Umbral Flash Research Articles

First Observation of Chromospheric Waves in a Sunspot by DKIST/ViSP: The Anatomy of an Umbral Flash

The Visible Spectro-Polarimeter of the NSF Daniel K. Inouye Solar Telescope collected its Science Verification data on 2021 May 7–8. The instrument observed multiple layers of a sunspot atmosphere simultaneously, in passbands of Ca ii 397 nm (H line), Fe i 630 nm, and Ca ii 854 nm, scanning the region with a spatial sampling of 0.″041 and an average temporal cadence of 7.76 s, for a duration of 38.8 minutes. The slit moved southward across the plane of sky at 3.83 km s−1. The spectropolarimetric scans exhibit prominent oscillatory “ridge” structures that lie nearly perpendicular to the direction of slit motion (north to south). These ridges are visible in the maps of line intensity, central wavelength, line width, and both linear and circular polarization. Contemporaneous Atmospheric Imaging Assembly observations indicate that these ridges are purely temporal in character and are likely attributed to the familiar chromospheric 3 minute umbral oscillations. We observe in detail a steady umbral flash near the center of the sunspot umbra. Although bad seeing limited the spatial resolution, the unique high signal-to-noise ratio data enable us to estimate the shock Mach numbers (≈2), propagation speeds (≈9 km s−1), and their impacts on the longitudinal magnetic field (ΔB ≈ 50 G), gas pressure, and temperature (ΔT/T ≈ 0.1) of subshocks over 30 s. We also find evidence for rarefaction waves situated between neighboring wave train shocks. The Ca ii 854 nm line width is fairly steady throughout the umbral flash, except for a sharp 1.5 km s−1 dip immediately before, and a comparable spike immediately after, the passage of the shock front. This zigzag in line width is centered on the subshock and extends over 0.″4.

Downflowing umbral flashes as evidence of standing waves in sunspot umbrae

Context. Umbral flashes are sudden brightenings commonly visible in the core of some chromospheric lines. Theoretical and numerical modeling suggests that they are produced by the propagation of shock waves. According to these models and early observations, umbral flashes are associated with upflows. However, recent studies have reported umbral flashes in downflowing atmospheres. Aims. We aim to understand the origin of downflowing umbral flashes. We explore how the existence of standing waves in the umbral chromosphere impacts the generation of flashed profiles. Methods. We performed numerical simulations of wave propagation in a sunspot umbra with the code MANCHA. The Stokes profiles of the Ca II 8542 Å line were synthesized with the NICOLE code. Results. For freely propagating waves, the chromospheric temperature enhancements of the oscillations are in phase with velocity upflows. In this case, the intensity core of the Ca II 8542 Å atmosphere is heated during the upflowing stage of the oscillation. However, a different scenario with a resonant cavity produced by the sharp temperature gradient of the transition region leads to chromospheric standing oscillations. In this situation, temperature fluctuations are shifted backward and temperature enhancements partially coincide with the downflowing stage of the oscillation. In umbral flash events produced by standing oscillations, the reversal of the emission feature is produced when the oscillation is downflowing. The chromospheric temperature keeps increasing while the atmosphere is changing from a downflow to an upflow. During the appearance of flashed Ca II 8542 Å cores, the atmosphere is upflowing most of the time, and only 38% of the flashed profiles are associated with downflows. Conclusions. We find a scenario that remarkably explains the recent empirical findings of downflowing umbral flashes as a natural consequence of the presence of standing oscillations above sunspot umbrae.

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

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 II8542 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.

Inversions of synthetic umbral flashes: a selection of wavelength sampling

Context. Imaging spectrographs are popular instruments used to obtain solar data. They record quasi-monochromatic images at selected wavelength positions. By scanning the spectral range of the line, it is possible to obtain bidimensional maps of the field-of-view with a moderate spectral resolution. Aims. In this work, we evaluate the quality of spectropolarimetric inversions obtained from various wavelength samplings during umbral flashes. Methods. We computed numerical simulations of nonlinear wave propagation in a sunspot and constructed synthetic Stokes profiles in the Ca II 8542 Å line during an umbral flash using the NLTE code NICOLE. The spectral resolution of the Stokes profiles was downgraded to various cases with differences in the wavelength coverage. A large set of wavelength samplings was analyzed and the performance of the inversions was evaluated by comparing the inferred chromospheric temperature, velocity, and magnetic field with the actual values at the chromosphere of the numerical simulation. Results. The errors in the inverted results depend to a large extent on the location of the wavelength points across the profile of the line. The inferred magnetic field improves with the increase of the spectral resolution. In the case of velocity and temperature, low spectral resolution data produce a match of the inverted atmospheres with the actual values comparable to wavelength samplings with finer resolution, while providing a higher temporal cadence in the data acquisition. Conclusions. We validated the NLTE inversions of spectropolarimetric data from the Ca II 8542 Å during umbral flashes, during which the atmosphere undergoes sudden dramatic changes due to the propagation of a shock wave. Our results favor the use of fine spectral resolution for analyses that focus on the inference of the magnetic field, whereas the estimation of temperature and velocity fluctuations can be performed with lower spectral resolution.

Fine wave dynamics in umbral flash sources

Context. Umbral flashes (UFs) are most common phenomenon of wave processes in sunspots. Studying the relationship between wave time dynamics and the origin of UFs requires further investigation of their fine spatial and height structure. Aims. We investigated the association between a short time increase in the variations of 3-min extreme ultraviolet (EUV) emission at footpoints of coronal magnetic loops and the UF emergence in sunspot umbra. The spatial structure of magnetic field lines and their inclination determine the cut-off frequency and visibility of sunspot waves. Methods. We applied the pixelized wavelet filtering (PWF) technique to analyse a cube of the images obtained by SDO/AIA at 1600 Å, 304 Å, and 171 Å to study the spatio-temporal oscillation power distribution in individual magnetic loops. Time–distance plots were used to obtain the wave front propagation velocity. Results. For the first time, we obtained the 2D images of the fine wave processes in magnetic structures of different spatial scales related to the UFs in sunspot. We revealed two types of the UFs as background and local. The background UFs associated with random increasing of separate parts of wave fronts. These UFs are seen as weak and diffuse details that ride the wave fronts without stable shapes and localization in space. The local UFs sources mainly localize near to the footpoint of magnetic loops, anchored in an umbra, along which the propagation of 3-min waves was observed. The time dynamics of flash evolution shows an increase in 3-min oscillations before the UFs peak value within one low-frequency wave train. It is shown that the maxima of 3-min oscillation trains coincide with the peak intensity of UFs. During the UFs evolution, a fine wave spatial and temporal dynamics in UFs local sources was observed. Conclusions. The sunspot 3-min wave dynamics showed a relation between the localization of oscillations power peak at the coronal magnetic loop footpoints and the UFs origin. The spatial structure of the UFs sources, their power, and lifetime are determined by the cut-off frequency of the waves for the detected waveguides. We concluded that UFs are a global process of short-time increase of the wave activity.

Inversions of synthetic umbral flashes: Effects of scanning time on the inferred atmospheres

Context. The use of instruments that record narrowband images at selected wavelengths is a common approach in solar observations. They allow scanning of a spectral line by sampling the Stokes profiles with two-dimensional images at each line position, but require a compromise between spectral resolution and temporal cadence. The interpretation and inversion of spectropolarimetric data generally neglect changes in the solar atmosphere during the scanning of line profiles. Aims. We evaluate the impact of the time-dependent acquisition of various wavelengths on the inversion of spectropolarimetric profiles from chromospheric lines during umbral flashes. Methods. Numerical simulations of nonlinear wave propagation in a sunspot model were performed with the code MANCHA. Synthetic Stokes parameters in the Ca II 8542 Å line in NLTE were computed for an umbral flash event using the code NICOLE. Artificial profiles with the same wavelength coverage and temporal cadence from reported observations were constructed and inverted. The inferred atmospheric stratifications were compared with the original simulated models. Results. The inferred atmospheres provide a reasonable characterization of the thermodynamic properties of the atmosphere during most of the phases of the umbral flash. The Stokes profiles present apparent wavelength shifts and other spurious deformations at the early stages of the flash, when the shock wave reaches the formation height of the Ca II 8542 Å line. These features are misinterpreted by the inversion code, which can return unrealistic atmospheric models from a good fit of the Stokes profiles. The misguided results include flashed atmospheres with strong downflows, even though the simulation exhibits upflows during the umbral flash, and large variations in the magnetic field strength. Conclusions. Our analyses validate the inversion of Stokes profiles acquired by sequentially scanning certain selected wavelengths of a line profile, even in the case of rapidly changing chromospheric events such as umbral flashes. However, the inversion results are unreliable during a short period at the development phase of the flash.

Direct Observations of Different Sunspot Waves Influenced by Umbral Flashes

Abstract We report the simultaneous presence of chromospheric umbral flashes and associated umbral waves, and propagating coronal disturbances, in a sunspot and related active region. We have analyzed time–distance maps obtained using the observations from the Atmospheric Imaging Assembly on board the Solar Dynamics Observatory. These maps show the simultaneous occurrence of different sunspot oscillations and waves such as umbral flashes, umbral waves, and coronal waves. Analysis of the original light curves, i.e., without implementing any Fourier filtering on them, shows that the amplitudes of different sunspot waves observed at different atmospheric layers change in synchronization with the light curves obtained from the umbral flash region, thus demonstrating that these oscillations are modulated by umbral flashes. This study provides the first observational evidence of the influence of sunspot oscillations within the umbra on other sunspot waves extending up to the corona. The properties of these waves and oscillations can be utilized to study the inherent magnetic coupling among different layers of the solar atmosphere above sunspots.

A Flare and Umbral Flashes in a Sunspot

We studied the evolution and dynamic processes in the chromosphere above a sunspot umbra. A relatively rarely occurring phenomenon of bright long-lasting emission observed in the umbra of a unipolar sunspot of the AR 9570 group on August 11, 2001 was investigated. It was found that during the course of the observation, emission was spreading, gradually occupying nearly the entire sunspot umbra. Based on the analysis of the observations from other observatories, we arrived at the conclusion that the bright emission was a sympathetic flare that occurred in the sunspot umbra. It was assumed that there occurred an interaction with a neighboring, rapidly evolving group that exhibited subflares on the day of observation. In the same umbra, there was taking place an oscillatory process of the type of ‘umbral flash’ (observations from August 11 and 12, 2001). The characteristics of the oscillatory process in the presence of the flare were studied. As the bright emission propagated in the sunspot umbra, brightness fluctuations ceased to be seen in the umbral flashes against the background of this brighter emission. The character of velocity variations did not change substantially, although the oscillation amplitude did decrease.

$\ion{Ca}{ ii}$ 8542 Å sunspot oscillations observed with THEMIS

Oscillations in the umbra and the penumbra of an isolated sunspot located near the solar disk centre were investigated. The observations were obtained with the Multichannel Subtractive Double Pass (MSDP) spectrograph operating in the $\ion{Ca}{ii}$ 8542 A line and installed at the focus of THEMIS (Tenerife). From the MSDP data, two-dimensional intensity and Doppler shift images were computed at different wavelengths within the line. Intensity and Doppler shift oscillations in the umbra and the penumbra of the sunspot showing up as umbral flashes and penumbral waves were analyzed using a 23 min time series with a cadence of 46 s. The $\ion{Ca}{ii}$ umbral flash intensity profile shows an emission core in its blue wing. We investigate the relation between umbral flashes and running penumbral waves by a power spectrum analysis which shows a 6 mHz frequency for the standing umbral oscillations (flashes) which are observed only on the upper half part of the umbra. The running penumbral waves propagate with an average phase velocity of 16 and their frequency is constant in the penumbra and equal to 3 mHz. Although the time slice images suggest that umbral flashes and running penumbral waves are probably due to the same resonator, the power analysis shows no direct relationship between the two phenomena.

Anomalous polarization profiles in sunspots: possible origin of umbral flashes

We present time-series spectropolarimetric observations of sunspots in the Ca II infrared triplet lines, which show a periodic occurrence of anomalous, asymmetric, circular polarization profiles in the umbral chromosphere. The profiles may be caused by the periodic development of an unresolved atmospheric component in a downward flowing magnetized environment. This active component with upward directed velocities as high as 10 kilometers per second is connected to the umbral flash (UF) phenomenon. We can explain the observations with a semiempirical model of the chromospheric oscillation and of the sunspot magnetized atmospheric plasma during a UF event.

The umbral flash as a magneto-acoustic wave phenomenon

The suggestion that an ‘umbra flash’ may be caused by a magneto-acoustic wave phenomenon is examined. It is suggested that the flash in Ca ii lines is formed during the compressional stage in a magneto-acoustic wave. The compression which is assumed to be adiabatic will produce a rise in temperature and a corresponding increase in number of Ca ii atoms. The variations in line emission (absorption) coefficient of the Ca ii K-line are calculated on this assumption and are found to be in general agreement with the observed variations. Other observed quantities as proper motion, magnitude of line shift etc., also agree with the wave hypothesis. Further observations which may serve as tests on the wave hypothesis are suggested.