Vertical temperature structure in the active-region chromosphere

Observations of the solar chromosphere at millimeter wavelengths provide a picture of the solar atmosphere very different from other diagnostics such as UV spectral lines. In particular, millimeter wavelengths are optically thick in the chromosphere, so a given wavelength samples a relatively narrow layer that is isolated from other heights in the atmosphere. In this paper we compare the observations of two active regions at two frequencies, 230 and 100 GHz, that sample distinct heights in the chromosphere. The comparison looks quite different in the two cases, resulting from different physical processes occurring at different heights. Temperature differences of up to 5000 K are seen along a given line of sight between the two levels, but the differences vary greatly across the fields of view. We compare these results with the expectations from 3D radiative MHD models of the chromosphere, and with observations at other wavelengths from IRIS and SDO.