Modelling galactic cosmic ray drifts in the presence of a Fisk-type heliospheric magnetic field

Abstract

Drift effects due to gradients and curvature in the heliospheric magnetic field (HMF) play a role in the modulation of galactic cosmic rays, or GCRs (e.g., Isenberg & Jokipii, 1979; Jokipii & Thomas, 1981). The only HMF model thus far for which drift effects have self-consistently been implemented in GCR modulation studies is the model of Parker (1958a). Other HMF models have been proposed; these include the models of Fisk (1996; see also Zurbuchen et al., 1997) and Schwadron (2002; see also Schwadron & McComas, 2003), as well as the Fisk-Parker hybrid model of Burger & Hitge (2004; see also Burger et al., 2008) and the Schwadron-Parker hybrid model of Hitge & Burger (2010). The drift velocity profiles associated with the Fisk-Parker hybrid HMF (Burger et al., 2008) and Schwadron-Parker hybrid HMF (Hitge & Burger, 2010) are shown to differ considerably from what is known for the Parker HMF, especially in the weakscattering limit. Differences between the heliospheric current sheet drift models of Burger (2012) and Engelbrecht et al. (2019) are also discussed. For comparatively high-rigidity (P = 1 GV) GCR protons, Fisk-type drift effects are seen to remain when drift reduction due to turbulence is taken into account (Engelbrecht et al., 2017). A Schwadron-Parker hybrid HMF signature in GCR intensities computed from the ab initio, steady-state, three-dimensional modulation code introduced by Engelbrecht & Burger (2015b) is found, and it is shown that the implementation of HMF-specific drift effects leads to intensities which differ from those calculated when the Parkerian drift effects described by Burger (2012) are used in conjunction with diffusion effects due to the Schwadron-Parker hybrid HMF. Azimuthal variations due to this HMF model are found to be of the order of several percent, similar to the recurrent amplitudes calculated from Ulysses observations by Zhang (1997).