Log-normal intensity distribution of the quiet-Sun FUV continuum observed by SUMER

Abstract

We analyse observations of the quiet-Sun far ultraviolet (FUV) continuum at various wavelengths near 1430 A obtained by the SUMER instrument on SOHO. According to semi-empirical atmospheric models this continuum originates from the layers in the chromosphere where the temperature rises from low values at near-radiative equilibrium to a plateau of about 6000 K. We study raster images and intensity distribution histograms and find that a single log-normal distribution matches these observations very well, and that the spatial structure observed corresponds to a mixture of features at supergranular and smaller scales that probably correspond to granular clusters. Also, a log-normal distribution was found in the literature to correspond to other chromospheric features and we compare here with histograms obtained from a Hi Ly-α quiet-Sun image. Because the continuum around 1430 A is mainly produced by Si I recombination it is expected to respond well to deep chromospheric heating and not be directly affected by velocities. The data suggest that chromospheric heating originates through dissipation of free-energy fields of small size and magnitude in underlying photospheric intergranular lanes. It has been suggested that such fields can be produced by photospheric dynamos at the intergranular scale and/or by complex fields emerging in a magnetic carpet. Such fields are expected to have sufficient free-energy to power the chromospheric heating. Plasma instabilities, such as the Farley-Buneman instability, would allow this free-energy to be dissipated in the chromosphere.