Spiral-shaped wavefronts in a sunspot umbra

Abstract

Context. Solar active regions show a wide variety of oscillatory phenomena. The presence of the magnetic field leads to the appearance of several wave modes, whose behavior is determined by the sunspot thermal and magnetic structure. Aims. We aim to study the relation between the umbral and penumbral waves observed at the high photosphere and the magnetic field topology of the sunspot. Methods. Observations of the sunspot in active region NOAA 12662 obtained with the GREGOR telescope (Observatorio del Teide, Tenerife, Spain) were acquired on 2017 June 17. The data set includes a temporal series in the Fe i 5435 Å line obtained with the imaging spectrograph GREGOR Fabry-Pérot Interferometer (GFPI) and a spectropolarimetric raster map acquired with the GREGOR Infrared Spectrograph (GRIS) in the 10830 Å spectral region. The Doppler velocity deduced from the restored Fe i 5435 Å line has been determined, and the magnetic field vector of the sunspot has been inferred from spectropolarimetric inversions of the Ca i 10839 Å and the Si i 10827 Å lines. Results. A two-armed spiral wavefront has been identified in the evolution of the two-dimensional velocity maps from the Fe i 5435 Å line. The wavefronts initially move counterclockwise in the interior of the umbra, and develop into radially outward propagating running penumbral waves when they reach the umbra-penumbra boundary. The horizontal propagation of the wavefronts approximately follows the direction of the magnetic field, which shows changes in the magnetic twist with height and horizontal position. Conclusions. The spiral wavefronts are interpreted as the visual pattern of slow magnetoacoustic waves which propagate upward along magnetic field lines. Their apparent horizontal propagation is due to their sequential arrival to different horizontal positions at the formation height of the Fe i 5435 Å line, as given by the inclination and orientation of the magnetic field.

Shock Heating Energy of Umbral Flashes Measured with Integral Field Unit Spectroscopy

“…Rouppe van der Voort et al 2003;Nagashima et al 2007). In the figure 10 animation, the evolution of the shocks show spiral patterns (López Ariste et al 2013Su et al 2016;Jess et al 2017;Felipe et al 2019;Kang et al 2019). The radial propagation of the pattern may be apparent propagation rather than physical wave propagation (Bogdan & Judge 2006).…”

The dynamics of 3-min wavefronts and their relation to sunspot magnetic fields

“…It has been suggested that the single-spiral structure of the wavefronts may be caused by reflections at an umbral light bridge [12], while the multi-spiral structuring may be associated with swirling magnetic fields beneath the umbral photosphere. It has been shown that wavefronts initially move in a counter-clockwise direction in the interior of the umbra, before gradually developing into radial running penumbral waves (RPWs) [13]. The propagation of horizontal waves is related to the direction of magnetic fields, essentially dependent on the magnetic vortex created across the umbral space and with geometric height.…”

The Physical Nature of Spiral Wave Patterns in Sunspots

“…Interestingly, recent observational works reported that in the horizontal plane, 3 minute oscillations often appear in sunspot umbrae as one-and two-armed spiral wave patterns (SWPs; Sych & Nakariakov 2014;Su et al 2016;Felipe et al 2019). SWPs apparently propagate radially out at the velocity of around 20 km s −1 , and also propagate upward (Su et al 2016).…”