A long-term high-resolution study of the visual activity curve of the Perseid meteor stream 1953-1983

Perseid and sporadic meteor hourly rates and magnitudes observed in 1953-1983 by a team of visual observers at the Onsala Space Observatory, Sweden, are analysed. A high-resolution study of the zenithal hourly rates of bright Perseid meteors /(m<=2.5) versus solar longitude has been made using a step length of /0.05° in solar longitude. The present study, which is based on 147 Perseid hourly rates, observed between solar longitudes /138.70°-/141.65° (equinox 2000) covers the period of maximum Perseid activity. Somewhat surprisingly it reveals a multi-peak structure of the Perseid maximum with at least four separate peaks in the activity curve of bright Perseids. The first peak located at /139.38° corresponds to the crossing of the nodal plane of the parent comet, the second at /139.68° is the so-called ``new'' Perseid maximum, the third peak at /140.20° is the ``old'' or ``traditional'' maximum of the shower, while the fourth peak although less intense than the previous three peaks indicates a well-defined activity maximum centred on /140.78°. It is interesting to note that, for bright Perseids, the nodal maximum is of the same intensity as the ``new'' and ``old'' maxima, but is of a somewhat shorter duration than these two maxima. A similar study of the activity curve of all observed Perseid meteors (i.e. independent of apparent magnitude) shows the same multi-peak structure, but with slightly less pronounced peaks. In a previous study of the Perseid activity curve based on 605 photographic Perseid orbits obtained in various two-station programs 1937-1985 the multi-peak structure can be recognised in a number versus solar longitude diagram (Lindblad and Porubcan, /1994. Planet Space Sci. 42, 117-122.). The various peaks in the photographic data are located at the same solar longitudes as in the visual data. This agreement between the results of the present long-term visual study and a long-term photographic study of the Perseid activity curve strongly supports our conclusions as to the multi-peak structure of the Perseid shower