Understanding the Solar Wind

Thanks to the new Japanese satellite Hinode, solar scientists can finally begin understanding some of the basic mechanisms of the sun's structure and activity--and the powerful solar wind that buffets the inner planets of the solar system.

The mechanisms that cause the solar wind had baffled scientists for decades, but were revealed in observations by a Japanese satellite called Hinode orbiting Earth, the scientists said in research published in the journal Science.

"The magnificent thing about the success of Hinode is its unprecedented view of the dynamics of the sun," Jonathan Cirtain, a solar physicist at NASA's Marshall Space Flight Center, Huntsville, Ala., who helped in the research, said in a telephone interview.

The research was conducted by Japanese, European and U.S. scientists.

The solar wind is a stream of electrically charged gas -- mostly hydrogen -- blown outward from the sun in all directions at a speed of about 1.6 million km/h.

It buffets planetary atmospheres. On Earth, it can disrupt satellites, power grids and communications, under certain circumstances. Earth's magnetic field protects against the solar wind, creating a bubble around which the wind must flow.

... Hinode (pronounced hin-OH-day and named for the Japanese word for "sunrise") showed that two mechanisms appear to power the solar wind, Dr. Cirtain said.

The first involves the way the sun's magnetic field undergoes rapid changes in its shape, the researchers said. As the magnetic field changes shape, it generates these Alfven waves along its length that accelerate the charged gas and blow it into space, they said.

Another mechanism powering the solar wind involves the sun's chromosphere, the region sandwiched between the solar surface and its corona. Images from Hinode's Solar Optical Telescope found that the chromosphere is filled with Alfven waves, which when they leak into the corona are strong enough to trigger the solar wind.

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The solar wind:

• Shapes the Earth's magnetosphere
• Powers the aurora borealis and aurora astralis, northern and southern lights
• Unfurls the tail of comets broadly out in space away from the sun
• Appears to be involved in Earth's climate variability
• May be instrumental in exploration of the outer solar system (via solar sails)
• Creates the heliopause, the limits of the solar system
• Eventually escapes (in part) into interstellar space