STEREO Reveals the Anatomy of a Solar Storm in 3D

By Donna McKinney, Naval Research Laboratory, Public Affairs

WASHINGTON (NNS) -- Observations from NASA's twin Solar Terrestrial Relations Observatory (STEREO) spacecraft have allowed scientists to reveal for the first time the speed, trajectory, and 3-D shape of solar explosions known as coronal mass ejections, or CMEs.

CMEs are powerful eruptions of plasma and magnetic energy from the sun's outer atmosphere, or corona. When these sudden outbursts are directed toward Earth, they can have both breathtakingly beautiful and potentially damaging effects.

"We can now see a CME from the time it leaves the solar surface until it reaches Earth, and we can reconstruct the event in 3-D directly from the images," said Angelos Vourlidas, a solar physicist at the Naval Research Laboratory (NRL) and project scientist for the NRL-led Sun Earth Connection Coronal and Heliospheric Investigation (SECCHI) aboard STEREO.

The NRL team has always been a leader in CME research, beginning with the initial discovery by an NRL-built telescope in 1971 (OSO-7). The CME phenomenon was explored in more detail by another NRL-led experiment (P78-1/Solwind) in the 1980s, resulting in the discovery of the characteristic signature (a halo) of Earth-directed CMEs, as well as the association to interplanetary shocks. Then in 1995, the NRL-led LASCO experiment was launched, with greatly increased sensitivity enabling routine observations of Earth-directed CMEs, firmly establishing the solar origin of space weather events.

"The evolution in imaging capability since the discovery in 1971 to now is just astounding. It took 44 minutes to read out the 256x256 pixel image (worse than most cell phone images), whereas now it takes a few seconds to read out the current 2048x2048 pixel image," said Russ Howard, a solar physicist at NRL and principal investigator of the LASCO and SECCHI experiments.

Until now, CMEs were observed near the sun but the next measurements had to wait until the CME arrived at Earth three to seven days later. STEREO's ability to continuously image a CME from its explosive birth to arrival at Earth is the first time this has been achieved and represents a significant advance. Three-dimensional tracking of an evolving CME is critical to determining its structure and predicting if and how it will affect Earth.

STEREO consists of two nearly identical observatories that make simultaneous observations of CMEs from two different vantage points. One observatory "leads" Earth in its orbit around the sun, while the other observatory "trails" the planet.

"The larger the separation between the spacecraft, the larger the structures we can examine," said Vourlidas.

Using the SECCHI telescopes, solar physicists can examine a CME's structure, velocity, mass, and direction in the corona and track it through interplanetary space. Such measurements can help determine when a CME will reach Earth, at what speed and how much energy it will deliver to Earth's magnetosphere.

CMEs carry billions of tons of plasma into space at thousands of miles per hour. This plasma — which carries some of the magnetic field from the corona with it — can create a large, moving disturbance in space that also produces a shock wave. The shock can accelerate some of the surrounding particles to high energies, a form of "solar cosmic rays" that can be hazardous to spacecraft and astronauts. The shock and CME material, which arrive days later, can produce disruptive space weather during and following their interaction with Earth's magnetosphere and upper atmosphere.

The seriousness of the effects depends on the size and speed of the CME when it arrives at Earth. These sun storms can interfere with communications between ground controllers and satellites, pilots flying near Earth's poles and astronauts on the International Space Station. Radio noise from the storm can also disrupt cell phone service. Disturbances in the ionosphere related to CMEs can affect Global Positioning System navigation and, in extreme cases, induced currents in long cables and transformers on the ground can cause power outages.

The STEREO mission employs two nearly identical observatories to provide 3-D measurements of the sun to study the nature of coronal mass ejections. STEREO is the third mission in NASA's Solar Terrestrial Probes program.

Story at
http://www.navy.mil/search/display.asp?story_id=44798

NASA story at
http://www.nasa.gov/mission_pages/stereo/news/solarstorm3D.html

NASA Sees the 'Dark Side' of the Sun

An artist's concept of one of the STEREO spacecraft.

Note for my readers who have X-band capability, you can participate in monitoring these spacecraft. More in the storey below-N5FPW

January 23, 2009: Today, NASA researchers announced an event that will transform our view of the Sun and, in the process, super-charge the field of solar physics for many years to come.

"On February 6, 2011," says Chris St. Cyr of the Goddard Space Flight Center, "Super Bowl XLV will be played in Arlington, Texas."

Wait … that's not it.

And on the same day," he adds, "NASA's two STEREO spacecraft will be 180 degrees apart and will image the entire Sun for the first time in history."

STEREO's deployment on opposite sides of the Sun solves a problem that has vexed astronomers for centuries: At any given moment they can see only half of the stellar surface. The Sun spins on its axis once every 25 days, so over the course of a month the whole Sun does turn to face Earth, but a month is not nearly fast enough to keep track of events. Sunspots can materialize, explode, and regroup in a matter of days; coronal holes open and close; magnetic filaments stretch tight and—snap!—they explode, hurling clouds of hot gas into the solar system. Fully half of this action is hidden from view, a fact which places space weather forecasters in an awkward position. How can you anticipate storms when you can't see them coming? Likewise researchers cannot track the long-term evolution of sunspots or the dynamics of magnetic filaments because they keep ducking over the horizon at inconvenient times. STEREO's global view will put an end to these difficulties.

The global view is still two years away. Already, however, the two spacecraft are beaming back over-the-horizon images that have researchers and forecasters glued to their monitors.

"This is a perspective we've never had before," says STEREO mission scientist Lika Guhathakurta of NASA headquarters. "We're now monitoring more than 270 degrees of solar longitude—that's 3/4ths of the star."

"After all these years," she laughs, "we're finally getting to see the dark side of the Sun."

(Editor's note: The Sun has no dark side. That was a solar physics joke.)

STEREO's journey to the "dark side" began on Oct. 25, 2006, when the twin probes left Earth together onboard a Delta II rocket. High above the atmosphere, they separated and headed for the Moon. What happened next was a first in space navigation. The Moon acted as a gravitational slingshot, flinging the two probes in opposite directions—STEREO-A ahead of Earth and STEREO-B behind. They've been spreading apart ever since.

Because of the way the Sun spins (counterclockwise in the diagram above), STEREO-B gets a sneak preview of sunspots and coronal holes before they turn to face Earth—a boon for forecasters.

"I know forecasters at NOAA's Space Weather Prediction Center monitor STEREO-B very closely," says St Cyr. "It lets them know what's coming."

At the moment, STEREO-B enjoys a 3-day look-ahead advantage over Earth-based observatories. This has allowed researchers to predict geomagnetic storms as much as 72 hours earlier than ever before. On several occasions in late 2008, STEREO-B spotted a coronal hole spewing solar wind before any other spacecraft did.

St. Cyr notes that experienced ham radio operators can participate in this historic mission by helping NASA capture STEREO's images. The busy Deep Space Network downloads data from STEREO only three hours a day. That's plenty of time to capture all of the previous day's data, but NASA would like to monitor the transmissions around the clock.

"So we're putting together a 'mini-Deep Space Network' to stay in constant contact with STEREO," says Bill Thompson, director of the STEREO Science Center at Goddard.

The two spacecraft beam their data back to Earth via an X-band radio beacon. Anyone with a 10-meter dish antenna and a suitable receiver can pick up the signals. The data rate is low, 500 bits per second, and it takes 3 to 5 minutes to download a complete image.

So far, the mini-Network includes stations in the United Kingdom, France and Japan—and Thompson is looking for more: "NASA encourages people with X-band antennas to contact the STEREO team. We would gladly work with them and figure out how they can join our network."

The two STEREO spacecraft rank among most sophisticated solar observatories launched by NASA to date. They are equipped with sensors that measure the speed, direction and composition of the solar wind; receivers that pick up radio emissions from explosions and shock waves in the sun's atmosphere; telescopes that image the solar surface and all the tempests that rage there; and coronagraphs to monitor events in the sun's outer atmosphere.

"So, really," says Guhathakurta, "we're not only seeing the sun's dark side, we're feeling, tasting and listening to it as well."

Super Bowl Sunday may never be the same….

Graphics with this story at:
http://science.nasa.gov/headlines/y2009/23jan_darkside.htm?list1066509