Transition region oscillations above sunspots

Space Science & Technology Department, Rutherford Appleton Laboratory, Chilton, Didcot, Oxfordshire, OX11 0QX, UK e-mail: A.Fludra@rl.ac.uk

Corresponding author: A. Fludra, A.Fludra@rl.ac.uk

Received: 8 September 2000
Accepted: 22 December 2000

Abstract

We present observations of three sunspots made in the EUV wavelength range with the Coronal Diagnostic Spectrometer (CDS) on SOHO. We examine time series of line intensities obtained with a 15-s cadence and calculate their Fourier power spectra and the wavelet transforms. We find oscillations in the chromosphere and transition region above sunspots in the temperature range 2 10⁴-4 10⁵ K. Most of the spectral power is contained in the 5-8 mHz frequency range, and the dominant frequency is 5.9 mHz (170 s period). We find a relationship between the oscillations and sunspot plumes which are compact features located above sunspots and bright in the transition region lines emitted between 1.7 10⁵ and 4 10⁵ K. When the CDS slit crosses a sunspot plume, the bright O IV 554.5 Åand O V 629.7 Ålines show clear 3 min oscillations. An extreme alternative interpretation that excludes the presence of oscillations in sunspot plumes carries the requirement that the adjacent low-intensity plasma oscillate with high relative amplitudes greater than 50 percent. We also observe oscillations in a fainter area above a sunspot, outside the sunspot plume. The oscillations seen at different temperatures are compared. Using the wavelet transform, we find variability of the oscillation period in the range 120-200 s.