Coherence of charged particle oscillations in the heliosphere (f≈5 μHz):: Implications for a solar modulation source

Time series analysis of low-energy electron and high-energy proton cosmic ray fluxes, both measured on the Ulysses spacecraft, shows that they are quite coherent in some frequency bands in the energy range 0-6 muHz, especially near 5 muHz. Interplanetary particle fluxes with very different kinematic properties (e.g., gyrofrequencies) are modulated coherently in the heliosphere at discrete frequencies; one specific example discussed here has a period of similar to2.2 days. The analyzed data span the interval January 1, 1993, to April 20, 1994, when Ulysses traveled between 22degrees and 60degrees south heliographic latitude and was similar to4 AU from the Sun. Frequencies with high coherence appear to be associated either with solar rotation or with discrete solar modal frequencies. The frequency at 5.26 muHz is tentatively identified as a zonal harmonic mode with l = 2 and m = 0.