Statistics of quiet Sun extreme ultraviolet intensities

The frequency distribution of the extreme ultraviolet (EUV) emission line intensities in the quiet Sun has in the past often been modelled using two Gaussians. This gives adequate fits to observed distributions of average statistical significance. In this paper we test this and other distribution functions against observed distributions with exceptional statistical significance. The data were obtained in a number of spectral lines observed with two extreme ultraviolet spectrometers on board the Solar and Heliospheric Observatory (SOHO). In this way, the influence of spatial resolution and other instrument-specific parameters can be identified. The observations span a period of more than two years and provide a very large data set of radiance measurements of the quiet Sun at or near solar disk centre. We show that the frequency distribution of the radiance is best modelled by a lognormal distribution. The fact that the radiance distribution of the quiet Sun including the network and the intranetwork is better reproduced by a single lognormal distribution function than by two Gaussians suggests that the same heating processes are acting in both types of features. The parameters of the lognormal fit show a clear temperature dependence, with the transition region lines exhibiting the largest skewness of the distribution and the chromospheric intensity distributions being the most symmetric.