Isotropic and perpendicular energization of oxygen ions at energies below 1 eV
1998 (English)In: JOURNAL OF GEOPHYSICAL RESEARCH-SPACE PHYSICS, ISSN 0148-0227, Vol. 103, no A3, 4223-4239 p.Article in journal (Refereed) Published
We present observations of low-energy O+ ion distributions obtained by the Freja satellite at altitudes between 1100 and 1600 km in the dayside high-latitude magnetosphere. The particle observations are obtained by the Freja ion mass spectrometer designed to observe ion energies up to several keV. We show that such instruments can also be used to accurately measure temperatures less than 1 eV, by operating the instrument at a fixed energy level of a few eV. When the mass spectrometer is operated at an energy approximately equal to the energy of O+ ions moving with the satellite velocity (several km/s), the satellite motion and the detector viewing directions become important. The idea is that the satellite ram effect together with different instrument viewing directions related to the satellite spin can allow sampling of different regions of ion velocity space. We find that isotropic O+ energization dominates at mean energies below about 0.4 eV, while energization perpendicular to the geomagnetic field dominates at higher energies. The isotropic ion distributions may exhibit a bulk motion up or down along the geomagnetic field, while the perpendicularly heated distributions are moving upward. The perpendicular energization occurs in the dayside auroral region including the cusp/cleft, the source region of the so-called cleft ion fountain. The perpendicular heating to a few eV can be associated with weak broadband low-frequency electric wave fields, similar to the stronger emissions causing O+ energization to hundreds of eV. However, at low energies other mechanisms map also be important.
Place, publisher, year, edition, pages
1998. Vol. 103, no A3, 4223-4239 p.
Fusion, Plasma and Space Physics
IdentifiersURN: urn:nbn:se:kth:diva-39259DOI: 10.1029/97JA02445ISI: 000072401200024OAI: oai:DiVA.org:kth-39259DiVA: diva2:511049
NR 201408052012-03-192011-09-092012-03-19Bibliographically approved