by Georgios Nicolaou (MSSL, UCL)
The polytropic process determines a relationship between the plasma density and temperature, during the transition of the plasma from one equilibrium state to another under constant specific heat. This process is described by the effective polytropic index, which can be determined by the analysis of plasma density and temperature measurements, and is a crucial parameter in determining the dynamics of the plasma.
Over the last few decades numerous studies have shown that the velocities of the plasma particles often follow kappa distribution functions. The kappa index that labels and governs these distributions also becomes a key parameter to understand the plasma dynamics.
Interestingly, recent studies have shown that the polytropic indices and kappa indices of space plasmas are related, in the presence of potential energy. Moreover, the relationship between the two indices defines the potential degrees of freedom.
This is the first statistical study to analyze Wind spacecraft observations to derive the polytropic index and the kappa index of solar wind protons and investigate their relationship, over the last two solar cycles. We show that, most of the time, the two indices are related, exactly as predicted by the theory. When able, we quantify the relation in order to derive the potential degrees of freedom. Among others, we show that an enhanced solar activity and/or interplanetary magnetic field, reduces the potential degrees of freedom, and decrease the dimensionality of a typical electric field potential from dr = 3 in solar minimum, to dr = 2 in solar maximum (Figure 1).
Overall, these results identify fundamental properties of the solar wind plasma, that demonstrate clear dependences on solar cycle.
Figure 1. Dimensionality dr for a typical interplanetary potential as a function of sunspot number Sn. The linear fit to data points (black dash) is also shown. The results indicate that the potential dimensionality dr reduces with increasing Sn.
For more information, please see the paper:
Nicolaou G. and G. Livadiotis (2019). Long-term correlations of polytropic indices with Kappa distributions in solar wind plasma near 1 AU. The Astrophysical Journal, 884:52, https://iopscience.iop.org/article/10.3847/1538-4357/ab31ad/meta