Magnetosphere

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A rendering of the magnetic field lines of the magnetosphere of the Earth.

In astronomy and planetary science, a magnetosphere is a region of space surrounding an astronomical object in which charged particles are affected by that object's magnetic field. [1] [2] It is created by a celestial body with an active interior dynamo.

Contents

In the space environment close to a planetary body with a dipole magnetic field such as Earth, the field lines resemble a simple magnetic dipole. Farther out, field lines can be significantly distorted by the flow of electrically conducting plasma, as emitted from the Sun (i.e., the solar wind) or a nearby star. [3] [4] Planets having active magnetospheres, like the Earth, are capable of mitigating or blocking the effects of solar radiation or cosmic 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 of plasma physics, space physics, and aeronomy.

History

Study of Earth's magnetosphere began in 1600, when William Gilbert discovered that the magnetic field on the surface of Earth resembled that of a terrella, a small, magnetized sphere. In the 1940s, Walter M. Elsasser proposed the model of dynamo theory, which attributes Earth's magnetic field to the motion of Earth's iron outer core. Through the use of magnetometers, 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 study cosmic 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 the Van Allen radiation belt (located in the inner region of Earth's magnetosphere), with the follow-up Explorer 3 later that year definitively proving its existence. Also during 1958, Eugene Parker proposed the idea of the solar wind, with the term 'magnetosphere' being proposed by Thomas Gold in 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 the magnetopause. By 1983, the International Cometary Explorer observed the magnetotail, or the distant magnetic field. [4]

Structure and behavior

Magnetospheres are dependent on several variables: the type of astronomical object, the nature of sources of plasma and momentum, the period 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 of solar 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, and represents the velocity of the solar wind:

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, and Neptune, 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 no internal dynamo effect, the only magnetic field present is that formed by the solar wind's wrapping around the physical obstacle of Venus (see also Venus' induced magnetosphere). When , the planet itself and its magnetic field both contribute. It is possible that Mars is of this type. [5]

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. Magnetosphere Levels.svg
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 image and artist's concept of the bow shock around R Hydrae Red Giant Plunging Through Space.jpg
Infrared image and artist's concept of the bow shock around R Hydrae

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 the stellar wind and interstellar medium; for planets, the speed of the solar wind there decreases as it approaches the magnetopause. [6] Due to interactions with the bow shock, the stellar wind plasma gains a substantial anisotropy, leading to various plasma instabilities upstream and downstream of the bow shock. [7]

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. [8] It is an area exhibiting high particle energy 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 undergone thermalization. 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]

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. [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 the Mach number and beta of the plasma, as well as the magnetic field. [9] The magnetopause changes size and shape as the pressure from the solar wind fluctuates. [10]

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. [11]

Earth's magnetosphere

Artist's rendition of Earth's magnetosphere Magnetosphere rendition.jpg
Artist's rendition of Earth's magnetosphere
Diagram of Earth's magnetosphere Structure of the magnetosphere LanguageSwitch.svg
Diagram of Earth's magnetosphere

Over Earth's equator, 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 [12] and located about 90,000 kilometers (56,000 mi) from Earth. [13] The magnetopause exists at a distance of several hundred kilometers above Earth's surface. Earth's magnetopause has been compared to a sieve 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 a different velocity from the magnetosphere, causing the plasma to slip past. This results in magnetic reconnection, and as the magnetic field lines break and reconnect, solar wind particles are able to enter the magnetosphere. [14] 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 the polar aurora. [11] 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. [15]

Other objects

Many astronomical objects generate and maintain magnetospheres. In the Solar System this includes the Sun, Mercury, Earth, Jupiter, Saturn, Uranus, Neptune, [16] and Ganymede. The magnetosphere 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 of Saturn on the nightside. [17] Jupiter's magnetosphere is stronger than Earth's by an order of magnitude, and its magnetic moment is approximately 18,000 times larger. [18] Venus, Mars, and Pluto, 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 to photodissociation and the solar wind. A strong magnetosphere greatly slows this process. [16] [19]

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

Magnetospheres generated by exoplanets are thought to be common, though the first discoveries did not come until the 2010s. In 2014, a magnetic field around HD 209458 b was inferred from the way hydrogen was evaporating from the planet. [20] [21] In 2019, the strength of the surface magnetic fields of 4 hot Jupiters were estimated and ranged between 20 and 120 gauss compared to Jupiter's surface magnetic field of 4.3 gauss. [22] [23] In 2020, a radio emission in the 14-30 MHz band was detected from the Tau Boötis system, likely associated with cyclotron radiation from the poles of Tau Boötis b a signature of a planetary magnetic field. [24] [25] In 2021 a magnetic field generated by HAT-P-11b became the first to be confirmed. [26] The first unconfirmed detection of a magnetic field generated by a terrestrial exoplanet was found in 2023 on YZ Ceti b. [27] [28] [29] [30]

See also

Related Research Articles

<span class="mw-page-title-main">Magnetopause</span> Abrupt boundary between a magnetosphere and the surrounding plasma

The magnetopause is the abrupt boundary between a magnetosphere and the surrounding plasma. For planetary science, the magnetopause is the boundary between the planet's magnetic field and the solar wind. The location of the magnetopause is determined by the balance between the pressure of the dynamic planetary magnetic field and the dynamic pressure of the solar wind. As the solar wind pressure increases and decreases, the magnetopause moves inward and outward in response. Waves along the magnetopause move in the direction of the solar wind flow in response to small-scale variations in the solar wind pressure and to Kelvin–Helmholtz instability.

<span class="mw-page-title-main">Solar wind</span> Stream of charged particles from the Sun

The solar wind is a stream of charged particles released from the Sun's outermost atmospheric layer, the corona. This plasma mostly consists of electrons, protons and alpha particles with kinetic energy between 0.5 and 10 keV. The composition of the solar wind plasma also includes a mixture of particle species found in the solar plasma: trace amounts of heavy ions and atomic nuclei of elements such as carbon, nitrogen, oxygen, neon, magnesium, silicon, sulfur, and iron. There are also rarer traces of some other nuclei and isotopes such as phosphorus, titanium, chromium, and nickel's isotopes 58Ni, 60Ni, and 62Ni. Superimposed with the solar-wind plasma is the interplanetary magnetic field. The solar wind varies in density, temperature and speed over time and over solar latitude and longitude. Its particles can escape the Sun's gravity because of their high energy resulting from the high temperature of the corona, which in turn is a result of the coronal magnetic field. The boundary separating the corona from the solar wind is called the Alfvén surface.

<span class="mw-page-title-main">Magnetic sail</span> Proposed spacecraft propulsion method

A magnetic sail is a proposed method of spacecraft propulsion where an onboard magnetic field source interacts with a plasma wind to form an artificial magnetosphere that acts as a sail, transferring force from the wind to the spacecraft requiring little to no propellant as detailed for each proposed magnetic sail design in this article.

<span class="mw-page-title-main">Bow shock</span> Shock wave caused by blowing stellar wind

In astrophysics, bow shocks are shock waves in regions where the conditions of density and pressure change dramatically due to blowing stellar wind. Bow shock occurs when the magnetosphere of an astrophysical object interacts with the nearby flowing ambient plasma such as the solar wind. For Earth and other magnetized planets, it is the boundary at which the speed of the stellar wind abruptly drops as a result of its approach to the magnetopause. For stars, this boundary is typically the edge of the astrosphere, where the stellar wind meets the interstellar medium.

<span class="mw-page-title-main">Interplanetary medium</span> Material which fills the Solar System

The interplanetary medium (IPM) or interplanetary space consists of the mass and energy which fills the Solar System, and through which all the larger Solar System bodies, such as planets, dwarf planets, asteroids, and comets, move. The IPM stops at the heliopause, outside of which the interstellar medium begins. Before 1950, interplanetary space was widely considered to either be an empty vacuum, or consisting of "aether".

<span class="mw-page-title-main">Magnetosheath</span> Region of a magnetosphere which cannot fully deflect charged particles

The magnetosheath is the region of space between the magnetopause and the bow shock of a planet's magnetosphere. The regularly organized magnetic field generated by the planet becomes weak and irregular in the magnetosheath due to interaction with the incoming solar wind, and is incapable of fully deflecting the highly charged particles. The density of the particles in this region is considerably lower than what is found beyond the bow shock, but greater than within the magnetopause, and can be considered a transitory state.

<span class="mw-page-title-main">Magnetosphere of Saturn</span> Cavity in the solar wind the sixth planet creates

The magnetosphere of Saturn is the cavity created in the flow of the solar wind by the planet's internally generated magnetic field. Discovered in 1979 by the Pioneer 11 spacecraft, Saturn's magnetosphere is the second largest of any planet in the Solar System after Jupiter. The magnetopause, the boundary between Saturn's magnetosphere and the solar wind, is located at a distance of about 20 Saturn radii from the planet's center, while its magnetotail stretches hundreds of Saturn radii behind it.

<span class="mw-page-title-main">Cluster II (spacecraft)</span> European Space Agency space mission

Cluster II was a space mission of the European Space Agency, with NASA participation, to study the Earth's magnetosphere over the course of nearly two solar cycles. The mission was composed of four identical spacecraft flying in a tetrahedral formation. As a replacement for the original Cluster spacecraft which were lost in a launch failure in 1996, the four Cluster II spacecraft were successfully launched in pairs in July and August 2000 onboard two Soyuz-Fregat rockets from Baikonur, Kazakhstan. In February 2011, Cluster II celebrated 10 years of successful scientific operations in space. In February 2021, Cluster II celebrated 20 years of successful scientific operations in space. As of March 2023, its mission was extended until September 2024. The China National Space Administration/ESA Double Star mission operated alongside Cluster II from 2004 to 2007.

Atmospheric escape is the loss of planetary atmospheric gases to outer space. A number of different mechanisms can be responsible for atmospheric escape; these processes can be divided into thermal escape, non-thermal escape, and impact erosion. The relative importance of each loss process depends on the planet's escape velocity, its atmosphere composition, and its distance from its star. Escape occurs when molecular kinetic energy overcomes gravitational energy; in other words, a molecule can escape when it is moving faster than the escape velocity of its planet. Categorizing the rate of atmospheric escape in exoplanets is necessary to determining whether an atmosphere persists, and so the exoplanet's habitability and likelihood of life.

<span class="mw-page-title-main">Heliophysics</span> Science of the heliosphere

Heliophysics is the physics of the Sun and its connection with the Solar System. NASA defines heliophysics as "(1) the comprehensive new term for the science of the Sun - Solar System Connection, (2) the exploration, discovery, and understanding of Earth's space environment, and (3) the system science that unites all of the linked phenomena in the region of the cosmos influenced by a star like our Sun."

<span class="mw-page-title-main">Geotail</span> NASA/ISAS spacecraft

Geotail was a satellite that observed the Earth's magnetosphere. It was developed by Japan's ISAS in association with the United States' NASA, and was launched by a Delta II rocket on 24 July 1992 from Cape Canaveral Air Force Station.

<span class="mw-page-title-main">Magnetosphere of Jupiter</span> Cavity created in the solar wind

The magnetosphere of Jupiter is the cavity created in the solar wind by Jupiter's magnetic field. Extending up to seven million kilometers in the Sun's direction and almost to the orbit of Saturn in the opposite direction, Jupiter's magnetosphere is the largest and most powerful of any planetary magnetosphere in the Solar System, and by volume the largest known continuous structure in the Solar System after the heliosphere. Wider and flatter than the Earth's magnetosphere, Jupiter's is stronger by an order of magnitude, while its magnetic moment is roughly 18,000 times larger. The existence of Jupiter's magnetic field was first inferred from observations of radio emissions at the end of the 1950s and was directly observed by the Pioneer 10 spacecraft in 1973.

<span class="mw-page-title-main">Comet tail</span> Dust or gases blown off a comet by solar wind in the inner solar system, leaving a visible trail

A comet tail and coma are visible features of a comet when they are illuminated by the Sun and may become visible from Earth when a comet passes through the inner Solar System. As a comet approaches the inner Solar System, solar radiation causes the volatile materials within the comet to vaporize and stream out of the nucleus, carrying dust away with them.

<span class="mw-page-title-main">Energetic neutral atom</span> Technology to create global images of otherwise invisible phenomena

Energetic Neutral Atom (ENA) imaging is a technology used to create global images of otherwise invisible phenomena in the magnetospheres of planets and throughout the heliosphere.

The Arctowski Medal is awarded by the U.S. National Academy of Sciences "for studies in solar physics and solar-terrestrial relationships." Named in honor of Henryk Arctowski, it was first awarded in 1969.

<span class="mw-page-title-main">Mercury's magnetic field</span> Mercurys small magnetic field

Mercury's magnetic field is approximately a magnetic dipole apparently global, on planet Mercury. Data from Mariner 10 led to its discovery in 1974; the spacecraft measured the field's strength as 1.1% that of Earth's magnetic field. The origin of the magnetic field can be explained by dynamo theory. The magnetic field is strong enough near the bow shock to slow the solar wind, which induces a magnetosphere.

Waves (<i>Juno</i>) Experiment on the Juno spacecraft to study radio and plasma waves

Waves is an experiment on the Juno spacecraft for studying radio and plasma waves. It is part of a collection of various types of instruments and experiments on the spacecraft; Waves is oriented towards understanding fields and particles in the Jupiter's magnetosphere. Waves is on board the uncrewed Juno spacecraft, which was launched in 2011 and arrived at Jupiter in the summer of 2016. The major focus of study for Waves is Jupiter's magnetosphere, which if could be seen from Earth would be about twice the size of a full moon. The magnetosphere has a tear drop shape, and that tail extends away from the Sun by at least 5 AU. The Waves instrument is designed to help understand the interaction between Jupiter's atmosphere, its magnetic field, its magnetosphere, and to understand Jupiter's auroras. It is designed to detect radio frequencies from 50 Hz up to 40,000,000 Hz (40 MHz), and magnetic fields from 50 Hz to 20,000 Hz (20 kHz). It has two main sensors: a dipole antenna and a magnetic search coil. The dipole antenna has two whip antennas that extend 2.8 meters and are attached to the main body of the spacecraft. This sensor has been compared to a rabbit-ear TV antenna. The search coil is overall a Mu-metal rod 15 cm (6 in) in length with a fine copper wire wound 10,000 times around it. There are also two frequency receivers that each cover certain bands. Data handling is done by two radiation-hardened systems on a chip. The data handling units are located inside the Juno Radiation Vault. Waves is allocated 410 Mbits of data per science orbit.

The Solar-Terrestrial Observer for the Response of the Magnetosphere (STORM) was one of five mission proposals selected to proceed to Phase A concept studies as part of the 2019 NASA Heliophysics Medium Class Explorer Announcement of Opportunity. STORM will provide the first-ever global view of the Sun-Earth system. STORM takes simultaneous observations of the solar wind and the response of Earth’s magnetosphere, including the magnetopause, auroral oval, and ring current dynamics, using global multi-spectral and neutral atom imaging to quantify the global circulation of the energy that powers space weather.

<span class="mw-page-title-main">Dungey Cycle</span>

The Dungey cycle, officially proposed by James Dungey in 1961, is a phenomenon that explains interactions between a planet's magnetosphere and solar wind. Dungey originally proposed a cyclic behavior of magnetic reconnection between Earth's magnetosphere and flux of solar wind. This reconnection explained previously observed dynamics within Earth's magnetosphere. The rate of reconnection in the beginning of the cycle is dependent on the orientation of the interplanetary magnetic field as well as the resultant plasma conditions at the site of reconnection. On Earth, the reconnection cycle takes around 1 hour, but this differs from planet to planet.

<span class="mw-page-title-main">James Dungey</span> British space scientist

James Wynne Dungey (1923–2015) was a British space scientist who was pivotal in establishing the field of space weather and made significant contributions to the fundamental understanding of plasma physics.

References

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