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Coronal Magnetic Fields Above Active Regions

Published online by Cambridge University Press:  14 August 2015

David M. Rust  and
J.-René Roy
David M. Rust
Affiliation:
Sacramento Peak Observatory, Air Force Cambridge Research Laboratories, Sunspot, New Mex., U.S.A.
J.-René Roy
Affiliation:
Dept. of Astronomy, University of Western Ontario, London, Ontario, Canada

Abstract

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We have made detailed comparisons of coronal structure, as photographed at λ5303 Å, with magnetic lines of force, as computed from measurements of the longitudinal component of the underlying photospheric magnetic fields. Coronal fields were computed under the assumption that the space above the active regions is current-free. Out of 36 regions for which there exist both magnetic and coronal data at the Sacramento Peak Observatory, we found only four suitable for analysis. Using a light-pen attachment to a digital computer, we were able to choose lines of force whose footpoints in the photosphere best matched those of the lines of force which were suggested by local intensity variations in the corona. The lines of force thus computed give excellent agreement with the apparent height and shape of coronal fieldlines. In particular the magnetic field in the loop structures of 2 November 1969 and 18–19 November 1968 falls from several hundred gauss at 10000–20000 km above the photosphere to 2–20 G at heights of 100000–150000 km. These computed values are in general agreement with other measurements. Spectra of the loops reveal that the direction of the line-of-sight component of the motion agrees with that anticipated from a comparison of the computed fieldline orientation and the observed motion in the plane of observation. We conclude that coronal magnetic fields above slowly changing active regions are nearly the same as the vacuum potential fields derived from underlying photospheric field sources (principally the larger sunspots in a region) and that calculations of this type may be relied upon to give the magnetic fields for studies of coronal dynamics.

Type
Part VI: Optical and Radio Observations of Large Scale Magnetic Fields on the Sun
Information
Symposium - International Astronomical Union , Volume 43: Solar Magnetic Fields , 1971 , pp. 569 - 579
Copyright
Copyright © Reidel 1971 

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