Measurement of lower-hybrid frequency range turbulence in a reconnecting current sheet

Physics

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7831 Laboratory Studies, 7835 Magnetic Reconnection, 7863 Turbulence, 7871 Waves And Instabilities

Scientific paper

Measurements in the Magnetic Reconnection Experiment (MRX) reveal fast reconnection [H. Ji et.al., PRL 80, 3256 (1998)] and anomalously high ion heating rates [S.C. Hsu et.al., PRL 84, 3859 (2000)]. Current sheets formed in MRX involve large gradients in density, flow, and magnetic field, all of which can drive unstable fluctuations. Turbulence resulting from these free energy sources could possibly explain the observed anomalous dissipation and ion heating in MRX. Double Langmuir and magnetic pick-up probes utilizing wide bandwidth amplifiers built into the probe tips have been used to measure electrostatic and magnetic fluctuations in null-helicity MRX current sheets. Using floating double Langmuir probes, we have measured broadband fluctuations in the lower hybrid frequency range (ω LH ≈ 13~MHz) which arise with the formation of the current sheet in MRX. These fluctuations are seen to have a hollow amplitude profile with respect to the current sheet and are strongest early in the reconnection process. Correlation length and phase velocity estimates are obtained for these fluctuations using multipoint probe techniques, and have revealed a tendency for propagation in the electron diamagnetic direction. Magnetic fluctuations are also observed in and near MRX current sheets. A high frequency magnetic fluctuation is seen concomitant in time and space with the high-frequency electrostatic fluctuations. Additional lower frequency (300-800~kHz) fluctuations are seen outside the current sheet early in time and also peaked at the center of the current sheet late in time. The late time magnetic mode is correlated in time with what appears to be current disruptions in MRX current sheets. We will present detailed characterization of these fluctuations including any correlation to the reconnection reconnection process in MRX.

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