solar flare
The sun on Oct. 23 as seen by NASA's Solar Dynamics Observatory. The dark sunspot cluster in AR2192 is obvious in the HMI Intensitygram (left), which represents the sun's photosphere -- known, colloquially, as the solar 'surface' -- whereas the EUV images to the right (at wavelengths 171A -- top -- and 304A) show emissions from the multimillion degree solar corona (where coronal loops shine bright) and chromosphere.
Just as the US prepares to watch the partial solar eclipse today, nearly 100 million miles away on the sun a possible solar storm is brewing. Amateur astronomers have been wowed by a vast sunspot that has rotated to face Earth, the largest since this solar cycle began in 2008, and solar observatories (on the ground and orbiting Earth) are closely monitoring the region.

The sunspot, a dark patch in the sun's photosphere, represents intense solar magnetism bursting from the sun's interior known as an active region. This particular active region, designated AR2192, has been rumbling with intense flare activity, recently exploding with 2 X-class flares, causing some short-lived high-frequency (HF) radio black outs around the globe.

Such blackouts are triggered by the intense extreme ultraviolet and X-ray radiation that solar flares can generate, causing ionization effects in the Earth's upper atmosphere - a region known as the ionosphere. HF radio can be strongly hindered by this activity, triggering blackouts that can effect air traffic and amateur radio operators.

Currently, the sunspot located at the base of AR2192 has swelled to over 80,000 miles across - Jupiter could almost fit inside the sunspot's mottled diameter.

While making for a spectacular astronomy target, especially as it coincides with today's partial solar eclipse, space weather forecasters are trying to gauge whether the active region could explode with more powerful solar flares.
x1.6 class flare
© NASA/SDO An X1.6 class flare erupted from the lower half of the sun, as seen in the bright flash of light in this image from NASA’s SDO. This image shows extreme ultraviolet light with a wavelength of 131 Angstroms, which highlights the intense heat of a solar flare and which is typically colorized in teal.
Since the start of this week, AR2192 has generated 27 C-class flares, 8 M-class flares and 2 X-class flares. Most recently, on Oct. 22 (Wednesday), an X1.6 flare erupted, creating an extremely bright eruption in the sun's lower corona (the solar atmosphere) that was captured by NASA's Solar Dynamics Observatory (SDO):

According to, there's a "95 percent chance of M-class flares and a 55 percent chance of X-flares during the next 24 hours." Since rotating toward the Earth, AR2192 has not generated any Earth-directed coronal mass ejections (CMEs).

Solar flares and CMEs are both related magnetic solar phenomena. Flares are generated when huge magnetic fieldlines erupt from the sun's interior and are forced together, particularly within active regions. Superheated solar plasma flows around these huge loops - aptly known as coronal loops - causing them to shine brightly in extreme ultraviolet (EUV) wavelengths. When forced together, however, and if the conditions are right, a phenomenon known as reconnection may occur. Reconnection causes magnetic fieldlines to "snap" and reconnect, causing the plasma trapped within coronal loops to be rapidly accelerated. It's this acceleration that generates huge amounts of energy, blasting powerful EUV and X-ray radiation into space as a flare.

CMEs can also be generated over active regions in the lower corona when magnetic bubbles containing energetic solar plasma expand and are hurled into space. Though CMEs can take hours to days to reach Earth (in other words, we can see them coming, whereas flares travel at the speed of light), the delivery of huge quantities of energetic particles from the sun (mainly protons) can boost the radiation environment around Earth and interact with our planet's magnetosphere. These geomagnetic storms are responsible for beautiful auroral displays at high latitiudes and can cause power outages on the ground and satellite damage in orbit.

Although flares and CMEs are rooted in magnetic eruptions, they are not necessarily generated at the same time. Flares can occur without generating a CME and vice versa.

As to whether AR2192 will unleash a large flare or CME at Earth, that remains to be seen, but if there's one thing history has taught us about the sun, it's worth being prepared.

Interestingly, this month marks the 11 year anniversary of the Hallowe'en Solar Storms. In 2003, through October and November, a series of flares and CMEs struck Earth generating vast aurorae and causing damage to satellites. Aircraft were advised not to travel through polar regions (due to the high-altitude uptick in radiation and possible communications outages) and astronauts and cosmonauts on the International Space Station had to shelter inside well-shielded portions of the orbiting outpost. Parts of Sweden even experienced short power outages due to atmospheric currents overloading the national grid.

When the sun erupts, it has no regard for our planet or our technology, so it is up to space weather forecasters to learn as much as we can about solar conditions so the impact of the next great solar storm causes minimal damage to our technological civilization.