Ulysses Observations of Short-Period (<30 days) Modulation in Galactic Cosmic Rays

Earlier a frequency analysis was carried out of the short--period GCR modulation as seen by Ulysses (EPAC, KET) for the time period from December 1993 to May 1994 (Blake et al. GRL 24, 671, 1997). During the five month period, the modulation of GCR protons with E>230 MeV was not the usual 26 days but more than 29 days, and the CIR--associated ∼MeV ions did not exhibit this period shift. Using data from the Energetic Particle Composition (EPAC) instrument on Ulysses, we have searched for other intervals in which the short period modulation of the GCRs deviates from its average value of ∼26 days. The data examined in this study were acquired between January 1992 and December 2001. Using more than one algorithm, we have derived frequency spectra from the data and have compared the results. We have also examined the behavior of the CIR--associated ∼MeV He flux from EPAC during the same time periods. Finally, we have looked for periodicity in the ratios of GCR proton fluxes from the 4 telescopes in the EPAC detector. We discuss the implications of the combined results from each of these analyses. This work was supported at The Aerospace Corporation by JPL under contract 958748 and at MPAe by the Max--Planck--Gesellschaft zur Förderung der Wissenschaften e.V.