On the nature of network oscillations

We examine time-series of spectral data obtained from the Solar Ultraviolet Measurements of Emitted Radiation instrument (SUMER), on board SOHO in the period 10-31 July 1996. Observations were obtained in lines, ranging in temperature from 12,000 K to 10⁶K, covering the low chromosphere to the corona. In this short contribution we report on the time series analysis on one of these dataset, 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 minutes. The low chromospheric and transition region lines show intensity and velocity power in the 2.4 mHz range. The observed 2-4 mHz network oscillations can be interpreted in terms of kink and sausage waves propagating upwards along thin magnetic flux tubes. The kink waves can be generated by random foot-point motions, e.g. by exploding granules, at the photospheric level. As they propagate within flux tubes, their amplitude grows exponentially with height and becomes non-linear. The waves can thereby undergo a mode transformation and become sausage type waves, which are more easily detected on the disk.