Although Mars does not have a global magnetic field, it also has auroras - so-called proton auroras. They are created by the interaction of the solar wind with the upper Martian atmosphere and were first detected in 2018 using data from NASA's MAVEN probe. Planetary researchers have now found out more about this proton aurora. Contrary to what was previously assumed, this luminous phenomenon, which is visible in ultraviolet light, does not appear to be evenly distributed over the entire day side of the planet, but varies greatly in terms of time and space. The scientists suspect that this type of luminous phenomenon is due to turbulence in the Martian environment. However, the exact mechanisms behind this still need to be clarified.
Aurora Borealis is not uncommon on Earth. The greenish or reddish streaks in the sky are created when high-energy particles of the solar wind - mostly electrons - are deflected and accelerated by the earth's magnetic field and then interact with the gas particles in the ionosphere. The gas molecules are excited by collisions with the high-energy electrons and release excess energy in the form of light when they return to their ground state. The magnetic field lines running to the poles lead to the terrestrial auroras being concentrated in the polar latitudes – there the protective “magnetic cage” is more permeable to the solar wind. Auroras have also been observed on other planets with a strong magnetic field, such as Jupiter or Saturn. Mars, on the other hand, has no global magnetic field, so according to conventional wisdom it should not form auroras.
Mars: Aurora even without a magnetic field
But back in 2018, Mars proved Earth scientists wrong: data from NASA's MAVEN spacecraft revealed that during solar storms, the entire dayside of the planet emits a faint ultraviolet glow -- a widespread aurora. More detailed analyzes showed that the protons of the solar wind play a decisive role in this. When they encounter the Red Planet's ionosphere, they react with the charged particles present there and temporarily transform into neutral hydrogen. This, in turn, allows them to penetrate the Martian atmosphere despite the deflecting "bow wave" of the ionosphere. There, these particles give off part of their energy again and thus generate the UV light. In previous observations by MAVEN and the European Space Agency's Mars Express spacecraft, the Martian proton aurora appeared as a uniformly bright expanse stretching across the entire day side of the planet.
Michael Chaffin from the University of Colorado in Boulder and his colleagues have now discovered what lies behind this apparently uniform glow. For their study, they evaluated new measurement data from the MAVEN probe and the UV spectrograph (EMUS) of the Mars probe "Hope" from the United Arab Emirates. "The global observations of the upper Martian atmosphere by this probe provide us with a unique perspective on a region that is crucial for MAVEN observations," explains co-author Shannon Curry from the University of California at Berkeley. "These types of simultaneous observations shed light on the fundamental physics of atmospheric dynamics."
First detection of a "blotchy" proton aurora
The evaluation of the data from both probes showed that the proton aurora of Mars is significantly less uniform than previously thought. Instead, there are strong differences in the temporal and spatial intensity of UV radiation. "We thus present the first definitive evidence for a spatially localized, 'blotchy' proton aurora on Mars," reports the research team. They hypothesize that this patchy distribution of the aurora is due to turbulence caused by the interaction of the solar wind and the Martian ionosphere. "The observations indicate that the plasma environment of Mars must be quite disturbed," explains Chaffin. "This is so far that the solar wind was able to penetrate directly into the upper Martian atmosphere at the points where we observed the strong aurora emission. So what we saw as the Aurora was essentially a map showing us where the solar wind is raining down on the planet.”
It has not yet been clarified in detail how these turbulences come about in the Martian ionosphere and in the proton aurora. "The effects of these conditions on the Martian atmosphere are also still unknown," says Chaffin. But he and his team hope that the two space probes MAVEN and Hope will contribute to further clarification with their complementary data.
Source: Michael Chaffin (University of Colorado, Boulder) et al., Geophysical Research Letters, doi: 10.1029/2022GL099881