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[9,10]. These phenomena were thought to be due to the impact of a swarm of very small particles into the ionosphere. We shall analyse the small particle peak of Leonid shower of 1998 in the following section.

2. OBSERVATIONS

An overview of the optical observations of the 1998 Leonid activity was given by Arlt [11]. An unexpected component rich in bright meteoroids appeared about 16 hours before the predicted maximum of the main shower. The actual "storm" component was observed as expected with a peak at November 17, 20:30UT.

We had a radio observation at Electric Wave Propagation Institute of China in Xinxiang, Henan. The result was also consistent with the prediction, and the peak was over 2500 h'\

Several ionosphere observational stations in China surveyed the ionization effect of the meteor shower during Nov. 14-20. The ionosphere characteristic value fbEs was selected as the observational parameter. About 18 hours after the main shower, an abnormal peak of the fbEs value was detected simultaneously by two ionosphere observational stations, Guangzhou and Hainan. The very high fbEs value was maintained for over one hour. The abnormal phenomenon showed that the ionosphere was bombarded by a swarm dust that could not be observed by optical and radio.

We can understand this phenomenon as follows. According to Hughes' [12] suggestion that typical stream meteoroids have a mass distribution index s=2.21 and the number of meteoroids N, with masses greater than m is proportional to m(l s\ we have the number of meteoroids N, with radii greater than b proportional to b3(Us\ So N ~ m'L27~ b~3&\ If a region of the stream contains 1 meteoroid with radius greater than 1 centimeter, it should contain 103 81 meteoroids with radii greater than 0.1 centimeter, 107 62 meteoroids with radii greater than 0.01 centimeter and so on. The large population of tiny grains bombarding the ionosphere can produce enough positive ions to create abnormal ionization effects in the ionosphere [9,10], There was no abnormal ionization effect during the main shower because the number of the grains was insufficient.

3. THE EFFECTS OF EJECTION VELOCITY AND RADIATION PRESSURE

The ejection velocity of the meteoroid from the comet and the solar radiation pressure upon it are obviously dependent on the mass of the meteoroids, which can cause the meteoroids of different sizes to be separated.

The dust swarm 18 hours after the main shower illustrates that the longitude of the ascending node of the tiny grains in the Leonid stream has a 0.74° difference from that of the visible ones. If they suffer the same planetary perturbation, the node can be changed only by ejection. From the fundamentals of the celestial mechanics, we can get [13]

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