Abstract
The paper derives the polytropic indices over the last two solar cycles (years 1995–2017) for the solar wind proton plasma near Earth (~1 AU). We use ~92-s datasets of proton plasma moments (speed, density, and temperature), measured from the Solar Wind Experiment instrument onboard Wind spacecraft, to estimate the moving averages of the polytropic index, as well as their weighted means and standard errors as a function of the solar wind speed and the year of measurements. The derived long-term behavior of the polytropic index agrees with the results of other previous methods. In particular, we find that the polytropic index remains quasi-constant with respect to the plasma flow speed, in agreement with earlier analyses of solar wind plasma. It is shown that most of the fluctuations of the polytropic index appear in the fast solar wind. The polytropic index remains quasi-constant, despite the frequent entropic variations. Therefore, on an annual basis, the polytropic index of the solar wind proton plasma near ~1 AU can be considered independent of the plasma flow speed. The estimated all-year weighted mean and its standard error is γ = 1.86 ± 0.09.
Highlights
A polytrope is a thermodynamic process that follows a specific relationship among the thermodynamic variables included in the equation of state, such as, density n, temperature T, and thermal pressure P
We study the variation of the polytropic index over the last two solar cycles for the solar wind proton plasma near Earth (~1 AU)
For the year 1995, we find γ ≈ 1.69 ± 0.03 (Table 1), while for the first 70 days of that year the polytropic index is γ ≈ 1.63 ± 0.05 [22]. (There is a chance that the polytropic index decreases with the heliocentric distance rs, since the Helios-1 datasets span rs from ~0.3AU to ~1 AU)
Summary
A polytrope is a thermodynamic process that follows a specific relationship among the thermodynamic variables included in the equation of state, such as, density n, temperature T, and thermal pressure P. The solar wind protons near 1 AU exhibits polytropes with sub-adiabatic indices (1 < γ < 5/3), e.g., Totten et al [20] using Helios-1 data. The estimated mean polytropic index is near zero, indicating dominance of isobaric thermodynamic processes in the sheath, similar to other previously published analyses [13].
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