Active region oscillations

¹ ESA Space Science Department, ESTEC Solar System Division, Keplerlaan 1, 2201 AZ, Noordwijk, The Netherlands
² Centre for Plasma Astrophysics, K.U. Leuven, Celestijnenlaan 200 B, 3001, Heverlee, Belgium
³ Armagh Observatory, College Hill, Armagh BT61 9DG, N. Ireland
⁴ ESA Space Science Department, NASA/GSFC, Mailcode 682.3, Greenbelt, MD 20771, USA
⁵ School of Computer Science, The Queen's University of Belfast, Belfast BT7 1NN, N. Ireland e-mail: dipu@wis.kuleuven.ac.be, jgd@star.arm.ac.uk, bfleck@esa.nascom. nasa.gov, f.murtagh@qub.ac.uk

Corresponding author: E. O'Shea, eoshea@so.estec.esa.nl

Received: 21 February 2000
Accepted: 4 January 2001

Abstract

We report here on an investigation of high frequency oscillations in active regions, carried out using high cadence observations of Ov 629 Å, Mgix 368 Åand Fexvi 335 Åwith the Coronal Diagnostic Spectrometer (cds) on soho. Using the techniques of wavelet analysis on various temporal series datasets, we find that certain oscillation frequencies are favoured for each line. We find furthermore that a ~5 min oscillation signature is commonly present in all lines, suggesting a coupling of the photospheric driver with the transition region and coronal loop modes. We report on the tendency for higher frequency oscillations to be present at lower intensity values, suggesting that higher frequency oscillations occur in interloop regions or at loop boundaries, possibly as a result of some resonant absorption process. In addition, we find that the coronal lines of Fexvi and Mgix show more significant oscillations in the velocity than in the intensity, which suggests that in the velocity we measure additional non-compressive wave modes not visible in the intensity. As this effect is not seen in the transition region line of Ov it would seem that these additional non-compressive modes are produced in and limited to the corona. We suggest that there are two main mechanisms responsible for the observed oscillations; either resonant Alfvén and/or fast kink waves or propagating slow magnetoacoustic waves, both present in coronal loops.