Space weather on Earth has long been thought to be largely a measure of the Sun's output. But mounting research reveals it is metered more by our own planet's changing magnetic field than was known.

When the Sun hiccups as it did last week, huge blasts of radiation and matter can be flung into space. Storms arrive with a magnetic charge, plus or minus. Our own planet has a varying magnetic field.

Scientists knew that the alignment of these fields had something to do with the odds of a satellite being disabled or a colorful Northern Lights sky show ensuing.

The new study shows that the Northern Lights, also called aurora, and other space weather near Earth are driven by the rate at which the Earth's and Sun's magnetic fields connect, or merge, and not just by the solar wind's electric field.

The merging occurs way out in space, at a spot between the Earth and Sun, roughly 40,000 miles above our planet's surface. Researchers have now developed a formula that describes the merging rate of the magnetic field lines and accurately predicts 10 different types of near-Earth space weather activity, such as the aurora and magnetic disturbances.

"Having this formula is a big step forward for understanding how the Sun and Earth interact," said study leader Patrick Newell of the Johns Hopkins University Applied Physics Laboratory (APL).

Space weather scientists had long assumed that near-Earth space weather phenomena could best be predicted by the behavior of the solar wind electric field. But Newell and his colleagues were the first to put this theory to a rigorous test with many data sets from a number of years.

They looked at NASA satellite observations of global auroral activity, NOAA satellite observations of the stretching of the Earth's magnetic field lines on our planet's nightside, and Air Force satellite observations of the access of solar wind particles to the Earth's upper atmosphere.

The research further disabuses the notion that space is empty. The region between Earth and the Sun is full of energetic particles, most of which are generated Sun. Temperatures of a few million degrees accelerate a stream of these particles, called the solar wind, to roughly one million mph.

This article is part of SPACE.com's weekly Mystery Monday series.

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