From Max Planck Institute for Solar System Research: “A new twist on stellar rotation”

From Max Planck Institute for Solar System Research

September 21, 2018

Prof. Dr. Laurent Gizon
Max Planck Institute for Solar System Research, Göttingen
+49 551 384979-439
gizon@mps.mpg.de

Dr. Birgit Krummheuer
Press Office
Max Planck Institute for Dynamics and Self-Organization, Göttingen
+49 551 5176-668
birgit.krummheuer@ds.mpg.de

Like our sun, distant stars are rotating spheres of hot gas.

1
Sun-like stars rotate differentially, with the equator rotating faster than the higher latitudes. The green arrows in the figure represent rotation speed in the stellar convection zone. Differential rotation is inferred from the oscillatory motions of the star seen as orange/blue shades on the right side of the picture. Differential rotation is thought to be an essential ingredient for generating magnetic activity and starspots. © MPS / MarkGarlick.com

Stars, however, do not rotate like solid spheres: regions at different latitudes rotate at different rates. A group of researchers from New York University and the Max Planck Institute for Solar System Research (MPS) in Germany has now measured the rotational patterns of a sample of Sun-like stars.

They have identified 13 stars that rotate in a similar fashion as our Sun: their equators rotate faster than their mid latitudes. This rotation pattern is, however, much more pronounced than in the Sun: the stars’ equators are found to rotate up to twice as quickly as their mid-latitudes. This difference in rotation speed is much larger than theories had suggested.

What do we know about distant stars aside from their brightness and colors? Is our Sun a typical star? Or does it show certain properties that make it special, or maybe even unique? One property that is not fully understood is rotation. In its outer layers the Sun has a rotation pattern that scientists refer to as `latitudinal differential rotation’. This means that different latitudes rotate at different rates. While at the Sun’s equator one full rotation takes approximately 25 days, the higher latitudes rotate more slowly. Near the Sun’s poles, one full rotation takes approximately 31 days.

In their new work the scientists studied the rotation of 40 stars that resemble the Sun with respect to mass. Among those, the 13 stars for which differential rotation could be measured with confidence all show solar-like differential rotation: equators rotate faster than higher latitudes. In some cases, however, the difference in rotational speed between the equator and the mid-latitudes is much larger than in the Sun.

Classically, stellar rotation is determined by tracking starspots at different latitudes in photometric light curves. This method is limited, however, because we do not know the latitudes of the starspots. “Using observations from NASA’s Kepler mission we can now probe the interior of stars with asteroseismology and determine their rotational profiles at different latitudes and depths”, says Laurent Gizon, director at MPS.

Stars are too far away to be resolved in astronomical images. They are point like. However scientists can indirectly obtain spatial information about stellar interiors using stellar oscillations. Stars undergo global acoustic oscillations that are excited by convective motions in their outer layers. Different modes of oscillations probe different regions in a star. Thus the frequencies of oscillation inform us about different regions. In this study the scientists used stellar oscillations to measure rotation at different latitudes in the outer convection zone. “Modes of oscillation that propagate in the direction of rotation move faster than the modes that propagate in the opposite direction, thus their frequencies are slightly different”, says Gizon.

“Our best measurements all reveal stars with solar-like rotation”, says Gizon. The most surprising aspect of this research is that latitudinal differential rotation can be much stronger in some stars than in the Sun. The scientists did not expect such large values, which are not predicted by numerical models.

This work is important as it shows that asteroseismology has fantastic potential to help us understand the inner workings of stars. “Information about stellar differential rotation is key to understanding the processes that drive magnetic activity”, says Gizon. Combining information about internal rotation and activity, together with modeling, will most likely reveal the root causes of magnetic activity in stars. However, many more Sun-like stars must be studied for this to happen. In 2026 the European Space Agency will launch the PLATO mission (an exoplanet mission, like Kepler) to characterize tens of thousands of bright Sun-like stars using precision asteroseismology.

ESA/PLATO

Large-number statistics will be key to studying the physics of stars and their evolution.

Science paper:
Asteroseismic detection of latitudinal differential rotation in 13 Sun-like stars
Science, 21. September 2018

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Max Planck Institute for Solar System Research

The Max Planck Institute for Solar System Research has had an eventful history – with several moves, changes of name, and structural developments. The first prototype of the current institute was founded in 1934 in Mecklenburg; it moved to Katlenburg-Lindau in 1946. Not just the location of the buildings changed – the topic of research also moved, from Earth to outer space. In the first decades the focus of research was the stratosphere and ionosphere of the Earth, but since 1997 the institute exclusively researches the physics of planets and the Sun. In January 2014 the Max Planck Institute for Solar System Research has relocated to it’s new home: a new building in Göttingen close to the Northern Campus of the University of Göttingen.

#latitudinal-differential-rotation, #asteroseismology, #latitudinal-differential-rotation-can-be-much-stronger-in-some-stars-than-in-the-sun, #max-planck-institute-for-solar-system-research, #new-york-university, #photometric-light-curves, #solar-equators-rotate-faster-than-higher-latitudes, #solar-research, #stars-are-too-far-away-to-be-resolved-in-astronomical-images-however-scientists-can-indirectly-obtain-spatial-information-about-stellar-interiors-using-stellar-oscillations, #stellar-rotation, #stellar-rotation-is-determined-by-tracking-starspots-at-different-latitudes-in-photometric-light-curves

From Max Planck Institute for Solar System Research: “A deep look into the hearts of stars”


Max Planck Institute for Solar System Research

January 02, 2018

Dr. Birgit Krummheuer
Press and Public Relations
Max Planck Institute for Solar System Research, Göttingen
+49 551 384979-462
Krummheuer@mps.mpg.de

Earl Bellinger
Max Planck Institute for Solar System Research, Göttingen
+49 551 384979-518
Bellinger@mps.mpg.de

Dr. Saskia Hekker
Max Planck Institute for Solar System Research, Göttingen
+49 551 384979-264
Hekker@mps.mpg.de

Researchers measure the inner structure of distant suns from their pulsations

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A glimpse into the heart: Artist’s impression of the interior of the star, which was studied through its surface oscillations.
© Earl Bellinger / ESA

At first glance, it would seem to be impossible to look inside a star. An international team of astronomers, under the leadership of Earl Bellinger and Saskia Hekker of the Max Planck Institute for Solar System Research in Göttingen, has, for the first time, determined the deep inner structure of two stars based on their oscillations.

Our Sun, and most other stars, experience pulsations that spread through the star’s interior as sound waves. The frequencies of these waves are imprinted on the light of the star, and can be later seen by astronomers here on Earth. Similar to how seismologists decipher the inner structure of our planet by analyzing earthquakes, astronomers determine the properties of stars from their pulsations—a field called asteroseismology. Now, for the first time, a detailed analysis of these pulsations has enabled Earl Bellinger, Saskia Hekker and their colleagues to measure the internal structure of two distant stars.

The two stars they analyzed are part of the 16 Cygni system (known as 16 Cyg A and 16 Cyg B) and both are very similar to our own Sun.

2
http://www.daviddarling.info/encyclopedia/A/16Cyg.html

“Due to their small distance of only 70 light years, these stars are relatively bright and thus ideally suited for our analysis,” says lead author Earl Bellinger [The Astrophysical Journal]. “Previously, it was only possible to make models of the stars’ interiors. Now we can measure them.”

To make a model of a star’s interior, astrophysicists vary stellar evolution models until one of them fits to the observed frequency spectrum. However, the pulsations of the theoretical models often differ from those of the stars, most likely due to some stellar physics still being unknown.

Bellinger and Hekker therefore decided to use the inverse method. Here, they derived the local properties of the stellar interior from the observed frequencies. This method depends less on theoretical assumptions, but it requires excellent measurement data quality and is mathematically challenging.

Using the inverse method, the researchers looked more than 500,000 km deep into the stars—and found that the speed of sound in the central regions is greater than predicted by the models. “In the case of 16 Cyg B, these differences can be explained by correcting what we thought to be the mass and the size of the star,” says Bellinger. In the case of 16 Cyg A, however, the cause of the discrepancies could not be identified.

It is possible that as-yet unknown physical phenomena are not sufficiently taken into account by the current evolutionary models. “Elements that were created in the early phases of the star’s evolution may have been transported from the core of the star to its outer layers,” explains Bellinger. “This would change the internal stratification of the star, which then affects how it oscillates.”

This first structural analysis of the two stars will be followed by more. “Ten to twenty additional stars suitable for such an analysis can be found in the data from the Kepler Space Telescope,” says Saskia Hekker, who leads the Stellar Ages and Galactic Evolution (SAGE) Research Group at the Max Planck Institute in Göttingen. In the future, NASA’s TESS mission (Transiting Exoplanet Survey Satellite) and the PLATO (Planetary Transits and Oscillation of Stars) space telescope planned by the European Space Agency (ESA) will collect even more data for this research field.

NASA/Kepler Telescope

NASA/TESS

ESA/PLATO

The inverse method delivers new insights that will help to improve our understanding of the physics that happens in stars. This will lead to better stellar models, which will then improve our ability to predict the future evolution of the Sun and other stars in our Galaxy.

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Max Planck Institute for Solar System Research

The Max Planck Institute for Solar System Research has had an eventful history – with several moves, changes of name, and structural developments. The first prototype of the current institute was founded in 1934 in Mecklenburg; it moved to Katlenburg-Lindau in 1946. Not just the location of the buildings changed – the topic of research also moved, from Earth to outer space. In the first decades the focus of research was the stratosphere and ionosphere of the Earth, but since 1997 the institute exclusively researches the physics of planets and the Sun. In January 2014 the Max Planck Institute for Solar System Research has relocated to it’s new home: a new building in Göttingen close to the Northern Campus of the University of Göttingen.

#16-cyg-a-and-16-cyg-b, #a-deep-look-into-the-hearts-of-stars, #asteroseismology, #astronomy, #astrophysics, #basic-research, #cosmology, #inverse-method-derived-the-local-properties-of-the-stellar-interior-from-the-observed-frequencies, #max-planck-institute-for-solar-system-research

From Queens University Belfast and Max Planck Institute for Solar System Research via Motherboard: “At Least 9 Exoplanets Could See Earth With Present-Day Human Technology”…


Max Planck Institute for Solar System Research

QUB bloc

Queens University Belfast (QUB)

motherboard

Motherboard

…But that doesn’t mean anybody’s looking.

Since the first exoplanet was discovered in 1995, well over 3,500 planets orbiting stars other than our own have been detected. This explosion in exoplanet discovery has largely happened in the last decade due to drastically improved methods of observation. Today, the main instrument in the exoplanet hunter’s toolbox is transit photometry, which detects exoplanets by measuring the decrease in a star’s brightness as a planet passes in front of it.

Planet transit. NASA/Ames

Now, a team of scientists from Queen’s University Belfast and the Max Planck Institute for Solar System Research want to know if the same methods could be used by aliens to observe Earth. Based on their initial research [MNRAS], it seems at least nine known exoplanets have a good view of Earth—although none of these are capable of sustaining life as we know it. Still, the researchers estimate that there are ten other planets that are ideally situated to observe Earth and habitable.

1
This illustration depicts how Earth causes light from the Sun to dim as it passes in front of it from the vantage point of an observer on an exoplanet. Image: Robert Wells/Queen’s University Belfast

To understand how an alien on one of these exoplanets might see Earth, the researchers first identified the areas in the sky in which the transit zones—where a planet passes in front of the Sun—of Mercury, Venus, Earth and Mars could be seen. The researchers only focused on the four innermost planets of our solar system because these are the most likely to be observed by an ET using transit photometry.

“Larger planets would naturally block out more light as they pass in front of their star,” said Robert Wells, a graduate student at Queen’s University Belfast and the paper’s lead author. “However the more important factor is actually how close the planet is to its parent star. Since the terrestrial planets are much closer to the sun than the gas giants, they’ll be more likely to be seen in transit.”

To determine which exoplanets would have the best chance of observing our solar system, the researchers determined which parts of the sky would be able to see more than one planet’s transit in front of the Sun. As Wells and his colleagues discovered, at most three of the four terrestrial planets could be observed in transit from any point outside of our solar system.

2
The image depicts where in our galaxy an observer would be able to see planetary transits in our solar system (the blue line represents Earth’s transit). The points where these lines converge are our best bets for being seen. Image: Robert Wells/Queen’s University Belfast

Statistically speaking, this means that a randomly placed alien outside the solar system has a 1 in 40 chance of observing a single terrestrial planet in our solar system. “The probability of detecting two planets would be about ten times lower, and to detect three would be a further ten times smaller than this,” said Katja Poppenhaeger, an astrophysicist at Queen’s University Belfast.

Of the 3,500 known exoplanets, the team calculated that only 68 are situated such that they could observe at least one planet in our solar system. Of these, nine are ideally situated to observe Earth, but none of these nine planets are habitable.

All hope is not lost for cosmic voyeurism, however. The team also estimated that based on the current distribution of exoplanets, there may be dozens of yet-to-be-discovered planets in the habitable zones of their star that can also see Earth.

The team hopes to confirm this based on data from NASA’s K2 mission, which is hunting for exoplanet transits in certain areas of the sky.

NASA/Kepler Telescope

Each K2 campaign, or the time the orbital telescope spends observing a certain region of the sky, lasts for around 83 days. The researchers expect K2 to discover around a dozen exoplanets that would be able to see planetary transits in our solar system during each campaign.

With any luck, one of those exoplanets might be gazing back at us.

The future is wonderful, the future is terrifying. We should know, we live there. Whether on the ground or on the web, Motherboard travels the world to uncover the tech and science stories that define what’s coming next for this quickly-evolving planet of ours.

Motherboard is a multi-platform, multimedia publication, relying on longform reporting, in-depth blogging, and video and film production to ensure every story is presented in its most gripping and relatable format. Beyond that, we are dedicated to bringing our audience honest portraits of the futures we face, so you can be better informed in your decision-making today.

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The Research Excellence Framework (REF) 2014 results placed Queen’s joint 8th in the UK for research intensity, with over 75 per cent of Queen’s researchers undertaking world-class or internationally leading research.

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The University has won the Queen’s Anniversary Prize for Higher and Further Education on five occasions – for Northern Ireland’s Comprehensive Cancer Services programme and for world-class achievement in green chemistry, environmental research, palaeoecology and law.

Max Planck Institute for Solar System Research

The Max Planck Institute for Solar System Research has had an eventful history – with several moves, changes of name, and structural developments. The first prototype of the current institute was founded in 1934 in Mecklenburg; it moved to Katlenburg-Lindau in 1946. Not just the location of the buildings changed – the topic of research also moved, from Earth to outer space. In the first decades the focus of research was the stratosphere and ionosphere of the Earth, but since 1997 the institute exclusively researches the physics of planets and the Sun. In January 2014 the Max Planck Institute for Solar System Research has relocated to it’s new home: a new building in Göttingen close to the Northern Campus of the University of Göttingen.

#astronomy, #astrophysics, #at-least-9-exoplanets-could-see-earth-with-present-day-human-technology, #basic-research, #cosmology, #exoplanets, #max-planck-institute-for-solar-system-research, #motherboard, #queens-university-belfast, #transit-photometry

From MPG: “Unleashed magnetic power”


Max Planck Institute for Solar System Research

August 22, 2017
Helmut Hornung
Administrative Headquarters of the Max Planck Society, München
Phone:+49 89 2108-1404
hornung@gv.mpg.d

Even the ancient Chinese saw them: dark spots on the Sun. Sunspots reveal impressive details in the telescope. And they represent the key to the activity of our star, its seething and boiling. For a long time, the scientists didn’t know what this phenomenon meant.

“Be calm my son and trust in God. I assure you, the spots are nothing more than flaws in your lenses.” The Jesuit priest Christoph Scheiner (1573 to 1650) from Ingolstadt found himself in a quandary when he read the lines penned by his Provincial Superior: had God not created the Sun as a pure and flawless light? On the other hand, Scheiner knew that the black regions on the star were anything but flaws in his telescope. His contemporaries, such as Galileo Galilei and Johannes Fabricius, also observed them in the early 17th century. And even ancient Chinese chronicles report on the sunspots, which are occasionally visible with the naked eye.

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Power play: Plasma at several thousand degrees rises from the Sun’s interior, cools and sinks back down to the depths. Wherever strong magnetic fields restrain the plasma, dark sunspots are generated. Filamentary structures can be seen at the edges. The fields in these regions should actually be strong enough to prevent currents; they should therefore appear darker. Here, scientists at the Max Planck Institute for Solar System Research were able to demonstrate that the magnetic field is weakened in places. The plasma circulates and generates extended highly luminous structures that appear to rotate around their axes.
© Max Planck Institute for Solar System Research, Göttingen/Johann Hirzberger

What is this phenomenon all about? The Sun does not have a solid shell. What astronomers refer to as the surface, is a layer, only around 400 kilometres thick, from which the light visible to us originates: the photosphere. The entire Sun’s orb has enormous dimensions. At a diameter of 1.39 million kilometres, Earth would fit into it 1.3 million times. And at 2,000 quadrillion tonnes (a two followed by 27 zeros!), the Sun has 330,000 times the mass of our planet – and yet, compared to many other stars, it is a dwarf.

The sunspots appear to float in the photosphere like black islands. As early as the 17th century, researchers recognized that at least the larger of them possessed a dark core, the umbra (from the Latin: shadow). This umbra is generally surrounded by a paler semi-shadow, the penumbra. The majority of sunspots are larger than Earth; some spot groups reach an extent of 300,000 kilometres, which corresponds to two-thirds of the distance between the Earth and the Moon.

During the 18th century, even serious scientists believed that the dark spots represented holes in the Sun’s atmosphere, allowing a view to the underlying, solid surface inhabited by alien beings. Later, however, researchers determined the temperature of the photosphere to be 5,500 degrees Celsius, and that of the umbra at 4,000 degrees Celsius. This difference means the spots appear considerably darker, almost black, in contrast to the undisturbed photosphere. But why are they cooler?

The key to the spots lies beneath the skin, so to speak. The Sun is a gas balloon, in the centre of which is a fusion reactor, constantly converting hydrogen to helium at temperatures around 15 million degrees. Here, the solar power station produces 380 sextillion kilowatts of power every hour. Two mechanisms transport it to the surface: radiation and convection. In the outer convection zone – not even a quarter of the Sun’s radius – hot plasma bubbles climb to the photosphere at an average speed of 3,000 kilometres per hour, cool off and sink back down a few minutes later. This constant bubbling and boiling lends the photosphere a grainy structure. The individual grains, referred to as granules, have diameters up to 1,500 kilometres. On images of the Sun, this granulation resembles a regular pattern of corn grains.

But not only the photosphere is moving. Hot material also circulates in the Sun’s interior. This plasma – a gas comprising ions and electrons, either partially or entirely – is electrically conductive. The Sun possesses a magnetic field originating deep in its interior. The mechanism involved in producing the sunspots is still a matter of discussion: one scenario assumes that plasma rises by convection from a depth of approximately 200,000 kilometres and draws the magnetic field lines with it like a teaspoon dipped in honey and then raised to the mouth. At those locations where the bundled field lines penetrate the surface, the strong magnetic fields prevent additional hot plasma from rising: a sunspot is born.

The photo shows two such sunspots. Johann Hirzberger from the Max Planck Institute for Solar System Research recorded it using the Swedish Solar Telescope (SST) at the Roque de los Muchachos observatory on La Palma.

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The Swedish 1m Solar Telescope (SST), at the Roque de los Muchachos observatory on La Palma, Spain

With an aperture of 98 centimetres, the SST is the world’s second largest refracting telescope. The telescope tube is evacuated to prevent air turbulence, which would impair its resolving power. In addition, adaptive optics compensate for the constant flicker caused by the Earth’s atmosphere. To achieve this, the instrument analyses the Sun’s image 1000 times per second and adapts the optics correspondingly. In this way, the Swedish Solar Telescope delivers high-definition images.

At the edges of the sunspots in the image, brighter, filamentary structures can be seen. These should in fact also appear dark, because the magnetic fields should be strong enough to hinder energy replenishment and cool the region. Hirzberger and the research team he heads have already demonstrated that the local magnetic field is weakened in places. The plasma circulates and generates extended luminous structures, which appear to rotate around their axes.

Spots often occur in groups. The number is a measure of solar activity – and is not always constant: every eleven years, on average, the Sun suffers from “chicken pox”, as it were. Then, a particularly large number of sunspots cover its glaring face. The last solar maximum was predicted for May 2013. However, there were only a few sunspots at that time; the Sun currently appears to be faltering slightly.
HOR

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Max Planck Institute for Solar System Research


The Max Planck Institute for Solar System Research has had an eventful history – with several moves, changes of name, and structural developments. The first prototype of the current institute was founded in 1934 in Mecklenburg; it moved to Katlenburg-Lindau in 1946. Not just the location of the buildings changed – the topic of research also moved, from Earth to outer space. In the first decades the focus of research was the stratosphere and ionosphere of the Earth, but since 1997 the institute exclusively researches the physics of planets and the Sun. In January 2014 the Max Planck Institute for Solar System Research has relocated to it’s new home: a new building in Göttingen close to the Northern Campus of the University of Göttingen.

The Max Planck Society is Germany’s most successful research organization. Since its establishment in 1948, no fewer than 18 Nobel laureates have emerged from the ranks of its scientists, putting it on a par with the best and most prestigious research institutions worldwide. The more than 15,000 publications each year in internationally renowned scientific journals are proof of the outstanding research work conducted at Max Planck Institutes – and many of those articles are among the most-cited publications in the relevant field.

What is the basis of this success? The scientific attractiveness of the Max Planck Society is based on its understanding of research: Max Planck Institutes are built up solely around the world’s leading researchers. They themselves define their research subjects and are given the best working conditions, as well as free reign in selecting their staff. This is the core of the Harnack principle, which dates back to Adolph von Harnack, the first president of the Kaiser Wilhelm Society, which was established in 1911. This principle has been successfully applied for nearly one hundred years. The Max Planck Society continues the tradition of its predecessor institution with this structural principle of the person-centered research organization.

The currently 83 Max Planck Institutes and facilities conduct basic research in the service of the general public in the natural sciences, life sciences, social sciences, and the humanities. Max Planck Institutes focus on research fields that are particularly innovative, or that are especially demanding in terms of funding or time requirements. And their research spectrum is continually evolving: new institutes are established to find answers to seminal, forward-looking scientific questions, while others are closed when, for example, their research field has been widely established at universities. This continuous renewal preserves the scope the Max Planck Society needs to react quickly to pioneering scientific developments.

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From MPISSR: “Jupiter’s whirlwinds: Turning the other way”

Max Planck Institute for Solar System Research bloc

Max Planck Institute for Solar System Research

November 30, 2015
No Writer Credit

1
Comparison of an image of Jupiter and the new computer simulations. The image (left) shows Jupiter’s clouds patterned by strong winds. East- and westward wind bands produce the colored stripes. Anti-cyclonic whirlwinds are recognizable as brighter spots in the lower part of the image. With a diameter of 16,000 kilometers, the Great Red Spot is the largest whirlwind in our solar system. In the computer simulation (right) anti-cyclonic winds are shown in blue, cyclonic winds in red. The cyclonic rings are also visible as darker rings in the Jupiter image (left).© NASA/JPL/University of Alberta/MPS

The numerous whirlwinds covering Jupiter are caused by upward gas flows originating deep within the giant planet. This is the conclusion reached by scientists at the University of Alberta (Canada) and the Max Planck Institute for Solar Research (MPS) in Germany after extensive computer simulations. The ascending flows are deflected in higher-lying, stable gas layers and swirled by the Coriolis force. For the first time, the new model succeeds to simulate that Jupiter-whirlwinds occur predominantly in wide bands north and south of the equator. There, the Great Red Spot can be found, a giant anticyclone in the planet’s atmosphere that has been stable for centuries. The model also explains why Jupiter’s storms rotate in the opposite direction from those on Earth. The researchers report their results today in the journal Nature Geoscience.

The atmosphere of gas giant Jupiter is a turbulent place. Broad east- and westbound jet streams drive clouds of frozen ammonia grains around the planet at speeds of 550 kilometers per hour. Other regions are dominated by huge, long-lived whirlwinds. The largest of these is the Great Red Spot, a giant anticyclone, which measures up to two times Earth’s diameter and has existed for at least 350 years. Until now, how exactly these weather phenomena originate, could not be explained comprehensively.

Jupiter’s whirlwinds rotate opposite to the rotation of the planet, i.e. clockwise on the northern and anti-clockwise on the southern hemisphere. On Earth hurricanes rotate in the opposite sense. How Jupiter’s storms are formed and why they are so different from those on Earth has long been controversial. “Our high-resolution computer simulation now shows that an interaction between the movements in the deep interior of the planet and an outer stable layer is crucial,” sums up Johannes Wicht from the MPS.

Jupiter consists essentially of hydrogen and helium. Due to the high pressure of the overlying masses, this mixture becomes metallic and thus electrically conductive at about 90 percent of the planet’s radius. Further outside, the gas exists in its non-metallic “normal state”. Measurements suggest that the outermost part of that layer, home to the observable weather events, is stably stratified.

The new simulation performed by the Canadian and German researchers for the first time consider this stable layer in an elaborate computer model. “We simulate only the topmost 7,000 kilometers of the non-metallic layer, because the magnetic field significantly slows down the dynamics in deeper regions. The outer 5 percent of this layer corresponding to the outer 350 kilometers are stably stratified”, says MPS-scientist Thomas Gastine.

Driven by the heat further inside the core of the giant planet, gas rises upwards in packages – similar to water boiling in a pot. However, the overlying stable air layers provide a kind of barrier. “Only when the buoyancy of the gas package is strong enough, it can penetrate into this layer and spreads out horizontally. Under the influence of planetary rotation, the horizontal movement is swirled, just as is observed for hurricanes on Earth”, says Wicht. When the gas has cooled off enough, it sinks again into the depths of the atmosphere. “The interplay of buoyancy, horizontal motion, rotational motion, and subsidence gives rise to a characteristic signature which corresponds well to actual observations of the planet”, Wicht says. This includes a colder anticyclonic core with a typical diameter and a cyclonic ring which arises where the gas sinks back down again.

“Cyclones on Earth form in a similar way”, says Wicht. There, too, the Coriolis force from the planet’s rotation swirls air masses rising upwards. However, the cyclones on Earth rotate in the opposite direction from those on Jupiter. The reason: On Jupiter, the vortices are formed when rising gas strives apart in the upper atmosphere. On Earth, however, they start at the bottom, where air converges and then rises.

“Simulating the conditions in Jupiter’s atmosphere is tricky since many properties of this region are not well known,” explains Gastine. The researchers rely on data of NASA’s Galileo mission.

NASA Galileo
NASA/Galileo

A small probe released from the space craft penetrated more than 100 kilometers below the cloud layer until it was destroyed at a pressure of 24 bar.

As a result, the new calculations offer a very realistic picture of the uppermost layers of Jupiter’s atmosphere: the currents from the inside don’t produce the anticyclones randomly, but preferably in the vicinity of the poles as well as in certain bands above and below the equator. the size of these features diminishes with increasing distance from the equator. This is consistent with observations. “The pattern is determined by the dynamics within the planet, and in particular by the interaction of rising gas packages with the eastward and westward jet streams, which the computer model also reproduces realistically,” says Wicht.

“However, we were not able to capture the actual lifetime of the anticyclones correctly,” he adds. While typical Jupiter-anticyclones last for up to a few years, the model storms dissolve after only days. This is most likely due to the unrealistic value for the viscosity of Jupiter gases that the researchers assumed for their calculations. It was deliberately chosen too high in order to limit the required computing time.

But even with a more realistic viscosity and unlimited computing power, the amazing stability of the Great Red Spot could not be achieved. “We are just beginning to understand Jupiter’s weather phenomena”, Wicht explains. “In addition to its size and durability, the Red Spot has other special features such as its characteristic color. Additional processes seem to be involved here that we don’t yet comprehend.”

See the full article here .

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Stem Education Coalition

The Max Planck Institute for Solar System Research has had an eventful history – with several moves, changes of name, and structural developments. The first prototype of the current institute was founded in 1934 in Mecklenburg; it moved to Katlenburg-Lindau in 1946. Not just the location of the buildings changed – the topic of research also moved, from Earth to outer space. In the first decades the focus of research was the stratosphere and ionosphere of the Earth, but since 1997 the institute exclusively researches the physics of planets and the Sun. In January 2014 the Max Planck Institute for Solar System Research has relocated to it’s new home: a new building in Göttingen close to the Northern Campus of the University of Göttingen.

#astronomy, #basic-research, #jupiter, #max-planck-institute-for-solar-system-research

From Max Planck: “The sun – a mercurial star”

Max Planck Institute for Solar System Research bloc

Max Planck Institute for Solar System Research

March 17, 2015
Tim Schröder

The Sun is the Earth’s principal source of energy and climate driver. Yet sometimes it sends more light to the Earth than other times. Astronomers working with Natalie Krivova at the Max Planck Institute for Solar System Research in Göttingen take these fluctuations in solar radiation into account in their models to find out whether they contribute to global warming or counteract it.

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Climate engine with cyclic operation: The solar irradiance varies over an 11-year cycle, but also over longer periods. When the Sun is particularly active, many dark sunspots and bright faculae appear on its surface.
© Shutterstock

I actually need just two things for my work,” says Natalie Krivova, with a laugh, “a computer and time.” That’s surprising, as Natalie Krivova is an astronomer and focuses on the celestial body that is most crucial for life on Earth: the Sun. “Nevertheless, I’ve rarely ever looked through a telescope.” The researcher works in a small office at the Max Planck Institute for Solar System Research in Göttingen. On the wall hangs a whiteboard. Krivova has drawn a smiling sun on it with a green felt-tip marker. The Sun is her passion.

Natural scientists have been observing this star for more than 400 years, since Galileo Galilei developed the first powerful telescope. Since that time, mankind has learned a lot about the gigantic, hot, gaseous balloon. Some details, however, are still unclear. It was previously believed that the intensity of solar emitted radiation did not vary with time, and this was dubbed the solar constant. But now we know better: the radiant intensity of the Sun fluctuates – and this is significant for planet Earth, for life here is dependent on solar radiation.

And with the discussion surrounding climate change, the topic of solar radiation has gained additional significance in recent years. The question is whether the Sun, too, plays a role in the slow process of global warming – and if so, to what extent – or whether diminishing solar activity may even counteract anthropogenic climate change.

Climate models must account for solar activity

Natalie Krivova and her colleagues want to help answer this question. On their computers, they have developed physical computational models that simulate changes in solar activity over many centuries. This is crucial for climate researchers: “If I want to find out how severely the climate will change as a result of the emission of greenhouse gases, then of course I have to be able to assess all of the influences correctly,” says Krivova. “And as the Earth’s main source of energy, the Sun just happens to be the most important natural influencing factor.” No climate model can deliver reliable data if the solar activity isn’t computed correctly, she says.

Of course scientists today know the most important solar activity characteristics. Where solar light is vertically incident, around 1,360 watts of power strike one square meter of the Earth’s atmosphere. This value, which is calculated across all wavelengths of light, from ultraviolet to infrared, is called total solar irradiance (TSI). However, just how much energy reaches the Earth’s surface – on the continents and the ocean surface – depends on the wavelength of the solar light. Ultraviolet light, for example, is almost entirely absorbed in the upper layers of the atmosphere. It is therefore important to consider wavelengths individually.

The intensity of the solar radiation fluctuates over an approximately 11-year cycle. This up and down coincides with the increased occurrence and disappearance of sunspots – dark spots on the Sun. The largest ones are visible from Earth with the naked eye. Chinese scientists even described sunspots many centuries ago. German pharmacist and amateur astronomer Samuel Heinrich Schwabe was the first to study them systematically, starting in 1843. However, it wasn’t until the 1970s, when satellites were sent into space with measuring instruments on board, that astronomers noticed that the Sun’s radiant flux also changes with the sunspot cycle. Solar radiative flux is highest at the peak of the 11-year cycle, when particularly many sunspots are visible. Today, we know that the TSI increases by about one watt at this time. During a sunspot minimum, in contrast, hardly any spots can be seen. Radiant flux decreases during this period.

Long-term trends besides the 11-year cycle

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The Sun at the peak (left) and at a minimum of its activity: The Kodaikanal Observatory in India photographed our star in 1928 (left) and 1933. The images show the intensity in the calcium II line of the optical spectrum.
© Kodaikanal Observatory, India

One watt – that sounds negligible, but the impact on the Earth is apparently considerable. In the 17th century, there was a particularly cold period in Europe that is now known as the Little Ice Age. Dutch artist Hendrick Avercamp captured winter impressions in his well-known paintings – ice skaters, villages enveloped in snow. At that time, rivers froze over until well into spring. In the mountains, the snow didn’t melt, even in summer. In historical astronomical records, hardly any sunspots are mentioned for this period. Accordingly, the solar activity at that time is likely to have been very low for several decades.

So there isn’t just the 11-year cycle, but also a long-term trend that may change the climate on Earth over longer periods. For instance, astronomers have found indications that, over the course of the past 300 to 400 years, radiative flux may have increased by roughly another one watt. The exact figure isn’t yet known.

Interestingly, during the solar cycle, radiative flux doesn’t fluctuate with the same amplitude across the entire solar spectrum. More than 50 percent of the variation in the radiative flux comes from the ultraviolet range. And for a long time, this wasn’t taken into account in solar and climate models. In the atmosphere, the ultraviolet radiation reacts with ozone molecules, thus governing the ozone balance. In addition, it reacts with nitrogen and many other molecules. “We don’t know exactly how these reactions change over the course of the solar cycle,” says Natalie Krivova. “But there are indications that reactions take place in the atmosphere that further increase the effect of the solar irradiance,” says Krivova.

That is why Krivova’s model SATIRE (Spectral And Total Irradiance Reconstruction) also takes the fluctuations in the UV light into account. “Although the UV light makes up just 8 percent of the total solar irradiance,” she says, “the fluctuations are considerable, and if the effect of the UV changes amplifies solar influence on the atmosphere, we have to account for this in our models.”

In order for models describing natural phenomena such as climate change and solar radiation to reflect reality accurately and make reliable forecasts for the future, they have to be fed with measurement data from the past. Simulating the sea level requires level measurements, simulating solar activity, radiation measurements and many other solar observations from satellites.

Isotope measurements serve as substitute data

However, the researchers have a fundamental problem with the data situation. The physical reconstructions have to cover long periods: if you want to know how the climate and Sun will change in the coming decades and centuries, then you also need data that covers long periods of time – centuries, or better yet, millennia. But we have reliable measurements for only a few decades to feed into the models.

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The Sun’s total radiative flux fluctuates over an 11-year cycle. The SATIRE-S model (red dots) reproduces over 92 percent of measured irradiance variations (black dots). When the short, deep drops in TSI occur, dark spots wander across the Sun’s visible surface.
© MPI for Solar System Research

The data that Natalie Krivova feeds into her models goes back to 1974, and the sunspot counts, back to Galileo Galilei. But then? If there are no actual measurements, researchers rely on substitute data, known as proxies. That’s what Natalie Krivova does, as well. The astronomer uses measurements of the heavy carbon isotope 14C, or of the beryllium isotope 10Be, as proxies. These two radioactive isotopes are produced in the atmosphere by bombardment with high-energy cosmic particles, 14C, for example, when a nitrogen isotope decays. 14C is incorporated into the global carbon cycle after a few years by plants absorbing it as carbon. Plants always absorb 14C in a proportion corresponding to that in the air. The 14C uptake ends when the plant dies. Then its proportion decreases, for instance in the wood of a dead tree, due to the radioactive decay of the isotope – in the case of 14C, with a half-life of 5,730 years.

From the 14C content of wood samples today, it is possible to calculate the 14C concentration in the atmosphere at the time the carbon was incorporated into the wood. To do this, researchers must know the age of the sample. This can be determined based on the characteristic growth rings found in tree trunks, for which there are now complete profiles that go far back.

The solar irradiance over the past 11,000 years

The atmospheric 14C concentration at a given time, as ascertained from tree ring samples, stands in direct relation to how severely the Earth is bombarded with energetic charged particles. The driver of the fluctuations in particle bombardment is the Sun’s magnetic field. It acts as a protective shield for the Earth, weakening the flux of high-energy cosmic particles. When the Sun’s magnetic field is weaker, the Earth is less well protected. The solar magnetic field is also responsible for the formation of sunspots and the solar irradiance changes. Thus, 14C measurements from tree ring samples can also be used to reconstruct, indirectly, via the strength of the magnetic field, the solar irradiance. In a similar way, the 10Be data can serve as proxy data for the solar irradiance. However, beryllium precipitates out of the atmosphere and eventually falls to the ground. Historical traces of beryllium can be found today, for example, deep in the icy armor of glaciers on Greenland and in the Antarctic.

Together with other researchers, Krivova succeeded, with the aid of these proxies, in computing, in detail, the variability of the solar irradiance for the past 11,000 years since the last ice age. Compared with the models of other research groups, Krivova’s simulation tool proved to be very reliable. Climate modelers therefore use it also for those simulations that are included in the Intergovernmental Panel on Climate Change (IPCC) climate report. But it can still be better, Krivova believes. Sunspots and 14C proxies aren’t everything – the variability of the solar radiation depends on many factors. Sunspots emerge primarily in regions in which the Sun’s magnetic field is particularly strong. Here, the strong magnetic field hinders the heat transfer from the Sun’s interior to its outer boundary. Sunspots are thus areas on the Sun’s surface where less radiation is emitted, which is why they appear darker.

One would expect the radiative flux of the Sun to decrease when particularly many sunspots occur at the cycle’s peak, but the opposite is the case. This is because, simultaneously, during the active phase, many smaller, bright regions appear that are best visible in UV light. The number of these faculae, as they are called – torches – increases much more than that of spots, which compensates for the radiation attenuation in the sunspots.

Unlike sunspots, faculae are not well seen in visible light. Researchers use magnetographs for this, special instruments on satellites that make the changes in the magnetic field clearly visible – and in this way discern not only sunspots, but also faculae, because they also harbor magnetic fields.

Unexploited treasure: Photos in the calcium II line

Krivova feeds her model with the images from the magnetograph, the so-called magnetograms, and the information they contain on the size of the faculae. In this way, together with her doctoral student Kok Leng Yeo, she succeeded in refining the model in such a way that it is currently considered to be the most precise irradiance model available.

But there is one problem: unlike with the sunspots, there is, as yet, no usable faculae data from the time before the satellite era. Magnetograms of sufficient quality have been researched only since the early 1970s, so not yet long enough. Therefore, together with her doctoral student Theodosios Chatzistergos, Krivova wants to take advantage of an as-yet-unexploited treasure: around 100 years ago, astronomers began using a special method to photograph the Sun. They used photo plates that are sensitive only in a certain region of the solar spectrum, in the so-called calcium II line. At this wavelength, faculae are particularly bright.

The network could explain long-term trends

The calcium II photographs haven’t yet been thoroughly analyzed. Theodosios Chatzistergos intends to do this – an enormous task. He aims to systematically study 60,000 individual images from three observatories for faculae structures. To do this, he wrote software that automatically detects the faculae areas in the images. By comparing images from three different observatories, he hopes to detect artifacts and image errors. “We hope that this unique faculae data will help us gain an even better understanding of the variability of the solar irradiance,” says Krivova.

And then Natalie Krivova has yet another faint hope: in addition to the sunspots and the faculae, there is a third structure on the Sun’s surface that influences solar brightness. A fine network of even smaller bright spots that astronomers refer to simply as the network. “We know little about the network,” says Krivova. “We suspect that it likewise has a cycle, which is, however, weaker and extended in time compared with the sunspot cycle.” Krivova and also other researchers believe that this network contributes to the gradual long-term changes in solar irradiance characterized by extended periods during which there are especially many or few sunspots, such as the Little Ice Age. “Secular change” is the term experts use for this longterm trend – “slow, systematic change.”

“The role the network plays in this is still poorly understood – so we hope that, in the calcium II images, we will also be able to recognize and analyze the network.” As far as the long-term change in the solar activity is concerned, the Sun is evidently currently in what, from the perspective of Earth’s inhabitants, is a very interesting phase. Sunspot counts in the past years indicate that solar activity is on the decline again after 60 very active years. For the coming decades, the researchers expect a decrease in solar activity. Climate change skeptics now claim that this cooling could counterbalance the global warming caused by human emissions of greenhouse gases. But Krivova dismisses this: “Current scientific work and the reports of the IPCC clearly show that greenhouse gases have contributed many times more than the Sun to the change in the Earth’s heat balance in the past decades.”

UV irradiance to be studied in greater detail

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Dazzling solar research: The model that Natalie Krivova and her colleagues have developed describes the fluctuations in solar radiative flux very reliably. It is therefore also used in climate simulations for the IPCC’s world climate reports.
© David Ausserhofer

Krivova plans to continue her endeavors to understand the Sun’s capricious nature. For her, this also, and especially, includes a more precise investigation of ultraviolet radiation – which, after all, contributes significantly to the variability of solar irradiance. UV irradiance is modulated primarily by the magnetic field in the Sun’s upper atmosphere, the chromosphere. The chromosphere floats above the photosphere, which we humans see from Earth as the apparent surface of the gaseous balloon that is the Sun. However, the processes that take place in the chromosphere are so complicated that it is very difficult to incorporate them in models. Now, though, Natalie Krivova aims to embed in her models a sort of calculation module for the chromosphere.

Her working group at the Max Planck Institute for Solar System Research isn’t alone with its studies on solar irradiance. She and her colleagues cooperate closely with other groups in the Sun and Heliosphere Department headed by Sami Solanki. This work, in turn, is part of the ROMIC (Role of the Middle atmosphere in Climate) research program, which is sponsored by the Federal Ministry of Education and Research, and in which the middle Earth atmosphere is being studied in greater detail.

Although the weather and the climate on Earth take place in the atmospheric layers near the ground – the troposphere – the processes that take place in the layers above that have a major impact on the troposphere. Even today, researchers don’t really understand the processes taking place in the middle atmosphere. Knowledge about the Sun’s impact is also fragmentary. Natalie Krivova and her colleagues at the Max Planck Institute for Solar System Research will therefore continue, again and again, to explore uncharted solar territory.

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Stem Education Coalition

The Max Planck Institute for Solar System Research has had an eventful history – with several moves, changes of name, and structural developments. The first prototype of the current institute was founded in 1934 in Mecklenburg; it moved to Katlenburg-Lindau in 1946. Not just the location of the buildings changed – the topic of research also moved, from Earth to outer space. In the first decades the focus of research was the stratosphere and ionosphere of the Earth, but since 1997 the institute exclusively researches the physics of planets and the Sun. In January 2014 the Max Planck Institute for Solar System Research has relocated to it’s new home: a new building in Göttingen close to the Northern Campus of the University of Göttingen.

#astronomy, #basic-research, #max-planck-institute-for-solar-system-research, #solar-research