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 the top and bottomside F‐region ionosphere under certain conditions. The European Incoherent Scatter (EISCAT) UHF radar was directed in magnetic zenith on October 18th and 19th, 2017 while stepping the pump frequency of the EISCAT Heating facility both upward and downward across the double resonance frequency of the fourth harmonic of the electron gyrofrequency and the local upper hybrid frequency, in a 2‐min‐on, 5‐min‐30‐s‐off pump cycle. 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, https://doi.org/10.1029/2018JA025822) 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 THFIL are consistent with transionospheric propagation through artificial radio windows by L mode guiding of the pump wave, facilitated by large scale density ducts present in the plasma. Key Points Observations of high frequency enhanced ion line spectra at the topside ionosphere The appearance of the topside enhancements are affected by the proximity to the topside double resonance frequency We interpret the observations as a result of L mode wave propagation through artificial radio windows