Temporal and radial variation of the solar wind temperature‐speed relationship

Abstract

The solar wind temperature (T) and speed (V) are generally well correlated at ∼1 AU, except in Interplanetary Coronal Mass Ejections where this correlation breaks down. We perform a comprehensive analysis of both the temporal and radial variation in the temperature‐speed (T‐V) relationship of the non‐transient wind, and our analysis provides insight into both the causes of the T‐V relationship and the sources of the temperature variability. Often at 1 AU the speed‐temperature relationship is well represented by a single linear fit over a speed range spanning both the slow and fast wind. However, at times the fast wind from coronal holes can have a different T‐V relationship than the slow wind. A good example of this was in 2003 when there was a very large and long‐lived outward magnetic polarity coronal hole at low latitudes that emitted wind with speeds as fast as a polar coronal hole. The long‐lived nature of the hole made it possible to clearly distinguish that some holes can have a different T‐V relationship. In an earlier ACE study, we found that both the compressions and rarefactions T‐V curves are linear, but the compression curve is shifted to higher temperatures. By separating compressions and rarefactions prior to determining the radial profiles of the solar wind parameters, the importance of dynamic interactions on the radial evolution of the solar wind parameters is revealed. Although the T‐V relationship at 1 AU is often well described by a single linear curve, we find that the T‐V relationship continually evolves with distance. Beyond ∼2.5 AU the differences between the compressions and rarefactions are quite significant and affect the shape of the overall T‐V distribution to the point that a simple linear fit no longer describes the distribution well. Since additional heating of the ambient solar wind outside of interaction regions can be associated with Alfvénic fluctuations and the turbulent energy cascade, we also estimate the heating rate radial profile from the solar wind speed and temperature measurements.