The Astrophysical Journal, 862:18 (18pp), 2018 July 20 https://doi.org/10.3847/1538-4357/aac8e3 © 2018. The American Astronomical Society. The Highly Structured Outer Solar Corona C. E. DeForest1 , R. A. Howard2 , M. Velli3 , N. Viall4 , and A. Vourlidas5,6 1 Southwest Research Institute, 1050 Walnut Street, Suite 300, Boulder, CO 80302, USA;
[email protected] 2 Naval Research Laboratory, Washington, DC, USA 3 University of California, Los Angeles, CA, USA 4 NASA/Goddard Space Flight Center, Greenbelt, MD, USA 5 Johns Hopkins University Applied Physics Laboratory, Laurel, MD, USA Received 2018 March 6; revised 2018 April 28; accepted 2018 May 22; published 2018 July 18 Abstract We report on the observation of fine-scale structure in the outer corona at solar maximum, using deep-exposure campaign data from the Solar Terrestrial Relations Observatory-A (STEREO-A)/COR2 coronagraph coupled with postprocessing to further reduce noise and thereby improve effective spatial resolution. The processed images reveal radial structure with high density contrast at all observable scales down to the optical limit of the instrument, giving the corona a “woodgrain” appearance. Inferred density varies by an order of magnitude on spatial scales of 50 Mm and follows an f −1 spatial spectrum. The variations belie the notion of a smooth outer corona. They are inconsistent with a well-defined “Alfvén surface,” indicating instead a more nuanced “Alfvén zone”—a broad trans-Alfvénic region rather than a simple boundary. Intermittent compact structures are also present at all observable scales, forming a size spectrum with the familiar “Sheeley blobs” at the large-scale end.