Chromospheric backradiation in ultraviolet continua and Hα

¹ Sterrekundig Instituut, Utrecht University, Postbus 80 000, 3508 TA Utrecht, The Netherlands
e-mail: r.j.rutten@uu.nl
² Institute of Theoretical Astrophysics, University of Oslo, PO Box 1029, Blindern, 0315 Oslo, Norway
³ Lockheed-Martin Solar and Astrophysics Laboratory, 3251 Hanover Street, Palo Alto, CA 94304, USA
⁴ National Solar Observatory / Sacramento Peak, PO Box 62, Sunspot, NM 88349, USA
Operated by the Association of Universities for Research in Astronomy, Inc. (AURA), for the National Science Foundation.

Received: 26 November 2011
Accepted: 29 February 2012

Abstract

A recent paper states that ultraviolet backradiation from the solar transition region and upper chromosphere strongly affects the degree of ionization of minority stages at the top of the photosphere, i.e., in the temperature minimum of the one-dimensional static model atmospheres presented in that paper. We show that this claim is incompatible with observations and we demonstrate that the pertinent ionization balances are instead dominated by outward photospheric radiation, as in older static models. We then analyze the formation of Hα in the above model and show that it has significant backradiation across the opacity gap by which Hα differs from other strong scatttering lines.