Energy Dependence of Forbush Decrease Recovery

I. Usoskin1, T. Jamsen1, G. Kovaltsov2, O. Gladysheva2, I. Braun3, J.R. Hoerandel4, S. Starodubtsev5

1 University of Oulu, Finland
2 Ioffe Physical-Technical Institute, St. Petersburg, Russia
3 Max-Planck-Institut fuer Kernphysik, Heidelberg, Germany
4 Radboud University, Nijmegen, The Netherlands
5 Shafer Institute of Cosmophysical Research and Aeronomy of RAS SB, Yakutsk, Russia

Although the nature of Forbush decreases of cosmic rays is generally understood, physical modeling of their recovery phase remains poor. Earlier results of theoretical and fragmentary statistical studies are inconsistent and lead to contradictory conclusions: some studies forecast that the recovery rate is the same for cosmic rays of all energies, while others suggest that more energetic cosmic rays recover faster.

Here we present a statistical study of the recovery phase for 39 isolated strong Forbush decreases for the last 40 years. We use data from the World Neutron Monitor Network since 1964 and ground based muon telescopes since 1973. Clear energy dependence of the recovery rate is found in 24 cases (60%) while the remaining 15 events are consistent with no energy dependence. The recovery rate depicts strong energy dependence for all analyzed strong Forbush decreases, with the magnitude exceeding 10%, while smaller events can be of either type. No apparent relation between the occurrence of energy dependent/independent recovery and the IMF polarity is 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.