A dynamic, rotating ring current around Saturn

Abstract

The idea that there is a ring current of trapped particles encircling the Earth at high altitudes first emerged in the early part of the twentieth century. The idea proved right, and measurements of the current's extent and composition were made in 1957. Ring currents of a different nature were later observed at Jupiter and inferred at Saturn. The magnetospheric imaging instrument on the Cassini probe has now obtained images of the ring current at Saturn. The current is highly variable, with strong longitudinal asymmetries that corotate nearly rigidly with the planet. This contrasts with Earth's ring current, where there is no rotational modulation and initial asymmetries depend on local effects. The concept of an electrical current encircling the Earth at high altitudes was first proposed in 1917 to explain the depression of the horizontal component of the Earth’s magnetic field during geomagnetic storms1,2,3,4. In situ measurements of the extent and composition of this current were made some 50 years later5 and an image was obtained in 2001 (ref. 6). Ring currents of a different nature were observed at Jupiter7,8 and their presence inferred at Saturn9,10. Here we report images of the ring current at Saturn, together with a day–night pressure asymmetry and tilt of the planet’s plasma sheet, based on measurements using the magnetospheric imaging instrument (MIMI) on board Cassini. The ring current can be highly variable with strong longitudinal asymmetries that corotate nearly rigidly with the planet. This contrasts with the Earth’s ring current, where there is no rotational modulation and initial asymmetries are organized by local time effects.