The IRIS$^{2+}$ inversion tool: recovering the radiative losses and the thermodynamics in the lower solar atmosphere

Abstract: We introduce an improved and fast inversion tool that is able to provide the thermodynamics of the solar atmosphere from the photosphere to the top of the chromosphere, as well as the integrated radiative losses in the chromosphere for data observed by the Interface Region Imaging Spectrograph (IRIS). This NASA mission has been observing the Sun and providing, among other kinds of data, multi-line spectral observations sensitive to changes in the lower solar atmosphere since 2013. In this paper, we explain the new inversion tool IRIS$<sup>{2+}$ based on the IRIS$<sup>{2+}$ database, which is based on 135,472 synthetic representative profiles (RP), each of them consisting of 6 chromospheric lines and 6 photospheric lines, their corresponding representative model atmospheres (RMA), and the integrated radiative losses (IRL) associated with these atmospheres. A nearest neighbor (k-nn) model algorithm is trained with the synthetic representative profiles to predict the closest RP in the database to the one observed, at which point IRIS$<sup>{2+}$ assigns the RMA and the IRL to the location of that observed profile. We have compared the results obtained by IRIS$<sup>{2+}$ with results obtained from the state-of-the-art inversion code STiC, which is also used to build the IRIS$<sup>{2+}$ database. We find that the thermodynamics and the IRL obtained with both methods are comparable in most cases. Therefore, IRIS$<sup>{2+}$ is a fast and reliable inversion tool that provides approximate values of the thermodynamic state and the radiative losses in the lower solar atmosphere for a large variety of solar scenes observed with IRIS.