  In the past few years, the ALMA radio telescope has become available for
solar observations. ALMA diagnostics of the solar atmosphere are of high
interest because of the theoretically expected linear relationship between the
brightness temperature at mm wavelengths and the local gas temperature in the
solar atmosphere. Key for the interpretation of solar ALMA observations is
understanding where in the solar atmosphere the ALMA emission originates.
Recent theoretical studies have suggested that ALMA bands at 1.2 (band 6) and 3
mm (band 3) form in the middle and upper chromosphere at significantly
different heights. We study the formation of ALMA diagnostics using a 2.5D
radiative MHD model that includes the effects of ion-neutral interactions
(ambipolar diffusion) and non-equilibrium ionization of hydrogen and helium.
Our results suggest that in active regions and network regions, observations at
both wavelengths most often originate from similar heights in the upper
chromosphere, contrary to previous results. Non-equilibrium ionization
increases the opacity in the chromosphere so that ALMA mostly observe spicules
and fibrils along the canopy fields. We combine these modeling results with
observations from IRIS, SDO and ALMA to suggest a new interpretation for the
recently reported "dark chromospheric holes", regions of very low temperatures
in the chromosphere.
