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Title:
A Study of Oscillations in Solar Active Regions
Authors:
Maurya, Ram Ajor
Affiliation:
AA(Udaipur Solar Observatory, Physical Research Laboratory, India )
Publication:
Ph.D. Thesis, Mohan Lal Sukhadiya University, India, 221 pages
Publication Date:
08/2010
Origin:
AUTHOR
Keywords:
flare, helioseismology
Bibliographic Code:
2010PhDT.........2M

Abstract

Solar active regions (ARs) are three-dimensional magnetic structures extending from deep sub-photosphere to coronal heights. These ARs are responsible for producing most of the energetic transients, such as flares and Coronal Mass Ejections (CMEs). The energetic charged particles released during these transients may also affect the measurements of magnetic and Doppler velocity fields. In addition, the energetic transients are expected to excite p-mode oscillations in ARs by imparting a mechanical impulse associated with their thermal expansion on the photosphere.

While studying the magnetic and velocity fields in AR NOAA 10486, we detected some puzzling moving transient features during the X17/4B and the X10/2B flares of 28 and 29 October 2003, respectively. We have examined the origin of these features and their relationship with various other aspects of the flares, viz., hard X-ray emission sources and flare kernels observed at different layers of solar atmosphere.

We have determined the characteristic properties of local oscillation modes by applying the ring diagram technique to 3-D power spectra of NOAA 10486. Strong evidence of substantial increase in mode amplitude and systematic variations in sub-surface flows are found from comparison of the pre- to the post-are phases of energetic flares. Furthermore, we have found statistically significant association between the mode energy and flare energy from the study of several ARs of Solar Cycle 23.

Our study has revealed strongly twisted, sheared flows in the interior of flaring ARs having complex magnetic fields. Using the Doppler data obtained for a sample of 74 ARs, we discovered steep gradients in meridional velocity at depths ranging from 1.5 to 5 Mm in flare productive ARs. The gradients showed an interesting hemispheric trend of negative (positive) sign in the northern (southern) hemispheres. An important inference derived from our analysis is that the location of the deepest zero vertical vorticity is correlated with the remaining lifetime of ARs. These new findings may be employed as important tool for predicting the life expectancy of an AR and space weather predictions. Finally, we have found using the data for 91 ARs of solar cycle 23 that kinetic helicity in sub-photospheric flows and magnetic helicity in photospheric magnetic fields show very weak correlation.

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