Mars is constantly losing small amounts of its atmosphere into space, but a new study shows that the loss rate may be enhanced by pulses of solar wind energy.

The researchers analysed solar wind data from the Advanced Composition Explorer (ACE) and Mars Express observations that track the flux of heavy ions leaving Mars' atmosphere, and found that bursts of atmospheric loss correlate with solar events known as corotating interaction regions (CIRs).

"CIRs form when a fast solar wind stream catches up a slower stream and they collide," explains Hans Nilsson of the of the Swedish Institute of Space Physics. "The density of the solar wind plasma and magnetic field increase in these regions and will therefore also have a stronger influence on any planetary atmosphere it may interact with." That is, when the CIR pulses pass by the red planet, it can drive away particles from Mars' atmosphere.

The researchers found that the outflow of atmospheric particles from Mars was about 2.5 times the outflow when CIRs were present than when these events were not occurring. Furthermore, about one third of the material lost from Mars into space is thought to be lost during CIRs.

The study will help scientists better understand the evolution of Mars' atmosphere. "It is possible that solar wind interaction was important to remove atmosphere and oceans from Mars," Nilsson tells Astronomy Now. "The main importance of the CIR study is to show how much the atmospheric loss increases when the plasma and magnetic field density of the solar wind increases. The total loss during the period of observation is not that significant, but if losses were 2.5 times higher in the past when the solar wind was more active, in addition to the fact that it may be several times higher during high solar activity, then the increased loss during CIR events is important."

The effect seems to be absent from Venus and Earth. Even though the solar wind dynamic pressure is greater at Venus than Mars and the transfer of energy between the solar wind and Venus' plasma is more efficient, the CIR region may not have enough time to form at the orbit of Venus, resulting in weaker, if any, 'pressure pulses'. Earth also escapes the effect because of its strong magnetic field that protects the atmosphere.

The team hope to continue studying plasma escape mechanisms at Mars until the next solar cycle maximum, in order to see how the outflow rates vary with the phase of the solar cycle. “These observations were made during a very quiet period in the eleven year solar cycle and so we would expect the effect of these and other large scale disturbances to be higher at other times in the solar cycle,” comments Mark Lester, Head of the Department of Physics and Astronomy at the University of Leicester.

The team's results are published in the journal Geophysical Research Letters.