HAO Colloquium - Tom Berger, University of Colorado

Blending of Ground-based and Space-based Magnetograms: Applications to Solar Wind Modeling and L5 Mission Studies

Solar magnetogram data are the primary inputs to models of solar wind transport in the heliosphere. For example, the WSA/Enlil model commonly used for solar wind and CME transport predictions by operational space weather forecast offices relies on NSO/Global Oscillations Network Group (GONG) solar magnetic field measurements to estimate coronal flux tube expansion factors that determine the boundary conditions for the model. But because all current magnetograms are limited to the Sun-Earth line-of-sight, the full-Sun magnetic field is never accurately determined; so-called “synoptic maps” of the solar sphere are temporal averages from a single viewpoint that are at best rough estimates of the solar magnetic field. This has led to calls for magnetogram measurements off of the Sun-Earth line, for example at the L5 Lagrangian point, in order to increase the data available for synoptic map construction. However current plans for L1 missions do not include magnetogram instrumentation, based on the assertion that L5 data can be combined "accurately enough” with ground-based data to improve synoptic maps. In this seminar we discuss the technical challenges of cross-calibrating and combining magnetogram data from different instruments. We use magnetogram data from GONG and SDO/HMI to demonstrate typical issues in magnetogram cross-calibration and to show that unavoidable errors in polar field measurements lead to errors in the WSA/Enlil solar wind predictions, for example the unexpected High-Speed-Stream events of October 2016. Our results confirm that combining magnetogram data from different instruments, even with the same solar view, is very difficult and that cross-calibration of different instruments, with different spatial and spectral resolutions, over varying flux levels and view angles is impossible. The implication is that an accurate measurement of the true full-Sun magnetic field can only be achieved by identical instruments at multiple locations around the Sun, including out-of-the-ecliptic locations that can view the polar regions. A corollary conclusion is that combining space-based L5 magnetogram data with ground-based data to construct an improved synoptic map is likely not feasible. 


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Wednesday, November 29, 2017 - 1:30pm to 2:30pm