Astronomy

Magnetars and the Dynamo Effect

Magnetars and the Dynamo Effect


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How do magnetars create/sustain such strong magnetic fields? If a dynamo effect did create a magnetar, then what creates the magnetic field? The dynamo effect requires an electrically conductive fluid, but magnetars are purely composed of neutrons. In Earth, the dynamo effect is supported with evidence, but how does it apply to magnetars?


Magnetars (and neutron stars in general) don't need a dynamo to create their magnetic fields. Their magnetic fields are "frozen in" at the time of their formation. To really see why this is, you have to understand a lot about electromagnetism, but I can boil it down to the basics. Keep in mind that neutron stars are highly mysterious objects and we don't have a huge amount of good, observational evidence behind our theories since they're so hard to find and observe.

Neutron stars (and magnetars) are superconductors (or at least most theory and objective evidence suggests that is the case). What this means is that there is literally no resistance to the motion of electric charges moving throughout the star. To point out, even if the entire star was all neutrons (which zibadawa timmy points out is not the case) you'd still have charges since neutrons themselves are composed of charged particles. In any case, since there's no resistance, there cannot exist any electric fields inside the star. Any field which arose would induce a force on the charged particles which would be able to immediately move to cancel said field out. All electric fields destroy themselves immediately in a superconductor.

If we venture into electromagnetic theory equations, you'll find the very useful equation:

$$ abla imes extbf{E} = -frac{partial extbf{B}}{partial t}$$

Without getting into the details of this equation, the general idea behind it is that electric fields result in time varying magnetic fields and vice versa. But we just said above that our neutron star was superconducting and thus has no electric fields. This means the left hand side of that equation is zero. The right side represents the change in magnetic field over time, but we know that has to be zero now so we're forced to conclude magnetic fields in neutron stars (and magnetars) cannot change. They're frozen in at the point of creation (when the star becomes superconducting).

We can venture one step further and say that the magnetic fields of neutron stars and magnetars is so strong simply because of magnetic flux conservation. That is, you have a huge star of a few solar masses which has a massive field in its own right. That star then collapses into a neutron star but in the process, the magnetic field flux through its surface has to stay conserved. Magnetic field flux is a function of both the field strength and the radius of the star. Since the radius is decreasing so immensely, the magnetic field strength has to increase in proportion to the radius decrease (squared) to compensate and keep the overall flux the same, at which point that magnetic field is locked into place due to the superconducting nature of the star.


There are, in general, two classes of explanations for neutron star magnetic fields: fossilized magnetic fields and active magnetic fields (see here for an early overview on some of the internal battery models).

The "fossilized" field theory - which is well-accepted, as far as I know - states that neutron star magnetic fields are leftovers from the magnetic fields of the progenitor stars. This seems plausible, and some (e.g. Spruit (2008)) have suggested that the supernova that formed the neutron stars may have endowed them with exceptionally strong fields during core collapse, leaving behind magnetars. This is what zephyr means by "frozen in": the magnetic fields remains the same after the star becomes a neutron star.

The "active" theories - and I'm using "active" as a non-technical term - posit that neutron stars continue to generate magnetic fields. This makes it possible for the magnetic fields to grow in strength, which can explain why magnetars have exceptionally strong fields; fossilized fields may not be sufficient to explain this in all cases. There have been several suggestions over the years for changes in magnetic fields, some of which are no longer accepted but some of which are still possible:

  • The battery model. This was originally proposed as a mechanism for generating magnetic fields in normal stars. It holds that electrons inside a star drift outward slightly relative to the ions, due to different effects from the gravitational field and any centrifugal force. The resulting pressures cause the electrons to move in ways similar to a battery, which generates a magnetic field.

    As zephyr mentioned, the important equation is $$-frac{partialmathbf{B}}{partial t}= abla imesmathbf{E}$$ In the battery model, $ abla imesmathbf{E}$ turns out to be non-zero, meaning that the magnetic field can, in fact, change, thanks to thermal effects. I believe, however, that the battery model has been ruled out for neutron stars. Taking degeneracy pressure into account does, in fact, lead to a vanishing $ abla imesmathbf{E}$, and therefore there is no time-varying field.

  • The thermoelectric mechanism. This is actually a variation on the pure battery model, and is applicable only in the neutron star's crust. If there is a non-zero radial heat gradient and a small magnetic field, electrons will create a small pressure gradient, which in turn causes an opposing thermoelectric field to arise - which leads to a non-zero $ abla imesmathbf{E}$! The precise equation is $$frac{partialmathbf{B}}{partial t}=overbrace{ abla imesleft(mathbf{V} imesmathbf{B} ight)}^{ ext{Field convection term}}-overbrace{ abla Q_0 imes abla T}^{ ext{Battery term}}-overbrace{ abla imesleft[frac{ abla imesmathbf{B}}{4pisigma_0} ight]}^{ ext{Ohmic decay term}}$$ The thermoelectric model is much better than the traditional battery model, and does allow for the magnetic field to grow.
  • Accretion from a companion. This is a slightly newer idea, which has gained traction after observations of binary systems (see a discussion by Bhattacharya (1999)). Matter from a companion star follows the neutron star's pre-existing magnetic field lines. Pressure causes the matter to "drag" the field lines along the neutron star's surface until magnetic reconnection occurs, "screening" the field underneath the accreted matter. This actually weakens the field, making it decay over time - which does match observations of some neutron stars. However, instabilities make it a difficult possibility.

When William Gilbert published de Magnete in 1600, he concluded that the Earth is magnetic and proposed the first hypothesis for the origin of this magnetism: permanent magnetism such as that found in lodestone. In 1919, Joseph Larmor proposed that a dynamo might be generating the field. [2] [3] However, even after he advanced his hypothesis, some prominent scientists advanced alternative explanations. Einstein believed that there might be an asymmetry between the charges of the electron and proton so that the Earth's magnetic field would be produced by the entire Earth. The Nobel Prize winner Patrick Blackett did a series of experiments looking for a fundamental relation between angular momentum and magnetic moment, but found none. [4] [5]

Walter M. Elsasser, considered a "father" of the presently accepted dynamo theory as an explanation of the Earth's magnetism, proposed that this magnetic field resulted from electric currents induced in the fluid outer core of the Earth. He revealed the history of the Earth's magnetic field through pioneering the study of the magnetic orientation of minerals in rocks.

In order to maintain the magnetic field against ohmic decay (which would occur for the dipole field in 20,000 years), the outer core must be convecting. The convection is likely some combination of thermal and compositional convection. The mantle controls the rate at which heat is extracted from the core. Heat sources include gravitational energy released by the compression of the core, gravitational energy released by the rejection of light elements (probably sulfur, oxygen, or silicon) at the inner core boundary as it grows, latent heat of crystallization at the inner core boundary, and radioactivity of potassium, uranium and thorium. [6]

At the dawn of the 21st century, numerical modeling of the Earth's magnetic field has not been successfully demonstrated, but appears to be in reach. Initial models are focused on field generation by convection in the planet's fluid outer core. It was possible to show the generation of a strong, Earth-like field when the model assumed a uniform core-surface temperature and exceptionally high viscosities for the core fluid. Computations which incorporated more realistic parameter values yielded magnetic fields that were less Earth-like, but also point the way to model refinements which may ultimately lead to an accurate analytic model. Slight variations in the core-surface temperature, in the range of a few millikelvins, result in significant increases in convective flow and produce more realistic magnetic fields. [7] [8]

Dynamo theory describes the process through which a rotating, convecting, and electrically conducting fluid acts to maintain a magnetic field. This theory is used to explain the presence of anomalously long-lived magnetic fields in astrophysical bodies. The conductive fluid in the geodynamo is liquid iron in the outer core, and in the solar dynamo is ionized gas at the tachocline. Dynamo theory of astrophysical bodies uses magnetohydrodynamic equations to investigate how the fluid can continuously regenerate the magnetic field. [9]

It was once believed that the dipole, which comprises much of the Earth's magnetic field and is misaligned along the rotation axis by 11.3 degrees, was caused by permanent magnetization of the materials in the earth. This means that dynamo theory was originally used to explain the Sun's magnetic field in its relationship with that of the Earth. However, this hypothesis, which was initially proposed by Joseph Larmor in 1919, has been modified due to extensive studies of magnetic secular variation, paleomagnetism (including polarity reversals), seismology, and the solar system's abundance of elements. Also, the application of the theories of Carl Friedrich Gauss to magnetic observations showed that Earth's magnetic field had an internal, rather than external, origin.

There are three requisites for a dynamo to operate:

  • An electrically conductive fluid medium
  • Kinetic energy provided by planetary rotation
  • An internal energy source to drive convective motions within the fluid. [10]

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 (also see Taylor columns) aligned with the rotation axis. Induction or creation of magnetic field is described by the induction equation:

where u is velocity, B is magnetic field, t is time, and η = 1 / ( σ μ ) is the magnetic diffusivity with σ electrical conductivity and μ 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 magnetic field to diffusion.

Tidal heating supporting a dynamo Edit

Tidal forces between celestial orbiting bodies cause friction that heats up their interiors. This is known as tidal heating, and it helps keep the interior in a liquid state. A liquid interior that can conduct electricity is required to produce a dynamo. Saturn's Enceladus and Jupiter's Io have enough tidal heating to liquify their inner cores, but they may not create a dynamo because they cannot conduct electricity. [11] [12] Mercury, despite its small size, has a magnetic field, because it has a conductive liquid core created by its iron composition and friction resulting from its highly elliptical orbit. [13] It is theorized that the Moon once had a magnetic field, based on evidence from magnetized lunar rocks, due to its short-lived closer distance to Earth creating tidal heating. [14] An orbit and rotation of a planet helps provide a liquid core, and supplements kinetic energy that supports a dynamo action.

In kinematic dynamo theory the velocity field is prescribed, instead of being a dynamic variable: The model makes no provision for the flow distorting in response to the magnetic field. This method cannot provide the time variable behaviour of a fully nonlinear chaotic dynamo, but can be used to study how magnetic field strength varies with the flow structure and speed.

Using Maxwell's equations simultaneously with the curl of Ohm's law, one can derive what is basically a linear eigenvalue equation for magnetic fields ( B ), which can be done when assuming that the magnetic field is independent from the velocity field. One arrives at a critical magnetic Reynolds number, above which the flow strength is sufficient to amplify the imposed magnetic field, and below which the magnetic field dissipates.

Practial measure of prossible dynamos Edit

The most functional feature of kinematic dynamo theory is that it can be used to test whether a velocity field is or is not capable of dynamo action. By experimentally applying a certain velocity field to a small magnetic field, one can observe whether the magnetic field tends to grow (or not) in response to the applied flow. If the magnetic field does grow, then the system is either capable of dynamo action or is a dynamo, but if the magnetic field does not grow, then it is simply referred to as “not a dynamo”.

An analogous method called the membrane paradigm is a way of looking at black holes that allows for the material near their surfaces to be expressed in the language of dynamo theory.

Spontaneous breakdown of a topological supersymmetry Edit

Kinematic dynamo can be also viewed as the phenomenon of the spontaneous breakdown of the topological supersymmetry of the associated stochastic differential equation related to the flow of the background matter. [15] Within stochastic supersymmetric theory, this supersymmetry is an intrinsic property of all stochastic differential equations, its interpretation is that the model’s phase space preserves continuity via continuous time flows. When the continuity of that flow spontaneously breaks down, the system is in the stochastic state of deterministic chaos. [16] In other words, kinematic dynamo arises because of chaotic flow in the underlying background matter.

The kinematic approximation becomes invalid when the magnetic field becomes strong enough to affect the fluid motions. In that case the velocity field becomes affected by the Lorentz force, and so the induction equation is no longer linear in the magnetic field. In most cases this leads to a quenching of the amplitude of the dynamo. Such dynamos are sometimes also referred to as hydromagnetic dynamos. [17] Virtually all dynamos in astrophysics and geophysics are hydromagnetic dynamos.

The main idea of the theory is that any small magnetic field existing in the outer core creates currents in the moving fluid there due to Lorenz force. These currents create further magnetic field due to Ampere's law. With the fluid motion, the currents are carried in a way that the magnetic field gets stronger (as long as u ⋅ ( J × B ) cdot (mathbf imes mathbf )> is negative [18] ). Thus a "seed" magnetic field can get stronger and stronger until it reaches some value that is related to existing non-magnetic forces.

Numerical models are used to simulate fully nonlinear dynamos. The following equations are used:

  • The induction equation, presented above.
  • Maxwell's equations for negligible electric field:
  • The continuity equation for conservation of mass, for which the Boussinesq approximation is often used:
  • The Navier-Stokes equation for conservation of momentum, again in the same approximation, with the magnetic force and gravitation force as the external forces:

These equations are then non-dimensionalized, introducing the non-dimensional parameters,

Energy conversion between magnetic and kinematic energy Edit

> (where one of Maxwell's equations was used). This is the local contribution to the magnetic energy due to fluid motion.

From the diagram above, it is not clear why this term should be positive. A simple argument can be based on consideration of net effects. To create the magnetic field, the net electric current must wrap around the axis of rotation of the planet. In that case, for the term to be positive, the net flow of conducting matter must be towards the axis of rotation. The diagram only shows a net flow from the poles to the equator. However mass conservation requires an additional flow from the equator toward the poles. If that flow was along the axis of rotation, that implies the circulation would be completed by a flow from the ones shown towards the axis of rotation, producing the desired effect.

Order of magnitude of the magnetic field created by Earth's dynamo Edit

Of those, the gravitational force and the centrifugal force are conservative and therefore have no overall contribution to fluid moving in closed loops. Ekman number (defined above), which is the ratio between the two remaining forces, namely the viscosity and Coriolis force, is very low inside Earth's outer core, because its viscosity is low (1.2–1.5 ×10 −2 pascal-second [19] ) due to its liquidity.

The current density J is itself the result of the magnetic field according to Ohm's law. Again, due to matter motion and current flow, this is not necessarily the field at the same place and time. However these relations can still be used to deduce orders of magnitude of the quantities in question.

The exact ratio between both sides is the square root of Elsasser number.

Note that the magnetic field direction cannot be inferred from this approximation (at least not its sign) as it appears squared, and is, indeed, sometimes reversed, though in general it lies on a similar axis to that of Ω > .

For earth outer core, ρ is approximately 10 4 kg/m 3 , [19] Ω = 2 π /day = 7.3×10 −5 /second and σ is approximately 10 7 Ω −1 m −1 . [20] This gives 2.7×10 −4 Tesla.

The magnetic field of a magnetic dipole has an inverse cubic dependence in distance, so its order of magnitude at the earth surface can be approximated by multiplying the above result with <<<1>>> giving 2.5×10 −5 Tesla, not far from to the measured value of 3×10 −5 Tesla at the equator.

Broadly, models of the geodynamo attempt to produce magnetic fields consistent with observed data given certain conditions and equations as mentioned in the sections above. Implementing the magnetohydrodynamic equations successfully was of particular significance because they pushed dynamo models to self-consistency. Though geodynamo models are especially prevalent, dynamo models are not necessarily restricted to the geodynamo solar and general dynamo models are also of interest. Studying dynamo models has utility in the field of geophysics as doing so can identify how various mechanisms form magnetic fields like those produced by astrophysical bodies like Earth and how they cause magnetic fields to exhibit certain features, such as pole reversals.

The equations used in numerical models of dynamo are highly complex. For decades, theorists were confined to two dimensional kinematic dynamo models described above, in which the fluid motion is chosen in advance and the effect on the magnetic field calculated. The progression from linear to nonlinear, three dimensional models of dynamo was largely hindered by the search for solutions to magnetohydrodynamic equations, which eliminate the need for many of the assumptions made in kinematic models and allow self-consistency.

The first self-consistent dynamo models, ones that determine both the fluid motions and the magnetic field, were developed by two groups in 1995, one in Japan [21] and one in the United States. [22] [23] The latter was made as a model with regards to the geodynamo and received significant attention because it successfully reproduced some of the characteristics of the Earth's field. [18] Following this breakthrough, there was a large swell in development of reasonable, three dimensional dynamo models. [18]

Though many self-consistent models now exist, there are significant differences among the models, both in the results they produce and the way they were developed. [18] Given the complexity of developing a geodynamo model, there are many places where discrepancies can occur such as when making assumptions involving the mechanisms that provide energy for the dynamo, when choosing values for parameters used in equations, or when normalizing equations. In spite of the many differences that may occur, most models have shared features like clear axial dipoles. In many of these models, phenomena like secular variation and geomagnetic polarity reversals have also been successfully recreated. [18]

Observations Edit

Many observations can be made from dynamo models. Models can be used to estimate how magnetic fields vary with time and can be compared to observed paleomagnetic data to find similarities between the model and the Earth. Due to the uncertainty of paleomagnetic observations, however, comparisons may not be entirely valid or useful. [18] Simplified geodynamo models have shown relationships between the dynamo number (determined by variance in rotational rates in the outer core and mirror-asymmetric convection (e.g. when convection favors one direction in the north and the other in the south)) and magnetic pole reversals as well as found similarities between the geodynamo and the Sun's dynamo. [18] In many models, it appears that magnetic fields have somewhat random magnitudes that follow a normal trend that average to zero. [18] In addition to these observations, general observations about the mechanisms powering the geodynamo can be made based on how accurately the model reflects actual data collected from Earth.

Modern modelling Edit

The complexity of dynamo modelling is so great that models of the geodynamo are limited by the current power of supercomputers, particularly because calculating the Ekman and Rayleigh number of the outer core is extremely difficult and requires a vast number of computations.

Many improvements have been proposed in dynamo modelling since the self-consistent breakthrough in 1995. One suggestion in studying the complex magnetic field changes is applying spectral methods to simplify computations. [24] Ultimately, until considerable improvements in computer power are made, the methods for computing realistic dynamo models will have to be made more efficient, so making improvements in methods for computing the model is of high importance for the advancement of numerical dynamo modelling.


A new theory of magnetar formation

Figure 1: 3D snapshots of the magnetic field lines in the convective zone inside a newborn neutron star. Inward (outward) flows are represented by the blue (red) surfaces. Left: strong field dynamo discovered for fast rotation periods of a few milliseconds, where the dipole component reaches 1015 G. Right: for slower rotation, the magnetic field is up to ten times weaker. Credit: CEA Sacley

Magnetars are neutron stars endowed with the strongest magnetic fields observed in the universe, but their origin remains controversial. In a study published in Science Advances, a team of scientists from CEA, Saclay, the Max Planck Institute for Astrophysics (MPA), and the Institut de Physique du Globe de Paris developed a new and unprecedentedly detailed computer model that can explain the genesis of these gigantic fields through the amplification of pre-existing weak fields when rapidly rotating neutron stars are born in collapsing massive stars. The work opens new avenues to understand the most powerful and most luminous explosions of such stars.

Magnetars: what are they?

Neutron stars are compact objects containing one to two solar masses within a radius of about 12 kilometers. Among them, magnetars are characterized by eruptive emission of X-rays and gamma rays. The energy associated with these bursts of intense radiation is probably related to ultra-strong magnetic fields. Magnetars should thus spin down faster than other neutron stars due to enhanced magnetic braking, and measurements of their rotation period evolution have confirmed this scenario. We thus infer that magnetars have a dipole magnetic field of the order of 10 15 Gauss (G), i.e., up to 1000 times stronger than typical neutron stars! While the existence of these tremendous magnetic fields is now well established, their origin remains controversial.

Neutron stars generally form after the collapse of the iron core of a massive star of more than nine solar masses, while the outer layers of the star are expelled into interstellar space in a gigantic explosion called a core-collapse supernova. Some theories therefore assume that neutron star and magnetar magnetic fields could be inherited from their progenitor stars, which means that the fields could be entirely determined by the magnetization of the iron core before collapse. The problem with this hypothesis is, however, that very strong magnetic fields in the stars could decelerate the rotation of the stellar core so that the neutron stars from such magnetized stars would rotate only slowly.

"This would not allow us to explain the huge energies of hypernova explosions and long-duration gamma-ray bursts, where rapidly rotating neutron stars or rapidly spinning black holes are considered as the central sources of the enormous energies," remarks team member H.-Thomas Janka of MPA. Therefore, an alternative mechanism appears more favorable, in which the extreme magnetic fields could be generated during the formation of the neutron star itself.

Figure 2: Strength of the dipolar component of the magnetic field as a function of the rotation period. The vertical dashed line corresponds to the rotation period where centrifugal forces would disrupt the newborn neutron star. The blue dots mark the ordinary amplification of magnetic fields when the neutron star spins slowly. The red dots correspond to the strong dynamo branch appearing for the fastest rotation rates. The properties of the magnetic field generated on this branch are compatible with the properties of galactic magnetars and the conditions to power the most extreme stellar explosions. Credit: CEA Sacley

In the first few seconds following stellar core collapse, the newborn hot neutron star cools down by emitting neutrinos. This cooling triggers strong internal convective mass flows, similar to the bubbling of boiling water in a pot on a stove. Such violent motions of the stellar matter could lead to the enhancement of any pre-existing weak magnetic field. Known as the dynamo effect, this field-amplification mechanism is at work, for instance, in the liquid iron core of the Earth or in the convective envelope of the Sun.

To test such a possibility for neutron stars, the team of researchers used a supercomputer of the French National Computing Center for Higher Education to simulate the convection in a newborn, very hot and rapidly spinning neutron star. Indeed, they found by this new modeling approach, which was more detailed than any other treatment used before, that the weak initial magnetic fields can be amplified up to values reaching 10 16 G for sufficiently fast rotation periods (see Fig. 1).

"Our models demonstrate that spin periods shorter than about 8 milliseconds allow for a more efficient dynamo process than slower rotation," says Raphaël Raynaud of CEA, Saclay, the lead author of the publication. "Slower rotating models do not display the enormous fields created by this strong dynamo."

In addition to shedding light on galactic magnetar formation, these results open new avenues to understand the most powerful and most luminous explosions of massive stars. For instance, superluminous supernovae emit a hundred times more light than usual supernovae, while others, called hypernovae, are characterized by a kinetic energy larger by a factor of ten and sometimes associated with a gamma-ray burst lasting several tens of seconds. These outstanding explosions constrain us to imagine non-standard processes that must extract tremendous amounts of energy from a "central engine."

The "millisecond magnetar" scenario is currently one of the most promising models for the central engine of such extreme events. It considers the rotational energy of a fast rotating neutron star as the additional energy reservoir that increases the power of the explosion. By exerting a braking torque, a strong dipole magnetic field of 10 15 G can transfer the neutron star's rotational energy to the explosion. "For this mechanism to be efficient, the field strength must be of the order of 10 15 G," explains coauthor Jérôme Guilet of CEA, Saclay. "This closely matches the values reached by convective dynamos for millisecond rotation periods" (see Fig. 2).

Until now, the main weakness of the millisecond magnetar scenario was to assume an ad hoc magnetic field, independent of the fast rotation rate of the neutron star. The results obtained by the research team thus provide theoretical support that was missing to this central engine scenario powering the strongest explosions observed in the universe.


Magnetars and the Dynamo Effect - Astronomy

Discovering a pulsar in the Milky Way&rsquos center has been one of the biggest and most exciting challenges recently in galactic astronomy. Pulsars can be used as precise clocks to investigate the interesting conditions found near the supermassive black hole at the galactic center, Sgr A*. Just recently, a new magnetar was discovered in the center of the Milky Way. Magnetars are very similar to pulsars, and therefore can also be used as clocks to learn about their surroundings. But as exciting as this discovery is, the road that led to the discovery is also very interesting.

Magnetars, how do they work?

Before diving into the discovery, it helps to have an understanding of what makes magnetars different from pulsars 1 . Magnetars and pulsars both originate from neutron stars. Neutron stars are stars a little more massive than the Sun supported by neutron degeneracy pressure, with a radius of about ten kilometers (approximately the size of a small city). They are the end products of type II supernovae, left behind after the explosion has taken place.

If the neutron star is spinning fast enough, it can develop a strong magnetic field through a process called dynamo action. Ionized gas inside the star circulates by convection (as the hot gases rise and cool gases fall towards the center of the star), and the movement of this gas can create a magnetic field. The spin of the star itself can help increase the magnetic field by facilitating the dynamo action inside the star. This process converts some of the star&rsquos rotational energy into energy stored in the magnetic field. In a pulsar, the spin is not strong enough to cause this conversion to happen significantly, while in magnetars, this process gives them their very strong magnetic fields: 10 15 &ndash 10 17 Gauss, over a 1000 times stronger than that of a pulsar. This property is the key differentiator between magnetars and pulsars, and also provides their name.

When the magnetic field of the star shifts, it can bend the crust and heat up the interior of the star, leading to starquakes on the star&rsquos crust. Gamma rays can be released during this process. Occasionally, the magnetar&rsquos magnetic field undergoes a huge rearrangement, leading to large flares (similar to, but much more powerful than, the solar flares on the Sun). These flares can cause fireballs that emit a large amount of X-ray radiation. It is with one of these large X-ray flares that the magnetar in the galactic center was first discovered.

It&rsquos a black hole &hellip it&rsquos a gas cloud &hellip no, it&rsquos a magnetar!

On April 24, a large X-ray flare was detected from the region surrounding the supermassive black hole in the galactic center by the Swift satellite. Immediately, predictions for what caused the flare jumped to the G2 gas cloud. G2 is a gas cloud discovered last year approaching towards an encounter with Sgr A*. The only problem is that this encounter is predicted for sometime late this year or early next year, and not this quickly.

Follow-up observations were quickly conducted using other telescopes. Using the superior timing resolution of the NuSTAR satellite on April 26, astronomers were able to measure a fluctuating signal coming in X-ray coming from the location, with a 3.76 second period. This period (and a later measurement of a spin-down rate) led to the discovery of the magnetar, one spinning every 3.76 seconds. Meanwhile, the Chandra X-ray Observatory was able to better resolve the location of the magnetar on April 29, identifying its location to be just 0.12 parsecs from Sgr A*.

Tick-Tock

So why is this small distance from the supermassive black hole very exciting? The answer stems from a pulsar or a magnetar&rsquos precise measurements of period, allowing them to behave as clocks (I discussed this briefly for pulsars in this previous post). If we could go out to Sgr A* and place a precise clock in an elliptical orbit around the supermassive black hole, we would observe the clock&rsquos ticking rate to fluctuate due to effects caused by general relativity as its distance from the black hole changes. Unfortunately, we can&rsquot do that 2 , but if we find a magnetar or a pulsar in an elliptical orbit around the supermassive black hole whose period we can precisely measure, it would be as if we conveniently happened to find a precise clock just as we wanted. As the pulsar or magnetar orbits around the supermassive black hole, we would observe different spin rates due to relativistic effects. In the process, we can conduct tests on general relativity. Hopefully, the new magnetar in the galactic center will prove useful in allowing these relativistic effects to be seen in future observations.

Sources and Further Exploration

  • Degenaar, N., M. T. Reynolds, J. M. Miller, et al. &ldquoLarge Flare from Sgr A* Detected by Swift&rdquo, 25 April 2013, The Astronomer&rsquos Telegram.
  • Gotthelf, Eric V., Kaya Mori, Jules P. Halpern, et al. &ldquoSpin-down Measurement of PSR J1745-2900: a New Magnetar&rdquo, 4 May 2013, The Astronomer&rsquos Telegram.
  • Kouveliotou, Chryssa, Robert C. Duncan, and Christopher Thompson. &ldquoMagnetars&rdquo, 2003, Scientific American 0203:34-41.
  • Mori, Kaya, Eric V. Gotthelf, Nicolas M. Barriere, et al. &ldquoNuSTAR discovery of a 3.76 second pulsar in the Sgr A* region&rdquo, 27 April 2013, The Astronomer&rsquos Telegram.
  • Rea, N., P. Esposito, G. L. Israel, et al. &ldquoChandra localization of the soft gamma repeater in the Galactic Center region&rdquo, 30 April 2013, The Astronomer&rsquos Telegram.
  • Reich, Eugenie Samuel. &ldquoMagnetar found at giant black hole&rdquo, 14 May 2013, Nature News & Comment.

I wrote a brief background on pulsars previously, available here. ↩︎


NASA Blueshift

Remember that time when we were all like Oooooh and Aaaaah? I miss those days. Since we began exploring these voids I’ve been starting to get a little bored. I mean look over there, nothing. And look over there, nothing. It really is just a big void. So like a moth to a flame, let’s head to that very bright light that’s flashing in the distance! This seems promising. This seems like a neutron star, a very special neutron star, a magnetar.

*Warning* Under no circumstances should any person approach a neutron star unless under the protection of technologies that have yet to or may never be discovered. Neutron Stars may lead to burning, major trauma, and spaghetification. Do not taunt neutron star.

This is an artistic image of all that is awesome. Credit: NASA/Dana Berry

All joking aside, neutron stars are very intense objects. Created from the collapse of stars 4 to 8 times the size of our Sun, these guys are the densest things in the universe next to black holes. Black holes need the collapse of a much larger star than does a neutron star to form. During the collapse, some mass is lost, resulting in neutron stars around 1.4 to 3.2 solar masses. Our sun is 1 solar mass. So they have more mass than the sun, but they are considerably smaller. In fact, neutron stars average about 12 kilometers in diameter. Two neutron stars could sit side by side in Chicago! The sun is 60,000 times larger than this but less massive, how can that be?

A neutron star compared to Manhattan. Credit: NASA/Goddard Space Flight Center

The amount of mass in a neutron star causes an immense amount of gravity. So immense that matter can degenerate into a pool of neutrons. What does that even mean? Take a sponge for example. Use one of those cool sponges you use in the bathroom – or a loofah. A loofah would work too. Hold your sponge in your hand and observe its structure. All the holes and empty space are badly analogous to the space in atoms that build matter.

Now, there are forces at play that hold the structure of the sponge but if you take your hands, and name them gravity, and squish the sponge as hard as you can, you’ll have a much smaller object with the same mass. Similarly, our neutron star builds a strong enough gravitational force to compress all the empty space out of matter leaving a pool of neutrons that is much smaller than our sun but much more massive.

Stars can balance this gravitational compression with pressure from the nuclear reactions that occur inside of them, but when the fuel is up gravity takes over. More fun examples to blow your mind! A Boeing 747 would be smaller than the size of a grain of sand under the same circumstances. Earth would be about the size of a basketball.

Like an m&m, a chocolate neutron center with a candy shell. Credit: NASA/Marshall Space Flight Center.

Along with the tremendous gravity, neutron stars also have a strong magnetic field. Actually, that doesn’t do it justice, the magnetic field around a neutron star is ridiculous and in some situations absolutely ridiculous! So let’s talk about magnetic fields. Magnets are awesome, and you have all noticed how you can manipulate objects with a magnet without them touching, right? Magnets have this non-contact force that builds around itself that we call a magnetic field. This field begins at one end of the magnet and ends at the other. That’s all I’m giving you, if you need a better recap click here!

Earth creates its own magnetic field, and it acts like a shield against the Sun. And the Sun has a magnetic field too. We can measure a magnetic field by the force it exerts on a charged particle the unit scientists use for this is called the Tesla (T). And a refrigerator magnet is about 0.01T. Earth’s magnetic field is around 0.004T. An MRI can produce a magnetic field of 10T, and the strongest magnetic field created by man (without destroying anything) was 100.75T. Neutron stars average a million T (10 6 )! And under unique circumstances a neutron star can reach a billion T (10 9 ), and is thus named a magnetar.

Artistic image of the magnetic field of a magnetar. Magnetic fields are actually not visible, but its effects are dire. Getting to close will cause your atoms to flatten along the field and all your credit cards will wiped! Credit: NASA/Goddard Space Flight Center Conceptual Image Lab

So what can I say about magnetars? It almost sounds straight out of science fiction. Neutron stars are born spinning really fast. To create a magnetar, we hypothesize that a neutron star is born spinning just a bit faster than your typical neutron star. This rotation sets up a “dynamo effect” that ramps up the magnetic field. As the magnetic field increases it starts to produce a drag on the star. This causes the rotation of the magnetar to slow and the magnetic field to stabilize. Even though the magnetar starts out faster than a typical neutron star, it spins down much faster since the magnetic field is so strong and provides more resistance.

There is still a lot of research needed on magnetars to fully understand them, and there aren’t too many orbiting around. Out of roughly 1000 observed neutron stars only a little over 20 have been discovered to be magnetars. Our 1 st recorded observation of a magnetar was in 1979 as a wave of gamma rays rushed through our solar system peaking instruments in satellites and causing small anomalies in our atmosphere. Over a year later it was found to originate outside our galaxy in a super novae remnant within the Large Magellanic Cloud (our neighboring galaxy). This also meant that the gamma wave was over 180,000 years old. Since then we’ve been on the lookout.

The Setting of the Sun Over the Pacific Ocean and a Towering Thundercloud, July 21, 2003 As Seen From the International Space Station (Expedition 7) Image Science and Analysis Laboratory, NASA-Johnson Space Center.

As with all good things, we have come to our last stop on our trip through the cosmos and should return to the land of blue skies and sunshine. But don’t think of it as returning home, think of it as the next stop to the most interesting place in our known universe. Of the rarest and most unique things we’ve discovered, Earth and its history is by far most unique to it all. I want to thank everyone who came on our journey, please leave any tips in the comments below. Keep looking up!


“Magnetar Power” –Chandra Observatory Captures Exotic Object With Magnetic Field a Million-Billion Times Earth’s

When stars more than thirty times bigger than our sun explode, they produce a type of young neutron star called a magnetar –the most magnetic stars in the universe, with gravity a billion times Earth’s and a magnetic field one-quadrillion times stronger than our Sun’s. A blast from magnetar could blow our atmosphere into space, leaving Earth a lifeless rock. Astronomer Phil Plait describes death by a magnetar should you venture too close as “the tides tearing you to pieces, the fierce heat vaporizing you, the magnetic field tearing your atoms apart, or the intense gravity crushing you into a thin paste an atom high.”

Biggest Stars Become the Strongest Magnets

In 2005, astrophysicist Bryan Gaensler in a study with Harvard-Smithsonian Center for Astrophysics and colleagues announced that they linked two of astronomy’s extremes, showing that some of the biggest stars in the cosmos “become the strongest magnets when they die.” The source of these very powerful magnetic objects –a city-sized ball of neutrons created when a massive star’s core collapses which forms from the collapsed core of a massive star during a supernova at the end of its lifetime–was a mystery since the first one was discovered in 1998.

A magnetar, typically less than nine hundred years old, possesses a magnetic field more than one quadrillion times (one followed by 15 zeroes) stronger than the earth’s magnetic field. If a magnetar were located a sixth of the way to the Moon –about 40,000 miles– it could wipe the data from every credit card on earth.

Magnetars are the rare, short-lived`white tigers’ of stellar astrophysics

“Magnetars are the rare, short-lived `white tigers’ of stellar astrophysics,” observed Gaensler, who specializes in exotic objects that change, flicker and explode. “We estimate that the magnetar birth rate will be only about a tenth that of normal pulsars.”

Same Regions of the Milky Way

“Both radio pulsars and magnetars tend to be found in the same regions of the Milky Way, in areas where stars have recently exploded as supernovae,” explained Gaensler in 2005. “The question has been: if they are located in similar places and are born in similar ways, then why are they so different?” Magnetars spit out bursts of high-energy X-rays or gamma rays. Normal pulsars emit beams of low-energy radio waves.

A magnetar goes through a cosmic extreme makeover and ends up very different from its less exotic radio pulsar cousins

A clue to the pulsar/magnetar difference may lie in how fast neutron stars are spinning when they form –heavy stars will form neutron stars spinning at up to 500-1000 times per second. Such rapid rotation should power a dynamo and generate superstrong magnetic fields. `Normal’ neutron stars are born spinning at only 50-100 times per second, preventing the dynamo from working and leaving them with a magnetic field 1000 times weaker.

“Astronomers used to think that really massive stars formed black holes when they died,” said Gaensler colleague Simon Johnston (CSIRO Australia Telescope National Facility). “But in the past few years we’ve realized that some of these stars could form pulsars, because they go on a rapid weight-loss program before they explode as supernovae.”

Fast forward to 2020, astronomers added a new member to an exclusive family of exotic objects with the discovery of a magnetar. New observations from NASA’s Chandra X-ray Observatory help support the idea that this magnetar is also a pulsar, meaning it emits regular pulses of light.

Youngest Known Magnetar

The image below shows an exceptional magnetar, a type of neutron star with very powerful magnetic fields. CfA astronomers have found evidence that this object may be the youngest known magnetar–about 500 years old in Earth’s timeframe. It is also the fastest rotating one yet discovered (spinning about 1.4 times per second). This image shows the magnetar in X-rays from Chandra (purple) at the center of the image in combination with Spitzer and WISE infrared data showing the wider field of view. Magnetars form when a massive star runs out of nuclear fuel and its core collapses onto itself.

What sets magnetars apart from other neutron stars is their magnetic fields. For context, the strength of our planet’s magnetic field has a value of about one Gauss, while a refrigerator magnet measures about 100 Gauss. Magnetars, on the other hand, have magnetic fields of about a million billion Gauss. If a magnetar was located a sixth of the way to the Moon (about 40,000 miles), it would wipe the data from all of the credit cards on Earth.

On March 12, 2020, astronomers detected a new magnetar with NASA’s Neil Gehrels Swift Telescope . This is only the 31st known magnetar, out of the approximately 3,000 known neutron stars.

After follow-up observations, researchers determined that this object, dubbed J1818.0-1607, was special for other reasons. First, it may be the youngest known magnetar, with an age estimated to be about 500 years old. This is based on how quickly the rotation rate is slowing and the assumption that it was born spinning much faster. Secondly, it also spins faster than any previously discovered magnetar, rotating once around every 1.4 seconds.

Chandra’s observations of J1818.0-1607 obtained less than a month after the discovery with Swift gave astronomers the first high-resolution view of this object in X-rays. The Chandra data revealed a point source where the magnetar was located, which is surrounded by diffuse X-ray emission, likely caused by X-rays reflecting off dust located in its vicinity. (Some of this diffuse X-ray emission may also be from winds blowing away from the neutron star.)

Close to the Milky Way’s Plane

The composite image above contains a wide field of view in the infrared from two NASA missions, the Spitzer Space Telescope and the Wide-Field Infrared Survey Explorer (WISE), taken before the magnetar’s discovery. X-rays from Chandra show the magnetar in purple. The magnetar is located close to the plane of the Milky Way galaxy at a distance of about 21,000 light-years from Earth.

Other astronomers have also observed J1818.0-1607 with radio telescopes, such as the NSF’s Karl Jansky Very Large Array (VLA), and determined that it gives off radio waves. This implies that it also has properties similar to that of a typical “rotation-powered pulsar,” a type of neutron star that gives off beams of radiation that are detected as repeating pulses of emission as it rotates and slows down. Only five magnetars including this one have been recorded to also act like pulsars, constituting less than 0.2% of the known neutron star population.

The Chandra observations may also provide support for this general idea. Safi-Harb and Blumer studied how efficiently J1818.0-1607 is converting energy from its decreasing rate of spin into X-rays. They concluded this efficiency is lower than that typically found for magnetars, and likely within the range found for other rotation-powered pulsars.

Searching for a Detectable Supernova Debris Field

The explosion that created a magnetar of this age would be expected to have left behind a detectable debris field. To search for this supernova remnant, Safi-Harb and Blumer looked at the X-rays from Chandra, infrared data from Spitzer, and the radio data from the VLA. Based on the Spitzer and VLA data they found possible evidence for a remnant, but at a relatively large distance away from the magnetar. In order to cover this distance the magnetar would need to have traveled at speeds far exceeding those of the fastest known neutron stars, even assuming it is much older than expected, which would allow more travel time.

Harsha Blumer of West Virginia University and Samar Safi-Harb of the University of Manitoba in Canada recently published results from the Chandra observations of J1818.0-1607 in The Astrophysical Journal Letters.

The Daily Galaxy, Jackie Faherty , astrophysicist, Senior Scientist with AMNH . via Harvard CfA and Chandra Observatory . Jackie was formerly a NASA Hubble Fellow at the Carnegie Institution for Science


Chryssa Kouveliotou, professor and chair of the Department of Physics in the Columbian College of Arts and Sciences (CCAS), was awarded the Shaw Prize in Astronomy for her contributions to our understanding of magnetars, a class of highly magnetized neutron stars that are linked to a wide range of spectacular, transient astrophysical phenomena.

The Shaw Prize is an international award to honor individuals who are currently active in their fields and who have recently achieved distinguished and significant advances, made outstanding contributions in academic and scientific research or applications, or who in other domains have achieved excellence. The award is dedicated to furthering societal progress, enhancing quality of life and enriching humanity’s spiritual civilization.

“I am very honored and at the same time very humbled for this great award,” Dr. Kouveliotou said. “I am grateful to all my colleagues who helped me along the way and believed and supported me in my journey. I feel that this honor is the best way I could celebrate my entire life and career.”

Dr. Kouveliotou, a member of the National Academy of Sciences, is George Washington University’s first recipient of the Shaw Prize. She was honored alongside Victoria M. Kaspi, a professor of Physics at McGill University in Canada. Through the development of new and precise observational techniques, the researchers confirmed the existence of neutron stars with ultra-strong magnetic fields and characterized their physical properties. Their work has established magnetars as a new and important class of astrophysical objects.

“Dr. KouveIiotou’s research in the field of astrophysics is nothing short of groundbreaking,” said CCAS Dean Paul Wahlbeck. “Her work has served to enlighten our understanding of the very origins and expansion of our universe. I congratulate her on this well-deserved honor.”

Neutron stars are the ultra-compact remnants of stellar explosions. Most are rapidly rotating with periods of milli-seconds to seconds and emit powerful beams of electromagnetic radiation, observed as pulsars. They are accurate “cosmic clocks” that enable tests of fundamental physics in the presence of a gravitational field many billion times stronger than that on Earth.

Pulsars also have strong magnetic fields. Magnetic field lines in the progenitor star are “frozen in” in the stellar remnant as it collapses to become a neutron star. These magnetic fields funnel jets of particles along the magnetic poles, but classical radio pulsars are powered mainly by rotational energy and slowly spin down over their lifetimes.

The magnetar discoveries by Dr. Kouveliotou and Dr. Kaspi were preceded by the theoretical prediction that neutron stars with extreme magnetic fields up to one thousand times stronger than those in regular pulsars could form if dynamo action would be efficient during the first few seconds after gravitational collapse in the core of the supernova. Such objects—termed magnetars—would be powered by their large reservoirs of magnetic energy and not by rotational energy losses.

From observations of a class of X-ray/γ-ray sources called “soft gamma-ray repeaters” (SGRs), Dr. Kouveliotou and her colleagues established the existence of magnetars and provided a stunning confirmation of the magnetar model in the late 1990s. The analysis of the data from the Rossi X-ray timing satellite (RXTE), obtained with her proposal, enabled Dr. Kouveliotou in 1998 was able to detect X-ray pulses with a period of 7.5 seconds in the persistent X-ray emission of SGR 1806-20. She then measured a spin-down rate for the pulsar and derived both the pulsar age and the dipolar magnetic field strength—which lay within the range of values predicted for magnetars. The spin-down measurements were extremely challenging because of the faintness of the pulsed signal, according to a Shaw Prize press release.

The Shaw Prize was established in 2002 and is managed and administered by the Shaw Prize Foundation, based in Hong Kong. Dr. Kouveliotou and Dr. Kaspi will each receive a $600,000 award.


Core Dynamics

8.05.5.7 Scaling Laws and Dynamo Simulations

In order to overcome the problem of the inaccessibility of the geodynamo parameter regime, there has been recent work on how the more important outputs from geodynamo simulations scale with the input parameters. If an asymptotic regime in which the role of the small diffusion coefficients can be identified, it may be possible to extrapolate to the very small values that occur in the Earth. Two approaches have been tried Starchenko and Jones (2002) used results from plane layer models and the general understanding of rotating magnetoconvection to estimate typical velocities and magnetic field strengths expected from very low E, low q dynamos. An alternative approach ( Christensen and Aubert, 2006 ) is to analyze the data from a large number of dynamo simulations and see whether asymptotic trends are evident in the data. In practice, these two approaches are not so different, as theoretical ideas inevitably affect the way the dynamo simulation data are analyzed.

Starchenko and Jones started by assuming that at very low E the Coriolis and buoyancy forces would be in balance, and that the magnetic field would bring the horizontal length scale ℓ appearing in [112] to a fixed ratio with d = r cmb − r icb . The idea here is that as E is reduced, the magnetic field prevents the roll-width reducing as E 1/3 . Then [112] is replaced simply by

Then eliminating the temperature perturbation using [113] ,

If the ratio d / ℓ tends to a fixed limit at small E due to magnetic field, as postulated by Starchenko and Jones, U* is proportional to F conv 1 / 2 . Interestingly, Christensen and Aubert (2006) find that the exponent of 2/5 gives a better fit to their data than the exponent of 1/2 in [149] . Of course, the difference is not that great, but it may be connected with the observation made in Section 8.05.5.6.1 that in dynamo simulations there is not much evidence of the magnetic field controlling the roll-width, as suggested by Starchenko and Jones (2002) . It remains possible, though, that this control will start to occur at E less than 10 −6 . Christensen and Aubert (2006) suggest replacing [149a] by

which is the same formula as in the nonmagnetic case, [115a] . They note that the best fit with their simulation data can be expressed in dimensionless variables

very close to [150] . Here Qadv is the total convective heat flux over a spherical surface of radius r, assumed constant with r throughout the core. Note that with this assumption, the convective heat flux Fconv in [148]–[150] is best taken as

The Rayleigh number R a Q * introduced by Christensen and Aubert is essentially the R a Q * defined in [126] . Equation [152] therefore expresses the independence of the velocity scaling on any diffusion coefficient. The emergence of [152] , with no dependence on any Prandtl numbers, expresses the fact that the simulations do appear to be approaching an asymptotic regime which is independent of diffusion, a very encouraging result.

Christensen and Aubert (2006) also go on to estimate the typical magnetic field strength in dynamo simulations. The rate of working of the buoyancy forces is g α F / c p , and assuming that the dissipation is primarily ohmic, this must balance the rate of ohmic dissipation, so

To convert this into a formula for B 2 we need to know the magnetic dissipation time, that is,

or equivalently the magnetic dissipation length scale

Christensen and Tilgner (2004) argue that δ B ∼ d R m − 1 / 2 , again on the basis of an analysis of numerical simulations. This is consistent with nonrotating flux expulsion arguments ( Galloway et al. (1977) ). Then [154] becomes

with the Starchenko and Jones scaling [149] this gives

and with the Christensen and Aubert scaling [150] we get

Note that the small difference in the velocity scalings has led to a significant difference in the scaling of B* with Ω, because the Christensen and Aubert scaling has a remarkably weak scaling of B* with Ω. Indeed, their best fit of the simulation data is

where L o = B * / μ ρ 1 / 2 Ω d and fohm is the fraction of the total dissipation (ohmic + viscous) which is ohmic. This gives

so that B* is completely independent of the rotation.

When compositional convection is present, in the above formulas the thermal buoyancy flux g α Q conv is replaced by the compositional buoyancy flux g Q buoy , where Q buoy is the mass of light material released per unit time.

The scalings [152] and [161] can be used to find the ratio of kinetic to magnetic energy, which is close to R a Q * 0.15 . Since R a Q * is very small in the core, this implies that magnetic energy is indeed larger than kinetic energy in the core, but the weak power shows that it is not surprising that in simulations the two forms of energy have similar magnitudes. Also, although ohmic dissipation is predicted to dominate over viscous dissipation at very low E, simulations have great difficulty getting down to this regime (at Pr = 1), and generally at Pr = 1 they show comparable ohmic and viscous dissipations.


The August 27th Event

NASA's Rossi X-ray Timing Explorer (RXTE), another Earth-orbiting X-ray observatory, was pointed away from SGR 1900+14 when the burst occured, but it nevertheless recorded a strong signal. High-energy photons were diffusing through the metal shields surrounding its X-ray detectors. However, one proven workhorse for SGR studies, the Burst and Transient Source Experiment (BATSE) aboard NASA's orbiting Compton Gamma-ray Observatory, detected nothing. The BATSE team, led by mild-mannered Charles Meegan (who is BATSE-MAN ) ran out of luck that day: the Compton Observatory was on the far side of the Earth at the time of the flare.

The flare hit the Earth on it's night side, in the zenith over the western Pacific Ocean, at 1:22 A.M. Hawaii time. It was intense enough to strongly ionize the Earth's outer atmosphere, affecting radio communications.

This requires some explanation. Radio waves, especially long-wavelength ones like those on the AM dial, bounce between the "ionosphere" and the Earth's surface as they propagate around our planet. The ionosphere is a layer of diffuse, ionized gas -- atoms of air which have lost electrons and become positively-charged ions -- in the upper reaches of the atmosphere. High-energy photons from the Sun keep the thin air up there well-ionized during the day, so the daytime ionosphere lies about 60 kilometers above the Earth's surface. At night, electrons recombine with ions, causing the inner edge of the ionosphere to recede upward, to 80 - 90 kilometers. This is why you can pick up very distant AM stations on your radio at night: radio signals generally travel farther if they must make fewer (power-sapping) bounces.

In the early morning of August 27th 1998, Stanford University engineers monitoring very-long-wavelength U.S. Navy radio transmissions (which carry coded messages for nuclear submarines) found that the altitude of the ionosphere plummeted for a five-minute period beginning at 3:22 A.M. PDT. Some mysterious source of ionization was apparently driving the ionized layer down to daytime altitudes (about 60 km). Curiously, the height of the ionosphere was also observed to vary cyclically over a period of 5.16 seconds. Of course, they had detected the rotation period of SGR 1900+14 in a remarkable new way, proving that you don't need a sensitive X-ray telescope to measure the spin period of a SGR. If you can wait for a giant flare, you can use a "Whole Earth Telescope"-- the bulk ionosphere of the whole planet -- to see the rotation period of a tiny neutron star, twenty thousand light years away.

As the wavefront of gamma-rays swept out of the solar system, the last spacecraft it reached was NASA's Near-Earth Asteroid Rendevous (NEAR) space probe, enroute to a rendevous with the asteroid Eros. The flare was bright enough to force NEAR's gamma-ray detectors into a protective shut-down mode.

Although the August 27 event was intrinsically less powerful than March 5th (roughly by a factor

The four-peaked pattern persisted through much of the rest of the flare, as shown in the following graph. (Note: each box below represents one 5.16-second rotation cycle of the star. The Beppo-SAX spacecraft recorded the detailed pattern of flare emission for 7 1/2 more cycles than did Ulysses .)



The occurrence of magnetism in the universe

Flows of molten metal can generate magnetic fields. This so-called dynamo effect creates cosmic magnetic fields, like those found on planets, moons and even asteroids. Over the coming years, a globally unique experiment, in which a steel drum containing several tons of liquid sodium rotates around two axes, is intended to demonstrate this effect. It will be carried out in the new DRESDYN facility at the Helmholtz-Zentrum Dresden-Rossendorf (HZDR), an independent German research laboratory. A recently published study in the scientific journal Physical Review Letters confirms the experiment's chances of success.

Similarly to how a bicycle dynamo converts motion into electricity, moving conductive fluids can generate magnetic fields. The so-called magnetic Reynolds number (the product of the fluid's flow velocity, expansion and conductivity) primarily determines whether a magnetic field is actually generated.

During a spectacular experiment, scientists in Frank Stefani's team at the HZDR's Institute of Fluid Dynamics aim to achieve the critical value required for the occurrence of the dynamo effect. For this purpose, a two-meter diameter steel cylinder containing eight tons of liquid sodium will rotate around one axis up to ten times per second and once per second around another, which is tilted with respect to the first. The technical term for this movement, which is often compared to a tilted spinning top, is precession.

"Our experiment at the new DRESDYN facility is intended to demonstrate that precession, as a natural driver of flow, is sufficient to create a magnetic field," says André Giesecke, lead author of the study. In his simulations and during accompanying water experiments -- the mock-up was six times smaller than the large dynamo -- scientists examined the structure of precession-driven flow.

"To our surprise, we observed a symmetrical double role structure in a specific range of the precession rate, which should provide a dynamo effect at a magnetic Reynolds number of 430," says the physicist.

Unresolved: the role of precession in the geodynamo

The center of the Earth consists of a solid core surrounded by a layer of molten iron. "The molten metal induces an electric current, which in turn generates a magnetic field," explains Giesecke. The common belief is that buoyancy-driven convection, together with Earth's rotation, is responsible for this geodynamo. However, the role played by precession in the formation of Earth's magnetic field is still completely unclear.

The Earth's rotational axis is tilted by 23.5 degrees from its orbital plane. The rotational axis changes position over a period of approximately 26,000 years. This precessing motion through space is thought to be one of the possible sources of energy for the geodynamo. Millions of years ago, the Moon also had a powerful magnetic field, as indicated by rock samples from the Apollo missions. According to experts, precession could have been the main cause of this.

The liquid sodium experiments at HZDR are expected to start in 2020. Unlike earlier geodynamo laboratory experiments, there will be no propeller inside the steel drum, as was used in the first successful dynamo experiment in Riga, Latvia in 1999, in which HZDR scientists were heavily involved. This and other experiments in Karlsruhe, Germany and Cadarache, France provided ground-breaking research for a better understanding of the geodynamo.

"In principle, we can define three different parameters for the experiments at DRESDYN: rotation, precession and the angle between the two axes," says Giesecke. On the one hand, he and his colleagues expect to get answers to the fundamental question of whether precession actually produces a magnetic field in a conductive fluid. On the other hand, they are interested in finding out which flow components are responsible for the creation of the magnetic field, and the point at which saturation occurs.

Double role in the Container

"In simulations, we discovered that stationary inertia waves occur in a wide parameter range. Within a certain range, however, we have now noticed a characteristic double role structure that proves to be extremely efficient for the dynamo effect. In principle, we are already aware of such a velocity structure thanks to the French dynamo experiment, in which it was artificially produced by two propellers, while in our precession experiment it should emerge naturally."

The HZDR researchers used special ultrasound technology to measure the flow structure. "We were very surprised at how well the data from the experiment matches the results of the simulation. We therefore have an extremely robust prediction for the major DRESDYN experiment. For example, we know at which rotational rates the dynamo effect occurs and which magnetic field structures we can expect," says Giesecke.

The scientific community involved with dynamos is eagerly awaiting the results of the planned experiment, which will operate at the limits of technical feasibility in many respects. "We also expect detailed insights into the general dynamics of liquid metal flows under the influence of magnetic fields. This will allow us to draw conclusions about flows in the industrial sector," according to Giesecke.

And last but not least, the magnetic flow tomography developed at the HZDR as part of its dynamo research is of interest to many areas of steel casting and crystal growing. The work has been partially funded by the Helmholtz Alliance "Liquid Metal Technologies" (LIMTECH).