2013 IR&D Annual Report

Validating CSSS model to Investigate Slow Solar Wind Origins, 15-R8373

Principal Investigators
Bala Poduval
Craig E. DeForest

Inclusive Dates: 02/18/13 – 06/18/13

Background — The existence of the solar wind — the supersonic expansion of corona out into the heliosphere — was established more than a half-century ago. Still, the nuances of the mechanism that gives rise to the two distinct components with differing physical properties, the slow and the fast winds, are not completely understood. The fast wind has been identified as emanating from the coronal holes, but the origin of slow solar wind remains controversial and is the most debated topic in solar physics. Fluctuations in solar wind, termed space weather, triggered by solar transient events cause magnetospheric and ionospheric disturbances leading to disruption and damage to electrical and electronics systems on Earth. Monitoring space weather and making accurate forecasts well ahead of time is required to protect them. WSA-ENLIL, the two-part space weather prediction model, is the state-of-the-art prediction scheme. The semi-empirical WSA (Wang-Sheeley-Arge) model provides an approximate outflow as the inner boundary condition for ENLIL, a sophisticated three-dimensional magnetohydrodynamic (MHD) model that predicts space weather conditions in the near-Earth environment. However, because of discrepancies between the predictions of WSA and the observed solar wind, there are ongoing efforts to improve the inner boundary conditions for ENLIL. The CSSS (Current Sheet Source Surface) model has been shown to perform better than the WSA model and will be a better alternate for driving ENLIL.

Approach — To validate the CSSS model, the predicted solar wind was compared with the observed solar wind near Earth's orbit. For the solar wind predictions, the inverse correlation between solar wind speed and the flux tube expansion (FTE) factor was used and is defined as:
f = (Rsun/Rss)2 * (Br(phot)/{Br(ss))2, where Rsun is the radius of the Sun, Rss is the radius of the source surface, and Br(phot) and Br(ss) are the magnetic field on the photosphere and the source surface. The validated CSSS model was then used to investigate the slow solar wind-pseudostreamer (regions of close magnetic field configuration between open field regions of like polarity) association.

Accomplishments — Flux tube expansion was computed at the source surface and a best-fit curve was obtained, vsw = af2 + bf + c, where vsw is the solar wind speed and f, the flux expansion factor. The coefficients a, b and c are 90.1, -473.2 and 808.2, respectively. The CSSS model (dashed line, left panels) predicts the solar wind with greater accuracy, by nearly a factor of two, than the WSA model (dotted line, right panels) as shown in the illustration, for selected Carrington rotations (CR) as marked. Here the solid line depicts the observed solar wind speed. The correlation coefficients between the observed and predicted speeds are shown inside each panel. Using the validated CSSS model, the slow solar wind-pseudostreamer relationship was studied by inverse mapping the observed slow solar wind to the source surface and obtaining the magnetic configuration of the corresponding region. Results showed about 70 percent of the slow solar wind in the range 350 to 450km/s was found to correspond to pseudostreamers, while about 30 percent was traced to the center of a unipolar open field region, which was unexpected. To pursue this aspect further a proposal has been submitted to NASA.

Graphs of observed solar wind speed in comparison with speed predicted by the CSSS
Observed solar wind speed in comparison with speed predicted by the CSSS (red dashed lines) and PFSS (blue dotted lines) models, using WSO (panel a) and NSO synoptic maps (panel b). The WSA/ENLIL predictions (blue lines) in comparison with observed (OMNI: red lines) solar wind speed is depicted in panel c; see text for details.
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