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Ann. Geophys., 20, 1117-1136, 2002
www.ann-geophys.net/20/1117/2002/
© European Geosciences Union 2002
Downward ion acceleration at auroral latitudes: cause of parallel electric field
B. Hultqvist
Swedish Institute of Space Physics Kiruna, Sweden
Correspondence to: B. Hultqvist (hultqv@irf.se)
Abstract. Observations with the
Freja satellite at about 1700 km altitude of downward accelerated ions in the
keV and sub-keV energy range are described and analysed. The observations show
the following: (1) Processes involving velocity dispersion are not important;
(2) Ion pitch-angle distributions are mostly somewhat field aligned but not far
from isotropic, so the ions are effectively spread in pitch-angle; (3) As all
ion species, H +, O +, and He +, are found to be accelerated to the same
energy, the only possible known acceleration mechanism is a potential
difference along the magnetic field lines; (4) No significant Birkeland current
features are associated with the ion precipitation; (5) Precipitation of
energetic electrons from the plasma sheet is always present when the downward
accelerated ions are observed; (6) Ion precipitation is generally not seen in
regions with primary auroral Birkeland currents associated with electron
inverted-V distributions; (7) Precipitated ions are mostly observed at low and
medium disturbance levels, but they are also found in strongly disturbed
conditions; (8) Downward accelerated ions occur fairly frequently at auroral
latitudes near Freja apogee altitudes and are seen at all local times. The
present investigation is limited to the nightside. The above observational
results are found to be consistent with the physical mechanism for producing a
downward-pointing parallel electric field proposed by Hultqvist (1971). That
mechanism is basically one of an ambipolar potential difference set up by the
energetic electrons from the plasma sheet.
Key words. Magnetospheric physics
(electric fields; energetic particles, precipitating; magnetosphere –
ionosphere interactions)
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