Forbush decreases of cosmic rays: Energy dependence of the recovery phase

Abstract

Cause and general shape of Forbush decreases of cosmic rays are relatively well understood, however, the knowledge of their recovery times remains rather poor. Earlier results of theoretical and fragmentary statistical studies are in disagreement whether the recovery time does or does not depend on the energy of cosmic rays. A thorough empirical study of the recovery phase of strong isolated Forbush decreases is presented here, based on the ground based data from the World Neutron Monitor Network since 1964 and three ground based muon telescopes since 1973. In total 39 strong Forbush decreases, suitable for the analysis, have been identified for the period 1964–2006, 24 of them depicting a clear energy dependence of the recovery time and 15 consistent with no energy dependence. All analyzed Forbush decreases with magnitudes exceeding 10% demonstrate an energy dependence of the recovery time, while smaller events can be of either type. No apparent relation between the occurrence of energy dependent/independent recovery and the IMF polarity has been found. This result provides an observational constraint for more detailed modeling of the propagation of interplanetary transients and their dynamic effects on cosmic ray transport.