[IGPP Everyone] TODAY - EPSS Space Physics seminar announcement - Friday November 12, 2021 - 03:30 PM Pacific Time (US and Canada)

[Sowmendran, Margie (IGPP)]

R e m I n d e r - T o d a y

SPACE PHYSICS SEMINAR

DEPARTMENT OF EARTH, PLANETARY, AND SPACE SCIENCES
DEPARTMENT OF ATMOSPHERIC AND OCEANIC SCIENCES
UNIVERSITY OF CALIFORNIA, LOS ANGELES

ZOOM LINK PROVIDED BELOW

 https://ucla.zoom.us/j/92101918782?pwd=Z2o5RmI4OEpBWW4zcG1DZStIUWgrZz09





Charged Particle Isotropization in Collisionless Environments: the Role of Turbulence as well as Very Short Radius-of-Curvature Toroidal Field Geometries


Professor William I. Newman

Departments of Earth, Planetary, and Space Physics; Physics and Astronomy; and Mathematics


The kinematics of individual particles responding to magnetic fields with very short radius-of-curvature R lines of force is highly complex, and the statistical nature of observed kinetic energy partitioning is often noted as being isotropic. Suppose a charged particle with a velocity v is injected into an environment with an associated gyrofrequency Om where with no electric field present. Northrop (1963) demonstrated, when the dimensionless constant Psi = v/Omega R << 1, that the behavior would be adiabatic and largely field-aligned. A natural question, then, is how the particle motion would evolve if its kinetic energy were increased making Psi  very large. We approach this question in two steps.

(a) Consider the situation where the magnetic field undergoes abrupt changes in orientation, as a model for turbulence, intermittently rendering its radius-of-curvature R=0  This produces to a field configuration that could be described literally as a ``stick-model.'' We show analytically via orbit integration methods in conjunction with Brownian motion methodologies that the particle's velocity distribution becomes isotropic.

(b) Consider a toroidally symmetric electric field with radius-of-curvature R  as might be encountered in a tokamak or as an approximation to field geometries encountered in space-based observations as well as their simulations. Schmidt (1979) noted that the associated Hamiltonian is "integrable" and every particle satisfies three conservation laws. He showed that these presented bounds on how the kinetic energy would evolve in time. Others have followed suit and in some instances presented numerical solutions to illustrate the complex behavior that emerged. However, an explicit analytic solution for the particle trajectories has eluded investigations owing to technical complexities. Exploiting the scaling properties of the field, we are able obtain analytically the evolution of the field and calculate explicitly how the distribution of its kinetic energy components varies as a function of Psi and become isotropic as Psi becomes infinite, including situations where the relativistic Lorentz factor must be included. While this latter result employs a specific model for the field geometry, the KAM theory for perturbations to that integrable model provides substantial assurance that its conclusions remain robust in other toroidal geometries.

Friday, November 12,  2021
3:30 - 5:00 PM

In-Charge:  Marco Velli

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