The nature of network oscillations

¹ Centre for Plasma Astrophysics, Katholieke Universiteit Leuven, Celestijnenlaan 200B, 3001 Heverlee, Belgium
² ESA Space Science Department, ESTEC Solar System Division, Keplerlaan 1, 2201 AZ, Noordwijk, The Netherlands
³ Armagh Observatory, College Hill, Armagh BT61 9DG, N. Ireland e-mail: eoshea@so.estec.esa.nl; jgd@star.arm.ac.uk; marcel.goossens@wis.kuleuven.ac.be

Corresponding author: D. Banerjee, dipu@wis.kuleuven.ac.be

Received: 10 January 2001
Accepted: 15 March 2001

Abstract

We examine time-series of spectral data obtained from the Coronal Diagnostic Spectrometer (CDS) and the Solar Ultraviolet Measurements of Emitted Radiation instrument (SUMER) onboard the Solar Heliospheric Observatory (SOHO) spacecraft, in the period 30-31 July 1996. The observations were obtained in lines, ranging in temperature from 12 000 K to 10⁶ K, covering the low chromosphere to the corona. We report here on a time series analysis, using wavelet methods, of small individual network regions in the quiet Sun. The wavelet analysis allows us to derive the duration as well as the periods of the oscillations. The statistical significance of the oscillations was estimated by using a randomisation method. The oscillations are considered to be due to waves, which are produced in short bursts with coherence times of about 10-20 min. The low chromospheric and transition region lines show intensity and velocity power in the 2-4 mHz range. The coronal line Mg x does not show any statistically significant power in this range. In general, it is thought likely that the chromosphere and possibly the transition region oscillates in response to forcing by the p-modes, but they are also influenced strongly by the presence of magnetic fields. The observed 2-4 mHz network oscillations can thus be interpreted in terms of kink and sausage waves propagating upwards along thin magnetic flux tubes. We perform a linear numerical computation comparing the results with our observations.