Wavelength measurement of auroral hiss

Abstract

The average wavelengths of VLF emissions in the auroral ionosphere were measured as a function of frequency using an orthogonal antenna technique. The measurements, made by an auroral sounding rocket, were sensitive to wavelengths between ∼5 m and ∼300 m for wave frequencies from 20 Hz to 10 kHz. The emissions just above the lower hybrid frequency had wavelengths of 12 m or less in many cases, increasing with increasing frequency. The measured wavelengths were such that the VLF emissions were resonant with electrons whose energies were a few hundred eV, whereas the inverted‐V electrons had energies of ∼20 keV. Bursts of low‐energy (<300 eV to ∼3 keV), field‐aligned electron fluxes were observed often during the flight, but no direct correlation between the flux intensity and the VLF intensity or wavelength could be found. The shorter‐than‐expected wavelengths were found in different types of VLF spectra. The steady state VLF spectra generally displayed a typical sharp cutoff below the lower hybrid frequency with a maximum at several times the lower hybrid frequency. These spectra also had structure at hydrogen cyclotron harmonics above the lower hybrid frequency. At other times, 25‐s to 50‐s bursts of lower hybrid emissions were observed. During the periods of the most intense emissions, the spectral peak was very near the lower hybrid frequency while hydrogen Bernstein emissions were seen below the lower hybrid frequency.