Tag: Magnetic Fields

Jupiter’s Magnetic Field Has Weird Structure

Jupiter as seen from the Juno spacecraft.

Jupiter has the strongest magnetic field of any of the planets in the solar system. Like the field that shelters Earth, it’s essentially dipolar, which means it has a north pole and a south pole, like the field created by a bar magnetic.

A really, really big bar magnetic.

Earth’s magnetic field is produced by churning liquid iron in the planet’s outer core. Iron conducts electricity, and a changing electrical current creates a magnetic field.

So as the liquid iron cycles up and down, carrying heat from the planet’s center up to the mantle and then sinking again, it creates powerful electrical currents that in turn produce the planet’s global field.

But Jupiter doesn’t have an iron core. In fact, it’s unclear if it has a core at all — Juno’s observations suggest the core might be “fuzzy,” a concentration of rock and ice that has dissolved (or is still dissolving) into the surrounding hydrogen.

Instead, the source of the global field is the overlying mantle of metallic hydrogen, where hydrogen molecules trade electrons, creating currents. The planet’s rotation organizes the resulting magnetic field into a dipole.

Or, at least it kind of does. Reporting in the September 6th Nature, Kimberly Moore (Harvard) and colleagues have discovered a strange plume of magnetic field shooting up from a region in Jupiter’s northern hemisphere and reentering the planet at its equator.

And it’s three times stronger than the main dipole field.

Detecting the Invisible

As it flies around Jupiter, the Juno spacecraft measures the planet’s magnetic field using two instruments called fluxgate magnetometers.

At each magnetometer’s core lie two rings, made of a material that soaks up magnetic field. Think of it like a magnetic sponge. Like a sponge, the material can only hold so much before it saturates.

The scientists can magnetically “fill up” the rings by running current through wires coiled around them, first one direction, then the other, explains John Connerney (NASA Goddard Space Flight Center), who heads up Juno’s magnetometer investigations and is a coauthor on the new study.

But if there’s another magnetic field in the environment, the rings will soak it up, too.

That will reduce how much of the applied field the rings can absorb from the wires in one direction, but increase the amount absorbed from current flowing the other direction.

When the magnetometer cancels out this imbalance using another wire-wound structure around each of the rings, the instrument measures how strong the environmental field is based on how much current it takes to push the field in the rings back to zero.

The coils’ orientations give the external field’s direction. But the magnetometer only detects the magnetic field the spacecraft is flying through.

The researchers have to extrapolate from those measurements, using detailed calculations to map the field at the planet’s cloudtops and below.

Combining data from eight of Juno’s flybys, the scientists confirmed the existence of the bizarre magnetic feature, hints of which had shown up in an analysis last year from Juno’s first orbit.

The structure looks like a ponytail shooting out from the planet’s forehead and reentering through the nose, at a location the team is calling the Great Blue Spot (for its color in a map of the planet’s field).

There’s nothing like this ponytail in the southern hemisphere. Why does this magnetic ponytail exist? Scientists don’t know.

The team considers several ideas in their paper, the most likely being that there’s some sort of layering in the metallic hydrogen mantle that’s messing with the convection pattern.

Layering could naturally arise with a dissolving core: Rock and ice mixed in with hydrogen would raise the density, and if that mixing isn’t uniform, it could create layers of different density that could destabilize the cyclic convection patterns or spur different convection patterns in distinct layers.

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Pass it on: Popular Science

Scientists Say Earth’s Magnetic Field Isn’t About To Flip, But It’s Definitely Doing Something Weird

Every few hundred thousand years, Earth’s magnetic field flips, and considering the huge impact that would have on everything from satellite systems to electrical grids, scientists are very keen to work out when the next one might be.

Hopefully it won’t be for a while yet, according to the latest study – by analysing recent near-reversals of the our planet’s magnetic field, researchers have concluded that we’re not in line for a reversal in the near future… at least based on what’s happened in the past.

The international team of experts compared the current state of Earth’s magnetic field with conditions during the Laschamp event (about 41,400 years ago) and the Mono Lake event (about 34,000 years ago).

On both those previous occasions the magnetic field recovered without a flip, and the scientists think the same will happen now.

There has been speculation that we are about to experience a magnetic polar reversal or excursion,” says one of the team, Richard Holme from the University of Liverpool in the UK.

However, by studying the two most recent excursion events, we show that neither bear resemblance to current changes in the geomagnetic field and therefore it is probably unlikely that such an event is about to happen.

Our research suggests instead that the current weakened field will recover without such an extreme event, and therefore is unlikely to reverse.

Both the Laschamp and Mono Lake events were smaller shifts that didn’t result in complete flips, as we can tell from magnetised volcanic rocks, particularly those embedded under the ocean floor.

The research matches our current magnetic field scenario with two other points at 49,000 years and 46,000 years in the past, prior to the previous events.

If a complete flip didn’t happen on those occasions, the hypothesis goes, then a flip isn’t about to happen now either.

There are in fact two outcomes to look out for: a geomagnetic reversal, where magnetic north and magnetic south change places, and a geomagnetic excursion, where there are short-lived changes in the field intensity rather than the field orientation.

Both reversals and excursions can weaken Earth’s magnetic field, allowing more solar radiation to hit the surface.

While this wouldn’t be damaging enough to affect us (human beings have survived through past events), it could cause serious problems with satellite, communications, and power systems.

There’s also the possibility it might interfere with the planet’s temperature and climate, but scientists just aren’t sure at the moment what the effects will be – the last full flip was 780,000 years ago, after all.

The general consensus is that these changes in Earth’s magnetic field are caused by movements of molten iron and nickel deep in the planet’s core.

In fact, smaller fluctuations in field strength and magnetic poles are happening on a regular basis, so scientists are keen to collect as much data as possible on them.

In particular, experts are keeping an eye on the South Atlantic Anomaly (SAA), currently the weakest part of Earth’s magnetic field. It’s slowly weakening and slowly moving westward at the same time.

The more data we have, the more accurately we can predict when a reversal is likely to happen – and make sure we’re ready for it.

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Pass it on: Popular Science

Neutron Star Smash-Up Produces Gravitational Waves And Light In Unprecedented Stellar Show​

The 2015 detection of gravitational waves – ripples in the very fabric of space and time – was one of the biggest scientific breakthroughs in a century.

But because it was caused by two black holes merging, the event was all but invisible, detectable indirectly via the LIGO facility.

Now a team of scientists has announced the fifth detection of gravitational waves, but there’s a groundbreaking difference this time around.

The ripples were caused by the collision of two neutron stars, meaning the event was accompanied by light, radio, and other electromagnetic signals for the first time.

First predicted by Albert Einstein over 100 years ago, gravitational waves are caused by cosmic cataclysms like the collision of two black holes, but because of the immense distance.

By the time they reach us here on Earth the distortions are occurring on the subatomic scale.

To observe waves that tiny, LIGO beams lasers down a 4-km (2.5-mi) long tunnel and measures how gravitational waves might warp the beam as they wash over our local corner of spacetime.

That delicate process is effective at confirming the phenomenon, but still somewhat indirect.

This is the first time that the collision of two neutron stars has been detected, and this is the closest and most precisely located gravitational wave signal we’ve received,” says Susan Scott, the Leader of the General Relativity Theory and Data Analysis Group at Australian National University (ANU), which played a key role in the observation.

It is also the loudest gravitational wave signal we’ve detected.

The collision occurred in a galaxy called NGC 4993, which lies about 130 million light-years away – that might sound far, but it’s much closer than previous observations, which occurred at distances of billions of light-years.

As well as producing gravitational waves, the neutron stars’ collision sent a host of electromagnetic signals sweeping across the universe, including a short gamma ray burst, X-rays, light and radio waves.

These were picked up by observatories all over the world, helping pinpoint the source.

ANU was among those, using SkyMapper and the Siding Spring Observatory in New South Wales, Australia, to observe the brightness and color of the light signals given off.

Along with learning more about gravitational waves, the discovery can teach astronomers about neutron stars.

Created when larger stars collapse, neutron stars are relatively tiny – only about 10 km (6.2 mi) wide – and incredibly dense, with very strong magnetic fields. Other than that, not a whole lot is known about them.

With this discovery we have the opportunity to learn so much more about neutron stars, which have been quite a mystery to us,” says Scott.

Unlike black holes, neutron star collisions emit other signals such as gamma rays, light and radio waves so astronomers around the world were able to observe the event through telescopes. This is an amazing time to be a scientist.

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Pass it on: New Scientist

Detecting Magnetic Fields On Brown Dwarfs And Exoplanets

Mysterious objects called brown dwarfs are sometimes called “failed stars.

They are too small to fuse hydrogen in their cores, the way most stars do, but also too large to be classified as planets.

But a new study in the journal Nature suggests they succeed in creating powerful auroral displays, similar to the kind seen around the magnetic poles on Earth.

This is a whole new manifestation of magnetic activity for that kind of object,” said Leon Harding, a technologist at NASA’s Jet Propulsion Laboratory, Pasadena, California, and co-author on the study.

On Earth, auroras are created when charged particles from the solar wind enter our planet’s magnetosphere, a region where Earth’s magnetic field accelerates and sends them toward the poles.

There, they collide with atoms of gas in the atmosphere, resulting in a brilliant display of colors in the sky.

As the electrons spiral down toward the atmosphere, they produce radio emissions, and then when they hit the atmosphere, they excite hydrogen in a process that occurs at Earth and other planets,” said Gregg Hallinan, assistant professor of astronomy at the California Institute of Technology in Pasadena, who led the team.

We now know that this kind of auroral behavior is extending all the way from planets up to brown dwarfs.

Brown dwarfs are generally cool, dim objects, but their auroras are about a million times more powerful than auroras on Earth, and if we could somehow see them, they’d be about a million times brighter, Hallinan said.

Additionally, while green is the dominant color of earthly auroras, a vivid red color would stand out in a brown dwarf’s aurora because of the higher hydrogen content of the object’s atmosphere.

The foundation for this discovery began in the early 2000s, when astronomers began finding radio emissions from brown dwarfs.

This was surprising because brown dwarfs do not generate large flares and charged-particle emissions the way the sun and other kinds of stars do. The cause of these radio emissions was a big question.

Harding, working as part of Hallinan’s group while pursuing his doctoral studies, found that there was also periodic variability in the optical wavelength of light coming from brown dwarfs that pulse at radio frequencies.

He published these findings in the Astrophysical Journal.

Harding built an instrument called an optical high-speed photometer, which looks for changes in the light intensity of celestial objects, to examine this phenomenon.

In this new study, researchers examined brown dwarf LSRJ1835+3259, located about 20 light-years from Earth.

Scientists studied it using some of the world’s most powerful telescopes the National Radio Astronomy Observatory’s Very Large Array, Socorro, New Mexico, and the W.M. Keck Observatory’s telescopes in Hawaii in addition to the Hale Telescope at the Palomar Observatory in California.

Given that there’s no stellar wind to create an aurora on a brown dwarf, researchers are unsure what is generating it on LSRJ1835+3259.

An orbiting planet moving through the magnetosphere of the brown dwarf could be generating a current, but scientists will have to map the aurora to figure out its source.

The discovery reported in the July 30, 2015 issue of Nature could help scientists better understand how brown dwarfs generate magnetic fields.

Additionally, brown dwarfs will help scientists study exoplanets, planets outside our solar system, as the atmosphere of cool brown dwarfs is similar to what astronomers expect to find at many exoplanets.

It’s challenging to study the atmosphere of an exoplanet because there’s often a much brighter star nearby, whose light muddles observations. But we can look at the atmosphere of a brown dwarf without this difficulty,” Hallinan said.

Hallinan also hopes to measure the magnetic field of exoplanets using the newly built Owens Valley Long Wavelength Array, funded by Caltech, JPL, NASA and the National Science Foundation.

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Pass it on: New Scientist