Interplanetary magnetic field control of high-latitude activity on July 29, 1977

A number of intense substorms on July 29, 1977, have been under study as a special International Magnetospheric Study (IMS) period. A large interplanetary shock was observed at 0027 UT and was followed by four major substorms spaced over the first 12 hours. Surface magnetometers in the north auroral zone (and the associated AE index), which recorded very disturbed conditions during the first half of July 29 (with AE values sometimes exceeding 1000 nT), observed a sharp reduction in the disturbance level beginning at 1300 UT, aproximately at the same time that the IMF began to increase in the northard direction. The northern auroral zone magnetometers observed quiet conditions throughout the end of July 29, when the IMF also remained strongly northward. However, surface magnetometers at higher latitudes in the north polar cap (at Thule and Resolute Bay) observed intense disturbances (up to 700 nT) during this same period. This was in marked contrast to magnetometers in the southern polar cap (at Vostok and Mirny) which recorded almost no disturbances during the end of July 29. This pronounced polar cap asymmetry was also observed in a series of nine TRIAD passes which recorded intense transverse magnetic disturbances in the northern hemisphere almost no disturbances over the south polar cap. In contrast to the magnetic field observations, precipitating low-energy electrons (<1 keV) accompanied by precipitating protons were observed at high latitude (>800 invariant) in both hemispheres by S3-3 (in the north) and AE-C (in the south). Regions of accelerated electrons (>10 keV) accompanied by the absence of protons were also observed by S3-3 and AE-C during these orbits