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

Abstract

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. This paper is concerned with observations during the last half of July 29 when the AE index indicated very quiet conditions and when the IMF was strongly northward. Data used in this study are ground‐based magnetograms, magnetic disturbances from TRIAD, and charged particle measurements from AE‐C over the southern polar cap and from S3‐3 over the northern polar cap. 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, approximately at the same time that the IMF began to increase in the northward 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 (sometimes exceeding 800 nT at latitudes above 85° invariant) with 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 (>80° 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. The surface and TRIAD magnetic field disturbances are interpreted as being due to a system of intense Birkeland and ionospheric currents confined to the north polar cap during the last half of July 29, 1977. The fact that this current system is confined only to the north polar cap may be due to the relationship between the intense northward IMF and the tilt of the north pole toward the sun during this period. This geometry would favor the connection of antiparallel magnetic field lines between the IMF and the nighttime geomagnetic field lines in the northern hemisphere and not in the south. Furthermore, the enhanced ionospheric conductivity in the sunlit northern polar regions would favor completion of Birkeland currents in the north rather than the south pole. The presence of particles in both hemispheres may be due to the fact that they are trapped on closed field lines during this period of strongly northward IMF.