Multipoint observations of global magnetospheric compressions

The magnetic field experiment on the Viking satellite detected long-period compressional oscillations with amplitudes ranging from 2 to 5 nT and perio ds ranging from 7 to 23 min on nine Viking orbits covering a wide range of polar and subauroral latitudes during the period from March to August 1986. We present a case study of an event on June 18, 1996, in which similar lon g-period pulsations were also observed by a global network of more than 50 ground magnetometers acid by the GOES-5 and GOES-6 spacecraft. The magnetic field variations throughout the auroral, midlatitude, and equatorial regio ns, at Viking, GOES-5, and GOES-B were approximately in phase, whereas the variations at some morningside stations were a half-wave period out of phas e. We interpret these long-period oscillations as evidence for periodic com pressions and inflations of the magnetosphere driven by solar wind pressure pulses. The relative time delays of the peak variations observed by GOES-5 , GOES-6, Viking, and the ground-based stations are consistent with an even t originating on the prenoon magnetopause whose signatures spread both pole ward and antisunward on both sides of local noon.