Study of Earth's magnetosphere began in 1600, whenWilliam Gilbert discovered that the magnetic field on the surface of Earth resembled that of aterrella, a small, magnetized sphere. In the 1940s,Walter M. Elsasser proposed the model ofdynamo theory, which attributesEarth's magnetic field to 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 3 later that year definitively proving its existence. Also during 1958,Eugene Parker proposed the idea of thesolar wind, with the term 'magnetosphere' being proposed byThomas Gold in 1959 to explain how the solar wind interacted with the Earth's magnetic field. The later mission ofExplorer 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 Explorer observed themagnetotail, or the distant magnetic field.[4]
The structure of magnetospheres are dependent on several factors: the type of astronomical object, the nature of sources ofplasma andmomentum, theperiod of 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 represents the radius of the planet, represents the magnetic field on the surface of the planet at the equator, represents thevelocity of the solar wind, is the particle density of solar wind, and is thevacuum permeability constant:
A magnetosphere is classified as "intrinsic" when, 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, 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).Venus has 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, the planet itself and its magnetic field both contribute. It is possible thatMars is of this type.[6]
The bow shock forms the outermost layer of the magnetosphere; the boundary between the magnetosphere and the surrounding medium. For stars, this is usually the boundary between thestellar wind andinterstellar medium; for planets, the speed of the solar wind there decreases as it approaches the magnetopause.[7] Due to interactions with the bow shock, thestellar windplasma gains a substantialanisotropy, leading to variousplasma instabilities upstream and downstream of the bow shock.[8]
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.[9] 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.[4]
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.[3] 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 number andbeta ratio of the plasma, as well as the magnetic field.[10] The magnetopause changes size and shape as the pressure from the solar wind fluctuates.[11]
Opposite the compressed magnetic field is the magnetotail, where the magnetosphere extends far beyond the astronomical object. It contains two lobes, referred to as thenorthern andsouthern 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.[12]
Artist's rendition of Earth's magnetosphereDiagram 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) thick[13] and located about 90,000 kilometers (56,000 mi) from Earth.[14] The magnetopause exists at a distance of several hundred kilometers above Earth's surface. Earth's magnetopause has been compared to asieve because it allows solar wind particles to enter.Kelvin–Helmholtz instabilities occur when large swirls of plasma travel along the edge of the magnetosphere at different velocities 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.[15] On Earth's nightside, the magnetic field extends in the magnetotail, which lengthwise exceeds 6,300,000 kilometers (3,900,000 mi).[3] Earth's magnetotail is the primary source of thepolar aurora.[12] 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.[16]
Many astronomical objects generate and maintain magnetospheres. In the Solar System this includes the Sun,Mercury,Earth,Jupiter,Saturn,Uranus,Neptune,[17] andGanymede. Themagnetosphere of Jupiter is 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 ofSaturn on the nightside.[18] Jupiter's magnetosphere is stronger than Earth's by anorder of magnitude, and itsmagnetic moment is approximately 18,000 times larger.[19]Venus,Mars, andPluto, on the other hand, have nointrinsic magnetic field. This may have had significant effects on their geological history. It is hypothesized that Venus and Mars may have lost their primordial water tophotodissociation and the solar wind. A strong magnetosphere, were it present, would greatly slow down this process.[17][20]
Artist impression of the magnetic field around Tau Boötis b detected in 2020.
Magnetospheres generated byexoplanets are thought to be common, though the first discoveries did not come until the 2010s. In 2014, a magnetic field aroundHD 209458 b was inferred from the wayhydrogen was evaporating from the planet.[21][22] In 2019, the strength of the surface magnetic fields of 4hot Jupiters were estimated and ranged between 20 and 120gauss compared to Jupiter's surface magnetic field of 4.3 gauss.[23][24] In 2020, a radio emission in the 14-30 MHz band was detected from theTau Boötis system, likely associated withcyclotron radiation from the poles ofTau Boötis b which might be a signature of a planetary magnetic field.[25][26] In 2021 a magnetic field generated by thehot NeptuneHAT-P-11b became the first to be confirmed.[27] The first unconfirmed detection of a magnetic field generated by a terrestrial exoplanet was found in 2023 onYZ Ceti b.[28][29][30][31]
^Stern, David P.; Peredo, Mauricio (20 November 2003)."The Magnetopause".The Exploration of the Earth's Magnetosphere. NASA. Archived fromthe original on 19 August 2019. Retrieved19 August 2019.
^HAT-P-11 Spectral Energy Distribution Signatures of Strong Magnetization and Metal-poor Atmosphere for a Neptune-Size Exoplanet, Ben-Jaffel et al. 2021
^Trigilio, Corrado; Biswas, Ayan; et al. (May 2023). "Star-Planet Interaction at radio wavelengths in YZ Ceti: Inferring planetary magnetic field".arXiv:2305.00809 [astro-ph.EP].
