The effects of two solar flares that hit Earth and Mars together have been recorded for the first time.
Twice during April 2001, the Mars Global Surveyor (MGS) was in the right place at the right time to provide scientists with the first measurements of the changes that occur in the Red Planet's ionosphere – an outer layer of ions and electrons – after experiencing a solar flare.
"This is the first observation of the effect of solar flares upon an ionosphere other than Earth's own," says Paul Withers, at Boston University, US, and a co-author of the new study.
Solar flares are explosive hiccups that spew high energy particles and radiation away from the Sun. Changes in the Earth's own ionosphere caused by such flares can affect the strength and quality of radio communications and introduce uncertainty in satellite-based navigation tools, such as the Global Positioning System.
As NASA looks to Mars as a target for manned missions, scientists hope to better understand its ionosphere and the effect solar flares might have on future communication and navigational devices there.
The team, led by Michael Mendillo, also at Boston University, examined hundreds of profiles of electron-density distribution in the Martian ionosphere taken by the MGS, looking for a link to solar flares.
"You have to be lucky to see these sorts of things," Withers says. The solar flare must be large enough to have a substantial effect; the planet must be in the right position to be bathed in the radiation from the flare; and the instrument must be taking a measurement within about an hour of the event.
The team managed to find two sets of measurements corresponding to a very strong flare on 15 April 2001 and a weaker event on 26 April 2001. MGS measured the electron density just 90 seconds after the flare's leading-edge reached the Martian ionosphere during the weaker event and 20 minutes after the peak of the 15 April flare.
In each case, the electron densities at and below the lower of Mars' two main ionospheric layers (110 km above the surface) were increased by 50% to 200%. But the upper layer (130 km) was largely unaffected.
Withers says that is because the upper layer is mostly generated by extreme ultraviolet radiation, while the lower layer, which is much more variable, is influenced by shorter wavelength X-rays, which are more energetic and can therefore penetrate deeper into the Martian atmosphere.
"The flux of ultraviolet radiation from the Sun doesn't change very much, but the Sun's X-ray flux changes a lot during a solar flare," says Withers. So the increased ionisation of the lower layer in reaction to an increased dose of solar photons, particularly X-rays, chimed well with scientists' expectations.
In the so-called E layer of the Earth's ionosphere – which the team uses as a comparison with Mars's lower layer – electron densities increased by about 45% after the 26 April flare, as measured from Sondrestrom, Greenland.
The team concludes that the electron-density increase at Mars was consistent with that measured at the corresponding site on Earth and nearly simultaneous – photons from the flare reached Mars only about four minutes after the Earth.
Journal reference: Science (vol 311, p 1135)
