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R. Komm, R. Howe, F. Hill (NSO)
The GONG project has now analysed 3-month velocity time series covering the period late 1995 to mid-1998, covering the end of solar cycle 22 and the beginning of cycle 23. We here present an analysis of the highly significant shifts in the central frequencies and the a2 and a4 coefficients of the modes, and relate them to magnetic activity indices and to the corresponding coefficients of a Legendre polynomial decomposition of the surface magnetic field. These changes are confirmed by an analysis of the MDI-SOI time series for some of the equivalent time periods using the GONG peakfinding algorithm, and are consistent with available data from previous solar cycles.
In addition, we study mode widths and amplitudes derived from the GONG data and search for a variation of these mode parameters with the solar cycle. With increased activity from cycle minimum to mid-1998, we find a small increase in mode width of about 2% on average and about 4% in the frequency range from 2.9 to 3.3 mHz. We find a decrease of similar size in mode amplitude and mode area (width times amplitude). The change in background amplitude is not significant and is consistent with a zero change.
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