Modeling of field‐aligned guided echoes in the plasmasphere

Ray tracing modeling is used to investigate the plasma conditions under which high‐frequency ( f ≫ f uh ) extraordinary mode waves can be guided along geomagnetic field lines. These guided signals have often been observed as long‐range discrete echoes in the plasmasphere by the Radio Plasma Imager (RPI) onboard the Imager for Magnetopause‐to‐Aurora Global Exploration satellite. 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 <10 wavelengths wide can guide nearly field‐aligned propagating high‐frequency X mode waves. Effective guidance of a wave at a given frequency and wave normal angle (Ψ) 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.