Monday

A fraction of the magnetic flux threading the solar photosphere extends to sufficient altitude that it is dragged out by the solar wind. Understanding this open solar flux (OSF) component is central to space weather, as the OSF forms the heliosphere and magnetically connects the Sun to the planets, allowing the transfer of energy and channelling of energetic particles. Quantification of OSF is also a key means of verifying global coronal models. However, OSF estimates derived from extrapolating the magnetic field from photospheric observations consistently underestimate the OSF derived from heliospheric observations by a factor of two or more. Heliospheric estimates of OSF are made using in situ magnetic field measurements. This requires both an assumption of latitudinal invariance in OSF and that integrating/averaging can filter out structures that are not immediately connected to the Sun, such as local magnetic field inversions or “switchbacks”. We use in situ electron and magnetic data to relax this last assumption, by determining the topology of the magnetic field and directly correcting for switchbacks, which would otherwise contribute to the overestimation of the OSF from heliospheric observations. We apply this methodology to the interval 1994-2020 and combine Wind and ACE to quantify the uncertainty in the resulting OSF estimates. Switchbacks cannot explain the discrepancy between heliospheric and photospheric observations, with the best estimate of OSF still, on average, a factor 1.7 higher than the values extrapolated from photospheric observations.