How electron two-stream instability drives cyclic Langmuir collapse and continuous coherent emission

Abstract

Continuous plasma coherent emission is maintained by repetitive Langmuir collapse driven by the nonlinear evolution of a strong electron two-stream instability. The Langmuir waves are modulated by solitary waves in the linear stage and electrostatic whistler waves in the nonlinear stage. Modulational instability leads to Langmuir collapse and electron heating that fills in cavitons. The high pressure is released via excitation of a shortwavelength ion acoustic mode that is damped by electrons and reexcites small-scale Langmuir waves; this process closes a feedback loop that maintains the continuous coherent emission.electron beams | nonlinear wave interaction | Langmuir collapse | coherent emission | plasma turbulence E lectron beams accelerated by solar flares and nanoflares are believed to be responsible for several types of solar radio bursts observed in the corona and interplanetary medium, including flare-associated coronal type U and J and interplanetary type III radio bursts, and nanoflare-associated weak coronal type III bursts (1-4). In 1958, Ginzburg and Zhelezniakov first proposed a basic framework for such bursts, which was subsequently refined by others (refs. 5 and 6 and references therein). In essence, the scenario is one in which the electron two-stream instability (ETSI), driven by electron beams, generates Langmuir waves that are converted into plasma coherent emission via nonlinear three-wave coupling [e.g., two Langmuir waves and one ion acoustic wave (IAW)]. However, the mechanism whereby the nonlinear ETSI produces coherent emission with a duration of several orders of magnitude longer than the linear saturation time is not well understood (6-9). Nonlinear evolution of ETSI is a fundamental problem in nonlinear wave theory in which disparate three-wave couplings dominate the energy transport and dissipation (10). It has broad applications in plasma physics, planetary physics, and astrophysics, such as terahertz emission in laser beam experiments, radio bursts from Jupiter, pulsars, and the formation of exotic astrophysical objects.In the classical Kolmogorov turbulence scenario, the balance between energy input and its final absorption is controlled by a nonlinear cascade from large spatial scales (the region of external forcing) to viscosity-dominated short wavelengths. In plasmas, the source of instability is often beams of charged particles that generate Langmuir waves. At shorter wavelengths, the natural candidate to provide the sink of wave energy is Landau damping. However, nonlinear disparate wave interactions, it follows from direct calculation of basic three-wave coupling, can only lead to inverse cascades (to longer wavelengths) through modulational instability (11), and away from the Landau damping region of the spectrum. The eventual nonlinear process capable of overriding this inverse cascade was suggested by Zakharov, namely Langmuir collapse (LC), which is analogous to a selffocusing of the Langmuir waves packets, or cavitons (12, 13). LC has been discovered in both experimen...

Another look at zonal flows: Resonance, shearing, and frictionless saturation

“…It is analogous to Landau damping absorption of plasmons during collapse of Langmuir turbulence. 17,18 In the latter case, plasmon Landau damping arrests collapse, leaving an "empty cavity," without its "filling" of Langmuir wave pressure. Table I compares these two processes.…”

How anomalous resistivity accelerates magnetic reconnection

“…Observations and laboratory experiments have discovered the Hall field and Hall current associated with fast collisionless magnetic reconnection 10,12 , but no direct experimental or observational evidence has been found to show that the rate is irrelevant to the dissipation mechanism [22][23][24][25] . On the other hand, observations of solar flares found that magnetic reconnection may commonly be turbulent as evidenced by the filamentary structure of magnetic field, non-thermal heating observed in X-ray, and coherent radio emissions that are likely results of electron two-stream instability [26][27][28] . The reconnection rate in solar flares estimated by the variability of magnetic flux around magnetic null-points varies in a wide range, from the moderate 0.1 V A to very fast 0.5-0.6 V A 29 -much higher than the maximum Hall-reconnection rate from simulations.…”

Ultrafast optical field–ionized gases—A laboratory platform for studying kinetic plasma instabilities

“…Simulations show that the most unstable k of the instability changes as the plasma evolves, which can be confirmed by simultaneously collecting the scattered light from multiple angles. One can simultaneously measure the time evolution of the electron and ion features at several ks, which will enable the hierarchy of kinetic instabilities to be established (41,42) and track the evolution of the plasma from a quiescent to turbulent state (15).…”

“…The mechanisms and time scale by which plasma electrons evolve from an anisotropic to a thermal state are important but unsolved experimental problems in basic plasma science. The kinetic plasma instabilities due to anisotropic plasmas are thought to occur in recombination x-ray lasers based on OFI plasmas (7), gamma ray flashes (8) and bursts (9), electron-positron plasmas (10), collisionless shocks (11), growth and filamentation of magnetic fields in plasmas (12), electron cloud effects in storage rings (13) and proton synchrotrons (14), solar corona and interplanetary medium (15), solar wind trapped in the ionosphere by Earth's magnetic field (16,17), neutrino-plasma interactions that may occur in supernova explosions (18), quark-gluon plasmas produced in heavy-ion beam collisions (19,20), and hot-electron transport in fast ignition-inertial confinement fusion (21,22). Given the extremely broad range of situations that give rise to kinetic instabilities, a voluminous theoretical body of work exists on the kinetic theory of plasmas.…”