Cosmic Rays Near the Equator and to >50° S Latitude in 2004-2006; Observations from the COSPIN High Energy Telescope on Ulysses and the CRNC Telescope on IMP- 8.

Cosmic ray modulation models that include gradient and curvature drifts predict that, for the present sign of the heliospheric magnetic field, the modulated cosmic ray intensity should decrease from the equator towards the poles near solar minimum, corresponding to negative latitude gradients. In the previous high latitude phases of the Ulysses mission near solar minimum in 1994-95, positive latitude gradients were found, consistent with predictions for the sign of the heliospheric field observed at that time. Also, quasi-periodic intensity modulations produced by CIRs in the current sheet were observed to persist to near-polar latitudes. Since its aphelion and distant Jupiter flyby near the heliospheric equator in 2004 Ulysses has been climbing steadily in latitude. Ulysses remained in the region swept by the heliospheric current sheet through 2005. It exited the current sheet region in early 2006 at a latitude of about 38° S. By July 2006 it had reached a latitude of >50° S, well above the current sheet. In the same period, solar activity has steadily decreased, though a few large solar energetic particle/CME events continued to occur through 2005. Since early 2006, no significant solar events have disrupted measurement of modulated cosmic ray intensities, which continue rising towards solar minimum levels. With measurements of >35 MeV/n protons and helium from the COSPIN High Energy Telescope on Ulysses compared to observations in nearly identical energy ranges from the IMP-8 CRNC experiment near Earth, we have been attempting to measure the magnitude and sign of latitude gradients as Ulysses rises in latitude. In the current sheet region, we find no evidence for latitude gradients in either protons or helium, and we find much smaller intensity variations in response to CIRs than observed in the similar phase of the previous solar cycle. We will report continuing observations to latitudes >50° S relating to our search for latitude gradients and also for evidence of the persistence of intensity modulations produced by CIRs to high latitudes. This work was supported in part by NASA/JPL Contract 1247101.