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Title:
Radiative Hydrodynamic Simulations of Acoustic Waves in Sunspots
Authors:
Bard, S.; Carlsson, M.
Affiliation:
AA(Institute of Theoretical Astrophysics, University of Oslo, P.O. Box 1029 Blindern, N-0315 Oslo, Norway), AB(Center of Mathematics for Applications, University of Oslo, P.O. Box 1053, Blindern, N-0316 Oslo, Norway.)
Publication:
The Astrophysical Journal, Volume 722, Issue 1, pp. 888-898 (2010). (ApJ Homepage)
Publication Date:
10/2010
Origin:
IOP
Astronomy Keywords:
Sun: atmosphere, Sun: chromosphere, Sun: oscillations, sunspots
DOI:
10.1088/0004-637X/722/1/888
Bibliographic Code:
2010ApJ...722..888B

Abstract

We investigate the formation and evolution of the Ca II H line in a sunspot. The aim of our study is to establish the mechanisms underlying the formation of the frequently observed brightenings of small regions of sunspot umbrae known as "umbral flashes." We perform fully consistent NLTE radiation hydrodynamic simulations of the propagation of acoustic waves in sunspot umbrae and conclude that umbral flashes result from increased emission of the local solar material during the passage of acoustic waves originating in the photosphere and steepening to shock in the chromosphere. To quantify the significance of possible physical mechanisms that contribute to the formation of umbral flashes, we perform a set of simulations on a grid formed by different wave power spectra, different inbound coronal radiation, and different parameterized chromospheric heating. Our simulations show that the waves with frequencies in the range 4.5-7.0 mHz are critical to the formation of the observed blueshifts of umbral flashes while waves with frequencies below 4.5 mHz do not play a role despite their dominance in the photosphere. The observed emission in the Ca II H core between flashes only occurs in the simulations that include significant inbound coronal radiation and/or extra non-radiative chromospheric heating in addition to shock dissipation.
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