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.
The sun may be an average star when compared to the menagerie of stars that exist in our galaxy, but to Earth and all life on our planet, the sun is the most important object in the Universe. However, regardless of its importance and close proximity, our nearest star holds many mysteries that continue to fox solar physicists after decades of modern studies with cutting-edge observatories. One of the biggest mysteries surrounding the sun is the underlying mechanisms that drive solar flares and coronal mass ejections (CMEs). Monday evening (EST), the sun reminded us that it hasn't quite finished with the current solar maximum (of solar cycle 24), unleashing a powerful X4.9 solar flare -- the biggest of 2014. An armada of space telescopes witnessed the event, including NASA's Solar Dynamics Observatory that can spy the sun's temper tantrums in astounding high definition. Shown here, 5 of the 10 filters from the SDO's Atmospheric Imaging Assembly (AIA) instrument are featured, showing the sun's lower corona (the solar multimillion degree atmosphere) through 5 wavelengths; each wavelength of extreme ultraviolet light representing a different plasma temperature and key coronal features -- such as coronal loops (highlighted here in the 'yellow' 171A filter) and ejected plasma that formed a CME.
At 7:13 p.m. EST (00:13 UT, Feb. 25) -- pictured here on the far left -- the active region (AR) 1990 was crackling with activity. Then, as magnetic field lines from the sun's interior forced together and through the solar photosphere, large-scale reconnection events occurred. Reconnection is a magnetic phenomena where field lines "snap" and reconnect, releasing huge quantities of energy in the process. At 7:44 p.m. EST (00:44 UT) -- second frame from the right -- a kinked coronal loop can be seen rising into the corona. At 7:59 p.m. EST (00:59 UT) -- far right -- solar plasma contained within the magnetic flux is accelerated to high energy, generating powerful x-rays and extreme ultraviolet radiation, creating the X-class flare.
The X4.9 flare was caught through the range of SDO fliters, including this dramatic view as seen through the 131A filter. The flare was so bright that photons from the flare overloaded the SDO's CCD inside the AIA instrument, causing the signal to "bleed" across the pixels. This bleeding effect is common for any optical instrument observing powerful solar flares.
Intense coronal activity is often associated with active regions -- the active lower corona is pictured here, left. In this case, the flare erupted from AR1990, at the limb of the sun. Also associated with active regions are sunspots, dark patches observed in the sun's photosphere (colloquially known as the sun's "surface") -- pictured right. The sun's cooler photosphere has been imaged by a different SDO instrument called Helioseismic and Magnetic Imager (HMI), which detects the intensity of magnetic fields threading though the sun's lower corona and photosphere.
In the case of AR1990, a large sunspot can be seen at the base of the coronal loops that erupted to generate the powerful flare. This is a prime example of how sunspots can be used to gauge solar activity and how they are often found at the base of intense coronal activity and flares.
The HMI monitors magnetic activity across the disk of the sun and can also generate a picture on the direction of the magnetic field polarity. In this observation of the sun's magnetic field around the time of the recent X-class flare, other active regions can be easily seen -- intense white and black regions highlighting where magnetic field lines emerge and sink back into the sun's interior in active regions.
The joint NASA/ESA Solar and Heliospheric Observatory (SOHO), which has been watching the sun since 1996, also spotted the flare, tracking a CME that was generated shortly after. Seen here by SOHO's LASCO C2 instrument, that monitors the interplanetary environment surrounding the sun for CMEs and comets, a growing bubble of solar plasma races away from the sun.
Approximately an hour after the flare, the CME grew and continued to barrel into interplanetary space. Space weather forecasters don't expect that this CME will interact with the Earth's atmosphere as it is not Earth-directed. This observation was captured by SOHO's LASCO C3 instrument -- an occulting disk covers the sun to block out any glaring effect. By combining observations by the SDO, SOHO and other solar observatories, the connection between the sun's internal magnetic "dynamo", the solar cycle, flares and CMEs, solar physicists are slowly piecing together what makes our nearest star tick, hopefully solving some of the most persistent mysteries along the way.
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.
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):
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.NASA/SDO
According to Spaceweather.com, 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.