We tend to think of these scary objects as having voracious appetites, gobbling up everything that gets in their way. But in reality, most black holes surround themselves with discs of gas and dust that swirl around, heat up, and emit lots of radiation. All this activity makes it difficult to determine what’s going on near the black hole itself. Which includes figuring out the black hole’s diet.

If we want to understand something that’s invisible (such as a black hole), we usually have to find creative ways of detecting it. But in order to learn about the black hole at the center of our galaxy, astronomers study the behavior of its surroundings—specifically nearby stars.

They hit the jackpot once they discovered a pulsar near the galactic center. Not only do these rare stars act as precise cosmic clocks, but this one in particular emits an abnormally strong magnetic field (called a magnetar).

Pulsar PSR J1745-2900 was the first of its kind to be found near the galactic center, and at only 0.3 light years away from our black hole (a.k.a. Sagittarius A* or Sgr A*), it tells us a lot about black hole characteristics.

A research team with the Max Planck Institute for Radio Astronomy (MPIfR) was able to measure the pulsar’s magnetic field, which revealed how magnetic fields affect black hole behavior.

“In order to understand the properties of Sgr A*, we need to comprehend the accretion of gas into the black hole,” says Michael Kramer, director at MPIfR. Although black holes have infamously strong gravity, material doesn’t typically fall directly into a black hole; instead, the material forms an accretion disc before slowly flowing toward the black hole at the center.

“However, up to now,” according to Kramer, “the magnetization of the gas, which is a crucial parameter determining the structure of the accretion flow, remains unknown. Our study changes that by using the discovered pulsar to probe the strength of the magnetic field at the start of this accretion flow of gas into the central object.”

Knowledge of pulsars’ consistent frequencies illustrates the effect of the black hole on the pulsar. Radio waves usually polarized along a plane are now rotating in a corkscrew motion, similar to the radiation emitted from black holes themselves.

“The rotation is way higher than anything seen in the Galaxy with the exception of Sagittarius A*,” says Ralph Eatough of MPIfR who measures the black hole’s radio waves and outward streaming matter.

The paper, published last week in Nature, concludes that the twisted magnetic fields might slow black holes’ diet, putting the brakes on infalling gas. Sgr A* is “not feeding to its full potential,” says Eatough.

Alyssa Keimach is an astronomy and astrophysics student at the University of Michigan and interns for the Morrison Planetarium.

Image: NASA

Early last week I wrote an article about how new data from Voyager 1 shed light on the structure of the Sun’s heliosphere and solar wind. Just a few days after that, researchers announced evidence of Earth’s own space wind!

Scientists proposed the existence of a space wind surrounding Earth about 20 years ago, but direct detection has eluded scientists until now.

Earth is surrounded by a magnetic field that encloses our magnetosphere. The plasmasphere is the inner part of that magnetosphere, and it looks a giant donut made of electrically-charged particles called (as its name suggests) plasma.

The Sun’s magnetic activity can accelerate plasma toward Earth, which impacts our magnetosphere. During such solar storms, we have observed plumes of material transfer between the plasmasphere and the outer magnetosphere, but researchers also proposed the existence of a steady flow of plasma that occurs around the clock. After years of theoretical work, Iannis Dandouras of the Research Institute in Astrophysics and Planetology in Toulouse, France, has directly detected this wind in data from the European Space Agency’s Cluster spacecraft.

Dandouras measured the properties of charged particles in the plasmasphere to find that the forces governing plasma motion exist slightly out of balance, forming a steady wind.

“After long scrutiny of the data, there it was, a slow but steady wind, releasing about one kilogram of plasma every second into the outer magnetosphere. This corresponds to almost 90 tons every day. It was definitely one of the nicest surprises I’ve ever had!” said Dandouras.

Don’t worry, the plasmasphere won’t evaporate away: it also refills. Dandouras reassured everyone that “due to the plasmaspheric wind, supplying plasma—from the upper atmosphere below it—to refill the plasmasphere is like pouring matter into a leaky container.”

Michael Pinnock, Editor-in-Chief of Annales Geophysicae, recognizes the importance of the new result. “It is a very nice proof of the existence of the plasmaspheric wind. It’s a significant step forward in validating the theory. Models of the plasmasphere, whether for research purposes or space weather applications (e.g. GPS signal propagation) should now take this phenomenon into account.”

We can even apply our understanding of Earth’s plasmaspheric wind to other places. Why wouldn’t another planet such as Jupiter or Saturn experience the exact same phenomenon? The Solar System could be a very windy place!

Alyssa Keimach is an astronomy and astrophysics student at the University of Michigan and interns for the Morrison Planetarium.

Image: NASA

How large is the heliosphere? The region of interstellar space dominated by the Sun? The Voyager 1 spacecraft has a partial answer: much larger than expected!

The heliosphere, composed of the sun’s magnetic field and a high-velocity stream of charged particles called the solar wind, creates an enormous bubble around our solar system. The charged particles move at about a million miles per hour, only slowing down when they near the region where the pressure of interstellar gas dominates. We thought Voyager 1, our farthest spacecraft, had arrived at edge of the heliosphere, but there is something fishy about Voyager 1’s new data.

Launched in 1977, the twin spacecraft Voyager 1 and Voyager2 have both entered an area called the heliosheath, where the solar wind slows, even though they’re headed in different directions away from the Sun. Voyager 1 lies farthest away, 11 billion miles from Earth, and at this distance it encountered a “magnetic highway.” Here the Sun’s magnetic field connects with the interstellar magnetic field, allowing for an exchange of charged particles between inside and outside the heliosphere.

Voyager 1 measured the highest rate of change so far between incoming and outgoing particles. “We saw a dramatic and rapid disappearance of the solar-originating particles. They decreased in intensity by more than 1,000 times, as if there was a huge vacuum pump at the entrance ramp onto the magnetic highway,” said Stamatios Krimigis, the low-energy charged particle instrument’s principal investigator at the Johns Hopkins University Applied Physics Laboratory in Laurel, Maryland. In this same region, scientists first detected the low-energy cosmic rays that originate from dying stars.

This should indicate that the spacecraft has reached interstellar space, except scientists have not yet seen the final indicator: an abrupt change in the direction of the magnetic field.

“If you looked at the cosmic ray and energetic particle data in isolation, you might think Voyager had reached interstellar space, but the team feels Voyager 1 has not yet gotten there because we are still within the domain of the Sun’s magnetic field,” said Edward Stone, Voyager project scientist at the California Institute of Technology in Pasadena.

So how much farther does Voyager 1 need to travel until it reaches interstellar space? Scientists estimate several months or even years until Voyager 1 experiences a change in magnetic field direction. For now, they have named this strange zone the heliosheath depletion region. Catchy, eh?

Stay tuned for more Voyager discoveries from the edge of interstellar space!

Alyssa Keimach is an astronomy and astrophysics student at the University of Michigan and interns for the Morrison Planetarium.

Image: NASA/JPL-Caltech

For years, researchers have suspected that the salmon use the Earth’s magnetic field to get back home, but scientists found little evidence to support this theory. Until now…

Researchers, reporting last week in Current Biology, took 56 years worth of data of salmon migration to and from the Fraser River in British Columbia. They then compared migration routes to the intensity of Earth’s magnetic field at pivotal locations in the salmons’ migratory routes.

See, Earth has a magnetic field at its surface that weakens with proximity to the equator and distance from the poles and gradually changes on a yearly basis. Therefore, the intensity of the magnet field in any particular location is unique and differs slightly from year to year.

The trick for these Fraser River salmon is that to get to the open ocean and back they need to navigate around Vancouver Island. The sockeye can travel north via the Queen Charlotte Strait or from the south via the Juan de Fuca Strait.

The researchers discovered that the intensity of the magnetic field largely predicted which route the salmon used to detour around Vancouver Island; in any given year, the salmon were more likely to take whichever route had a magnetic signature that most closely matched that of the Fraser River years before, when the salmon initially swam from the river into the Pacific Ocean. (See image, above right.)

“These results are consistent with the idea that juvenile salmon imprint on (i.e. learn and remember) the magnetic signature of their home river, and then seek that same magnetic signature during their spawning migration,” says Nathan Putman, a post-doctoral researcher at Oregon State University and the lead author of the study.

Other factors, in addition to Earth’s magnetism, also influence the route, Putman says. Once the salmon reach their home river, they probably use their sense of smell to find the particular tributary in which they were born.

Because salmon populations are so important to Northwest economies, the authors hope this study will aid in forecasting fish movement for better fisheries management and conservation.

Image courtesy of Nathan F. Putman et al.

Launched in 1977, Voyager 1 is the most distant human-made object in our galaxy, and is close to passing beyond the limits of our solar system.  This week, the team announced that Voyager 1 has entered the “magnetic highway,” a region between the heliosphere and interstellar space. Scientists coined the new term “magnetic highway” to describe the place where the Sun’s magnetic field lines connect with interstellar magnetic field lines. As Stone said at yesterday’s meeting, “The new region isn’t what we expected, but we’ve come to expect the unexpected from Voyager.”

Voyager has three instruments on board to measure changes in the magnetic environment—one that detects the low-energy particles that come from the solar wind within the heliosphere; one that detects the high-energy particles from interstellar space (remnants from supernovae explosions millions of years ago); and a magnetometer, which measures the strength and direction of magnetic fields.

How do the scientists know the magnetic highway isn’t just interstellar space? First, both low- and high-energy particles are detected.  Also, the magnetic field from the Sun runs east to west, and that should change dramatically once the spacecraft enters interstellar space.

Voyager 1 first merged onto this highway in late July, but then quickly exited. The same thing happened in early August, and finally Voyager entered for good in late August. Stone predicts that interstellar space can’t be too far for Voyager 1. “We believe this is the last leg of our journey to interstellar space,” Stone said. “Our best guess is it’s likely just a few months to a couple years away.” (Hopefully well before the spacecraft’s power is due to shut off in 2025.)

Because Voyager 1 is now located about 11 billion miles away from the Sun, the signal from the spacecraft takes approximately 17 hours to travel to Earth. Voyager 2, the longest continuously operated spacecraft, is about 9 billion miles away from our sun, headed in a completely different direction. While Voyager 2 has seen changes similar to those seen by Voyager 1, the changes are much more gradual. Scientists do not think Voyager 2 has yet reached the magnetic highway.

So imagine the surprise, forty years ago, when the Apollo astronauts brought back moon rocks with magnetic properties. How is that possible?

This week, two teams of scientists attempt to solve the mystery with separate papers in Nature.

Christina Dwyer, of UC Santa Cruz, and her team offer one theory. Early in its history, the Moon orbited Earth at a much closer distance than it does now, and it continues to gradually recede from Earth—even today! At close distances, tidal interactions between Earth and the Moon caused the Moon’s mantle to rotate slightly differently than the core. This differential motion of the mantle relative to the core stirred the liquid core, creating fluid motions that could give rise to a magnetic field.

Michael Le Bars, of Non-Equilibrium Phenomena Research Institute in Marseille, France, and his team have another theory. Large impact events like asteroids a few billion years ago could have caused sloshing within the lunar core for up to 10,000 years at a time.

So is it the asteroids’ fault or Earth’s? New Scientist doesn’t take sides:

Both models offer “a way out of a pretty major conundrum,” says Ben Weiss at the Massachusetts Institute of Technology.

Both theories produce a magnetic field of the right strength—about one fiftieth of what we experience here on Earth’s surface—but how do we decide which one is correct?  Sky & Telescope explains:

Distinguishing between these theories will depend in part on figuring out which rocks were magnetized when. Big bull’s-eyes happened pretty rarely in lunar history. If an impact created a dynamo, any molten surface rock around the time of the crash—such as lava created by the hit itself—would record the magnetic field created. But lava that erupted on the surface between these infrequent events wouldn’t. If most lunar rocks everywhere were magnetized during a particular time period, including rocks not made by impacts, that would sway the balance toward the precession argument, Weiss says. If impact melts are always associated with a magnetic field, the balance swings the other way.

Or maybe a combination of both? Wired makes the point that the two ideas aren’t mutually exclusive:

Dwyer herself has suggested that both models could have some parts correct, with tidal forces pushing the mantle steadily for a time and giant impacts speeding up the motion occasionally.

Image: Luc Viatour / www.Lucnix.be

Scientists from Brown University and Japan report that echolocating bats traveling in large groups minimize sound wave interference by tweaking the frequencies of the sounds they emit — their broadcasts — to detect and maneuver around obstacles. The scientists also found that bats make mental templates of each broadcast and the echo it creates, to differentiate one broadcast/echo set from another. According to James Simmons of Brown, “They’ve evolved this, so they can fly in clutter. Otherwise, they’d bump into trees and branches.”

Meanwhile, in Germany, researchers from the Max Planck Institute for Ornithology found that the greater mouse-eared bat uses an internal compass and the Earth’s magnetic field to navigate in the dark. The fact that the greater mouse-eared bat does not use echolocation, even when hunting for food, makes this finding even more surprising.

The scientists don’t know how the bats detect the magnetic field, but according to New Scientist, “By exposing bats to a short pulse of skewed magnetic field during and after sunset,” they found that exposure “during sunset confused the bats, causing them to fly in the wrong direction, while experiencing it after the sun had set had no effect. This is strong evidence that the bats rely on the magnetic field while flying at night, after calibrating it by noting where the sun has set.”

So add radio broadcaster and astronomer to the list of bats’ amazing feats!

Creative Commons image by Vermin Inc