Observations of Detailed Structure in the Solar Wind at 1 AU with STEREO/HI-2

TL;DR

This paper presents a new data processing method for STEREO/HI-2 images, enabling detailed visualization of solar wind and CME structures at 1 AU, revealing features previously obscured by background noise.

Contribution

Developed a background subtraction technique for HI-2 data, allowing high-sensitivity imaging of solar wind structures at 1 AU with detailed spatial resolution.

Findings

Revealed detailed CME features such as leading-edge pileup and interior voids.

Identified quiet solar wind structures like V-shaped current sheet features and plasmoids.

Demonstrated the ability to analyze heliospheric structures at 1 AU using visible light imagery.

Abstract

Heliospheric imagers offer the promise of remote sensing of large-scale structures present in the solar wind. The STEREO/HI-2 imagers, in particular, offer high resolution, very low noise observations of the inner heliosphere but have not yet been exploited to their full potential. This is in part because the signal of interest, Thomson scattered sunlight from free electrons, is ~1000 times fainter than the background visual field in the images, making background subtraction challenging. We have developed a procedure for separating the Thomson-scattered signal from the other background/foreground sources in the HI-2 data. Using only the Level 1 data from STEREO/HI-2, we are able to generate calibrated imaging data of the solar wind with sensitivity of a few times 1e-17 Bsun, compared to the background signal of a few times 1e-13 Bsun. These images reveal detailed spatial structure in CMEs and the solar wind at projected solar distances in excess of 1 AU, at the instrumental motion-blur resolution limit of 1-3 degree. CME features visible in the newly reprocessed data from December 2008 include leading-edge pileup, interior voids, filamentary structure, and rear cusps. "Quiet" solar wind features include V shaped structure centered on the heliospheric current sheet, plasmoids, and "puffs" that correspond to the density fluctuations observed in-situ. We compare many of these structures with in-situ features detected near 1 AU. The reprocessed data demonstrate that it is possible to perform detailed structural analyses of heliospheric features with visible light imagery, at distances from the Sun of at least 1 AU.