Relationship between photospheric magnetic fields and coronal activities
Federated Physics Department of NJIT and Rutgers-Newark
Doctor of Philosophy
Gary, Dale E.
Coronal emission comes in two forms, a steady component where the corona is heated to million degrees and a much hotter transient component of solar flares. Both components are known to be related to the evolution of surface magnetic fields. This dissertation studies the evolution of photospheric magnetic fields and flow fields and their relation to the properties of these two coronal emission components.
The key issue in the study of the steady coronal emission is the coronal heating problem: how the corona is heated to millions of degrees while the underlying solar photosphere is only a few thousand degrees. Although there is theoretical and observational evidence to support many aspects of certain heating models, the general agreement is not yet reached. Even the location of the heating source is still under debate. In this dissertation, the correlations between some photospheric magnetic parameters and coronal soft X- ray brightness are statistically evaluated to contribute to resolving the problems of coronal heating. The key findings include: (1) The energy of the Poynting flux is sufficient to heat the corona due to footpoint random motions of magnetic flux tubes. (2) Close correlation is established between coronal brightness and various magnetic parameters. (3) Evolution of 3-D magnetic structure in the form of free magnetic energy plays an additional role in the heating of corona. (4) The coronal holes (lower temperature region) shows more stable magnetic structure than the surrounding areas, demonstrating that the magnetic reconnection frequently occurs in the coronal hole boundary to increase the temperature outside the holes.
For the transient coronal activity, for example solar flare, the linkage between flare productivity and the free magnetic energy of active regions is explored. The key findings are: (1) For the first time, a positive correlation is found between the available free magnetic energy and flare productivity. (2) Based on the study of the temporal variation of free magnetic energy in flaring and flare-quiet active regions, free magnetic energy is not found to exhibit a clear and consistent pre-flare pattern. Therefore, the triggering mechanism of flares is as important as the energy storage in active regions. (3) As a case study, the topology changes of active region NOAA10930 magnetic fields before and after an X3.4 class flare on December 13, 2006 are studied. For the first time, rapid and permanent changes of optical penumbral and shear flows before and after the flares are found.
This dissertation took the advantage of comprehensive data from several solar space mission such as SoHO (MDI, EIT), Hinode (SOT, XRT) and Yohkoh (SXT) and ground- based data, e.g. SOLIS. Some most advanced data analysis tools were utilized, such as local correlation tracking, Stokes inversion, 180° ambiguity resolution, potential/non-potential field extrapolation and line ratio technique to extract coronal temperature.
njit-etd2010-037 (148 pages ~ 9,495 KB pdf)
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Created March 20, 2011