Decorrelation Times of Photospheric Fields and Flows

Details Decorrelation Times of Photospheric Fields and Flows Decorrelation Times of Photospheric Fields and Flows

Dec 2011

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2011agufmsh31a1989w&link_type=abstract

American Geophysical Union, Fall Meeting 2011, abstract #SH31A-1989

Astronomy and Astrophysics

Astrophysics

[7524] Solar Physics, Astrophysics, And Astronomy / Magnetic Fields, [7529] Solar Physics, Astrophysics, And Astronomy / Photosphere, [7594] Solar Physics, Astrophysics, And Astronomy / Instruments And Techniques

Scientific paper

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 structure as a function of 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 Δ t between magnetogram pairs tracked; spatial binning applied to the magnetograms; and windowing parameter σ used in FLCT. In addition, we tracked both boxcar-averaged and unaveraged magnetograms. Because flow structures are thought to vary over a range of spatial and temporal scales in the photospheric plasma (including unresolved scales), we suggest that estimated flows necessarily represent a local average of the flow pattern over space and time, and argue that flow decorrelation times should be at least as long as Δ t to meaningfully estimate plasma velocities. When Δ t exceeds the flow decorrelation time, then displacements inferred from tracking represent the average velocity over more than one flow lifetime. We also analyze decorrelation times of flow components, divergences, and curls as functions of spatial scale and magnetic field strength.

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