University of Cambridge > > DAMTP Astrophysics Seminars > Kinetic effects in collisionless and turbulent space plasmas: 'in situ' data and Vlasov simulations

Kinetic effects in collisionless and turbulent space plasmas: 'in situ' data and Vlasov simulations

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Living on Earth, and thanks to the support of many space missions, we have the unique opportunity to analyse directly the features of the dynamical behaviour of a natural plasma: the solar wind. A puzzling aspect of solar-wind dynamics consists in the empirical evidence that it is hotter than expected for an adiabatic expanding gas. Understanding the physical mechanisms of dissipation, and the related heating, in such turbulent collision-free system represents nowadays one of the key issues of plasma physics but also for thermodynamics in general. Plasma turbulence is a challenging problem, involving a variety of complex phenomena: the energy transferred towards smaller scales by turbulence produces localized regions (coherent structures) where dissipation increases, and particle distribution functions are strongly distorted. The absence of an equilibrium attractor leaves the plasma state free to explore the dual spatial-velocity phase space. In this scenario, the use of both spacecraft measurements and kinetic numerical simulations becomes crucial.

Here, a novel theory of space plasma turbulence is described to investigate the possibility of a velocity-space cascade, by means of a three-dimensional Hermite decomposition applied to both numerical and ‘in situ’ measurements. A broad-band, power-law Hermite spectrum for the velocity space fluctuations of the ion velocity distribution is recovered. Moreover, by using high-time resolution magnetic field data of multi-point measurements, the nature of the turbulent fluctuations close to the ion scales is investigated. However, due to the limitation on in-situ particle measurements, a support from self-consistent fully nonlinear Vlasov models is needed. High resolved hybrid Vlasov-Maxwell simulations have been used to investigate the details of the strong deformation of the ion distribution function. The phase-space cascade might be of general relevance for the nonlinear dynamics of a weakly-collisional plasma in a wide variety of conditions.

This talk is part of the DAMTP Astrophysics Seminars series.

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