Resonant vibration of the magnetosphere observed from Explorer 26

This paper examines in detail a set of large-amplitude pulsations in magnetic-field magnitude and proton fluxes (E > 98 kev) observed by Explorer 26 in connection with a bay event during the magnetic storm of April 18, 1965. The peak-to-peak amplitude was about 40 γ and the period about 300 sec. These pulsations were such that the field magnitude was high when the proton fluxes were low and vice versa. They were accompanied by field-direction changes corresponding to an elliptical polarization. The observations are compared with four models involving (1) drifting plasma clumps, (2) ripples traveling in longitude on the surface of a particle belt, (3) a particle belt in periodic radial motion, and (4) a particle population sloshing back and forth along the field lines in the acoustic mode. The last of these models is found to be the most attractive one, and its consequences in terms of periodic modulation of the particle energy are examined. The study also indicates that periodic field-aligned currents accompanied the event.