A generalized Fisk-type HMF: implications of spatially dependent photospheric differential rotation

Abstract

The existence of a Fisk-type heliospheric magnetic field (HMF) has been debated ever since Fisk proposed an alternative to the traditional view of the HMF first proposed by Parker. Several modifications of the original Fisk field model have been published in the past, for example, the Schwadron field and the Fisk-Parker hybrid HMF model. This study presents a new generalized Fisk HMF model that implements for the first time a spatially dependent differential rotation rate of the photosphere, by mapping magnetic field lines from the solar wind source surface to the photosphere. Data analysis methods of Forsyth et al. are used to search for a signature of a Fisk-type field during solar minimum conditions, using the magnetic field data from the first solar orbit of the Ulysses spacecraft. The new generalized Fisk field agrees better with the observed magnetic field winding angle than a standard Parker field during the majority of intervals scanned by Ulysses