J. P. Heppner

Auroral and Polar Cap Electric Fields from Barium Releases

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 (Foppl 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.

EARLY RESULTS FROM ISEE-1 ELECTRIC FIELD MEASUREMENTS

The double probe, floating potential instrumentation on ISEE-1 is producing reliable direct measurements of the ambient DC electric field at the bow shock, at the magnetopause, and throughout the magnetosheath, tail plasma sheet and plasmasphere. In the solar wind and in middle latitude regions of the magnetosphere spacecraft sheath fields obscure the ambient field under low plasma flux conditions such that valid measurements are confined to periods of moderately intense flux. Initial results show: (a) that the DC electric field is enhanced by roughly a factor of two in a narrow region at the front, increasing B, edge of the bow shock, (b) that scale lengths for large changes in E at the sub-solar magnetopause are considerably shorter than scale lengths associated with the magnetic structure of the magnetopause, and (c) that the transverse distribution of B-aligned E-fields between the outer magnetosphere and ionospheric levels must be highly complex to account for the random turbulent appearance of the magnetospheric fields and the lack of corresponding time-space variations at ionospheric levels. Spike-like, non-oscillatory, fields lasting <0.2 s are occasionally seen at the bow shock and at the magnetopause and also intermittently appear in magnetosheath and plasma sheet regions under highly variable field conditions. These suggest the existence of field phenomena occurring over dimensions comparable to the probe separation and ⩽c/ωpe (the ‘characteristic’ electron cyclotron radius) where ωpe is the electron plasma frequency.

Instrumentation for DC and Low-Frequency Electric-Field Measurements on ISEE-A

The double-probe floating-potential technique is applied using long-wire antenna-probes with an effective electric-field baseline of 179 meters. 14-bit A/D conversion is used for the prime dc-field measurements. An 8-channel spectrometer is used for ac-field measurements over the range 0.19 to 1900 Hz.

Recent measurements of the magnetic field in the outer magnetosphere and boundary regions.

Magnetic field measurements in outer magnetosphere, emphasizing boundary regions and shock front characteristics

The world magnetic survey

The mathematical and graphical description of the earths main field has been, and is, a “data limited” problem. The World Magnetic Survey (WMS) is an endeavor to minimize this limitation by rapidly and comprehensively blanketing the earth with magnetic field measurements. Satellite surveys, which will play a key role in the WMS, are the principal topic of this paper. Existing magnetic field descriptions, the expected results from new surveys, and the methods of obtaining these results with the POGO satellite are emphasized. It is anticipated on the basis of extrapolation from Vanguard 3 results and other considerations that a factor of 10 improvement will be obtained. This means that the average errors of 1 to 3 percent now present in field charts and spherical harmonic descriptions should be reduced to 0.1 to 0.3 percent as a result of the survey.

Magnetic Field Observations in High β Regions of the Magnetosphere

For a description of the distortions of the geomagnetic field in the magnetosphere ΔB provides a useful measure. Figure 1 shows approximate equi-ΔB contours for the two 90° sectors centered at the noon and midnight meridian half planes, based on a preliminary sampling of a much greater body of OGO 3 and 5 rubidium magnetometer data.