From astrobites: “Cloudy with a chance of coronal mass ejections”

Astrobites bloc

astrobites

Nov 9, 2017
Kerrin Hensley

Title: Using the Coronal Evolution to Successfully Forward Model CMEs’ In Situ Magnetic Profiles
Authors: Christina Kay and Nat Gopalswamy
First Author’s Institution: NASA Goddard Space Flight Center

Status: Accepted to the Journal of Geophysical Research – Space Physics, open access

Coronal Mass Ejections and You

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An eruption on April 16, 2012 was captured here by NASA’s Solar Dynamics Observatory in the 304 Angstrom wavelength, which is typically colored in red. Credit: NASA/SDO/AIA

NASA/SDO

Coronal mass ejections (CMEs) are immense eruptions of solar plasma and magnetic fields. When a CME strikes a planet, it can have huge effects; over billions of years, CMEs can strip away a planet’s atmosphere. In the short term, CMEs wreak havoc at Earth by causing dangerous and costly geomagnetic storms.

In 1859, a CME impacted the Earth and caused the most intense geomagnetic storm ever recorded, resulting in stunning auroral displays over much of the northern hemisphere (Figure 1) and widespread failure of telegraph systems. An event of this magnitude today would cause huge damage to power grids, satellites, and oil pipelines—resulting in a trillion dollars of damage in the United States alone. So, how can we prevent this from occurring?

Enter the growing field of space weather forecasting. Although we can’t stop the Sun from ejecting CMEs, we can try to figure out if a given CME will hit the Earth, and how severe the resulting geomagnetic storm will be if it does. The severity of a geomagnetic storm is linked to the CME’s magnetic field conditions, especially the magnitude of the southward-pointing magnetic field (i.e. the component of the magnetic field that opposes the Earth’s magnetic field at the equator). If the CME’s properties can be accurately estimated, the severity of the resulting geomagnetic storm can be estimated too, allowing for power grids and satellites to be put into safe mode if necessary.

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What do we do?

Astrobites is a daily astrophysical literature journal written by graduate students in astronomy. Our goal is to present one interesting paper per day in a brief format that is accessible to undergraduate students in the physical sciences who are interested in active research.
Why read Astrobites?

Reading a technical paper from an unfamiliar subfield is intimidating. It may not be obvious how the techniques used by the researchers really work or what role the new research plays in answering the bigger questions motivating that field, not to mention the obscure jargon! For most people, it takes years for scientific papers to become meaningful.
Our goal is to solve this problem, one paper at a time. In 5 minutes a day reading Astrobites, you should not only learn about one interesting piece of current work, but also get a peek at the broader picture of research in a new area of astronomy.

#astrobites, #astronomy, #astrophysics, #basic-research, #cloudy-with-a-chance-of-coronal-mass-ejections, #coronal-mass-ejections, #cosmology, #sun-studies

From AAS NOVA: “Carrying Energy to the Corona with Waves”

AASNOVA

AAS NOVA

8 November 2017
Susanna Kohler

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How does the solar corona, the Sun’s outer atmosphere visible in this image, get so hot? [Luc Viatour]

The solar corona has a problem: it’s weirdly hot! A new study explores how magnetic waves might solve the mystery of the unusually hot corona by transporting energy to the outer atmosphere of the Sun.

The Problem with the Corona

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The temperatures of different layers of the Sun. [ISAS/JAXA]

The corona, the outer layer of the Sun’s atmosphere, has typical temperatures of 1–3 million K — significantly hotter than the cool 5,800 K of the photosphere, the surface of the Sun far below it. Since temperatures ordinarily drop the further you get from the heat source (in this case, the Sun’s atom-fusing center), this so-called “coronal heating problem” poses a definite puzzle.

As is the case for many astronomical mysteries, the answer may have something to do with magnetic fields. Alfvén waves, magnetohydrodynamic waves that travel through magnetized plasma, could potentially carry energy from the convective zone beneath the Sun’s photosphere up into the solar atmosphere. There, the Alfvén waves could turn into shock waves that dissipate their energy as heat, causing the increased temperature of the corona.

Daniel K. Inouye Solar Telescope, DKIST under construction by the National Solar Observatory atop the Haleakala volcano on the Pacific island of Maui, Hawaii, USA, at an altitude of 3,084 m (10,118 ft), with a planned completion date of 2018

Predicting Observations

Alfvén waves as a means of delivering heat to the corona makes for a nice picture, but there’s a lot of work to be done before we can be certain that this is the correct model. Observational evidence of Alfvén waves has thus far been limited to specific conditions — and the observations have not yet been enough to convince us that Alfvén waves can deliver enough energy to explain the corona’s temperature.

Lucas Tarr, a scientist at the Naval Research Laboratory, argues that upcoming solar telescopes may make it easier to detect these waves — but first we need to know what to look for! In a recent study, Tarr uses a simplified analytic model to show which frequencies of waves are likely to carry power when magnetic field lines in the corona are pertubed.

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The power carried by Alfvén waves as a function of frequency, as a result of an initial perturbation, plotted for several different initial conditions (such as the size of the perturbation or the length of the loop on which it is introduced). [Tarr 2017]

A Promising Future

Tarr modeled the effects of a minor perturbation — like a local magnetic reconnection event in the corona — on a coronal arcade, a common structure of magnetic field loops found in the corona. Tarr determined that such a disturbance would peak in power at a low frequency (maybe tens of millihertz, or oscillations on scales of minutes), but a substantial portion of the power is carried by waves of higher frequencies (0.5–4 Hz, or oscillations on scales of seconds).

Tarr’s findings confirm that with the cadence and sensitivity of current instrumentation, we would not expect to be able to detect these Alfvén waves. The results do indicate, however, that high-cadence observations with future telescope technology — like the instrumentation at the upcoming Daniel K. Inouye Solar Telescope, which should be completed in 2018 — may have the ability to reveal the presence of these waves and confirm the model of Alfvén waves as the means by which the Sun achieves its mysteriously hot corona.
Citation

Lucas A. Tarr 2017 ApJ 847 1. doi:10.3847/1538-4357/aa880a

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AAS Mission and Vision Statement

The mission of the American Astronomical Society is to enhance and share humanity’s scientific understanding of the Universe.

The Society, through its publications, disseminates and archives the results of astronomical research. The Society also communicates and explains our understanding of the universe to the public.
The Society facilitates and strengthens the interactions among members through professional meetings and other means. The Society supports member divisions representing specialized research and astronomical interests.
The Society represents the goals of its community of members to the nation and the world. The Society also works with other scientific and educational societies to promote the advancement of science.
The Society, through its members, trains, mentors and supports the next generation of astronomers. The Society supports and promotes increased participation of historically underrepresented groups in astronomy.
The Society assists its members to develop their skills in the fields of education and public outreach at all levels. The Society promotes broad interest in astronomy, which enhances science literacy and leads many to careers in science and engineering.

Adopted June 7, 2009

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From CfA: “The Secret of Magnetic Cycles in Stars”

Harvard Smithsonian Center for Astrophysics


Center For Astrophysics

Megan Watzke
Harvard-Smithsonian Center for Astrophysics
+1 617-496-7998
mwatzke@cfa.harvard.edu

Peter Edmonds
Harvard-Smithsonian Center for Astrophysics
+1 617-571-7279
pedmonds@cfa.harvard.edu

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This combination of images and artist’s impression shows changes in the Sun’s appearance and magnetic fields during part of the solar cycle. The Sun’s magnetic field flips approximately every 11 years, defining this cycle. The switch happens around at the maximum peak of magnetic activity, when sunspot and flare activity reaches its peak. We show images of the Sun captured by NASA’s Solar Dynamics Observatory (SDO) obtained on 10th October 2010 (solar minimum), 25th December 2013 (solar maximum) and on 25th June 2017 (solar minimum), combined with artist’s impressions to show the magnetic field of the Sun. Images: NASA/SDO/A. Strugarek et al; Illustrations: L. Almeida, Federal University of Rio Grande do Norte (UFRN), Brazil.

NASA/SDO

Using new numerical simulations and observations, scientists may now be able to explain why the Sun’s magnetic field reverses every eleven years. This significant discovery explains how the duration of the magnetic cycle of a star depends on its rotation, and may help us understand violent space weather phenomena around the Sun and similar stars.

During what is known as the solar cycle, the magnetic field of the Sun has reversed every 11 years over the past centuries. This flip, where the south magnetic pole switches to north and vice versa, occurs during the peak of each solar cycle and originates from a process called a “dynamo”. Magnetic fields are generated by a dynamo, which involves the rotation of the star as well as convection and the rising and falling of hot gas in the star’s interior.

For the Sun, scientists know that magnetic fields originate in its turbulent outer layers and have a complex dependency upon how quickly the Sun is rotating. Scientists have also measured magnetic cycles for distant stars with fundamental properties similar to those of the Sun. By studying the characteristics of these magnetic properties, scientists have a very promising way to better understand the magnetic evolution in our Sun associated with the dynamo process.

An international collaboration that includes the University of Montréal, the Harvard-Smithsonian Center for Astrophysics, the Commissariat à l’énergie atomique et aux énergies alternatives and the Universidade Federal do Rio Grande do Norte, carried out a set of 3D simulations of the interiors of stars similar to the Sun to explain the origin of their magnetic field cycles. The scientists found that the period of the magnetic cycle depends on the rotation rate of a star. The trend is that more slowly rotating stars have a magnetic cycle that repeats more quickly.

“The trend we found differs from theories developed in the past. This really opens new research avenues for our understanding of the magnetism of stars,” said Antoine Strugarek of the Commissariat à l’énergie atomique et aux énergies alternatives, France, the lead author of a paper published in the July 14th issue of Science Magazine.

An important advance is that the scientists’ model can explain the cycle of both the Sun and stars that astronomers categorize as Sun-like. Previously scientists thought that the Sun’s cycle might differ in behavior from those of Sun-like stars, with a shorter magnetic cycle than expected.

“Our work supports the idea that our Sun is an average, middle-aged yellow dwarf star, with a magnetic cycle compatible with cycles from its stellar cousins,” said co-author Jose-Dias Do Nascimento of the Harvard-Smithsonian Center for Astrophysics (CfA) and the University of Rio G. do Norte (UFRN), Brazil. “In other words we confirm that the Sun really is a useful proxy for understanding other stars in many ways.”

By observing more and more stars and exploring stellar structures different from those of the Sun with numerical simulations, the team of researchers hopes to refine their new scenario for the origin of stellar magnetic cycles.

One long-term goal of this work is to gain a better understanding of “space weather”, a term used to describe the wind of particles that blows away from the Sun and other stars. The acceleration mechanism for this wind is likely related to magnetic fields in the atmospheres of stars. In extreme cases, space weather can interrupt electrical power on Earth, and it can be very dangerous to satellites and astronauts.

“The changes throughout a magnetic cycle have effects throughout the Solar System and other planetary systems thanks to the influence of space weather,” said Do Nascimento.

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The Center for Astrophysics combines the resources and research facilities of the Harvard College Observatory and the Smithsonian Astrophysical Observatory under a single director to pursue studies of those basic physical processes that determine the nature and evolution of the universe. The Smithsonian Astrophysical Observatory (SAO) is a bureau of the Smithsonian Institution, founded in 1890. The Harvard College Observatory (HCO), founded in 1839, is a research institution of the Faculty of Arts and Sciences, Harvard University, and provides facilities and substantial other support for teaching activities of the Department of Astronomy.

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From ESA: “Science backing for formation-flying Sun-watcher Proba-3”

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European Space Agency

17 July 2017
No writer credit

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Proba-3, ESA’s amazing testbed
Released 30/03/2013 3:10 pm
Copyright ESA-P. Carril

ESA’s Science Programme has agreed to support the technology-demonstrating Proba-3, a double-satellite formation-flying mission tasked with observing a region of the Sun normally hidden from view.

Set for launch in late 2020, the two satellites making up Proba-3 will fly at a precise separation to cast a shadow across space, blocking out the disc of the Sun to reveal details of its ghostly surrounding ‘corona’ – usually masked by dazzling sunlight.

Proba-3, like all the missions in the Proba series, is first and foremost a technology demonstrator, exploring precision formation-flying techniques so that future multiple satellites flying together could perform equivalent tasks to a single giant spacecraft.

But, following a longstanding Proba tradition, the mission has also been given an ambitious scientific goal: returning scientifically useful data is a good way of proving the technology works as planned.

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Proba-3 satellites form artificial eclipse
Released 12/09/2016. Copyright ESA.

Proba-3 will offer solar scientists a window on the inner segment of the solar corona – a mysterious region because it is more than a million degrees hotter than the surface of the Sun it surrounds.

Up until now, the best way to observe the corona has been during a solar eclipse, although stray light through Earth’s atmosphere is a limiting factor.

As an alternative, space-flown ‘coronagraphs’ create artificial eclipses inside Sun-watching satellites such as SOHO and Stereo, but stray light still bends around their blocking discs, limiting access to the all-important inner corona.

ESA/NASA SOHO

NASA/STEREO spacecraft

Proba-3 will get around this by flying the disc of its coronagraph on a separate satellite, exactly 150 m apart, lined up with the Sun. This should open up a new view of dynamic regions extremely close to the solar surface, where the solar wind and the eruptions called ‘coronal mass ejections’ are born. Coronal mass ejections are primary sources of disturbed space weather at the Earth.

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Solar eclipses. Released 12/06/2008. Copyright Wendy Carlos & Fred Espenak.

Proba-3 is funded through ESA’s Directorate of Technology, Engineering and Quality, but in June the Agency’s Science Programme Committee endorsed the mission for additional backing through the Directorate of Science.

“It was clear that it would be very beneficial to have this mission supported in the Science programme,” explains Andrei Zhukov of the Royal Observatory of Belgium, serving as Principal Investigator for Proba’s coronagraph.

“There was widespread enthusiasm in the solar physics community. The Science Programme Committee is advised in turn by its advisory committees composed of scientists from all around Europe, giving independent endorsements, and they recommended Proba-3 be supported as a ‘mission of opportunity’.

“In plain terms, the running of Proba-3’s Science Operations Centre, which will process and distribute scientific data to scientists across Europe will be funded by the Science programme. This centre will be hosted here in Belgium, with contributions to the data processing pipeline made by Germany, Poland and Romania.

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A fiery solar explosion. Released 16/09/2013. Copyright SOHO (ESA/NASA)/S. Hill.
A coronal mass ejection observed by the ESA/NASA SOHO space mission on 4 January 2002 has been coloured to indicate the intensity of the matter being ejected by the Sun. White represents the greatest intensity, red/orange somewhat less, and blue the least.

An extreme-ultraviolet image of the Sun captured by SOHO’s EIT (Extreme ultraviolet Imaging Telescope) instrument is superimposed on the image. The shaded blue disc surrounding the Sun at the centre is a mask in SOHO’s LASCO instrument that blots out direct sunlight to allow study of the details in the Sun’s corona.

“During each highly elliptical 19.6 hour orbit, Proba-3 will be imaging the corona for about six hours at a time, at a typical rate of one image per minute, although we have the ability to increase this rate to once every two seconds for phenomena of special interest.

“So we will be returning lots of unique data, increasing scientific knowledge of the Sun and its surrounding corona.”

Proba-3 project development continues to progress well, with a structural and thermal model version of the coronagraph built, ahead of its critical design review due to take place this autumn, followed by that of the entire mission.

The challenge is in keeping the satellites safely controlled and correctly positioned relative to each other. This will be accomplished using various new technologies, including bespoke formation-flying software, GPS information, intersatellite links, startrackers, optical visual sensors and optical metrologies for close-up manoeuvring.


Published on Mar 23, 2015
Dancing is probably the oldest human artform – and now ESA’s Proba-3 precision formation-flying mission intends to extend the art of dance to space.
Like dancers, a pair of minisatellites will move around each other, their relative positions maintained to millimetre-scale precision, as if they were both parts of one giant spacecraft.
Their mission is to cast a shadow from one minisatellite onto another, in order to form an artificial total solar eclipse in space – then study the fine details of the Sun’s wispy atmosphere, the solar corona.
Franco Ongaro, ESA Director of Technical and Quality Management; Frederic Teston, Head of System and Cost Engineering; Andrea Santovincenzo, ESA engineer and the project’s manager Agnes Mestreau-Garreau, explain how to go about teaching a space mission to dance. Credit: European Space Agency, ESA.

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The European Space Agency (ESA), established in 1975, is an intergovernmental organization dedicated to the exploration of space, currently with 19 member states. Headquartered in Paris, ESA has a staff of more than 2,000. ESA’s space flight program includes human spaceflight, mainly through the participation in the International Space Station program, the launch and operations of unmanned exploration missions to other planets and the Moon, Earth observation, science, telecommunication as well as maintaining a major spaceport, the Guiana Space Centre at Kourou, French Guiana, and designing launch vehicles. ESA science missions are based at ESTEC in Noordwijk, Netherlands, Earth Observation missions at ESRIN in Frascati, Italy, ESA Mission Control (ESOC) is in Darmstadt, Germany, the European Astronaut Centre (EAC) that trains astronauts for future missions is situated in Cologne, Germany, and the European Space Astronomy Centre is located in Villanueva de la Cañada, Spain.

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From Goddard: “NASA Satellites Ready When Stars and Planets Align” A NASA Tour de Force

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NASA Goddard Space Flight Center

March 17, 2017
Mara Johnson-Groh
mara.johnson-groh@nasa.gov
NASA’s Goddard Space Flight Center, Greenbelt, Md.

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No image caption. No image credit

The movements of the stars and the planets have almost no impact on life on Earth, but a few times per year, the alignment of celestial bodies has a visible effect. One of these geometric events — the spring equinox — is just around the corner, and another major alignment — a total solar eclipse — will be visible across America on Aug. 21, with a fleet of NASA satellites viewing it from space and providing images of the event.

To understand the basics of celestial alignments, here is information on equinoxes, solstices, full moons, eclipses and transits:

Equinox

Earth spins on a tilted axis. As our planet orbits around the sun, that tilt means that during half of the year, the Northern Hemisphere receives more daylight — its summertime — and during the other half of the year, the Southern Hemisphere does. Twice a year, Earth is in just the right place so that it’s lined up with respect to the sun, and both hemispheres of the planet receive the same amount of daylight. On these days, there are almost equal amounts of day and night, which is where the word equinox — meaning “equal night” in Latin — comes from. The equinox marks the onset of spring with a transition from shorter to longer days for half the planet, along with more direct sunlight as the sun rises higher above the horizon. In 2017, in the Northern Hemisphere, the spring equinox occurs on March 20. Six months later, fall begins with the autumnal equinox on Sept. 22.

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During the equinoxes, both hemispheres receive equal amounts of daylight. Image not to scale. Credits: NASA’s Goddard Space Flight Center/Genna Duberstein

Solstice

As Earth continues along in its orbit after the equinox, it eventually reaches a point where its tilt is at the greatest angle to the plane of its orbit — and the point where one half of the planet is receiving the most daylight and the other the least. This point is the solstice — meaning “sun stands still” in Latin — and it occurs twice a year. These days are our longest and shortest days, and mark the change of seasons to summer and winter.

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During the solstices, Earth reaches a point where its tilt is at the greatest angle to the plane of its orbit, causing one hemisphere to receive more daylight than the other. Image not to scale.
Credits: NASA’s Goddard Space Flight Center/Genna Duberstein

Full Moon and New Moon

As Earth goes around the sun, the moon is also going around Earth. There is a point each month when the three bodies align with Earth between the sun and the moon. During this phase, viewers on Earth can see the full face of the moon reflecting light from the sun — a full moon. The time between full moons is about four weeks — 29.5 days to be exact. Halfway between full moons, the order of the three bodies reverses and the moon lies between the sun and Earth. During this time, we can’t see the moon reflecting the sun’s light, so it appears dark. This is the new moon.

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When the moon’s orbit around Earth lines up on the same plane as Earth’s orbit around the sun, its shadow is cast across the planet. Image not to scale. Credits: NASA’s Goddard Space Flight Center/Genna Duberstein

Lunar Eclipse

Sometimes, during a full moon, Earth lines up perfectly between the moon and the sun, so its shadow is cast on the moon. From Earth’s viewpoint, we see a lunar eclipse. The plane of the moon’s orbit around Earth isn’t precisely aligned with the plane of the Earth’s orbit around the sun so on most months we don’t see an eclipse. The next lunar eclipse — which will be visible throughout much of Asia, Europe, Africa and Australia — will occur on Aug. 7.

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When the moon falls completely in Earth’s shadow, a total lunar eclipse occurs. Only light travelling through Earth’s atmosphere, which is bent into the planet’s shadow, is reflected off
the moon, giving it a reddish hue. Image not to scale. Credits: NASA’s Goddard Space Flight Center/Genna Duberstein

Solar Eclipse

A solar eclipse happens when the moon blocks our view of the sun. This can only happen at a new moon, when the moon’s orbit positions it between the sun and Earth — but this doesn’t happen every month. As mentioned above, the plane of the moon’s orbit around Earth isn’t precisely aligned with the plane of the Earth’s orbit around the sun so, from Earth’s view, on most months we see the moon passing above or below the sun. A solar eclipse happens only on those new moons where the alignment of all three bodies are in a perfectly straight line.

When the moon blocks all of the sun’s light, a total eclipse occurs, but when the moon is farther away — making it appear smaller from our vantage point on Earth — it blocks most, but not all of the sun. This is called an annular eclipse, which leaves a ring of the sun’s light still visible from around the moon. This alignment usually occurs every year or two, but is only visible from a small area on Earth.

On Aug. 21, a total solar eclipse will move across America. While lunar eclipses are visible from entire hemispheres of Earth, a total solar eclipse is visible only from a narrow band along Earth’s surface. Since this eclipse will take about an hour and a half to cross an entire continent, it is particularly important scientifically, as it allows observations from many places for an extended duration of time. NASA has funded 11 projects to take advantage of the 2017 eclipse and study its effects on Earth as well as study the sun’s atmosphere.

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When the moon’s orbit around Earth lines up on the same plane as Earth’s orbit around the sun, its shadow is cast across the planet. Image not to scale. Credits: NASA’s Goddard Space Flight Center/Genna Duberstein

Transits


Planet transit. NASA/Ames

An eclipse is really just a special kind of transit — which is when any celestial body passes in front of another. From Earth we are able to watch transits such as Mercury and Venus passing in front of the sun. But such transits also offer a way to spot new distant worlds. When a planet in another star system passes in front of its host star, it blocks some of the star’s light — making it appear slightly dimmer. By watching for changes in the amount of light over time, we can deduce the presence of a planet. This method has been used to discover thousands of planets, including the TRAPPIST-1 planets.

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The seven planets that orbit the Trappist-1 star, in order of their distance from the star, compared to Earth’s solar system. https://www.thestar.com/news/world/2017/02/22/what-to-know-about-the-newly-discovered-trappist-1-solar-system.html

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During a transit, a planet passes in between us and the star it orbits. This method is commonly used to find new exoplanets in our galaxy. Image not to scale.
Credits: NASA’s Goddard Space Flight Center/Genna Duberstein

For more information about how NASA looks at these events, visit:

http://www.nasa.gov/sunearth

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NASA’s Goddard Space Flight Center is home to the nation’s largest organization of combined scientists, engineers and technologists that build spacecraft, instruments and new technology to study the Earth, the sun, our solar system, and the universe.

Named for American rocketry pioneer Dr. Robert H. Goddard, the center was established in 1959 as NASA’s first space flight complex. Goddard and its several facilities are critical in carrying out NASA’s missions of space exploration and scientific discovery.

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From Eos: “Scientists Get First Glimpse of Solar Wind as It Forms”

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Eos

9.9.16
JoAnna Wendel

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An extreme ultraviolet light image of the Sun and its corona from NASA’s Solar Terrestrial Relations Observatory (STEREO). Credit: NASA/STEREO

NASA/STEREO spacecraft
NASA/STEREO spacecraft

What does solar wind look like when it first forms from the Sun’s corona? Now, with new satellite images manipulated to remove background light, scientists can answer that question.

“This is part of the last major connection we need to make to understand how [the Sun] influences the environment around the Earth,” Craig DeForest, an astrophysicist at the Southwest Research Institute in Boulder, Colo., told Eos. DeForest is the lead author on a new paper describing the novel technique, published last week in the Astrophysical Journal.

A Tricky Search

Back in the 1960s, scientists discovered the solar wind, a constant flow from the Sun of extremely high temperature plasma that’s so hot the Sun’s gravity can’t hold it. Scientists knew that the solar wind was somehow connected to the Sun’s corona—the bright layer of the Sun’s atmosphere that can be seen during a solar eclipse—but until now, scientists weren’t sure how one transitioned into the other.

This transition is important because “we’re trying to understand, among other things, why the solar wind near the Earth is variable and gusty,” DeForest said. This gustiness can affect things like the trajectory of coronal mass ejections—huge magnetic explosions from that Sun that, when they hit Earth, can knock out telecommunications, short out satellite circuitry, and damage electrical transmission lines.

But studying the transition between the corona and the solar wind is difficult—the solar wind is very faint against a background full of stars and interplanetary dust, DeForest said, making it hard to discern exactly what is happening as the solar wind gets created.

When scientists looked at previous images and “saw the [corona] fade, it was difficult to tell whether it was fading in an absolute sense or dropping below stellar background,” DeForest continued.

Unfixing the View

With computer-processed images from NASA’s Solar Terrestrial Relations Observatory (STEREO), the scientists finally observed this transition. The processing removed objects of “fixed brightness,” DeForest said, like the dust cloud that fills the inner solar system and the background stars themselves. That left the moving and variable features of the solar wind itself.

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Two views of the solar wind: STEREO’s images (left) before computer processing and (right) after processing. Scientists used an algorithm to dim the light coming from the background star field. Credit: NASA/STEREO, data from Craig DeForest, SwRI

Scientists already knew that masses of particles in the corona are controlled by magnetic fields, which gives the Sun its “rays”—similar to those in a child’s drawing, DeForest said. The new images revealed the farthest reaches of the magnetically controlled corona, showing that once the material travels about a third of the distance from the Sun to the Earth, the magnetic fields weaken enough that solar wind particles can disperse from the field lines and fan out more like an Earthly wind.

The video below, from NASA, compares this transformation of the solar wind from rays to dispersed particles to the way water shoots from a water gun or hose: Closer to the water gun, the water is one mass, but as it moves farther from the gun, it disperses into a spray of individual droplets.

Investigating this transition region will help scientists to predict the arrival and strength of the Sun’s outbursts— Earth-bound coronal mass ejections—after they pass through a full astronomical unit of the existing solar wind, DeForest said.

See the full article here .

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Eos is the leading source for trustworthy news and perspectives about the Earth and space sciences and their impact. Its namesake is Eos, the Greek goddess of the dawn, who represents the light shed on understanding our planet and its environment in space by the Earth and space sciences.

#astronomy, #basic-research, #eos, #solar-wind, #sun-studies

From Chandra: “Astronomers Gain New Insight into Magnetic Field Of Sun and Its Kin”

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NASA Chandra Telescope

NASA Chandra

July 27, 2016
Molly Porter
Marshall Space Flight Center, Huntsville, Ala.
256-544-0034
molly.a.porter@nasa.gov

Megan Watzke
Chandra X-ray Center, Cambridge, Mass.
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An artist’s illustration depicts the interior of a low-mass star, such as GJ 3253, a low-mass red dwarf star about 31 light years away from Earth, seen in an X-ray image from Chandra in the inset.
Credits: X-ray: NASA/CXC/Keele Univ./N. Wright et al; Optical: DSS

Astronomers have used data from NASA’s Chandra X-ray Observatory to make a discovery that may have profound implications for understanding how the magnetic field in the Sun and stars like it are generated.

Researchers have discovered that four old red dwarf stars with masses less than half that of the Sun are emitting X-rays at a much lower rate than expected.

A new study of four low-mass stars may have important implications for understanding the magnetic field of the Sun.

Magnetic fields are responsible for solar storms that can generate auroras, knock out satellites, and affect astronauts in space.

X-ray emission is an excellent indicator of a star’s magnetic field strength.

Two low-mass stars observed with Chandra and two by ROSAT showed their X-ray emission was similar to that of stars like the Sun.

NASA/ROSAT satellite
DLR/NASA ROSAT satellite

X-ray emission is an excellent indicator of a star’s magnetic field strength so this discovery suggests that these stars have much weaker magnetic fields than previously thought.

Since young stars of all masses have very high levels of X-ray emission and magnetic field strength, this suggests that the magnetic fields of these stars weakened over time. While this is a commonly observed property of stars like our Sun, it was not expected to occur for low-mass stars, as their internal structure is very different.

The Sun and other stars are giant spheres of superheated gas. The Sun’s magnetic field is responsible for producing sunspots, its 11-year cycle, and powerful eruptions of particles from the solar surface. These solar storms can produce spectacular auroras on Earth, damage electrical power systems, knock out communications satellites, and affect astronauts in space.

“We have known for decades that the magnetic field on the Sun and other stars plays a huge role in how they behave, but many details remain mysterious,” said lead author Nicholas Wright of Keele University in the United Kingdom. “Our result is one step in the quest to fully understand the Sun and other stars.”

The rotation of a star and the flow of gas in its interior both play a role in producing its magnetic field. The rotation of the Sun and similar stars varies with latitude (the poles versus the equator) as well as in depth below the surface. Another factor in the generation of magnetic field is convection. Similar to the circulation of warm air inside an oven, the process of convection in a star distributes heat from the interior of the star to its surface in a circulating pattern of rising cells of hot gas and descending cooler gas.

Convection occurs in the outer third (by radius) of the Sun, while the hot gas closer to the core remains relatively still. There is a difference in the speed of rotation between these two regions. Many astronomers think this difference is responsible for generating most of the magnetic field in the Sun by causing magnetic fields along the border between the convection zone and the core to wind up and strengthen. Since stars rotate more slowly as they age, this also plays a role in how the magnetic field of such stars weakens with time

“In some ways you can think of the inside of a star as an incredibly complicated dance with many, many dancers,” said co-author Jeremy Drake of the Harvard-Smithsonian Center for Astrophysics in Cambridge, Mass. “Some dancers move with each other while others move independently. This motion generates magnetic field, but how it works in detail is extremely challenging to determine.”

For stars much less massive than the Sun, convection occurs all the way into the core of the star. This means the boundary between regions with and without convection, thought to be crucial for generating magnetic field in the Sun, does not exist. One school of thought has been that magnetic field is generated mostly by convection in such stars. Since convection does not change as a star ages, their magnetic fields would not weaken much over time.

By studying four of these low-mass red dwarf stars in X-rays, Wright and Drake were able to test this hypothesis. They used NASA’s Chandra X-ray Observatory to study two of the stars and data from the ROSAT satellite to look at two others.

“We found that these smaller stars have magnetic fields that decrease as they age, exactly as it does in stars like our Sun,” said Wright. “This really goes against what we would have expected.”

These results imply that the interaction along the convection zone-core boundary does not dominate the generation of magnetic field in stars like our Sun, since the low mass stars studied by Wright and Drake lack such a region and yet their magnetic properties are very similar.

A paper describing these results by Wright and Drake appears in the July 28th issue of the journal Nature. NASA’s Marshall Space Flight Center in Huntsville, Alabama, manages the Chandra program for NASA’s Science Mission Directorate in Washington. The Smithsonian Astrophysical Observatory in Cambridge, Massachusetts, controls Chandra’s science and flight operations.

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NASA’s Marshall Space Flight Center in Huntsville, Ala., manages the Chandra program for NASA’s Science Mission Directorate in Washington. The Smithsonian Astrophysical Observatory controls Chandra’s science and flight operations from Cambridge, Mass.

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