‘Proton Aurora’ is Most Common Type of Aurora on Mars, Researchers Say

Proton aurora, a type of Martian aurora first identified in 2016, forms when protons from the solar wind interact with hydrogen in the extended portions of the Martian atmosphere and travel to lower regions. According to multi-year observations made using the Imaging UltraViolet Spectrograph (IUVS) onboard NASA’s MAVEN (Mars Atmosphere and Volatile Evolution) spacecraft, this phenomenon is actually the most common form of aurora on Mars; it is observed on the dayside of Mars in 14% of the MAVEN/IUVS data, which is far more often than initially expected.

Images of proton aurora on Mars: MAVEN’s IUVS instrument observes the atmosphere of Mars, making images of neutral hydrogen and proton aurora simultaneously (left); observations under normal conditions show hydrogen on the disk and in the extended atmosphere of the planet from a vantage point on the nightside (middle); proton aurora is visible as a significant brightening on the limb and disk (right); with the contribution of neutral hydrogen subtracted, the distribution of proton aurora is revealed, showing that it peaks in brightness just off the Martian disk as energetic neutrals slam into the atmosphere. Image credit: Embry-Riddle Aeronautical University / Laboratory for Atmospheric and Space Physics, University of Colorado, Boulder.

“At first, we believed that proton aurora events were rather rare because we weren’t looking at the right times and places,” said Dr. Mike Chaffin, a researcher in the Laboratory for Atmospheric and Space Physics at the University of Colorado, Boulder.

“But after a closer look, we found that proton aurora are occurring far more often in dayside southern summer observations than we initially expected.”

MAVEN’s mission is to investigate how Mars lost much of its atmosphere and water, transforming its climate from one that might have supported life to one that is cold, dry, and inhospitable.

Since the proton aurora is generated indirectly by hydrogen derived from the Martian water that’s in the process of being lost to space, this phenomenon could be used to help track ongoing water loss.

“In this new study using MAVEN/IUVS data from multiple Mars years, we found that periods of increased atmospheric escape correspond with increases in proton aurora occurrence and intensity,” said Dr. Andréa Hughes, from the Center for Space and Atmospheric Research and the Department of Physical Sciences at the Embry-Riddle Aeronautical University.

“Perhaps one day, when interplanetary travel becomes commonplace, travelers arriving at Mars during southern summer will have front-row seats to observe Martian proton aurora majestically dancing across the dayside of the planet.”

On Earth, the Northern and Southern lights occur when the solar wind (electrically charged particles from the Sun) follow our planet’s geomagnetic field lines to the poles and collide with the upper atmosphere. Mars lacks a global magnetic field, so instead the solar wind piles up in front of Mars in a bow shock, which blocks charged particles from reaching the bulk of the atmosphere. However, in a process first observed by NASA’s MAVEN spacecraft, some solar wind protons can slip past the bow shock by first bonding with electrons from the Martian upper atmosphere to form hydrogen atoms. Because these hydrogen atoms are electrically neutral, they can pass through the bow shock and go on to create an ultraviolet proton aurora on the dayside of Mars. Image credit: NASA’s Goddard Space Flight Center.

The study authors found proton aurora in about 14% of their dayside observations, which increases to more than 80% of the time when only dayside southern summer observations are considered.

“By comparison, IUVS has detected diffuse aurora on Mars in a few percent of orbits with favorable geometry, and discrete aurora detections are rarer still in the dataset,” said Dr. Nick Schneider, also from the Laboratory for Atmospheric and Space Physics at the University of Colorado, Boulder.

“All the conditions necessary to create Martian proton aurora (e.g., solar wind protons, an extended hydrogen atmosphere, and the absence of a global dipole magnetic field) are more commonly available at Mars than those needed to create other types of aurora,” Dr. Hughes said.

“Also, the connection between MAVEN’s observations of increased atmospheric escape and increases in proton aurora frequency and intensity means that proton aurora can actually be used as a proxy for what’s happening in the hydrogen corona surrounding Mars, and therefore, a proxy for times of increased atmospheric escape and water loss.”

The study was published in the Journal of Geophysical Research: Space Physics.

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Andréa Hughes et al. Proton Aurora on Mars: A Dayside Phenomenon Pervasive in Southern Summer. Journal of Geophysical Research: Space Physics, published online October 12, 2019; doi: 10.1029/2019JA027140