| 1. |
- Marklund, Göran T., et al.
(författare)
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Characteristics of the auroral particle acceleration in the upward and downward current regions
- 2001
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Ingår i: Physics and Chemistry of the Earth, Part C. - 1464-1917 .- 1873-4685. ; 26:03-jan, s. 81-96
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Tidskriftsartikel (refereegranskat)abstract
- To understand auroral particle acceleration it is necessary to consider the entire auroral current circuit, from the generator in the outer magnetosphere to the auroral ionosphere, decoupled from each other by field-aligned potential drops in both the upward and downward current regions. This paper focusses on the characteristics of such potential drops responsible for the accelerated electrons that produce the aurora and for the upward acceleration of ions and electrons. First, some features of the converging electric field structures in the upward current region are discussed based predominantly on results from Viking observations within and above the acceleration region. These include the relationship between the high-altitude potential and the ionospheric potential associated with auroral area; ways to estimate the field-aligned potential drop from in situ satellite measurements of fields and particles; the role of low-frequency electric field fluctuations for acceleration and heating of auroral particles. ii brief review is then given of recent Freja results on characteristics of the downward curl ent region, such as the intense diverging electric field structures that give rise to upward acceleration of ionospheric electrons and perpendicular ion heating. Different theoretical models have been proposed to explain these phenomena. It is clear that the plasma density on auroral field lines and the ionospheric density play a fundamental role for where and when potential structures a reformed in the upward and downward current region. For a current generator feeding the auroral currents, particle acceleration is necessary to maintain the current through regions of low plasma density. The altitude distribution of the field-aligned potential will thus depend strongly on the local ambient plasma conditions, and this vary with local time, season and magnetic activity level. These characteristics could be investigated in detail by combining: measurements from various spacecraft (such as Viking, Polar, Freja, FAST, and Astrid-2) at different altitudes with respect to the the field-aligned potential distribution.
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| 2. |
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| 3. |
- Raadu, Michael A.
(författare)
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Particle acceleration mechanisms in space plasmas
- 2001
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Ingår i: Physics and Chemistry of the Earth, Part C. - 1464-1917 .- 1873-4685. ; 26:03-jan, s. 55-59
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Tidskriftsartikel (refereegranskat)abstract
- There are many possible mechanisms for particle acceleration in space plasmas. Among these are two broadly defined types that may be characterised on the one hand by large scale electric field structures and on the other hand by stochastic mechanisms and enhanced wave activity. Double layers (DLs) are an example of the first type. They sustain a net potential difference, across which particles are accelerated. Crucial questions concern the generator maintaining the potential, the rate of energy transfer, equilibrium and matching conditions. Magnetic reconnection at an X-type neutral line is another example and shares many characteristics with DLs, such as the cutting of field lines and constraints set by external MHD motions. Particle acceleration by the growth of the modified two- stream instability (MTSI) is an example of the second type. The MTSI is driven by cross magnetic field ion motion and rapidly accelerates electrons. It has been seen as a link in the critical ionisation velocity (CIV) mechanism, which has been extensively studied both in experiments and theory. Crucial questions concern the evolution of the particle energy distribution, the wave source and the saturation of the driving instability.
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| 4. |
- Torvén, Staffan, et al.
(författare)
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Experimental Investigations and Simulations of Quasi-Steady Potential Drops
- 2001
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Ingår i: Phys. Chem. of the Earth C. - 1464-1917. ; 26, s. 71-80
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Tidskriftsartikel (refereegranskat)abstract
- Magnetic-field-aligned potential drops in the auroral zone have recently been interpreted in terms of "quasi-steady" states which are evaluated for a fixed, given ion density profile. Here we present experimental studies of such states, observed when a voltage drop is suddenly applied to an inhomogeneous plasma column with a homogeneous, axial magnetic field. The quasi-steady potential drops have a spatial extension related to the gradient length of the initial ion density and the potential profile steepens slowly to a U-shaped double layer on the ion time scale. They exist only when the applied voltage drop does not exceed a critical value which depends on the maximum difference in ion density. Higher voltage drops concentrate in a cathode sheath. Results from PIC-simulations and from a theoretical model, based on steady electron motion, are also presented. They agree excellently for times up to about an ion plasma period (omega (-1)(pi)) The theoretical current-voltage characteristic, which is determined by electron reflection at a virtual cathode formed in the plasma, agrees with the experiments up to about one omega (-1)(pi). However, already at about 4 omega (-1)(pi) the measured currents have dropped much below the theoretical values, and also the measured average potential profiles are inconsistent with steady acceleration of the electrons. The increased resistivity is associated with strong fluctuations.
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| 5. |
- Yamauchi, M., et al.
(författare)
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Acceleration signatures in the dayside boundary layer and the cusp
- 2001
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Ingår i: Physics and Chemistry of the Earth, Part C. - 1464-1917 .- 1873-4685. ; 26:03-jan, s. 195-200
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Tidskriftsartikel (refereegranskat)abstract
- Freja data show various electron acceleration signatures in the cusp and the dayside boundary layer: (1) time dispersive super-Alfvénic electrons followed by strong wave activity which accompanies transient downward super-thermal electron burst in both the boundary layer and the cusp; (2) quasi-stationary bidirectional electron burst coinciding with localized intense field-aligned current in the boundary layer; (3) downgoing electron burst without visible time dispersion in the cusp; and (4) thermal electrons accelerated by electrostatic potential in both the boundary layer and the cusp. The first and last signatures are different between two regions for typical energies and fluxes, and these differences probably reflect the different auroral emission in the cusp proper (red) and the boundary layer (green). Contributions of these electrons to the large-scale field-aligned currents are also different between two regions. The dispersed electron burst is probably accelerated within 1 Re above the ionosphere. From this result we believe that the cusp red aurora is caused mainly by accelerated electrons, but not by the smoothly entering magnetosheath electrons without acceleration. This also requires revisions of flux transfer event models for the structured cusp red aurora.
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| 6. |
- Hoymork, SH, et al.
(författare)
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The shape and evolution of Lower Hybrid density cavities observed by FREJA
- 2001
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Ingår i: PHYSICS AND CHEMISTRY OF THE EARTH PART C-SOLAR-TERRESTIAL AND PLANETARY SCIENCE. - : PERGAMON-ELSEVIER SCIENCE LTD. - 1464-1917. ; 26:1-3, s. 213-217
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Tidskriftsartikel (refereegranskat)abstract
- A large number of narrow density cavities (50 m wide in the direction perpendicular to the magnetic field) are observed by the FREJA satellite. These cavities are correlated with electric field enhancements in the frequency range of Lower Hybrid waves. By
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| 7. |
- Janhunen, P, et al.
(författare)
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Auroral potential structures and current-voltage relationship: Summary of recent results
- 2001
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Ingår i: PHYSICS AND CHEMISTRY OF THE EARTH PART C-SOLAR-TERRESTIAL AND PLANETARY SCIENCE. - : PERGAMON-ELSEVIER SCIENCE LTD. - 1464-1917. ; 26:1-3, s. 107-111
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Tidskriftsartikel (refereegranskat)abstract
- Discrete auroral arcs are caused by precipitating accelerated electrons, and there is a lot of observational evidence for the associated potential structure being U-shaped below about 15000 km altitude. However, according to our studies using Polar data a
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| 8. |
- Yamauchi, M, et al.
(författare)
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Acceleration signatures in the dayside boundary layer and the cusp
- 2001
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Ingår i: PHYSICS AND CHEMISTRY OF THE EARTH PART C-SOLAR-TERRESTIAL AND PLANETARY SCIENCE. - : PERGAMON-ELSEVIER SCIENCE LTD. - 1464-1917. ; 26:1-3, s. 195-200 Language: English
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Tidskriftsartikel (refereegranskat)abstract
- Freja data show various electron acceleration signatures in the cusp and the dayside boundary layer: (1) time dispersive suger-Alfvenic electrons followed by strong wave activity which accompanies transient downward super-thermal electron burst in both th
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| 9. |
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