Abstract
This article presents our implementation of a non-LTE solver in spherical symmetry for molecular rotational transition in static or expanding atmospheres. The new open-source code relies on the Gauss–Seidel Accelerated Lambda Iteration methodology that provides a rapid and accurate convergence of the non-LTE problems, which is now routinely used in astrophysical and planetary research. The non-LTE code is interfaced with the widely used package, the Atmospheric Radiative Transfer Simulator (ARTS), to facilitate spectral line simulations for various viewing geometries. In this paper we describe the numerical implementation, provide the first validation results for the populations against two other non-LTE codes, and then discuss the possible application. The quantitative comparisons are performed using an established ortho-water non-LTE model applied to cases of optical thick and thin conditions of Ganymede’s atmosphere. The differences in populations expressed as excitation temperatures show very good agreement in both cases. Finally, we also apply this model to a sample of data from the Microwave Instrument for the Rosetta Orbiter (MIRO) instrument. The new non-LTE package is demonstrated to be fast and accurate, and we hope that it will be a useful addition to the planetary community. In addition, being open source and part of the ARTS, it will be further improved and developed.
Highlights
The past two decades have seen numerous advances in heterodyne radio technologies and their application to modern themes in atmospheric science of planets, small bodies, and astrophysics
Several observatories use this transition for detection, for example the Submillimeter Wave Astronomical Satellite (SWAS; Neufeld et al 2000; Bensch et al 2007), the Odin satellite equipped with a 1.1 m submillimetre telescope (Biver et al 2007), and the Heterodyne Instrument for the Far Infrared (HIFI) on the Herschel Space Observatory (Hartogh et al 2009; de Graauw et al 2010)
We presented an implementation of an accurate and efficient numerical method of the solution for the non-LTE problem in spherical symmetry
Summary
The past two decades have seen numerous advances in heterodyne radio technologies and their application to modern themes in atmospheric science of planets, small bodies, and astrophysics These instruments, operating typically in in the submillimetre wave range up to frequencies of 5 THz, provide a very high frequency resolution such that photons from individual rotational transitions of molecules can be measured. The advantage of EP is that the problem is treated as local (with photon escape probabilities calculated without radiative transfer), and that it is a deterministic method Another popular method relies on the Monte Carlo (MC) method to obtain the mean intensity (Hogerheijde & Van Der Tak 2000; Zakharov et al 2007; Lee et al 2011), which can cope in principle with more complex geometry, but the calculated populations show intrinsic random noise.
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