Searching for cosmic ray latitude gradients with the Ulysses COSPIN High Energy Telescope (HET) during Ulysses' fast latitude scan in 2007-2008
Abstract
Between February 2007 and January 2008, Ulysses traversed all heliographic latitudes between 79.7° S and 79.7° N at radii between 2.35 and 1.39 AU. This so-called fast latitude scan (FLS) provided a snapshot of the latitudinal intensity structure of modulated galactic cosmic rays in the inner heliosphere in a time short compared to significant evolution of the solar activity cycle, which is now near its minimum phase. Previous fast latitude scans were performed near solar maximum in 2000-2001, and near solar minimum in 1994-1995, when the sign of the solar dipole was opposite to that observed during the current solar minimum. In 1994-1995, small but clear increases in intensity from the equator towards the poles were measured by the HET for the integral intensity of cosmic rays >92 MeV and for helium above about 35 MeV/n, consistent in sign, if not in magnitude, with the positive latitudinal gradients predicted for that sign of the solar dipole by then-current modulation models that included the effects of gradient and curvature drifts. The opposite polarity of the solar dipole in this solar minimum was expected to lead to gradients of opposite sign, with intensities decreasing towards the poles. However in a preliminary report of our results through October 2007 (McKibben et al., AGU Fall Mtg, 2007), based on comparison of measurements of protons, helium, and heavy ions from the Ulysses HET with similar measurements near 1 AU and in the ecliptic from the SOHO/EPHIN and ACE/CRIS instruments no gradients of either sign were apparent. In this report we extend our results to include the complete 2007-2008 fast latitude scan, perform more extensive analysis to search for evidence of even small gradients, and discuss the significance of our results. This work was supported in part by NASA Contract 1247101.
- Publication:
-
37th COSPAR Scientific Assembly
- Pub Date:
- 2008
- Bibcode:
- 2008cosp...37.1976M