Conditions for Topside Ion Line Enhancements

Enhanced ion line spectra as a response to magnetic field-aligned high frequency (HF) pumping of the overdense polar ionosphere with left-handed circular polarization, can be observed at top and bottomside F-region under certain conditions. The European Incoherent Scatter (EISCAT) UHF radar was directed in zenith on October 18th 19th, 2017 while stepping pump EISCAT Heating facility both upward downward across double resonance fourth harmonic electron gyrofrequency local upper hybrid frequency, 2-min-on, 5-min-30-s-off cycle. We present observations two separate cases topside HF-enhanced lines (THFIL). THFIL simultaneous HFIL (BHFIL), conditioned by relative proximity HF consistent previous (Rexer et al., 2018, https://doi.org/10.1029/2018JA025822) were for pulses 19th October. Recurring appearing after BHFIL have faded ionosphere, are transionospheric propagation through artificial radio windows L mode guiding wave, facilitated large scale density ducts plasma.