Power Fluctuations in Meteor Head Echoes Observed with the EISCAT VHF Radar

Meteor head echoes are radio wave reflections from the plasma generated by the interaction of meteoroids with the atmosphere at about 80-120 km altitude. The echoes are characterized by being transient and Doppler shifted. The received power is confined in range, as from a point source, and it moves with the line-of-sight velocity of the meteoroid. Head echoes arise and disappear as meteoroids pass through the radar beam, and usually last from one tenth to a few tenths of a second. Some meteor echoes in the data from a meteor experiment carried out with the EISCAT VHF radar at 224 MHz contain peculiar pulsations in received power. These pulsations are in the frequency range 50-100 Hz. The interpulse period of the experiment was 2.167 ms, i.e., meteoroid line-of-sight velocity and echo power was monitored at 461 Hz. This limits the observable power fluctuation frequency range to about 10-230 Hz. The process causing the echo power pulsations is at present not identified. Plasma effects are the most likely cause, a presumable mechanism is for instance asymmetrical dust grains in rotation causing a modulation of the ionization rate.