Simultaneous observation of an intense 65 keV field-aligned proton beam and ULF-waves during a break-up event

Measurements of nearly monoenergetic 65 keV protons above active auroral forms were obtained on a rocket flight from Andenes (Norway) on October 13, 1977. Analysis of the pitch angle distribution of the greater than 30 keV protons and the azimuthal anisotropy of the proton fluxes support a model of a proton beam of finite spatial extent. The particle observations occur simultaneously with ULF-waves in the frequency range 0.5-5 Hz. The particle distribution function is unstable and causes an amplification of obliquely propagating Alfven waves. It is argued that the waves are amplified by resonant interaction with the proton beams.