Comparing Different Methods for Estimating Total Open Heliospheric Magnetic Flux

There are extended periods over the solar cycle where significant discrepancies occur between the observed open magnetic flux (i.e., those based on spacecraft observations) and that determined from coronal models. One explanation for the source of these discrepancies is the magnetic fields in CMEs, which have yet to magnetically disconnect from the Sun. These “closed” flux sources can be included in open flux estimates, because open and closed magnetic field lines are not easily distinguished in spacecraft data. Another possibility is that a portion of the open flux measured by in situ spacecraft originates from the time-dependent evolution of solar magnetic fields that is not captured by static or steady state coronal model solutions. In this research, the total open heliospheric magnetic flux is computed using three different methods and then compared with results obtained using in situ interplanetary magnetic field observations. The first two methods make use of the Potential Field Source Surface (PFSS) model to calculate the total open magnetic flux using as its input: 1) traditional Carrington or diachronic maps and 2) Air Force Data Assimilative Photospheric Flux Transport (ADAPT) model synchronic maps. The diachronic and synchronic photospheric magnetic field maps are derived from magnetograms from the same source, namely the National Solar Observatory (NSO) Kitt Peak Vacuum Telescope (KPVT) and Vector Spectromagnetograph (VSM) magnetographs. The third method involves the use of observationally derived Helium and EUV coronal hole maps overlain on the above mentioned magnetic field maps to compute total open magnetic flux. The results of this work show that alternative approaches using observationally derived coronal holes to compute the open flux match well with what the model derives, especially near solar minimum. Both deviate from the spacecraft data especially near solar maximum. This suggests that the models are determining coronal hole boundaries well, but are unable to capture open flux resulting from the opening and closing of field lines during solar maximum. A primary suspicion also is that spacecraft instruments could be mistaking the field’s tangential component for the radial component due to oscillations in the field lines. Future research will work to filter out the field’s tangential component that could be causing inaccuracies in the observed radial field