Compressible Turbulence in the Heliosphere

The solar wind is a high speed continuous expulsion of matter originating from the Sun. Due to the high speeds, the solar wind is often observed to be in a turbulent state associated with multiscale energy transfer from large fluid scales towards small kinetic scales. The solar wind plasma is nearly collisionless, therefore, kinetic processes other than viscosity are necessary to describe turbulent dissipation over small scales. The project deals primarily with measurement and analysis of compressive fluctuations in the solar wind. While the power of compressive fluctuations is small at large fluid scales (frequencies below 0.1Hz), at the smaller kinetic (frequencies above 1Hz) scales, the power of compressive fluctuations approaches a third of the total fluctuation power. At kinetic scales the intensity of energy exchanges, wave particle interactions is enhanced. The compressive magnetic power at small scales can be estimated by using the magnitude of the magnetic field when the magnetic fluctuations are small. The power of compressive fluctuations can also be determined through density measurements. However, often, density measurements performed using plasma instruments are slow. A novel alternative is to use the high resolution measurement of the spacecraft potential. The value of the spacecraft potential is related to the ambient plasma density. The objectives of the project are (1) to understand the origin of the nature of compressive fluctuations in terms of density and magnetic field measurements. (2) to determine what are the control parameters that govern those fluctuations. (3) to understand how turbulence evolves in the heliosphere. To answer these questions, the project has three work packages. WP1: Fluctuations at Earths: Data from Magnetospheric MultiScale mission will be used to understand the density fluctuations using both single spacecraft and multi-spacecraft methods WP2: Fluctuations in the inner Heliosphere: Data from the Solar Orbiter mission will be used to study the evolution of the density fluctuations while data from BepiColombo and Parker Solar Probe missions will be used to study the evolution of magnetic fluctuations with heliocentric distances WP3: Numerical simulations of compressible plasma turbulence. Surveying a large number of intervals, as well as numerically simulated data for a variety of different cases, will allow us to determine which control parameters are important and, consequently, what models of compressive fluctuations best explain the data.