Spatial and time variations of the interplanetary microparticle flux analysed from deep space probes pioneers 8 and 9

The first results of a comprehensive computer analysis of over 300 front film and grid coincidence events is presented using statistical tests on the observed data. The short term time dependence of the observed flux is entirely commensurate with a random Poisson distribution and any possible contributions from discrete “cometary showers” must certainly be of relatively minor significance compared to the sporadic background for mass > 10^?13 g. Periodic seasonal variations of ～ 20 per cent of the average rate are observed common to Pioneers 8 and 9. These variations could reflect on the cometary nature of the source or alternatively indicate the presence of an interstellar component. The mass spectrum of the flux in the range 10^?11?10^?13 g indicates an increasing flux of particles to the lowest limits of mass detected, with a derived flux of Φ = 1·4 × 10^?12m^?0·68 (g) m^?2 sec^?1(2π ster.)^?1.