The polarization-free approximation

The development of effective iterative methods capable of accurately solving NLTE Stokes transfer problems is of considerable importance for the investigation of solar and stellar magnetic fields. After briefly indicating the iterative approach which is being presently pursued for the exact solution of such problems, the particular regime where polarization signals can only be due to the Zeeman effect is considered in some detail. By means of NLTE Stokes transfer calculations for a two-level atomic model it is first shown that the currently-used field-free approximation (Rees, 1969) cannot be safely applied in the presence of magnetic field gradients. Such gradients lead to changes in the shape and width of the line profiles and they can produce non-negligible effects on the atomic level populations and line source functions. A new approximate method is then proposed, which does not require the actual solution of the Stokes vector transfer equation and is practically as fast as the field-free one. This polarization-free approximation provides a fairly good account of the effects of homogeneous and inhomogeneous magnetic fields on the statistical equilibrium and is very easy to implement in any existing non-magnetic, multi-level transfer code.On leave from the Dipartimento di Astronomia e Scienza dello Spazio, Università di Firenze, Largo E. Fermi 5, I-50125 Firenze, Italia     

The radiative energy budget of the middle atmosphere and its parameterization in general circulation models

The thermal regime of the middle atmosphere is determined to a great extent by the balance between the incoming solar and outgoing infrared radiation. To account for these processes in numerical models of the middle atmosphere, parameterizations that are capable of quickly and accurately calculating infrared cooling and solar heating rates are required. These parameterizations should include the breakdown of local thermodynamic equilibrium (LTE) conditions and allow for feedbacks by ensuring that dependencies on all input parameters are accounted for. This paper discusses the major mechanisms responsible for maintaining the radiative energy budget of the middle atmosphere and presents a brief review of approaches and numerical schemes currently available for use in general circulation models. The main focus of the paper is on the approaches and schemes designed for non-LTE treatment.