The nature of network oscillations

Abstract

We examine time-series of spectral data obtained from the Coronal Diagnostic Spectrometer (CDS) andthe Solar Ultraviolet Measurements of Emitted Radiation instrument (SUMER) onboard the Solar HeliosphericObservatory (SOHO) spacecraft, in the period 30{31 July 1996. The observations were obtained in lines, rangingin temperature from 12 000 K to 106K, covering the low chromosphere to the corona. We report here on atime series analysis, using wavelet methods, of small individual network regions in the quiet Sun. The waveletanalysis allows us to derive the duration as well as the periods of the oscillations. The statistical signi canceof the oscillations was estimated by using a randomisation method. The oscillations are considered to be dueto waves, which are produced in short bursts with coherence times of about 10{20 min. The low chromosphericand transition region lines show intensity and velocity power in the 2{4 mHz range. The coronal line Mgxdoesnot show any statistically signi cant power in this range. In general, it is thought likely that the chromosphereand possibly the transition region oscillates in response to forcing by the p-modes, but they are also influencedstrongly by the presence of magnetic elds. The observed 2{4 mHz network oscillations can thus be interpretedin terms of kink and sausage waves propagating upwards along thin magnetic flux tubes. We perform a linearnumerical computation comparing the results with our observations.