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tokens: we only observe in the vertical direction, meteor ablation height generally correlates very well with the vector velocity of a meteor. The average ablation height for the three groups of meteors is 105, 103, and 99 km, respectively. Meteors observed on the back-side of the Earth are largely prograde meteors with a heliocentric velocity larger than 30 km s −1 . Their detection rate is very low because of small atmospheric entry velocity. 6 DISCUSSION AND CONCLUSION The most important aspect of the current report is the conclusion that the majority of the meteors observed by the Arecibo radar are in nearly circular orbits. This points to the importance of Poynting–Robertson and solar wind drags. The nature of the two drags is similar, with the latter being about 30 per cent of the former (Gustafson 1994 ). In order for the drag forces to be effective, meteoroid mass is typically smaller than 10 −11 kg. On the other hand, when β, the ratio of radiation pressure to solar gravitation force, is larger than 0.5, a particle will be blown out of the solar system. β = 0.5 corresponds to a particle size of 1 μm (Dermott et al. 2001 ) and a mass of 10 −14 kg, which is the lower boundary of the meteors observed at Arecibo as discussed above. Thus, Arecibo meteors appear to have the ‘Goldilocks’ size, small enough for the drags to force them into nearly circular orbits and being pulled inward to the Sun but large enough not to be blown away by the radiation pressure. The simulations by Borin et al ( 2017 ) show that it takes several million years to reduce the semimajor axis from 3.4 to 1 au for a 10 μm in radius particle. The evolution time for the smaller Arecibo particles is expected to be much shorter. As the Earth lies relatively close to the Sun, the majority of the meteoroids have their initial orbits outside that of the Earth whether they are of asteroidal or cometary origin. For meteoroids whose size is right for drag forces to be important, their orbits will evolve into nearly circular ones at 1 au if not rejected by gravitational scattering by outer planets. Meteoroids with a diameter of 1–9 μm originating from as far as the Kuiper belt have a 20 per cent chance of reaching 1 au and once a meteoroid's orbit reaches within that of Jupiter, it will largely spiral onto the Sun (Liou, Zook Dermott 1996 ). As all meteoroids in the mass range of 10 −14 –10 −11 kg generated outside Earth's orbit have a good chance to be on nearly circular orbits when they reach the inner solar system, their population is thus expected to be uncharacteristically large at 1 au. Further, meteoroids in the ecliptic plane have a higher probability ejected by outer planets than those on inclined planes. Although the distribution shown in Fig. 4 is a convolution of instrument aspect sensitivity and the true distribution, it indicates at least that a large number of meteoroids exist in inclined orbits. The ‘inclined’ population discussed by Divine ( 1993 ) has low