National Aeronautics and Space Administration

Marshall Space Flight Center



Scientific Objective

One of the key factors in improving our understanding of the solar corona has been the continued development of high-resolution imaging, particularly in the X-ray and extreme ultraviolet (EUV) spectral regime. Instruments on rocket flights in the 1960s directly imaged the on-disk X-ray corona for the first time, leading to a clearer understanding of the close connection between surface magnetic fields and X-ray emission (Vaiana et al., 1973). The improved spatial resolution of the Skylab instruments (Zombeck et al., 1978) led to the view that the corona is composed of loop-like structures representing isolated mini-atmospheres (Rosner et al., 1978) with thermodynamic-type scaling laws describing their physical properties. Additional improvements in both spatial and temporal resolution from the Transition Region and Coronal Explorer (TRACE) revealed a highly dynamic corona (Schrijver et al., 1999), and led to the discovery of various wave modes propagating throughout the solar corona (e.g., De Moortel, 2005).

The High Resolution Coronal Imager (Hi-C) instrument's first launch occurred on July 11, 2012 from White Sands Missile Range (WSMR). The instrument consisted of a single narrowband EUV channel centered on 193 Å. The experiment design meets the requirements for an EUV telescope with 0.25" resolution (~0.1"/pixel); hence the images collected during this first launch are the highest resolution images of the ~1-2 million-degree solar corona to date. The second successful flight occurred on May 29, 2018, again from WSMR. The mirror was recoated for reflectivity centered on 172 Å, which is sensitive to slightly cooler temperatures (~0.6 million degrees). Observations from these flights provide the highest resolution images of the corona to date and better constrain the spatial scale for structures in the solar corona and offer a brief look at the evolution of structures at this scale size.

Our basic understanding of magnetic reconnection has recently advanced dramatically: it is now recognized that fast Petschek reconnection is inevitable provided significant non-ideal effects are restricted to a small region of space, possibly even within a Sweet-Parker current sheet. Without this localization fast reconnection cannot occur (Erkaev, Semenov & Jamitsky 2000, Kulsrud 2001, Biskamp & Schwarz 2001). This theoretical insight may help explain why the corona is composed of distinct loops but it also raises a host of new questions. Where in the corona does the localized reconnection occur? At what scale does it occur and by what mechanism is it triggered and localized? To capitalize on our newly achieved theoretical understanding of reconnection and to answer these questions, higher spatial resolution coronal imagery is required.

Our understanding of coronal dynamics will be further advanced by understanding how localized reconnection fits into the three-dimensional topology of the field. For this to be accomplished, we must compare coronal structures to the relevant calculated topological features which are predicted to play a role in coronal activity. We must then be certain that we are observing the elementary structures in the corona which participate in the interactions between the plasma and the magnetic field. Analysis of the TRACE data shows that the scale size of coronal structures extends down to the resolution limit of the instrument (Golub et al 1998), and there is substantial evidence that additional fine structure is present. This instrument program, therefore, takes the view that the first step in understanding the corona is to see what is there. We have designed the Hi-C instrument accordingly. Our flight data demonstrate that at 150km resolution we are at or near the scale size of coronal structures, and our 5-minute data set provides tantalizing clues for the outstanding science capabilities of this next generation in astrophysical instrumentation.