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

Abstract

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 μHz, especially near 5 μHz. 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 ~2.2 days. The analyzed data span the interval January 1, 1993, to April 20, 1994, when Ulysses traveled between 22° and 60° south heliographic latitude and was ~4 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 μHz is tentatively identified as a zonal harmonic mode with l=2 and m=0.