LAUSR

A fraction of the magnetic flux which threads the photosphere reaches sufficient coronal altitude to be dragged out by the solar wind and form the heliospheric magnetic field (HMF). Directly measuring this “open solar flux” (OSF) component, however, is difficult. While OSF can be extrapolated from photospheric magnetic field measurements, the most direct method is from in-situ spacecraft measurements of the HMF. The difficultly is unambiguously distinguishing between HMF which connects directly back to the Sun (the OSF) and that which is locally distorted by waves, turbulence and near-Sun reconnection. Suitable temporal filtering of the data can remove such “noise”, but the level of filtering cannot be known a priori and varies with solar cycle, solar wind types, etc. Here, we use the suprathermal electron beam, or “strahl”, to distinguish between different HMF topologies. As strahl moves antisunward on global scales, times when strahl is observed to be moving sunward indicate that the HMF is locally inverted. By subtracting the inverted HMF we compute the OSF without need for arbitrary filtering of the data. We find that the OSF obtained in this manner is slightly larger than the proposed “kinematic correction” based on observed solar wind velocity structure, though in general agreement. Our new OSF estimate agrees with methods based wholly on HMF data, if the data are first used to compute approximately 1-day averages during solar minimum and approximately 3-day averages during solar maximum, stressing the point that the filter method is unreliable because the required characteristics vary.