Near-Earth magnetic disturbance in total field at high latitudes 2. Interpretation of data from Ogo 2, 4, and 6

Abstract

Variations in the scalar magnetic field (ΔB) from the polar-orbiting Ogo 2, 4, and 6 spacecraft, with supporting vector magnetic field data from surface observatories, are analyzed at dipole latitudes above 55°. Data from all degrees of magnetic disturbance are included, the emphasis being on periods when Kp= 2− to 3+. Although individual satellite passes at low altitudes confirm the existence of electrojet currents, neither individual satellite passes nor contours of average ΔB are consistent with latitudinally narrow electrojet currents as the principal source of ΔB at the satellite. The total field variations at the satellite form a region of positive ΔB between about 2200 and 1000 MLT and a region of negative ΔB between about 1000 and 2200 MLT. Further characteristics are given by Langel lpar;1974a). The ratio of ΔB magnitudes in these positive and negative regions is variable. The characteristics of the negative ΔB region indicate a latitudinally broad ionospheric source. Equivalent current systems were derived from satellite data in the negative ΔB region for summer, winter, and equinox seasons for Kp=2− to 3+. Comparison of the surface magnetic disturbance caused by the equivalent current with the measured average surface disturbance shows good agreement except in localized details. Because ΔB decreases very slowly with altitude in the positive ΔB region, it is not possible to account for this disturbance in terms of ionospheric currents. The contribution to the satellite ΔB due to a model electrojet that reproduced the measured average surface horizontal disturbance was computed. When the contribution to ΔB from this model electrojet is removed from the measured satellite ΔB, the remaining ΔB is constant with altitude within experimental error. This nonelectrojet ΔB is estimated to be about 50–80, 20–30, and 7–15 γ for Kp ≥ 4−, 2− to 3+, and 1− to 1+, respectively. It is concluded that most, if not all, of the nonelectrojet positive ΔB is extraionospheric in origin. Some of this nonionospheric ΔB is caused by the equatorial current sheet (ring current), but some data cannot be accounted for in terms of this source. Definitive identification of all sources of positive ΔB is not possible at this time.