Variation in altitude of high-frequency enhanced plasma line by the pump near the 5th electron gyro-harmonic

Abstract. During an ionospheric heating campaign carried out at the European Incoherent Scatter Scientific Association (EISCAT), the ultra high frequency incoherent scatter (IS) radar observed a systematic variation in the altitude of the high-frequency enhanced plasma line (HFPL), which behaves depending on the pump frequency. Specifically, the HFPL altitude becomes lower when the pump lies above the 5th gyro-harmonic. The analysis shows that the enhanced electron temperature plays a decisive role in the descent in the HFPL altitude. That is, on the traveling path of the enhanced Langmuir wave, the enhanced electron temperature can only be matched by the low electron density at a lower altitude so that the Bragg condition can be satisfied, as expected from the dispersion relation of Langmuir wave.