J. Dudík, C. Denker, H. Balthasar, J. Jurčák, W. Liu, and the GREGOR Team

A small flare ribbon above a sunspot umbra in active region 12205 was observed on November 7, 2014, at 12:00 UT in the blue imaging channel of the 1.5-m GREGOR telescope, using a 1 A Ca II H interference filter. Context observations from SDO/AIA, Hinode/SOT, and IRIS show that the ribbon is a part of a larger one that extends through the neighboring positive polarities and also participates in several other flares within the active region. A 140 second long time series of Ca II H images was reconstructed by means of the Multi-Frame Blind Deconvolution method, giving the respective spatial and temporal resolutions of 0.1 arcsec and 1 s. Light curves and horizontal velocities of small-scale bright knots in the observed flare ribbon were measured. Some knots are stationary but three move along the ribbon with speeds of 7 - 11 km/s. Two of them move in the opposite direction and exhibit highly correlated intensity changes, providing evidence for the presence of slipping reconnection.

P. Heinzel, M. Švanda, J. Jurčák, D. del Moro, and F. Berrilli

Several mechanisms may heat the solar chromosphere: acoustic waves, magnetoacoustic waves (slow, fast, and Alfven waves), and small-scale magnetic reconnections. Based on observations in the Ca II 854.2 nm line, the contribution of acoustic waves to the heating of quiet and plage regions in the chromosphere is discussed. The point is to compare the energy released by radiative losses with the energy deposited by acoustic waves. Radiative losses are computed using a grid of semi-empirical chromospheric models. The deposited acoustic flux is calculated using power spectra of Doppler oscillations measured in the Ca II line core. The comparison shows that the spatial correlation of maps of radiative losses and acoustic flux is 72 %. The deposited acoustic flux covers only 15 % of radiative losses in quiet chromosphere but 23 % in network and 54 % in plage areas. This estimate is a lower limit of the real acoustic energy flux.

Reza Rezaei, IAC

We present high-resolution full-Stokes measurements of sunspots, using the highly-sensitive full-split (g=3) infrared line Fe I 1564.85 nm, and compare them with simultaneous continuum intensity measurements. We extend the classical work of Kopp & Rabin (1992, Solar Phys. 141, 253) from the intensity to the full Stokes vector. The data were obtained with the GREGOR Infrared Spectrograph (GRIS) on May 11, 2015, using the adaptive optics system. The observations include three large sunspots of active region 12339, with spatial resolution better than 0.5" in the spectral scans. The continuum intensity is corrected for instrumental scattered light and the brightness temperature is calculated. Magnetic field strength and inclination are derived directly from the line split and ratio of Stokes components. The continuum intensity relation to the field strength and inclination are studied separately in the umbra, light bridges, and penumbra. The results agree with previous studies but the higher spatial resolution leads to a larger scatter of values in the continuum intensity (temperature) and magnetic-field relations.

Michal Švanda, Petr Heinzel

An EST science case concerning the possible heating of solar chromosphere by magneto-acoustic waves is elaborated. The case is based on observations of intensity, polarisation, and Doppler velocity of selected spectral lines with high temporal resolution to detect fluctuations with frequencies up to 40 mHz. The required observing parameters, including the list of proposed spectral lines, together with the tools for velocity and magnetic-field measurements are described. An important point is the determination of time-dependent and time-averaged model atmospheres under the non-LTE conditions in quiet and magnetic regions. These models are used to calculate the energy flux transported by magneto-acoustic waves and net radiative cooling rates at different heights in the atmosphere. Finally, an example of a comparison between acoustic energy fluxes and radiative losses in quiet and magnetised chromosphere is given.