Abstract
The phenomenon of MHD wave refraction is useful in interpreting the properties of the magnetic fluctuations in certain parcels of solar wind. In the physics of MHD wave refraction, variations in the Alfvén speed V Alf play a dominant role. Here, we compile statistics of the 1-min averages of V Alf at the location of the ACE spacecraft during its first 5 years of operation. We find that monthly distributions of V Alf are close to log-normal, with standard deviations σ V as small as 0.11 in the logarithm. Variations in the monthly mean V Alf are correlated significantly with sunspot number. We also compile monthly distributions of the plasma β parameter. The distributions of both V Alf and β are significantly narrower than they would be if the various solar wind parameters were statistically independent. In the T p–V Alf plane, we find a zone of avoidance at low V Alf: for V Alf ≤10 – 15 km/s, there are no samples in the 1-min data that are cooler than T p = 10 000 – 15 000 K. This feature can be understood in the context of MHD wave refraction, although other explanations are also possible.
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Mullan, D.J., Smith, C.W. Solar Wind Statistics at 1 AU: Alfven Speed and Plasma Beta. Sol Phys 234, 325–338 (2006). https://doi.org/10.1007/s11207-006-2077-y
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DOI: https://doi.org/10.1007/s11207-006-2077-y