University of Minnesota
School of Physics & Astronomy

MN Institute for Astrophysics Colloquium

Friday, November 15th 2019
2:30 pm:
Speaker: Yan Song, University of Minnesota
Subject: How Do Cosmic Plasmas Create High-Energy Particle Accelerators in Space?

Interaction between magnetized cosmic plasmas and electromagnetic fields and waves can lead to various collective behaviors, including particle acceleration, explosive energy conversion and structure formation. The acceleration of charged particles to high energy often occurs in space, solar and cosmic plasmas. Among other mechanisms for particle acceleration, parallel electrostatic fields related to charge separation are the most powerful means to directly and efficiently accelerate particles.
We have proposed a dynamical theory of the generation of parallel electric fields (Song and Lysak, 2006), and pointed out that the E||-generation is described by the displacement current term in the parallel component of Ampere’s law. The generation of these fields is favored by a low plasma density, enhanced magnetic stress and strong Alfvenic interaction. In this talk, I will review advances in developing the theory of the E||-generation. I will show that, the displacement current term provides a new mechanism of the generation of powerful high energy electrostatic fields, i.e., charge separation. In fact, the discovery of the role of the displacement current in particle acceleration has opened the door to understand the mechanism of high energy particle acceleration in cosmic plasmas.
We also realize that once the parallel electric fields are produced, they will disappear right away due to the high mobility of charged particles, unless the electric fields can be continuously generated and sustained for a fairly long time. Thus, the generation of a long-lasting parallel electrostatic field is needed for the acceleration of auroral particles to high energy. We have developed a preliminary theory to explain the formation of Alfvenic Double Layers (DL) which are electrostatic plasma structures, and serve as high-energy particle accelerators in auroral acceleration regions. I will describe the properties and characteristics of such high energy particle accelerators.
The Alfvenic DL consists of localized electrostatic fields related to charge separation embedded in low density cavities surrounded by enhanced reactive stresses. The enhanced magnetic shear carrying free energy serves as the local dynamo. The generated electrostatic fields will quickly deepen previously produced weak low density cavities, which can further enhance the generation of stronger electrostatic fields, causing auroral particle acceleration. The Poynting flux carried by Alfven waves is required to continuously supply energy to the auroral acceleration region, supporting Alfvenic DLs, leading to strong long-lasting electrostatic fields. The Alfvenic electrostatic plasma structure acts as powerful high energy particle accelerators for the formation of quasi-static and Alfvenic auroras, covering small to large scales.
In addition, I will briefly discuss some problems related to magnetic reconnection concept, which has been considered as one of the mechanism to cause charge particle acceleration to high energy, and to suggest using fundamental physical laws and principles to study “reconnection” related physical processes.

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