Bayesian inference of ion velocity distribution function from laser-induced fluorescence spectra

The velocity distribution function is a statistical description that connects particle kinetics and macroscopic parameters in many-body systems. Laser-induced fluorescence (LIF) spectroscopy is utilized to measure the local velocity distribution function in spatially inhomogeneous plasmas. However, the analytic form of such a function for the system of interest is not always clear under the intricate factors in non-equilibrium states. Here, we propose a novel approach to select the valid form of the velocity distribution function based on Bayesian statistics. We formulate the Bayesian inference of ion velocity distribution function and apply it to LIF spectra locally observed at several positions in a linear magnetized plasma. We demonstrate evaluating the spatial inhomogeneity by verifying each analytic form of the local velocity distribution function. Our approach is widely applicable to experimentally establish the velocity distribution function in plasmas and fluids, including gases and liquids.

Temporal Ion Velocity Variation with Obliqueness in Magnetized Plasma Sheath

A narrow region having sharp gradients in physical parameters is formed whenever plasma comes into contact with a material wall. In this work, the temporal velocity variation of ions in such a sheath has been studied in the presence of an external oblique magnetic field. The Lorentz force equation has been solved for the given boundary conditions using Runge-Kutta method. In order to satisfy the Bohm criterion, ions enter the sheath region with ion acoustic velocity. It is observed that all components of the velocity waves are damped in plasma in the time scale of one second. The computed oscillatory part of ion velocity match with the equation of the damped harmonic oscillator. Thus obtained damping constants as well as the frequency of all three components are nearly equal for obliqueness less than 600 after which they are distinctly different. This is due to the fact that the magnetic field becomes almost parallel to the wall. In earlier studies, only the final velocity profiles are reported and hence this study is useful in understanding how the ion velocities evolve in time as they move from sheath entrance towards the wall.

Statistical Study of ICW Events and Associated Ion Velocity Distributions in the Inner Heliosphere

<p>Ion cyclotron waves (ICWs) frequently occur in the solar wind and are detected by PSP within 0.3 AU (Bowen et al. 2020), by MESSENGER from 0.3 AU to 0.7 AU (Jian et al. 2010, Boardsen et al. 2015) and by STEREO at 1 AU (Jian et al. 2009; He et al. 2011). However, the relation between the wave properties and the kinetic features of different ion components (proton core, proton beam and helium) are not widely discussed in the existing literature. We statistically analyze the polarization and propagation properties of hundreds of ICW events using measurements from the Solar Orbiter spacecraft. We find three types of ICW events in terms of their occurrence and duration: clustering ICW events with long durations; sporadic ICW events immersed in a quiet background magnetic field; and ICW events alongside discontinuities. We perform an investigation of the ion velocity distribution functions (VDFs) and draw comparisons of the kinetic behavior of each ion component during intervals with and without ICWs. The plasma parameters of the different ion components are acquired by our newly developed fitting program.</p>

Foreshocks of the terrestrial planets: Simulations of kinetic effects

<p>Planetary foreshocks and magnetosheaths are regions which include many small-scale kinetic processes. Therefore, terrestrial planets Mercury, Venus, Earth and Mars provide interesting laboratories to investigate how the kinetic effects depend on the properties of the solar wind and on the properties of the planet.</p><p>The kinetic effects can be investigated with a 3D hybrid model where ions are modelled as particles accelerated by the Lorentz force. Recent studies based on our parallel hybrid model have shown that the simulation has an adequate spatial resolution to investigate, in detail, the ion 3D velocity distributions and the properties of the ULF waves at the foreshocks of Mercury, Venus and Mars.  </p><div> <p>In this presentation, we focus on the simulated 3D ion velocity distributions at various sites around terrestrial planetary bodies and discuss their role near the planets, especially at the foreshocks. We also introduce methods to automatically analyze basic properties of the ion velocity distributions in the simulation.</p> </div>