Researchers explain how auroras form on Mars without a global magnetic field

In July 2021, we reported how the Hope probe sent by the United Arab Emirates to study the Martian atmosphere had published images of the night side dawn on Mars. “They’re not easy to catch, and that’s why seeing them right away with (Emirates Mars Mission) was kind of exciting and unexpected,” said Justin Deighan, planetary scientist at the University of Colorado and deputy chief science officer of the mission. , Told Space.com at the time.

Aurora without global magnetic field

It was a rarely seen event and raised the question: how do auroras form on the Red Planet without a global magnetic field? Now physicians led by the University of Iowa have found the answer, according to a press release from the institution released Wednesday.

The Mars aurora is a light show in the sky that occurs mostly at night in the Red Planet’s southern hemisphere. Although their existence has been known for some time, scientists are puzzled about their formation because Mars does not have a global magnetic field like Earth, which is the main source of auroras on our precious planet.

Physicists now claim that new research has discovered that auroras on Mars are created by the interaction between the solar wind and magnetic fields generated by the crust in the Red Planet’s southern latitudes.

“We have the first detailed study of how solar wind conditions affect auroras on Mars,” said Zachary Girazian, associate researcher in the Department of Physics and Astronomy and corresponding author of the study.

“Our main finding is that inside the strong crustal field region, the rate of aurora occurrence mainly depends on the orientation of the solar wind magnetic field, while outside the crustal field region strong, the occurrence rate mainly depends on the dynamic pressure of the solar wind.”

To reach this conclusion, the researchers had to study more than 200 observations of discrete auroras 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 flux of protons from the solar wind and the magnetosheath around Mars and limits the nature of the interactions of the solar wind with the upper atmosphere.

“Now is a very fruitful and exciting time for aurora research on Mars. The database of discrete aurora sightings we have from MAVEN is the first of its kind, allowing us to understand for the first time the basic characteristics of auroras,” Girazian concluded.

Proton aurora?

The new results complete a study that was published last year and found the Mars aurora to be a proton aurora.

“Proton auroras are a distinct class of auroral phenomena caused by energetic protons rushing through a planetary atmosphere. The defining observational signature is the emissions of atomic hydrogen from the rushing particles after obtaining an electron from the neutral atmospheric gas, a process known as charge exchange,” the researchers wrote at the time of their study.

The new study is published in the Geophysical Research Journal: Space Physics.

Abstract:

Discrete auroras on Mars, characterized by their small spatial scale and their tendency to form near strong crustal magnetic fields, are emissions produced by the precipitation of particles in the Martian upper atmosphere. Since 2014, the Mars Atmosphere and Volatile EvolutioN (MAVEN) Ultraviolet Imaging Spectrograph (IUVS) has obtained a large collection of discrete UV aurora observations during its routine analyzes of nocturnal periapsis limbs. Initial analysis of these observations showed that near the strongest crustal magnetic fields in the southern hemisphere, the detection frequency of discrete UVS auroras is very sensitive to the clock angle of the interplanetary magnetic field. (MFI). However, the role of other properties of the solar wind in controlling the discrete frequency of aurora detection has yet to be determined. In this work, we use observations of discrete auroras from the UVS, together with MAVEN observations of the solar wind upstream, to determine how the frequency of detection of discrete auroras varies with the dynamic pressure of the solar wind, the strength of the IMF and the cone angle of the IMF. We find that outside the Strong Crustal Field Region (SCFR) in the Southern Hemisphere, aurora detection frequency is relatively insensitive to IMF orientation, but increases significantly with dynamic pressure of the solar wind and increases moderately with the strength of the IMF. Interestingly though, although high dynamic solar wind pressures cause more auroras to form, they have little impact on the brightness of auroral emissions. Alternatively, inside the SCFR, the detection frequency is only moderately dependent on dynamic solar wind pressure and is much more sensitive to IMF clock and cone angles. In the SCFR, auroras are unlikely to occur when the IMF points near 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 upwind solar wind conditions affect the formation of discrete auroras on Mars.


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