Meteor observations with the European Incoherent Scatter UHF Radar

The European Incoherent Scatter (EISCAT) UHF radar, which operates at a nominal frequency of 930 MHz, is introduced as a powerful meteor radar. Its high sensitivity is utilized to detect transient enhanced ionization trails caused by meteors of all orientations, in contrast to conventional HF and VHF backscatter radars, which observe only the meteor trails oriented approximately normal to the radar beam. A comparison shows that EISCAT observes almost as high hourly rates of meteors as meteor radars do, in spite of its beamwidth being only 0.5° compared to their 100°. Two different kinds of echoes are seen in the data. About three fourths of them are strongly Doppler-shifted transient echoes, which we interpret as head echoes, since they move with meteor velocities. The remaining echoes are long-lived. They come from the ionized trails left behind the meteors, which drift slowly or not at all. There are fundamental differences between the scattering process producing both types of echoes and the scattering from trails observed with the meteor radars. For example, we have never seen the trail echoes appear after a head echo. All our shower echoes are also strongly nonspecular. We postulate that some time is required for the trails to expand and for the ionization within them to approach the thermal equilibrium state in order to give detectable incoherent echoes. Due to the very small beamwidth our radar cannot follow the motion of an individual meteor, as the conventional radars can, but we can obtain a statistical profile of what happens at different heights as the meteoroids penetrate the atmosphere. The altitude distribution of ionized trails shows that they are observable over the whole measurement range from about 65 to 165 km, while the head echoes can be detected only within a narrow altitude range. The latter effect is probably related to the radio meteor ceiling effect.