Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Surface Flows From Magnetograms   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 $B_z$ 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 $B_z$.
Consequently, additional information is required to accurately specify the
perpendicular flow field. Tracking methods, which analyze $\partial_t B_z$ 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 $\mathbf{B}$ and parallel to contours of
$B_z$. This work has been supported by NASA Heliophysics Theory grant
NNG05G144G.

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The categories of this paper are: chem-ph
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The categories of this paper are: patt-sol
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The categories of this paper are: plasm-ph
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The categories of this paper are: mtrl-th
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The categories of this paper are: hep-ex
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The categories of this paper are: cond-mat
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The categories of this paper are: hep-lat
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The categories of this paper are: q-alg
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The categories of this paper are: q-bio
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The categories of this paper are: funct-an
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The categories of this paper are: ao-sci
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The categories of this paper are: nucl-th
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The categories of this paper are: econ
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The categories of this paper are: math
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The categories of this paper are: dg-ga
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The categories of this paper are: nlin