Comparative aeronomy: Molecular ionospheres at Earth and Mars

Abstract

The ionospheres in our solar system vary not only in their electron densities but also in the dominance of atomic versus molecular ions at their altitudes of peak plasma density. With the exception of Earth's F layer composed of atomic oxygen ions and electrons, all other planets have their peak ionospheric layers composed of molecular ions and electrons embedded in a dense neutral atmosphere. At Mars, both of its ionospheric layers have molecular ions, with the M1 layer at a lower altitude than the more robust M2 layer above it. The terrestrial ionosphere has a prominent region of molecular ions (the E layer) below the dominant F layer. In this paper, we explore the production and loss of molecular ion layers observed under the same solar irradiance conditions at Mars and Earth. We compare observations of M1 and M2 electron densities with terrestrial ionosonde data for the peak densities of the E and F layers during low, moderate, and high solar flux conditions. The subsolar peak densities of molecular ion layers have high correlations at each planet, as well as between planets, even though they are produced by separate portions of the solar spectrum. We use photochemical‐equilibrium theory for layers produced by soft X‐rays (M1 and E) versus the M2 layer produced by extreme ultraviolet (EUV) to identify the key parameters that cause similarities and differences. The yield of our comparative study points to the roles of secondary ionization and temperature‐dependent plasma recombination rates as areas most in need of further study at each planet.

Sources of Ionospheric Variability at Mars

“…A very substantial yield came from these studies of solar‐induced variability associated with the full range of time scales of solar activity. These included (a) solar flare effects (Gurnett et al, , ; Lollo et al, ; Mendillo et al, ) on time scales of minutes, (b) day‐to‐day solar irradiance effects (Martinis et al, ; Mendillo et al, , ; Rishbeth & Mendillo, ), (c) monthly solar active regions (Mendillo et al, ; Venkateswara Rao et al, ; Withers & Mendillo, ), and (d) solar cycle time frames (Mendillo et al, , ; Sanchez‐Cano et al, ; Withers et al, ). The MAVEN mission, with its suite of plasma, neutral atmosphere, and solar measurements, overcame the limitation of studying ionospheric variability at Mars by using only solar observations from Earth.…”

Comparative ionospheres: Terrestrial and giant planets

“…To address longer time scales, a recent study of NmM2-layer and E-layer observations spanning six years was described in Mendillo et al (2016) . For Mars, the MGS radio occultation data base was used to obtain daily values of NmM2 from 1 November 2001 to 9 June 2005.…”

Comparison of terrestrial and Martian TEC at dawn and dusk during solstices

“…Ionospheres are the charged regions that exist within the upper atmospheres of planets, moons, and minor bodies of the Solar and other Stellar Systems. Each Solar System body that has sufficient neutral atmosphere, especially within the inner part of the Solar System, has an ionosphere that is mainly formed by the solar ionising radiation at extreme-ultraviolet (EUV) and X-ray wavelengths (e.g., Witasse et al 2008;Mendillo et al 2016). The Earth and Mars are often considered to be the most similar of the terrestrial planets, with similar axial tilts (similar seasons, although longer at Mars), similar length of the day, and both having water at (or near) its surface, albeit on different scales and with different distributions (Orosei et al 2018).…”

“…This terrestrial layer is the so-called F-region, found at about 300 km. The F-region is controlled by a mix of photochemistry and dynamic processes, and is where the maximum electron density of terrestrial ionosphere occurs (e.g., Mendillo et al 2016).…”