Tidal Effects on the Longitudinal Structures of the Martian Thermosphere and Topside Ionosphere Observed by MAVEN

Abstract

Longitudinal structures in the Martian thermosphere and topside ionosphere between 150 and 200 km altitudes are studied using in situ electron and neutral measurements from the NASA Mars Atmosphere and Volatile EvolutioN (MAVEN) mission. Four time intervals are selected for comparison, during which MAVEN sampled similar local time (9.3–10.3 h) and latitude (near 20°S) regions but at different solar longitude positions (two near northern summer solstice, one each at northern vernal and autumnal equinoxes). Persistent and pronounced tidal oscillations characterize the ionosphere and thermosphere, whose longitudinal variations in density are generally in‐phase with each other. Our analysis of simultaneous and collocated neutral and electron data provides direct observational evidence for thermosphere‐ionosphere coupling through atmospheric tides. We conclude that the ionosphere is subject to modulation by upward‐propagating thermal tides, via both tide‐induced vertical displacement and photochemical reactions. Atmospheric tides constitute a ubiquitous and significant perturbation source to the ionospheric electron density, up to ∼15% near 200 km.Plain Language SummaryVertically propagating tides are a type of global‐scale periodic oscillation caused by solar heating in a rotating planetary atmosphere. Martian atmospheric tidal waves have been extensively observed and studied at low altitudes, but our knowledge concerning their behavior at high altitudes is sparse. In particular, little is known about tidal oscillations in the ionosphere, which is the part of the upper atmosphere that is ionized by absorption of solar EUV and X‐ray irradiance. Using in situ neutral and electron measurements from the NASA Mars Atmosphere and Volatile EvolutioN (MAVEN) mission, we investigate thermal tidal signatures in both the Martian upper atmosphere and ionosphere. Our analysis of simultaneous and collocated ionospheric and atmospheric data provides direct observational evidence that the charged and neutral regimes are tightly coupled, not only through well‐understood photochemical reactions but also by tidal waves forced from below. This study elucidates the role of planetary‐scale tidal waves in coupling various elements of the near‐Mars space environment. Besides our directly examined neutral‐electron density coupling, this study also sheds light on the nature of vertical coupling between the lower and upper atmospheric regimes of Mars.Key PointsDirect observational evidence for Martian thermosphere‐ionosphere coupling by atmospheric tides is presentedThe ionosphere below 200 km altitude is controlled by photochemistry and modulated by tide‐induced vertical displacementAtmospheric tides constitute a ubiquitous, significant source of ionospheric electron density variability (up to ∼15% at 200 km altitude)