Auroral and Polar Cap Electric Fields from Barium Releases

Abstract

High latitude magnetic disturbances and accompanying auroral displays are obvious evidence of the release of large amounts of energy, presumably from the magnetosphere and magnetotail regions. Until recently experimental efforts towards understanding these phenomena were directed toward measurement of magnetic fields, energetic particles, optical emissions, and the morphology of the observables. The importance of ionospheric and magnetospheric electric fields was recognized in theoretical studies, but actual measurements were not available due to the lack of suitable techniques. In the past several years valid electric field measurements have been made from sounding rockets and satellites with long antennas (Aggson, 1969; Maynard and Heppner, 1970). Another method more suited for making E field observations simultaneously at several points and for extended times at a given location was developed at the Max Planck Institut (Föppl et al., 1967). Barium vapor released from a sounding rocket above about 150 km partially ionizes and produces a visible ion cloud during twilight which can be tracked photographically. The ion cloud drifts under the influence of the electric and magnetic fields, and it can be shown that the velocity of a small cloud above 200 km altitude is given essentially by v = E x B/B ². Inverting this equation to E= - v x B, the measurement of v in a known magnetic field B gives E. The GSFC-NASA Ba release experiments have produced high latitude observations of 23 ion clouds in the auroral zone and 12 in the polar cap region. From these observations we can draw conclusions about the nature of the ionospheric electric fields and the tensor conductivity elements that are most effective in producing ionospheric currents. The measurements also test the usual assumption that the surface magnetic disturbance is explained by ionospheric currents. From ground based magnetometers a typical equivalent ionospheric current pattern for the polar cap and auroral zone disturbance can be inferred. Figure 1 shows a diagram illustrating the essential features. Our E field investigations have sampled four regions of interest: (a) in the westward electrojet or negative magnetic bay region, (b) in the eastward electrojet region, (c) in the transition region where the westward electrojet passes polewards of the eastward jet, and (d) in the polar cap region. Visible auroral displays were observed in the Ba release region or close by, in all flights except those in the polar cap where auroras were seen only near the southern horizon.