Umbral chromospheric fine structure and umbral flashes modelled as one: The corrugated umbra

Abstract

Context. The chromosphere of the umbra of sunspots features an assortment of dynamic fine structures that are poorly understood and often studied separately. Small-scale umbral brightenings (SSUBs), umbral microjets, spikes or short dynamic fibrils (SDFs), and umbral dark fibrils are found in any observation of the chromosphere with sufficient spatial resolution performed at the correct umbral flash stage and passband. Understanding these features means understanding the dynamics of the umbral chromosphere. Aims. We aim to fully understand the dynamics of umbral chromosphere through analysis of the relationships between distinct observed fine features and to produce complete models that explain both spectral profiles and the temporal evolution of the features. We seek to relate such understanding to umbral flashes. Methods. We studied the spatial and spectral co-evolution of SDFs, SSUBs, and umbral flashes in Ca II 8542 Å spectral profiles. We produced models that generate the spectral profiles for all classes of features using non-local thermodynamic equilibrium radiative transfer with a recent version of the NICOLE inversion code. Results. We find that both bright SSUBs and dark SDF structures are described with a continuous feature in the parameter space that is distinct from the surroundings even in pixel-by-pixel inversions. We find a phase difference between such features and umbral flashes in both inverted line-of-sight velocities and timing of the brightenings. For umbral flashes themselves we resolve, for the first time in inversion-based semi-empirical modelling, the pre-flash downflows, post-flash upflows, and the counter-flows present during the umbral flash phase. We further present a simple time-dependent cartoon model that explains the dynamics and spectral profiles of both fine structure, dark and bright, and umbral flashes in umbral chromospheres. Conclusions. The similarity of the profiles between the brightenings and umbral flashes, the pattern of velocities obtained from the inversions, and the phase relationships between the structures all lead us to put forward that all dynamic umbral chromospheric structures observed to this date are a locally delayed or locally early portion of the oscillatory flow pattern that generates flashes, secondary to the steepening large-scale acoustic waves at its source. Essentially, SSUBs are part of the same shock or merely compression front responsible for the spatially larger umbral flash phenomenon, but out of phase with the broader oscillation.

Downflowing umbral flashes as evidence of standing waves in sunspot umbrae

“…Our model offers an alternative explanation, interpreting downflowing UFs as a natural feature of the global process of sunspot oscillations. This is also the recent interpretation of Henriques et al (2020), but here we show that such downflows are evidence of the standing-wave nature of these oscillations. Henriques et al (2017) reported strong downflowing atmospheres for UFs, with velocities up to 10 km s −1 , whereas our model produces downflowing UFs with velocities around 1 km s −1 .…”

“…The analysis is also challenged by the sudden variations in the atmosphere during the time required by Fabry-Perot spectrographs, such as the Interferometric BIdimensional Spectrometer (IBIS, Cavallini 2006) and the CRisp Imaging SpectroPolarimeter (CRISP, Scharmer et al 2008), to scan the line (Felipe et al 2018b). However, observations of smallscale umbral brightenings using a cadence of 14 s exhibit some indication of a temporal evolution consistent with that predicted by our modeling (Henriques et al 2020). Upcoming observations with the National Science Foundation's Daniel K. Inouye Solar Telescope (DKIST, Rimmele et al 2020) will provide the ideal data for studying the temporal variations of UFs.…”

“…The temperature maximum precedes the highest value of the upflow velocity by 9 s. Prior to the appearance of the flash (Fig. 1b), the atmosphere is downflowing, in accordance with observations (Henriques et al 2020), but the temperature fluctuation is too low to produce an emission core (panel d). When the UF starts to develop (panel e), the atmosphere is upflowing at the optical depth where the response of the line is at its maximum (log τ ≈ −5.1, Fig.…”

“…This Letter provides a missing piece of the puzzle in what has been a convergence of observations and simulations: We have tied together the evidence for downflowing UFs and the existence of cavities in the chromosphere of the umbra as the latter will naturally generate the former. Finally, the case for a corrugated surface where downflows are significant (Henriques et al 2020), together with the importance of cavities for such downflows, implies that future observations constraining the transition region of sunspots should find the transition region itself to be highly corrugated.…”

Critical Science Plan for the Daniel K. Inouye Solar Telescope (DKIST)

“…Even emission features are difficult to observe with spatial and temporal resolutions sufficient to distinguish the responsible physical mechanisms. For example, much of the spatial and temporal structuring of umbral micro-jets that was originally thought to be reconnection related (Bharti, Hirzberger, and Solanki, 2013;Nelson et al, 2017b) may instead be the signature of a shock/compression front forming in the corrugated sunspot atmosphere, with the time varying emission reflecting local density/velocity structures that enhance or delay the shock along a steepening compression front (Henriques et al, 2020). DKIST will enable spectropolarimetric measurements of such dim small-scale features with sufficient polarimetric sensitivity to constrain their physical properties.…”