Radiation from an electron beam in a magnetized plasma: Whistler mode wave packets

Abstract

[1] Experimental studies are reported of oscillations and radiation that is spontaneously excited by an electron beam which is shot along a diverging magnetic field into a plasma from a hot cathode. In the present study we focus on oscillations below the electron gyrofrequency, where we find that whistler mode radiation appears in the form of bursts, or wave packets, each with typically 0.1-1 ms time duration, and which together cover typically a few percent of the full time. Wave packets are found in a broad frequency range of 7-40 MHz, while each individual wave packet is dominated by a single frequency. There is propagation along two routes: at the group velocity resonance cone angle, away from the central channel where the waves are excited, and in a channel along the magnetic field. Features of the whistler mode wave packets that are studied include (1) the statistics of amplitudes, frequencies, and time durations; (2) the propagation and decay of wave packets with different frequencies; (3) the group and phase velocities; and (4) how the wave packet production varies with the energy, and the current density, in the electron beam.

Vlasov simulations of parallel potential drops

“…Later, double layers were studied through laboratory experiments and numerical simulations, showing how waves near the local plasma frequency can be concentrated in narrow oscillating electric field spikes (Gunell et al, 1996a,b;Gunell and Löfgren, 1997;Löfgren and Gunell, 1998), which are associated with whistler emissions (Brenning et al, 2006). The electron beams and whistlers in the latter experiment are similar to those recently observed at Saturn's moon Enceladus (Gurnett et al, 2011).…”

Nonlinear radiation generation processes in the auroral acceleration region

“…The observations performed in the AKR source region strengthen those previously acquired from laboratory experiments (Gunell and Löfgren, 1997;Brenning et al, 2004). Actually, part of the AKR radiation appears to be generated thanks to the existence of nonlinear structures, located on the high-potential side of a DL, which act as a sender antenna.…”

“…Electromagnetic radiation from double layers (DLs) has been extensively studied in laboratory plasma experiments (Volwerk, 1993;Lindberg, 1993;Brenning et al, 2004). The spectrum was found to contain characteristic peaks around the electron gyrofrequency and electron plasma frequency (Gunell and Löfgren, 1997;Löfgren and Gunell, 1998).…”

Vlasov simulations of trapping and loss of auroral electrons

“…The emission mechanism they suggested is an electron cyclotron maser mechanism, where the electron distribution function is governed by the hole and may be different from the much studied horseshoe distribution (Ergun et al, 2000a). Similar systems have been seen to emit radio waves in laboratory plasmas (Lindberg, 1993;Volwerk, 1993;Brenning et al, 2006), although the electron beam energies were far below those used in dedicated cyclotron maser experiments (McConville et al, 2008) and simulations (Speirs et al, 2014). The simulations reported here are electrostatic and can therefore not be used to study radiation processes, which would require electromagnetic models.…”

“…Laboratory experiments on parallel electric fields in magnetic mirror fields have been carried out in Q-machines (Sato et al, 1986(Sato et al, , 1988 and modelled using particle in cell simulations (Ishiguro et al, 1995). It was found in experiments, simulations, and theory of discharge plasmas that electrons accelerated in an electric double layer can cause oscillating electric field spikes when entering a density gradient on its high potential side (Gunell et al, 1996a, b;Gunell and Löfgren, 1997;Löfgren and Gunell, 1998;Brenning et al, 2006). Song et al (1992) derived a stability criterion for double layers in a converging magnetic field, showing that a stable double layer position is possible when electrons are accelerated into a stronger magnetic field.…”