Interstellar dust flux measurements by the Galileo dust instrument between the orbits of Venus and Mars

We present an analysis of the data obtained by the Galileo in situ dust instrument for interstellar dust (ISD). Between December 1989 and the end of 1993, three orbit segments were favorable for the detection of ISD at less than 3 AU heliocentric distance. After removing background events from the data sample, which were mostly due to interplanetary dust impactors, we infer that the flux of ISD grains between 0.7 AU and 3 AU is about 4 × 10^-5 m^-2 s^-1. This result is compatible with the ISD flux of 3 × 10^-5 m^-2 s^-1 (grain size ≈ 0.4 μm) derived from the Cassini measurements at about 1 AU. Using a new concept called "ISD β spectroscopy," we are able to estimate at different locations in the inner solar system the ISD flux alteration resulting from the competing effects of radiation pressure and gravitation. In particular, we confirm results of previous Ulysses dust data analysis showing that radiation pressure prevents smaller ISD grains (radius smaller than ≈ 0.3 μm) from reaching the innermost region of the solar system. Furthermore, our analysis shows the relevance of gravitational focusing in the dynamics of bigger ISD grains (micron-sized grains). The Galileo measurements were performed 10 years before the Cassini measurements. Thus the available ISD data now cover almost a full 11-year solar cycle. Nonetheless, the flux of ISD grains with radius bigger than 0.4 μm shows no significant temporal variation. This suggests that the dynamics of these ISD grains is not influenced much by the interaction with the time-dependent solar magnetic field.