The Dynamic Magnetosphere

Abstract

The shape of the magnetosphere is defined by a complicated system of electric currents which flow within it in response to the interaction of the Earth’s magnetic field with the solar wind. Changes in the solar wind pressure, such as occur during SSCs (storm sudden commencements), alter these complicated current systems and excite a variety of magnetohydrodynamic waves. The magnetic fields associated with these phenomena have been observed on the ground for over a century. For example, Stewart [1] reported on “pulsations” in the geomagnetic field observed during a major magnetic storm in 1859, and Birkeland [2] is credited as being the first to report on magnetic field oscillations later called “giant pulsations” [3]. The periods of these oscillations, in almost all cases, are shorter than about 10 min. Examples have been found in the Viking magnetic field measurements in which the total magnetic field varied in a quasi periodic manner with periods from 7 to 23 min (with an average of 12.2 min). Similar long-period fluctuations were also observed by a global network of ground-based magnetic observatories during two of the Viking examples. These long-period oscillations are interpreted as the periodic compression and relaxation of the magnetosphere, a long-period “breathing mode” of the magnetosphere. These long period variations are difficult to detect because their amplitudes are small (about 4 nT at Viking) and consequently may be overwhelmed by the higher frequency and stronger amplitude ULF waves often present in ground-based and satellite observations. However, several examples were readily found in the Viking data set, due to the unusually quiet conditions that prevailed during Viking’s lifetime. Therefore, these long-period breathing modes may not be uncommon. These observations provide the opportunity to study long-period “equilibrium” global changes of the magnetosphere uncomplicated by transient effects, which exist at periods shorter than about 10 min (longer than most known resonances of the magnetosphere).