The magnetosphere of a planet, as the name implies, is the region surrounding the planet in which the planetary magnetic field plays a dominant role in determining the behaviour of the medium. The inner boundary of a magnetosphere is the surface of the planet if it has no significant atmosphere (as in the case of Mercury), or the lower ionosphere in the case of planets with atmospheres. The outer boundary, usually termed the “magnetopause”, is shaped by stresses exerted by the solar wind, being blunt on the upstream side, with normal stresses playing a dominant role, and extending in a comet-like “magnetotail” on the downstream side away from the Sun largely as a result of the action of shear stresses. The characteristic size of a magnetosphere, namely the distance Lm (in planetary radii) to the subsolar point is determined approximately by balancing solar wind ram pressure and the magnetic pressure:
where n is the ion number density and Vs the speed of the solar wind, m is the average ion mass, Bo the (dipole) field strength at the surface of the planet and C is a constant of order unity.