Turbulent dissipation in space plasmas

Turbulence is a ubiquitous process in the laboratory plasma, as well as in space and astrophysical plasmas. The analysis of turbulence in the terrestrial plasma environment is the key to understanding the impact of the energy, mass and momentum transport from the solar wind to the magnetosphere and ionosphere. The most scientifically investigated space plasma is the solar wind. However, the planetary environments represent other types of space plasma, with field and plasma parameters differing from those in the solar wind. Contrarily to the solar wind case, the terrestrial plasmas are constrained by the available volumes between large-scale boundaries, such as the bow-shock and the magnetopause or the plasma sheet boundaries in the magnetotail. As a consequence, the analysis of fluid-scale processes is often not fully accessible. Despite the large variability of solar wind driven processes, both the magnetospheric and solar wind fluctuations are associated with the formation of coherent structures, and exhibit rather similar kinetic range spectral scalings and breaks near the ion scales. In this study the emphasis is on the observations of turbulence in the terrestrial magnetosheath and magnetotail. Dr Vörös will present a review on the experimental findings on magnetospheric turbulence, with the main focus being on the observations of coherent structures and various channels of energy conversion. On this basis, they aim to outline the possible strategies for further statistical investigations of magnetospheric plasma turbulence.

Compressible turbulence has been a subject of active research within the space physics community for the last three decades and is actually believed to be essential for understanding the physics of the solar wind (for instance the heating of the fast wind), of the interstellar medium (in cold molecular clouds) and other astrophysical and space phenomena. In this talk Dr Hadid will give an overview of the different studies that the group have done regarding the compressible and incompressible cascade rates in the interplanetary space. Firstly, using the exact law of compressible isothermal magnetohydrodynamic (MHD) turbulence [Banerjee & Galtier, PRE, 2013], they give an estimation of the compressible energy cascade rate (|εC|) in the Earth’s magnetosheath using THEMIS and CLUSTER spacecraft data and show that it is at least three orders of magnitude larger than its value in the solar wind. Moreover, they show the role of the density fluctuations in increasing the spatial anisotropy in the Earth's magnetosheath [Hadid et al., PRL, 2018]. Secondly, using the exact law of compressible Hall MHD turbulence [Andrés & Sahraoui, PRE, 2017] they give a complete estimation of |εC | at the MHD and the sub-ion scales in the Earth's magnetosheath using MMS data [Andrés et al., PRL, 2019]. Finally we show the radial evolution of the turbulent cascade rate from the Sun (~0.2 A.U.) up to Mars (~1.5 A.U.), using Parker Solar Probe and Maven data [Andrés et al. in prep].

Dr Chasapis will be discussing measurements of dissipation in turbulence observed in the Earth’s magnetosheath and the near-Earth solar wind. Such turbulence is characterised by an abundance of small-scale intermittent structures, where significant dissipation and particle energisation occurs through a series of different kinetic-scale mechanisms. The Magnetospheric MultiScale mission is able to carry out very high-resolution multi-point measurements in near-Earth space, giving us a unique opportunity to study these mechanisms. The high time resolution plasma and fields measurements allow us to directly observe dissipative processes occurring at kinetic scales. In parallel, we can implement and test new methods to identify pathways of dissipation using multi-point measurements. Over the last few years, the four spacecraft have gathered a significant amount of data, both in the strong turbulence of the Earth’s magnetosheath, as well as out in the pristine solar wind, allowing us to carry out statistical studies of turbulent dissipation. Additionally, we can adequately resolve the behaviour of suprathemral particles and their statistical behaviour in different turbulence conditions, helping us understand the role of turbulence in generating such populations.

Finally, Dr Chasapis will discuss novel configurations of the MMS spacecraft constellation, and some of their scientific results so far. Ultimately, they are paving the way for a new generation of multi-spacecraft missions, that will be able to carry out simultaneous multi-scale measurements, giving us new insights into plasma turbulence and the open questions of kinetic dissipation and particle energisation. 

Turbulence in plasmas involves a complex cross-scale coupling of fields and distortions of particle distribution functions, with the emergence of non-thermal features. The heliosphere, strongly characterized by nonlinear processes, represents the best natural laboratory to study in-situ plasma turbulence and, thanks to new solar missions, it is finally possible to study the radial evolution of the solar wind as it expands in the inner heliosphere, from the solar corona out to 1 AU.  

Solar wind turbulence is not homogeneous but is highly space-localized and the degree of non-homogeneity increases as the spatial/time scales decrease (intermittency). Such an intermittent nature also evolves, in fast streams, during the wind expansion, possible due to the emergence of strong non-homogeneities of the magnetic field over a broad range of scales (coherent structures). Here, the nature of turbulent fluctuations close to the ion scales, is investigated by using high-time resolution magnetic field data in different regions of the heliosphere and in different solar wind conditions. The ion scales are characterized by the presence of non-compressive coherent structures, such as current sheets, vortex-like structures, and wave packets identified as ion cyclotron modes, responsible for solar wind intermittency and strongly related to the energy dissipation. Both in-situ data and numerical simulations show that particle energization is observed in and near coherent structures. Understanding the physical mechanisms that produce coherent structures and how they contribute to dissipation in collisionless plasma can provide key insights into the general problem of solar wind heating. 

The solar wind has many forms of anisotropy: it has a background magnetic field; it expands roughly radially; and it is nearly collisionless, supporting non-Maxwellian particle velocity distributions. In this talk Dr Wicks will discuss how these forms of anisotropy combine together in the evolution of solar wind turbulence and hypothesise on how they may affect the dissipation of energy from fluid-like flows into kinetic particle motions. To do this, he will examine results from Parker Solar Probe and Wind, showing how turbulence interacts with fluctuations at the dissipation scale and how the thermodynamics of the solar wind and the evidence for dissipation change with radial distance from the Sun. He finds that the solar wind, overall, undertakes a close-to-3D adiabatic expansion, but locally energy flows look one dimensional. Local plasma fluctuations are dominantly perpendicular to the magnetic field and result in kinetic Alfven waves at small scales, leading to parallel proton heating, but strong perpendicular temperature anisotropies arise and drive proton cyclotron wave instabilities. These are proposed to arise from different processes, turbulence in the case of KAWs but local, cyclical instability for ICWs. The turbulence itself generates local compressions and rarefactions that drive temperature anisotropy, but only in one (roughly radial) dimension. He will therefore discuss the implications of a roughly one dimensional outcome of turbulence on the multi-dimensional and anisotropic heating / cooling of the solar wind.