Sunspot oscillations in the 3 and 5 mHz (i.e. 5 and 3 minute) bands have been observed in both the intensity and the Doppler shift by a number of observers, and their properties have been investigated in detail (see the review by Lites 1992).
The effect of the underlying magnetoacoustic oscillations on the magnetic field are expected to be small (rms value of at the most a few G for a monolithic sunspot, Lites et al. 1998). Consequently, the results are controversial. In the recent literature only Horn et al. (1997) and Lites et al. (1998) report on sunspot magnetic field oscillations. The former authors present oscillations that are marginally significant, while the latter authors do not consider their own observations to be sufficiently reliable to represent a true detection. Ulrich (1996) also reported on magnetogram oscillations in active region, but at the very low spatial resolution of 20 20 arcsec2.
Here we explore velocity and magnetic field oscillations observed in an active region with the Michelson Doppler Interferometer (MDI) onboard the Solar and Heliospheric Observatory (SOHO). In the present paper we concentrate on the oscillations of the two main spots of the region.
© European Southern Observatory (ESO) 1998
Online publication: June 26, 1998