Speaker
Mark E. Koepke
Description
See the full Abstract at http://ocs.ciemat.es/EPS2018ABS/pdf/P1.4002.pdf
Excitation of a whistler mode wave packet by
interacting, higher-frequency, electrostatic-wave eigenmodes
M. Koepke1,2, N. Brenning2, I. Axnäs2, M. Raadu2, E. Tennfors2
1 West Virginia University, Morgantown WV, USA
2 Royal Institute of Technology, Stockholm, Sweden
Infrequent, bursty, electromagnetic, whistler-mode wave packets, excited spontaneously in
the laboratory by an electron beam from a hot cathode, appear transiently, each with a time
duration around ~1 μs. The ensemble of wave-packet frequency f_W is broadly distributed
in the range 7 MHz < f_W < 40 MHz. Wave-packet excitation takes place in the plasma
volume which is filled with an ensemble of separate electrostatic (es) plasma oscillations,
having frequency f_hf, 200 MHz < f_hf < 500 MHz, that are hypothesized to match
eigenmode frequencies of an axially localized hf es field in a restricted subvolume attached
to the cathode. Features of these es-eigenmodes that are studied include: the mode
competition at times of transitions from one dominating es-eigenmode to the next, the
amplitude and spectral distribution of simultaneous, independent es-eigenmodes that do not
lead to a transition, and the correlation of these features with the excitation of whistler
mode waves. It is concluded that transient coupling of es-eigenmode pairs having f_1,hf
and f_2,hf, such that | f_1,hf - f_2,hf | = f_W < electron gyrofrequency, can explain both the
transient lifetime and the frequency spectra of the whistler-mode wave packets (f_W) as
observed in lab. The generalization of the results to bursty whistler-mode excitation in
space from electron beams, created on the high potential side of double layers, is discussed.
This research on radiation from an electron beam in magnetized plasma [1] strives to
identify ways for a double layer in space to produce electromagnetic radiation that
propagates over a long distance.
This work was supported by the Swedish Research Council, the Alfven Laboratory Center
for Space and Fusion Plasma Physics, and the US National Science Foundation
(ATM-0201112, PHYS-0613238, and PHYS-1301896).
1. N Brenning et al 2006 J. Geophys. Res. 111, A11212; 2017 Plasma Phys. Control. Fusion 59 124006.
