A spacecraft sending data from Mars to Earth had its signal repeatedly interrupted in 2005 and scientists say they've learned the culprit -- the sun.

In August of that year, the Mars Express spacecraft was dutifully sending back data on the stratigraphy, or layering, of the upper regions of the martian crust, when its signals were knocked off the air.

Researchers in the Boston University Astronomy Department say solar flares, and the high-energy particles they emit, can strongly interfere with radio signals at certain heights in the martian atmosphere.

Analyzing ion and electron densities at various altitudes in the martian atmosphere following a solar flare, they discovered very high-energy protons ejected from the sun during a flare can strike neutral atoms in the atmosphere so hard that they knock loose an electron, creating a free electron and a positive ion.

If enough of these electrons and ions build up in the atmosphere, they can interfere with radio waves.

That's what was happening to the Mars Express, they said, although it requires strong flares and lots of particles.

"There must be lots of ionization to account for losing radio waves," Professor Paul Withers said.

A model created during their study will help scientists better understand and predict the effect of highly charged protons on the martian atmosphere, researchers said.