In July of 2021, we reported how the Hope probe sent by the United Arab Emirates to study the Martian atmosphere had released images of the nightside aurora on Mars. "They're not easy to catch, and so that's why seeing them basically right away with (Emirates Mars Mission) was kind of exciting and unexpected," Justin Deighan, a planetary scientist at the University of Colorado and deputy science lead of the mission, told Space.com at the time.
Aurora without a global magnetic field
This was a rarely seen event and brought up the question: how do aurora form on the Red planet without a global magnetic field? Now, physicists led by the University of Iowa have found the answer, according to a press release by the institution published on Wednesday.
Mars' aurora is a light-in-the-sky display that occurs mostly during the night in the Red Planet's southern hemisphere. While their existence has been known for a while, scientists have been perplexed as to how they form because Mars does not have a global magnetic field like Earth, which is the main source for aurora on our precious planet.
The physicists now claim that new research has uncovered that aurora on Mars is created through the interaction between the solar wind and magnetic fields generated by the crust at southern latitudes on the Red Planet.
"We have the first detailed study looking at how solar wind conditions affect aurora on Mars," said Zachary Girazian, an associate research scientist in the Department of Physics and Astronomy and the study's corresponding author.
"Our main finding is that inside the strong crustal field region, the aurora occurrence rate depends mostly on the orientation of the solar wind magnetic field, while outside the strong crustal field region, the occurrence rate depends mostly on the solar wind dynamic pressure."
To come to this conclusion, the researchers had to study more than 200 observations of discrete aurora on Mars by the NASA-led Mars Atmosphere and Volatile Evolution (MAVEN) spacecraft using a tool called the Solar Wind Ion Analyzer. This tool measures the solar wind and magnetosheath proton flow around Mars and constrains the nature of solar wind interactions with the upper atmosphere.
"Now is a very fruitful and exciting time for researching aurora at Mars. The database of discrete aurora observations we have from MAVEN is the first of its kind, allowing us to understand basic features of the aurora for the first time," Girazian concluded.
The new findings complement a study that was released last year and revealed that Mars' aurora was proton aurora.
"Proton aurora are a distinct class of auroral phenomena caused by energetic protons precipitating into a planetary atmosphere. The defining observational signature is atomic hydrogen emissions from the precipitating particles after they obtain an electron from the neutral atmospheric gas, a process known as charge exchange," wrote the researchers at the time of their study.
The new study is published in the Journal of Geophysical Research: Space Physics.
Discrete aurora at Mars, characterized by their small spatial scale and tendency to form near strong crustal magnetic fields, are emissions produced by particle precipitation into the Martian upper atmosphere. Since 2014, Mars Atmosphere and Volatile EvolutioN's (MAVEN's) Imaging Ultraviolet Spectrograph (IUVS) has obtained a large collection of UV discrete aurora observations during its routine periapsis nightside limb scans. Initial analysis of these observations has shown that, near the strongest crustal magnetic fields in the southern hemisphere, the IUVS discrete aurora detection frequency is highly sensitive to the interplanetary magnetic field (IMF) clock angle. However, the role of other solar wind properties in controlling the discrete aurora detection frequency has not yet been determined. In this work, we use the IUVS discrete aurora observations, along with MAVEN observations of the upstream solar wind, to determine how the discrete aurora detection frequency varies with solar wind dynamic pressure, IMF strength, and IMF cone angle. We find that, outside of the strong crustal field region (SCFR) in the southern hemisphere, the aurora detection frequency is relatively insensitive to the IMF orientation, but significantly increases with solar wind dynamic pressure, and moderately increases with IMF strength. Interestingly however, although high solar wind dynamic pressures cause more aurora to form, they have little impact on the brightness of the auroral emissions. Alternatively, inside the SCFR, the detection frequency is only moderately dependent on the solar wind dynamic pressure, and is much more sensitive to the IMF clock and cone angles. In the SCFR, aurora are unlikely to occur when the IMF points near the radial or anti-radial directions when the cone angle (arccos(Bx/|B|)) is less than 30° or between 120° and 150°. Together, these results provide the first comprehensive characterization of how upstream solar wind conditions affect the formation of discrete aurora at Mars.