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The sun's atmosphere regularly develops leaky holes, where the solar magnetic field opens up and unleashes a 2-million-miles-per-hour flow of protons and electrons. That hot blast of particles, which moves twice as fast as the normal solar wind, has a paradoxical effect: When it blows most fiercely, it actually cools Earth.

Climatologist Eric Posmentier of Long Island University in New York and astrophysicist Willie Soon of the Harvard-Smithsonian Center for Astrophysics collected two decades' worth of temperature data from weather satellites and correlated the readings with the size and number of the sun’s coronal holes. The holes are least common when the sun is at the peak of its 11-year activity cycle, as is the case right now. During this "solar maximum," giant sun storms trigger auroral displays , but the sun's strengthened magnetic field snaps the coronal holes shut. When solar activity is lowest, as it was around 1995, the holes expand to cover about 20 percent of the sun's surface. During those periods, the researchers found, Earth’s lower atmosphere is about 2 degrees cooler than during solar max.

The reason for this is unclear. "One idea is that the fast solar wind creates a lot of ions in the upper atmosphere, which act as condensation nuclei for water droplets, and lead to cloud formation," says Posmentier. "During solar mininum, you have bigger coronal holes, more ions, and more clouds." Those overcast skies blot out sunlight, putting a surprise chill in the air.

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