WHAT'S UP WITH THE SUN?
With our nearest star recently displaying even more erratic behaviour, solar physicists have turned their attention to its tempestuous surface – and, for the first time, could have found answers to its greatest puzzle
The summer of 2017 treated the North American continent to a rare total solar eclipse. Keen observers could see – even with the naked eye – neon tongues of solar flares licking the sky from behind the Moon’s limb. The flares have been part of a recent trend in solar activity, a trend that affects power grids, communications, weather satellites and even the aurora in our night sky.
On 6 September, 2017, two titanic flares exploded from the Sun. The first, rocketing into space at 9.10am GMT, was an X-class flare (the most powerful type). Just three hours later, a second, more powerful flare erupted, this one four-times as fierce as the earlier event. “This event started out with a lone sunspot sitting out there on the
Sun, wishing it had company,” says Dean Pesnell, lead project scientist for NASA’s Solar Dynamics Observatory. “But in the course of a little over two days, it got way too much company. It grew to an enormous active region. Normally we see these things spread out, but this thing bent around in a
‘U’ around the original sunspot.”
The timing of the dramatic 'geomagnetic storm' is somewhat puzzling, as our nearest star is headed toward the quiet phase in its 11-year cycle. The most recent cycle began in 2008, and promises to reach its most quiescent stage around 2018 or 2019. But, such off-season activity has happened before, says Pesnell. “We had a solar maximum in 2001, and late 2003 saw some enormous flares.”
Solar flares are fairly common events. Gusts of high energy, including X-rays, flash from the Sun’s interior as its magnetic fields twist and realign, discharging enormous amounts of energy. The flash of a flare may last anywhere from a few minutes to several hours. A flare event releases high-energy particles in all directions, and some of those particles arrive at the Earth soon after.
A different kind of energy release is stronger, more directed, and more significant to our own world. This type of event is called a coronal mass ejection, or CME. CMEs actually eject material from the Sun, sending a wad of radioactive matter into space. The blaze of a flare can be likened to the flash of a rifle muzzle, while a CME is the bullet that shoots out. “A flare is essentially a bright flash of light when a magnetic field from the Sun is converted into heat,” Pesnell explains. “We see very rapid particles in the area moving mostly down toward the surface of the Sun in the region of the flare. They last 20 or 30 minutes, and then they’re done. A CME, on the other hand, occurs when a filament that has been on the Sun for some period of time (it could be hours, could be months) is no longer able to stay on the surface of the Sun, and erupts off. That looks like a big rope extending into the Solar System.” Rather than a flash of light, a coronal mass ejection is a burst of particles in a magnetic field coming off the Sun. In the 6 September event, a magnetic field line buried deep inside the Sun bubbled up to the surface and
ejected solar material into space. At the time, observers clocked approximately
28 fairly large flares, and some number of coronal mass ejections. Then, on 10
September, researchers charted a large
CME associated with a flare on the edge
– or limb – of the Sun. It was fortuitously placed, as the CME was visible as it departed on a trajectory at a right angle to the Earth’s observers. Pesnell likens the event’s geometry to baseball: “It’s like the pitcher throwing to first base instead of home.”
Thanks to orbital platforms, Pesnell and other scientists across the world are no longer limited to Earth-based solar observations. An armada of orbiting observatories keeps track of these energetic solar events. These include the Solar and Heliospheric Observatory (SOHO), the twin spacecraft of the STEREO (Solar Terrestrial Relations Observatory) mission, the international Hinode (formerly Solar-B) spacecraft and the Solar Dynamics Observatory. SOHO has recorded a host of discoveries, not only about the Sun, but also pertaining to comets that impact or graze it. The twin STEREO spacecraft were launched in October of 2006, and continue to investigate the nature of CMEs. The STEREO observatories are shedding light on what initiates CMEs, what primary force drives them (is their source principally magnetic or other forces?) and what other coronal phenomena accompany them (flares and other formations,
for example). Japan’s Hinode passed its ten-year anniversary of solar observation in 2016, charting the initiation, transport and dissipation of magnetic energy from the Sun’s deep photosphere to its upper solar atmosphere, the corona. The Solar Dynamics Observatory rounds out our current toolbox with a suite of instruments designed to enhance our comprehension of the Sun's influence on Earth and near-Earth space, by studying the solar atmosphere on much finer scales of space and time.
This flotilla of solar observatories enables organisations like the United Kingdom’s Met Office Space Weather Operations Centre (MOSWOC) and the US National Oceanic and Atmospheric Administration (NOAA) to carry out cosmic meteorology, a discipline implemented by the Space Weather Prediction Center (SWPC). The center carries out computer simulations of solar activity, enabling them to predict when a CME will arrive at the Earth. According to NOAA, the center services areas of aviation, radio communications, electric
“We study how the magnetic field emerged from inside the Sun”
Dean Pesnell
power grids, satellites, emergency management and Global Positioning Systems.
Europe opened a new space weather forecasting facility in Brussels in 2013. The Space Weather Coordination Centre (SSCC) issues alerts and offers support to various government, industry and research agencies that may be affected by solar events. Japan’s own NICT Space Weather Information Center serves a similar role for the eastern hemisphere.
Once a flare or CME is spotted, NOAA and other Space Weather organisations send out alerts to applicable groups so that power utility companies, airlines, and communications satellite operators can take precautions. It’s an important system, as the energy flowing from CMEs can actually damage the electronics in communications and weather satellites. While a flare only affects the sunlit side of the Earth, sending relatively low levels of radiation, a coronal mass ejection injects currents into the Earth’s magnetosphere, a protective bubble of magnetic fields surrounding the planet. These currents pump up the electrical fields surrounding Earth, endangering satellites that travel through them. CMEs also heat and expand the upper atmosphere of the Earth, which can cause satellites to deorbit.
As energetic particles from CMEs charge up the Earth’s magnetic field, they can cause problems with the transmission lines near the surface of the Earth by inducing currents in those lines.
“Those currents actually go into transformers and muck about with the transformers,” Pesnell says. “The real problem that we worry about is the geomagnetic storms interfering with the largepower transmissions that they have at both ends of high-tension wires. When this happens, the power company has to prevent the induced current from getting into the transformer. There are a variety of ways to do that. You want to dial down the susceptible lines for just a while, so the current can fade. So you try to pull power from other parts of the grid so you can disconnect some of the wires. There are special things you can also do inside the transformer, like filtering out low-frequency current.”
Power companies are able to react fairly quickly. Space weather organisations like NOAA’s Space Weather Prediction Center see the CME coming towards us with hours, or even days, of warning. CMEs typically take several days to reach the
Earth’s distance. Fast ones will take a day, and the most powerful ones arrive on Earth in a matter of hours, but the average event affords some warning for organisations to take preventive measures. Even so, damage does occur. In an incident in August of 2006 at the Lake Eerie Loop (in the eastern
US), power lines actually heated up and sagged far enough that they touched the ground and shorted. The power companies were not able to shunt power very effectively.
One of the most powerful tools in our orbiting armada of observatories is the Solar Dynamics Observatory. SDO brings the newest tools to the space weather community. It gives researches and engineers rapid response images, so observers can see where the flare is taking place, or where the CME occurs, so they can have more overall knowledge of what’s going on across the entire Sun. Observers involved in forecasting space weather are primarily concerned with taking action in the wake of a solar event. Scientists, on the other hand, try to explain the cause of phenomena, along with its site of origin. “We want to know more than simply that it happened,” Pesnell explains. “We want to know why it happened. So we study how the magnetic field emerged from inside the Sun.”
Today, solar meteorologists have more time to prepare for the problems caused by geomagnetic storms than in the past, because scientists are now constantly observing the Sun, says Dirk Terrell, researcher at the Southwest Research Institute in Boulder, Colorado. “The really detailed and long-term observations from the various solar missions like SOHO and STEREO, and modelling improvements have to have played a role. Models of the magnetic fields and solar wind have gotten much more sophisticated in the last decade.”
Astronomers are gaining a better understanding of the relationship between flare events and
CMEs as well. One scenario used to describe the
“This event started out with a lone sunspot sitting out there on the Sun, wishing it had company,”
Dean Pesnell
possible relationship between the two is that a flare causes an instability in the Sun’s magnetic field, allowing the ejection of material. The flare changes the magnetic field, causing a filament (a stream of magnetised gas) to be ejected. Observers believed they could see CMEs without a flare, but now that orbiting satellites like the Solar Dynamics Observatory are capable of imaging the entire face of the Sun, researchers are questioning whether this is actually true. Pesnell sites “a very nice example of where a flare takes place on one side of the Sun, and a CME takes place on the other side. People model the situation, and they can see that the entire Sun participated in that magnetic field readjusting after the flare, and that readjustment caused the CME to happen.” Researchers also witness CMEs when new magnetic fields arise at the surface. However, they do see large, active flares without CMEs.
The knowledge that scientists gain about our nearest star has implications far beyond the theoretical. As Pesnell puts it, “The things that affect our life here on the Earth are typically the flares and coronal mass ejections, so distinguishing between those effects is important for understanding how the Sun affects our lives.”