magnetosheath

The magnetosheath is the region of space between a planet’s bow shock and its magnetopause, where solar wind plasma has been slowed, heated, and deflected by the planetary magnetic field. It forms downstream of the bow shock as solar wind flows around the magnetosphere and then continues toward the magnetopause.

Formation and properties: When the supersonic solar wind encounters a planetary magnetosphere, a bow shock forms ahead of the planet. Crossing the shock increases the plasma’s temperature and density and reduces its velocity. The shocked solar wind then flows around the magnetosphere in a turbulent, magnetized flow—the magnetosheath. The magnetic field in this region becomes draped around the magnetosphere, and the plasma remains highly dynamic and structured, more turbulent than the pristine solar wind upstream.

Structure and boundaries: The magnetosheath occupies the space between the bow shock and the magnetopause, extending on the dayside and around the flanks of the magnetosphere. The subsolar region lies directly sunward, while flank regions wrap around toward the night side. The magnetopause marks the outer boundary of the magnetosphere proper, and the bow shock marks the outer boundary of the magnetosheath relative to the solar wind.

Dynamics and significance: The magnetosheath is a site of enhanced plasma temperature, density, and magnetic activity, and it hosts processes such as magnetic reconnection and shear-driven instabilities at the magnetopause. These processes facilitate energy and plasma transfer from the solar wind into the magnetosphere, influencing space weather, auroral activity, and radiation belts.

Observations: The magnetosheath has been studied by numerous space missions (for example, satellites in Earth’s magnetosphere). Similar regions exist around planets with intrinsic magnetic fields; for planets with induced magnetospheres, a magnetosheath-like boundary layer can form as well.