Electron anisotropy driven by kinetic Alfven waves in the Earth magnetotail

<p>Thermal and subthermal electron populations in the Earth&#8217;s magnetotail is usually characterized by pronounced field-aligned anisotropy that contributes to generation of strong electric currents within the magnetotail current sheet. Formation of this anisotropy requires electron field-aligned acceleration, and thus likely involves field-aligned electric fields. Such fields can be carried by various electromagnetic waves generated by fast plasma flows interacting with ambient magnetotail plasma. In this presentation we consider one of the most intense observed wave emissions, kinetic Alfven waves, that accompany all fast plasma flows in the magnetotail.</p><p>Using two tail seasons (2018, 2019) of MMS observations we have collected statistics of 80 fast plasma flows (or BBF) events with distinctive enhancement of intensity of broadband electromagnetic waves sharing properties of kinetic Alfven waves. We show that a direct correlation the intensity of electric fields of kinetic Alfven waves and electron anisotropy distribution: the parallel electron anisotropy significantly increases with magnitude of the wave parallel electric field. The energy range of this electron anisotropic population is well within the range of resonant energies for observed kinetic Alfven waves. Our results show that kinetic Alfven waves can significantly contribute to shaping the magnetotail electron population.</p>

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Thermal electron anisotropy driven by kinetic Alfven waves in the Earth's magnetotail

10.5194/angeo-2020-76 ◽

2020 ◽

Author(s):

Alexander Lukin ◽

Anton Artemyev ◽

Evgeny Panov ◽

Rumi Nakamura ◽

Anatoly Petrukovich ◽

...

Keyword(s):

Electric Fields ◽

Electromagnetic Waves ◽

Alfven Waves ◽

Alfvén Waves ◽

Plasma Flows ◽

Thermal Electron ◽

Electron Population ◽

Electron Field ◽

Kinetic Alfvén Waves

Abstract. Thermal and subthermal electron populations in the Earth's magnetotail are usually characterized by pronounced field-aligned anisotropy that contributes to generation of strong electric currents within the magnetotail current sheet. Formation of this anisotropy requires electron field-aligned acceleration, and thus likely involves field-aligned electric fields. Such fields can be carried by various electromagnetic waves generated by fast plasma flows interacting with ambient magnetotail plasma. In this paper we consider one of the most intense observed wave emissions, kinetic Alfven waves, that often accompany fast plasma flows in the magnetotail. Using two tail seasons (2017, 2018) of MMS observations we have collected statistics of 80 fast plasma flows (or bursty bulk flows) events with distinctive enhancement of intensity of broadband electromagnetic waves (kinetic Alfven waves). We show correlation the intensity of electric fields of kinetic Alfven waves and characteristics of electron anisotropy distributions: the parallel electron anisotropy increases with magnitude of the wave parallel electric field. Also the energy range of this electron anisotropic population is well within the expected acceleration range for assumed kinetic Alfven waves. Our results indicate an important role of KAWs in production of thermal field-aligned electron population typically observed in the Earth's magnetotail.

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Electron acceleration and parallel electric fields due to kinetic Alfvén waves in plasma with similar thermal and Alfvén speeds

Advances in Space Research ◽

10.1016/j.asr.2007.03.030 ◽

2008 ◽

Vol 42 (5) ◽

pp. 964-969 ◽

Cited By ~ 14

Author(s):

Clare E.J. Watt ◽

Robert Rankin

Keyword(s):

Electric Fields ◽

Electron Acceleration ◽

Alfven Waves ◽

Alfvén Waves ◽

Kinetic Alfvén Waves

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"Kinetic" Alfven Waves and Parallel Electric Fields.

Journal of Plasma and Fusion Research ◽

10.1585/jspf.78.1043 ◽

2002 ◽

Vol 78 (10) ◽

pp. 1043-1048

Author(s):

Tadas K. NAKAMURA

Keyword(s):

Electric Fields ◽

Alfven Waves ◽

Alfvén Waves ◽

Kinetic Alfvén Waves

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Field-aligned particle acceleration on auroral field lines by interaction with transient density cavities stimulated by kinetic Alfvén waves

Annales Geophysicae ◽

10.5194/angeo-24-2313-2006 ◽

2006 ◽

Vol 24 (8) ◽

pp. 2313-2329 ◽

Cited By ~ 5

Author(s):

P. A. Bespalov ◽

V. G. Misonova ◽

S. W. H. Cowley

Keyword(s):

Electric Fields ◽

Planck Equation ◽

Initial Energy ◽

Alfven Waves ◽

Distribution Functions ◽

Energy Change ◽

Alfvén Waves ◽

Time Varying ◽

Field Lines ◽

Kinetic Alfvén Waves

Abstract. 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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