Cold Plasma Source of Upflowing Ionospheric Ions in the Nightside Auroral Ionosphere.

DOI Web Site 19 References
  • Yau A. W.
    Department of Physics and Astronomy, University of Calgary Herzberg Institute of Astrophysics, National Research Council
  • Abe T.
    College of Science and Engineering, Aoyama Gakuin University
  • Whalen B. A.
    Herzberg Institute of Astrophysics, National Research Council

Bibliographic Information

Other Title
  • Cold Plasma Source of Upflowing Ionosph

Search this article

Abstract

The frequent occurrence of energetic O+ upflowing ionospheric ions (UFI) at high altitude during auroral substorms raises the question of the cold plasma source and acceleration altitude for the O+ ions, and their possible effects on the substorm-time plasma sheet. Ion composition observations on Akebono in the nightside auroral ionosphere reveal the significant presence of thermal-energy (a few eV) O+ ions in the 6000-10, 000 km altitude region both during and between auroral substorms. Their upward flux normalized to 2000 km altitude is about 2 × 108 cm-2s-1. They are believed to be a significant source of cold plasma for the energetic UFI. During auroral substorms, Akebono occasionally observes molecular (N2+ and NO+) upflowing ions up to ∼60 eV, in or near regions of auroral electron precipitation up to 10, 000 km altitude. This suggests the occurrence of a fast ion acceleration process in the F-region or topside ionosphere, where freshly created molecular N2+ and NO+ ions are accelerated to several eV or greater within their dissociative recombination lifetime (∼ a few minutes). Ground photometric observations and simultaneous particle measurements on sounding rockets confirm the presence of transversely accelerated ions (TAI) up to ∼200 eV in the topside ionosphere (near 600 km altitude) within tens of seconds of substorm expansion onset. Such hundred-eV TAI are frequently observed on Akebono in latitudinally confined regions of the nightside auroral ionosphere down to ∼2000 km altitude. They can reach the so-called “parallel accceleration region” at ∼-2 RE altitude within a few minutes, where they are often accelerated further to keV energy and can typically reach the plasma sheet during a substorm. Their flux is an order of magnitude smaller than the thermal ion flux. They are believed to be a minor source of plasma for the energetic UFI. In contrast, the lower-energy (≤10-eV) O+ TAI are typically too slow to reach the parallel acceleration region during the substorm, as they are decelerated by gravitation or trapped by it and traverse repeatedly along the magnetic field line. Hence they constitute a possible source of quiet-time thermal ions in the parallel acceleration region. Another possible source of thermal ions in the region is the polar wind O++ ions convected anti-sunward along the auroral oval from the dawn and dusk sectors. The thermal O+ ions in the parallel acceleration region at ∼1-2 RE altitude of the nightside auroral ionosphere are believed to be the dominant source of cold plasma for energetic UFI at high altitude; they can reach the plasma sheet during a substorm, thereby modifying its composition significantly.

Journal

References(19)*help

See more

Keywords

Details 詳細情報について

Report a problem

Back to top