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Paper: Oscillations in Active Regions - Diagnostics and Seismology
Volume: 184, Third Advances in Solar Physics Euroconference: Magnetic Fields and Oscillations
Page: 151
Authors: Kosovichev, A. G.
Abstract: Oscillations in solar active regions provide a unique tool for probing the structure and dynamics of the active regions in the solar interior. The oscillations are excited stochastically by turbulent convection and also by strong localized events, such as flares. Oscillation power maps of stochastically excited waves show a deficit of acoustic power in the frequency range 2-5 mHz, and enhanced power at higher frequencies. These power variations are stronger in regions with stronger magnetic field, and, probably, are due to transformation of lower-frequency acoustic waves into higher-frequency MHD waves. An impulsive excitation event was observed as a result of a solar flare on July 9, 1996, from SOHO/MDI. The amplitude of the seismic waves on the surface was sufficiently high to determine the characteristics of the energy and momentum impact, and also to construct time-distance diagrams of the seismic response and to detect the deviation from the axial symmetry. This deviation probably resulted from the interaction with sunspots. Thus, in principle, the flare seismic waves can be used to probe the structure of active regions by time-distance helioseismology. The current methods of seismic diagnostics, acoustic imaging and time-distance tomography, are based on accumulating the seismic signal by averaging stochastically excited waves over a period of several hours. These methods have provided interesting information about the structure and evolution of active regions in the Sun's interior. As an example, I present the results of acoustic imaging of sunspots, and helioseismic tomography for large-scale imaging of active complexes of January 1991, and for an active region which emerged near the center of the solar disk in July 1996 and which studied from SOHO/MDI for nine days, both before and after its emergence at the surface. The initial results show complicated structures of the emerging regions in the interior, and suggest that the emerging flux ropes travel very quickly through the depth range of these observations.
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