Origins of the Ambient Solar Wind: Implication for Space Weather

Thursday, September 7, 2017

The Sun’s outer atmosphere is heated to temperatures of millions of degrees, and solar plasma flows out into interplanetary space at supersonic speeds. This paper reviews our current understanding of these interrelated problems: coronal heating and the acceleration of the ambient solar wind.

Carrington Rotation Stack Plots-Wind image
Carrington rotation stack plots showing (a) in-ecliptic OMNI wind speeds and (b) surface features from the McIntosh archive, both for the duration of solar cycle 23 (June 1996 to July 2009). In panel (a), white denotes u ≤ 450 km s−1 and increasingly darker shades of purple eventually saturate at the darkest color for u ≥ 750 km s−1. Longitudes have been offset by 50.55◦, or 3.83 days, to account for propagation from the Sun to 1 AU at a mean speed of 450 km s−1. Panel (b) shows equatorial (±20◦ from equator) features, with blue [red] showing coronal holes of positive [negative] polarity, cyan [gray] showing quiet regions with predominantly positive [negative] polarity, orange indicating sunspots, and green indicating filaments. The occurrences of these streams line up quite well with the presence of large equatorial coronal holes as recorded in the McIntosh synoptic image archive. The long-lived coronal holes (blue/red) seen in panel (b) are rotating at a rate somewhat faster than the 27.275 day Carrington rotation, and thus they have a positive slope in this plot. This correlates well with the slopes seen in the fast wind streams indicated in panel (a).

We also discuss where the community stands in its ability to forecast how variations in the solar wind (i.e., fast and slow wind streams) impact the Earth. Although the last few decades have seen significant progress in observations and modeling, we still do not have a complete understanding of the relevant physical processes, nor do we have a quantitatively precise census of which coronal structures contribute to specific types of solar wind. Fast streams are known to be connected to the central regions of large coronal holes. Slow streams, however, appear to come from a wide range of sources, including streamers, pseudostreamers, coronal loops, active regions, and coronal hole boundaries. Complicating our understanding even more is the fact that processes in the solar wind can make it difficult to unambiguously map a parcel measured at 1 AU back down to its coronal source. We also review recent progress—in theoretical modeling, observational data analysis, and forecasting techniques that sit at the interface between data and theory—that gives us hope that the above problems are indeed solvable.

Publication Name: Space Science Reviews