VHF Radar Images of Artificial Field‐Aligned Ionospheric Irregularities in the Subauroral E Region

Abstract

AbstractArtificial E region field‐aligned plasma density irregularities (AFAIs) have been generated using the High‐frequency Active Auroral Research Program (HAARP) ionospheric modification facility in Gakona and observed with a 30 MHz coherent scatter radar imager in Homer, Alaska. The AFAIs were generated using a distinctive, twisted‐beam antenna pattern that illuminated a particularly broad volume overhead. The broad beam facilitates studies of natural sporadic E layer patches when they are present. The center of the pattern was pointed at different angles between zenith and magnetic zenith to examine the effects on the AFAI morphology. Radar images of AFAIs generally resemble the radiation pattern of the high‐frequency source, but the irregularities are strongest within a narrow range of zenith angles bounded approximately by the Spitze angle. A number of factors which might influence AFAI generation and detection are examined. The most important is most likely the requirement for the pump mode to have a standing‐wave component for thermal parametric instability to operate.