Temperature and density anti‐correlations in solar wind fluctuations

We summarize recent theoretical investigations of low Mach number flows, that describe two distinct approaches by fluids to the incompressible regime. The first includes the effects of relatively strong density and temperature fluctuations (Type I), while the second places fluctuations in mechanical pressure, density and temperature on an equal footing (Type II). In the latter case, the pseudosound relations between density and pressure, in slightly generalized form, are recovered, whereas the former case yields entirely different small departures from incompressible behavior in that density and temperature fluctuations are predicted to be anti‐correlated. It is suggested that nearly incompressible fluids be classified as either Type I or II and it is shown that the well‐known pressure‐balanced structures represent a sub‐class of static solutions within this classification. Two examples from Voyager data illustrate the potential for observing these distinct nearly incompressible dynamical orderings in the solar wind.