Day To Night And Back Again: Earth’s Ionosphere During The Total Solar Eclipse

On Aug. 21, 2017, the Moon will slide in front of the Sun and for a brief moment, day will melt into a dusky night. Moving across the country, the Moon’s shadow will block the Sun’s light, and weather permitting, those within the path of totality will be treated to a view of the Sun’s outer atmosphere, called the corona.

But the total solar eclipse will also have imperceptible effects, such as the sudden loss of extreme ultraviolet radiation from the Sun, which generates the ionized layer of Earth’s atmosphere, called the ionosphere. This ever-changing region grows and shrinks based on solar conditions, and is the focus of several NASA-funded science teams that will use the eclipse as a ready-made experiment, courtesy of nature.

NASA is taking advantage of the Aug. 21 eclipse by funding 11 ground-based science investigations across the United States. Three of these will look to the ionosphere in order to improve our understanding of the Sun’s relationship to this region, where satellites orbit and radio signals are reflected back toward the Earth.

“The eclipse turns off the ionosphere’s source of high-energy radiation,” said Bob Marshall, a space scientist at University of Colorado Boulder and principal investigator for one of the studies. “Without ionizing radiation, the ionosphere will relax, going from daytime conditions to nighttime conditions and then back again after the eclipse.”

Stretching from roughly 50 to 400 miles above Earth’s surface, the tenuous ionosphere is an electrified layer of the atmosphere that reacts to changes from both Earth below and space above. Such changes in the lower atmosphere or space weather can manifest as disruptions in the ionosphere that can interfere with communication and navigation signals.

“In our lifetime, this is the best eclipse to see,” said Greg Earle, an electrical and computer engineer at Virginia Tech in Blacksburg, Virginia, who is leading another of the studies. “But we’ve also got a denser network of satellites, GPS and radio traffic than ever before. It’s the first time we’ll have such a wealth of information to study the effects of this eclipse; we’ll be drowning in data.”

Pinning down ionospheric dynamics can be tricky. “Compared to visible light, the Sun’s extreme ultraviolet output is highly variable,” said Phil Erickson, a principal investigator of a third study and space scientist at Massachusetts Institute of Technology’s Haystack Observatory in Westford, Massachusetts.

“That creates variability in ionospheric weather. Because our planet has a strong magnetic field, charged particles are also affected along magnetic field lines all over the planet—all of this means the ionosphere is complicated.”

But when totality hits on Aug. 21, scientists will know exactly how much solar radiation is blocked, the area of land it’s blocked over and for how long. Combined with measurements of the ionosphere during the eclipse, they’ll have information on both the solar input and corresponding ionosphere response, enabling them to study the mechanisms underlying ionospheric changes better than ever before.

Tying the three studies together is the use of automated communication or navigation signals to probe the ionosphere’s behavior during the eclipse. During typical day-night cycles, the concentration of charged atmospheric particles, or plasma, waxes and wanes with the Sun.

“In the daytime, ionospheric plasma is dense,” Earle said. “When the Sun sets, production goes away, charged particles recombine gradually through the night and density drops. During the eclipse, we’re expecting that process in a much shorter interval.”

The denser the plasma, the more likely these signals are to bump into charged particles along their way from the signal transmitter to receiver. These interactions refract, or bend, the path taken by the signals. In the eclipse-induced artificial night the scientists expect stronger signals, since the atmosphere and ionosphere will absorb less of the transmitted energy.

“If we set up a receiver somewhere, measurements at that location provide information on the part of the ionosphere between the transmitter and receiver,” Marshall said. “We use the receivers to monitor the phase and amplitude of the signal. When the signal wiggles up and down, that’s entirely produced by changes in the ionosphere.”

Using a range of different electromagnetic signals, each of the teams will send signals back and forth across the path of totality. By monitoring how their signals propagate from transmitter to receiver, they can map out changes in ionospheric density. The teams will also use these techniques to collect data before and after the eclipse, so they can compare the well-defined eclipse response to the region’s baseline behavior, allowing them to discern the eclipse-related effects.

Probing the Ionosphere

The ionosphere is roughly divided into three regions in altitude based on what wavelength of solar radiation is absorbed: the D, E and F, with D being the lowermost region and F, the uppermost. In combination, the three experiment teams will study the entirety of the ionosphere.

Marshall and his team, from the University of Colorado Boulder, will probe the D-region’s response to the eclipse with very low frequency, or VLF, radio signals. This is the lowest and least dense part of the ionosphere—and because of that, the least understood.

“Just because the density is low, doesn’t mean it’s unimportant,” Marshall said. “The D-region has implications for communications systems actively used by many military, naval and engineering operations.”

Marshall’s team will take advantage of the U.S. Navy’s existing network of powerful VLF transmitters to examine the D-region’s response to changes in solar output. Radio wave transmissions sent from Lamoure, North Dakota, will be monitored at receiving stations across the eclipse path in Boulder, Colorado, and Bear Lake, Utah. They plan to combine their data with observations from several space-based missions, including NOAA’s Geostationary Operational Environmental Satellite, NASA’s Solar Dynamics Observatory and NASA’s Ramaty High Energy Solar Spectroscopic Imager, to characterize the effect of the Sun’s radiation on this particular region of the ionosphere.

Erickson and team will look further up, to the E- and F-regions of the ionosphere. Using over 6,000 ground-based GPS sensors alongside powerful radar systems at MIT’s Haystack Observatory and Arecibo Observatory in Puerto Rico, along with data from several NASA space-based missions, the MIT-based team will also work with citizen radio scientists who will send radio signals back and forth over long distances across the path.
MIT’s science team will use their data to track travelling ionospheric disturbances—which are sometimes responsible for space weather patterns in the upper atmosphere—and their large-scale effects. These disturbances in the ionosphere are often linked to a phenomenon known as atmospheric gravity waves, which can also be triggered by eclipses.

“We may even see global-scale effects,” Erickson said. “Earth’s magnetic field is like a wire that connects two different hemispheres together. Whenever electrical variations happen in one hemisphere, they show up in the other.”

Earle and his Virginia Tech-based team will station themselves across the country in Bend, Oregon; Holton, Kansas; and Shaw Air Force Base in Sumter, South Carolina. Using state-of-the-art transceiver instruments called ionosondes, they will measure the ionosphere’s height and density, and combine their measurements with data from a nation-wide GPS network and signals from the ham radio Reverse Beacon Network. The team will also utilize data from SuperDARN high frequency radars, two of which lie along the eclipse path in Christmas Valley, Oregon, and Hays, Kansas.

“We’re looking at the bottom side of the F-region, and how it changes during the eclipse,” Earle said. “This is the part of the ionosphere where changes in signal propagation are strong.” Their work could one day help mitigate disturbances to radio signal propagation, which can affect AM broadcasts, ham radio and GPS signals.

Ultimately, the scientists plan to use their data to improve models of ionospheric dynamics. With these unprecedented data sets, they hope to better our understanding of this perplexing region.

“Others have studied eclipses throughout the years, but with more instrumentation, we keep getting better at our ability to measure the ionosphere,” Erickson said. “It usually uncovers questions we never thought to ask.”

Sun’s Core Rotates Four Times Faster Than Its Surface

The Sun’s core rotates nearly four times faster than the sun’s surface, according to new findings by an international team of astronomers. Scientists had assumed the core was rotating like a merry-go-round at about the same speed as the surface.

“The most likely explanation is that this core rotation is left over from the period when the Sun formed, some 4.6 billion years ago,” said Roger Ulrich, a UCLA professor emeritus of astronomy, who has studied the sun’s interior for more than 40 years and co-author of the study that was published today in the journal Astronomy and Astrophysics. “It’s a surprise, and exciting to think we might have uncovered a relic of what the Sun was like when it first formed.”

The rotation of the solar core may give a clue to how the sun formed. After the Sun formed, the solar wind likely slowed the rotation of the outer part of the Sun, he said. The rotation might also impact sunspots, which also rotate, Ulrich said. Sunspots can be enormous; a single sunspot can even be larger than the Earth.

The researchers studied surface acoustic waves in the Sun’s atmosphere, some of which penetrate to the Sun’s core, where they interact with gravity waves that have a sloshing motion similar to how water would move in a half-filled tanker truck driving on a curvy mountain road. From those observations, they detected the sloshing motions of the solar core. By carefully measuring the acoustic waves, the researchers precisely determined the time it takes an acoustic wave to travel from the surface to the center of the Sun and back again. That travel time turns out to be influenced a slight amount by the sloshing motion of the gravity waves, Ulrich said.

The researchers identified the sloshing motion and made the calculations using 16 years of observations from an instrument called GOLF (Global Oscillations at Low Frequency) on a spacecraft called SoHO (the Solar and Heliospheric Observatory)—a joint project of the European Space Agency and NASA. The method was developed by the researchers, led by astronomer Eric Fossat of the Observatoire de la Côte d’Azur in Nice, France. Patrick Boumier with France’s Institut d’Astrophysique Spatiale is GOLF’s principal investigator and a co-author of the study.

The idea that the solar core could be rotating more rapidly than the surface has been considered for more than 20 years, but has never before been measured.

The core of the Sun differs from its surface in another way as well. The core has a temperature of approximately 29 million degrees Fahrenheit, which is 15.7 million Kelvin. The sun’s surface is “only” about 10,000 degrees Fahrenheit, or 5,800 Kelvin.

Ulrich worked with the GOLF science team, analyzing and interpreting the data for 15 years. Ulrich received funding from NASA for his research. The GOLF instrument was funded primarily by the European Space Agency.

SoHO was launched on Dec. 2, 1995 to study the Sun from its core to the outer corona and the solar wind; the spacecraft continues to operate.

Eclipse On August 21 Offers Unique Research Opportunities

In a briefing today on solar eclipse science, leading U.S. scientists highlighted research projects that will take place across the country during the upcoming August 21 solar eclipse. The research will advance our knowledge of the sun’s complex and mysterious magnetic field and its effects on Earth’s atmosphere and land.

Experts at the National Science Foundation (NSF), the National Aeronautics and Space Administration (NASA) and the National Center for Atmospheric Research (NCAR) discussed how scientists from coast to coast are preparing to deploy an array of technologies and methodologies to gain unprecedented views of the sun.

The experiments, led by specialized researchers, will also draw on observations by amateur sky watchers and students.

“This total solar eclipse across the United States is a unique opportunity in modern times, enabling the entire country to be engaged through modern technology and social media,” said Carrie Black, a program director in NSF’s Division of Atmospheric and Geospace Sciences. “Images and data from as many as millions of people will be collected and analyzed by scientists for years to come.”

“This is a generational event,” agreed Madhulika Guhathakurta, NASA lead scientist for the 2017 Eclipse. “This is going to be the most documented, the most appreciated, eclipse ever.”

The scientific experiments will take place along the path of totality, a 70-mile-wide ribbon where the moon will completely cover the sun; it stretches from Oregon to South Carolina.

Viewers in any one location may experience the total eclipse for as long as two minutes and 40 seconds. It will take about an hour and a half for the eclipse to travel across the sky from the Pacific Coast to the Atlantic.

For scientists, the celestial event is a rare opportunity to observe the elusive solar corona, the sun’s outer atmosphere, which is usually obscured by the sun’s bright surface.

Many scientific questions focus on the corona: Why is it much hotter than the sun’s surface? What role does it play in spewing large streams of charged particles, known as coronal mass ejections, which strike Earth’s atmosphere and can disrupt GPS systems and other sensitive technologies?

Black noted that during the eclipse the moon will align exactly with the sun’s surface and enable observations of the entire corona, including regions that are rarely detectable. “The moon is about as perfect an occulter as one can get,” she said.

Obtaining observations from the ground will play a particularly important part in the experiments, she explained, because far more data can be transmitted than would be possible from space-based instruments.

In addition to focusing ground-based instruments on the sun, scientists will also deploy aircraft to follow the eclipse, thereby increasing the amount of time they can make observations.

An NCAR research team, for example, will use the NSF/NCAR Gulfstream-V research aircraft to take infrared measurements for about four minutes, helping scientists better understand the solar corona’s magnetism and thermal structure.

Scientists at the Southwest Research Institute in Boulder, Colorado, will use visible and infrared telescopes on NASA’s twin WB-57 airplanes to enable a unique look at both the solar corona and Mercury for about eight minutes. The goals are to better understand the movement of energy through the corona and to learn more about the composition and properties of Mercury’s surface.

During the eclipse, scientists will also study Earth’s outer atmosphere, the ionosphere, a region of the atmosphere containing particles that are charged by solar radiation. Disturbances in the ionosphere can affect radio waves. Because the eclipse blocks energy from the sun, scientists can study the ionosphere’s response to a sudden drop in solar radiation.

For example, a Boston University research team will use off-the-shelf cellphone technology to construct a single-frequency GPS array of sensors to study the ionospheric effects of the eclipse. This project could lay the foundation for using consumer smartphones to help monitor the outer atmosphere for disturbances caused by solar storms.

In another experiment, a Virginia Tech team will use a network of radio receivers and transmitters across the country to observe the ionosphere, while researchers at the University of Virginia and George Mason University will use transmitters broadcasting at low frequencies to probe various regions of the ionosphere.

Citizen scientists are expected to play a major role in making valuable observations during the eclipse. “This is a social phenomenon, and we have a significant opportunity to promote this and do all the science we can,” Guhathakurta said. Black added, “What makes this an even more valuable opportunity is that everyone has access to it.”

The Citizen Continental-America Telescopic Eclipse (CATE) Experiment by the National Solar Observatory, for example, will rely on volunteers from universities, high schools, informal education groups, and national labs for an eclipse “relay race.” Participants spaced along the path of totality will use identical telescopes and digital camera systems to capture high-quality images that will result in a dataset capturing the entire 93-minute eclipse across the country.

And a project led by the University of California, Berkeley, will assemble a large number of solar images, obtained along the eclipse path by students and amateur observers, to create educational materials as part of an “Eclipse Megamovie.”

“As these projects show, the eclipse will place the sun firmly in the forefront of the national eye,” said Scott McIntosh, director of NCAR’s High Altitude Observatory. “This is a unique opportunity to communicate the fact that our star is complex, beautiful and mysterious. At the same time, it’s more critical than ever to study it, as solar activity can pose significant threats to our technologically driven society.”

The 2017 Solar Eclipse May Prove the Sun Is Bigger Than We Think

A growing number of researchers think that the Sun is actually larger than commonly thought.

Scientists don’t know the Sun’s size as precisely as the details of the Earth and moon, making it a sticking point for perplexed eclipse modelers.

Xavier Jubier creates detailed models of solar and lunar eclipses that work with Google Maps to show precisely where the shadow of the Sun will fall on the Earth, and what the eclipse will look like at each point. He came to realize there was something off about the sun’s measurements as he matched his eclipse simulations with actual photos. The photos helped him identify exactly where an observer had been for historical eclipses — but those precise eclipse shapes only made sense if he scaled up the Sun’s radius by a few hundred kilometers.

“For me, something was wrong somewhere, but that’s all I could say,” Jubier told Space.com.

Scientists’ knowledge of the Earth’s and moon’s contours weren’t exact enough to highlight this discrepancy until about 10 years ago — the same time that modern eclipse simulations became possible through computer power and precision mapping. So it was around then that Jubier began to realize something was amiss.

NASA researcher Ernie Wright came to a similar conclusion as he began to create increasingly precise models of solar eclipses, starting about two years ago. He, too, had to scale up the sun slightly from the traditional size for his calculations to match reality.

“How can you not know this?” Wright recalls thinking. “You just hold a ruler up to the sky, and you say it’s this big.”

But as it turns out, it’s not that simple, Wright told .

Where did it come from?

Historically, researchers have used the value 696,000 km as the radius of the sun’s photosphere — the body of the sun whose wavelengths are visible to the naked eye on Earth. The value was first published in 1891 by the German astronomer Arthur Auwers, Wright said, and it was taken as a standard value for quite some time. In 2015, the International Astronomical Union defined a “unit” based on the sun’s radius as a similar 695,700 km, based on a 2008 study, so researchers can use that value to compare the sizes of other stars in the universe.

But efforts to measure the sun’s radius have never been accurate enough to match our knowledge of the moon’s and the Earth’s contours, the researchers said. Scientists have tried measuring it through transits of Mercury and Venus — when those planets cross the face of the sun — and through images taken from sun-observing satellites like the Solar Dynamics Observatory. Each pixel on SDO images covers about 90 miles (150 km), Wright said, which means there’s a limit to how precisely the size of the photosphere can be measured with this method. In addition, orbiting solar telescopes like SDO generally collect wavelengths of light emitted deeper inside or further outside the Sun, rather than its visible photosphere.

“It’s harder than you think just to put a ruler on these images and figure out how big the Sun is — [SDO] doesn’t have enough precision to nail this down,” Wright said. “Similarly, with the Mercury and Venus transits, it turns out [a measurement based on those is] not quite as precise as you’d like it to be.”

Different papers trying to pin down the sun’s radius, using planet transits, space-based sensors as well as ground observations, have produced results that differ by as many as 930 miles (1,500 km), and can’t seem to be reconciled with one another, Wright said. And for eclipse modelers, it’s a critical and irritating problem.

Eclipse viewers might find the uncertainty of interest, as well, as they plot out where they’ll be in the path of totality. A slightly larger sun means the period of total blackout can be a few seconds shorter in the center of the path, and the path itself would warp, as well.

“For most people, yes, it doesn’t really matter; it won’t change everything,” Jubier said. “But the closer you get to the edge of the [eclipse] path, the more risk you take.” If the sun is indeed bigger, the path is narrower than projections made with the usual value would suggest. So those chasing the effects on the eclipse’s edge could be in trouble if they’re not using a large enough value for their calculations.

Few people do eclipse predictions, Jubier added, and the precise value isn’t necessary to a lot of researchers. Because of that, definitions can vary and it’s hard to compare different values to one another, including the original 1891 value. It can be hard to tell for a given study what assumptions went into their answer for the Sun’s diameter, and so they can’t be adapted easily to match each other or the eclipse. Any discrepancies in eclipse measurements can be attributed to not fully understanding the values, Jubier added.

“It is definitely still an area of ongoing research, and something that the field itself is interested in getting a better handle on,” C. Alex Young, a solar astrophysicist at NASA’s Goddard Space Flight Center in Maryland, told Space.com. “Probably a little esoteric for many people, and I would say that the calculation is not as important for a lot of areas, for example in solar physics, in terms of the accuracy needed. But especially the eclipse community is very interested in the accuracy.”

Figure it out

Michael Kentrianakis, an avid eclipse chaser and a member of the American Astronomical Society’s Solar Eclipse Task Force, learned about the confusion over the sun’s size from his colleague Luca Quaglia, a physicist and eclipse researcher.

“The straw that broke the camel’s back,” Kentrianakis said, came during an expedition to Argentina in February, where he positioned himself outside what should have been the edge of an annular eclipse — where the moon is circled by a bright “ring of fire.” A larger Sun would make the “ring of fire” effect visible to a wider area.

“Technically, I should have been outside of annularity, [but the unfiltered photographs show] we were still in the path of annularity, and we have this beautiful chromosphere circling around at the edge,” Kentrianakis said. That experience fully convinced him the Sun was larger than generally thought.

This upcoming eclipse — which will very likely be the most-watched total solar eclipse in history, NASA officials have said — will provide a chance for others inside and outside the path of totality to help verify its size.

While researchers would ordinarily use the radius of the Sun to compute exactly when the moon will cover and uncover the sun for a given location, called contact times, the opposite strategy is required here, Quaglia told Space.com. “If we can measure contact times accurately, everything else being the same, the only thing that can change is the solar radius. We can actually compute the solar radius that way,” he said.

Kentrianakis, Jubier, Quaglia and others want to pin it down by positioning researchers inside and outside where totality should be, armed with the equipment for what’s called a “flash spectrum” photograph. The process uses a textured grating over a camera, which splits incoming light into component wavelengths — making it easy to determine precisely when the entire photosphere has been covered by the moon, revealing a more limited set of wavelengths emitted by the chromosphere. Combined with accurate timestamps, that process would provide strong evidence for the Sun’s size. (Such a process has been used before, but on a limited scale, Quaglia said.)

Such measurements would also provide another benefit, Jubier said — investigating what some think is a thin layer in between the photosphere and chromosphere called the mesosphere. That thin layer can be visible for a moment after the photosphere is blotted out during an eclipse, which means observers may make measurements that confuse the mesosphere for more of the photosphere. A flash spectrum can help distinguish between the two, although it must be a high enough resolution so the signals from each can be clearly separated.

A group involving Quaglia, Kentrianakis and Jubier was unable to get funding for as broad a flash-spectrum experiment as they would have liked — something like 30 separate measurement stations arrayed just inside and just outside the predicted eclipse path. But researchers could still use crowdsourced data and measurements during the eclipse to learn more.

“The more observations we have the better even if they are not providing the kind of quality we expected to get from the cinematographic spectroscopy,” Jubier said. “Time will tell what we can make of all this.”

Jubier said that flash spectrum measurements would be most useful, but so would (safely!) unfiltered views of the eclipse. Most filters cut out details of the images, making it much more difficult to determine precisely when the Sun fully covers the moon.

Other groups will also be using the eclipse to try and measure the Sun’s diameter, Quaglia said, including the International Occulting Timing Association, which will analyze smartphone videos taken at intervals perpendicular to the eclipse path in Nebraska.

“The more people, the more techniques, the more teams involved will get us there as a whole,” Quaglia said. “If, then, the International Astronomical Union makes the decision to change the value, they will probably not change the value lightly.”

Understanding the visible sun’s exact size will be possible only by combining careful solar measurements with the simulations and precise understanding of the moon’s and Earth’s elevations that exist now, Jubier said. But the pieces are in place to make that determination, if enough people get on board to measure the most common sight in the sky during those uncommon moments of eclipse.

“It’s big, and it will take many eclipses — it may take until 2024 — but at least we’re starting it now,” Kentrianakis said.

JUST IN: Large CME on Farside of Sun

A large CME occurred on the farside of the Sun beginning at approximately 03:20 UTC (July 23rd). Shortly afterwards, a fast moving, asymmetric halo coronal mass ejection (CME) became visible in LASCO coronagraph imagery.

Because the flare location was situated on the farside of the Sun, the energetic plasma cloud was directed away from Earth. Had it been directed our way, severe geomagnetic storming would have been likely.

This event is actually helpful in disrupting the far more dangerous galactic cosmic rays from entering Earth’s atmosphere. It is the very reason the Sun is in an extreme solar minimum, that is allowing up to a 20% increase in dose rates of cosmic rays.

Thank you for helping support this project to keep us informed of the latest research and breaking news. I need to register with specific journals and research sites which average about $100 each. I would also like to attend one or two symposiums attended by the top scientists in the world, who will present their latest research regarding these topics – and before it ever hits the journals or news organizations.

My next article will outline the geographic areas most vulnerable to coming events.

Watch for ongoing reports as information comes in. I also plan to present greater outlines to the science behind by research, especially for those who may be new to my work.

Cheers, Mitch

 

Major Solar Flare Detected In Dwarf Star Close To Sun

The Sun’s closest star neighbour, Proxima Centauri, a cool dwarf star situated a little over four light years away, may not be inclined to harbour habitable planets, if its temperament is anything to go by. AstroSat, along with other space and earth-based observatories, has detected a powerful solar flare sent out by this star.

At an energy of 10-raised-to-30 ergs, this explosion is about 100 times a typical flare. “If [such a flare] occurs in our Sun, it might have a devastating effect on power grids, interrupt broadcasts and electricity, affect electronic instruments, and cause excess UV radiation in space,” Professor K.P. Singh of the Tata Institute of Fundamental Research (TIFR), Mumbai, who was involved in the research, said in a joint press release issued by the observatories.

Observation campaign

On 31 May 2017, three space-based observatories, the Astrosat, Chandra and Hubble Space Telescope, and the ground-based High Accuracy Radial velocity Planet Searcher (HARPS) observatory, participated in a multi-wavelength simultaneous observation campaign.

“The mission teams of all satellites agreed to point to this star and spend a whole day watching this particular star. Also, AstroSat is sensitive enough to easily catch a flare from a star that is so close to us, if it happens during the night time of the satellite and if the telescope is pointed towards this star, as was the case here,” Professor Singh said in an email to The Hindu.

Last year’s discovery of Proxima Centauri b — a planet orbiting Proxima Centauri and, more importantly, lying in its habitable ‘Goldilocks’ zone — had set everyone wondering if it could host life.

“It [the solar flare] makes it quite improbable for Proxima Centauri b to host a life form as we know it,” said Professor Singh.