Temperature analysis of solar prominences by multi-wavelength observations

Kinetic temperature of solar prominences is the key parameter to be determined by observations for understanding the energy balance and energy dissipation at work in prominences or in outer solar atmosphere. The aim of this work is (1) to evaluate the accuracy of the kinetic temperature determined from the width of emission lines and identify the most reliable scheme of the data analysis, and (2) to study the spatial correlation between the non-thermal velocity and temperature in prominences to investigate the possible role of the non-thermal motions on the heating. Spectroscopic observations of prominences were carried out by using the horizontal spectrograph of the Domeless Solar Telescope at Hida observatory. Spectral profiles of four emission lines, i.e., Ha 656.3nm, Hb 846.1nm, CaII 854.2nm, and CaII K 393.4nm, were recorded simultaneously over the 2D plane covering two prominences on the limb. We found systematic differences of temperatures derived from different pairs of spectral lines. The reason of difference is that the intrinsic line width determined with a single slab model is larger for optically thick lines than that of optically thin lines. Numerical experiment confirmed that the single slab model with a Gaussian profile of turbulence fails and a model with multiple components can explain the observation. The most accurate evaluation of the temperature is then obtained by the pair of Hb 846.1nm and CaII 854.2nm. Based on these results, we found the typical temperature of prominence to be 8000-10000K, which is significantly higher than the temperature determined by radiative balance and suggests the presence of mechanical heating in prominences. Also found is a positive correlation between non-thermal motion and temperature. We review the past results on prominence temperature by various authors and discuss the reason of discrepancy among them.