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. The response of ionospheric plasma to pumping by powerful HF(high frequency) electromagnetic waves transmitted from the ground into theionosphere is the strongest in the direction of geomagnetic zenith. Wepresent experimental results from transmitting a left-handed circularlypolarized HF beam from the EISCAT (European Incoherent SCATter association)Heating facility in magnetic zenith. The CASSIOPE (CAScade, Smallsat andIOnospheric Polar Explorer) spacecraft in the topside ionosphere above theF-region density peak detected transionospheric pump radiation, although thepump frequency was below the maximum ionospheric plasma frequency. The pumpwave is deduced to arrive at CASSIOPE through L -mode propagation andassociated double ( O to Z , Z to O ) conversion in pump-induced radiowindows. L -mode propagation allows the pump wave to reach higher plasmadensities and higher ionospheric altitudes than O -mode propagation so thata pump wave in the L -mode can facilitate excitation of upper hybridphenomena localized in density depletions in a larger altitude range. L -mode propagation is therefore suggested to be important in explaining themagnetic zenith effect. Keywords. Space plasma physics (active perturbation experiments)

Evidence of &lt;i&gt;L&lt;/i&gt;-mode electromagnetic wave pumping of ionospheric plasma near geomagnetic zenith

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