Modeling of Field-Aligned Guided Echoes in the Plasmasphere

Shing F. Fung and James L. Green 
NASA/Goddard Space Flight Center, Greenbelt, MD


The conditions under which high frequency (f >> fuh) long-range extraordinary-mode 
discrete field-aligned echoes observed by the Radio Plasma Imager (RPI) on board 
the Imager for Magnetopause-to-Aurora Global Exploration (IMAGE) satellite in the 
plasmasphere are investigated by ray tracing modeling. Field-aligned discrete 
echoes are most commonly observed by RPI in the plasmasphere although they are 
also observed over the polar cap region. The plasmasphere field-aligned echoes 
appearing as multiple echo traces at different virtual ranges are attributed to 
signals reflected successively between conjugate hemispheres that propagate along 
or nearly along closed geomagnetic field lines. The ray tracing simulations show 
that field-aligned ducts with as little as 1% density perturbations (depletions) 
and less than 10 wavelengths wide can guide nearly field-aligned propagating high 
frequency X mode waves. Effective guidance of wave at a given frequency and wave 
normal angle (psi) depends on the cross-field density scale of the duct, such that 
ducts with stronger density depletions need to be wider in order to maintain the 
same gradient of refractive index across the magnetic field. While signal guidance 
by field aligned density gradient without ducting is possible only over the polar 
region, conjugate field-aligned echoes that have traversed through the equatorial 
region are most likely guided by ducting.

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Submitted for publication, Journal of Geophysical Research, 2004