The Leonid meteor shower is a well-known periodic meteor shower. The Leonid parent comet, 55P/Tempel-Tuttle, has an orbital period of about 33.2 yr, and storms usually occur in years around the perihelion passage of the parent comet. Since comet 55P/Tempel-Tuttle passed perihelion on February 28, 1998, the Leonid meteor shower of 1998 was predicted [1,

2, 3, 4, 5] to produce a strong shower. Such strong displays were seen and details can be found in many published articles. Showers have not always appeared when expected although the general behaviour of meteor streams has been reasonably well modelled. In 1998 a strong component, rich in bright meteors, appeared about 16 hours before the expected maximum of the main shower. An explanation for this has been given by Asher et al. [6]. An unexpected new peak in the Perseids occurred in the early nineties slightly separated in time from the traditional main peak [7], and a similar case occurred in the Quadrantids [8]. We found another unusual phenomenon, which was an abnormal level of ionization detected in the ionosphere about 18 hours after the main Leonid shower of 1998. The same phenomena occurred in the ionosphere around the strong showers of the Leonids, Perseids and Draconids

* Supported by National Natural Science Foundation of China (No. 19873020 \& No.49474242) and the Royal Society KC Wong fellowship.

direction of ejection and the orbital plane, so vsin <p is the component of the ejection velocity perpendicular to the orbital plane.

Suppose the ejection occurred at perihelion, Eq.(l) becomes

From Eq.(2), substituting the orbital parameters of 55P/Tempel-Tuttle, we can get A(vsin</>) = v^ sin</>j - v0 sin</>0 = 1.67 x 103 AQ (m s"1) (3)

vi and vo are ejection velocity of different sized particles, and AQ is in degrees. Following Whipple [14] and Wu and Williams [15], the ejection velocity can be given as v = —^jjr-U25 (4)

where C is a constant. If they were ejected simultaneously in the same direction, we have sin 0 _ _ jVo (5)

It is well known that the small meteoroids are principally affected by the solar gravity and an anti-gravity force arising from solar radiation pressure after they were ejected from their parent body, and they move basically along the original orbit. Different sized particles will have different orbital periods due to the different influence of solar radiation force on them. They will therefore separate gradually in the process of orbital movement. Since the force from solar radiation pressure is opposite to the direction of solar gravity, its effect is to 'weaken' the gravity so that the quantity GM is replaced by GM(\-J3), where fi=F/Fs (Fr: the force from solar radiation pressure and Fg: the solar gravity) has the value about fl-105/be [16], b being the radius and c the bulk density of the grain, both in cgs units. From the Kepler's third law, and remembering that the meteoroids move basically along the same orbit, we have the orbital period of the meteoroids p = -^-y (6)

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