Localized Ionization Hypothesis for Transient Ionospheric Layers

The persistent two-peaked vertical structure of the Martian ionosphere is created by extreme and far ultraviolet radiation whose energies, respectively, determine their ionization altitude. A third low-altitude transient layer (previously referred to as M3 or M-m) has been observed by radio occultation techniques and attributed to meteor ablation. However, recent remote sensing and in situ observations disfavor a meteoric origin. Here we propose an alternative hypothesis for these apparent layers associated with impact ionization from penetrating solar wind ions, previously observed as proton aurora. Localized ionization, occurring nonglobally at a given altitude range, breaks the symmetry assumed by the radio occultation technique, and creates electron layers apparently lower in the ionosphere than their true altitude. This may occur when the upstream bow shock is altered by a radial interplanetary magnetic field configuration, which allows the solar wind to penetrate directly into the thermosphere. This localized ionization hypothesis provides an explanation for apparent layers' wide variation in heights and their transient behavior. Moreover, this hypothesis is testable with new observations by the Mars Atmospheric and Volatile EvolutioN Radio Occultation Science Experiment in future Mars years. This hypothesis has implications for the ionospheres of Venus and Titan, where similar transient layers have been observed. Plain Language Summary The ionosphere of a planet couples the neutral atmosphere to the exosphere and solar wind. Created by solar ionizing radiation, the ionosphere on the dayside of planet is expected to be hemispherically symmetric. When Mars Express discovered transient low-altitude ionospheric layers in 2005, they were initially predicted to be due to meteor ablation. However, recent observations indicate that they are due to a geometric observational effect related to local, not hemispheric, ionization. Observations from three instruments on Mars Atmospheric and Volatile EvolutioN are used concurrently to understand this phenomenon. These transient layers have been observed at Venus and Titan as well, and this hypothesis indicates novel physics for those ionospheric systems.