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.
Abstract. Previous studies have shown that conjugate auroral features are displaced in the two hemispheres when the interplanetary magnetic field (IMF) has a transverse (Y) component. It has also been shown that a BY component is induced in the closed magnetosphere due to the asymmetric loading of magnetic flux in the lobes following asymmetric dayside reconnection when the IMF has a Y component. The magnetic field lines with azimuthally displaced footpoints map into a “banana”-shaped convection cell in one hemisphere and an “orange”-shaped cell in the other. Due to the Parker spiral our system is most often exposed to a BY-dominated IMF. The dipole tilt angle, varying between ±34∘, leads to warping of the plasma sheet and oppositely directed BY components in dawn and dusk in the closed magnetosphere. As a result of the Parker spiral and dipole tilt, geospace is asymmetric most of the time. The magnetic storm on 17 August 2001 offers a unique opportunity to study the dynamics of the asymmetric geospace. IMF BY was 20–30 nT and tilt angle was 23∘. Auroral imaging revealed conjugate features displaced by 3–4 h magnetic local time. The latitudinal width of the dawnside aurora was quite different (up to 6∘) in the two hemispheres. The auroral observations together with convection patterns derived entirely from measurements indicate dayside, lobe and tail reconnection in the north, but most likely only dayside and tail reconnection in the Southern Hemisphere. Increased tail reconnection during the substorm expansion phase reduces the asymmetry.
We present 3-D excitation rate estimates of artificial aurora in the ionospheric F layer, induced by high-frequency radio waves from the European Incoherent Scatter heating facility. Simultaneous imaging of the artificial aurora was done with four separate Auroral Large Imaging System stations, permitting tomography-like 3-D auroral reconstruction of the enhanced atomic oxygen emissions at 6,300, 5,577, and 8,446 Å. Inspection of the 3-D reconstructions suggests that the distribution of energized electrons is less extended in altitude than predicted by transport calculations of electrons accelerated to 2-100 eV. A possible reason for this discrepancy is that high-frequency pumping might induce an anisotropic distribution of energized electrons.Plain Language Summary Auroral lights can be artificially generated by transmitting high-frequency radio waves with high power into the upper atmosphere. In this article, we use multiple viewpoint imaging of artificially produced aurora to estimate the 3-D distribution of the auroral lights by employing tomography-like techniques. The 3-D distribution is estimated in the red, green, and infrared auroral emission lines with wavelengths of 630.0, 557.7, and 844.6 nm, respectively. These emissions are excited by energetic electrons, which have been accelerated through interaction processes between the transmitted radio waves and plasma in the upper atmosphere, at an altitude of about 220-250 km. We observe that the estimated 3-D auroral distributions are less extended in altitude than indicated by previous theoretical work. A possible reason for this disagreement is that the radio wave-plasma interaction processes might lead to a direction dependent electron acceleration.
<p>High frequency (HF) enhanced ion line spectra as a response to magnetic field aligned HF pumping of the polar ionosphere in an O-mode polarization can be observed at the top and bottomside F-region ionosphere under certain conditions. The European Incoherent Scatter (EISCAT) UHF radar was directed in magnetic zenith on 18th and 19th October 2017 while stepping the pump frequency of the EISCAT Heating facility across the double resonance frequency of the fourth harmonic of the electron gyrofrequency and the local upper hybrid frequency, in a 2-min-on, 2-min-off pump cycle, stepping both upward and downward in frequency. 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 to the double resonance frequency, consistent with previous observations \citep{Rexer2018} were observed for heating pulses on 19th October. Recurring THFIL with a second set of characteristics were observed on 18th October, appearing independently from BHFIL and possibly conditioned by the proximity of the topside double resonance frequency. Propagation of the pump wave to the topside ionosphere is consistent with L-mode wave propagation facilitated by density striations in the plasma. We consider the conditions for the occurrence of THFIL for two cases/types of observations.&#160;</p>
Abstract. High-frequency electromagnetic pumping of ionospheric F-region plasma at high and mid latitudes gives the strongest plasma response in magnetic zenith, antiparallel to the geomagnetic field in the Northern Hemisphere. This has been observed in optical emissions from the pumped plasma turbulence, electron temperature enhancements, filamentary magnetic field-aligned plasma density irregularities, and in self-focusing of the pump beam in magnetic zenith. We present results of EISCAT (European Incoherent SCATter association) Heating-induced magnetic-zenith effects observed with the EISCAT UHF incoherent scatter radar. With heating transmitting a left-handed circularly polarized pump beam towards magnetic zenith, the UHF radar was scanned in elevation in steps of 1.0 and 1.5∘ around magnetic zenith. The electron energy equation was integrated to model the electron temperature and associated electron heating rate and optimized to fit the plasma parameter values measured with the radar. The experimental and modelling results are consistent with pump wave propagation in the L mode in magnetic zenith, rather than in the O mode.
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