The Thermodynamic Impact of Compressive Fluctuations on the Solar Wind in the Inner Heliosphere

The solar wind plasma is observed to fluctuate over a broad range of space and time scales, extending from scales above the magnetic field correlation scale to below those associated with the particle gyration. At scales larger than the gyroscale, the fluctuations are typically categorised as 1) non-compressive fluctuations that have Alfvénic correlation, 2) compressive fluctuations that perturb the plasma density and pressure. While the amplitude of the compressive fluctuations are subdominant to the Alfvénic component, they have unique dynamics that drastically alter the plasma. For example, compressive fluctuations perturb the pressure anisotropy and beam drift speeds. This may drive the perturbed plasma unstable, generating microscale waves that scatter particles and alter the effective mean free path. In addition, compressive fluctuations perturb the magnetic field strength, leading to stochastic heating and transit time damping. Therefore, an understanding of compressive fluctuations is vital to a complete picture of the plasma thermodynamics. To build on our understanding of the solar wind in the inner heliosphere, we combine observations from Solar Orbiter, Parker Solar Probe, and the Wind spacecraft to study compressive fluctuations. We compare amplitude ratios and polarisations to numerical models to understand the efficiency of various generation mechanisms of compressive fluctuations and how they heat and modify the thermodynamics of the solar wind plasma.