The Highly Structured Outer Solar Corona

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 e , 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.

Radio Occultation Observations of the Solar Corona Over 1.60–1.86 R⊙: Faraday Rotation and Frequency Shift Analysis

“…A steep rise in electron concentrations below = SO 1.7 R ⊙ was found. We speculate that this density pattern may be related to steep gradients near a streamer boundary (Guhathakurta & Fisher, ) but also note the order‐of‐magnitude density changes recently reported in within white light streamer structures (DeForest et al, ). Further studies using the frequency shift analysis in the middle corona should contribute new insights about variations in plasma density structure.…”

Coronal Density and Temperature Profiles Calculated by Forward Modeling EUV Emission Observed by SDO/AIA

“…Fisher & Guhathakurta 1995;Esser et al 1999), multifrequency radio emission (e.g Mercier & Chambe 2015), or more recently the using the Interface Region Imaging Spectrograph (IRIS; Kayshap et al 2018). DeForest et al (2018) recently investigated on the existence of fine structure in the corona extending to several times the solar radius and so there is potential to apply models to become significantly more detailed than the demonstrations in this paper. The simultaneous use of other observational data to reconstruct the coronal density and temperature profiles would also be useful since our method using SDO/AIA EUV emission is most sensitive to low heights r 1.2R ⊙ .…”

The Source, Significance, and Magnetospheric Impact of Periodic Density Structures Within Stream Interaction Regions

“…It is important to note that these periodic density structures in the solar wind are not propagating waves. Rather, they are coherent mesoscale structures created very early in the formation of the solar wind, which then advect outward in quasi pressure balance (DeForest et al, 2018;Di Matteo et al, 2019;Kepko et al, 2016;Viall & Vourlidas, 2015;Viall, Spence, & Kasper, 2009;Viall, Spence, Vourlidas, & Howard, 2010).…”