Measurements of transverse cosmic ray particle density gradient at high rigidities in the Ulysses era

Transverse cosmic ray gradient at high rigidities can only be determined from diurnal anisotropy data obtained with the neutron monitors and muon detectors based on a unique spinning platform at 1AU, namely, the Earth. This fortunate circumstance has several distinct advantages. Use of high counting rate detectors ensures excellent statistics. The detector performance is checked periodically to ensure stability which makes it possible to obtain long data strings with 100% coverage and reliability, over a wide range of primary rigidities. Very surprisingly, there is an impressive qualitative agreement between measurements made on Earth in the inner heliosphere and those made on board the various spacecrafts in the outer heliosphere, at much lower rigidities, at different short time intervals. This argues for the existence of long lived, spatially extensive, electromagnetic structures in the heliosphere, which affect a very large rigidity range in the galactic cosmic ray spectrum. We describe the results of our analysis of the data obtained with detectors with median rigidity of response (R(m)) spanning the range, 16 GV less than or equal to R(m) less than or equal to 134 GV, over nearly three decades encompassing three epochs each of solar activity maxima, minima, and polar field reversals, including the era of pole-to-pole scan of Sun by the spacecraft Ulysses.