ASHI Instrument Image Analysis using Ground-Based, Extant NASA Spacecraft, and Balloon-Flight Images

We propose to ready the ASHI (All-Sky Heliospheric Imager) balloon flight instrument’s imagery from a balloon flight, for scientific use and in preparation for a long duration spacecraft flight. The ASHI instrument (see Figure 1) was flown on a topside balloon flight on June 1, 2021. The ASHI system, largely unscathed through its balloon landing after cut-down from its 100,000 float altitude, has been refurbished, and is again invited to fly on a topside flight in the autumn of 2022. Here we propose to ready ground-based, extant NASA imagery, and the images from the refurbished balloon-flight ASHI, to provide a definitive basis for the science ASHI can do best. ASHI addresses heliospheric science from the general question: “What are the shapes and time histories of heliospheric structures, in two basic plasma parameters: density and velocity?” This has been studied to some extent in the same way from extant NASA spacecraft images using Solar Mass Ejection Imager (SMEI) and STEREO Heliospheric Imager data (for a recent review of SMEI analysis see Jackson et al., 2020, Frontiers, doi: 10.3389/fspas.2020.568429). However, we here concentrate on the ubiquity of the near-Earth structure observed in high-resolution 3-D reconstructions presented recently from SMEI and STEREO analysis, as heliospheric structures approach near and pass the spacecraft. From the Earth passage of the brightest CME structures in SMEI, we have found that CME density structures do not appear as a uniform front but have a corrugated or striated appearance. This differs from CME structures or their shocks, when presented by most 3-D MHD or kinematic model simulations, where the background solar wind and CME structure are both presumed uniform. If these corrugated shapes are ubiquitous to all types of solar structures, and are present in the less bright and less dense solar wind surrounding Earth, they have a profound implication for solar wind physics, and for that matter, Earth’s solar wind magnetospheric interactions. The ASHI ability to 3-D reconstruct faint solar wind structures passing Earth will uniquely explore this science. To enable the 3-D reconstruction of density starting from near the Sun, but especially events passing Earth, a key photometric specification for ASHI is 0.1% differential photometry in one-degree sky bins 90 degrees from the Sun. Although some aspects of the ASHI imagery concept have already been proven using extant SMEI and STEREO data, the ASHI instrument is a far more compact design than either of these previously flown instruments. We here extend this concept to include velocity; a heliospheric imager viewing Thomson-scattered light will yield high-resolution 3-D analyses of velocity as well as density that will be compared with in-situ measurements from spacecraft near Earth. This concept can be proven using the ASHI imagery from a topside balloon flight, but with less throughput than is ultimately envisioned for a spaceborne instrument. The ultimate ASHI to be proposed for a long-duration spaceflight promises to provide an order of magnitude improved light throughput over time than SMEI or STEREO, and can thus yield far better near-Earth 3-D reconstructions as well as velocity.

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