Descents in high-frequency enhanced plasma and ion lines during ionospheric heating at EISCAT

Abstract An observation of altitude descents in high-frequency enhanced ion line (HFIL) and plasma line (HFPL) are examined, and an alternative explanation are presented. The HFIL and HFPL altitudes should be governed by the dispersion relations of ion acoustic and Langmuir waves respectively, and those altitude descents in HFIL and HFPL may be in consequence of the significantly enhanced electron temperature and the thermal conduction. When heating the ionosphere, the HFIL should be observed at a lower altitude, at which a larger mass of ion may match the enhanced electron temperature so that the enhanced ion acoustic wave may meet the Bragg condition. Similarly, the enhanced electron temperature should match a smaller electron density at a lower altitude, where the enhanced Langmuir wave may meet the Bragg condition and the HFPL is observed. Some previously observed altitude descents in the HFIL and HFPL demonstrate the consistency with the suggested theory.