## 4 Results  
According to Figure 1, Wind observed several stream-interaction regions in 1995. Within these regions, the observed bulk velocity increases to \(V\sim\)750 kms\({}^{-1}\) over the  
Figure 1: Time series of solar wind proton bulk parameters as measured by the Wind spacecraft in 1995. From top to bottom, we show the proton number density, speed, scalar temperature, temperature anisotropy, and the magnetic field. background solar wind speed \(V\sim\)300 kms\({}^{-1}\). The scalar temperature also increases by an order of magnitude within the interaction regions. The temperature anisotropy seems to increase with increasing speed and temperature. In the bottom left panel of Figure 2, we show the 2D histogram of log(\(\alpha\)) and \(V\), as observed in 1995. We normalize the occurrence in each column (\(V\) bin) to its maximum value. The top panel of Figure 2 shows the 1D histogram of \(V\), while the right panel shows the 1D histogram of log(\(\alpha\)) for the same interval. Although the most frequent temperature anisotropy is below 1, there is a sharp increase at \(V\sim\)600 kms\({}^{-1}\). The peak of the most frequent value (magenta) is \(\alpha\sim\)2.5. The anisotropy decreases again for \(V>650\) kms\({}^{-1}\). We interpret this as an anisotropic heating/cooling of the plasma protons within stream-interaction regions. The top panels of Figure 3 show scatter plots of the logarithms of the temperature tensor elements and the scalar temperature in the slow solar wind (\(V<500\) kms\({}^{-1}\)), while the bottom panels show the corresponding plots for the fast solar wind (\(V>500\) kms\({}^{-1}\)). In each panel, we plot the identity line (\(y=x\)) and show the calculated linear Pearson correlation coefficient of the parameters. We observe that in the slow solar wind, log(\(T\)) is strongly correlated with both log(\(T_{\perp}\)) and log(\(T_{\parallel}\)). The Pearson correlation coefficients are \(\sim\)0.98 and \(\sim\)0.96, respectively. Additionally, log(\(T_{\perp}\)) and log(\(T_{\parallel}\)) are well-correlated (Pearson correlation coefficient \(\sim\)0.88) in the slow solar wind. In the fast solar wind, the correlation coefficients are smaller than the corresponding coefficients in the slow solar wind. The largest correlation is still observed between log(\(T_{\perp}\)) and log(\(T\)), which gives a Pearson correlation coefficient of \(\sim\)0.94. The coefficients between the other tensor elements are significantly smaller (\(<\)0.64). We note that the number of analyzed data points in the fast solar wind (\(V>500\) kms\({}^{-1}\)) is significantly smaller than the number of analyzed data points in the slow solar wind (\(V<500\) kms\({}^{-1}\); see the speed histogram in Figure 2). According to this result, we infer that the proton temperature measured along a single direction could  
Figure 2: Two-dimensional histogram of log(\(\alpha\)) and \(V\), normalized to the maximum value in each column (\(V\) bin). The magenta line shows the most frequent (mode) value in each column. In the top panel, we show the 1D histogram of \(V\); in the right panel, we show the 1D histogram of log(\(\alpha\)). be used successfully in the studies of some of the plasma thermodynamic properties. Such an application will be more accurate in the slow solar wind. We also investigate the long-term relationship between plasma density and temperature. In the left panel of Figure 4, we show the 2D histogram of \(\log(n)\) and \(\log(T)\). The middle panel of Figure 4 shows the 2D histogram of \(\log(n)\) and \(\log(T_{\perp})\), while the right panel shows the 2D histogram of \(\log(n)\) and \(\log(T_{\parallel})\). We normalize each column of the histograms to the maximum value within the column. Besides some obvious differences, the three histograms have a similar general structure. The large-scale fluctuations of \(\log(n)\) and the logarithm of the temperature tensor elements are anti-correlated. Specifically, the general trend of the large-scale variations follow the grey dashed lines with a slope of \(-2\).