Amplitude evolution and rigidity dependence of the 26-day recurrent cosmic ray decreases: COSPIN/KET results
Kunow, H.; Bothmer, V.; Potgieter, M. S.; Posner, A.; Heber, B.; Ferrando, P.; Müller-Mellin, R.; Raviart, A.; Paizis, C.; Falconi, B.; Marzolla, S.
Italy, Germany, France, South Africa, Switzerland
Abstract
In the time interval extending from July 1992 to July 1994, Ulysses climbed from 10°S heliographic latitude up to over 70°S. During this time lapse, solar minimum conditions were gradually approached, which, in turn, led to stable and long-lasting corotating interaction regions (CIRs). The corotating particle events observed during this period, associated with ~30 registered CIRs, offer a unique opportunity to probe the three-dimensional structures of the heliosphere. In this work we use data from the Cosmic Ray and Solar Particle Investigation Kiel Electron Telescope (COSPIN/KET) instrument on board Ulysses to study the amplitude evolution of the 26-day recurrent cosmic ray decreases, generated by these CIRs, at different energies and derive its rigidity dependence. We find that the amplitude has a maximum around 25°-30° heliolatitude. We also find that the rigidity dependence of both the latitudinal gradient as well as the 26-day variation amplitude show a remarkable similarity. We discuss these observations within the framework of our current understanding of heliospheric phenomena.