VLF WAVE GROWTH FROM DISPERSIVE BURSTS OF FIELD-ALIGNED ELECTRON-FLUXES

Large-amplitude (>200 mV/m) electrostatic whistler waves near the lower hybrid frequency (approximately 5 kHz to approximately 20 kHz) were observed by an auroral sounding rocket during substorm breakup. The measured wavelengths (10 m to 40 m) indicate that the emissions were electrostatic and resonant with electrons that had parallel energies of a few hundred electron volts. We propose that the intense emissions drew their energy from dispersive bursts of low-energy (less than or similar to 100 eV to approximately 3 keV), field-aligned electron fluxes. The dispersive bursts are known to cause a brief (approximately 100 ms), but intense instability that results in large-amplitude Langmuir emissions (approximately 1 MHz). The high-frequency emissions can rapidly form a plateau in the one-dimensional electron distribution. We show, however, that these distributions remain unstable to electrostatic whistler waves near the lower hybrid frequency. The amplitude and wavelength of the observed emissions were sufficient to accelerate the hydrogen ions with energies between approximately 50 eV and approximately 200 eV.