Modulation over a 22-year cosmic ray cycle: On the tilt angles of the heliospheric current sheet

The 'tilt angle' of the wavy current sheet in the heliosphere has been used very successfully as a proxy for solar activity in cosmic ray (CR) modulation models with particle drifts included. The "Hoeksema tilt angles alpha" are calculated using two potential field methods: (1) The classic 'line-of-sight' computation and (2) the more recent 'radial' photospheric field computation. The question of which set of 'tilt angles' is the more appropriate to use in modulation models is. addressed by utilizing them in a comprehensive time-dependent drift model applied to the 22-year modulation cycle of 1.2 GeV cosmic ray electrons. We found that the 'classic' alpha are better suited than the 'new' alpha as a proxy for solar activity for periods of decreasing alpha and vice versa for periods of increasing alpha. By comparing the model with data it was shown that the set of alpha with the smallest rate of change over a period of decreasing or increasing solar activity provides the best compatibility with CR observations. (C) 2003 COSPAR. Published by Elsevier Ltd. All rights reserved.