Observations of Plasma Densities in the Polar Cap during Theta Auroras

William W. L. Taylor (1), Venku Jayanti (1), and Harald Frey(2)

(1) RITSS/Goddard Space Flight Center, Greenbelt, MD 20771
(2) SSL, University of California, Berkeley, CA 94720

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.