Echo Signal Strength on Low-Latitude High-Altitude Digital ISIS 1 Topside-Sounder Ionograms: Significance for Magnetopause Sounding Using the Radio-Plasma Imager on IMAGE R. F. Benson, S. F. Fung, J. L. Green NASA Goddard Space Flight Center Laboratory for Extraterrestrial Physics NASA/Goddard Space Flight Center Greenbelt, MD 20771, USA Phone: (301) 286-4037, Fax: (301) 286-1683, e.mail: u2rfb@lepvax.gsfc.nasa.gov H. G. James Communications Research Centre B. W. Reinisch University of Massachusetts, Lowell Background The radio plasma imager (RPI) is one of the instruments to be included on the Imager for Magnetopause-to-Aurora Global Exploration (IMAGE) which is scheduled for a January, 2000 launch. The main IMAGE scientific objective is to understand the global dynamics of the terrestrial magnetosphere and its response to changing solar-wind conditions. The RPI will play a key role in achieving this objective by performing radio sounding over distances of several earth radii (RE) in the magnetosphere. This paper investigates the problem of long-distance sounding in space plasmas by analyzing the echo signal strength observed during some of the longest vertical propagation paths recorded by ionospheric topside sounders. Data The most suitable topside-sounder satellite for this investigation is ISIS I. The investigation uses digital ISIS-I ionograms (see http://nssdc.gsfc.nasa.gov/space/isis/isis-status.html) recorded near apogee (3500 km) at low latitudes in order to obtain the longest possible propagation paths across the direction of the terrestrial magnetic field. Earlier investigations of echo signal strength were based on much shorter distances using either Alouette I at 1,000 km [1] or Alouette II at about 1,500 km [2]. The geometry used in the present study approximates the reflection conditions expected to be encountered during long-distance sounding to the magnetopause, where the wave reflection will occur in a region where the electron density gradient will be approximately normal to the magnetic- field direction. In addition, this geometry represents a difficult sounding condition because there is no field-aligned propagation to the ionospheric target below. (Such field-aligned propagation can increase the echo signal by 20 to 40 dB [3]). This ISIS-I sounding distance of approximately 1/2 RE is about 1/4 of the RPI sounding distance to the magnetopause when IMAGE is near its 8 RE apogee location. Observations Digital ISIS-I ionograms recorded at Quito when ISIS-I was near apogee in the early afternoon reveal strong ordinary (O) and extraordinary (X) mode echoes from near the F-region peak. On a given ionogram, the amplitude of the O-mode echo can be less than, comparable to, or stronger than the X-mode echo. Either (or both) can be of short duration, suggesting a simple mirror reflection, or they can have a duration much greater than the sounder-pulse duration suggesting the presence of irregularities in the vicinity of the reflection level. The reception of these strong long-distance ionospheric echoes will be discussed in the context of experimental conditions anticipated by the RPI on the IMAGE mission (see http://image.gsfc.nasa.gov/). References [1] L. E. Petrie et al., J. Geophys. Res., 70, pp. 4347 -4356 1965. [2] C. A. Franklin and M. A. Maclean, Proc. IEEE, 57, pp. 897-929, 1969. [3] G. E. K. Lockwood, J. Geophys. Res., 78, pp. 2244-2250, 1973.