Semi-empirical models of spicule from inversion of CaII 8542 Å line

Abstract

<p><em>[eng] We study a solar spicule observed off-limb using high-resolution imaging spectroscopy in the Ca II 8542 Å line</em></p><p><em>obtained with the CRisp Imaging SpectroPolarimeter (CRISP) on the Swedish 1 m Solar Telescope. Using a new</em></p><p><em>version of the non-LTE code NICOLE specifically developed for this problem we invert the spicule single- and</em></p><p><em>double-peak line profiles. This new version considers off-limb geometry and computes atomic populations by</em></p><p><em>solving the 1D radiative transfer assuming a vertical stratification. The inversion proceeds by fitting the observed</em></p><p><em>spectral profiles at 14 different heights with synthetic profiles computed in the model by solving the radiative</em></p><p><em>transfer problem along its length. Motivated by the appearance of double-peak Ca II 8542 Å spicule profiles, which</em></p><p><em>exhibit two distinct emission features well separated in wavelength, we adopt a double-component scenario. We</em></p><p><em>start from the ansatz that the spicule parameters are practically constant along the spicule axis for each component,</em></p><p><em>except for a density drop. Our results support this ansatz by attaining very good fits to the entire set of 14 Å~ 4</em></p><p><em>profiles (14 heights and 4 times). We show that the double-component model with uniform temperature of 9560 K,</em></p><p><em>exponential decrease of density with a height scale of 1000–2000 km, and the counter-oriented line-of-sight</em></p><p><em>velocities of components reproduce the double-peak line profiles at all spicule segments well. Analyses of the</em></p><p><em>numerical response function reveals the necessity of the inversions of spectra at multiple height positions to obtain</em></p><p><em>height-dependent, degeneracy-free reliable models with a limited number of free parameters.</em></p>