Synthetic Spectra of Radio, Millimeter, Sub-millimeter and Infrared Regimes with NLTE approximation

TL;DR

This paper presents a new NLTE-based numerical code for synthesizing solar emission spectra across radio to infrared wavelengths, analyzing the impact of different opacity functions on emission mechanisms.

Contribution

The work introduces PAKALMPI, a 3D NLTE radiative transfer code that incorporates multiple opacity functions to improve synthetic spectra modeling of the solar atmosphere.

Findings

H- opacity dominates low-altitude emission below 500 km.

Classical bremsstrahlung is the main emission mechanism at higher altitudes.

Inverse bremsstrahlung has negligible effect on radio emission at these heights.

Abstract

We use a numerical code called PAKALMPI to compute synthetic spectra of the solar emission in quiet conditions at millimeter, sub-millimeter and infrared wavelengths. PAKALMPI solves the radiative transfer equation, with Non Local Thermodynamic Equilibrium (NLTE), in a three dimensional geometry using a multiprocessor environment. The code is able to use three opacity functions: classical bremsstrahlung, H-and inverse bremsstrahlung. In this work we have computed and compared two synthetic spectra, one in the common way: using bremsstrahlung opacity function and considering a fully ionized atmosphere; and a new one considering bremsstrahlung, inverse bremsstrahlung and H- opacity functions in NLTE. We analyzed in detail the local behavior of the low atmospheric emission at 17, 212, and 405 GHz (frequencies used by the Nobeyama Radio Heliograph and the Solar Submillimeter Telescope). We found that the H- is the major emission mechanism at low altitudes (below 500 km) and that at higher altitudes the classical bremsstrahlung becomes the major mechanism of emission. However the brightness temperature remains unalterable. Finally, we found that the inverse bremsstrahlung process is not important for the radio emission at these heights.