A laboratory model of the Harris magnetic field

Abstract

Summary form only given. An often-used approximation to the magnetotail field and the reconnection layer magnetic fields is the modified Harris field which consists of a magnetic field profile B/sub x/(z)x along with a superimposed normal field B/sub z/z. We have designed a configuration of current-carrying conductors and Helmholtz coil fields aimed at producing the topological characteristics of this field. As a criterion of validity of this simulation we appeal to the energy resonance phenomenon which 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. We have re-examined earlier particle orbit data, restricting the analysis to particles whose gyroradii are consistent with this requirement. We present details of the creation of the field in the laboratory which includes realistic design considerations related to the method of producing the field. In addition the results of test-particle simulations showing the effect of the finite thickness of conducting wires are shown. Finally we present data from experiments performed in the NRL Space Physics Simulation Chamber including the field construction and detected particle orbits.