Latitudinal extent of the heliospheric current sheet and modulation of galactic cosmic rays

Abstract

The possible relation between changes in the maximum heliographic latitude of the heliospheric current sheet during solar maximum and cosmic ray modulation has been investigated. A causal relation between the intensity of galactic cosmic rays and the inclination or maximum latitude of the current sheet is an important feature of the gradient drift model. The maximum latitudinal extent in the northern and southern solar hemispheres during 1976–1982 was determined from current sheet contours at the solar source surface (2.5 solar radii) derived by Stanford University by extrapolating the photospheric magnetic fields into the corona using potential theory. The maxima in the north and south were found to be reasonably symmetrical about the solar equator except for an interval of about one year near sunspot maximum when the sun's polar caps were reversing. It was therefore possible to take the difference between the maximum latitudes in the north and south as a measure of current sheet inclination. The latitude difference so derived showed the evolution of the current sheet to high latitudes near sunspot maximum with a subsequent return to lower inclinations, but with reversed polarity, following maximum. In addition to this general behavior, a number of episodes occurred in which the latitude difference changed significantly over intervals of one year or less. When the changing latitude differences from 1976 through 1982 were compared with Deep River neutron monitor observations (corresponding to cosmic rays with E ≥ 1 GeV), a significant correlation was found. Changes in maximum latitude also correlate well with cosmic ray measurements being made at lower energies (E > 200 MeV) by Pioneer 10 between 10 and 30 AU. The correlation is improved by introducing a delay corresponding to the time required for the solar wind to propagate from the sun to the spacecraft. The sensitivity of the cosmic ray intensity to changes in the current sheet is significantly different before and after the reversal in the polar cap and heliospheric magnetic field, the sensitivity being enhanced after the reversal when the field is inward in the north. The existence of this correlation and its sensitivity to the polarity of the solar heliospheric field provide significant support for the gradient drift model of cosmic ray modulation.