On the electric field in the Earth’s distant magnetotail

Solar electrons of energy Ee ≳ 50 kev are used as test particles for studying electric and magnetic fields in the distant magnetotail of the earth. During the prolonged solar electron event November 10-22, 1967 (J. A. Van Allen and N. F. Ness, 1969), simultaneous observations were made with the earth-orbiting satellite Explorer 33 in interplanetary space and with the moon-orbiting satellite Explorer 35 as the latter crossed the magnetotail. On the basis of the fact that the intensity of electrons was nearly identical at successive pairs of observational points during a wide range of geomagnetic conditions, it is inferred (a) that where E is the vector electric field, from any cause, at a vector element ds of the trajectory of the particle and the line integral is taken along trajectories from source points A′ outside the magnetosphere to a succession of points B across the magnetotail at about 64 RE (earth radii) downstream; and (b) that, by using (a), data from Van Allen and Ness, and observations of the very short delay time (±100 sec) in access of solar electrons into the central part of the magnetotail (impulsive event of August 14, 1968, as an example), solar electrons enter the magnetotail at downstream distances between 64 and 900 RE (cf. R. P. Lin and K. A. Anderson, 1966). Otherwise stated, it appears that the magnetic topology of the distant magnetotail is an ‘open one’ (dynamic interconnection to the interplanetary field) and that there are no closed electrical equipotential surfaces beyond 64 RE. This evidence supports the idea that the motional electromotive force that results from the movement of the interplanetary magnetic field with respect to the earth plays an essential role in magnetospheric physics, e.g., in driving magnetospheric convection, electrical currents in the polar ionosphere, etc.