Beam-driven turbulence and localized structures

Daniel Graham
School of Physics, University of Sydney

*Iver Cairns
School of Physics, University of Sydney

*Peter Robinson
School of Physics, University of Sydney

*Olaf Skjaeraasen
ProsTek, Institute for Energy Technology

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     Last modified: July 27, 2011

Abstract
In the solar wind and Earth’s foreshock electron beams are able to drive Langmuir waves. These waves undergo nonlinear wave-wave interactions and produce transverse waves with frequencies at the local plasma frequency and its harmonics. Localized Langmuir waves trapped in density wells have also been observed during type III events, and are possibly involved in producing transverse waves by an antenna mechanism. Here, simulations are performed using the three-dimensional electromagnetic Zakharov equations to study the evolution of beam-driven turbulence in plasmas and the structure of wave packets. Langmuir waves driven at nonzero wavenumbers are found to undergo a series of decays to form a low wavenumber condensate. For all electron thermal speeds, beam-driven Langmuir waves with suitable wavenumbers undergo electrostatic decay. Electromagnetic decay is also shown to occur. As the driven modes decay, chains of propagating beat waves develop. These decay processes lead to a low wavenumber condensate forming, from which localized wave packets form, and in the absence of damping at low wavenumbers these wave packets collapse. The structure of these wave packets are investigated in detail, with the Langmuir packets predominantly oblate, while the transverse packets are predominantly prolate. The structure of these packets is shown to be at most weakly affected by the driving parameters, while the statistical properties of the system are strongly dependent on the driving parameters. For conditions applicable to the solar wind and Earth’s foreshock, the electric fields associated with these wave packets have only a weak electromagnetic component.