Modulation Near Solar Maximum at High Solar Latitudes: Observations From the Ulysses Cospin High Energy Telescope

Abstract

The three-dimensional structure of the solar maximum modulation of cosmic rays in the heliosphere can be studied for the first time by comparing observations from Ulysses at high solar latitudes to those from in-ecliptic spacecraft, such as IMP-8. Observations through mid-2000 show that changes in modulation remain well correlated at Earth and Ulysses up to latitudes of ∼60° south. The observed changes seem to be best correlated with changes in the inclination of the heliospheric current sheet. The spectral index of the proton spectra at energies <100 MeV in the ecliptic and at high latitudes remain roughly consistent with the T ⁺¹ spectrum expected from modulation models, while the spectral index of the helium spectrum at both locations has changed smoothly from the flat or even negative index spectra characteristic of anomalous component fluxes toward the T ⁺¹ galactic spectrum with increasing modulation. Intensities near the equator and at high latitude remain nearly equal, and latitudinal gradients for nucleonic cosmic rays thus remain small (<1% deg^-1) at solar maximum. In the most recent data fluxes of protons and helium with energies less than ∼100 MeV nucl^-1 measured by Ulysses are smaller than those measured at IMP-8, suggesting that the gradients may have switched to become negative toward the poles even before a clear reversal of polarity for the solar magnetic dipole has been completed.