Astronomy and Astrophysics – Astrophysics
Scientific paper
Dec 2009
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2009agufmsh51a1264c&link_type=abstract
American Geophysical Union, Fall Meeting 2009, abstract #SH51A-1264
Astronomy and Astrophysics
Astrophysics
[7524] Solar Physics, Astrophysics, And Astronomy / Magnetic Fields, [7529] Solar Physics, Astrophysics, And Astronomy / Photosphere
Scientific paper
Much work is being carried out to calculate magnetic energy flux through the photosphere using observational data. The theoretical underpinning of these calculations is the determination of the Poynting flux S = (c/4\pi)E x B in terms of observed quantities. The standard approach is to use the ideal MHD equation, E + v x B, which leads to S = (1/4 \pi)B^2 v. B and v are determined using vector magnetogram and Doppler data, respectively. These remote sensing techniques, however, do not directly measure electric or magnetic fields. The formulation of the calculations currently carried out make a number of unrealistic fundamental assumptions that render them irrelevant to photospheric signatures. In particular, the calculations do not include the physical properties of the plasma motion or magnetic field dynamics in the photosphere. One basic implicit assumption is that the magnetic field is coherent and well resolved. The calculated results cannot account for contributions from incoherent, unresolved magnetic field features that emerge and overestimate the observable effects of emerging magnetic energy. The second key assumption is the neglect of the rapid increase in the optical transparency of the atmosphere above the photosphere. We construct a model incorporating the incoherence length scale L_i relative to the instrumental pixel size L_p and incoherence time scale, tau_i. We construct model line profiles relating detectable magnetic energy flux to the degree of incoherence in space and time. It is shown that unless all the magnetic energy appears as coherent features throughout an observed region, the above assumptions fail and the Doppler velocity v is uncorrelated with the speed at which magnetic energy B2/8π is transported through the photosphere. This is traced to the fact that the equation of motion for the photospheric plasmas is not taken into account and the ideal MHD equation does not constrain v_parallel.
Chen Jiahua
Kunkel Valbona
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