The magnetic field of the equatorial magnetotail from 10 to 40 RE

A statistical study of IMP 6, 7, and 8 magnetotail magnetic field measurements near the equatorial plane reveals new information about various aspects of magnetospheric structure. More magnetic flux crosses the equatorial plane on the dawn and dusk flanks of the tail than near midnight, but no evidence is found for a dependence on the interplanetary magnetic field sector polarity. Field magnitudes within 3 R E of the equatorial plane near dawn are more than twice as large as those near dusk for −20 < X sm < −10 R E . The frequency of occurrence of southward fields is greatest near midnight, and such fields are seen almost twice as often for −20 < X sm < −10 R E as for X sm beyond −20 R e . This latter result supports the idea that the midnight region of the tail between 10 and 20 R E is a special location where neutral lines are particularly apt to form. Such a neutral line will approach nearest the earth in the midnight and premidnight region, where substorms are thought to have their onset.