Electrostatic decay in a magnetized plasma with applications to polarized Langmuir waves in the solar wind

Andrew Layden
School of Physics, University of Sydney

*Iver Cairns
School of Physics, University of Sydney

*Peter Robinson
School of Physics, University of Sydney

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     Last modified: July 29, 2011

Abstract
Langmuir waves with strong perpendicular polarization, associated with electron beams and type III solar radio bursts, were recently observed by the STEREO spacecraft. Such polarization requires wave numbers substantially lower than those of the initial beam-driven Langmuir waves. One mechanism which can downshift the Langmuir wave number is electrostatic (ES) decay, in which a Langmuir wave decays into a backscattered Langmuir wave and an ion sound wave. However, assuming an unmagnetized plasma, a sufficient reduction in Langmuir wave number is kinematically prohibited. We investigate the effects of including a background magnetic field on the kinematics of ES decay, by calculating the dispersion relation for Langmuir waves in a warm magnetized plasma. We find that for finite magnetizations decay to much lower wave numbers is permitted, and that a low wave number condensate can result. These changes are due to magnetization leading to z-mode character of the Langmuir mode at low wave numbers, allowing wave frequencies less than the plasma frequency. The lowest attainable wave number and number of successive decays allowed both depend on magnetization, with the unmagnetized results recovered in that limit. For the STEREO data the observed magnetic field permits the decay to proceed to essentially zero wave numbers, not inconsistent with the observed Langmuir wave packets. These fundamental modifications to the kinematics of ES decay may be important in other space physics contexts such as Langmuir turbulence.