The mass distribution of comets and meteoroid streams and the shower/sporadic ratio in the incident visual meteoroid flux

Summary The large dust particles that comets emit as they decay produce meteoroid streams. If the Earth passes close to the centre of a meteoroid stream, a shower of meteors is produced in the atmosphere and the intensity of this shower can be quantified by the maximum zenithal hour rate (ZHR) of the meteors that are observed. If, as seems reasonable, the dust/snow mass ratio and the mass distribution of the dust particles are similar in all comets, then comets and meteoroid streams are expected to have similar mass distribution indices. This expectation has been confirmed for the more massive comets and streams by comparing the distribution of shower ZHRs with the distribution of cometary masses. The later distribution is also used to predict the expected numbers of minor meteor showers, with low ZHRs, that are present in what was previously referred to as the sporadic background. It is concluded that on a typical ‘non-shower’ night, (20 ±3) per cent of the observed visual meteors belong to minor showers, and the remaining percentage are truly sporadic. Statistical variations in the numbers of massive short-period comets and thus massive meteoroid streams will lead to significant variations in the decay products of these objects. It is thus expected that this effect will cause the sporadic meteor flux and the brightness of the zodiacal cloud to vary by around 30-40 per cent over time periods of the order of 103-105 yr.