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