Solar Physics Division Meeting 2000, June 19-22
Session 1. Helioseismology, Magnetic Fields, Chromosphere and Transition Region
Display, Chair: C. U. Keller, Monday-Thursday, June 19, 2000, 8:00am-6:00pm, Forum Ballroom

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[1.09] Local Fractional Frequency Shifts of Helioseismic Modes Associated With Magnetic Activity Using Ring-Diagram Analyses

B. W. Hindman, D. A. Haber, J. Toomre (JILA / Univ. of Colorado), R. S. Bogart (HEPL-CSSA, Stanford Univ.)

Using full-disk velocity data from SOI-MDI during the advancing solar cycle from 1996 through 1999, we have computed the local frequencies of high-degree p modes and f modes over localized regions of the sun. The frequencies are obtained through ring-diagram mode fitting over Dense-Pack data sets consisting of mosaics of 189 overlapping tiles, each tracked separately at the surface rotation rate over 1664-minute time intervals during the MDI Dynamics Programs. Each tile is 16 degrees square, and the tile centers are separated by 7.5 degrees in latitude and longitude. For each observational day and for each tile, we have computed the frequency shift measured relative to the temporal and spatial average of the entire set of frequencies sampled over a full rotation. The magnetic field is computed for each of the 189 regions using the magnetograms supplied every 90 min by MDI. The motion of active regions as they rotate across the solar disk is vividly traced by changes in the frequencies. Active regions appear as locations of large positive frequency shifts. Depending on the radial order and wavenumber of the observed acoustic modes the frequency shifts can be as much as 10 to 30 microHz.

The magnitude and frequency dependence of the large frequency shifts are consistent with the far smaller changes measured in global oscillation frequencies over the solar cycle, provided the local frequency shifts are averaged over the solar disk and are scaled to the appropriate wavenumber regimes. The frequency dependence of the shifts indicates that the physical phenomena inducing the shifts is largely confined to the surface layers of the sun, although there is some indication that there may be a deeper structural component as well. This research was supported by NASA through grants NAG 5-8133 and NAG 5-7996, and by NSF through grant ATM-9731676.

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