Excess open solar magnetic flux from satellite data: 1. Analysis of the third perihelion Ulysses pass

Abstract

We use the third perihelion pass by the Ulysses spacecraft to illustrate and investigate the “flux excess” effect, whereby open solar flux estimates from spacecraft increase with increasing heliocentric distance. We analyze the potential effects of small‐scale structure in the heliospheric field (giving fluctuations in the radial component on timescales smaller than 1 h) and kinematic time‐of‐flight effects of longitudinal structure in the solar wind flow. We show that the flux excess is explained by neither very small‐scale structure (timescales < 1 h) nor by the kinematic “bunching effect” on spacecraft sampling. The observed flux excesses is, however, well explained by the kinematic effect of larger‐scale (>1 day) solar wind speed variations on the frozen‐in heliospheric field. We show that averaging over an interval T (that is long enough to eliminate structure originating in the heliosphere yet small enough to avoid cancelling opposite polarity radial field that originates from genuine sector structure in the coronal source field) is only an approximately valid way of allowing for these effects and does not adequately explain or account for differences between the streamer belt and the polar coronal holes.