Heavy ion beam-ionosphere interactions - Electron acceleration

Operation of a divergent 25-eV Ar+ gun within an auroral arc produced dramatic changes in the flux of electrons with energies between 1 keV and the 88-eV detector cutoff. The observations suggest that intense return currents flowed parallel to to neutralize the Ar+ beam, particularly within a few meters of the rocket. These neutralization currents were carried above and below the rocket by the few-eV electrons which were emitted by the gun and by colder ionospheric electrons. Such low-energy electrons could not be measured directly by detectors on the rocket. We concluded that generation of strong field-aligned return currents was the most important effect of ion gun operation, and that these field aligned currents were responsible for many other observable effects. Downgoing hectovolt electrons appear to have been accelerated because of interactions with waves or quasi-stationary electric fields that were generated by the field-aligned current. This acceleration took place throughout a cylinder centered on the rocket, with a radius of at least several meters. Acceleration of hectovolt electrons depended surprisingly little on the direction in which the Ar+ gun was pointing. All downgoing electrons were accelerated, whether or not they passed through or near the heavy ion beam. The most likely energy source for acceleration of downgoing electrons involves an interruption of the auroral current system. Strong low-frequency waves or quasi-stationary electric fields impede the flow of the principal current carrying electrons. Energy is transferred to the more energetic observed electrons, which can pass through the strong low-frequency electric fields. These downgoing energetic electrons are then pitch angle scattered through Doppler resonance with the low-frequency waves. This process produces an approximately isotropic Maxwellian distribution of downgoing electrons. Those electrons with sufficiently large gyroradii appear to scatter past 90° through a perpendicular resonance, or a finite gyroradius effect. Electrons which scatter through 90° below the rocket then are detected as upgoing electrons. The resulting upgoing electron distribution has a depression at low energies because electrons with small gyroradii cannot scatter efficiently through 90°. Stability calculations identify waves that are expected to be produced directly by the heavy ion beam and by the depression in the electron distribution function, and suggest features to look for in future flights.