C. La Hoz¹, M. T. Rietveld^2,3, B. Isham^3,4, T. Grydeland¹

¹Auroral Observatory, Tromsø, Norway
²Max-Planck-Institut für Aeronomie, Germany
³EISCAT Scientific Association, Ramfjordmoen, Norway
⁴formerly at Interamerican University, Bayamón, Puerto Rico, USA

An event was measured by the VHF EISCAT radar containing enhanced upshifted and downshifted Langmuir and ion acoustic waves plus a zero frequency enhancement all occurring simultaneously and in the same altitude interval in the auroral ionosphere under natural conditions. The event lasted about 50 seconds and had an altitude span of about 14 km. It was coincident with auroral precipitation as evidenced by electron density measurements made by the radar and by optical observations. It is suggested that the theory of François Forme (Ann. Geophysicae, Vol 17, p. 1172, 1999) can explain these measurements whereby a beam of electrons excites Langmuir waves via the bump in tail instability which subsequently decay into other Langmuir waves and ion acoustic waves. The excitation is sufficiently strong to reach the regime of the modulational instability hence producing an enhancement at zero frequency as well. It is of interest to emphasize that this mechanism is the same as the one that accounts for the wave enhancements observed during ionospheric HF heating experiments as as well as the waves that produce solar radio bursts of type III. The Zakharov equations offer a model framework to explain the three phenomena.