A. Mohan, Indian Institute of Technology (BHU), amohan.app@iitbhu.ac.in

Abstract: Solar atmosphere is the seat of highly ionized atomic species. Because of its proximity, the high temperature solar atmosphere is the only astrophysical plasma source that can be studied with greater spatial and temporal resolution. Diagnostics of inhomogeneous structures on the Sun embedded in a complex solar magnetic field is a problem of universal importance. In some cases, it serves to refrigerate the gas (e.g., sunspots) while in other cases it serves to heat the gas to millions of degrees (e.g., active corona). The knowledge of the physical parameters such as electron densities, temperatures, elemental abundances and emission measures of cosmic plasma enables us to understand the generation and transport of mass, momentum and energy. The solar atmosphere and its composition is examined using the spectroscopic diagnostic techniques. The main source of our understanding of stellar atmospheres is based on our knowledge concerning the plasma radiation processes in the solar atmosphere.
Although some of these processes can be studied in high-resolution images of the solar atmosphere, much of our knowledge concerning temperature, density and dynamics of different regions of the solar atmosphere is obtainable through the technique of high resolution spectroscopy in the UV, EUV, and X-ray spectral regions. Systematic spectroscopic analysis of the solar ions, namely N III, Ne IV, Ne V, Mg V, Ne VI, Mg VI, Si VII, Mg VII, Si VIII, Mg VIII, Si IX, Mg IX, S X and Si X in the temperature range 8 x 104 K to 1.6 x 106 K has been carried out to understand the physical properties of solar plasma. These ions belonging to various iso-electronic sequences carry the radiation signature of the chromosphere-corona transition region and the corona. For the atomic processes involved in hot (Te > 105 K) and low density (Ne < 1012 cm-3) plasma, we assume that the spectral lines are optically thin, which is valid assumption for the outer atmosphere of the Sun and other stars. Some results will be presented in light of high-resolution ultraviolet observations obtained with Coronal Diagnostic Spectrometer (CDS) and Solar Ultraviolet Measurements of Emitted Radiation (SUMER) instruments onboard the SOHO (Solar and Heliospheric Observatory) spacecraft covering the spectral range 150-1616 .