Ray tracing analysis of L mode pumping of the ionosphere, with implications for the magnetic zenith effect

Abstract

Abstract. Using ray tracing of ordinary mode HF waves in an ionosphere with kilometre-scale field aligned density depletions, or ducts, we find that transmission across the plasma resonance becomes possible at new locations as rays are guided into the so-called L mode. Stronger transmitted fields are seen in some directions, notably at inclinations close to the vertical or the magnetic zenith (MZ). It is argued that the results could have implications for the magnetic zenith effect, i.e., the increased plasma response that has been observed around the MZ.

Evidence of <i>L</i>-mode electromagnetic wave pumping of ionospheric plasma near geomagnetic zenith

“…These treatments too are concerned with large-scale structuring of the plasma by bunching of small-scale density striations, similar to the formation of ducts discussed by Leyser and Nordblad (2009);Nordblad and Leyser (2010). However, whereas Gurevich et al (2001Gurevich et al ( , 2002 propose self-focusing of an O-mode pump wave, the present results, those of Leyser and Nordblad (2009) and Nordblad and Leyser (2010) suggest the importance of L-mode propagation deeper into the plasma where an O-mode wave cannot reach. Further experimental investigations and modelling of the magnetic zenith effect are needed to find out the importance of O-mode versus L-mode pumping for the magnetic zenith effect.…”

“…Ducted waves therefore do not have wave vectors that are strictly parallel to ±B 0 . Nevertheless, ray tracing computations show the ducted waves to propagate in the L-mode where f p > f 0 (Leyser and Nordblad, 2009;Nordblad and Leyser, 2010). Further, strictly speaking the dispersion properties of the L-mode discussed in the present treatment concern a homogeneous plasma.…”

“…For increasing θ beyond θ c , an O-mode wave reflects at successively lower f p , as in magnetic zenith at θ = 90 • − I ≈ 12 • . Based on ray-tracing computations, it has been shown that a LHCP wave can be guided in the L-mode in magnetic zenith by kilometre-scale magnetic field-aligned density ducts in an otherwise horizontally stratified ionosphere (Leyser and Nordblad, 2009;Nordblad and Leyser, 2010). A wave in the L-mode has a wave vector parallel or antiparallel to the ambient magnetic field.…”

“…Note that the O-and Z-mode dispersion surfaces only touch at f p = f 0 for a wave vector parallel to the ambient magnetic field (Nordblad and Leyser, 2010). A real wave field propagating along ±B 0 involves a range of wave vectors around the parallel direction.…”

“…The square of the refractive index for the L-mode as obtained from the dispersion equation for a cold magnetized and homogeneous plasma is given by n 2 = 1−f 2 p /f (f −f e ), where f is the wave frequency and f e is the electron gyrofrequency (Stix, 1992). Based on ray tracing analyses, it has been proposed that the pump wave can be guided in the Lmode by pump-induced kilometre-scale geomagnetic fieldaligned plasma density ducts (Leyser and Nordblad, 2009;Nordblad and Leyser, 2010).…”