Alfvén wave dissipation in the solar chromosphere

Abstract

Magnetohydrodynamic Alfvén waves1 have been a focus of laboratory plasma physics2 and astrophysics3 for over half a century. Their unique nature makes them ideal energy transporters, and while the solar atmosphere provides preferential conditions for their existence4, direct detection has proved difficult as a result of their evolving and dynamic observational signatures. The viability of Alfvén waves as a heating mechanism relies upon the efficient dissipation and thermalization of the wave energy, with direct evidence remaining elusive until now. Here we provide the first observational evidence of Alfvén waves heating chromospheric plasma in a sunspot umbra through the formation of shock fronts. The magnetic field configuration of the shock environment, alongside the tangential velocity signatures, distinguish them from conventional umbral flashes5. Observed local temperature enhancements of 5% are consistent with the dissipation of mode-converted Alfvén waves driven by upwardly propagating magneto-acoustic oscillations, providing an unprecedented insight into the behaviour of Alfvén waves in the solar atmosphere and beyond.

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Fig. 1: The building blocks of the magnetized solar atmosphere observed on 24 August 2014.
Fig. 2: A statistical insight into the magnetic, velocity and occurrence relationships between shock phenomena in a sunspot umbra.
Fig. 3: A cartoon representation of a sunspot umbral atmosphere demonstrating a variety of shock phenomena.
Fig. 4: The distribution of temperature enhancements resulting from Alfvén shocks close to the umbral boundary.

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Acknowledgements

S.D.T.G. and S.J.H. thank the Northern Ireland Department for Employment and Learning (now the Northern Ireland Department for the Economy) for the awards of PhD studentships. D.B.J. wishes to thank the UK Science and Technology Facilities Council for the award of an Ernest Rutherford Fellowship alongside a dedicated Research Grant. S.D.T.G. and D.B.J. also wish to thank Invest NI and Randox Laboratories Ltd for the award of a Research & Development Grant (059RDEN-1) that allowed this work to be undertaken. T.Z. was supported by the Austrian Science Fund (FWF) project P30695-N27 and by the Georgian Shota Rustaveli National Science Foundation project DI-2016-17. H.S.-N. acknowledges support from the Spanish Ministry of Economy and Competitivity through project AYA2014-60476-P (Solar Magnetometry in the Era of Large Solar Telescopes). P.H.K. is grateful to the Leverhulme Trust for the award of an Early Career Fellowship. The NSO is operated by the Association of Universities for Research in Astronomy under cooperative agreement with the National Science Foundation. The magnetic field measurements employed in this work are courtesy of NASA/SDO and the AIA, EVE and HMI science teams.

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S.D.T.G., D.B.J. and P.H.K. performed analysis of the observations. S.D.T.G., D.B.J., T.V.Z., M.J.A., D.J.C., S.J.H. and R.L.H. interpreted the observations. S.D.T.G, D.B.J., C.B., H.S.-N. and M.J.A. prepared and processed all data products. All authors discussed the results and commented on the manuscript.

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Correspondence to Samuel D. T. Grant.

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Grant, S.D.T., Jess, D.B., Zaqarashvili, T.V. et al. Alfvén wave dissipation in the solar chromosphere. Nature Phys 14, 480–483 (2018). https://doi.org/10.1038/s41567-018-0058-3

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