Hessi principal investigator Robert Lin, UC Berkeley professor of physics. Courtesy of Solar Max 2000

Solar flares, the most powerful explosions in the solar system, typically are associated with sunspots in "active regions" of strong magnetic field in the solar atmosphere. Sunspots form where the sun's magnetic field lines arc out of the surface in bright loops, and flare explosions seem to emanate from these loops.

One possible explanation for solar flares dates from the 1950s and involves magnetic reconnection. As the sun's strong magnetic field lines reach out into space they sometimes cross or reconnect within the corona or atmosphere of the sun. In seconds, the short circuit heats the gas to tens of millions of degrees, and perhaps as high as 100 million Kelvin, accelerating electrons and protons to speeds approaching the speed of light. The electrons and protons slamming into gas particles, mostly hydrogen, in the lower corona and chromosphere produce X-rays and gamma-rays, respectively.

HESSI principal investigator Robert Lin discusses the mysteries of solar flares.
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While microflares last for seconds, larger flares may emit X-rays for tens of minutes and remain visible for hours. The large ones extend for as much as 100,000 km above the solar surface, nearly 10 times the diameter of the Earth.

Most of what scientists know about flares has come from ground-based observations at visible and radio wavelengths and from instruments aboard Skylab, the Solar Maximum Mission, the Japanese/U.S. Yohkoh spacecraft and other spacecraft. X-rays have been recorded from flares for more than 30 years.

HESSI will have the finest angular and spectral resolution of any hard X-ray or gamma-ray instrument flown in space, providing scientists with the first high fidelity color movies of flares in their highest energy emissions. The data will help scientists pinpoint where and how flares form.

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