Image of the Month

Every month, an image (or video) related to the WaLSA Team’s activities is showcased on this page. For a greater detail, please visit the original source given at the bottom of each image's caption.

Co-spatial images revealing the structure of a sunspot observed at 13:00 UT on 24 August 2014. The lower image shows the magnitude of the photospheric magnetic field from HMI/SDO, revealing high umbral field strengths (colour bar relates to the field strengths in gauss). The image above is taken from the blue wing of the Ca II 8542 Å spectral line from DST, displaying the photospheric representation of the sunspot. Above this is the photospheric plasma temperature of the region derived from CAISAR at log(τ500nm) ~ −2 (or ~ 250 km above the photosphere), showing the clear temperature distinction between the umbra, penumbra and surrounding quiet Sun (colour bar in units of kelvin). The upper image shows the chromospheric core of the Ca II 8542 Å spectral line from DST, highlighting the strong intensity gradient between the umbra and penumbra at these heights. In each of these images, the red contours represent the inner and outer boundaries of the plasma-β = 1 region at the height where shocks first begin to manifest (~ 250 km), where magneto-acoustic and Alfvén waves can readily convert.

Copyright: Grant, S. D. T., Jess, D. B., Zaqarashvili, T. V., et al. 2018, Nature Physics, 14, 480, doi: 10.1038/s41567-018-0058-3

This stack image shows the basis of designation of a sausage surface mode in a pore. The bottom panel shows the LOS magnetogram from HMI indicating the magnetic field of the pore and the sharp boundary in terms of magnetic field at the pore's edge. Above this is the full FOV ROSA G-band image showing the photospheric appearance of the pore taken on 2011 December 10. The blue box indicates the expanded region shown in the top three panels. The expanded G-band image has blue contours indicating the pore boundary established for that particular frame. Above this is the time-averaged pore boundary map showing the variation in boundary location during this observation sequence, where the arrows indicate the sausage mode oscillations present. The top panel is a two-dimensional power plot of the power across the pore obtained with wavelet transforms of the data filtered at a frequency of ~4.6 mHz. The blue contour shows the time average pore boundary location. Peaks in power at this boundary indicate that a sausage mode is observed at this frequency.

Copyright: Keys, P. H., Morton, R. J., Jess, D. B., Verth, G., et al. 2018, The Astrophysical Journal, 857, 28, doi: 10.3847/1538-4357/aab432

Signatures of running penumbral waves in sunspot photospheres:
Three-dimensional view of intensities and peak periods of intensity wave power of a sunspot at various wavelength positions. The intensities (left) show the sunspot at August 21st 2013 at 15:00:06 UTC (±3 s). The images along the z-axis belong to several line core and wing positions of Fe I 630.15 nm, Na I 589.6 nm, and Ca II 854.2 nm. The corresponding time-averaged (≈1h) distribution of peak periods TPEAK of the intensity wave power is shown on the right. The periods are scaled from 2.5 min (dark blue) to 8 min (dark red). The black contours indicate the location of the umbra (inner) and penumbra (outer) in continuum intensity (bottom panel). Whereas the length of the axis arrows represent distances around 1.5 Mm, the image positions along the z-axis are not to scale.

Copyright: J. Löhner-Böttcher and N. Bello González 2018, Astronomy & Astrophysics, 580, A53, doi: 10.1051/0004-6361/201526230

Observations of ubiquitous compressive waves in the Sun’s chromosphere:
(a) Typical ROSA Hα example of a pair of relatively large dark flux tubes at t=1,536 s measured from the beginning of the data series. (b) Time-distance plot revealing the dynamic motion. The position of the cross-cut is shown in (a), with the given distance starting at the top of the cross-cut. Times are given in seconds from the start of the data set. The results from a Gaussian fitting are over-plotted and show the nonlinear fast MHD kink wave (red line shows the central axis of the structure) and the fast MHD sausage mode (yellow bars show the measured width of structure). The transverse motion has a period of 232±8 s and we detect multi-directional propagating transversal wave trains in the MFT travelling with speeds of 71±22 km/s upwards and 87±26 km/s downwards. The typical velocity amplitudes are 5 km/s. The fast MHD sausage mode has a period of 197±8 s, a phase speed of 67±15 km/s and apparent velocity amplitudes of 1–2 km/s. (c) Comparison of MFTs intensity (blue) and width (red) perturbations from the Gaussian fitting. The data points have been fitted with a smoothed 3-point box-car function. The observed out-of-phase behaviour is typical of fast MHD sausage waves. The error bars plotted are the one-sigma errors on each data point calculated from the Gaussian fitting.

Copyright: Richard J. Morton, Gary Verth, David B. Jess, David Kuridze, Michael S. Ruderman, Mihalis Mathioudakis, and Robertus Erdélyi 2012, Nature Communications, 3, 1315, doi: 10.1038/ncomms2324

Transverse Oscillations in Slender Ca II H Fibrils Observed with SUNRISE/SuFI:
Left: A dense forest of slender bright fibrils near a small solar active region seen in high-quality narrowband Ca II H images from the SuFI instrument onboard the SUNRISE balloon-borne solar observatory.
Right: Phase speeds in an example slender Ca II H fibril (SCF) with an indication of wave propagation. The curves in panels (a)–(e) represent displacement of the SCF along a series of cuts across the fibril shown in the upper panel, from left to right, respectively. The red triple-dot–dashed lines are the centroids of a Gaussian fit to the oscillations smoothed by convolving with a boxcar filter of 0.05 arcsec width. The green lines indicate waves propagation in the SCF in a direction corresponding to from right to left in the top panel.

Copyright: Shahin Jafarzadeh, Sami K. Solanki, Ricardo Gafeira, Michiel van Noort, et al. 2017, The Astrophysical Journal Supplement Series, 229, 1, 9, doi: 10.3847/1538-4365/229/1/9

Semi-empirical model atmospheres for the chromosphere of the sunspot penumbra and umbral flashes:
Columns showing the observed (black dots; from the Swedish 1- Solar Telescope, SST) and synthetic (red) full Stokes spectra in units of normalized HSRA (Gingerich et al. 1971) continuum intensity at disk center at a wavelength in the middle of the spectral range, and their atmospheric parameters such as temperature, LOS magnetic field, velocity, and microturbulent velocity for three different models: hot penumbra (asterisk), cool penumbra ('cross') and umbral flash ('plus sign'), respectively. The continuous line overplotted on the dashed line for the LOS magnetic field, shows the variation of the magnetic field in the region log(tau) = [-6,-2] where the Ca 8542 spectra is most sensitive.

Copyright: S. Bose, Vasco M.J. Henriques, L. Rouppe van der Voort, and T.M.D. Pereira 2019, Astronomy & Astrophysics, 627, A46, doi: 10.1051/0004-6361/201935289

Second meeting of the WaLSA Team
Oslo, Norway; 12-16 August 2019

Spiral-shaped wavefronts in a sunspot umbra:
Temporal evolution of the Doppler velocity, inferred from the GFPI Fe I 5435 Å restored images (from observations with GREGOR; a 1.5 m solar telescope), during the appearance of a two-armed spiral wavefront, filtered in frequency bands. From left to right columns: 2–8 mHz, 2–3.5 mHz, 3.5–4.7 mHz, 4.7–8 mHz. T The black lines mark the umbral and penumbral boundaries. The green circles highlight the location of the spiral wavefronts.

Copyright: Tobías Felipe, C. Kuckein, Elena Khomenko, and I. Thaler 2019, Astronomy & Astrophysics, 621, A43, doi: 10.1051/0004-6361/201834367