Pitch-Angle features in cosmic rays in advance of severe magnetic storms: neutron monitor observations

The behavior of cosmic rays in advance of the severe geomagnetic storms of 1978-1982 (from the Gosling list of severe storms) is analyzed on the basis of the cosmic ray anisotropy and pitch angle distribution derived from neutron monitor observations. This analysis aims to reveal cosmic ray predictive effects (loss cone and pre-increase) before arrival of the shock and solar wind disturbed region. Essentially in all cases the changes in cosmic rays, in particular their anisotropy, start well before the beginning of major magnetic storms, which do not always coincide with the time of shock arrival; the severe magnetic storm may start later (up to 20 hours after), depending on the structure, orientation and magnetic topology of solar ejecta. We show examples for which the characteristic changes in CR anisotropy and pitch angle distribution (as a narrow decrease oriented along the magnetic force line) are revealed. These loss cone effects are predictors of strong geomagnetic disturbances.