Magnetosphere

A rendering of themagnetic field linesof the magnetosphere of the Earth.

Inastronomyandplanetary science,amagnetosphereis a region of space surrounding anastronomical objectin whichcharged particlesare affected by that object'smagnetic field.It is created by acelestial bodywith an active interiordynamo.

In the space environment close to a planetary body with adipole magnetic fieldsuch as Earth, the field lines resemble a simplemagnetic dipole.Farther out,field linescan be significantly distorted by the flow ofelectrically conductingplasma,as emitted from the Sun (i.e., thesolar wind) or a nearby star.Planets having active magnetospheres, like the Earth, are capable of mitigating or blocking the effects ofsolar radiationorcosmic radiation;in Earth's case, this protects living organisms from harm. Interactions of particles and atmospheres with magnetospheres are studied under the specialized scientific subjects ofplasma physics,space physics,andaeronomy.

History

Main article:Magnetosphere chronology

Study of Earth's magnetosphere began in 1600, whenWilliam Gilbertdiscovered that the magnetic field on the surface of Earth resembled that of aterrella,a small, magnetized sphere. In the 1940s,Walter M. Elsasserproposed the model ofdynamo theory,which attributesEarth's magnetic fieldto the motion of Earth'sironouter core.Through the use ofmagnetometers,scientists were able to study the variations in Earth's magnetic field as functions of both time and latitude and longitude.

Beginning in the late 1940s, rockets were used to studycosmic rays.In 1958,Explorer 1,the first of the Explorer series of space missions, was launched to study the intensity of cosmic rays above the atmosphere and measure the fluctuations in this activity. This mission observed the existence of theVan Allen radiation belt(located in the inner region of Earth's magnetosphere), with the follow-upExplorer 3later that year definitively proving its existence. Also during 1958,Eugene Parkerproposed the idea of thesolar wind,with the term 'magnetosphere' being proposed byThomas Goldin 1959 to explain how the solar wind interacted with the Earth's magnetic field. The later mission of Explorer 12 in 1961 led by the Cahill and Amazeen observation in 1963 of a sudden decrease in magnetic field strength near the noon-time meridian, later was named themagnetopause.By 1983, theInternational Cometary Explorerobserved the magnetotail, or the distant magnetic field.

Structure and behavior

Magnetospheres are dependent on several variables: the type of astronomical object, the nature of sources of plasma and momentum, theperiodof the object's spin, the nature of the axis about which the object spins, the axis of the magnetic dipole, and the magnitude and direction of the flow ofsolar wind.

The planetary distance where the magnetosphere can withstand the solar wind pressure is called the Chapman–Ferraro distance. This is usefully modeled by the formula wherein$R_{\rm {P}}$represents the radius of the planet,$B_{\rm {surf}}$represents the magnetic field on the surface of the planet at the equator, and$V_{\rm {SW}}$represents the velocity of the solar wind:

$R_{\rm {CF}}=R_{\rm {P}}\left({\frac {B_{\rm {surf}}^{2}}{\mu _{0}\rho V_{\rm {SW}}^{2}}}\right)^{\frac {1}{6}}$

A magnetosphere is classified as "intrinsic" when$R_{\rm {CF}}\gg R_{\rm {P}}$,or when the primary opposition to the flow of solar wind is the magnetic field of the object.Mercury,Earth,Jupiter,Ganymede,Saturn,Uranus,andNeptune,for example, exhibit intrinsic magnetospheres. A magnetosphere is classified as "induced" when$R_{\rm {CF}}\ll R_{\rm {P}}$,or when the solar wind is not opposed by the object's magnetic field. In this case, the solar wind interacts with the atmosphere or ionosphere of the planet (or surface of the planet, if the planet has no atmosphere).Venushas an induced magnetic field, which means that because Venus appears to have nointernal dynamo effect,the only magnetic field present is that formed by the solar wind's wrapping around the physical obstacle of Venus (see alsoVenus' induced magnetosphere). When$R_{\rm {CF}}\approx R_{\rm {P}}$,the planet itself and its magnetic field both contribute. It is possible thatMarsis of this type.

Structure

An artist's rendering of the structure of a magnetosphere: 1) Bow shock. 2) Magnetosheath. 3) Magnetopause. 4) Magnetosphere. 5) Northern tail lobe. 6) Southern tail lobe. 7) Plasmasphere.

Bow shock

Infrared imageand artist's concept of the bow shock aroundR Hydrae

Main article:Bow shock

The bow shock forms the outermost layer of the magnetosphere; the boundary between the magnetosphere and the ambient medium. For stars, this is usually the boundary between thestellar windandinterstellar medium;for planets, the speed of the solar wind there decreases as it approaches the magnetopause.Due to interactions with the bow shock, thestellar windplasmagains a substantialanisotropy,leading to variousplasma instabilitiesupstream and downstream of the bow shock.

Magnetosheath

Main article:Magnetosheath

The magnetosheath is the region of the magnetosphere between the bow shock and the magnetopause. It is formed mainly from shocked solar wind, though it contains a small amount of plasma from the magnetosphere.It is an area exhibiting high particleenergy flux,where the direction and magnitude of the magnetic field varies erratically. This is caused by the collection of solar wind gas that has effectively undergonethermalization.It acts as a cushion that transmits the pressure from the flow of the solar wind and the barrier of the magnetic field from the object.

Magnetopause

Main article:Magnetopause

The magnetopause is the area of the magnetosphere wherein the pressure from the planetary magnetic field is balanced with the pressure from the solar wind.It is the convergence of the shocked solar wind from the magnetosheath with the magnetic field of the object and plasma from the magnetosphere. Because both sides of this convergence contain magnetized plasma, the interactions between them are complex. The structure of the magnetopause depends upon theMach numberandbetaof the plasma, as well as the magnetic field.The magnetopause changes size and shape as the pressure from the solar wind fluctuates.

Magnetotail

Opposite the compressed magnetic field is the magnetotail, where the magnetosphere extends far beyond the astronomical object. It contains two lobes, referred to as the northern and southern tail lobes. Magnetic field lines in the northern tail lobe point towards the object while those in the southern tail lobe point away. The tail lobes are almost empty, with few charged particles opposing the flow of the solar wind. The two lobes are separated by a plasma sheet, an area where the magnetic field is weaker, and the density of charged particles is higher.

Earth's magnetosphere

See also:Earth's magnetic field § Magnetosphere,andVan Allen radiation belt

Further information:Plasmasphere

Artist's rendition of Earth's magnetosphere

Diagram of Earth's magnetosphere

Over Earth'sequator,the magnetic field lines become almost horizontal, then return to reconnect at high latitudes. However, at high altitudes, the magnetic field is significantly distorted by the solar wind and its solar magnetic field. On the dayside of Earth, the magnetic field is significantly compressed by the solar wind to a distance of approximately 65,000 kilometers (40,000 mi). Earth's bow shock is about 17 kilometers (11 mi) thickand located about 90,000 kilometers (56,000 mi) from Earth.The magnetopause exists at a distance of several hundred kilometers above Earth's surface. Earth's magnetopause has been compared to asievebecause it allows solar wind particles to enter.Kelvin–Helmholtz instabilitiesoccur when large swirls of plasma travel along the edge of the magnetosphere at a different velocity from the magnetosphere, causing the plasma to slip past. This results inmagnetic reconnection,and as the magnetic field lines break and reconnect, solar wind particles are able to enter the magnetosphere.On Earth's nightside, the magnetic field extends in the magnetotail, which lengthwise exceeds 6,300,000 kilometers (3,900,000 mi).Earth's magnetotail is the primary source of thepolar aurora.Also, NASA scientists have suggested that Earth's magnetotail might cause "dust storms" on the Moon by creating a potential difference between the day side and the night side.

Other objects

Many astronomical objects generate and maintain magnetospheres. In the Solar System this includes the Sun,Mercury,Jupiter,Saturn,Uranus,Neptune,andGanymede.Themagnetosphere of Jupiteris the largest planetary magnetosphere in the Solar System, extending up to 7,000,000 kilometers (4,300,000 mi) on the dayside and almost to the orbit ofSaturnon the nightside.Jupiter's magnetosphere is stronger than Earth's by anorder of magnitude,and itsmagnetic momentis approximately 18,000 times larger.Venus,Mars,andPluto,on the other hand, have no magnetic field. This may have had significant effects on their geological history. It is theorized that Venus and Mars may have lost their primordial water tophotodissociationand the solar wind. A strong magnetosphere greatly slows this process.

Artist impression of the magnetic field around Tau Boötis b detected in 2020.

Magnetospheres generated byexoplanetsare thought to be common, though the first discoveries did not come until the 2010s. In 2014, a magnetic field aroundHD 209458 bwas inferred from the way hydrogen was evaporating from the planet.In 2019, the strength of the surface magnetic fields of 4hot Jupiterswere estimated and ranged between 20 and 120gausscompared to Jupiter's surface magnetic field of 4.3 gauss.In 2020, a radio emission in the 14-30 MHz band was detected from theTau Boötissystem, likely associated withcyclotron radiationfrom the poles ofTau Boötis ba signature of a planetary magnetic field.In 2021 a magnetic field generated byHAT-P-11bbecame the first to be confirmed.The first unconfirmed detection of a magnetic field generated by a terrestrial exoplanet was found in 2023 onYZ Ceti b.

See also

• Geospace
• Plasma (physics)