INBOUND WAVES IN THE SOLAR CORONA: A DIRECT INDICATOR OF ALFV\ EN SURFACE LOCATION

Author
Keywords
Abstract
<p>The tenuous supersonic solar wind that streams from the top of the corona passes through a natural boundary\textemdashthe Alfv\ en surface\textemdashthat marks the causal disconnection of individual packets of plasma and magnetic flux from the Sun itself. The Alfv\ en surface is the locus where the radial motion of the accelerating solar wind passes the radial Alfv\ en speed, and therefore any displacement of material cannot carry information back down into the corona. It is thus the natural outer boundary of the solar corona and the inner boundary of interplanetary space. Using a new and unique motion analysis to separate inbound and outbound motions in synoptic visible-light image sequences from the COR2 coronagraph on board the STEREO-A spacecraft, we have identified inbound wave motion in the outer corona beyond 6 solar radii for the first time and used it to determine that the Alfv\ en surface is at least 12 solar radii from the Sun over the polar coronal holes and 15 solar radii in the streamer belt, well beyond the distance planned for NASA\textquoterights upcoming Solar Probe Plus mission. To our knowledge, this is the first measurement of inbound waves in the outer solar corona and the first direct measurement of lower bounds for the Alfv\ en surface.</p>
Year of Publication
2014
Journal
The Astrophysical Journal
Volume
787
Number
Number of Pages
124
Date Published
06/2014
ISSN Number
0004-637X
URL
http://stacks.iop.org/0004-637X/787/i=2/a=124?key=crossref.8ca79a982204ddd2b4922cc108364616
DOI
10.1088/0004-637X/787/2/124