| 1. |
- Erlandson, R. E., et al.
(författare)
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Freja observations of electromagnetic ion cyclotron ELF waves and transverse oxygen ion acceleration on auroral field lines
- 1994
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Ingår i: Geophysical Research Letters. - 0094-8276. ; 21:17, s. 1855-1858
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Tidskriftsartikel (refereegranskat)abstract
- Extremely low-frequency (ELF) magnetic and electric field plasma wave emissions were recorded on 2 October 1993 on auroral field lines by the Magnetic Field Experiment during Freja orbit 4770. The ELF wave frequencies were below the local oxygen gyrofrequency (25 Hz) and between the helium and proton gyrofrequencies (100 to 400 Hz). The ELF waves, interpreted as electromagnetic ion cyclotron (EMIC) waves, were observed in a region of inverted-v-type electron precipitation. The EMIC waves were correlated over time with auroral and lower energy (≈ 100 eV) electrons, which are both possible sources of free energy, and also with transversely accelerated oxygen ions. The waves above the helium gyrofrequency were more closely correlated with the transverse oxygen ion acceleration than the waves below the oxygen gyrofrequency. These observations are consistent with a scenario in which electron beams generate EMIC waves, which then produce transverse oxygen ion acceleration through a gyroresonant interaction.
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| 2. |
- Knudsen, D. J., et al.
(författare)
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Sub-kilometer thermal plasma structure near 1750 km altitude in the polar cusp/cleft
- 1994
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Ingår i: Geophysical Research Letters. - 0094-8276. ; 21:17, s. 1907-1910
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Tidskriftsartikel (refereegranskat)abstract
- We present Freja Cold Plasma Analyzer (CPA) measurements from an encounter with the low altitude (approx.1750 km) polar cusp during which the CPA measured 2-D images of the thermal (0-16 eV) particle distributions at 1.2 s time resolution, and simultaneously made rapid estimates (600/s) of integrated thermal particle flux into the instrument. The high resolution data show bursty ion flux enhancements of the order of tens of percent on time scales of tens of ms, or alternatively, hundreds of m spatial scales. The flux of electrons from 0-16 eV also varied by tens of percent and on temporal/spatial scales comparable to those in the ion cases. There is some evidence that the thermal particle flux variations are associated with intense low-frequency electromagnetic fluctuations with temporal/spatial scales identical to those seen by the CPA (tens of ms. hundreds of m).
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| 3. |
- Persson, M. A. L., et al.
(författare)
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Near-Earth substorm onset - a coordinated study
- 1994
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Ingår i: Geophysical Research Letters. - 0094-8276 .- 1944-8007. ; 21:17, s. 1875-1878
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Tidskriftsartikel (refereegranskat)abstract
- We present simultaneous satellite and ground-based measurements of a substorm. Throughout the initial substorm expansion, southward drifting arcs are observed poleward of the expanding substorm aurora, indicating two independent systems of particle precipitation. Freja passes the brightening onset arc in the topside ionosphere near the moment of the substorm onset, observing an Alfven wave, field aligned current and oxygen ion outflow. The substorm onset occurs at low magnetospheric L-shells, near the poleward edge of the region of trapped particles. The location and time for the substorm injection are confirmed by geostationary spacecraft together with magnetometers, all-sky cameras and radar on the ground. We believe that the substorm onset may be triggered by modification of the oxygen content of the inner magnetosphere during the growth-phase caused by ionospheric ion outflow.
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