Ring Currents in Planetary Magnetospheres

Abstract:

Nearly a century ago, Schmidt [1917] and Chapman [1919] first proposed that the magnetic field depression during magnetic storms was caused by electrical currents flowing near the Earth, fed by charged particles of solar origin. In the early 1930s, Chapman and Ferraro suggested that a solar charged particle flow (later established by Parker as the “solar wind”) could leak into the magnetosphere and drift around the Earth, creating an electric current whose field would oppose the intrinsic terrestrial field. Today, we know well how the interaction of planets with the solar wind produces a variety of current systems, usually classified as ionospheric currents, magnetospheric boundary currents, magnetotail currents, and currents flowing inside the magnetosphere, such as the ring current, the plasma sheet current and the field-aligned currents. The inhomogeneity (gradient and curvature) of the planetary magnetic field produces an azimuthal drift of charged particles with ions and electrons moving in opposite directions. The combined gradient and curvature drifts, result in a net motion of current charges, a planetary “ring” current. Its average structure and intensity are functions of the plasma population that carry the current and the particular characteristics of each system. In rapidly rotating magnetospheres, such as those of Jupiter and Saturn, the ring current is strongly modified by the inertial forces that also tend to form a disc-shaped outer magnetosphere. If the magnetospheric plasma beta is close to 1, the azimuthal current produces magnetic perturbations that can significantly modify the planetary magnetic field, an effect that has been well observed in Earth, Jupiter and Saturn, but is relatively small (weak ringcurrent) for Uranus and Neptune, although these two planets also have radiation belts. In this talk, we focus on the description, particular characteristics (plasma sources, rotational and plasma beta regime, anisotropies) and interplanetary variability of the magnetospheric ring currents, based on in-situ and remote (Energetic Neutral Atoms) observations, where available. We also examine the relative strength of different ring current contributors and we attempt a qualitative comparison between different magnetospheric systems, as far as their ring currents are concerned.