the FD amplitude in time indicate that the FD is formed in a magnetic cloud for all energies. However, the calculated FD spectrum differs from the obtained one from measurements. The reasons for the difference can be: 1) the mechanism of formation is not the electromagnetic one; 2) the method for determining the FD spectrum, using the notion of mean or median energies, needs additional studies.