Magnetotails

A magnetotail is the elongated extension of a planet's magnetosphere on the nightside, formed as the solar wind drags and stretches the planetary magnetic field lines. In Earth's case, the magnetotail reaches many tens to hundreds of Earth radii into space, varying with solar wind pressure and activity.

Structure of the magnetotail consists of two opposing magnetic-field lobes separated by a thinner central plasma sheet. The lobes contain relatively low-density plasma with fields pointing in opposite directions, while the plasma sheet hosts hotter, denser plasma and carries the cross-tail current that helps balance forces in the tail. The tail may also contain magnetic flux ropes and plasmoids formed by dynamic reconnection processes.

Dynamics and processes in the magnetotail include magnetic reconnection, which dissipates stored magnetic energy and can trigger substorms and power auroras. Reconnection can produce fast plasma flows, eject plasmoids tailward, and create dipolarization fronts that propagate through the tail. The ongoing interaction with the solar wind keeps the tail in a constantly evolving state.

Occurrence and study: Earth’s magnetotail is the best studied, but most magnetized planets in the Solar System—including Mercury, Jupiter, Saturn, Uranus, and Neptune—also possess magnetotails, whose size and behavior depend on their magnetic fields and solar wind conditions. Observations from spacecraft (such as THEMIS and Cluster) and theoretical magnetohydrodynamic models are used to investigate the tail’s structure, dynamics, and role in space weather.

Overall, the magnetotail is a key feature of magnetized planetary environments, linking solar wind forcing to magnetospheric storage and release of energy.