Speaker
Description
First Name: Costas
Last Name: Alissandrakis
Affiliation: University of Ioannina, Department of Physics
All Authors: C. E. Alissandrakis, J.-C. Vial and T. S. Bastian
Abstract: Spicules are small, nearly vertical jets of chromospheric plasma intruding into the hot corona up to a height of about 20 Mm. They are readily observed in chromospheric and transition region lines and continua in the optical, EUV and short wavelength radio ranges. Although known for more than a century, the nature of spicules is still not fully understood, as the sub-arcsecond width of spicules requires observations near the limit of available instruments and their optical and EUV emission is formed under conditions far from Local Thermodynamic Equilibrium (LTE). In a recent work Bastian et al. (2025) presented spicule observations in a coronal hole at the N limb with the Atacama Large Millimeter/submillimeter Array (ALMA) at two wavelengths: 3 mm (100 GHz, Band3) and 1.25 mm (239 GHz, Band 6), with a spatial resolution as good as 1.5" by 0.7" in Band 6. As the spicule radio emission is free-free, it is immune to non-LTE effects, hence the observed brightness temperature can be associated to the plasma temperature and density in a straightforward way. Assuming a constant electron temperature of 15000 K, Bastian et al. deduced electron densities in the range of 1.7 x 10^11 cm^-3 and 2.5 x 10^10 cm^-3 at heights of 2 and 8 Mm, respectively. Here we present the results of a joint analysis of these observations and simultaneous IRIS spectral and slit-jaw data in the MgII k and h lines. We fitted the IRIS spectra to a non-LTE model, using the observed ALMA brightness as a constraint, and thus alleviating the need for additional assumptions, as was done in previous works. We processed both the average spectrum and spectra of individual spicules. We found that the spicule temperature rises from about 10000 K at a height of 2 Mm to about 15000 at 10 Mm, whereas the electron density drops from about 2.1 x 10^11 cm^-3 to 2.1 x 10^10 cm^-3 in the same height range; we note that in this height range the degree of ionization of hydrogen increases from 60% to 100 %. We will discuss further the implications of these results. This work has been supported by the European Research Council through the Synergy grant No. 810218 (“The Whole Sun,” ERC-2018-SyG).