Amplitude evolution and rigidity dependence of the 26‐day recurrent cosmic ray decreases: COSPIN/KET results

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.