Incoherent Scatter Radar (ISR) Observations of High-Frequency Enhanced Ion and Plasma Lines Induced by X/O Mode Pumping Around the Critical Altitude

Analysis of Incoherent Scatter Radar (ISR) data collected during an experiment involving alternating O/X mode pumping reveals that the high-frequency enhanced ion line (HFIL) and plasma line (HFPL) did not appear immediately after the onset of pumping, but were delayed by a few seconds. By examining the initial behaviors of the ion line, plasma line, and electron temperature, as well as ionosphere conditions, we find that (1) the HFIL and HFPL were delayed not only in the X mode pumping but also in the O mode pumping and (2) the HFIL was not observed prior to enhancement of the electron temperature. Our analysis suggests that (1) leakage of the X mode to the O mode pumping may not be ignored and (2) spatiotemporal uncertainties and spatiotemporal variations in the profiles of ion mass and electron density may have played important roles in the apparent failure of the Bragg condition to apply; (3) nevertheless, the absence of parametric decay instability (PDI) cannot be ruled out, due to our inability to match conditions caused by the spatiotemporal uncertainties.