Astronomy and Astrophysics – Astronomy
Scientific paper
Oct 2010
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2010apj...722..888b&link_type=abstract
The Astrophysical Journal, Volume 722, Issue 1, pp. 888-898 (2010).
Astronomy and Astrophysics
Astronomy
1
Sun: Atmosphere, Sun: Chromosphere, Sun: Oscillations, Sunspots
Scientific paper
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.
Bard Steven
Carlsson Marcus
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