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Paper: Surface Flows From Magnetograms
Volume: 383, Subsurface and Atmospheric Influences on Solar Activity
Page: 19
Authors: Welsch, B.T.; Fisher, G.H.
Abstract: Estimates of velocities from time series of photospheric and/or chromospheric vector magnetograms can be used to determine fluxes of magnetic energy (the Poynting flux) and helicity across the magnetogram layer, and to provide time-dependent boundary conditions for data-driven simulations of the solar atmosphere above this layer. Velocity components perpendicular to the magnetic field are necessary both to compute these transport rates and to derive model boundary conditions. Here, we discuss some possible approaches to estimating perpendicular flows from magnetograms. Since Doppler shifts contain contributions from flows parallel to the magnetic field, perpendicular velocities are not generally recoverable from Doppler shifts alone. The induction equation’s vertical component relates evolution in Bz to the perpendicular flow field, but has a finite null space, meaning some “null” flows, e.g., motions along contours of normal field, do not affect Bz. Consequently, additional information is required to accurately specify the perpendicular flow field. Tracking methods, which analyze tBz in a neighborhood, have a long heritage, but other approaches have recently been developed. In a recent paper, several such techniques were tested using synthetic magnetograms from MHD simulations. Here, we use the same test data to characterize: 1) the ability of the induction equation’s normal component, by itself, to estimate flows; and 2) a tracking method’s ability to recover flow components that are perpendicular to B and parallel to contours of Bz.
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