JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 90, NO. A1, PAGES 505-515, JANUARY 1, 1985 The Excitation of Plasma Waves by a Current SourceMoving in a Magnetized Plasma: The MHD Approximation C. E. RASMUSSEN,' P.M. BANKS, AND K. J. HARKER Space, Telecommunications and Radio Science Laboratory, Stanford University, California This is the first of a series of three papers which examinethe excitationof low-frequency plasma waves arising from a current source moving througha cold,magnetized plasma. In particular,theeffects of motion of the source upon the radiation pattern are studied. In the present paper the general theory of wave excitation by a movingsource is developed and specific approximations are made which are valid for frequencies much below the ion-cyclotron frequency. The amplitudeand pattern of shear-Alv6n waves excited by a current source are studied for sources whose size in the direction of motion is large in comparison to the size in otherdirections and also for sources whose size islargein the directionof current flow (also in comparison to the size of the source in otherdirections). It isshown that the radiationpatternof the shear waves arises naturally asa consequence of the motion of the source and that any modulationof the source currenthaslittle effectupon the waves, other than to reduce their amplitude. 1. INTRODUCTION Soon after the first artificial satellites were orbited in the late 1950's,researchers becameinterestedin vehicle charging and the dragupon satellites created by theinduced emf of the v x B0 interaction of satellites with the terrestrialmagneticfield, B0, where v is the satellite velocity [Beard and Johnson, 1960; A1]vert, 1965,Osborne and Kasha, 1967]. From these studies it was realized that low-frequencyshear-Alv6nwaves could be excited by the motion of conducting bodies, setting up current systems connecting vastregions of theionosphere [Drell et al., 1965; Chu and Gross,1966]. Another feature associated with the satellite v x B0interaction is the presence of an induced,dc electric field, perpendicular to the directionof the background magnetic field. When one considers the Lorentz transforma- tion, E' = E + v x B0/c, where E' is the electricfield in the reference frame of the satellite, it is seenthat an electric field must bepresent either withinthereference frameof the satellite (E') or within the reference frame of the plasma (E), or in some combination of the two. The presence of the perpendicular electric field and parallel current (with reference to B) associated with the shear-Alfv/:n wave isimportantbecause it relates to manytopics of interest in space-plasma physics. For instance, the shear-Alfv6n wave is important in understanding thedecametric radiation from Jupi- ter. The motion of the satellite Io, relativeto the Jovian magne- tospheric plasma,excites a shear wavewhichpropagates to the Jovian ionosphere.The ionosphericcurrents created by the impact of the Alfv/:n wave upon the Jovian ionosphere are believed to be the source of the Jovian decametric radiation [Piddingto•n and Drake, 1968; Neubauer, 1980]. The shear- Alfv/:n waveis alsoimportant to theories of magnetospheric- ionospheric coupling. Goertz and Boswell [1979] have pro- 1Now at Center for Atmospheric andSpace Sciences, Utah State University, Logan. Copyright 1985by the American Geophysical Union. Paper number 4A1090. 0148-0227 / 85/ 004A- 1090505.00 posed that the electric field structure over auroral arcs is due to a transverse electric field in the magnetosphere propagating to the auroral zoneasan Alfv/:n wave.Another topic of interest to space-plasma physics is the relationship of a parallel currentto electricdoublelayersand to anomalous resistivity. Kindel and Kennel [ 1971] have shown that parallel-current densities ex- ceeding certainthresholds are unstable to microscopic, plasma wave growth. Goertz [1979] has discussed the importance of parallel current and perpendicular electric fields as the energy source of doublelayers. In view of the importanceof Alfv/:n waves in natural-space phenomena, it is not surprising to note the interest in studying these waves by performingactive experiments in space plasmas. One proposedmethod to study these waves is the use of an electrodynamic tether[ Williamson and Banks, 1976; Wu, 1978; Banks et al., 1981]. As currently envisioned, the electrodynamic tether consists of a subsatellite with a conducting outer surface connected to the space shuttle by an insulator-clad, conducting wire about 20 kilometersin length. If the subsatellite and the shuttle arein electrical contact with the ionospheric plasma, the emf generated by the motion of the tether across the geomag- netic field will act to drive a current through the tethered system. This current in the tetheracts asa source for exciting the shear-Alfv/:n wave. Much of the original effort in regards to understanding an orbiting, electrodynamic tether hasgoneinto studying the col- lection of currentby the tether wire (if uninsulated) and by the conducting surfaces on the subsatellite and orbiter bodies [Dobrowolny, 1978; -Morrison et al., 1978; Arnold and Dobrow- olny, 1980;Banks et al., 1981 ]. To this point in time, relatively little attention hasbeenpaid to the type of wave modes excited by an electrodynamic tether and to the amplitude and location in space of the excited waves. Belcastro et al. [1982] have studied the radiation of low-frequency waves from ULF up to the electron-cyclotron frequencybut considered only the radia- tion resistance of these modes. The pioneering study by Drell et al. [1965] looked at ULF radiation emitted by a large satellite, but the model used for the current source is not applicable'to tethered systems. In particular it was assumed that the sizeof the satellite in the directionof motion waslargeenough sothat fringing of the electric field pattern could be neglected. In fact, 505