The Highly Structured Outer Solar Corona

Abstract

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. We use these structures to track overall flow and acceleration, finding that it is highly inhomogeneous and accelerates gradually out to the limit of the COR2 field of view. Lagged autocorrelation of the corona has an enigmatic dip around 10 R ⊙, perhaps pointing to new phenomena near this altitude. These results point toward a highly complex outer corona with far more structure and local dynamics than has been apparent. We discuss the impact of these results on solar and solar-wind physics and what future studies and measurements are necessary to build upon them.