Physics and Astronomy Colloquium

Week of Monday, September 18th 2017

Thursday, September 21st 2017
3:35 pm:
Speaker: Yan Song, University of Minnesota
Subject: Formation of Alfvenic Electromagnetic Plasma Structures and Auroral Particle Acceleration: -- Exploring Theory of High Energy Plasma Physics

The acceleration of charged particles to high energy and the associated emission of electromagnetic (EM) radiation produced by the accelerated electrons and ions, occur throughout space and cosmic plasmas. During these processes, a large part of stored free magnetic energy can rapidly and efficiently convert into the kinetic energy of charged particles producing non-thermal high energy particles and EM radiation. To find the mechanism of such high energy particle acceleration is one of the most important unsolved problems in space and cosmic plasmas.

In general, applying parallel electrostatic electric fields associated with charge separation is the simplest and powerful method to directly accelerate particles to high energy. However, once the electric fields are produced, they will quickly short themselves out by the motion of free charges. Thus, a central question in auroral physics is to find the mechanism by which long-lasting parallel electrostatic electric fields can be generated.

I will present the theory of the generation of parallel electrostatic electric fields (Song and Lysak, 2001, 2006), and point out that the generation of parallel electric fields is favored by a low plasma density and high magnetic shear. In the auroral current system, nonlinear Alfvenic interactions between Alfven wave packets can produce EM plasma structures, such as Alfvenic Double Layers. The Alfvenic Double Layer consists of localized long-lasting electrostatic electric fields, which are embedded in low density cavities and surrounded by enhanced magnetic stresses. These structures are dynamical in nature, where the Poynting flux carried by Alfven waves continuously supplies energy to the Alfvenic Double Layers to maintain strong electrostatic electric fields for a fairly long time. These structures become a new fundamental dynamical state in cosmic plasmas, which constitute powerful high energy particle accelerators.

It has been broadly considered that the magnetic reconnection is a fundamental physical process which is responsible for almost all high energy plasma processes in space and cosmic plasmas. Magnetic reconnection is described as a process occurring when oppositely directed magnetic field lines in a plasma are cut and rejoined. I will show that the crucial components of magnetic reconnection lack support from fundamental physical theory, and suggest that we should use the fundamental physical laws and principles to study “reconnection” related processes.

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