Planetary Magnetodiscs: Some Unanswered Questions

Characteristic of giant planet magnetospheres is a near equatorial region in which a radially stretched magnetic field confines a region of high density plasma. The structure, referred to as a magnetodisc, is present over a large range of local time. This introductory chapter describes some of the physics relevant to understanding the formation of this type of structure. Although many features of the magnetodisc are well understood, some puzzles remain. For example, Jupiter's magnetodisc moves north-south as the planet rotates. The displacement has been attributed to the motion of the dipole equator, but at Saturn the dipole equator does not change its location. This chapter argues that the reasons for flapping may be similar at the two planets and suggests a role for compressional waves in producing the displacement. The development of thermal plasma anisotropy and its role in the structure of Jupiter's magnetodisc are explored. Finally, localized plasma enhancements encountered by the New Horizons spacecraft at large downtail distances in Jupiter's nightside magnetodisc are noted and a firehose instability of stretched flux tubes is proposed as a possible interpretation of the observations.