p Modes in and Away from a Sunspot

A time series of GONG Dopplergrams for the period 10-14 May 1997 from Udaipur and Big Bear sites has been used to measure the velocity fluctuations in a sunspot (NOAA active region 8038) and quiet photosphere simultaneously. We observe that the power of pre-dominant p mode is reduced in the sunspot as compared to quiet photosphere by 39-52% depending on the location of the sunspot region on the solar disk. We also observe a relative peak frequency deviation of p modes in the sunspot, of the order of 80-310 μHz, which shows a linear dependence on the magnetic field gradient in the active region. The maximum frequency deviation of 310 μHz on 12 May appears to be an influence of a long-duration solar flare that occurred in this active region. We interpret this relative peak frequency deviation as either due to power re-distribution of p modes in the sunspot or a consequence of frequency modulation of these modes along the magnetic flux tubes due to rapidly varying magnetic field structure.[PUBLICATION ABSTRACT]