Observations of Plasma Densities in the Polar Cap during Orbits with 
Theta Aurora Observations

William W. L. Taylor, Venku Jayanti, and Harald Frey

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. The FUVI (Far Ultra Violet 
Imager) on the IMAGE satellite occasionally observes theta aurora. 
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. In a preliminary look at 
FUV and RPI data, we have found two orbits when theta auroras are 
observed during the same orbit as field aligned echoes, although 
separated in time. We will present plasma density measurements, both 
along the satellite orbit and along the field lines connected to the 
satellite. There were no unusual plasma density distributions found 
during orbits with theta auroras.

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To be presented at the Magnetospheric Imaging Workshop, 
Yosemite National Park, CA, Feb. 5-8, 2002.