Magnetic Free-Energy in the Solar Atmosphere

  1. J. H. Waite Jr.,
  2. J. L. Burch and
  3. R. L. Moore
  1. B. C. Low

Published Online: 18 MAR 2013

DOI: 10.1029/GM054p0021

Solar System Plasma Physics

Solar System Plasma Physics

How to Cite

Low, B. C. (1989) Magnetic Free-Energy in the Solar Atmosphere, in Solar System Plasma Physics (eds J. H. Waite, J. L. Burch and R. L. Moore), American Geophysical Union, Washington, D. C.. doi: 10.1029/GM054p0021

Author Information

  1. High Altitude Observatory, National Center for Atmospheric Research, Boulder, CO 80307

Publication History

  1. Published Online: 18 MAR 2013
  2. Published Print: 1 JAN 1989

ISBN Information

Print ISBN: 9780875900742

Online ISBN: 9781118664315

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Keywords:

  • Space plasmas;
  • Sun;
  • Magnetosphere;
  • Astrophysics

Summary

The solar atmosphere is an excellent electrical conductor and the electric current flowing in it is a source of stored energy which can, under the appropriate circumstance, fuel a variety of plasma and hydromagnetic instabilities in such spectacular phenomena as the flares, prominence eruptions, and coronal mass ejections. These electric currents are associated with magnetic fields which permeate the solar atmosphere. The available spectroscopic methods of detecting solar magnetic fields allow only the fields in the photosphere and chromosphere to be measured with useful spatial resolutions down to about an arc sec. The line-of-sight component of the field can be measured and interpreted with considerable confidence while the measurement of the transverse field component involves a different piece of physics, straining existing techniques in both instrumentation and data interpretation. It has long been recognized that measurement of the line-of-sight component of the magnetic field in the lower solar atmosphere by itself tells us nothing about the electric current in the solar atmosphere. Whereas, with measurement of all three field components, the prospect exists for extrapolating for the magnetic field and its electric current in the upper atmosphere. For long-lived structures in the solar active region, a reasonable assumption takes the field to be force-free. A program for extrapolating magnetic field in the force-free approximation has much to teach us about solar magnetic fields and this review surveys what can be done and the kind of pitfalls that beset the program. Of particular note is the possibility of detecting the magnetic free energy associated with the electric currents in the solar atmosphere directly from the vector field measurements alone. Many of the ideas described are not new but interest in these ideas has heightened with the current effort to build new magnetographs and to improve on the existing vector magnetographs.