Facts about Mars' magnetic field
Unlike the Earth, Mars has no inner dynamo to create a major global magnetic field. This, however, does not mean that Mars does not have a magnetosphere; simply that it is less extensive than that of the Earth.
The magnetosphere of Mars is far simpler and less extensive than that of the Earth. A magnetosphere is a kind of shield that prevents charged particles from reaching the planet surface. Since the particles borne by the solar wind through the Solar System are typically electrically charged, the magnetosphere acts as a protective shield against the solar wind.
In addition to particles, the solar wind carries magnetic field lines from the Sun. As magnetic field lines cannot pass through electrically conductive objects (such as Mars), they drape themselves around the planet creating a magnetosphere, even if the planet does not necessarily have a global magnetic field.
The outer boundary of the planet’s magnetic field is called ‘bow shock’. Bow shocks are similar to the waves that form at the bow of a ship, and occur around all magnetic planets.
Behind the bow shock is the MPB (Magnetic Pileup Boundary) which marks the outer boundary of the area where electrical particles are stored and where the magnetic field is therefore extremely strong. Below the MPB is the ionosphere or PEB (Photo-Electron Boundary), which is the boundary separating the particles travelling from the planet from those borne by the solar wind.
The ionosphere of Mars is not exactly the same as that of the Earth, and there is therefore some disagreement as to which term to use, but simply put it is understood to mean the outer region of the planet’s atmosphere.
Despite the fact that Mars no longer has an internal dynamo capable of generating a large global magnetic field as on Earth, there is evidence to suggest that Mars may once have had such a dynamo. This is mainly supported by observations from the American satellite mission MGS (Mars Global Surveyor), which from 1997 to 2006 measured the magnetic field of Mars using a small magnetometer from an altitude of 100-400 km above the planet’s surface. These measurements showed the existence of powerful magnetic crustal fields on the planet’s surface, far more powerful than those found on Earth.
The presence of these crustal fields gives rise to local mini-magnetospheres, i.e. small areas where the lines of the magnetic field locally protect the planet surface from electrically charged particles. Mini-magnetospheres occur when a magnetic field line is connected to two different points on the Martian surface, thus creating a kind of bubble. Between these ‘bubbles’, one end of the magnetic field lines can be connected to the planet and the other to the interplanetary magnetic field (IMF).
If these mini-magnetospheres are sufficiently powerful and reach high enough above the Martian surface, they can disturb the boundaries within the planet’s global magnetic field, thus affecting the interaction between the solar wind and the atmosphere, and this may help to protect what is left of Mars’ weak atmosphere.