Understanding and predicting processes that perturb planetary ionospheres is of paramount importance for long-distance radio communication. Perhaps the oldest known ionospheric disturbances are ‘sporadic E layers’1: unpredictable and short-lived concentrations of plasma2, which can bounce radio signals over the horizon for thousands of kilometres3. Consequentially, local radio broadcasts can become jammed by more distant transmissions, and thus sporadic E layers are a potentially serious complication for commercial radio, aviation, shipping or the military. According to the current theory of their formation, we should also expect an equal proportion of localized ionospheric density depletions to develop. However, no such ‘sporadic E rifts’ have been detected in over 85 years of ionospheric research. In addition, despite being common at Earth, no sporadic E layers have yet been reported at other planets. Here we report the detection of sporadic E-like phenomena in the ionosphere of Mars by NASA’s Mars Atmosphere and Volatile Evolution (MAVEN) spacecraft, providing a physical explanation for previous unexplained observations at Mars4,5,6,7. We observe enhanced-density layers that can be explained through the presence of a sporadic E-like mechanism, and we establish the existence of sporadic E rifts in nature. We find that, unlike the case at Earth, Martian sporadic E features are trapped in a near-perpetual state of dynamic formation and may form at predictable locations. Also unlike the case at Earth, Martian sporadic E features are readily accessible to satellites, and indeed MAVEN has already encountered more of the phenomena at Mars than have ever been explored in situ at Earth with suborbital rockets.
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MAVEN data are available from the Planetary Plasma Interactions Node of the NASA Planetary Data System (https://pds-ppi.igpp.ucla.edu/).
Software to analyse data from the MAVEN Particles and Fields package is available through the MAVEN Science Data Center (https://lasp.colorado.edu/maven/sdc/public/), and through the University of California at Berkeley’s Space Science Laboratory TPLOT package (http://sprg.ssl.berkeley.edu/data/maven/misc/socware/).
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This work was supported by the MAVEN mission. We thank C. Fowler for assistance in differentiating Martian sporadic E features from Martian ionospheric irregularities. We also thank A.P. McCall of the Kunyung Institute for Space Physics, Melbourne, Australia, for useful discussions.
The authors declare no competing interests.
Peer review information Nature Astronomy thanks Michael Pezzopane, Hiroyuki Shinagawa and the other, anonymous, reviewer(s) for their contribution to the peer review of this work.
Publisher’s note Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.
Comparison of observations of Sporadic-E-like features at Mars (red) and Earth (green). Each panel shows plasma density (cm-3) versus altitude (km). Top panels show observations of ion density (each species colour coded), Bottom panels show electron densities. Panels a,b.) MAVEN in situ observations (Fig. 2, main paper); Panels c,d.) remote-sensing observations by the Mars Global Surveyor (MGS); Panels e,f.) In situ observations of Sporadic E at Earth by suborbital rocketcraft. Consistent with previous remote-sensing results from Mars and at Earth, Martian Sporadic E appear to be narrow structures only a few kilometres across.
Expanded plot of additional supporting MAVEN data from our two prime events.
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Collinson, G.A., McFadden, J., Grebowsky, J. et al. Constantly forming sporadic E-like layers and rifts in the Martian ionosphere and their implications for Earth. Nat Astron 4, 486–491 (2020). https://doi.org/10.1038/s41550-019-0984-8
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