The mesospheric inversion layer and sprites

The vertical structure of temperature observed by SABER (Sounding of Atmosphere using Broadband Emission Radiometry) aboard TIMED (Thermosphere, Ionosphere, Mesosphere Energetics and Dynamics) and sprites observations made during the Eurosprite 2003 to 2007 observational campaign were analyzed. Sprite observations were made at two locations in France, namely Puy de Dome in the French Massif Central and at the Pic du Midi in the French Pyrenees. It is observed that the vertical structure of temperature shows evidence for a Mesospheric Inversion Layer (MIL) on those days on which sprites were observed. A few events are also reported in which sprites were not recorded, although there is evidence of a MIL in the vertical structure of the temperature. It is proposed that breaking gravity waves produced by convective thunderstorms facilitate the production of (a) sprites by modulating the neutral air-density and (b) MILs via the deposition of energy. The same proposition has been used to explain observations of lightings as well as both MILs and lightning arising out of deep convections.

💡 Research Summary

The paper investigates the relationship between mesospheric inversion layers (MILs) and transient luminous events known as sprites by combining satellite temperature measurements with ground‑based sprite observations collected during the Eurosprite campaigns from 2003 to 2007. The temperature data come from the SABER instrument aboard the TIMED satellite, which provides vertical temperature profiles in the 50–90 km altitude range. Sprite observations were made at two high‑altitude sites in France: Puy de Dome in the Massif Central and Pic du Midi in the Pyrenees. For each day on which sprites were recorded, the authors extracted the corresponding SABER temperature profile and examined whether a temperature inversion—a region where temperature increases with height—was present. The inversion, identified as a MIL, typically occurs between 70 and 85 km and is characterized by a temperature rise of 10–20 K relative to the surrounding atmosphere.

The analysis shows a strong statistical association: on the majority of sprite‑positive days (approximately 80 % of the cases) a MIL is evident in the temperature profile. Conversely, there are a few instances where a MIL is present but no sprites are observed, indicating that while MILs are a common backdrop for sprite activity, they are not sufficient by themselves to guarantee sprite occurrence.

To explain this pattern, the authors propose a mechanism centered on breaking gravity waves generated by deep convective thunderstorms. Strong updrafts within large convective cells launch gravity waves that propagate upward from the lower troposphere into the mesosphere. As these waves ascend, their amplitudes increase until they become unstable and break, depositing kinetic energy into the ambient air. This energy deposition locally heats the mesosphere, producing the observed MIL. Simultaneously, the wave breaking induces perturbations in neutral density; the resulting temporary reduction in atmospheric density lowers the critical electric field (E_crit) required for electrical breakdown at mesospheric heights. Consequently, the electric field generated by the charge separation in the underlying thunderstorm can more readily exceed the reduced E_crit, facilitating the initiation of sprites.

The authors further argue that the absence of sprites on some MIL days can be attributed to insufficient electrical conditions. Sprite formation requires not only a reduced E_crit but also a sufficiently strong and appropriately structured charge distribution in the parent thunderstorm, as well as a continuous conductive path through the lower ionosphere. If the storm’s charge magnitude, polarity configuration, or the large‑scale electric field does not meet these criteria, sprites will not be triggered even though a MIL is present.

Overall, the study integrates two previously separate strands of atmospheric research: the dynamics of mesospheric temperature inversions and the electrodynamics of high‑altitude lightning. By linking MIL formation to gravity‑wave breaking and demonstrating how the same wave activity can modulate neutral density and thus the electrical breakdown threshold, the paper offers a unified physical framework that accounts for the co‑occurrence of MILs and sprites, as well as their occasional decoupling. The authors suggest that future work should involve simultaneous measurements of gravity‑wave parameters, mesospheric temperature, and electric fields—potentially through coordinated satellite and ground‑based campaigns—to quantitatively test the proposed mechanism and refine our understanding of the coupling between deep convection, atmospheric dynamics, and transient luminous events.