European incoherent scatter VHF measurements of gyrolines

Abstract

We have observed the incoherent scatter gyrolines with the European Incoherent Scatter VHF radar (224 MHz) pointing northward at 45° elevation, which corresponds to an angle of 54.6° with the magnetic field at 200 km altitude. The lines can be seen after a 30‐min integration almost all the time. Most of the time they are observable after a 10‐min integration and often after only 1‐2 min when the radar is operating at full power. The lines are observed at altitudes of 100‐270 km and are most intense at about 180 km. Typical intensities are 0.5‐2.0% relative to the ion line. The symmetry between upshifted and downshifted lines is good, but the upshifted lines appear to be slightly more powerful. The experiment shows the effect of photoelectron enhancement, the lines being generally stronger in the morning than in the evening. Another indication of the photoelectron effect is the fact that there is a sudden drop in the power of the lines at about 1830 UT which corresponds to the local sunset in the plasma volume we are probing. The data also show examples of enhanced gyrolines in the absence of photoelectrons which we believe are caused by secondary electrons from particle precipitation.