STANFORD UNIVERSITY
EE 350 RADIOSCIENCE SEMINAR
Professor Umran Inan

Winter 2000-2001

Date: Wednesday, February 21, 2001
Time: 4:15-5:30 PM; Refreshments at 4:00 PM
Location:Bldg. 200, Rm. 34


Radio Imaging of Plasma Boundries in Near-Earth Space

Prof. Donald Carpenter
STAR Lab, Stanford University

Abstract

A new satellite, called IMAGE (Imager for Magnetopause-to-Aurora Global Exploration) was launched in March, 2000 and is now producing global-scale images of the Earth's space environment from various locations along a polar orbit with apogee about 8 Earth radii and perigee about 1200 km. Among the instruments on board are EUV, a photon imager operating at 30.4 nM (U. of Arizona) and the Radio Plasma Imager (RPI) (U. Mass. at Lowell), designed to act as a radio sounder of various important magnetospheric plasma boundaries. This talk will emphasize the operation of the RPI.

The Earth's plasmasphere is a major object of study for both EUV and RPI. By collecting scattered sunlight at 30.4 nM from He+, a minor but important constituent of the plasmasphere, EUV is able to obtain global images of that region during several-hour periods as IMAGE ascends toward apogee at high northern latitudes. Meanwhile, RPI is able to obtain data on the plasmasphere by the classical radio sounding technique developed during the "golden age" of ionospheric topside sounders in the 1960's and early 1970's. RPI is also able to make local measurements of plasma parameters along the IMAGE orbit.

The performance of RPI thus far has yielded many surprises. It was widely believed before launch that although it would be difficult to sound the magnetopause, due in part to the dynamic nature of the boundary, it should be easy to sound the plasmapause, the region of steep density gradients at the outer limit of the plasmasphere. In fact, most of the echoes from the plasmapause region are spread in range rather than discrete, as had originally been expected, for reasons that remain to be understood. Another surprise is the extent to which the sounder echoes are dominated by propagation along the geomagnetic field, rather than in the directions that ray tracing in simple models of the medium would predict.

I will present some of the first images of the plasmasphere from EUV, and also sequences of records from RPI showing the response to the sounder as IMAGE moves inbound toward the plasmasphere, passes through the plasmasphere to low altitudes over the southern polar region, and then moves outward through the other lobe of the plasmasphere and begins to probe the northern polar region.