GEOMETRY OF THE GEOMAGNETIC TAIL.

An analysis of magnetic field measurements in the geomagnetic tail from Explorers 33 and 35 during 1967–1968 has shown that there is a broad region of depressed field magnitude approximately 12 RE thick and centered on the neutral sheet. The solar magnetospheric Bz component is proportionately larger within that region than outside of it. Bz is found to decrease with distance from the earth but to be positive on the average within the depressed field region out to a distance of 70 RE, indicating that although for short periods of time the neutral line may be closer to the earth than the orbital distance of the moon, on the average it is beyond that distance. A negative Bz component was found in ⅔ of the measurements outside the depressed field region. This analysis has shown that the magnetotail field diverges from the tail axis with distance from the earth. This result, together with the Bz observations, supports a geometry in which for large |Zsm| the tail field is diverging in the Zsm as well as the Ysm direction, but is converging slightly toward the neutral sheet within the depressed field region. The combination of expanding tail and reconnection at the neutral sheet can account for the observed tail field magnitude gradient. The Explorer 33 and 35 measurements also indicate that the average angle between the tail axis and the earth-sun line is 3.1°±1.5°.

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