A comparison of ULF fluctuations in the solar wind, magnetosheath, and dayside magnetosphere: 2. Field and plasma conditions in the magnetosheath

Abstract

Data from three spacecraft (AMPTE IRM, AMPTE CCE, and ISEE 1 or 2) are used to study the correlation among the field and plasma conditions in the subsolar magnetosheath region, ULF wave activity in the magnetosphere, and the cone angle of the IMF. A disturbance parameter, R, which is the magnitude of the normalized resultant of unit vectors (calculated from measurements of the magnetic field or the plasma bulk velocity in a time interval), is used to describe the disturbance of the magnetosheath region. A “quiet” state has R values close to unity. We have studied five time intervals and found that when the R values of the magnetosheath magnetic field were below 0.8, indicative of a disturbed magnetosheath near local noon, transverse harmonic oscillations of magnetic field lines in the Pc 3, 4 range were observed in the magnetosphere and small cone angles were observed in the solar wind. We have also investigated the variation of other magnetosheath parameters (such as the magnetic pressure, the thermal pressure, the dynamic pressure, and the perturbation energy, etc.) under various magnetosheath conditions by comparing them with the disturbance parameter. It is found that the thermal beta (the ratio of the thermal pressure to the magnetic pressure) and the perturbation energy increase greatly as R decreases (i.e., as the magnetosheath region becomes more disturbed). The total pressure, which is the sum of the magnetic, thermal, and dynamic pressure of the subsolar magnetosheath region, decreases as the region becomes more disturbed. The dynamic pressure and the dynamic beta (the ratio of the dynamic pressure to the magnetic pressure) measured in the magnetosheath are poorly correlated with R, indicating that the changes in magnitude of the plasma bulk velocity in the subsolar magnetosheath have little effect on the occurrence of Pc 3‐4 waves in the outer magnetosphere. All magnetosheath parameters we examined became more variable during disturbed periods than during quiet ones. The implication of the above results to the transport of wave energy from the solar wind to the magnetosphere is discussed.