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No phenomenon in astronomy has been studied more closely than solar flares, gigantic eruptions on the Sun that can affect Earth’s climate and even disrupt power grids. Scientists have been watching the Sun with ground based instruments and orbiting satellites for years, so it might be thought that we know a lot about such eruptions. Well think again. A new report from NASA has revealed that, like earthquakes, solar flares often have aftershocks. Moreover, the aftershocks can emit bursts of ultraviolet (UV) radiation more powerful than the original eruption. Combine this new finding with the recently uncovered linkage between fluctuating UV levels and El Niño, and the Sun-Climate connection looks stronger than ever.

In a report, published in The Astrophysical Journal, NASA scientists are reporting that solar flares generate considerably more energy than previously suspected. About 1 flare in 7 experiences an “aftershock” around ninety minutes after the flare dies down. “We call it the ‘late phase flare,’” says Thomas N. Woods, a physicists at the University of Colorado and lead author. “The energy in the late phase can exceed the energy of the primary flare by as much as a factor of four.” The major findings are detailed in the article’s abstract below:

New solar extreme-ultraviolet (EUV) irradiance observations from the NASA Solar Dynamics Observatory (SDO) EUV Variability Experiment provide full coverage in the EUV range from 0.1 to 106 nm and continuously at a cadence of 10 s for spectra at 0.1 nm resolution and even faster, 0.25 s, for six EUV bands. These observations can be decomposed into four distinct characteristics during flares. First, the emissions that dominate during the flare’s impulsive phase are the transition region emissions, such as the He II 30.4 nm. Second, the hot coronal emissions above 5 MK dominate during the gradual phase and are highly correlated with the GOES X-ray. A third flare characteristic in the EUV is coronal dimming, seen best in the cool corona, such as the Fe IX 17.1 nm. As the post-flare loops reconnect and cool, many of the EUV coronal emissions peak a few minutes after the GOES X-ray peak. One interesting variation of the post-eruptive loop reconnection is that warm coronal emissions (e.g., Fe XVI 33.5 nm) sometimes exhibit a second large peak separated from the primary flare event by many minutes to hours, with EUV emission originating not from the original flare site and its immediate vicinity, but rather from a volume of higher loops. We refer to this second peak as the EUV late phase. The characterization of many flares during the SDO mission is provided, including quantification of the spectral irradiance from the EUV late phase that cannot be inferred from GOES X-ray diagnostics.

The Solar Dynamics Observatory was the first mission to be launched for NASA’s Living With a Star (LWS) Program, a program designed to understand the causes of solar variability and its impacts on Earth. The SDO was able to make the discovery because of its unique ability to monitor the sun’s extreme UV output in high resolution nearly 24 hours a day, 7 days a week. From these extensive and more accurate observations, scientists are beginning to piece together a more complete picture of solar flare activity. A late phase aftershock is thought to result when some of the sunspot’s magnetic loops re-form. A diagram prepared by team member Rachel Hock of the University of Colorado shows how it works.

The extra energy from the late phase can have a big effect on Earth. There has been speculation that UV radiation plays a larger part in warming Earth than commonly thought. It turns out that extreme ultraviolet wavelengths are particularly good at heating and ionizing Earth’s upper atmosphere. In a report in the August 28, 2009, issue of the journal Scienceentitled “Amplifying the Pacific Climate System Response to a Small 11-Year Solar Cycle Forcing,” Gerald A. Meehl et al. described a possible mechanism that could explain how seemingly small changes in solar output can have a big impact on Earth’s climate. Their work explained how the upper atmosphere can act as a solar heat amplifier when UV radiation from the Sun increases.

Scientists have long suspected that changes in solar output may have triggered the Little Ice Age that gripped Europe several centuries ago, as well as droughts that brought down Chinese dynasties. While total solar irradiance varies by only 0.1 percent during the normal 11 year solar cycle, the change in the intensity of ultraviolet light varies by much larger amounts. According to Judith Lean, a solar physicist at the U.S. Naval Research Laboratory in Washington, D.C., its possible that long-term patterns—operating over hundreds or thousands of years—could cause even more pronounced swings in solar irradiance (see “Scientists Discover The Sun Does Affect Earth’s Climate”).

Changes in UV can affect aspects of climate other than the atmosphere. In “Dynamical Response of the Tropical Pacific Ocean to Solar Forcing During the Early Holocene,” also published in Science, Thomas M. Marchitto, Raimund Muscheler, Joseph D. Ortiz, Jose D. Carriquiry and Alexander van Geen presented a high-resolution magnesium/calcium proxy record of Holocene sea surface temperature (SST) from off the west coast of Baja California Sur, Mexico. They concluded that variation in UV irradiance was having a big impact on the El Niño–Southern Oscillation (ENSO). The ENSO is a major driver of climate, as recent events world wide have shown. Now Woods et al. have shown that UV levels during periods of heavy solar flare activity may be even higher than previous estimates.

“We’ve just learned that some flares are many times stronger than previously thought,” says Woods, who led the research team. “Solar flares were already the biggest explosions in the solar system—and this discovery makes them even bigger.”

Combine these findings with the recent long-term NASA study that found Earth’s upper atmosphere was radiating more energy back into space than previously thought, and it becomes painfully clear that none of the current climate models can be correct. No one disputes the fact that the Sun’s energy drives Earth’s climate system. If you do not understand how much energy is coming in, how that energy is absorbed and how that energy is re-emitted you have no hope of correctly modeling Earth’s climate. For existing climate models it’s game over.

Climate change is not the only result of fluctuating UV levels. When our planet’s atmosphere is heated by extreme UV radiation, it puffs up, accelerating the decay of low-orbiting satellites. Furthermore, the ionizing action of extreme UV can bend radio signals, disrupt the normal operation of GPS and cause electrical power blackouts. Such is the power of our local star to influence things here on Earth.

Yet there are still those who would rather believe that minuscule changes in the level of a trace atmospheric gas controls Earth’s climate. This new NASA report is more bad news for the CO2 fanboys, the Sun is even more powerful than previously suspected. And unlike the ill-defined hypothetical feedbacks needed to amplify carbon dioxide’s marginal impact, the mechanisms through which UV radiation affect climate are based in real physics and empirical observation. I have said it before—when it comes to climate change, it’s the Sun, stupid.

Be safe, enjoy the interglacial and stay skeptical

Resilient Earth, 21 September 2011