Electric field statistics and modulation characteristics of bursty Langmuir waves observed in the cusp

Abstract

HF wave receivers on the Twin Rockets to Investigate Cusp Electrodynamics (TRICE) detect numerous 20-250 ms duration Langmuir wave bursts. Investigation of 41 bursts at 967-984 km, where the gyrofrequency exceeds the plasma frequency, reveals a characteristic signature whereby the Langmuir wave bandwidth and highest frequency maximize at the center of the bursts, suggesting that bursts coincide with density enhancements. The Langmuir waveforms are modulated at frequencies up to 50 kHz, with significant modulation at frequencies <1 kHz about 10% of the time. Where amplitudes of the modulated waves pass through zero, the phase of the carrier wave shifts by 180, consistent with a superposition of interfering waves. The corresponding wave spectrum has multiple <1 kHz bandwidth peaks separated by <1-50 kHz. For no averaging, the wave E field statistics, dominated by the carrier Langmuir wave, show an E+1 dependence at small E fields as expected for rectified sine waves. For moderate averaging, the statistics, dominated by the envelope field, show a spectral index of +2 at small E fields, consistent with numerical simulations of independent stochastically driven waves. The wave statistics, absence of VLF waves, lack of transition in the modulation occurrence as a function of frequency, and tendency of the spread spectrum to cut off at the ambient plasma frequency outside the density enhancement associated with the wave burst suggest that linear growth and mixing of Langmuir modes provides a more natural explanation of modulated Langmuir waves than do wave-wave processes previously proposed, at least for cusp waves encountered by TRICE. Copyright 2010 by the American Geophysical Union.