Variations in the rigidity spectrum and anisotropy of cosmic rays at the period of Forbush eect on 12-25 July 1982

Abstract

Ground-based cosmic-ray (CR) observational data from a worldwide network of stations were used to investigate the variation of the CR distribution function and the changes in geomagnetic cuto rigidities during the Forbush eect in July 1982. The data from 42 neutron monitors were employed in this study. The analysis was made by the method of spectrographic global survey. Using the worldwide network of stations as a single multichannel instrument, the method makes it possible to determine the parameters of the rigidity spectrum of CR variation and anisotropy in interplanetary space, as well as the changes of the planetary system of geomagnetic cuto rigidities for every hour of observation. It is shown that at some instances a high degree of anisotropy is observed (the amplitude of the fundamental and second spherical harmonics is 10%-25% and 5%, respectively). A bidirectional anisotropy, which is characterized by a deficiency of particles with large pitch angles, was observed after the passage of regions with an increased interplanetary magnetic field (IMF) strength during the phase of solar wind velocity decrease when the Earth entered the IMF structure either with the field orientation normal to Parker's spiral or with the field polarity opposite with respect to the background polarity. Maximum values of the fundamental spherical harmonic amplitudes are observed in the phase of CR intensity decrease. The phase of the fundamental harmonic is orthogonal to the IMF vector. The variations have a very hard spectrum, and the spectral index is 0.4-0.5 in the main phase of the Forbush eect. A modulation mechanism is suggested, which permits the results to be explained.