Equatorial current sheet in the magnetosphere

The ΔB distribution deduced from the Ogo 3 and 5 satellites shows large field depressions in the equatorial region inside the plasmasphere. The existence of this −ΔB region is established by demonstrating that contours of differential ΔB, derived by ΔB (Kp = 2–3) − ΔB (Kp = 0–1), have characteristics similar to those of ΔB contours, and thus the possibility is eliminated that the ΔB distribution in question may have arisen from errors in the reference fields used to calculate ΔB or systematic errors in the orbit determination. On the basis of the ΔB distribution it is shown that what is usually considered the quiet-time ring current is an equatorial sheet current that is an extension of the neutral sheet current in the magnetospheric tail. The primary source of the large field reductions inside the plasmasphere is a population of protons with energies of 0.1–1 Mev initially detected by Davis and Williamson (1963) and Davis (1965) on Explorer 12, 14, and 15. Low-energy protons extensively measured on Ogo 3 by Frank (1967, 1971) are primarily responsible for the current near and outside the plasmapause. Although the sources of the current are identified with the workers who discovered protons in these two different energy ranges, the division of the sheet current into two separate current belts is arbitrary and inadvisable unless future experiments indicate the necessity for such a division. The presently available magnetic-field data are concordant with a sheet current continuous from the tail to 2.5 RE. It is suggested that from the standpoint of particle dynamics in the magnetosphere an equatorial charged-particle sheet including all energies may be a more useful concept than the conventional concepts of the plasma sheet and the plasmasphere.