Identification

Title

Spectral lines for polarization measurements of the coronal magnetic field. I. Theoretical intensities

Abstract

Infrared emission lines are potentially sensitive probes of components of the coronal vector magnetic field, through the Zeeman effect, and on its direction projected onto the plane of the sky, through fluorescent polarization of scattered photospheric light. Prompted by the advent of sensitive infrared array detectors, existing atomic data were reexamined to compile a complete list of coronal lines that may yield a detectable Zeeman effect, through careful differential measurements of Stokes profiles, at typical coronal field strengths of order 10 G. "Average" intensities were computed for a subset of promising forbidden coronal lines. A representative coronal density structure was used. The distribution of plasma with temperature was, at all heights in the corona, assumed to be that described by a standard differential emission measure from extreme-ultraviolet observations of the solar disk. Effects of excitation by photospheric radiation were included, as well as cascades from collisionally excited higher levels having the same principal quantum number as the ground levels. The largest source of error in the computed intensities lies in the form assumed for the emission measure distribution. The assumed density and temperature structure is too simple for detailed comparisons with observations of a particular coronal structure. Nevertheless, existing observed intensities are consistent with the calculations, which suggests that the theoretical intensities of (as yet) unobserved lines can be used as a basis for further study. The strongest predicted lines arise from magnetic dipole transitions within the ground terms of the 2sm2pn and 3sm3pn, m = 1, 2, n = 1,...,5, configurations. The most promising lines lie between 1 and 10 m, the lower limit being set by the need to detect small field strengths. The upper limit is set by the small Einstein A-coefficients and the smaller intensities of the exciting photospheric light, both of which lead to smaller forbidden line intensities. The most promising lines include [Fe XIII] 1.0747, 1.0798 m; [Si X] 1.43 m; [Si IX] 2.58, 3.93 m; [Mg VIII] 3.03 m; and [Mg VII] 5.50, 9.03 m. An aircraft experiment is being prepared to obtain targeted portions of the coronal spectrum between 1 and 10 m during the 1998 February 26 eclipse, with the goal of detecting some of these promising lines. This work will help toward the planning and development of efficient magnetographs, perhaps space-borne, for the routine measurement of coronal magnetic fields in the quiet and active Sun.

Resource type

document

Resource locator

Unique resource identifier

code

http://n2t.net/ark:/85065/d7c24w83

codeSpace

Dataset language

eng

Spatial reference system

code identifying the spatial reference system

Classification of spatial data and services

Topic category

geoscientificInformation

Keywords

Keyword set

keyword value

Text

originating controlled vocabulary

title

Resource Type

reference date

date type

publication

effective date

2016-01-01T00:00:00Z

Geographic location

West bounding longitude

East bounding longitude

North bounding latitude

South bounding latitude

Temporal reference

Temporal extent

Begin position

End position

Dataset reference date

date type

publication

effective date

1998-06-20T00:00:00Z

Frequency of update

Quality and validity

Lineage

Conformity

Data format

name of format

version of format

Constraints related to access and use

Constraint set

Use constraints

Limitations on public access

None

Responsible organisations

Responsible party

contact position

OpenSky Support

organisation name

UCAR/NCAR - Library

full postal address

PO Box 3000

Boulder

80307-3000

email address

opensky@ucar.edu

web address

http://opensky.ucar.edu/

name: homepage

responsible party role

pointOfContact

Metadata on metadata

Metadata point of contact