p-modes in and away from a sunspot

Abstract

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 the 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 $\mu$Hz, which shows a linear dependence on the magnetic field gradient in the active region. The maximum frequency deviation of 310 $\mu$Hz 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.