Waves Similar To Those Controlling Weather On Earth Have Now Been Found On The Sun

A team of scientists led by the Max Planck Institute for Solar System Research (MPS) and the University of Göttingen has discovered new waves of vorticity on the Sun. As described in today’s issue of Nature Astronomy, these Rossby waves propagate in the direction opposite to rotation, have lifetimes of several months, and maximum amplitudes at the Sun’s equator. For forty years scientists had speculated about the existence of such waves on the Sun, which should be present in every rotating fluid system. Now, they have been unambiguously detected and characterized for the first time. The solar Rossby waves are close relatives of the Rossby waves known to occur in the Earth’s atmosphere and oceans.

In almost every weather map of the Earth’s northern hemisphere atmospheric Rossby waves are a prominent feature. They appear as meanders in the jet stream separating cold polar air in the north from warmer subtropical air farther to the south. Sometimes these waves reach the equatorial regions and can even affect weather in Australia. In principle, waves of this type (often referred to as planetary waves) arise on every rotating sphere due to the Coriolis force. Saturn’s hexagon, a stable cloud pattern at the planet’s north pole, may also be an expression of these waves.

The existence of Rossby waves in stars was predicted about forty years ago. “Solar Rossby waves have very small amplitudes and periods of several months, thus they are extremely difficult to detect”, says Prof. Dr. Laurent Gizon, coordinator of the team that made the discovery and director at the MPS. The study required high-precision observations of the Sun over many years. The scientists from MPS analyzed a six-year dataset from the Heliospheric and Magnetic Imager (HMI) onboard NASA’s Solar Dynamics Observatory (SDO), in operation since 2010.

“The HMI images have sufficiently high spatial resolution to allow us to follow the movement of photospheric granules on the Sun’s visible surface”, says Dr. Björn Löptien, scientist at the MPS and first author of the article. These granules are small convective cells that are roughly 1500 kilometers in size on the solar surface. In their new study, the researchers used the granules as passive tracers to uncover the underlying, much larger vortex flows associated with the Rossby waves. In addition, methods of helioseismology were used to confirm the discovery and to study the Rossby waves in the solar interior at depths up to 20000 kilometers.

“All in all, we find large-scale waves of vorticity on the Sun that move in the direction opposite to rotation. That these waves are only seen in the equatorial regions is completely unexpected”, Gizon explains. The vorticity patterns are stable for several months. The researchers were able to determine the relationship between the waves’ frequency and wavelength for the first time – thus clearly identifying them as Rossby waves.

“Solar Rossby waves are gigantic in size, with wavelengths comparable to the solar radius”, Gizon explains. They are an essential component of the Sun’s internal dynamics because they contribute half of the Sun’s large-scale kinetic energy.

New Magnetic Process In Turbulent Space

Though close to home, the space immediately around Earth is full of hidden secrets and invisible processes. In a new discovery reported in the journal Nature, scientists working with NASA’s Magnetospheric Multiscale spacecraft — MMS — have uncovered a new type of magnetic event in our near-Earth environment by using an innovative technique to squeeze extra information out of the data.

Magnetic reconnection is one of the most important processes in the space — filled with charged particles known as plasma — around Earth. This fundamental process dissipates magnetic energy and propels charged particles, both of which contribute to a dynamic space weather system that scientists want to better understand, and even someday predict, as we do terrestrial weather. Reconnection occurs when crossed magnetic field lines snap, explosively flinging away nearby particles at high speeds. The new discovery found reconnection where it has never been seen before — in turbulent plasma.

“In the plasma universe, there are two important phenomena: magnetic reconnection and turbulence,” said Tai Phan, a senior fellow at the University of California, Berkeley, and lead author on the paper. “This discovery bridges these two processes.”

Magnetic reconnection has been observed innumerable times in the magnetosphere — the magnetic environment around Earth — but usually under calm conditions. The new event occurred in a region called the magnetosheath, just outside the outer boundary of the magnetosphere, where the solar wind is extremely turbulent. Previously, scientists didn’t know if reconnection even could occur there, as the plasma is highly chaotic in that region. MMS found it does, but on scales much smaller than previous spacecraft could probe.

MMS uses four identical spacecraft flying in a pyramid formation to study magnetic reconnection around Earth in three dimensions. Because the spacecraft fly incredibly close together — at an average separation of just four-and-a-half miles, they hold the record for closest separation of any multi-spacecraft formation — they are able to observe phenomena no one has seen before. Furthermore, MMS’s instruments are designed to capture data at speeds a hundred times faster than previous missions.

Even though the instruments aboard MMS are incredibly fast, they are still too slow to capture turbulent reconnection in action, which requires observing narrow layers of fast moving particles hurled by the recoiling field lines. Compared to standard reconnection, in which broad jets of ions stream out from the site of reconnection, turbulent reconnection ejects narrow jets of electrons only a couple miles wide.

“The smoking gun evidence is to measure oppositely directed electron jets at the same time, and the four MMS spacecraft were lucky to corner the reconnection site and detect both jets,” said Jonathan Eastwood, a lecturer at Imperial College, London, and a co-author of the paper.

Crucially, MMS scientists were able to leverage the design of one instrument, the Fast Plasma Investigation, to create a technique to interpolate the data — essentially allowing them to read between the lines and gather extra data points — in order to resolve the jets.

“The key event of the paper happens in only 45 milliseconds. This would be one data point with the basic data,” said Amy Rager, a graduate student at NASA’s Goddard Space Flight Center in Greenbelt, Maryland, and the scientist who developed the technique. “But instead we can get six to seven data points in that region with this method, allowing us to understand what is happening.”

With the new method, the MMS scientists are hopeful they can comb back through existing datasets to find more of these events, and potentially other unexpected discoveries as well.

Magnetic reconnection occurs throughout the universe, so that when we learn about it around our planet — where it’s easiest for Earthlings to examine it — we can apply that information to other processes farther away. The finding of reconnection in turbulence has implications, for example, for studies on the Sun. It may help scientists understand the role magnetic reconnection plays in heating the inexplicably hot solar corona — the Sun’s outer atmosphere — and accelerating the supersonic solar wind. NASA’s upcoming Parker Solar Probe mission launches directly to the Sun in the summer of 2018 to investigate exactly those questions — and that research is all the better armed the more we understand about magnetic reconnection near home.

Eta Aquarid Meteor Shower 2018 Is Peaking Now! Here’s What to Expect

Interestingly, all of the major meteor showers occurring from April through October this year happen on a weekend. This may make it easier for you to look for “shooting stars” during the predawn hours without having to worry about getting up for work or school later that morning.

Just before daybreak on Sunday (May 6), for instance, is the peak of the Eta Aquarid meteor shower. This meteor display is active in the first week of May and produces long streaks whose paths are aimed away from the “water jar” of Aquarius. Their streaks are long for a good reason, for which I will explain in a moment.

But to curb your enthusiasm, I feel it necessary to also tell you that this year, the Eta Aquarids will be poorly seen, because of glare from a waning gibbous moon, which turned full on April 29. Although not as bright (66-percent illuminated) as when it was full, it will still serve to light up the morning sky and likely squelch most of the fainter streaks from being visible. In other years — without a bright moon — the Eta Aquarids are usually the richest meteor display for observers in the Southern Hemisphere, producing up to 60 meteors per hour.

Too low

Bright moonlight is only one of two obstacles to viewing this shower. The other problem is that if you live north of the equator, hourly meteor rates drop off rather rapidly. [A Gift from Halley’s Comet: The Eta Aquarid Meteor Shower in Photos]

This is especially true for north temperate latitudes because the Eta Aquarid radiant, from which the meteors appear to dart, never reaches a high altitude above the southeast horizon. It rises around 3 a.m. local daylight time, so rates are correspondingly low.

Observers typically report only 10 meteors per hour at 26 degrees north, or about the same latitude as the Florida Keys. A little farther north at 35 degrees latitude (or the southern border of Tennessee), viewers may see about five meteors per hour. Beyond the 40th parallel north—a line of latitude that lies on the Kansas and Nebraska—skywatchers will see little to no meteors.

Hope for a “grazer”

Even if you live in a far northern location, there is still reason to head outside and take a look, for it is possible that you just might luck out and spot an “Earth grazer.” These are meteorsemerging from the Aquarid radiant that will skim the atmosphere horizontally — much like a bug skimming the side window of an automobile. They also sometimes leave colorful, long-lasting trails.

Remember the long Aquarid trails? Well, Earth-grazing meteors tend to be extremely long and usually appear to hug the horizon rather than shooting overhead, where most night-sky photographers aim their cameras.

“Earth grazers are rarely numerous,” cautioned Bill Cooke, a member of the Space Environments team at NASA’s Marshall Space Flight Center. “But even if you only see a few, you’re likely to remember them.”

Halley’s legacy

And if for nothing else, be aware that if you catch sight of an Aquarid meteor, you will have seen a piece of space debris that was shed by the famous Halley’s Comet in past centuries. They remain traveling more or less along the comet’s 75-year orbit around the sun. The particles likely range in size from sand grains to pebbles, and they have the consistency of cigar ash.

Earth, in its annual orbit around the sun, passes through this thin “river of rubble” twice: once in late October, producing the annual Orionid meteor shower, and also in early May, causing the Eta Aquarids. Each meteoroid collides with Earth’s upper atmosphere at 41 miles per second (66 kilometers per second), creating an incandescent trail of shocked, ionized air. This hot trail, not the tiny meteoroid itself, is what you see.

And in case you’re wondering, Halley’s Comet itself will return to the sun’s vicinity in the summer of 2061.

Old Data, New Tricks: Fresh Results From NASA’s Galileo Spacecraft 20 Years On

Far across the solar system, from where Earth appears merely as a pale blue dot, NASA’s Galileo spacecraft spent eight years orbiting Jupiter. During that time, the hardy spacecraft — slightly larger than a full-grown giraffe — sent back spates of discoveries on the gas giant’s moons, including the observation of a magnetic environment around Ganymede that was distinct from Jupiter’s own magnetic field. The mission ended in 2003, but newly resurrected data from Galileo’s first flyby of Ganymede is yielding new insights about the moon’s environment — which is unlike any other in the solar system.

“We are now coming back over 20 years later to take a new look at some of the data that was never published and finish the story,” said Glyn Collinson, lead author of a recent paper about Ganymede’s magnetosphere at NASA’s Goddard Space Flight Center in Greenbelt, Maryland. “We found there’s a whole piece no one knew about.”

The new results showed a stormy scene: particles blasted off the moon’s icy surface as a result of incoming plasma rain, and strong flows of plasma pushed between Jupiter and Ganymede due to an explosive magnetic event occurring between the two bodies’ magnetic environments. Scientists think these observations could be key to unlocking the secrets of the moon, such as why Ganymede’s auroras are so bright.

In 1996, shortly after arriving at Jupiter, Galileo made a surprising discovery: Ganymede had its own magnetic field. While most planets in our solar system, including Earth, have magnetic environments — known as magnetospheres — no one expected a moon to have one.

Between 1996 and 2000, Galileo made six targeted flybys of Ganymede, with multiple instruments collecting data on the moon’s magnetosphere. These included the spacecraft’s Plasma Subsystem, or PLS, which measured the density, temperature and direction of the plasma — excited, electrically charged gas — flowing through the environment around Galileo. New results, recently published in the journal Geophysical Research Letters, reveal interesting details about the magnetosphere’s unique structure.

We know that Earth’s magnetosphere — in addition to helping make compasses work and causing auroras — is key to in sustaining life on our planet, because it helps protect our planet from radiation coming from space. Some scientists think Earth’s magnetosphere was also essential for the initial development of life, as this harmful radiation can erode our atmosphere. Studying magnetospheres throughout the solar system not only helps scientists learn about the physical processes affecting this magnetic environment around Earth, it helps us understand the atmospheres around other potentially habitable worlds, both in our own solar system and beyond.

Ganymede’s magnetosphere offers the chance to explore a unique magnetic environment located within the much larger magnetosphere of Jupiter. Nestled there, it’s protected from the solar wind, making its shape different from other magnetospheres in the solar system. Typically, magnetospheres are shaped by the pressure of supersonic solar wind particles flowing past them. But at Ganymede, the relatively slower-moving plasma around Jupiter sculpts the moon’s magnetosphere into a long horn-like shape that stretches ahead of the moon in the direction of its orbit.

Flying past Ganymede, Galileo was continually pummeled by high-energy particles — a battering the moon is also familiar with. Plasma particles accelerated by the Jovian magnetosphere, continually rain down on Ganymede’s poles, where the magnetic field channels them toward the surface. The new analysis of Galileo PLS data showed plasma being blasted off the moon’s icy surface due to the incoming plasma rain.

“There are these particles flying out from the polar regions, and they can tell us something about Ganymede’s atmosphere, which is very thin,” said Bill Paterson, a co-author of the study at NASA Goddard, who served on the Galileo PLS team during the mission. “It can also tell us about how Ganymede’s auroras form.”

Ganymede has auroras, or northern and southern lights, just like Earth does. However, unlike our planet, the particles causing Ganymede’s auroras come from the plasma surrounding Jupiter, not the solar wind. When analyzing the data, the scientists noticed that during its first Ganymede flyby, Galileo fortuitously crossed right over Ganymede’s auroral regions, as evidenced by the ions it observed raining down onto the surface of the moon’s polar cap. By comparing the location where the falling ions were observed with data from Hubble, the scientists were able to pin down the precise location of the auroral zone, which will help them solve mysteries, such as what causes the auroras.

As it cruised around Jupiter, Galileo also happened to fly right through an explosive event caused by the tangling and snapping of magnetic field lines. This event, called magnetic reconnection, occurs in magnetospheres across our solar system. For the first time, Galileo observed strong flows of plasma pushed between Jupiter and Ganymede due to a magnetic reconnection event occurring between the two magnetospheres. It’s thought that this plasma pump is responsible for making Ganymede’s auroras unusually bright.

Future study of the PLS data from that encounter may yet provide new insights related to subsurface oceans previously determined to exist within the moon using data from both Galileo and the Hubble Space Telescope.

The Galileo mission was funded by NASA’s Solar System Workings program and managed by NASA’s Jet Propulsion Laboratory in Pasadena, California, for the agency’s Science Mission Directorate in Washington.

Recent Work Challenges View Of Early Mars, Picturing A Warm Desert With Occasional Rain

The climate of early Mars is a subject of debate. While it has been thought that Mars had a warm and wet climate, like Earth, other researchers suggested early Mars might have been largely glaciated. A recent study by Ramses Ramirez from the Earth-Life Science Institute (Tokyo Institute of Technology, Japan) and Robert Craddock from the National Air and Space Museum’s Center for Earth and Planetary Studies (Smithsonian Institution, USA) suggests that the early Martian surface may not have been dominated by ice, but instead it may have been modestly warm and prone to rain, with only small patches of ice.

While there is little debate about whether water previously existed on Mars, the debate regarding what the climate of Mars was like around 4 billion years ago has persisted for decades. Mars has a surprisingly diverse landscape, made up of valley networks, lake basins and possible ocean shorelines. These ancient fluvial features all provide clues that early Mars may have had a warm and wet climate, similar to Earth’s.

However, this idea has challenges. First, the amount of solar energy entering the atmosphere at the time was considered to be too low to support a warm and wet climate. Secondly, recent climate studies have argued that Mars’ ancient fluvial features can be accounted for with an icy climate, where widespread surfaces of ice promoted cooling by reflecting solar radiation. Occasional warming events would have triggered large amounts of ice-melt, and fluvial activity as a result. However, Ramses Ramirez (Earth-Life Science Institute, Japan) and Robert Craddock (Smithsonian Institution, USA) suggest that early Mars was probably warm and wet, and not so icy, after a careful geological and climatological analysis revealed little evidence of widespread glaciation.

Recently, the authors’ study, published in Nature Geoscience, argues that volcanic activity on a relatively unglaciated planet could explain Mars’ fluvial features. Volcanic eruptions releasing CO2, H2, and CH4 may have contributed to the greenhouse effect, which in turn may have promoted warming, precipitation (including rain), and the flow of water that carved out the valleys and fluvial features. However, this climate would not have been as warm and wet as Earth’s, with precipitation rates of around 10 centimeters per year (or less), similar to Earth’s semi-arid regions. This drier climate suggests that small amounts of ice deposits could have also existed, though these would have been thin, and liable to melt, contributing to the fluvial system.

In the future, the authors will be using more complex models in their analysis to investigate their warm, semi-arid climate hypothesis further. They will also be aiming to find out what the climate was like before these fluvial features formed on Mars. This will involve investigating the earliest history of Mars, which is a mysterious subject since little is currently known about it.

Mercury’s Thin, Dense Crust

Mercury is small, fast and close to the sun, making the rocky world challenging to visit. Only one probe has ever orbited the planet and collected enough data to tell scientists about the chemistry and landscape of Mercury’s surface. Learning about what is beneath the surface, however, requires careful estimation.

After the probe’s mission ended in 2015, planetary scientists estimated Mercury’s crust was roughly 22 miles thick. One University of Arizona scientist disagrees.

Using the most recent mathematical formulas, Lunar and Planetary Laboratory associate staff scientist Michael Sori estimates that the Mercurial crust is just 16 miles thick and is denser than aluminum. His study, “A Thin, Dense Crust for Mercury,” will be published May 1 in Earth and Planetary Science Letters and is currently available online.

Sori determined the density of Mercury’s crust using data collected by the Mercury Surface, Space Environment and Geochemistry Ranging (MESSENGER) spacecraft. He created his estimate using a formula developed by Isamu Matsuyama, a professor in the Lunar and Planetary Laboratory, and University of California Berkeley scientist Douglas Hemingway.

Sori’s estimate supports the theory that Mercury’s crust formed largely through volcanic activity. Understanding how the crust was formed may allow scientists to understand the formation of the entire oddly structured planet.

“Of the terrestrial planets, Mercury has the biggest core relative to its size,” Sori said.

Mercury’s core is believed to occupy 60 percent of the planet’s entire volume. For comparison, Earth’s core takes up roughly 15 percent of its volume. Why is Mercury’s core so large?

“Maybe it formed closer to a normal planet and maybe a lot of the crust and mantle got stripped away by giant impacts,” Sori said. “Another idea is that maybe, when you’re forming so close to the sun, the solar winds blow away a lot of the rock and you get a large core size very early on. There’s not an answer that everyone agrees to yet.”

Sori’s work may help point scientists in the right direction. Already, it has solved a problem regarding the rocks in Mercury’s crust.

Mercury’s Mysterious Rocks

When the planets and Earth’s moon formed, their crusts were born from their mantles, the layer between a planet’s core and crust that oozes and flows over the course of millions of years. The volume of a planet’s crust represents the percentage of mantle that was turned into rocks.

Before Sori’s study, estimates of the thickness of Mercury’s crust led scientists to believe 11 percent of the planet’s original mantle had been turned into rocks in the crust. For the Earth’s moon — the celestial body closest in size to Mercury — the number is lower, near 7 percent.

“The two bodies formed their crusts in very different ways, so it wasn’t necessarily alarming that they didn’t have the exact same percentage of rocks in their crust,” Sori said.

The moon’s crust formed when less dense minerals floated to the surface of an ocean of liquid rock that became the body’s mantle. At the top of the magma ocean, the moon’s buoyant minerals cooled and hardened into a “flotation crust.” Eons of volcanic eruptions coated Mercury’s surface and created its “magmatic crust.”

Explaining why Mercury created more rocks than the moon did was a scientific mystery no one had solved. Now, the case can be closed, as Sori’s study places the percentage of rocks in Mercury’s crust at 7 percent. Mercury is no better than the moon at making rocks.

Sori solved the mystery by estimating the crust’s depth and density, which meant he had to find out what kind of isostasy supported Mercury’s crust.

Determining Density and Depth

The most natural shape for a planetary body to take is a smooth sphere, where all points on the surface are an equal distance from the planet’s core. Isostasy describes how mountains, valleys and hills are supported and kept from flattening into smooth plains.

There are two main types isostasy: Pratt and Airy. Both focus on balancing the masses of equally sized slices of the planet. If the mass in one slice is much greater than the mass in a slice next to it, the planet’s mantle will ooze, shifting the crust on top of it until the masses of every slice are equal.

Pratt isostasy states that a planet’s crust varies in density. A slice of the planet that contains a mountain has the same mass as a slice that contains flat land, because the crust that makes the mountain is less dense than the crust that makes flat land. In all points of the planet, the bottom of the crust floats evenly on the mantle.

Until Sori completed his study, no scientist had explained why Pratt isostasy would or wouldn’t support Mercury’s landscape. To test it, Sori needed to relate the planet’s density to its topography. Scientists had already constructed a topographic map of Mercury using data from MESSENGER, but a map of density didn’t exist. So Sori made his own using MESSENGER’s data about the elements found on Mercury’s surface.

“We know what minerals usually form rocks, and we know what elements each of these minerals contain. We can intelligently divide all the chemical abundances into a list of minerals,” Sori said of the process he used to determine the location and abundance of minerals on the surface. “We know the densities of each of these minerals. We add them all up, and we get a map of density.”

Sori then compared his density map with the topographic map. If Pratt isostasy could explain Mercury’s landscape, Sori expected to find high-density minerals in craters and low-density minerals in mountains; however, he found no such relationship. On Mercury, minerals of high and low density are found in mountains and craters alike.

With Pratt isostasy disproven, Sori considered Airy isostasy, which has been used to make estimates of Mercury’s crustal thickness. Airy isostasy states that the depth of a planet’s crust varies depending on the topography.

“If you see a mountain on the surface, it can be supported by a root beneath it,” Sori said, likening it to an iceberg floating on water.

The tip of an iceberg is supported by a mass of ice that protrudes deep underwater. The iceberg contains the same mass as the water it displaces. Similarly, a mountain and its root will contain the same mass as the mantle material being displaced. In craters, the crust is thin, and the mantle is closer to the surface. A wedge of the planet containing a mountain would have the same mass as a wedge containing a crater.

“These arguments work in two dimensions, but when you account for spherical geometry, the formula doesn’t exactly work out,” Sori said.

The formula recently developed by Matsuyama and Hemingway, though, does work for spherical bodies like planets. Instead of balancing the masses of the crust and mantle, the formula balances the pressure the crust exerts on the mantle, providing a more accurate estimate of crustal thickness.

Sori used his estimates of the crust’s density and Hemingway and Matsuyama’s formula to find the crust’s thickness. Sori is confident his estimate of Mercury’s crustal thickness in its northern hemisphere will not be disproven, even if new data about Mercury is collected. He does not share this confidence about Mercury’s crustal density.

MESSENGER collected much more data on the northern hemisphere than the southern, and Sori predicts the average density of the planet’s surface will change when density data is collected over the entire planet. He already sees the need for a follow-up study in the future.

The next mission to Mercury will arrive at the planet in 2025. In the meantime, scientists will continue to use MESSENGER data and mathematical formulas to learn everything they can about the first rock from the sun.

 

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