Brian T. Welsch, Kanya Kusano, Tetsuya T. Yamamoto
We use autocorrelation to investigate evolution in flow fields inferred by
applying Fourier Local Correlation Tracking (FLCT) to a sequence of
high-resolution (0.3"), high-cadence (~2 min) line-of-sight magnetograms of
NOAA AR 10930 recorded by SOT/NFI aboard the Hinode satellite over 12-13
December 2006. To baseline the timescales of flow evolution, we also
autocorrelated the magnetograms, at several spatial binnings, to characterize
the lifetimes of active region magnetic structures versus spatial scale.
Autocorrelation of flow maps can be used to optimize tracking parameters, to
understand tracking algorithms' susceptibility to noise, and to estimate flow
lifetimes. Tracking parameters varied include: time interval \Delta t between
magnetogram pairs tracked; spatial binning applied to the magnetograms; and
windowing parameter \sigma used in FLCT. Flow structures vary over a range of
spatial and temporal scales (including unresolved scales), so tracked flows
represent a local average of the flow over a particular range of space and
time. We define flow lifetime to be the flow decorrelation time, \tau. For
\Delta t > \tau, tracking results represent the average velocity over one or
more flow lifetimes. We analyze lifetimes of flow components, divergences, and
curls as functions of magnetic field strength and spatial scale. We find a
significant trend of increasing lifetimes of flow components, divergences, and
curls with field strength, consistent with Lorentz forces partially governing
flows in the active photosphere, as well as strong trends of increasing flow
lifetime and decreasing magnitudes with increases in both spatial scale and
\Delta t.
View original:
http://arxiv.org/abs/1110.6117
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