Nightside magnetosphere configuration as obtained from trapped electrons at 1100 kilometers

Using data from the polar orbiting satellite 1963 38C, we have obtained the diurnal variation of trapped electrons of energies Ee≥280 kev and ≥1.2 Mev during magnetic quiet. This diurnal variation is measured as a latitude shift for constant electron intensity and is obtained as a function of invariant magnetic latitude. All the data were obtained for dipole orientations within ±12° from the normal to the earth-sun line and for satellite positions within 8° of the noon-midnight meridian. Assuming conservation of the adiabatic invariants as these trapped electrons drift in the magnetosphere, it has been possible to obtain a nightside magnetic field configuration that fits the observed diurnal variations. A dayside configuration that agrees with experimental observations was used. The nightside configuration so determined displays an extended field line geometry and a current sheet in the magnetic equatorial plane. The field due to this current sheet is found to range from 20 to 40 gammas adjacent to the sheet, depending upon the radial extent of the sheet. A field line configuration in the noon-midnight meridian is presented. The nightside trapping boundary as defined by field line closure was found to occur at 1100 km at 67°, in agreement with observed boundaries at 1100 km of ∼67° for both ≥40- and ≥280-kev electrons. The situation on the dayside is different and is discussed.