The Harris magnetic field: A laboratory realization of the topology based on energy resonance

Abstract

An often‐used approximation to the magnetotail field and the reconnection layer magnetic fields is the modified Harris field which consists of a neutral sheet magnetic field profile Bx(z) along with a superimposed normal field Bz. We have designed a configuration of current‐carrying conductors and Helmholtz coil fields aimed at producing the topological characteristics of this field. In addition to mapping the geometry of the field we have appealed to another criterion of validity of this simulation, the energy resonance phenomenon. Energy resonance is an essential feature of nonlinear particle dynamics in magnetotail‐like magnetic fields. The phenomenon is characterized by periodic peaks and valleys occurring in scattered particle distribution functions when plotted versus energy. The “neutral sheet” of the simulated field is not actually plasma current which self‐consistently is responsible for the magnetic field, but rather the field is produced by planar arrays of current carrying wires. For this reason certain self‐consistency requirements on particle orbits in such fields do not arise. Particle injection into the field region, which is formed by 4 conducting wire grids and an external constant field, is accomplished with a programmable electron gun with energies in the range 200 eV to 10 keV. The basic physics of the resonance phenomenon which occurs if the average radius of curvature of the field and the Larmor radius of injected particles are comparable, is scalable from the experiment to the magnetotail environment and has no dimensional dependence. The identification of the resonance effect itself arose from analysis of possible particle phase space orbits in Harris‐type fields. A result of this analysis was that the orbits are divided into clearly separate classes from which the resonance phenomenon follows. The creation of the field in the laboratory model includes realistic design considerations related to the method of producing the field. The design is based on using a test particle code which selects a source distribution of particles to “push” through the modeled magnetic field. Detailed test‐particle simulation investigates particle orbits and the effect on resonance of the finite thickness of conducting wires in addition to the finiteness of the apparatus itself. The experimental effort was performed in the large Space Physics Simulation Chamber (SPSC) in the Plasma Physics Division at the Naval Research Laboratory.