Theory Magnetospheres In the Solar System

Magnetospheres are the result of a collision between a planet's intrinsic magnetic field and the supersonic solar wind. 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. Be them ionospheric or magnetic, magnetospheres often display a identical behavior, like, for example hot flow anomalies, or HFA. Those features may be spotted at Earth, Venus, Saturn, Mars, or Mercury, as they are very hot solar wind deflected off the bow shock -like a horn of it- and result of giant electric current sheets. HFAs come in a variety of scale sizes –from around 600 miles across at Venus to closer to 60,000 miles across at Saturn. Most important factor for determining HFA size is the geometry and size of the planet's magnetic bow shock. A better understanding of how works the outflow of oxygen ions coming from the upper atmosphere into a magnetosphere could shed light on why Mars, which has a very weak magnetic field, is losing its atmosphere, while Venus, which has no magnetic field at all, remains enshrouded in a thick atmosphere. Models suggest, in terms of the relation between a planet's axis of rotation and its magnetic axis, that there needs to be at least a slight offset between both for the planet to maintain a magnetic field. A magnetosphere generally is likely a key to sustaining life on a planet because it protect from radiations and atmosphere erosion. Charged particles and magnetic fields interact very close to gas-giant planets, generally

Ionospheric Magnetospheres

Venus, with a magnetic field 25,000 weaker than Earth's and Mars with a one 5,000 times weaker have "atmospheric" magnetospheres only. The slow rotation of Venus -despite a molten core- might be an explanation for the lack of a dynamo process as its considerable atmosphere and ionosphere act to modify the stream of particles emanating from the Sun. The ionosphere in particular creates a bowshock, preventing the particles of the solar wind from penetrating into the atmosphere. The lack of a molten core accounts for no magnetosphere 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). As the ionosphere's shape and density are partly controlled by the internal magnetic field of the planet and that Venus does not have its own internal magnetic field and relies instead on interactions with the solar wind to shape its ionosphere, the effect of a very low solar wind pressure on the ionosphere of an unmagnetised planet is that the planet’s ionosphere balloon outwards on the planet’s nightside. Astronomers generally were unsure how the strength of the solar wind affects how ionospheric plasma is transported from the dayside to the nightside of Venus. Usually, material flows along a thin channel in the ionosphere, but what happened under low solar wind conditions was unsure (did the flow of plasma particles increase as the channel widens due to the reduced confining pressure, or does it decrease because less force is available to push plasma through the channel?). The first effect indeed outweighs the second, and the ionosphere expands significantly during low solar wind density conditions. A similar effect is also expected to occur around Mars, the other non-magnetised planet in our inner Solar System. Even in times of reduced solar wind, the Sun can still significantly influence the environment of planets

At both planets, a ESA study by 2010 reveals, beams of electrically charged particles are flowing out of the planet's atmosphere as the particles are accelerated away by the interaction with the solar wind. It is that interaction which creates a weak magnetic field draping itself around the planet and stretching out, long tail-shaped behind the night side. Albeit the atmosphere of Venus is denser than the about residual one at Mars, the structures of both ionospheric magnetospheres have been deemed identical and the density of the ionosphere found similar at a altitude of 155 miles (250 km). The larger force of the solar wind at Venus, due to its larger proximity to the Sun makes that the particles are flowing away in a fluid mode as the escape of them at Mars is acted more individually. On Mars, further, the residual pockets of a magneto-induced magnetic field are helping too to funnel the atmosphere out, or the rate of espace rose by 10 times on Mars when a solar storm struck in December 2006. Venus ionosphere is a layer of the atmosphere filled with charged particles. The Venusian ionosphere is bombarded on the Sun-side of the planet by the solar wind. Consequently, the ionosphere is shaped to be a thin boundary in front of the planet and to extend into a long comet-like tail behind. As the solar wind plows into the ionosphere, it piles up like a big plasma traffic jam, creating a thin magnetosphere around Venus

->Magnetospheric Holes At Venus
When Pioneer Venus Orbiter moved into orbit around Venus by 1978, it noticed something very weird, a hole in the planet's ionosphere, a region where the density just dropped out. Nothing such was seen then in the following 30 years as the ESA Venus Express by 2014 showed that such holes are common and extend much deeper into the atmosphere. Such a phenomenon results from how Venus interacts with the solar wind with its ionospheric magnetosphere. The holes might be two long, fat cylinders of lower density material stretching from the planet's surface to way out in space and some magnetic structure probably causes the charged particles of the solar wind to be squeezed out of these areas. Magnetic field lines from the Sun move toward Venus like waves of water approaching a lighthouse at the ocean as the far sides of these lines then wrap around the planet leading to two long straight magnetic field lines trailing out directly behind Venus, yielding the magnetic forces to squeeze the plasma out of the holes. The holes however originate at both sides of Venus, as if they were coming straight up out of the surface. One model is that the magnetic fields do not stop at the edge of the ionosphere to wrap around the outside of the planet, but instead continue further, sinking right through the ionosphere down to the planet's surface and some ways into the planet; a alternate model is that magnetic fields drape themselves around the ionosphere, but they collide with a pile up of plasma already at the back of the planet. As the two sets of charged material jostle for place, it causes the required magnetic squeeze in the perfect spot. Either way, areas of increased magnetism would stream out on either side of the tail, pointing directly in and out of the sides of the planet, creating those long ionospheric holes whence the solar plasma is squeeze out

Asteroids have no magnetosphere of any kind, which allows the solar wind to interact with their surface. 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 and, the comet's coma in any case. Their magnetic field results from the solar wind interacting with the plasma generated in the coma as it is heated by the Sun. Titan, as Saturn, may also behaves much like Venus, Mars or a comet when its orbit brings it outside Saturn's magnetosphere (during 5 percent of the orbit), as without a magnetic field, it interacts with the solar wind however. Dwarf planet Pluto could also be concerned. At Moon, despite our satellite is devoided of any atmosphere, the solar wind is ionizing some atoms as it strikes the lunar surface. Such ionized atoms are creating areas of 'geological' ionization and the flow of the solar wind is disturbed by the Moon, with eddies on the side opposed to the Sun. The solar magnetic lines arriving at a celestial body may journey through it, like at the Moon, mostly made up of mantle with little to no atmosphere. The magnetic field lines go through the Moon's mantle and then hit what is thought to be an iron core. A same process also occurs at Venus

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. As far a gas giants are concerned, their magnetospheres are understood to require some degree of tilt to sustain currents flowing through the liquid metal deep inside the planets. With no tilt, the currents would eventually subside and the field disappear. 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. On a other hand, exotic magnetospheres, like at Earth, likely allows a continuous flow of solar wind into, like through, for example, the Kelvin–Helmholtz (KH) effect

It's interesting to know that Mars has remnants of a global magnetic field. Ground-level data yielded recently, by 2020, a more sensitive picture of magnetization over smaller areas as, for a given place, the magnetic field comes from nearby sources and likely from older, underground magnetized rocks. 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 and found mainly in the southern hemisphere's highlands crust. 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. Fossil magnetic field loops are seen remaining in ancient surfaces and other local areas on Mars. Mars, despite its weak magnetosphere, also has a plasma sheet, which consists of heavy ions, mainly oxygen. Mars magnetotail is a hybrid between the one of Venus and the one of the Earth as solar wind's magnetic lines reconnect with the remnant pockets of a magnetic field at Mars, and then trail behind. Such a magnetic reconnection causes further the Martian magnetotail to twist 45 degrees from what's expected based on the direction of the solar wind

A magnetosphere, generally, yields auroras. Atmopsheric chorus waves which play a role at Earth by generating pulsating auroras also have been observed at other planets. Planetary ionospheres or moon volcanism are also a cause of auroras in the solar system. Auroras usually occur on planets with powerful magnetic fields such at Jupiter or Saturn, as they can even occur on planets with no magnetic field, such as Venus and Mars. At Jupiter the aurora mostly works with particles from Io as some moons play a part at Saturn, with Mimas and Enceladus tidely locked with a persistent or intermittent bright patch, respectively. The auroras at the gas giants further are responsible for the high atmospheres of those planets being heated far beyond what could be expected by their distance to the Sun. As Earth's auroras are dominated by excited nitrogen and oxygen molecules, Saturn's auroras, for example, are dominated by excited hydrogen molecules, providing for colors red at the bottom and purple at the top. Colors of auroras can differ because of atmospheric density, the levels of the atomic version of an element versus the molecular version, and the energy of impacting electrons. A type of aurora is caused by hydrogen protons instead of electrons, proton auroras -- which is more rare but known to astronomers. Such protons are brought by the solar wind and pass through a weak magnetosphere, or, like at Mars, pass the magnetosphere's bowshock unimpeded because they turned neutral through capturing a electron from a huge hydrogen cloud surrounding the planet. Once in the planet's atmosphere neutral protons collides with gas molecules, causing the atom to emit ultraviolet light, hence a aurora not visible to the naked eye. Proton auroras at Mars, for example, are much more frequent than at Earth where they occur in limited regions near the poles for cause that our magnetosphere more efficiently divert the solar wind. However, proton auroras could be common on Venus and on Saturn's moon Titan. Like Mars, these two worlds lack their own magnetic fields, and have lots of hydrogen in their upper atmospheres

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. Our 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 ancient global magnetic field of our Moon resulted from the heat from crystallization of the lunar core as it met its end some 3 billion years ago after having been alive during 1 billion years. Crystallization came from that Moon likely had an iron/nickel core with only a small amount of sulfur and carbon, thus giving the lunar core a high melting point, as crystallization occurred early in lunar history. The lunar core is currently thought to be composed of a solid inner and liquid outer core since the Apollo mission as recent studies, by 2017, bring to that it would be partially solid and liquid. 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. On airless objects like moons and asteroids, generally, sunlight (which is the solar wind) ejects negatively charged electrons from matter, giving sunlit areas a strong positive electric charge. Areas in shadow get a strong negative charge when electrons in the solar wind rush in ahead of heavier ions to fill voids created as the solar wind flows. At a asteroid, the solar wind as it passes by is creating varied areas of interaction

check a table of the magnetospheres in the solar system