Boundary Determinations from Low Frequency Magnetic Field Measurements

Abstract

Objects in the interplanetary medium are subject to interactions with the solar wind. As the solar wind encounters magnetized or unmagnetized objects, a shock wave forms upstream of the object and the solar wind field is distorted and draped over the obstacle. For magnetic bodies, this interaction produces three distinct regions of space: the solar wind, the magnetosphere of the body, and the magnetosheath or shocked solar wind between the solar wind and the magnetosphere. Accurate characterization of magnetic bodies therefore depends on identification of crossing from the solar wind or magnetosheath into the magnetosphere of the object. Magnetic measurements in the < 1Hz to 100’s Hz range (denoted AC) have proven extremely useful in determining characteristic regions within the Earth’s magnetosphere. At low altitudes, magnetic fluctuation levels are particularly useful in identifying field-aligned currents of the ionosphere auroral zones. These currents are a necessary consequence of the solar wind and will occur at other bodies as well, connecting the various altitude regions of magnetospheres. At high altitudes, the AC levels have proven indicative of magnetosheath and magnetopause current layers. Magnetosheath regions are subject to ion cyclotron and mirror mode local instabilities which do not penetrate the magnetopause. Furthermore, the magnetopause boundary for large magnetic shear is the site of intense magnetic noise. Magnetic fluctuation levels may be used to assist in the identification, at various altitudes, of the boundaries between the regions of magnetic influence of the solar wind and interplanetary bodies.