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Downward Ion Acceleration at Auroral Latitudes: Cause of Parallel Electric Field : Volume 20, Issue 8 (30/11/-0001)

By Hultqvist, B.

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Book Id: WPLBN0004000939
Format Type: PDF Article :
File Size: Pages 20
Reproduction Date: 2015

Title: Downward Ion Acceleration at Auroral Latitudes: Cause of Parallel Electric Field : Volume 20, Issue 8 (30/11/-0001)  
Author: Hultqvist, B.
Volume: Vol. 20, Issue 8
Language: English
Subject: Science, Annales, Geophysicae
Collections: Periodicals: Journal and Magazine Collection, Copernicus GmbH
Publication Date:
Publisher: Copernicus Gmbh, Göttingen, Germany
Member Page: Copernicus Publications


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Hultqvist, B. (-0001). Downward Ion Acceleration at Auroral Latitudes: Cause of Parallel Electric Field : Volume 20, Issue 8 (30/11/-0001). Retrieved from

Description: Swedish Institute of Space Physics Kiruna, Sweden. 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)

Downward ion acceleration at auroral latitudes: cause of parallel electric field


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