The distribution of interplanetary dust between 0.96 and 1.04 AU as inferred from impacts on the STEREO spacecraft observed by the Heliospheric Imagers

TL;DR

This study infers the distribution and characteristics of interplanetary dust between 0.96 and 1.04 AU using impacts observed on the STEREO spacecraft's Heliospheric Imagers, revealing asymmetries and size distributions of dust particles.

Contribution

It provides the first detailed analysis of dust impact distributions in this region using HI camera data, highlighting directional asymmetries and size distribution differences.

Findings

Impacts predominantly originate from the apex direction.

Particles from the apex have masses >10^-17 kg and radii >0.1 μm.

Size distribution varies between apex and anti-apex directions.

Abstract

The distribution of dust in the ecliptic plane between 0.96 and 1.04 AU has been inferred from impacts on the two STEREO spacecraft through observation of secondary particle trails and unexpected off-points in the Heliospheric Imager (HI) cameras. This study made use of analysis carried out by members of a distributed web-based project, Solar Stormwatch. A comparison between observations of the brightest particle trails and a survey of fainter trails shows consistent distributions. While there is no obvious correlation between this distribution and the occurrence of individual meteor streams at Earth, there are some broad longitudinal features in these distributions that are also observed in sources of the sporadic meteor population. The asymmetry in the number of trails seen by each spacecraft and the fact that there are many more unexpected off-points in the HI-B than in HI-A, indicates that the majority of impacts are coming from the apex direction. For impacts causing off-points in the HI-B camera these dust particles are estimated to have masses in excess of 10-17 kg with radii exceeding 0.1 {\mu}m. For off-points observed in the HI-A images, which can only have been caused by particles travelling from the anti-apex direction, the distribution is consistent with that of secondary 'storm' trails observed by HI-B, providing evidence that these trails also result from impacts with primary particles from an anti-apex source. It is apparent that the differential mass index of particles from the apex direction is consistently above 2. This indicates that the majority of the mass is within the smaller particles of this population. In contrast, the differential mass index of particles from the anti-apex direction (causing off-points in HI-A) is consistently below 2, indicating that the majority of the mass is to be found in larger particles of this distribution.