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Sunspot umbra atmosphere from full Stokes inversion  [PDF]
R. Wenzel,S. V. Berdyugina,D. M. Fluri,J. Arnaud,A. Sainz-Dalda
Physics , 2010,
Abstract: Sunspots are prominent manifestations of the solar cycle and provide key constraints for understanding its operation. Also, knowing internal structure of sunspots allows us to gain insights on the energy transport in strong magnetic fields and, thus, on the processes inside the convection zone, where solar magnetic fields are generated and amplified before emerging at the surface on various scales, even during solar minima. In this paper, we present results of a spectropolarimetric analysis of a sunspot observed during the declining phase of the solar cycle 23. By inversion of full Stokes spectra observed in several spectral regions in the optical at the THEMIS facility we infer the height dependence of physical quantities such as the temperature and the magnetic field strength for different sunspot regions. The simultaneous use of atomic (Fe{\sc i} 5250.2 and 5250.6 \AA) and highly temperature sensitive molecular (TiO 7055 \AA and MgH 5200 \AA) lines allow us to improve a model of the sunspot umbra.
Three-minute oscillations above sunspot umbra observed with SDO/AIA and NoRH  [PDF]
V. E. Reznikova,K. Shibasaki,R. A. Sych,V. M. Nakariakov
Physics , 2011, DOI: 10.1088/0004-637X/746/2/119
Abstract: Three-minute oscillations over sunspot's umbra in AR 11131 were observed simultaneously in UV/EUV emission by SDO/AIA and in radio emission by Nobeyama Radioheliograph (NoRH). We use 24-hours series of SDO and 8-hours series of NoRH observations to study spectral, spatial and temporal variations of pulsations in the 5-9 mHz frequency range at different layers of the solar atmosphere. High spatial and temporal resolution of SDO/AIA in combination with long-duration observations allowed us to trace the variations of the cut-off frequency and spectrum of oscillations across the umbra. We found that higher frequency oscillations are more pronounced closer to the umbra's center, while the lower frequencies concentrate to the peripheral parts. We interpreted this discovery as a manifestation of variation of the magnetic field inclination across the umbra at the level of temperature-minimum. Possible implications of this interpretation for the diagnostics of sunspot atmospheres is discussed.
A Steady-state Supersonic Downflow in the Transition Region above a Sunspot Umbra  [PDF]
Thomas Straus,Bernhard Fleck,Vincenzo Andretta
Physics , 2015, DOI: 10.1051/0004-6361/201525805
Abstract: We investigate a small-scale ($\approx$ 1.5 Mm along the slit), supersonic downflow of about 90 km s$^{-1}$ in the transition region above the light-bridged sunspot umbra in AR 11836. The observations were obtained with the Interface Region Spectrograph (IRIS) on 2013 September 2, from 16:40 to 17:59 UT. The downflow shows up as red-shifted "satellite" lines of the Si IV and O IV transition region lines and is remarkably steady over the observing period of nearly 80 min. The downflow is not visible in the chromospheric lines, which only show an intensity enhancement at the location of the downflow. The density inferred from the line ratio of the red-shifted satellites of the O IV lines ($N_\mathrm{e} = 10^{10.6\pm0.25} \mathrm{cm}^{-3}$) is only a factor 2 smaller than the one inferred from the main components ($N_\mathrm{e} = 10^{10.95\pm0.20} \mathrm{cm}^{-3}$). Consequently, this implies a substantial mass flux ($\approx 5 \times 10^{-7}$ g cm$^{-2}$ s$^{-1}$), which would evacuate the overlying corona on time scales of the order of 10 s. We interpret these findings as evidence of a stationary termination shock of a supersonic siphon flow in a cool loop rooted in the central umbra of the spot.
Wave dynamics in a sunspot umbra  [PDF]
R. Sych,V. M. Nakariakov
Physics , 2014, DOI: 10.1051/0004-6361/201424049
Abstract: The high spatial and time resolution data obtained with SDO/AIA for the sunspot in active region NOAA 11131 on 08 December 2010 were analysed with the time-distance plot technique and the pixelised wavelet filtering method. Oscillations in the 3 min band dominate in the umbra. The integrated spectrum of umbral oscillations contains distinct narrowband peaks at 1.9 min, 2.3 min, and 2.8 min. The power significantly varies in time, forming distinct oscillation trains. The oscillation power distribution over the sunspot in the horizontal plane reveals that the enhancements of the oscillation amplitude, or wave fronts, have a distinct structure consisting of an evolving two-armed spiral and a stationary circular patch at the spiral origin, situated near the umbra centre. This structure is seen from the temperature minimum to the corona. In time, the spiral rotates anti-clockwise. The wave front spirality is most pronounced during the maximum amplitude phases of the oscillations. In the low-amplitude phases the spiral breaks into arc-shaped patches. The 2D cross-correlation function shows that the oscillations at higher atmospheric levels occur later than at lower layers. The phase speed is estimated to be about 100 km/s. The fine spectral analysis shows that the central patch corresponds to the high-frequency oscillations, while the spiral arms highlight the lower-frequency oscillations in the 3-min band. The vertical and horizontal radial structure of the oscillations is consistent with the model that interprets umbral oscillations as slow magnetoacoustic waves filtered by the atmospheric temperature non-uniformity in the presence of the magnetic field inclination from the vertical. The mechanism for the polar-angle structure of the oscillations, in particular the spirality of the wave fronts, needs to be revealed.
High Resolution Observations of Chromospheric Jets in Sunspot Umbra  [PDF]
V. Yurchyshyn,V. Abramenko,S. Kosovichev,P. Goode
Physics , 2014, DOI: 10.1088/0004-637X/787/1/58
Abstract: 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.
The chromosphere above sunspots at millimeter wavelengths  [PDF]
Maria A. Loukitcheva,Sami K. Solanki,Stephen M. White
Physics , 2014, DOI: 10.1051/0004-6361/201321321
Abstract: Aims: The aim of this paper is to demonstrate that millimeter wave data can be used to distinguish between various atmospheric models of sunspots, whose temperature structure in the upper photosphere and chromosphere has been the source of some controversy. Methods: We use observations of the temperature contrast (relative to the quiet Sun) above a sunspot umbra at 3.5 mm obtained with the Berkeley-Illinois-Maryland Array (BIMA), complemented by submm observations from Lindsey & Kopp (1995) and 2 cm observations with the Very Large Array. These are compared with the umbral contrast calculated from various atmospheric models of sunspots. Results: Current mm and submm observational data suggest that the brightness observed at these wavelengths is low compared to the most widely used sunspot models. These data impose strong constraints on the temperature and density stratifications of the sunspot umbral atmosphere, in particular on the location and depth of the temperature minimum and the location of the transition region. Conclusions: A successful model that is in agreement with millimeter umbral brightness should have an extended and deep temperature minimum (below 3000 K). Better spatial resolution as well as better wavelength coverage are needed for a more complete determination of the chromospheric temperature stratification above sunspot umbrae.
A closer look at a coronal loop rooted in a sunspot umbra  [PDF]
L. P. Chitta,H. Peter,P. R. Young
Physics , 2015,
Abstract: Extreme UV (EUV) and X-ray loops in the solar corona connect regions of enhanced magnetic activity, but usually they are not rooted in the dark umbrae of sunspots. This is because there the strong magnetic field suppresses convection and thus the Poynting flux of magnetic energy into the upper atmosphere is not significant within the umbra, as long as there are no light bridges, umbral dots. Here we report a rare observation of a coronal loop rooted in the dark umbra of a sunspot without any traces of light bridges or umbral dots. We used the slit-jaw images and spectroscopic data from the IRIS and concentrate on the line profiles of O IV and Si IV that show persistent strong redshifted components in the loop rooted in the umbra. Using the ratios of O IV, we can estimate the density and thus investigate the mass flux. The coronal context and temperature diagnostics of these observations is provided through the EUV channels of the AIA. The coronal loop, embedded within cooler downflows, is hosting supersonic downflows. The speed of more than 100 km s$^{-1}$ is of the same order of magnitude in the transition region lines of O IV and Si IV, and is even seen at comparable speed in the chromospheric Mg II lines. At a projected distance of within 1" from the footpoint, we see a shock transition to smaller downflow speeds of about 15 km s$^{-1}$ being consistent with mass conservation across a stationary isothermal shock. We see no (direct) evidence for energy input into the loop because the loop is rooted in the dark uniform part of the umbra, with no light bridges or umbral dots around. Thus one might conclude that we see a siphon flow driven from the footpoint at the other end of the loop. However, for a final result one would need data of similar quality at the other footpoint, which is too far away to be covered by the field-of-view of IRIS.
Dynamics in Sunspot Umbra as Seen in New Solar Telescope and Interface Region Imaging Spectrograph Data  [PDF]
Vasyl Yurchyshyn,Valentyna Abramenko,Ali Kilcik
Physics , 2014, DOI: 10.1088/0004-637X/798/2/136
Abstract: 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 2796.35\AA\ and Si IV 1393.76\AA\ line formation levels changes during the observed period and peak-to-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 \ha\ 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 non-uniformities and the structure of sunspot magnetic fields may play a role in wave propagation and heating of umbral loops.
Chromospheric seismology above sunspot umbrae  [PDF]
B. Snow,G. J. J. Botha,S. Regnier
Physics , 2015, DOI: 10.1051/0004-6361/201526115
Abstract: The acoustic resonator is an important model for explaining the three-minute oscillations in the chromosphere above sunspot umbrae. The steep temperature gradients at the photosphere and transition region provide the cavity for the acoustic resonator, which allows waves to be both partially transmitted and partially reflected. In this paper, a new method of estimating the size and temperature profile of the chromospheric cavity above a sunspot umbra is developed. The magnetic field above umbrae is modelled numerically in 1.5D with slow magnetoacoustic wave trains travelling along magnetic fieldlines. Resonances are driven by applying the random noise of three different colours---white, pink and brown---as small velocity perturbations to the upper convection zone. Energy escapes the resonating cavity and generates wave trains moving into the corona. Line of sight (LOS) integration is also performed to determine the observable spectra through SDO/AIA. The numerical results show that the gradient of the coronal spectra is directly correlated with the chromosperic temperature configuration. As the chromospheric cavity size increases, the spectral gradient becomes shallower. When LOS integrations is performed, the resulting spectra demonstrate a broadband of excited frequencies that is correlated with the chromospheric cavity size. The broadband of excited frequencies becomes narrower as the chromospheric cavity size increases. These two results provide a potentially useful diagnostic for the chromospheric temperature profile by considering coronal velocity oscillations.
Synthetic observations of wave propagation in a sunspot umbra  [PDF]
T. Felipe,H. Socas-Navarro,E. Khomenko
Physics , 2014, DOI: 10.1088/0004-637X/795/1/9
Abstract: Spectropolarimetric temporal series from Fe I $\lambda$ 6301.5 \AA\ and Ca II infrared triplet lines are obtained by applying the Stokes synthesis code NICOLE to a numerical simulation of wave propagation in a sunspot umbra from MANCHA code. The analysis of the phase difference between Doppler velocity and intensity core oscillations of the Fe I $\lambda$ 6301.5 \AA\ line reveals that variations in the intensity are produced by opacity fluctuations rather than intrinsic temperature oscillations, except for frequencies between 5 and 6.5 mHz. On the other hand, the photospheric magnetic field retrieved from the weak field approximation provides the intrinsic magnetic field oscillations associated to wave propagation. Our results suggest that this is due to the low magnetic field gradient of our sunspot model. The Stokes parameters of the chromospheric Ca II infrared triplet lines show striking variations as shock waves travel through the formation height of the lines, including emission self-reversals in the line core and highly abnormal Stokes V profiles. Magnetic field oscillations inferred from the Ca II infrared lines using the weak field approximation appear to be related with the magnetic field strength variation between the photosphere and the chromosphere.
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