What is heating the quiet-Sun chromosphere?

It is believed that the heating of the chromosphere in internetwork regions is due to the dissipation of acoustic waves that are excited by the convective motions in the top-layers of the convection zone and by the convective overshoot into the photosphere. The acoustic waves would provide ample energy to compensate the radiative losses in the chromosphere. But this view lately became challenged by observations of the intensity fluctuations in the continuum at 160 nm, suggesting that the acoustic energy flux into the chromosphere is too low, by a factor of at least ten (Fossum & Carlsson, 2005). Here, this problem is addressed with the help of three-dimensional simulations that include the height range from the top layers of the convection zone to the middle chromosphere. The simulations enable the production of synthetic intensity maps that can be directly compared with the observations. We present results from this comparison. We also present simulations with a weak internetwork magnetic field and discuss structure and dynamics of this field in the chromosphere. It is shown that this field leads to a meshwork of electric current sheets in the lower chromosphere. Their capability of Ohmic heating will be discussed.