Proper Motions of Sunspots’ Umbral Dots at High Temporal and Spatial Resolution (original) (raw)
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Umbral Fine Structures in Sunspots Observed with Hinode Solar Optical Telescope
Publications of the Astronomical Society of Japan, 2007
High resolution imaging observation of a sunspot umbra was done with Hinode Solar Optical Telescope (SOT). Filtergrams in wavelengths of blue and green continuum were taken during three consecutive days. The umbra consisted of a dark core region, several diffuse components and numerous umbral dots. We derived basic properties of umbral dots (UDs), especially their temperatures, lifetimes, proper motions, spatial distribution and morphological evolution. Brightness of UDs is confirmed to depend on the brightness of their surrounding background. Several UDs show fission and fusion. Thanks to the stable condition of space observation, we could first follow the temporal behavior of these events. The derived properties of internal structure of the umbra are discussed in viewpoint of magnetoconvection in a strong magnetic field.
Sol Phys, 1968
From an investigation of spectra in a magnetically sensitive (26173, g --2.5) and insensitive line (25576, g = 0), we derived the following properties for a symmetrical sunspot: (a) The magnetic field strength varies with the distance Q(O ~< 1) from the sunspot center like H(O) = H(0) (1 § 02) -I. (b) The zenith angle of the magnetic field varies like 90~ From this and from H(0)we find a height gradient of 0.5 gs/km at 0 --0. (c) The equivalent width and the half width of 25576 show an increase in penumbral regions of maximum Evershed flow. Most likely this is due to a combination of inhomogeneities in the Evershed flow and 'microturbulence'.
Properties of simulated sunspot umbral dots
Astronomy and Astrophysics, 2010
Realistic 3D radiative MHD simulations reveal the magneto-convective processes underlying the formation of the photospheric fine structure of sunspots, including penumbral filaments and umbral dots. Here we provide results from a statistical analysis of simulated umbral dots and compare them with reports from high-resolution observations. A multi-level segmentation and tracking algorithm has been used to isolate the bright structures in synthetic bolometric and continuum brightness images. Areas, brightness, and lifetimes of the resulting set of umbral dots are found to be correlated: larger umbral dots tend to be brighter and live longer. The magnetic field strength and velocity structure of umbral dots on surfaces of constant optical depth in the continuum at 630 nm indicate that the strong field reduction and high velocities in the upper parts of the upflow plumes underlying umbral dots are largely hidden from spectro-polarimetric observations. The properties of the simulated umbral dots are generally consistent with the results of recent highresolution observations. However, the observed population of small, short-lived umbral dots is not reproduced by the simulations, possibly owing to insufficient spatial resolution.
Spatial Distribution of the Origin of Umbral Waves in a Sunspot Umbra
The Astrophysical Journal, 2020
Umbral flashes (UFs) are emissions in the core of chromospheric lines caused by upward propagating waves steepening into shocks. UFs are followed by an expanding blue shifted umbral wave (UW) and red-shifted plasma returning to the initial state. Here we use 5 s cadence images acquired at ±0.04 nm off the Hα line center by the Visible Imaging Spectrometer (VIS) installed on the Goode Solar Telescope (GST) to detect the origin of UFs and UWs in a sunspot with a uniform umbra free of LBs and clusters of umbral dots. The data showed that UFs do not randomly originate over the umbra. Instead, they appear to be repeatedly triggered at locations with the lowest umbral intensity and the most powerful oscillations of Hα-0.04 nm intensity. GST magnetic field measurements using Near Infra-Red Imaging Spectropolarimeter (NIRIS) also showed that the dominant location of prevalent UF origin is co-spatial associated with the strongest fields in the umbra. Interface Region Imaging Spectrograph 149.0 nm images showed that no bright UV loops were anchored in the umbra in general and near the UF patches in particular suggesting that UFs and UWs alone can not be responsible for the origin of warm coronal loops. We thus conclude that the existence of locations with prevalent origin of UFs confirms the idea that they may be driven by a sub-surface source located near the axis of a flux rope, while the presence of several UFs trigger centers may indicate the complex structure of a sunspot umbra.
Magnetic Structure of Umbral Dots Observed with the Hinode Solar Optical Telescope
Publications of the Astronomical Society of Japan, 2009
A high-resolution, seeing-free spectroscopic observation of a decaying sunspot was made with the Solar Optical Telescope aboard the Hinode satellite. The target was NOAA 10944, located in the west side of the solar surface from 2007 March 2 to March 4. The umbra included many umbral dots (UDs) with a size of 300 km in continuum light. We report on the magnetic structures and Doppler velocity fields around UDs, based on the Milne-Eddington inversions of the two iron absorption lines at 6302Å. Histograms of the magnetic field strength (B), inclination angle (i), and Doppler velocity (v) of UDs showed a center-to-limb variation; observed at the disk center, the UDs had (i) slightly smaller field strength (ΔB = 17 Gauss) and (ii) relative blue shifts (Δv = 28 m s 1) compared to their surroundings. When the sunspot got close to the limb, UDs and their surroundings showed almost no difference in the magnetic and Doppler values. This center-to-limb variation can be understood by the formation height difference in a cusp-shaped magnetized atmosphere around UDs, due to the weakly magnetized hot gas intrusion. In addition, some UDs showed the oscillatory light curves with multiple peaks separated around 10 min, which may indicate the presence of the oscillatory convection. We discuss our results within the frameworks of two theoretical models: the monolithic model (
The Astrophysical Journal, 2015
We analyse sunspot oscillations using Interface Region Imaging Spectrograph (IRIS) slit-jaw and spectral data and narrow-band chromospheric images from the New Solar Telescope (NST) for the main sunspot in NOAA AR 11836. We report that the difference between the shock arrival times as measured the Mg II k 2796.35Å and Si IV 1393.76Å line formation levels changes during the observed period and peakto-peak delays may range from 40 s to zero. The intensity of chromospheric shocks also displays a long term (about 20 min) variations. NST's high spatial resolution Hα data allowed us to conclude that in this sunspot umbral flashes (UFs) appeared in the form of narrow bright lanes stretched along the light bridges and around clusters of umbral bright points. Time series also suggested that UFs preferred to appear on the sunspot-center side of light bridges, which may indicate the existence of a compact sub-photospheric driver of sunspot oscillations. The sunspot's umbra as seen in the IRIS chromospheric and transition region data appears bright above the locations of light bridges and the areas where the dark umbra is dotted with clusters of umbral dots. Co-spatial and co-temporal data from the Atmospheric Imaging Assembly on board Solar Dynamics Observatory showed that the same locations were associated with bright footpoints of coronal loops suggesting that the light bridges may play an important role in heating the coronal sunspot loops. Finally, the power spectra analysis showed that the intensity of chromospheric and transition region oscillations significantly vary across the umbra and with height, suggesting that umbral nonuniformities and the structure of sunspot magnetic fields may play a role in wave propagation and heating of umbral loops.
PROPERTIES OF UMBRAL DOTS AS MEASURED FROM THE NEW SOLAR TELESCOPE DATA AND MHD SIMULATIONS
The Astrophysical Journal, 2012
We studied bright umbral dots (UDs) detected in a moderate size sunspot and compared their statistical properties to recent MHD models. The study is based on high-resolution data recorded by the New Solar Telescope at the Big Bear Solar Observatory and three-dimensional (3D) MHD simulations of sunspots. Observed UDs, living longer than 150 s, were detected and tracked in a 46 minute long data set, using an automatic detection code. A total of 1553 (620) UDs were detected in the photospheric (low chromospheric) data. Our main findings are (1) none of the analyzed UDs is precisely circular, (2) the diameter-intensity relationship only holds in bright umbral areas, and (3) UD velocities are inversely related to their lifetime. While nearly all photospheric UDs can be identified in the low chromospheric images, some small closely spaced UDs appear in the low chromosphere as a single cluster. Slow-moving and long-living UDs seem to exist in both the low chromosphere and photosphere, while fast-moving and short-living UDs are mainly detected in the photospheric images. Comparison to the 3D MHD simulations showed that both types of UDs display, on average, very similar statistical characteristics. However, (1) the average number of observed UDs per unit area is smaller than that of the model UDs, and (2) on average, the diameter of model UDs is slightly larger than that of observed ones.
Fine structures in the atmosphere above a sunspot umbra
Astronomy & Astrophysics, 2013
We present simultaneous photospheric and chromospheric observations of the trailing sunspot in NOAA 10904, obtained with the Swedish Solar Telescope (SST) La Palma, Canary Islands. Time series of high resolution Ca ii H images show transient jet-like structures in sunspot umbrae are elongated, which we call umbral microjets. These jets are directed roughly parallel to nearby penumbral microjets, suggesting that they are aligned with the background magnetic field. In general, first a bright dot-like structure appears, from which a jet later emerges, although some jets appear without an associated chromospheric dot. Bright photospheric umbral dots are associated with umbral microjets arising in the outer umbra. Nevertheless, a one-to-one correspondence between jet-like events and underlying umbral dots is not seen. They are typically less than 1 long and less than 0. 3 wide. The typical lifetime of umbral microjets is around one minute. The brightness of these structures increases from the center of the umbra toward the umbra-penumbra boundary along with the brightness of the local background.
HIGH RESOLUTION OBSERVATIONS OF CHROMOSPHERIC JETS IN SUNSPOT UMBRA
The Astrophysical Journal, 2014
Recent observations of sunspot's umbra suggested that it may be finely structured at a sub-arcsecond scale representing a mix of hot and cool plasma elements. In this study we report the first detailed observations of the umbral spikes, which are cool jet-like structures seen in the chromosphere of an umbra. The spikes are cone-shaped features with a typical height of 0.5-1.0 Mm and a width of about 0.1 Mm. Their life time ranges from 2 to 3 min and they tend to re-appear at the same location. The spikes are not associated with photospheric umbral dots and they rather tend to occur above darkest parts of the umbra, where magnetic fields are strongest. The spikes exhibit up and down oscillatory motions and their spectral evolution suggests that they might be driven by upward propagating shocks generated by photospheric oscillations. It is worth noting that triggering of the running penumbral waves seems to occur during the interval when the spikes reach their maximum height. Subject headings:
Velocity fields in and around sunspots at the highest resolution
2010
The flows in and around sunspots are rich in detail. Starting with the Evershed flow along low-lying flow channels, which are cospatial with the horizontal penumbral magnetic fields, Evershed clouds may continue this motion at the periphery of the sunspot as moving magnetic features in the sunspot moat. Besides these well-ordered flows, peculiar motions are found in complex sunspots, where they contribute to the build-up or relaxation of magnetic shear. In principle, the three-dimensional structure of these velocity fields can be captured. The line-of-sight component of the velocity vector is accessible with spectroscopic measurements, whereas local correlation or feature tracking techniques provide the means to assess horizontal proper motions. The next generation of ground-based solar telescopes will provide spectropolarimetric data resolving solar fine structure with sizes below 50 km. Thus, these new telescopes with advanced post-focus instruments act as a "zoom lens" to study the intricate surface flows associated with sunspots. Accompanied by "wide-angle" observations from space, we have now the opportunity to describe sunspots as a system. This review reports recent findings related to flows in and around sunspots and highlights the role of advanced instrumentation in the discovery process.