| 1. |
- Andre, M, et al.
(författare)
-
Ion energization mechanisms at 1700 km in the auroral region
- 1998
-
Ingår i: JOURNAL OF GEOPHYSICAL RESEARCH-SPACE PHYSICS. - 0148-0227. ; 103:A3, s. 4199-4222
-
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.
|
|
| 2. |
- Eliasson, L, et al.
(författare)
-
Observations of electron conics by the Viking satellite
- 1996
-
Ingår i: JOURNAL OF GEOPHYSICAL RESEARCH-SPACE PHYSICS. - : AMER GEOPHYSICAL UNION. - 0148-0227. ; 101:A6, s. 13225-13238
-
Tidskriftsartikel (övrigt vetenskapligt/konstnärligt)abstract
- Electron angular distributions peaked at oblique angles to the magnetic field, electron conics, are frequently found in the Viking data at all magnetic local times, but with a maximum in the dusk sector. Several types of electron conics have been observed
|
|
| 3. |
- Fu, H. S., et al.
(författare)
-
Pitch angle distribution of suprathermal electrons behind dipolarization fronts : A statistical overview
- 2012
-
Ingår i: Journal of Geophysical Research. - : American Geophysical Union (AGU). - 0148-0227 .- 2156-2202. ; 117:12
-
Tidskriftsartikel (refereegranskat)abstract
- We examine the pitch angle distribution (PAD) of suprathermal electrons (> 40 keV) inside the flux pileup regions (FPRs) that are located behind the dipolarization fronts (DFs), in order to better understand the particle energization mechanisms operating therein. The 303 earthward-propagating DFs observed during 9 years (2001-2009) by Cluster 1 have been analyzed and divided into two groups according to the differential fluxes of the > 40 keV electrons inside the FPR. One group, characterized by the low flux (F < 500/cm(2) , s . sr . keV), consists of 153 events and corresponds to a broad distribution of IMF Bz components. The other group, characterized by the high flux (F >= 500/cm(2) . s . sr . keV), consists of 150 events and corresponds to southward IMF Bz components. Only the high-flux group is considered to investigate the PAD of the > 40 keV electrons as the low-flux situation may lead to large uncertainties in computing the anisotropy factor that is defined as A = F-perpendicular to/F-parallel to - 1 for F-perpendicular to > F-parallel to, and A = -F-parallel to/F-perpendicular to + 1 for F-perpendicular to < F-parallel to. We find that, among the 150 events, 46 events have isotropic distribution (vertical bar A vertical bar <= 0.5); 60 events have perpendicular distribution (A > 0.5), and 44 events have field-aligned distribution inside the FPR (A < -0.5). The perpendicular distribution appears mainly inside the growing FPR, where the flow velocity is increasing and the local flux tube is compressed. The field-aligned distribution occurs mainly inside the decaying FPR, where the flow velocity is decreasing and the local flux tube is expanding. Inside the steady FPR, we observed primarily the isotropic distribution of suprathermal electrons. This statistical result confirms the previous case study and gives an overview of the PAD of suprathermal electrons behind DFs.
|
|
| 4. |
- Hamrin, Maria, et al.
(författare)
-
A statistical study of wave properties and electron density at 1700 km in the auroral region
- 2002
-
Ingår i: Journal of Geophysical Research. - 0148-0227 .- 2156-2202. ; 107:A8, s. SIA 21-1-SIA 21-13
-
Tidskriftsartikel (refereegranskat)abstract
- We present a comprehensive overview of the electron density and six different wave types in the range similar to1 Hz to similar to1 MHz, and we investigate their occurrence, average wave frequency and amplitude as a function of location, Kp index, and solar illumination. Twenty-one months of Freja observations from the Northern Hemisphere obtained at similar to1700 km altitude and invariant latitudes 40degrees-75degrees are used. We find that waves around the lower hybrid frequency occur in one low-latitude dayside band and one high-latitude nightside band. The latter band correlates with precipitating auroral electrons and coexists with electromagnetic ion cyclotron (EMIC) waves. This indicates the importance of energetic electrons for the wave generation. Both broadband ELF waves and broadband high-frequency whistler mode waves are found at high latitudes, but whistler mode emissions are most common in regions of high electron densities on the dayside, while broadband ELF waves are found where the density is reduced on the nightside. Moreover, the average density in the presence of broadband ELF waves is more reduced when the ionosphere is dark than when it is sunlit. However, broadband whistler mode waves, Langmuir waves, and waves with an upper cutoff just below the proton gyrofrequency coincide with density enhancements when the ionosphere is dark. Ion heating correlated with auroral electrons coexists with EMIC waves and the high-latitude band of waves around the lower hybrid frequency. Furthermore, ion heating not correlated with downgoing electrons coexists with broadband ELF waves.
|
|
| 5. |
- Hamrin, Maria, et al.
(författare)
-
Inhomogeneous transverse electric fields and wave generation in the auroral region : A statistical study
- 2001
-
Ingår i: Journal of Geophysical Research. - 0148-0227 .- 2156-2202. ; 106:A6, s. 10803-10816
-
Tidskriftsartikel (refereegranskat)abstract
- We use data from the Freja satellite to investigate the importance of localized transverse DC electric fields for the generation of broadband waves responsible for ion heating in the auroral region. Theoretical models indicate that shear in the plasma Row perpendicular to the geomagnetic field can generate waves in a broad range around the ion gyrofrequency for parallel currents significantly below the threshold of the current-driven electrostatic ion cyclotron instability. We compare in situ data with laboratory measurements and theoretical predictions, and we find that inhomogeneous electric fields might well be important for the generation of waves in the auroral region.
|
|
| 6. |
- Hamrin, M, et al.
(författare)
-
Inhomogeneous transverse electric fields and wave generation in the auroral region: A statistical study
- 2001
-
Ingår i: JOURNAL OF GEOPHYSICAL RESEARCH-SPACE PHYSICS. - : AMER GEOPHYSICAL UNION. - 0148-0227. ; 106:A6, s. 10803-10816
-
Tidskriftsartikel (refereegranskat)abstract
- We use data from the Freja satellite to investigate the importance of localized transverse DC electric fields for the generation of broadband waves responsible for ion heating in the auroral region. Theoretical models indicate that shear in the plasma Row
|
|
| 7. |
- Lönnqvist, Håkan, et al.
(författare)
-
Generation of VLF saucer emissions observed by the Viking satellite
- 1993
-
Ingår i: Journal of Geophysical Research. - 0148-0227 .- 2156-2202. ; 98, s. 13565-13574
-
Tidskriftsartikel (refereegranskat)abstract
- Simultaneous observations by the Viking satellite of electric and magnetic fields as well as charged particles have been used to investigate V-shaped wave phenomena. The intensity of these VLF and ELF emissions is V-shaped when shown in a frequency versus time plot. Simultaneous observations of V-shaped so-called VLF saucer emissions, particles and field-aligned currents strongly suggest, for the first time, that upgoing electrons with energies less than a few hundred electron volts can generate these waves. Broadband waves observed inside the saucer generation region, form frequencies much less than the ion cyclotron frequency up to the plasma frequency, may also be generated by these electrons. Viking observations of VLF saucers at altitudes between 4000 km and 13,500 km show that these emissions occur at higher altitudes tha discussed in previous reports. The generation regions seem to be more extended at these higher altitudes than what has been reported at lower altitudes by other observations.
|
|
| 8. |
- Norqvist, P, et al.
(författare)
-
Ion cyclotron heating in the dayside magnetosphere
- 1996
-
Ingår i: Journal of Geophysical Research. - 0148-0227 .- 2156-2202. ; 101:A6, s. 13179-13193
-
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.
|
|
| 9. |
- Norqvist, P, et al.
(författare)
-
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
-
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
|
|
| 10. |
- Norqvist, P, et al.
(författare)
-
Isotropic and perpendicular energization of oxygen ions at energies below 1 eV
- 1998
-
Ingår i: JOURNAL OF GEOPHYSICAL RESEARCH-SPACE PHYSICS. - 0148-0227. ; 103:A3, s. 4223-4239
-
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.
|
|
