NASA Headquarters, Washington, DC
Goddard Space Flight Center, Greenbelt, MD
Montana State University, Bozeman
March 9, 1999
Using the Japanese Yohkoh spacecraft, NASA-sponsored scientists have discovered that an S-shaped structure often appears on the Sun in advance of a violent eruption, called a coronal mass ejection, that is as powerful as billions of nuclear explosions.
"Early warnings of approaching solar storms could prove useful to power companies, the communications industry and organizations that operate spacecraft, including NASA," said Dr. George Withbroe, science director for Sun-Earth Connection research at NASA Headquarters. "This is a major step forward in understanding these tremendous storms."
"S marks the spot," said Dr. Alphonse Sterling of Computational Physics, Inc., Fairfax, VA, detailed to the Institute of Space and Astronautical Science (ISAS), Japan. "We have found a strong correlation between an S-shaped pattern on the Sun, called a sigmoid, and the likelihood that an ejection will occur from that region within days. Each sigmoid is like a loaded gun that we now know has a high probability of going off."
"The S-shaped regions are the dangerous ones," said Dr. Richard Canfield, a research professor of physics at Montana State University-Bozeman, and lead author on a paper to be published in the March 15 issue of Geophysical Research Letters. "As soon as we can recognize an S-shaped region, we know that it is more likely to erupt. Other common structures look like a butterfly, quite symmetric, and these rarely erupt."
The sigmoid structures are likely the result of twisted solar magnetic fields, said Dr. Sarah Gibson of the University of Cambridge, UK. "The key to the coronal mass ejection is its magnetic field, which can structure and propel the mass outward," said Gibson.
Coronal mass ejections are violent discharges of electrically charged gas from the Sun's corona, or outer atmosphere. The largest explosions in the solar system, they hurl up to 10 billion tons of gas into space at speeds of one to two million miles an hour. The outbursts occur several times a day, but not all are hurled toward Earth.
Images from various spacecraft have provided often spectacular images and information after a coronal mass ejection had already erupted, but scientists have been trying for some time to identify a precursor for these events. Sterling and Dr. Hugh Hudson of the Solar Physics Research Corporation, Tucson, AZ, working at ISAS, first observed a relationship between a sigmoid shape before a coronal mass ejection, and an arch-shape afterwards. Later, Hudson and others found the same pattern in several other ejections.
That finding prompted Canfield, Hudson and Dr. David McKenzie, a research scientist at Montana State University, to look for a statistical correlation between the sigmoid shape and subsequent eruptions. They viewed a total of two years of daily X-ray images from the Japanese/US/UK Soft X-ray Telescope on Yohkoh. The composite pictures -- 50 images each day -- were made into movies for analysis.
"We need to get past simple classifications such as, 'Is it sigmoidal or not, is the sunspot big or small,' and get to quantitative measurements that answer, 'how twisted are the magnetic fields, how big is the spot'," Canfield said. "As well, we want to know in which direction the ejection is going to go and how many regions are likely to erupt."
Ultimately, Canfield continued, the National Oceanic and Atmospheric Administration (NOAA) may be able to include warnings of coronal mass ejections in its space weather forecasts. NOAA is building a Solar X-ray Imager similar to that on Yohkoh, scheduled for launch next year, he said.
Images and supporting material can be found on the Internet at:
California Institute of Technology
March 4, 1999
Work in Progress
Prominences typically just sit there, but sometimes they erupt violently, spewing out magnetic fields and energetic particles that travel all the way to Earth and beyond. They can wreak havoc on Earth's magnetosphere, sometimes even damaging spacecraft.
Solar prominences take on their dramatic shapes for pretty much the same reason that a magnet can make iron shavings form an arc on a sheet of paper. The sun has a substantial magnetic field and this field sometimes pokes out of the solar surface. It's like having a shrink-wrapped ball of string (the magnetic field) with occasional threads poking out of holes in the shrink- wrap. Plasma particles from the sun are trapped in this magnetic field and the light emitted by this plasma reveals the arched shape of the magnetic field. If electric currents flow along the arch, it twists up, and when it becomes too twisted, it erupts.
Caltech applied physics professor Paul Bellan thinks solar prominences are interesting for another reason. Because solar prominences involve dramatic interactions between plasma and twisted magnetic fields, they behave in a way remarkably similar to what would occur inside a fusion reactor. So understanding why prominences erupt could help lead the way to better control of fusion processes.
"If you're staring through a telescope at the sun in order to study solar prominences, you have to wait a long time to see something interesting," says Bellan. "You can't control the parameters, and you can't measure everything. But by making a miniature version of a prominence in a laboratory experiment, you have nearly complete control, and can arrange it to do interesting things which can then be carefully diagnosed."
Thus, Bellan's approach should be of interest to both solar research and fusion plasma research. At the root of both these areas is a quantity known as "magnetic helicity," which in nontechnical terms is a measure of the twist of the magnetic field (like the twist on a licorice stick). The key idea is that helicity, once created, tends to be conserved, no matter what crazy things are happening.
Helicity conservation leads to the concept of self-organization when the plasma undergoes complex instabilities. Bellan says that "no matter how you start and no matter how much turbulence and instability there is, you'll always head for a relatively simple minimum energy configuration that has the helicity you started out with."
Practical applications are apparent if one takes into consideration the history of fusion research over the last few decades. Fusion, the process that drives the sun's energy output, is the merging of two atomic nuclei to form one bigger and heavier nucleus. This happens when the nuclei have been slammed so closely together that the "strong force" takes over, thereby swamping the weaker repulsion each positively charged nucleus has for the other.
This creation of a newer and heavier nucleus releases a huge amount of energy, so scientists have long hoped that fusion can eventually become a cheap and clean source of almost limitless power.
But the reality has been plagued by many daunting engineering challenges. For one, the plasma has proved to be difficult to contain, and containment for a certain period of time is essential for getting back the energy invested into heating the plasma up in the first place.
Since a solar prominence is a natural container of plasma, Bellan's work could lead to a cheaper and easier way to confine plasma in fusion reactors.
Bellan's experiment takes place inside a large, evacuated, stainless-steel vessel nearly five feet in diameter and six feet long. The miniature prominence, formed by a specially designed plasma gun, is about a billion times smaller than actual solar prominences and lasts for a time that is more than a billion times shorter.
The plasma gun is essentially a contrivance for driving electric current through plasma, which lies in the gap between the poles of a horseshoe magnet. Hydrogen gas is injected between the magnet poles, and when several thousand volts are applied across the magnet poles, the gas suddenly turns into plasma and electric current flows from one pole to the other.
The current creates its own magnetic field, which interacts with the original magnetic field to cause twisting and instability.
"It's somewhat like blowing bubbles of magnetic field," says Bellan. "The more current you give it, the more it bulges out and the more twisted it gets."
This entire process happens in a few microseconds, which means that anyone looking into the window of the vacuum vessel sees just a bright flash of pink light. To really see what has happened -- that is, to see the geometry of the plasma arc -- Bellan and his graduate student Freddy Hansen capture the event with a digital camera that has a shutter speed of 10 billionths of a second.
In fact, they take pictures with two cameras to make stereo pictures of the twisted plasma arcs.
"The laboratory experiment mimics the actual three-dimensional dynamics on the sun and should be very helpful for understanding what is really going on; the experiment provides an excellent way to check the various theoretical models," says Bellan.
Besides helping to understand what is happening on the sun, Bellan thinks the research could eventually lead to performance improvements in the spheromak, a laboratory device that has physics remarkably similar to solar prominences and that is an important contender in the effort to develop a controlled fusion reactor.
According to Bellan, "The advantage of this prominence-spheromak approach is that you're not fighting nature -- you're leveraging it."
Images supporting this article are available at:
Goddard Space Flight Center
U.S. Air Force Space and Missile Systems Center (AFMC)
El Segundo, Calif. 90245-4687
February 25, 1999
This selection marks the first use of RSDO's unique spacecraft catalog by a U.S. government agency other than NASA. Jim Adams, RSDO Chief, said "It's important for NASA to pass along the efficiencies we've developed under the 'Faster, Better, Cheaper' paradigm to any agency that needs our help. We are proud to have been part of the team that got the Coriolis mission up and running."
U.S. Air Force Space Test Program personnel at Kirtland AFB (Albuquerque) N.M. selected a commercially available satellite from a group of 16 set forth in the RSDO's Rapid Spacecraft Acquisition Catalog of Indefinite Delivery/Indefinite Quantity contracts. The Space Test Program office at Kirtland will manage the delivery order, valued at approximately $36.4 million.
The U.S. Air Force will use the satellite for a test mission called Coriolis. Onboard Coriolis will be two payloads -- Windsat and a Solar Mass Ejection Imager (SMEI). Windsat is a Navy experiment being built by the Naval Research Laboratory (Washington) D.C. to passively measure ocean surface wind vector. SMEI is an U.S. Air Force Research Laboratory experiment to observe solar mass ejections in visible light. The spacecraft's two payloads will collect data continuously during its three-year mission.
"The Space Test Program is happy to be working with NASA on the Coriolis project by way of the Rapid Spacecraft Acquisition process," Lt. Col. Perry Ballard remarked. Ballard is the Deputy Program Manager, Space Test Program, at Kirtland. He said the Coriolis mission is important to Department of Defense and civilian researchers alike. "It will demonstrate passive remote sensing of ocean surface wind speed and direction. Weather satellites will be able to use this technology to significantly improve oceanic weather forecasting."
Coriolis will also monitor solar activity with the goal of more accurately predicting geomagnetic disturbances to orbiting satellites.
In Aug. 1998, the U.S. Air Force issued three studies valued at $150,000 each for the Coriolis accommodation assessments using Goddard's Rapid Spacecraft Acquisition IDIQ contracts, down selecting at that time from the eight RSDO vendors to three study vendors.
On Jan. 22, 1999, the three study vendors -- Ball Aerospace & Technologies Corp. (Boulder, Colo.), TRW Space and Electronics Group (Redondo Beach, Calif.) and Spectrum Astro, Inc. (Gilbert, Ariz.) -- were issued Requests for Offers for the spacecraft order.
The entire spacecraft acquisition cycle, from release of the draft Request for Offers to selection, took just two months, representing a dramatically shorter timeframe than most typical NASA and U.S. Air Force acquisition cycles.
The spacecraft is scheduled to launch from Vandenberg AFB, Calif. Dec. 15, 2001. Carried into space onboard a Titan II rocket, the spacecraft is intended for a 98.7 degree, 830 km circular orbit above the Earth.
ROYAL ASTRONOMICAL SOCIETY
31st March 1998
In addition, understanding how CMEs are produced is crucial to understanding the overall workings of the Sun. They are a dominant feature of the solar corona (the white halo seen around the Sun during solar eclipses) and may play a major role in the behaviour of the solar magnetic field.
Combined, the three coronagraphs of LASCO give images of the solar corona from 1.1 to 30 solar radii (from just above the visible surface to a distance of about 20 million km from the Sun). This wide angle view and its high sensitivity give LASCO a tremendous advantage over previous instruments.
In general, a CME is thought to occur when closed magnetic configurations in the solar corona are destabilised by some trigger. This destabilisation then leads to the expulsion of matter from the solar atmosphere. The latest research at Birmingham is revealing that the entire Sun can be affected by CMEs. This is displayed most strikingly by events observed by LASCO where an initial mass ejection is closely followed by a series of others. In some cases CMEs occur at widely separated points almost simultaneously. For the first time LASCO is showing us that the corona behaves as a single unit, capable of storing large amounts of magnetic energy which can be released from more than one point by some initial triggering mechanism.
Further evidence for a global reaction of the corona was provided by an event observed on the 23rd February 1997. LASCO C1 images (covering a region from 1.1 to 3 solar radii) showed the expansion of a CME in the lower corona moving with a speed of 880 km/s from the north-east limb of the Sun. This quickly destabilised a sequence of much larger magnetic loop structures to the south which then became the dominant feature of the CME. This sequence of events implies that a higher magnetic loop system spans the solar equator to physically connect regions in opposite hemispheres.
A study of CME events carried out by Professor George Simnett at Birmingham University has shown that they begin to undergo an acceleration at a distance of about 6 solar radii. So the LASCO observations indicate that this is probably where the solar wind begins.
The SOHO satellite was launched in December 1995. It orbits a stable point (the Lagrangian L1 point), situated approximately 1.5 million km from Earth towards the Sun at which the gravitational pull on the satellite from the Earth and the Sun are equal. This position ensures that LASCO has an uninterrupted view of the Sun.
LASCO observations will continue as solar activity moves towards a maximum in its 11 year cycle by the end of the century. Dr Mark Lyons is a research fellow working on the LASCO project at the University of Birmingham. The solar group at the University of Birmingham, headed by Prof. George Simnett, is using images from LASCO to provide new insights into the CME process.
More information about the LASCO research carried out by the solar group at the University of Birmingham and about the SOHO project, including images of CMEs, can be found on the following Web site.