Dust particles beyond the asteroid belt—a study based on recent results of the Ulysses dust experiment

Dust fluxes detected by the Ulysses dust experiment beyond the asteroid belt, their possible sources and the possibilities of comparing them to observations of dust in the Zodiacal light are discussed. The dust fluxes detected at solar distances about r < 2.5 AU are compatible with a population of interplanetary dust particles moving on low eccentricity and low inclination orbits. However, different orbital parameters are required in order to explain the data obtained beyond this region. The analysis of impact directions and related inclinations of orbits points to the existence of an interstellar dust stream (see Grün et al., Nature 362, 428–430, 1993) that makes up about 50% of the particles detected in the outer solar system. Further components of the dust flux seem to be randomly oriented and on high eccentricity or unbound orbits. The activity of long period comets yields a possible source: from the analysis of radiation pressure particles of submicron size are expected to be in unbound orbits, whereas larger particles stay coupled to the solar system. Comparison to the out-of-ecliptic distribution of dust derived from Zodiacal cloud models, shows that this outer dust cloud has only a small influence on the Zodiacal light brightness. However, it is shown, that detailed observations of the spectral variation in the infrared brightness could help for a better understanding. Also the compatibility of submicron dust, predominant in flux data, and larger particles, predominant in brightness data, is shown to be not as clear in the outer dust cloud as in the inner solar system dust cloud.