The Dark Side of Penumbral Microjets: Observations in Hα

Abstract

We present data of 10 penumbral microjets (PMJs) observed in Hα, Ca II 8542 Å, and Fe I 6302 Å line pair with the Swedish 1 m Solar Telescope (SST) with CRISP and Ca II K with SST/CHROMIS in active region NOAA 12599 on the 12 th October 2016 at µ = 0.68. All four Stokes parameters of the Ca II 8542 Å and Fe I 6302 Å lines were observed and a series of test pixels was inverted using the Stockholm inversion code. Our analysis revealed for the first time that PMJs are visible in Hα, where they appear as dark features with average line-of-sight (LOS) upflows of 1.1 ± 0.6 km/s, matching the LOS velocities from the inversions. Based on the Hα observations we extend the previous average length and lifetime of PMJs to 2815 ± 530 km and 163 ± 25 s, respectively. The plane-of-sky (POS) velocities of our PMJs of up to 17 km/s tend to give increased velocities with distance travelled. Furthermore, two of our PMJs with significant Stokes V signal indicate that the PMJs possess an increased LOS magnetic field of up to 100 G compared to the local pre-/post-PMJ magnetic field, which propagates as quickly as the PMJs' POS velocities. Finally, we present evidence that PMJs display an on average 1 minute gradual precursory brightening that only manifests itself in the cores of the Ca II lines. We conclude that PMJs are not ordinary jets but likely are manifestations of heat fronts that propagate at the local Alfvén velocity.

A multi-diagnostic spectral analysis of penumbral microjets

“…Figure 10 shows all individual Ca ii 8542 Å and H α PMJ profiles for these dark features, together with the PMJ mean profiles of these. The average PMJ profiles in the H α line for both dates confirm a slight decrease in intensity in the inner line wings of the studied PMJs, which is consistent with the profiles presented in Buehler et al (2019). The average Ca ii 8542 Å line profiles for our selected dark features show less distinct behaviour, and they have mean profiles that are overall slightly enhanced compared to the penumbral mean profile, even though the CRISP images at an offset of −350 mÅ clearly show features that are darker than their surroundings.…”

“…The average Ca ii 8542 Å line profiles for our selected dark features show less distinct behaviour, and they have mean profiles that are overall slightly enhanced compared to the penumbral mean profile, even though the CRISP images at an offset of −350 mÅ clearly show features that are darker than their surroundings. The darkenings found in the images of the inner line wing of the H α line appear very similar to those as presented in Buehler et al (2019). The spectral formation process of the dark features in the inner ling wings of both the Ca ii 8542 Å and H α lines has yet to be explained and warrants further investigation, possibly in the form of modelling.…”

“…In Sect. 4.8 we present our investigation of dark features observed in the inner line-wings of the H α line linked to PMJs (first observed in Buehler et al 2019), which we link to similar darkenings in the inner line wings of the Ca ii 8542 Å line. We further link the darkenings at these wavelengths to the brightenings observed in the Mg ii line pair.…”

“…PMJs have mostly been studied in the Ca ii lines, and not so much in the H α line. Recently, however, Buehler et al (2019) reported on PMJs that present with darkenings in H α images when studied by eye and are viewed in the inner line core, with accompanying H α line profiles exhibiting subtle decreases in intensity in the inner line core at sites adjacent to PMJs that are visible in concurrent Ca ii 8542 Å line observations. We investigated the appearance of PMJs in H α and Ca ii 8542 Å images together with their line profiles at selected locations.…”

A New View of the Solar Interface Region from the Interface Region Imaging Spectrograph (IRIS)

“…Evidence of such emission was independently found in the IRIS Si IV lines by Nóbrega-Siverio et al (2017) and Guglielmino, Young, and Zuccarello (2019). Other examples of chromospheric ejections with associated TR emission observed with IRIS are fan-shaped jets originating from light bridges (Yang et al, 2015;Bharti, 2015;Bai et al, 2019) and penumbral microjets Humphries et al, 2020), although penumbral microjets are not genuine jets with significant mass motions, but rather manifestations of heat fronts (Esteban Buehler et al, 2019; Rouppe van der Voort and Drews, 2019; Drews and Rouppe van der Voort, 2020). In order to properly understand the TR emission of the aforementioned phenomena, one has to take the highly dynamic nature of the chromosphere into account as well as the fact that some elements in the TR have long ionization and recombination timescales, leading to relevant non-equilibrium ionization effects (e.g.…”