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

Abstract

[1] 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.

Modeling the Solar Wind at the Ulysses, Voyager, and New Horizons Spacecraft

“…A similar pattern is observed around 1997.0 and 2006.0 during which Ulysses traverses mid-latitude regions between PCHs and the equa-torial band characterized by the interaction of slow wind and PCH fast wind streams [e.g., Gosling (1996)]. Occasional coronal mass ejection (CME) and comet tail passages also contributed to the discrepancy between the model and spacecraft data [see Ebert et al (2009), Elliott et al (2012) and references therein]. The model |B| at latitudes above ±60°-70°is also systematically lower than Ulysses data by up to 30 % suggesting that we may have underestimated magnetic field values near the poles at the inner boundary, but an accurate polar magnetic field estimation is beyond the scope of our study.…”

“…Furthermore, PCHs sometimes have large extensions to the equatorial region as they did in 2003 [see Elliott et al (2012)]. At the inner boundary, the solar wind velocity changes from 800 km s −1 at the center (poles) to ∼700 km s −1 at the edges of PCHs.…”

THE NEW HORIZONS SOLAR WIND AROUND PLUTO (SWAP) OBSERVATIONS OF THE SOLAR WIND FROM 11–33 au

“…This anti-correlation is opposite that of the normal positive correlation present in the inner heliosphere (Neugebauer & Snyder 1966;Elliott et al 2012). In Figure 22 we show SWAP speeds and temperatures from mid-2012 through late 2013 as recorded and running averages with color-coding.…”

“…In the quiet interval even though the solar wind speed is steady, enhancements in the distribution width are coincident with enhancements in temperature (e.g., the enhancement centered on 2013, March 2 (061)). In the inner heliosphere the solar wind temperature and speed are strongly positively correlated (Neugebauer & Snyder 1966;Elliott et al 2012). The solar wind speed being more steady and the temperature being elevated may reflect that dynamic interaction between fast wind parcels running into slower wind parcels emitted earlier in time in the inner heliosphere causes the fast wind to slow and the slow wind to speed up while simultaneously heating the wind.…”

Turbulence in the Solar Wind Measured With Comet Tail Test Particles

“…Many other single-point features have also been well-described, yielding a plausible picture of an active, evolving medium (Matthaeus & Velli 2011) in which turbulence plays a role in modifying the solar wind and scattering cosmic rays (Jokipii 1966;Shalchi 2009). Turbulence therefore may make crucial contributions to establishing the global structure of the heliosphere (Breech et al 2008;Elliott et al 2012). But this work relies heavily on interpretation of single-point measurements from in-situ probes.…”