Ministerio de Ciencia, Innovación y Universidades Gobierno de Canarias Universidad de La Laguna CSIC Centro de Excelencia Severo Ochoa

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Observations of heating produced by Alfvén waves in a sunspot chromosphere

Author/s: S. D. T. Grant, D. B. Jess, T. V. Zaqarashvili, C. Beck, H. Socas-Navarro, M. J. Aschwanden, P. H. Keys, D. J. Christian, S. J. Houston, R. L. Hewitt

Reference: 2018 Nature Physics 14 480 | Link

A cartoon representation of a sunspot umbral atmosphere demonstrating a variety of shock phenomena. A side-on perspective of a typical sunspot atmosphere, showing magnetic field lines (orange cylinders) anchored into the photospheric umbra (bottom of image) and expanding laterally as a function of atmospheric height. The light blue annuli highlight the lower and upper extents of the mode conversion region for the atmospheric heights of interest. The mode conversion region on the lefthand side portraits a schematic of non-linear Alfven waves resonantly amplifying magneto-acoustic waves, increasing the shock ´ formation efficiency in this location. The mode conversion region on the righthand side demonstrates the coupling of upwardly propagating magneto-acoustic oscillations (the sinusoidal motions) into Alfven waves ´ (the elliptical structures), which subsequently develop tangential blue- and red-shifted plasma during the creation of Alfven shocks. The central portion represents the traditional creation of UFs that result from ´ the steepening of magneto-acoustic waves as they traverse multiple density scale heights in the lower solar atmosphere. Image not to scale. Credit: Grant et al. (2018).
A cartoon representation of a sunspot umbral atmosphere demonstrating a variety of shock phenomena. A side-on perspective of a typical sunspot atmosphere, showing magnetic field lines (orange cylinders) anchored into the photospheric umbra (bottom of image) and expanding laterally as a function of atmospheric height. The light blue annuli highlight the lower and upper extents of the mode conversion region for the atmospheric heights of interest. The mode conversion region on the lefthand side portraits a schematic of non-linear Alfven waves resonantly amplifying magneto-acoustic waves, increasing the shock ´ formation efficiency in this location. The mode conversion region on the righthand side demonstrates the coupling of upwardly propagating magneto-acoustic oscillations (the sinusoidal motions) into Alfven waves ´ (the elliptical structures), which subsequently develop tangential blue- and red-shifted plasma during the creation of Alfven shocks. The central portion represents the traditional creation of UFs that result from ´ the steepening of magneto-acoustic waves as they traverse multiple density scale heights in the lower solar atmosphere. Image not to scale. Credit: Grant et al. (2018).

Understanding why the temperature increases from the surface outward is one of the classic problems in solar physics. We suspect that this phenomenon is due to the magnetic field but it is far from clear how energy is transported and deposited in the upper layers. We have observed chromospheric heating in the chromosphere above a sunspot produced by Alfvén waves, a special type of magnetic waves. n a similar fashion to ocean waves, Alfvén waves were believed to travel upwards from the solar surface, before eventually ‘crashing’ in the outermost regions of the Sun’s atmosphere and giving off immense heat energy. Within the last 10 years, the existence of Alfvén waves in the Sun’s atmosphere has been proven beyond doubt. However, to date there has been no conclusive direct evidence of these waves converting their motion into physical heat; something that has puzzled physicists since their confirmed existence. In this work we have detected, for the first time, the heating produced by Alfvén waves in a sunspot, a theoretical prediction from 75 years ago.

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