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| Titel |
Field-aligned particle acceleration on auroral field lines by interaction with transient density cavities stimulated by kinetic Alfvén waves |
| VerfasserIn |
P. A. Bespalov, V. G. Misonova, S. W. H. Cowley |
| Medientyp |
Artikel
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| Sprache |
Englisch
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| ISSN |
0992-7689
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| Digitales Dokument |
URL |
| Erschienen |
In: Annales Geophysicae ; 24, no. 8 ; Nr. 24, no. 8 (2006-09-13), S.2313-2329 |
| Datensatznummer |
250015629
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| Publikation (Nr.) |
 copernicus.org/angeo-24-2313-2006.pdf |
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| Zusammenfassung |
| We consider the field-aligned acceleration of energetic
ions and electrons which takes place on auroral field lines due to their
interaction with time-varying density cavities stimulated by the strong
oscillating field-aligned currents of kinetic Alfvén waves. It is shown
that when the field-aligned current density of these waves increases, such
that the electron drift speed exceeds the electron thermal speed, ion
acoustic perturbations cease to propagate along the field lines and instead
form purely-growing density perturbations. The rarefactions in these
perturbations are found to grow rapidly to form density cavities, limited by
the pressure of the bipolar electric fields which occur within them. The
time scale for growth and decay of the cavities is much shorter than the
period of the kinetic Alfvén waves. Energetic particles traversing these
growing and decaying cavities will be accelerated by their time-varying
field-aligned electric fields in a process that is modelled as a series of
discrete random perturbations. The evolution of the particle distribution
function is thus determined by the Fokker-Planck equation, with an energy
diffusion coefficient that is proportional to the square of the particle
charge, but is independent of the mass and energy. Steady-state solutions
for the distribution functions of the accelerated particles are obtained for
the case of an arbitrary energetic particle population incident on a
scattering layer of finite length along the field lines, showing how the
reflected and transmitted distributions depend on the typical "random walk"
energy change of the particles within the layer compared to their initial
energy. When this typical energy change is large compared to the initial
energy, the reflected population is broadly spread in energy about a mean
which is comparable with the initial energy, while the transmitted
population has the form of a strongly accelerated field-aligned beam. We
suggest that these processes are responsible for the occurrence of
accelerated field-aligned beams of ions and electrons that are commonly
observed on auroral field lines in planetary magnetospheres. |
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