On the Origin of Highly Alfvénic Slow Solar Wind

Journal
Date Issued
2015
Author(s)
•
DOI
Description
The authors are grateful to the following people and organizations for the provision of data used in this study: N. Papatashvilli and NASA/GSFC for the use of OMNI data (both plasma and magnetic field); H. R. Rosenbauer and Max-Planck-Institut fur Aeronomie for the use of data from the Helios /E1 plasma experiment, and N. F. Ness and the Bartol Research Institute for the use of the magnetometers aboard the Helios 2 spacecraft. Solar magnetic field maps at the source surface are available from http://wso.stanford.edu/ . The present work has been supported by the Italian Space Agency (ASI) under contract I/013/12/0 ASI/INAF.
Abstract
Alfvénic fluctuations are a common feature in the solar wind and are found especially in the trailing edges of fast wind streams. The slow wind usually has a lower degree of Alfvénicity, being more strongly intermixed with structures of non-Alfvénic nature. In the present paper we show the first evidence in the interplanetary space of two different kinds of slow solar wind: one coming from coronal streamers or active regions and characterized by non-Alfvénic structures and the other one being highly Alfvénic and originating from the boundary of coronal holes. The Alfvénic character of fluctuations, either outward or inward, can be studied by means of the normalized cross-helicity, {{σ }C}, which is an indicator of the {\boldsymbol{v}} -{\boldsymbol{b}} alignment. The evolution of {{σ }C} toward lower values with increasing radial distance is interpreted both as a decrease of the presence of the outward modes and as a continuous production of inward modes within those regions such as stream shears where some plasma instability is active. On the other hand, the decrease of {{σ }C} is often related also to magnetic field and/or density enhancements which specifically act on the destruction of the {\boldsymbol{v}} -{\boldsymbol{b}} alignment. In the present analysis we study the role of compressibility presenting both case studies and a statistical analysis over different phases of solar cycle 23. Our findings indicate that the presence of regions of magnetic field compression generally play a major role in the depletion of {{σ }C} and thus in the destruction of the {\boldsymbol{v}} -{\boldsymbol{b}} alignment.
Volume
805
Issue
1
Start page
84
Uri
Url
Issn Identifier
0004-637X
Ads BibCode
2015ApJ...805...84D
Rights
open.access
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