Non-LTE calculation of the Ni I 676.8 nanometer line in a flaring atmosphere

Document Type

Article

Publication Date

9-1-2002

Abstract

The Ni I 676.8 nm line is used by the Solar and Heliospheric Observatory Michelson Doppler Imager to measure the magnetic field and velocity field in the solar atmosphere. We make non-LTE calculations of this line in an atmosphere that is bombarded by an energetic electron beam. This case is associated with the occurrence of solar flares. The electron beam produces nonthermal ionization and excitation of the hydrogen atoms and redistributes the level populations. This results in an enhanced opacity near the Ni I line and an upward shift of its formation height, as well as an increased line source function. We find that the Ni I line may appear in emission in the presence of a fairly strong electron beam and preferentially in a cool atmosphere (i.e., sunspot umbrae/penumbrae). On the other hand, if there is no bombarding electron beam, the profile can hardly turn to emission even though the atmosphere may be heated to higher temperatures through other ways. This result implies that the sign reversal of the longitudinal magnetic field observed in some flare events may not be a true reversal but just an artifact associated with the production of an emission profile.

Identifier

0041730887 (Scopus)

Publication Title

Astrophysical Journal

External Full Text Location

https://doi.org/10.1086/343103

e-ISSN

15384357

ISSN

0004637X

First Page

L83

Last Page

L86

Issue

1 II

Volume

576

Grant

G20000784

Fund Ref

National Science Foundation

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