Solar wind polytropic index in the vicinity of stream interactions

In an ideal, 3‐D, closed system that experiences compression and expansion, the temperature (T) and density (n) of a monatomic gas are related adiabatically through the polytropic law; i.e., T ∝ nα−1, where α is the polytropic index and equals 5/3. This relationship has been examined over a variety of scale sizes in the solar wind, and empirical measurements of α vary a great deal. As shown in this paper, blanket applications of the polytropic law to ambient solar wind can return misleading measurements of α, due to inhomogeneities in the solar wind originating at coronal source regions. However, on a smaller and more localized scale, adiabatic compression and expansion is observed. In this paper, we measure α by examining parcels of originally homogeneous material that have become compressed at stream interactions. A superposed epoch analysis of 73 isolated stream interactions observed by PVO illustrates this adiabatic behavior. Observations of individual stream interactions show clear intervals where α ∼ 5/3 and also a few cases where α ∼ 2, suggesting that the number of degrees of freedom may occasionally be restricted.