Physics
Scientific paper
Sep 2008
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2008espm...12.2.34n&link_type=abstract
"12th European Solar Physics Meeting, Freiburg, Germany, held September, 8-12, 2008. Online at http://espm.kis.uni-freiburg.de/,
Physics
Scientific paper
The solar photosphere is the dynamical interface between the convection zone and the chromosphere. It is compressible, convectively stable, and affected by the overshooting granular flow. The photospheric dynamics must thus be investigated as the continuation of the granular dynamics as it spills over into the stable layers.
We investigate empirically the non-oscillatory small-scale velocity field of the photosphere. We are particularly interested in the temporal and height variations of the dynamics and its topological behavior, i.e. in the evolution of velocity patterns in comparison to the granular intensity patterns.
Our analysis is based on time series of 2D spectra taken with the triple etalon spectrograph TESOS at the VTT on Tenerife. Oscillations were filtered out in the Fourier domain. In a 2D time-series analysis, power spectra demonstrate the rapid decay of the vertical overshoot velocities with height by a factor 2 within less than 300 km above the surface, thus implying a decay of the associated kinetic energy flux density by nearly two orders of magnitude over the same height interval. As expected, this decay of the energy flux is accompanied by a change of the scales in the wavenumber domain. 2D coherence maps quantify the drastic change of the pattern of the velocity field with height: While the continuum layers are still governed by the typical granular-like structuring with small-scale isotropy, the higher layers show elongated patterns of upflow and downflow regions with short fragmentation and reorganization time scales. According to a cross-correlation analysis the extension of the granular upflows into the upper photosphere is a strongly local process, suggesting a burst-like nature of the granular velocity.
Over the scale of the field of view, the velocity field loses its horizontal isotropy with height. This suggests the action of a structural instability of the deeper layers. It is an open question which dynamical processes in the overshoot layers cause these effects. The fragmentation and immediate reorganization of the velocity field of the upper photosphere merit further study.
Hammer Reiner
Nesis Anastasios
Roth Marcel
Schleicher Helmhold
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