Cosmic Ray Observations from the COSPIN High Energy Telescope (HET) during Ulysses' Current Fast Latitude Scan

Details Cosmic Ray Observations from the COSPIN High Energy Telescope (HET) during Ulysses' Current Fast Latitude Scan Cosmic Ray Observations from the COSPIN High Energy Telescope (HET) during Ulysses' Current Fast Latitude Scan

Dec 2007

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2007agufmsh33a1094m&link_type=abstract

American Geophysical Union, Fall Meeting 2007, abstract #SH33A-1094

Physics

7514 Energetic Particles (2114), 7536 Solar Activity Cycle (2162), 7984 Space Radiation Environment

Scientific paper

Ulysses reached its maximum southern heliographic latitude in February, 2007 and is now performing its third fast latitude scan. This scan will cover all latitudes between 79.7° S and 79.7° N at radii between 2.35 and 1.39 AU within 1 year, a time short compared to significant evolution of the solar activity cycle, which is now near its minimum phase. During this solar minimum the solar dipole magnetic field is opposite that observed during the first solar minimum fast latitude scan in 1994-95, a fact which is expected to lead to observable differences in the latitudinal structure of modulated cosmic ray intensities. Measurements of protons, helium, and heavy ions during the recent slow latitude scan, when Ulysses rose from near the ecliptic to maximum southern latitude between 2004 and 2007, a period of declining solar activity, revealed no measurable latitude gradients in HET observations above a threshold (for protons) of about 35 MeV/n. Because of its short duration and restricted radial range, observations during the fast latitude scan provide the best chance for a definitive measure of latitudinal gradients in the cosmic intensity through comparison of Ulysses observations with observations from near-Earth spacecraft, fixed in radius and latitude. However the recent loss of the IMP-8 spacecraft, which had served as a 1 AU baseline for COSPIN HET cosmic ray measurements since the launch of Ulysses, has presented a difficulty for measurement of the small gradients that seem likely given previous observations and the lack of clearly visible latitude effects in the Ulysses cosmic ray data alone. We will present results for heavy ions from the fast latitude scan through the most recent data available, using measurements from Ulysses compared to ACE/CRIS as the 1 AU baseline. For protons and helium, we will present Ulysses observations and evaluate available possible 1 AU baselines, including ERNE data from SOHO, STEREO HET observations, and possible extrapolations from available lower or higher energy measurements which can be used to define the modulation parameter at 1 AU as a function of time, and thus the expected flux variations in Ulysses' energy range at 1 AU. (This work was supported in part by NASA/JPL Contract 1247101)

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