First Observations of Recurring HF‐Enhanced Topside Ion Line Spectra Near the Fourth Gyroharmonic

Abstract

We present first incoherent scatter radar observations of systematically recurring, high-frequency (HF)-enhanced ion line spectra at the topside F-region ionosphere, during magnetic field aligned HF pumping in an O-mode polarization. The European Incoherent Scatter UHF radar was directed in magnetic zenith on 9-11 March 2016 while stepping the pump frequency across the double resonance of the fourth harmonic of the electron gyrofrequency and the local upper hybrid frequency, in a 3-min-on, 3-min-off pump cycle. Topside and bottomside enhancements occur at the respective plasma resonance altitude and seem to be asymmetrically conditioned by the relative proximity of the pump frequency to the double resonance frequency. Further, the topside HF-induced ion line enhancements predominantly appear while the pump frequency is just below the double resonance frequency and only simultaneous to strong bottomside enhancements. A powerful, HF radio wave in O-mode, transmitted in the direction of magnetic zenith is reflected a few kilometers below the plasma resonance altitude, where the pump frequency is equal to the local plasma frequency, on the bottomside F-region in the ionosphere. Transionospheric propagation of the pump wave outside the radio window can be facilitated by density striations in the plasma, and we consider, in detail, the possible mechanisms proposed for propagation outside the standard radio window.

Conditions for Topside Ion Line Enhancements

“…We present observations of two separate cases of topside HF-enhanced ion lines (THFIL). THFIL simultaneous to bottomside HFIL (BHFIL), and conditioned by the relative proximity of the HF pump frequency to the bottomside double resonance frequency, consistent with previous observations (Rexer et al, 2018) were observed for HF pulses on 19th October. Recurring THFIL appearing after BHFIL have faded and conditioned by the relative proximity to the double resonance frequency on the topside ionosphere, were observed on 18th October.…”

“…The first consistently recurring topside HF enhanced ion line (THFIL) spectra and their relation to pump frequencies close to the 4th gyroharmonic frequency were shown by Rexer et al (2018). Observations of THFIL from an experiment in 2013 were also reported by Borisova et al (2020).…”

“…Several studies have since reported ISR measurements showing effects of HF pumping above the reflection altitude of the ionospheric F-region (Isham, Rietveld, et al, 1999;Isham, Hagfors, et al, 1999;Rietveld et al, 2002;Kosch et al, 2011). Leyser et al (2018) present signatures of transionospheric HF pump wave propagation into space in CASSIOPE spacecraft data.…”

Electron heating by HF pumping of high-latitude ionospheric F-region plasma near magnetic zenith

“…Evidence of L-mode propagation of the EISCAT Heating beam has previously been obtained as transionospheric propagation for f 0 < f oF 2 < f 0 +f e /2, in which case an Lmode wave would not be reflected but pass through the ionospheric plasma density peak. This was observed by direct measurement on the CASSIOPE spacecraft (Leyser et al, 2018) and indirectly by EISCAT UHF radar observations of ion acoustic lines in the topside ionosphere (Rexer et al, 2018). Figure 6 also displays the altitude of the plasma resonance (white lines in Fig.…”

Electron heating by HF-pumping of high-latitude ionospheric F-region plasma near magnetic zenith

“…These results are consistent with the pump wave propagating in the L mode in magnetic zenith and in the O mode at angles deviating from the zenith direction. Evidence of L-mode propagation of the EISCAT Heating beam has previously been obtained as transionospheric propagation for f 0 < f oF 2 < f 0 + f e /2, in which case an L-mode wave would not be reflected but pass through the ionospheric plasma density peak, by direct measurement on the CASSIOPE spacecraft (Leyser et al, 2018) and indirectly by EISCAT UHF radar observations of ion acoustic lines in the topside ionosphere (Rexer et al, 2018). Figure 6 also displays the altitude of the plasma resonance (white line in the right panels).…”