User:Marshallsumter/Radiation astronomy1/Magnetism

A coronal cloud is a cloud, or cloud-like, natural astronomical entity, composed of plasma and usually associated with a star or other astronomical object where the temperature is such that X-rays are emitted. While small coronal clouds are above the photosphere of many different visual spectral type stars, others occupy parts of the interstellar medium (ISM), extending sometimes millions of kilometers into space, or thousands of light-years, depending on the size of the associated object such as a galaxy.

Neutrals
"The sensors on the IBEX spacecraft are able to detect energetic neutral atoms (ENAs) at a variety of energy levels."

The satellite's payload consists of two energetic neutral atom (ENA) imagers, IBEX-Hi and IBEX-Lo. Each of these sensors consists of a collimator that limits their fields-of-view, a conversion surface to convert neutral hydrogen and oxygen into ions, an electrostatic analyzer (ESA) to suppress ultraviolet light and to select ions of a specific energy range, and a detector to count particles and identify the type of each ion.

"IBEX–Lo can detect particles with energies ranging from 10 electron–volts to 2,000 electron–volts (0.01 keV to 2 keV) in 8 separate energy bands. IBEX–Hi can detect particles with energies ranging from 300 electron–volts to 6,000 electron–volts (.3 keV to 6 keV) in 6 separate energy bands. ... Looking across the entire sky, interactions occurring at the edge of our Solar System produce ENAs at different energy levels and in different amounts, depending on the process."

Proton–hydrogen charge-exchange collisions [such as those shown at right] are often the most important process in space plasma because [h]ydrogen is the most abundant constituent of both plasmas and background gases and hydrogen charge-exchange occurs at very high velocities involving little exchange of momentum.

"Energetic neutral atoms (ENA), emitted from the magnetosphere with energies of ∼50 keV, have been measured with solid-state detectors on the IMP 7/8 and ISEE 1 spacecraft. The ENA are produced when singly charged trapped ions collide with the exospheric neutral hydrogen geocorona and the energetic ions are neutralized by charge exchange."

"The IMAGE mission ... High Energy Neutral Atom imager (HENA) ... images [ENAs] at energies between 10 and 60 keV/nucleon [to] reveal the distribution and the evolution of energetic [ions, including protons] as they are injected into the ring current during geomagnetic storms, drift about the Earth on both open and closed drift paths, and decay through charge exchange to pre‐storm levels."

"In 2009, NASA's Interstellar Boundary Explorer (IBEX) mission science team constructed the first-ever all-sky map [at right] of the interactions occurring at the edge of the solar system, where the sun's influence diminishes and interacts with the interstellar medium. A 2013 paper provides a new explanation for a giant ribbon of energetic neutral atoms – shown here in light green and blue -- streaming in from that boundary."

"[T]he boundary at the edge of our heliosphere where material streaming out from the sun interacts with the galactic material ... emits no light and no conventional telescope can see it. However, particles from inside the solar system bounce off this boundary and neutral atoms from that collision stream inward. Those particles can be observed by instruments on NASA’s Interstellar Boundary Explorer (IBEX). Since those atoms act as fingerprints for the boundary from which they came, IBEX can map that boundary in a way never before done. In 2009, IBEX saw something in that map that no one could explain: a vast ribbon dancing across this boundary that produced many more energetic neutral atoms than the surrounding areas."

""What we are learning with IBEX is that the interaction between the sun's magnetic fields and the galactic magnetic field is much more complicated than we previously thought," says Eric Christian, the mission scientist for IBEX at NASA's Goddard Space Flight Center in Greenbelt, Md. "By modifying an earlier model, this paper provides the best explanation so far for the ribbon IBEX is seeing.""

Protons
"The ‘bubble’ of solar wind plasma and frozen-in magnetic fields expanding out from the solar corona, within a few radii of the Sun, to boundaries with the local interstellar gas and plasma near about 100 AU is called the heliosphere. Dependent on points of origin at the Sun, and on time phase during the eleven year cycle of solar activity, the solar wind plasma expands radially outward at speeds of 300–800 km/s. Neutral atoms flowing into the heliosphere from the Very Local Interstellar Medium (VLISM) can be ionized by solar UV, and by charge exchange with solar wind ions, then picked up by magnetic fields in the outward plasma flow. Due to inverse-square fall-off of solar wind ion density with distance from the Sun, these interstellar pickup ions increasingly contribute to the plasma pressure and become the dominant component beyond the orbit of Saturn (Burlaga et al., 1996; Whang et al., 1996). Further out near 90–100 AU (Stone, 2001; Stone and Cummings, 2001; Whang and Burlaga, 2002) the outflowing plasma is expected to encounter the solar wind termination shock where flow speeds abruptly transition to sub-sonic values ∼100 km/s. The shock position is dependent in part on the plasma and neutral gas density in the Local Interstellar Medium (LISM) and could move into the giant planet region, or even nearer to the Earth’s orbit, if the Sun passed through a region of much higher LISM density (Zank and Frisch, 1999; Frisch, 2000). Further out at 120 AU or more should be the heliopause, the contact boundary between the diverted solar wind plasma flows and the in-flowing interstellar plasma. The intervening region between the termination shock and the heliopause is called the heliosheath. In this latter region the previously radial flow of the solar wind is diverted into a direction downstream from the ∼26 km/s flow of the interstellar gas to form a huge teardrop-shaped structure called the heliotail which extends hundreds to perhaps thousands of AU from the Sun into the VLISM."

"The initial solar wind conditions at the inner boundary at 1 AU are radial outward speed V = 441 km/s, solar wind proton density N = 7.0/cc and temperature T = 9.8 × 104 K, and interplanetary magnetic field = 7.0 × 10−5 Gauss. The interstellar hydrogen atoms at the solar wind termination shock are taken to have speed 20 km/s and temperature 1 × 104 K, while H0 density, and the energy partition ratio for ions, are varied to give good fits to radial speed and temperature profiles measured by the operational plasma spectrometer on Voyager 2. Good fits are obtained for a neutral density of 0.09/cc and a partition ratio of 0.05, which means that five percent of the total energy from the pickup process goes into solar wind protons. For the LISM plasma ions, which are not included in the Wang and Richardson model, we compute convecting maxwellian (Vasyliunas, 1971) distributions for the LISM parameters T ∼ 7000 K, u ∼ 26 km/s, and N ∼ 0.1/cc of interstellar protons as derived from Wood and Linsky (1997)."

"Near solar minimum the ACR ions, including protons, are dominant components of radiation dosage outward from ∼40 AU to the outer heliosphere, while these ions largely disappear at solar maximum. There is a 22-year cycle in the polarity of the solar dipole magnetic field, which is frozen into the solar wind plasma within several radii of the Sun and thereby carried outward into the heliosphere. Due to sign-dependent transport effects, the ACR ions accelerated at the termination shock have larger fluxes, and more positive radial gradients, at 40 to 85 AU near the Ecliptic when the solar dipole moment is directed southward (qA < 0 polarity) than when it is northward (qA > 0 polarity)."

Ultraviolets
"One of the fastest CMEs in years was captured by the STEREO COR1 telescopes on August 1, 2010. ... This CME is seen to be heading towards Earth at speeds well over 1000 kilometers per second."

"On August 1st, almost the entire Earth-facing side of the sun erupted in a tumult of activity. There was a C3-class solar flare, a solar tsunami, multiple filaments of magnetism lifting off the stellar surface, large-scale shaking of the solar corona, radio bursts, a coronal mass ejection and more. This extreme ultraviolet snapshot [at right] from the Solar Dynamics Observatory (SDO) shows the sun's northern hemisphere in mid-eruption. Different colors in the image represent different gas temperatures ranging from ~1 to 2 million degrees K."

Magnetism
Def. a mineral that is weakly attracted by the poles of a magnet but does not retain any permanent magnetism is called a paramagnet, or a paramagnetic mineral.

Def. a mineral that is weakly attracted by the poles of a magnet tending to become magnetized in a direction at 180° to the applied magnetic field but does not retain any permanent magnetism is called a diamagnet, or a diamagnetic mineral.

Def. a mineral that is exhibits the poles of a magnet even in zero applied magnetic field is called a ferromagnet, or a ferromagnetic mineral.

Def. a mineral that is exhibits the poles of a magnet even in zero applied magnetic field but with opposite directions is called a antiferromagnet, or a antiferromagnetic mineral.

Def. a mineral that is exhibits the poles of a magnet even in zero applied magnetic field with opposite directions but a net magnetic moment is called a ferrimagnet, or a ferrimagnetic mineral.

Alloys
Antitaenite is a meteoritic metal alloy mineral composed of iron and nickel, 20-40% Ni (and traces of other chemical elements) that has a face centered cubic crystal structure that exists as a new mineral species occurring in both iron meteorites and in chondrites The pair of minerals antitaenite and taenite constitute the first example in nature of two minerals that have the same crystal structure (face centered cubic) have the same chemical composition (same proportions of Fe and Ni) - but differ in their electronic structures: taenite has a high magnetic moment whereas antitaenite has a low magnetic moment. This difference arises from a high-magnetic-moment to low-magnetic-moment transition occurring in the Fe-Ni bi-metallic alloy series.

Taenite (Fe,Ni), an alloy of iron and nickel, with nickel proportions of 20% up to 65%, found naturally on Earth mostly in iron meteorites, one of four known Fe-Ni meteorite minerals: kamacite, taenite, tetrataenite, and antitaenite, that is opaque with a metallic grayish to white color, has an isometric-hexoctahedral structure, a density around 8 g/cm³ and a hardness 5 to 5.5 on the Mohs scale of mineral hardness (Mohs scale) that is magnetic, with a crystal lattice of the c≈a= 3.582 Å ± 0.002 Å. The Strunz classification is I/A.08-20, while the Dana classification is 1.1.11.2, with a hexoctahedral (cubic) structure.

Iron meteorites
As indicated in the image on the right, all or nearly all, iron meteorites are magnetic. Passage through the Earth's magnetic field and the natural electric field of the Earth may cause these iron meteorites to slow down sufficiently so as to land without a crater.

Sunquakes
"The phenomenon of flare induced sunquakes - waves in the photosphere - discovered by Kosovichev and Zharkova (1998) and now widely studied (e.g. Kosovichev 2006) should also result from the momentum impulse delivered by a cometary impact."

A Moreton wave is the chromospheric signature of a large-scale solar coronal shock wave. Described as a kind of solar 'tsunami', they are generated by solar flares.

The 1995 launch of the Solar and Heliospheric Observatory led to observation of coronal waves, which cause Moreton waves. (SOHO's EIT instrument discovered another, different wave type called 'EIT waves'.) The reality of Moreton waves (aka fast-mode MHD waves) has also been confirmed by the two STEREO spacecraft. They observed a 100,000-km-high wave of hot plasma and magnetism, moving at 250 km/second, in conjunction with a big coronal mass ejection in February 2009.

Moreton waves propagate at a speed of usually 500–1500 km/s. Yutaka Uchida interpreted Moreton waves as MHD fast mode shock waves propagating in the corona. He links them to type II radio bursts, which are radio wave discharges created when coronal mass ejections accelerate shocks.

Moreton waves can be observed primarily in the Hα band.

Coronal clouds
"Coronal clouds, type IIIg, form in space above a spot area and rain streamers upon it."

"[C]oronal magnetic bottles, produced by flares, [may] serve as temporary traps for solar cosmic rays ... It is the expansion of these bottles at velocities of 300–500 km/s which allows fast azimuthal propagation of solar cosmic rays independent of energy. A coronagraph on Os 7 observed a coronal cloud which was associated with bifurcation of the underlying coronal structure."

In a coronal cloud are magnetohydrodynamic plasma flux tubes along magnetic field lines.

Coronal heating
"The photosphere of the Sun has an effective temperature of 5,570 K yet its corona has an average temperature of 1–2 x 106 K. However, the hottest regions are 8–20 x 106 K. The high temperature of the corona shows that it is heated by something other than direct heat conduction from the photosphere.

It is thought that the energy necessary to heat the corona is provided by turbulent motion in the convection zone below the photosphere, and two main mechanisms have been proposed to explain coronal heating. The first is wave heating, in which sound, gravitational or magnetohydrodynamic waves are produced by turbulence in the convection zone. These waves travel upward and dissipate in the corona, depositing their energy in the ambient gas in the form of heat. The other is magnetic heating, in which magnetic energy is continuously built up by photospheric motion and released through magnetic reconnection in the form of large solar flares and myriad similar but smaller events—nanoflares.

Currently, it is unclear whether waves are an efficient heating mechanism. All waves except Alfvén waves have been found to dissipate or refract before reaching the corona. In addition, Alfvén waves do not easily dissipate in the corona. Current research focus has therefore shifted towards flare heating mechanisms.

Solar winds
"The sun spits out charged particles that hit our atmosphere two to four days later at 1.5-2.7 million kph, but it does not do so evenly."

"The solar wind is not continuous, it has slow and fast streams. Because the Sun rotates, these streams can be sent out behind each other - so if you have a fast solar wind catching up with a slow solar wind, it causes a concentration to occur."

"The slow phase is composed similarly to the solar corona while fast particles have a composition close to that of the photosphere, the outer layer of the sun that produces the light."

There is "a 31% increase in average lightning strikes over central England (422 to 321) in the 40 days after major solar wind events compared to the days beforehand. Lightning peaked 12-18 days after the wind's arrival. A matching increase in thundery days provided supporting evidence."

"It's unexpected, because these streams of particles bring with them an enhanced magnetic field - and this shields Earth from the very high-energy cosmic rays from outside of the Solar System.”

"The reduction in cosmic rays is only around 1%, but still noticeable. Cosmic rays emitted by supernovae are thought to trigger lighting strikes, and it was expected that the shielding effect of the solar wind would cause a reduction in lightning, rather than an increase."

Sunspot "numbers negatively correlate with thunder days in other parts of the world."

"High speed streams were found to occur after periods when the sun was putting out less light, but sunspot numbers increased. Scott and his fellow authors attribute this to the streams coming from an active region appearing on the eastern side of the sun."

"We propose that these particles, while not having sufficient energies to reach the ground and be detected there, nevertheless electrify the atmosphere as they collide with it, altering the electrical properties of the air and thus influencing the rate or intensity at which lightning occurs."

Earth
"Traces of magnetism in ancient rocks suggest that Earth had a magnetic field as far back as 4.2 billion years ago. That earlier field was likely generated by heat within the planet driving circulation within the molten core. But over time, computer simulations suggest, the heat-driven circulation wouldn’t have been strong enough alone to continue to power a strong magnetic field. Instead, the field began to shut down, signaled in the rock record by weakening intensities and rapid polarity reversals over millions of years. And then, at some point, Earth’s inner core began to crystallize, jump-starting the geodynamo and generating a new, strong magnetic field."

"Data from [seismic] waves, rotations and inertia of the whole Earth, magnetic-field dynamo theory, and laboratory experiments on melting and alloying of iron all contribute to the identification of the composition of the inner and outer core. The core is presumed to be composed principally of iron, with about 10 percent alloy of oxygen or sulfur or nickel, or perhaps some combination of these three elements."

"Radioactive potassium, uranium and thorium are thought to be the three main sources of heat in the Earth's interior, aside from that generated by the formation of the planet. Together, the heat keeps the mantle actively churning and the core generating a protective magnetic field."

"Modelers need heat, so this is one source, because the radiogenic isotope of potassium can produce heat and that can help power convection in the core and drive the magnetic field. They proved it could go in. What's important is how much is pulled out of the silicate. There's still work to be done."

"The heat generated in the core turns the iron into a convecting dynamo that maintains a magnetic field strong enough to shield the planet from the solar wind. This heat leaks out into the mantle, causing convection in the rock that moves crustal plates and fuels volcanoes."

In the case of the Earth, the magnetic field is induced and constantly maintained by the convection of liquid iron in the outer core. A requirement for the induction of field is a rotating fluid. Rotation in the outer core is supplied by the Coriolis effect caused by the rotation of the Earth. The Coriolis force tends to organize fluid motions and electric currents into columns aligned with the rotation axis. Induction or creation of magnetic field is described by the induction equation:


 * $$\frac{\partial \mathbf{B}}{\partial t} = \eta \nabla^2 \mathbf{B} + \nabla \times (\mathbf{u} \times \mathbf{B}),$$

where u is a velocity, B is the magnetic field, t is time, and $$\eta=1/\sigma\mu$$ is the magnetic diffusivity with $$\sigma$$ electrical conductivity and $$\mu$$ permeability. The ratio of the second term on the right hand side to the first term gives the Magnetic Reynolds number, a dimensionless ratio of advection of a magnetic field to diffusion.

"Of course, you need a force to move fluid towards the tangent cylinder. This could be provided by buoyancy, or perhaps more likely from changes in the magnetic field within the core."

"As the inner core cools, crystallizing iron releases impurities, sending lighter molten material into the liquid outer core. This upwelling, combined with the Earth's rotation, drives convection, forcing the molten metal into whirling vortices. These vortices stretch and twist magnetic field lines, creating Earth’s magnetic field. Currently, the center of the field, called an axis, emerges in the Arctic Ocean west of Ellesmere Island, about 300 miles (500 kilometers) from the geographic North Pole."

"The lopsided growth of the inner core makes convection in the outer core a little bit lopsided, and that then induces the geomagnetic field to have this lopsided or eccentric character too".

"Magnetic particles trapped and aligned in rocks reveal that the magnetic north pole wandered around the Western Hemisphere over the past 10,000 years, and circled the Eastern Hemisphere before that — a result mirrored by the numerical test."

"Earth’s magnetic field [...] is powered by circulation of iron-rich fluid in the core. [...] Earth’s solid inner core [may have] formed after 565 million years ago, saving a weakening magnetic field from collapse."

"We don’t have many real benchmarks for the thermal history of our planet."

"Proposed ages have been anywhere from 500 million years ago to older than 2.5 billion years."

"The interplay of the two layers drives the geodynamo, the circulation of iron-rich fluid that powers the magnetic field. That field, surrounding the planet, protects Earth from being battered by the solar wind, a constant flow of charged particles ejected by the sun. As the inner core cools and crystallizes, the composition of the remaining fluid changes; more buoyant liquid rises like a plume while the cooling crystals sink. That self-sustaining, density-driven circulation generates a strong magnetic field with two opposing poles, north and south, or polarity."

"Traces of magnetism in ancient rocks suggest that Earth had a magnetic field as far back as 4.2 billion years ago. That earlier field was likely generated by heat within the planet driving circulation within the molten core. But over time, computer simulations suggest, the heat-driven circulation wouldn’t have been strong enough alone to continue to power a strong magnetic field. Instead, the field began to shut down, signaled in the rock record by weakening intensities and rapid polarity reversals over millions of years. And then, at some point, Earth’s inner core began to crystallize, jump-starting the geodynamo and generating a new, strong magnetic field."

Magnetic "inclusions within a suite of rocks in Quebec, Canada, dating to about 565 million years ago [...] — needlelike iron-rich grains that align themselves with the orientation of the magnetic field that existed when the rocks formed — show that the planet’s magnetic field was extremely weak at that time. These paleo-intensity values were 10 times less than the present magnetic field, lower than anything observed previously. It suggested there’s something fundamental going on in the core."

"Combined with previous studies that have found that the magnetic field was also rapidly reversing polarity during that time period, the new result indicates that Earth’s field may have been on the point of collapse about 565 million years ago. That suggests that the inner core hadn’t yet solidified."

"Because the rocks bearing the magnetic grains didn’t cool instantaneously but over a long time, the data represent an average field intensity for about a 100,000-year period. [A] true, persistent signal [was found]. Computer simulations have suggested that the weak field phase may have lasted much longer, from about 900 million to 600 million years ago."

If "the core is cooling quickly, that means it was very hot in the recent past, and that the lower mantle was very hot in the recent past — so hot that both were molten just 1 billion to 2 billion years ago. We absolutely do not see that in the rock record."

Palaeointensity "data from the Ediacaran (~565 million years old) Sept-Îles intrusive suite measured on single plagioclase and clinopyroxene crystals that hosted single-domain magnetic inclusions [indicates] a time-averaged dipole moment of ~0.7 × 1022 A m2, the lowest value yet reported for the geodynamo from extant rocks and more than ten times smaller than the strength of the present-day field."

"Palaeomagnetic directional studies of these crystals define two polarities with an unusually high angular dispersion (S = ~26°) at a low latitude. Together with 14 other directional data sets that suggest a hyper-reversal frequency, these extraordinary low field strengths suggest an anomalous field behaviour, consistent with predictions of geodynamo simulations, high thermal conductivities and an Ediacaran onset age of inner core growth."

Magnetic field reversals
The graph in the center shows a comparison of the observed magnetic profile for the seafloor across the East Pacific Rise against a profile calculated from the Earth's known magnetic reversals, assuming a constant rate of spreading.

Theoretical magnetic field reversals
On the right is an image of a model for the formation of magnetic striping on Earth.

Here's a theoretical definition:

Def. a polarity reversal of the global magnetic field of an astronomical object or body is called a magnetic field reversal.

Remanent magnetization
The movement of electrical charges produces a magnetic field, where the intense currents of a lightning discharge create a fleeting but very strong magnetic field and the lightning current path passes through rock, soil, or metal these materials can become permanently magnetized, known as lightning-induced remanent magnetism, or LIRM which follow the least resistive path, often horizontally near the surface but sometimes vertically, where faults, ore bodies, or ground water offers a less resistive path. One theory suggests that lodestones, natural magnets encountered in ancient times, were created in this manner.

Lightning-induced magnetic anomalies can be mapped in the ground, and analysis of magnetized materials can confirm lightning was the source of the magnetization and provide an estimate of the peak current of the lightning discharge.

Magnetic fields generated by plasma may induce hallucinations in subjects located within 200 meters of a severe lightning storm.

Auroras
Auroras can be caused by electrons being absorbed into an atmosphere.

The "dramatic panorama [on the right shows a colorful], shimmering auroral curtain reflected in a placid Icelandic lake. The image was taken on 18 March 2015 by Carlos Gauna, near Jökulsárlón Glacier Lagoon in southern Iceland."

"The celestial display was generated by a coronal mass ejection, or CME, on 15 March. Sweeping across the inner Solar System at some 3 million km per hour, the eruption reached Earth, 150 million kilometres away, in only two days. The gaseous cloud collided with Earth’s magnetic field at around 04:30 GMT on 17 March."

"When the charged particles from the Sun penetrate Earth's magnetic shield, they are channelled downwards along the magnetic field lines until they strike atoms of gas high in the atmosphere. Like a giant fluorescent neon lamp, the interaction with excited oxygen atoms generates a green or, more rarely, red glow in the night sky, while excited nitrogen atoms yield blue and purple colours."

"Auroral displays are not just decorative distractions. They are most frequent when the Sun's activity nears its peak roughly every 11 years. At such times, the inflow of high-energy particles and the buffeting of Earth’s magnetic field may sometimes cause power blackouts, disruption of radio communications, damage to satellites and even threaten astronaut safety."

Jupiter
In the image at right is a diagram describing interaction with the local magnetic field. Jupiter's strong, rapidly rotating magnetic field (light blue lines in the figure) generates strong electric fields in the space around the planet. Charged particles (white dots), "trapped in Jupiter's magnetic field, are continually being accelerated (gold particles) down into the atmosphere above the polar regions, so auroras are almost always active on Jupiter. Electric voltages of about 10 million volts, and currents of 10 million amps - a hundred times greater than the most powerful lightning bolts - are required to explain the auroras at Jupiter's poles, which are a thousand times more powerful than those on Earth. On Earth, auroras are triggered by solar storms of energetic particles, which disturb Earth's magnetic field. As shown by the swept-back appearance in the illustration, gusts of particles from the Sun also distort Jupiter's magnetic field, and on occasion produce auroras."

Supporting the idea of water clouds are the flashes of lightning detected in the atmosphere of Jupiter that can be up to a thousand times as powerful as lightning on Earth. The water clouds are assumed to generate thunderstorms in the same way as terrestrial thunderstorms, driven by the heat rising from the interior.

As the spacecraft passed behind the planet, it observed flashes of lightning in the night side atmosphere.

Saturn
Starting in early 2005, scientists used Cassini to track lightning on Saturn. The power of the lightning is approximately 1,000 times that of lightning on Earth.

The Radio and Plasma Wave Science instrument (RPWS) on Cassini-Huygens studied the configuration of Saturn's magnetic field and its relationship to Saturn Kilometric Radiation (SKR), as well as monitoring and mapping Saturn's ionosphere, plasma, and lightning from Saturn's (and possibly Titan's) atmosphere.

Local Interstellar Mediums
In 2009, Voyager 2 data suggested that the magnetic strength of the local interstellar medium was much stronger than expected (370 to 550 picotesla (pT), against previous estimates of 180 to 250 pT). The fact that the Local Interstellar Cloud is strongly magnetized could explain its continued existence despite the pressures exerted upon it by the winds that blew out the Local Bubble.

White dwarfs
"White dwarfs are end-products of stellar evolution. The fundamental properties of the dominant group of nonmagnetic white dwarfs have been invaluable in constraining the theory of single star evolution." Of the 2551 white dwarf stars from the full spectroscopic white dwarf and hot subdwarf sample within the Sloan Digital Sky Survey (SDSS) first data release, DR1, 1888 are non-magnetic DA types and 171, non-magnetic DBs. "White dwarfs are the most readily studied of the end products of stellar evolution. Investigations of white dwarfs have generally focused on the dominant group of the nonmagnetic variety for which realistic model atmospheres can be constructed and stellar parameters deduced." "White dwarfs are intensively studied end products of stellar evolution. However, investigations of white dwarfs have generally focused on the dominant group of nonmagnetic stars for which realistic model atmospheres can be constructed and fundamental properties, such as their masses or interior chemical composition can be determined."

Satellites
"Three [Swarm] satellites of the European Space Agency (ESA) have measured the magnetic field of Earth more precisely than ever before."