Using Kappa Distributions to Identify the Potential Energy

This paper reports on the properties of kappa distributions in the presence of a potential energy. The constructed phase space kappa distribution leads to a polytropic relationship between the local density and temperature. The kappa and polytropic indices are connected via a characteristic and universal relationship: their sum is a constant depending specifically on the potential features. Determining this constant comprises a method essential for revealing the functional form and understanding the nature of the particle potential energy. The method is applied to the solar wind proton plasma at 1 AU. It is shown that a satisfactory physical interpretation of the potential affecting solar wind protons can be given by the interplanetary radial electric field.