The Boundary of the Polar Cap and its Relation to Electric Fields, Field-Aligned Currents, and Auroral Particle Precipitation

  1. S.-I. Akasofu and
  2. J.R. Kan
  1. R. B. Torbert1,
  2. C. A. Cattell1,
  3. F. S. Mozer1 and
  4. C.-I. Meng2

Published Online: 26 MAR 2013

DOI: 10.1029/GM025p0143

Physics of Auroral Arc Formation

Physics of Auroral Arc Formation

How to Cite

Torbert, R. B., Cattell, C. A., Mozer, F. S. and Meng, C.-I. (1981) The Boundary of the Polar Cap and its Relation to Electric Fields, Field-Aligned Currents, and Auroral Particle Precipitation, in Physics of Auroral Arc Formation (eds S.-I. Akasofu and J.R. Kan), American Geophysical Union, Washington, D. C.. doi: 10.1029/GM025p0143

Author Information

  1. 1

    Space Sciences Laboratory, University of California, Berkeley, California 94720

  2. 2

    Applied Physics Laboratory, the Johns Hopkins University, Laurel, Maryland 20810

Publication History

  1. Published Online: 26 MAR 2013
  2. Published Print: 1 JAN 1981

ISBN Information

Print ISBN: 9780875900506

Online ISBN: 9781118664360

SEARCH

Keywords:

  • Auroras—Addresses, essays, lectures

Summary

Simultaneous observations of electric fields, field-aligned currents, and auroral particles on the S3-3 spacecraft over the earth's polar regions have been used to illustrate how the low-altitude extent of field lines open to interplanetary space may be unambiguously identified. We define the polar cap to be the region of open field lines, which are only those containing plasma flowing anti-sunward and possessing precipitating low-energy electron distributions characteristic of the magnetosheath. Field-aligned current signatures are found near the polar cap boundary, but not always coincident with it. Within this polar cap, gradients in the observed soft particle flux are explained as a result of the density decrease previously observed in the magnetosheath as one progresses anti-sunward along the magnetotail. Presumably depending upon the interplanetary magnetic field orientation, the location of the polar cap is occasionally drastically shifted. At these times, there are high-latitude regions, extending even up to the magnetic pole, which are closed and convecting towards the sun. Since boundaries, identified by either the poleward extent of plasma sheet, particle populations or the 33 keV electron intensity cutoff or isotropy boundary, are well-separated at these times from the region of open field lines, we conclude that there is no physical connection between these boundaries and the edge of the polar cap; but, rather, that they are topologically often found near each other. The location of field-aligned current sheets appears to depend, in a more complex way, on the gradients in the electric field and ionospheric conductivity, as indicated by auroral precipitation.