Hyperspectral detection of spicules in Ca II K

Abstract

Solar spicules are chromospheric jets of up-flowing plasma observed all over the Sun, and can be categorized into type i and type ii spicules. Type ii spicules are more dynamic and more ubiquitous, yet their origin and ubiquity are not understood. We use, for the first time, high-resolution observations of type ii spicules in Ca ii K 393.4 nm to characterize the bright/dark (B/D) characteristics we observe in Ca ii K spicules. We also compare the apparent length of spicules in Ca ii K to spicules in Hα. The observations are of the quiet Sun close to the limb (µ = 0.41) and were acquired by the CHROMospheric Imaging Spectrometer (CHROMIS) and the CRisp Imaging SpectroPolarimeter (CRISP) at the Swedish 1-metre Solar Telescope on La Palma. We find that the B/D characteristics are evident due to suppression of the K 2 peak in the upper part corresponding to the K 3 Doppler shift, combined with significant enhancement of that same K 2 peak along the lower part of the spicule, yielding a direct demonstration of heating along the spicule. We often see the effect of enhanced K 2 peaks in upper parts of spicules due to the “opacity window” reported by Bose et al. (2019), but we also sometimes observe enhanced upper K 2 peaks from the bright surroundings or the spicule itself. Out of 39 spicules, we do not see this “window”-effect for the lower parts of spicules, except one spicule where we see enhancement of lower part K 2 due to neighbouring spicule in the background. We also found that spicules appear a little longer in Ca ii K than Hα due to the signal generally reaching higher in Ca ii K but they also appear to reach deeper due to the brightening in the bottom.