The electron–cyclotron maser for astrophysical application

Abstract

The electron-cyclotron maser is a process that generates coherent radiation from plasma. In the last two decades, it has gained increasing attention as a dominant mechanism of producing high-power radiation in natural hightemperature magnetized plasmas. Originally proposed as a somewhat exotic idea and subsequently applied to include non-relativistic plasmas, the electroncyclotron maser was considered as an alternative to turbulent though coherent wave-wave interaction which results in radio emission. However, when it was recognized that weak relativistic corrections had to be taken into account in the radiation process, the importance of the electron-cyclotron maser rose to the recognition it deserves. Here we review the theory and application of the electron-cyclotron maser to the directly accessible plasmas in our immediate terrestrial and planetary environments. In situ access to the radiating plasmas has turned out to be crucial in identifying the conditions under which the electron-cyclotron maser mechanism is working. Under extreme astrophysical conditions, radiation from plasmas may provide a major energy loss; however, for generating the powerful radiation in which the electron-cyclotron maser mechanism is capable, the plasma must be in a state where release of susceptible amounts of energy in the form of radiation is favorable. Such conditions are realized when the plasma is unable to digest the available free energy that is imposed from outside and stored in its particle distribution. The lack of dissipative processes is a common property of collisionless plasmas. When, in addition, the plasma density becomes so low that the amount of free energy per particle is large, direct emission becomes favorable. This can be expressed as negative absorption of the plasma which, like in conventional masers, leads to coherent emission even though no quantum correlations are involved. The physical basis of this formal analogy between a quantum maser and the electron-cyclotron maser is that in the electron-cyclotron maser the free-space radiation modes can be amplified directly. Several models have been proposed for such a process. The most famous one is the so-called loss-cone maser. However, as argued in this review, the loss-cone maser is rather inefficient. Available in situ measurements indicate that the loss-cone maser plays only a minor role. Instead, the main source for any strong electron-cyclotron maser is found in the presence of a magnetic-field-aligned electric potential drop which has several effects: (1) it dilutes the local plasma to such an extent that the plasma enters the regime in which the electron-cyclotron maser becomes effective; (2) it generates energetic relativistic electron beams and field-aligned currents; (3) it deforms, together with the magnetic mirror force, the electron distribution function, thereby mimicking a high energy level sufficiently far above the Maxwellian ground state of an equilibrium plasma; (4) it favors emission in the freespace RX mode in a direction roughl...

Numerical simulations of unbounded cyclotron-maser emissions

“…This trend is reflected in the results of unbounded PiC simulations of cyclotron-wave coupling, where radiation propagation has been observed with a distinctive backward-wave character to it, typically directed in a slightly backward direction, only a few degrees away from the perpendicular to the magnetostatic field. The corresponding rf conversion efficiencies are comparable with estimates for the astrophysical phenomena (Gurnett 1974;Treumann 2006;Bingham et al 2013). …”

“…PiC simulations have been conducted to study the electrodynamics and emission topology of the cyclotronmaser instability responsible for auroral magnetospheric radio emission and the generation of non-thermal radio emission in other astrophysical contexts (Treumann 2006). Initial waveguide-bounded PiC simulations and laboratory experiments show for numerous near-cutoff resonance regimes, a tendency for cyclotronwave coupling to favour a backward-wave character.…”

Jupiter’s magnetosphere and aurorae observed by the Juno spacecraft during its first polar orbits

“…Many of the features in the figure are organized by a fundamental mode of the plasma, the electron cyclotron frequency (simply related to magnetic field intensity by f ce [Hz] = Figure 2a-b (and near the top of 2c) are confined to frequencies at or above f ce . These radio emissions are thought to be generated by the cyclotron maser instability (CMI) at frequencies at or just below f ce [13]. Since these radio emissions closely approach the line at f ce in several locations, and show intensification as they do so, it appears that Juno passed very close to (if not through) the source regions for these emissions.…”

Mapping radio emitting-region on low-mass stars and brown dwarfs

“…The ECM was also applied to radio emissions from binary systems, and narrowband emission from dwarf M flare stars. The strong radio emission from some planets, such as AKR, DAM, HOM and SKR, are also attributed to the ECM instability [12,16].…”