The solar wind speed - expansion factor (v - fs) relationship at the inner boundary (18 R⊙) of the heliosphere
Abstract
The accuracy of data-driven magnetohydrodynamics (MHD) models depends on accurate boundary conditions specified at the inner heliosphere. However, all of the MHD parameters (B, v, ρ, T) close to the Sun are not measurable at the present time,except the total magnetic field (|B|) at the photosphere. The solar wind speed (v), which is probably most relevant to space weather forecasting, is often modeled by the standard Wang-Sheely-Arge (WSA) empirical formula. The WSA formula is based on an inverse relationship between the solar wind speed measured at 1 AU and the magnetic field expansion factor estimated at 2.5 solar radii (R⊙ ), with the following generic form: v = v1 +v2 fs -α (where v is the solar wind speed at 18 R⊙ , fs is the magnetic field expansion factor, and v1, v2, and α are three free parameters to be determined). Because it uses the solar wind speed at 1 AU, the formula ignores the transport of solar wind in the heliosphere. While the WSA formula uses "source projection" to account for the transport of the solar wind, it does not treat the solar wind as plasma. The purpose of this study is to rectify this omission by using numerical MHD simulations to find the optimal set of the free parameters that relate the magnetic properties at the source surface to the plasma parameters at 1 AU. In addition to the expansion factor, conservation of mass (ρv), magnetic flux (r2B), and total pressure along the stream line are assumed to obtain a complete set of MHD parameters at 18 R⊙ . These parameters are used as the inner boundary conditions of our global three-dimensional MHD (G3DMHD) code to simulate solar wind plasma and field parameters out to ~1 AU. The simulation results are compared with the in situ data from Wind to assess the accuracy. Such a procedure is repeated (880 times) to cover the three parameter regimes (100 < v1 < 350 km/s; 250 < v2 < 700 km/s; and 0.2 < α < 0.9) to find the optimal set. The simulation is performed for the period of CR2082. It is found that v = 189 + 679 fs -0.7 is the best formula to relate the solar wind speed at 18 R⊙ to the expansion factor. Strictly speaking, this formula applies only to periods around solar minimum.
* Work of CCW was partially supported by the Chief of Naval Research.- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2019
- Bibcode:
- 2019AGUFMSH41F3329L
- Keywords:
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- 7599 General or miscellaneous;
- SOLAR PHYSICS;
- ASTROPHYSICS;
- AND ASTRONOMY;
- 7899 General or miscellaneous;
- SPACE PLASMA PHYSICS;
- 7999 General or miscellaneous;
- SPACE WEATHER