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A Study Of The Viscous Interaction Between The Solar Wind And Earth's Magnetosphere Using An MHD Simulation

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A Study Of The Viscous Interaction Between The Solar Wind And Earth's Magnetosphere Using An MHD Simulation

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Title: A Study Of The Viscous Interaction Between The Solar Wind And Earth's Magnetosphere Using An MHD Simulation
Author: Bruntz, Robert Jeffrey
Abstract: The solar wind interacts with Earth's magnetosphere largely through magnetic reconnection and a “viscous-like” interaction that is not fully understood. The ionospheric cross-polar cap potential (ΦPC) component due to reconnection (ΦR) is typically much larger than the viscous component (ΦV) and very dynamic, making detailed studies of the viscous potential difficult. We used the Lyon-Fedder-Mobarry (LFM) magnetohydrodynamic (MHD) simulation to study the viscous potential by running LFM for a variety of solar wind density and velocity values and ionospheric Pedersen conductance (ΣP) values, but no solar wind magnetic field, so that ΦPC was entirely due to the viscous interaction. We found that ΦV increased with solar wind density, scaling as n0.439 (n in cm-3), and ΦV increased with solar wind velocity, scaling as V1.33 (V in km s-1); these results were combined to create a formula for ΦV in LFM, using a ΣP value that produces realistic potentials: ΦV = (0.00431)n0.439V1.33 (in kV), which matches simulation results very well. ΦV also varied inversely with ΣP, as predicted by previous theory. The form of this formula is similar to results from the Newell et al. [2008] empirical study, which tested a list of viscous coupling functions and found that n1/2V2 worked best (but did not create a formula to predict potentials, so actual viscous potential values could not be compared). The Bruntz et al. formula was also compared to LFM results from a run with real solar wind input, from the Whole Heliosphere Interval (WHI), which lasted from 20 March to 16 April 2008. LFM was first run with the full solar wind from the WHI, then with the same solar wind but zero interplanetary magnetic field (IMF), which meant that ΦPC = ΦV for that run. These runs were performed with the empirical ionospheric solver, using the average F10.7 flux value from the WHI as input. This empirical ionosphere is known to produce potentials that are higher than observations, so the output was scaled down to match the range of the Bruntz et al. formula with a scaling factor γ = 1.542, which was found from 11 steady periods in the WHI. Those same periods were also used to calibrate the Newell et al. viscous scaling factor, turning it into a predictive formula: ΦV = (6.39×10-5)n1/2V2 (in kV). Both viscous potential formulas were compared to ΦPC from the zero-IMF run, producing ΦV values that were very close to the LFM ΦPC values, differing in opposite ways in some places, but with essentially identical correlation coefficients. We also used the γ factor to scale ΦPC from the full-IMF LFM run down, then compared it to ΦPC from the Weimer05 empirical model. The two matched well in the higher ΦPC values, but the Weimer05 ΦPC values reached a minimum “floor” value, while the LFM ΦPC has no such floor, and so dropped much lower in some places. The fact that γ scaled the full-IMF LFM down to match the Weimer05 values, even though γ was derived from very different runs and conditions, is interpreted to support the idea that the cause of high LFM potentials is in the ionospheric conductivity, since γ is derived from the higher-conductivity-based Bruntz et al. formula.
URI: http://hdl.handle.net/10106/11084
Date: 2012-07-25

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