Context. The exact nature of the quiet solar chromosphere, and especially its temporal variations, are still subject of intense debate. One of the contentious issues is the possible role of magnetic field in structuring the quieter solar regions. Aims. We characterize the dynamics of the quiet inter-network chromosphere by studying the occurrence of acoustic shocks and their relation with the c...

We present spatially resolved spectra observed with the Space Telescope Imaging Spectrograph on the Hubble Space Telescope of the upper chromosphere and dust envelope of α Orionis (M2 Iab). In the fall of 2002 a set of five high-resolution near-UV spectra was obtained by scanning at intensity peak-up position and four off-limb target positions up to one arcsecond, using a small aperture (200 by...

The solar chromosphere consists of three classes which contribute differentially to ultraviolet radiation reaching the earth. We describe a data set of solar images, means of segmenting the images into the constituent classes, and a novel high-level representation for compact objects based on a triangulated spatial ‘membership function.’ Such representations are fitted in a variable-dimension M...

The overall structure and the fine structure of the solar photosphere outside active regions are largely understood, except possibly the important roles of a turbulent near-surface dynamo at its bottom, internal gravity waves at its top and small-scale vorticity. Classical one-dimensional static radiation-escape modelling has been replaced by three-dimensional time-dependent magento-hydrodynami...

Context. High resolution X-ray observations of classical T Tauri stars (CTTSs) show a soft X-ray excess due to high density plasma (ne = 1011 − 1013 cm−3). This emission has been attributed to shock-heated accreting material impacting onto the stellar surface. Aims. We investigate the observability of the shock-heated accreting material in the X-ray band as a function of the accretion stream pr...

Combining a variety of data with radiation hydrodynamic simulations, we examine the heating of the Sun’s internetwork chromosphere and the hypothesis that the chromospheric ‘‘ basal ’’ emission arises because of acoustic wave dissipation. We focus on the 2s 2p2 2D 2s22p 2Po multiplet of C ii near 1335 Å, whose basal level of chromospheric emission has been reliably determined for stars and the ...