Magnetic flux transfer associated with expansions and contractions of the dayside magnetosphere

Abstract

An empirical magnetopause model is developed by using Ogo 5 magnetometer data, solar wind data from three satellites, and auroral zone magnetic indices. By employing this model it is found that the average displacements of the dayside magnetopause due to changes in solar wind dynamic pressure psw are comparable with, but greater than, the average displacements associated with the transfer of magnetic flux between the nose and the tail of the magnetosphere. After correcting for changes in psw, multiple encounters of the magnetopause occurring over distances of ≥0.5 RE on inbound or outbound legs of the satellite orbit are termed contraction or expansion events, respectively. Events occurring during periods of significant ring current, as judged by the Dst index, are excluded from the study. The remaining 24 expansion and contraction events are attributed to net changes ΔΦ in the total magnetic flux present in the dayside magnetosphere. The model dependence of the results is discussed, and for the events observed it is found that 2 × 1015 < ΔΦ < 2 × 1016 Mx. For the expansion events, ΔΦ shows a positive correlation with the time integral of the auroral zone magnetic index AL over the temporal extent of the observation. A similar correlation between the net flux losses during contraction events with the southward component of the IMF flux impinging on the magnetosphere is complicated by flux returning from the tail during the erosion process. Displacements associated with net flux transfers were observed in all regions of the dayside magnetopause accessible to the satellite, a finding suggesting that the entire dayside magnetosphere is affected by the flux transfer process. The bearing of these results on magnetic reconnection in the earth's magnetosphere is briefly discussed.