Magnetospheres In the Solar System

A magnetosphere is a comet tail-shaped region of ionized and magnetized plasma, associated with a planet or a moon, linked to the interaction of the planet with the solar wind. As Earth's magnetosphere is produced by the internal dynamo between the molten core and the surrounding mantle, another to produce a magnetosphere is the mere interaction between the planet's upper atmosphere and ionosphere and the solar wind. Hence most objects in the solar system having one of these characteristics may have a magnetosphere. Studies of magnetospheres is a way too to understand the interior of planets and moons

Ionospheric Magnetospheres

Venus, with a magnetic field 25,000 weaker than Earth's and Mars with a one 5,000 times weaker has "atmospheric" magnetospheres only. The slow rotation of Venus -despite a molten core- might be an explanation for the lack of a dynamo process and the lack of a molten core is at Mars. Mars has localized points of magnetic field which might be remnants of a former global magnetic field. Such ionospheric magnetospheres have a structure similar to usual magnetosphere: a bow shock ahead of the planet, and a tail. The solar wind pressure may or may not be balanced by the thermal pressure of the ionosphere. In both cases solar wind is eroding the atmosphere. What mass of Venus atmosphere is taking away by the solar wind is still an enigma as data have been collected at Mars but have not been reduced yet. At Mars, a complement of protection is furnished by pockets of remnant magnetic field (see below)

Comets have small atmospheric magnetospheres working on a slow pace and it might that comets' various tails are associated partly or completely to such magnetic fields

Magneto-Induced Magnetospheres

As far as magneto-induced magnetic fields are concerned, the size of the magnetosphere depends on the solar wind dynamic pressure (which is decreasing with distance) and on the strength of the planet's magnetic field (which is linked to the planet's radius and to the rotation period of the planet). Earth's magnetosphere is typical due to its average conditions in the solar system (more about Earth's magnetosphere). Mercury magnetosphere is more efficient at extracting energy from the solar wind as Jupiter has the largest magnetosphere in the solar system. This is due to Jupiter's distance from the Sun and the strength of Jupiter magnetic field: solar wind does not shape it as much as at Earth. Jupiter magnetosphere extends up to beyond Saturn's orbit! On the other hand plasma inside Jupiter's magnetosphere is fed by particles from Io and result into a torus and intense radiation belts. At Saturn, the magnetosphere is the sole of the solar system not to be tiltred relative to planet's axis as gas from Titan's atmosphere is found in it. Saturn's magnetosphere is relatively weak as it lacks radiation belts due to the rings and inner moons defining particle-free regions. Both Uranus and Neptune magnetosphere are tilted by 50° to the planet's axis as they are both offset by about 30 percent of the radius from the planet's core. The weakness of the magnetospheres there might be due to they being produced by underlying ice or water oceans near the surface and not by a molten metallic core. Pluto condition is unknown until now as the planet's slow rotation might point to an absence of magnetic field

It's interesting to know that Mars has remnants of a global magnetic field. The fluid core of Mars having ceased to work as soon as 4 billion years ago, the planet lost its global magnetosphere. NASA's Mars Global Surveyor however, found East-West, 120 mi (200 km) wide loops of remnant magnetic field reaching high above the surface at more than 250 mi (400 km). Such loops are dating back to the disparition of Mars' magnetosphere. They are sheltering pockets of ionosphere from the solar wind. A Mars atmospheric dimorphism is seen linked to these magnetic structures. Where they exist -mostly in the southern hemisphere (except above Hellas and Argyre)- a ionosphere is found, extending up to 250 mi, and even some hundreds miles above. At the contrary, above the northern hemisphere such a ionosphere is found below 250 mi (400 km) of altitude only. There where this magnetically-proteced atmosphere exists, the atmosphere is protected from the erosion by the solar wind

As far as moons are concerned, our Moon, Jupiter's Ganymede and Callisto are the sole bodies to have been found with a magnetic field and/or a magnetosphere. All magnetic fields of these bodies are of type magneto. Moon's magnetic field is mostly absent at 107 times weaker than Earth's with localized region of higher magnetism. Moon lacks of a real dynamo. Our Moon interestingly produce a plasma umbra at the opposite of where solar radiation is reaching. The early Moon is believed to have had a global magnetic field generated by a molten core. At Jupiter, Ganymede was the first moon ever to have been detected with a magnetosphere. Ganymede is solar system largest moon (the moon is larger than Mercury) and due to tidal stress on an ancient orbit, it has a molten core. Callisto dynamo is due to Jupiter magnetophere currents flowing in the moon's underlying ocean

see a solar system magnetospheres synoptic table