Laboratory experiments on shear Alfvén waves and their relationship to space plasmas

Abstract

Alfvén waves are ubiquitous in space plasmas and are the means by which information about changing currents and magnetic fields are communicated. Shear Alfvén waves radiated from sources with cross-field scale size of the order of the electron inertial length, δ = c/ωpe, have properties which differ considerably from planar magnetohydrodynamic waves. Currents of cross-field size, δ, are common in space plasmas. A series of experiments in the Large Plasma Device at the University of California, Los Angeles is presented which illustrates that waves generated by small-scale fluctuating currents have a parallel electric field and radiate across magnetic field lines. Data and theory are presented for varying plasma collisionality and ratio of Alfvén speed to electron thermal velocity. Waves generated by two sources are observed to constructively interfere to produce large magnetic fields in spatial regions away from source field lines. The shear Alfvén wave is important in the Earth's auroral regions and may play a role in auroral dynamics. Copyright 1997 by the American Geophysical Union.