Ion velocity distributions in the auroral ionosphere

There are regions in the weakly-ionized plasma of the lower ionosphere where conditions may be such as to drive the ion velocity distributions away from the Maxwellian from. Such conditions are encountered in auroral regions where strong perpendicular electric fields are present. Large neutral winds are also the cause of similarly distorted distributions. A relaxation model for collisions enables Boltzmann's equation to be solved exactly. For infrequent collisions and strong electric fields the ions are distributed over a torus in velocity space. As the collision frequency is increased, the ions distribute themselves on a portion of this torus. Similar distributions are expected for other models of ion--neutral interactions which may be physically more accurate than a relaxation model. In particular, resonant charge-exchange reactions, which are dominant for 0+ above 200 km, drive the ions to the same kind of non-Maxwellian distribution. A comparison with results obtained with a polarization model for collisions in the limit of small departures from a Maxwellian also shows this description to be qualitatively correct. Evidence has been found for the existence of non-Maxwellian velocity distributions in some of the volt--ampere characteristics obtained with the retarding potential analyser carried by Atmospheric Explorer C. The non-Maxwellian behavior was detected when the electric field strength was greater than 50 mV/m. For weaker electric field strengths the presence of non-Maxwellian distributions still affects the shape of the characteristics. Use of Maxwell distributions to fit the data results in values that are less accurate. Non-Maxwellian effects must be considered if an accurate value for the ion temperature is desired