It's not just Earth weather that's erratic. Our own star, the Sun, also goes through changes.

It's known to go through an 11-year cycle, becoming more or less active. At its peak it explosively fires charged particles out into space, creating powerful sun storms that in turn trigger spectacular auroras on Earth.

Now it seems it has a second, shorter cycle, which lasts about 330 days. In other words, the Sun has a seasonal cycle that lasts almost one Earth year.

Earlier studies had suggested this. Researchers have confirmed it using records of solar activity spanning several decades.

The Sun is an enormous ball of extremely hot gas, which carries an electric charge. The gas is being churned around at very high speeds, and the Sun itself is in constant motion. This means the Sun is magnetic.

"If you move a charge you create a magnetic field. That's what's happening inside the Sun," says Louise Harra of University College London in the UK, who was not involved with the present study.

The magnetic field has bands of particularly strong activity, both in the Sun's northern hemisphere and in the south. The bands in the hemispheres can interact and collide. That's when the season changes.

"The activity bands on the sun have very slow-moving waves that can expand and warp," says co-author Robert Leamon of Montana State University in Bozeman.

"Sometimes this results in magnetic field leaking from one band to the other. In other cases, the warp drags magnetic field from deep in the solar interior and pushes it toward the surface."

These band's are unstable like the jet stream, says lead author Scott McIntosh of the National Center for Atmospheric Research in Boulder, Colorado.

"When they do kink, magnetic field surges through the surface and can destabilize what is already there. The surges last many months."

The new findings take us a step closer to being able to predict the changeable weather on the Sun.

It would be useful to be able to predict solar storms, as strong ones can seriously impact satellites and radio communications.

"It's another piece of the jigsaw in understanding how our star works," says Harra.

Knowing how the Sun's activity varies will also help us uncover how other stars work, and whether habitable planets may orbit them, she adds.

The team published their findings in the journal Nature Communications.

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