Cosmic ray transverse gradient for a Hale Cycle

Abstract

We have used neutron monitor data obtained at Deep River and Huancayo as well as the vertical underground muon telescope data obtained at Embudo to compute the cosmic ray transverse gradient (Gθ) for the 1965 to 1990 period. The computed mean value of Gθ, for a given qA epoch, changes sign with the change in the solar magnetic polarity in accordance with the predictions of some contemporary hypotheses. Also, we find that the magnitude of Gθ undergoes a significant time variation during a given polarity epoch. It has the largest value near the solar polar field reversal epoch boundaries, when magnetic polarity in the solar northern hemisphere is positive (qA>0). For each of the three solar polar field reversals, the change in the magnitude of Gθ is steeper across the epoch boundaries. There is a good correlation between Gθ and the tilt angle of the heliospheric neutral current sheet (HNCS). Our results are in agreement with the reported piecemeal measurements of the latitudinal gradient made by spacecrafts, at different times, at lower primary energies, at far off distances from Earth's orbit. As such, their global significance is appreciable. Even so, it is not clear yet how these findings reflect upon the drift hypothesis in its present form. However, they do affirm the important role played by the tilt angle of HNCS in the modulation processes.