CME Week: The Difference Between Flares and CMEs

There are many kinds of eruptions on the sun. Solar flares and coronal mass ejections both involve gigantic explosions of energy, but are otherwise quite different. The two phenomena do sometimes occur at the same time – indeed the strongest flares are almost always correlated with coronal mass ejections – but they emit different things, they look and travel differently, and they have different effects near planets.

Both eruptions are created when the motion of the sun’s interior contorts its own magnetic fields. Like the sudden release of a twisted rubber band, the magnetic fields explosively realign, driving vast amounts of energy into space. This phenomenon can create a sudden flash of light -- a solar flare. Flares can last minutes to hours and they contain tremendous amounts of energy. Traveling at the speed of light, it takes eight minutes for the light from a solar flare to reach Earth. Some of the energy released in the flare also accelerates very high energy particles that can reach Earth in tens of minutes.
The magnetic contortions can also create a different kind of explosion that hurls solar matter into space. These are the coronal mass ejections, also known as CMEs. One can think of the explosions using the physics of a cannon. The flare is like the muzzle flash, which can be seen anywhere in the vicinity. The CME is like the cannonball, propelled forward in a single, preferential direction, this mass ejected from the barrel only affecting a targeted area. This is the CME—an immense cloud of magnetized particles hurled into space. Traveling over a million miles per hour, the hot material called plasma takes up to three days to reach Earth. The differences between the two types of explosions can be seen through solar telescopes, with flares appearing as a bright light and CMEs appearing as enormous fans of gas swelling into space.

Flares and CMEs have different effects at Earth as well. The energy from a flare can disrupt the area of the atmosphere through which radio waves travel. This can lead to degradation and, at worst, temporary blackouts in navigation and communications signals.
On the other hand, CMEs can funnel particles into near-Earth space. A CME can jostle Earth’s magnetic fields creating currents that drive particles down toward Earth's poles. When these react with oxygen and nitrogen, they help create the aurora, also known as the Northern and Southern Lights. Additionally, the magnetic changes can affect a variety of human technologies. High frequency radio waves can be degraded: Radios transmit static, and GPS coordinates stray by a few yards. The magnetic oscillations can also create electrical currents in utility grids on Earth that can overload electrical systems when power companies are not prepared.
One thing is the same about flares and CMEs: A fleet of NASA heliophysics observatories in space are always on the watch for these explosions. Much like how we forecast thunderstorms and rain showers, the U.S. National Oceanic and Atmospheric Administration’s Space Weather Prediction Center runs simulations and can make predictions about when the CME will arrive at Earth based on this and other data. They then alert appropriate groups so that power companies, airlines, and other stakeholders can take precautions in the event of a solar storm. For example, if a strong CME is on its way—utility companies can redirect power loads to protect the grids.
NASA's heliophysics spacecraft observe flares and CMEs for another reason as well. Scientists want to understand exactly what causes these powerful explosions and some day predict them even before they erupt.