A new look into the Helios dust experiment data: presence of interstellar dust inside the Earth's orbit

An analysis of the Helios in situ dust data for interstellar dust (ISD) is presented in this work. Recent in situ dust measurements with impact ionization detectors on-board various spacecraft (Ulysses, Galileo, and Cassini) showed the deep penetration of an ISD stream into the Solar System. The Helios dust data provide a unique opportunity to monitor and study the ISD stream alteration at very close heliocentric distances. This work completes therefore the comprehensive picture of the ISD stream properties within the heliosphere. In particular, we show that gravitation focusing facilitates the detection of big ISD grains (micrometer-size), while radiation pressure prevents smaller grains from penetrating into the innermost regions of the Solar System. A flux value of about $2.6 \pm 0.3\ensuremath{\times 10^{-6}}$ m-2 s-1 is derived for micrometer-size grains. A mean radiation pressure-to-gravitation ratio (so-called β ratio) value of 0.4 is derived for the grains, assuming spheres of astronomical silicates to modelize the grains surface optical properties. From the ISD flux measured on the Helios trajectory, we infer a lower limit of $3\, \pm\,3\,\ensuremath{\times 10^{-25}}$ kg m-3 to the spatial mass density of micron-sized grains in the Local Interstellar Cloud (LIC). In addition, compositional clues for ISD grains are obtained from the data provided by the time-of-flight mass spectrometer subsystem of the Helios instrument. No clustering of single minerals is observed but rather a varying mixture of various minerals and carbonaceous compounds.