Forbush decreases and interplanetary magnetic field disturbances

Several Forbush decreases (Fd's) in the galactic cosmic radiation have been analyzed for which ground-based neutron monitor observations and spacecraft measurements of both the cosmic ray intensity and the interplanetary magnetic fields were available. This has enabled us to determine the propagation characteristics of the Fd modulating region and the azimuthal extent of the modulating region and to obtain some understanding of the interplanetary magnetic field configuration. We found that (1) the front, behind which there was a reduction in the cosmic ray intensity, propagated radially along the earth-sun line toward the earth in at least one case; (2) in several Fd's the time delay between the increase in the interplanetary magnetic field and the onset of the Fd varied from 1.5 to 10 hours; (3) the azimuthal extent of the modulating region varied but was about 45° in angular extent at the earth; and (4) modulating regions associated with Fd's were observed propagating several astronomical units beyond the earth.