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Although the Sun looks steady and constant, it can get violent every now and then. The surface of the Sun can sometimes explode, releasing a tremendous amount of energy – astronomers call these events solar flares.
Solar flares happen in the atmosphere of the Sun, in the corona and chromosphere. Plasma is heated up to tens of millions of kelvins, and particles are accelerated to nearly the speed of light. Within an instant, 6 x 1025 joules of energy are released. Spacecraft and telescopes watching the Sun observe a bright flash of X-ray and ultraviolet radiation.
The number and strength of solar flares varies with the 11-year solar cycle. When the Sun is active and dotted with sunspots, there can be frequent solar flares. And they can happens less than one a week when the Sun is in its quiet phase. Solar flares often go hand in hand with coronal mass ejections.
Solar flares pose one of the greatest risks to human spaceflight. When a powerful flare and coronal mass ejection are aimed at the Earth, a tremendous amount of radiation passes through our region. Since the particles have been accelerated to nearly the speed of light, a dangerous radiation storm will come just minutes after a flare occurs on the surface of the Sun. During a powerful solar storm, astronauts will have less than 15 minutes to seek protection, or receive a potentially lethal dose of radiation.
The most powerful solar flare ever recorded happened on November 4, 2003 during a high point in solar activity. The Sun produced a flare so powerful that it overloaded the sensors on one of NASA’s Geostationary Operational Environmental Satellites.
We have recorded an episode of Astronomy Cast just about the Sun called The Sun, Spots and All.