Summer solstice heats up solar mystery

A salvo from the sun spotted earlier this year by the Solar and Heliospheric Observatory

(CNN) -- The summer season arrived in the Northern Hemisphere on Friday, so it seems natural that the northern half of the planet soaks up more rays at this time of year.

Then why are scientists scratching their heads?

Solar activity has picked up around the June solstice in recent years, meaning that the sun is more likely to release energy bursts that, if directed our way, can disturb electrical systems on the ground and in orbit, NASA researchers said.

"The number of solar storms doesn't increase around the time of the June solstice, but over the past five or six years, the intensity of the storms does tend to be high at this time of year," said Walter Gonzalez, a visiting scientist at NASA's Jet Propulsion Laboratory.

Bruce Tsurutani, also a JPL researcher, said that solar storm activity picks up in the days around the equinoxes in March and September.

Curiously, there seems to be no corresponding rise in solar activity near the winter solstice in December, he said.

"We're not really sure why this happens, so this is one of the things we are investigating," said Gonzalez, who heads the space science program at Brazil's National Institute for Space Research.

Change is in the sky

GLOBE GRAPHIC The scoop on the summer solstice

In other ways, the boost in solar power at this time of year is perfectly natural. The summer solstice is the longest day of the year in the Northern Hemisphere.

The sun rises higher in the sky throughout the day and its rays hit the northern half of the planet at a more direct angle, beginning the seasonal trend of efficient heating that ushers in summer.

Near the time of a solstice, the sun appears to rise and set in the same location for a couple of days. Thus came the term solstice, which comes from Latin words meaning "the sun" and "to stop." For the next six months, the sun will gradually move south along the horizon when it rises and sets.

The seasons are reversed in the Southern Hemisphere, where the June solstice brings the beginning of winter and the shortest day of the year.

The North Pole tilts slightly more than 23 degrees toward the sun on the day of the solstice. The vertical noon rays are directly over the Tropic of Cancer.

In late December the conditions reverse, with the South Pole tilting toward the sun and the solar rays shining over the Tropic of Capricorn.

In other words, the Northern Hemisphere now "leans" toward the sun and the Southern Hemisphere away from it, a condition that bathes much of the Arctic in constant sunlight and leaves much of the Antarctic in prolonged darkness.