Abstract
Abstract. The response of ionospheric plasma to pumping by powerful HF (high frequency) electromagnetic waves transmitted from the ground into the ionosphere is the strongest in the direction of geomagnetic zenith. We present experimental results from transmitting a left-handed circularly polarized HF beam from the EISCAT (European Incoherent SCATter association) Heating facility in magnetic zenith. The CASSIOPE (CAScade, Smallsat and IOnospheric Polar Explorer) spacecraft in the topside ionosphere above the F-region density peak detected transionospheric pump radiation, although the pump frequency was below the maximum ionospheric plasma frequency. The pump wave is deduced to arrive at CASSIOPE through L-mode propagation and associated double (O to Z, Z to O) conversion in pump-induced radio windows. L-mode propagation allows the pump wave to reach higher plasma densities and higher ionospheric altitudes than O-mode propagation so that a pump wave in the L-mode can facilitate excitation of upper hybrid phenomena localized in density depletions in a larger altitude range. L-mode propagation is therefore suggested to be important in explaining the magnetic zenith effect. Keywords. Space plasma physics (active perturbation experiments)
Highlights
Electromagnetic high-frequency (HF) pumping of ionospheric plasma from ground-based transmitters yields the strongest plasma response when the HF beam is directed in magnetic zenith, antiparallel to the geomagnetic field (B0)in the Northern Hemisphere
We present experimental results from EISCAT Heating (Rietveld et al, 2016); this HF facility was used to pump the plasma with left-hand circularly polarized (LHCP) HF radio waves
The EISCAT Heating facility was used to transmit LHCP pump waves into the ionosphere in a beam directed towards magnetic zenith, with f0 a few hundred kilohertz below foF2 such that f0 < foF2 < f0 + fe/2
Summary
Electromagnetic high-frequency (HF) pumping of ionospheric plasma from ground-based transmitters yields the strongest plasma response when the HF beam is directed in magnetic zenith, antiparallel to the geomagnetic field (B0)in the Northern Hemisphere. Electromagnetic high-frequency (HF) pumping of ionospheric plasma from ground-based transmitters yields the strongest plasma response when the HF beam is directed in magnetic zenith, antiparallel to the geomagnetic field (B0). Electron temperature enhancements are the largest when pumping near magnetic zenith (Rietveld et al, 2003; Honary et al, 2011) and pump-excited geomagnetic field-aligned density striations give the strongest HF-radar backscatter, as observed in experiments with EISCAT Heating (Rietveld et al, 2003; Blagoveshchenskaya et al, 2006), and the mid-latitude Sura HF facility in Russia (Tereshchenko et al, 2004). The plasma response to the HF pumping was observed with the EISCAT UHF (ultra high frequency) incoherent scatter radar at the Ramfjordmoen site
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