Radars probe the plasma irregularities generated by ablating interplanetary dust grains in the upper atmosphere generally heights 80-120 km. From observational programmes we ultimately want to know about the physical and dynamical characteristics of the dust. There are certain properties of interplanetary dust for which radars are an especially valuable probing agent. Radar surveys, sampling individual meteoroids, can provide us with information about the space environment determining especially:
• the influx rate for a given mass hence spatial density and mass distribution;
• for discrete streams the time variations of rates and mass distributions with any associated fine structure are valuable signatures of processes like comet ejection mechanisms and dynamical history of streams where sampling in longitude is valuable;
• determining the velocity vector of a meteor's atmospheric trajectory provides the heliocentric orbit;
• measurements of meteoroid atmospheric decelerations or recording of body fragmentation are valuable in providing evidence of their physical characteristics and cohesive structure.
As in other dust observational techniques there are important biases that must be taken into account: for example to derive the dust heliocentric orbital distribution severe correction factors must be recognised: the impact probability with the Earth; Earth focusing; atmospheric effects and the radar detection function. The size of Earth-impacting dust that can be sampled by radar systems depends on radar transmitted powers available, operating frequency and antenna system used but has a lower useful size limit of some tens of fim (set by the radar transmitter power available and antenna gain) while the ultimate lower limit is set by the fact that very small grains (< 10 pm) suffer incomplete ablation. The upper size is set by the area of the atmosphere (acting as a detector) illuminated, and statistical sampling: for a single radar the meteoroid population of sizes > cms is sparsely sampled.
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