| 1. |
- Andre, M, et al.
(författare)
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Ion energization mechanisms at 1700 km in the auroral region
- 1998
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Ingår i: JOURNAL OF GEOPHYSICAL RESEARCH-SPACE PHYSICS. - 0148-0227. ; 103:A3, s. 4199-4222
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Tidskriftsartikel (refereegranskat)abstract
- Observations obtained by the Freja satellite at altitudes around 1700 km in the high-latitude magnetosphere are used to study ion energization perpendicular to the geomagnetic field. Investigations of ions, electrons, plasma densities, electric and magnetic wave fields, and field-aligned currents are used to study O+ heating mechanisms. Three ion heating events are studied in detail, and 20 events are used in a detailed statistical study. More than 200 events are classified as belonging to one of four major types of ion heating and are ordered as a function of magnetic local time. The most common types of ion heating are associated with broadband low-frequency electric wave fields occurring at all local times. These waves cover frequencies from below one up to several hundred hertz and correspond to the most intense O+ energization. Heating by these waves at frequencies of the order of the O+ gyrofrequency at 25 Hz seems to be the important energization mechanism, causing O+ ion mean energies up to hundreds of eV. The broadband waves are associated with Alfven waves with frequencies up to at least a few hertz and with field-aligned currents. Other types of O+ energization events are less common. During these events the ions are heated by waves near the lower hybrid frequency or near half the proton gyrofrequency. These waves are generated by auroral electrons or in a few cases by precipitating ions.
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| 2. |
- Norqvist, P, et al.
(författare)
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Ion cyclotron heating in the dayside magnetosphere
- 1996
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Ingår i: Journal of Geophysical Research. - 0148-0227 .- 2156-2202. ; 101:A6, s. 13179-13193
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Tidskriftsartikel (refereegranskat)abstract
- Observations of waves and particles obtained by the Freja satellite at altitudes around 1700 km in the dayside high-latitude magnetosphere are used to study ion energization. We find that ions, including O+, during several events of intense ion energization can be heated perpendicularly to the geomagnetic field to mean energies df up to about 20 eV via the process of cyclotron resonance by broadband waves around the ion gyrofrequencies. There is a good correlation between such broadband waves and the ion energization. The waves show no spectral features at the O+ gyrofrequency. The observed wave amplitudes are used as an input to a Monte Carlo simulation to obtain the observed ion energies. The waves around the ion gyrofrequencies may be generated either by field-aligned electrons or by nonlinear processes transferring energy from waves with lower frequencies. Not only the mean energy but also the shape of the particle distribution agrees with the cyclotron resonance heating mechanism. Other mechanisms, such as heating by lower hybrid waves or by a slowly varying electric field, are investigated but are found to be less important than cyclotron heating in this region of space.
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| 3. |
- Norqvist, P, et al.
(författare)
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Ion cyclotron heating in the dayside magnetosphere
- 1996
-
Ingår i: JOURNAL OF GEOPHYSICAL RESEARCH-SPACE PHYSICS. - : AMER GEOPHYSICAL UNION. - 0148-0227. ; 101:A6, s. 13179-13193
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Tidskriftsartikel (övrigt vetenskapligt/konstnärligt)abstract
- Observations of waves and particles obtained by the Freja satellite at altitudes around 1700 km in the dayside high-latitude magnetosphere are used to study ion energization. We find that ions, including O+, during several events of intense ion energizati
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| 4. |
- Norqvist, P, et al.
(författare)
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Isotropic and perpendicular energization of oxygen ions at energies below 1 eV
- 1998
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Ingår i: JOURNAL OF GEOPHYSICAL RESEARCH-SPACE PHYSICS. - 0148-0227. ; 103:A3, s. 4223-4239
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Tidskriftsartikel (refereegranskat)abstract
- 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.
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| 5. |
- Vaivads, A, et al.
(författare)
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Energy transport during O+ energization by ELF waves observed by the Freja satellite
- 1999
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Ingår i: Journal of Geophysical Research. - 0148-0227 .- 2156-2202. ; 104:A2, s. 2563-2572
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Tidskriftsartikel (refereegranskat)abstract
- We present a study of strong ELF waves correlated with Of ion heating. We investigate the kinetic energy flux of oxygen ions and electrons parallel to the ambient magnetic field and the Poynting flux along the magnetic field of waves in different frequency regions. We show that the Poynting flux of lower hybrid waves and waves with frequencies below the oxygen gyrofrequency is downward but much less than the upgoing oxygen ion energy flux. The Poynting flux of ELF waves with frequencies at about half the local proton gyrofrequency is also downward, and it is comparable to, or larger, than the upward oxygen energy flux. Our results are consistent with a scenario where ELF waves with frequencies around half the proton gyrofrequency are generated above the satellite and transfer energy to oxygen ions in a region below the satellite.
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