Observations of Plasma Densities in the Polar Cap During Theta Auroras

Theta auroras, auroras with sun-aligned transpolar arcs, are observed predominantly during intervals where the IMF is northward, AE < 150, and when the solar wind velocity is high. The FUVI (Far Ultra Violet Imager) on the IMAGE satellite occasionally observes theta aurora. The RPI (Radio Plasma Imager) on IMAGE measures the plasma density along the satellite orbit by observing the plasma frequency cutoff in passive observations of naturally occurring plasma waves. In addition, we can make estimates of the plasma density along the magnetic field through the spacecraft when the RPI observes field aligned echoes from its active sounding. In a preliminary look at FUV and RPI data, we have found two instances of theta auroras with field aligned echoes. We will present plasma density results, both along the satellite orbit and along the field lines connected to the satellite. We will also correlate the geomagnetic and solar wind conditions with the appearances of theta auroras.