Spectroscopic detection of terrestrial lightning from space by JUICE-MAJIS during Earth Gravity Assist

Abstract. A lightning event was detected by the MAJIS imaging spectrometer onboard the Jupiter Icy Moons Explorer (JUICE) spacecraft during its first Earth gravity assist maneuver. This serendipitous space-based spectroscopic observation represents the first detection of its kind for any planetary atmosphere. The event, composed of four flashes, was registered on 2024, August, 20^th in an area offshore of Sumatra island, during local nighttime, near to optically thick clouds probed by MAJIS thermal wavelengths. No coincident detection has been obtained by ground-based lightning sensor networks, yet MAJIS observations provide unambiguous evidence of neutral atomic oxygen and nitrogen emissions, identified through several diagnostic lines. A faint Hα signature may also tentatively be associated with lightning flashes.

As MAJIS is not optimized for such observations, a number of caveats related to spectral and temporal resolutions have been considered when deriving absolute quantities, such as lightning energy and temperature. Retrieved energies are overall consistent with known emission by lightning of average strength, ranging from (0.7 ± 0.2) to (1.3 ± 0.3) MJ in the 777 nm O I line and from (0.5 ± 0.2) to (1.5 ± 0.4) MJ in the 870 nm N I line. Temperature estimates, more sensitive to observing biases, yield a broad range of values, spanning between 5000 and 20000 K, with standard uncertainties of the order of 2000–3000 K depending on the retrieval method.

This observation represents a useful benchmark for guiding detection and interpreting possible lightning events on Jupiter, a primary target of the JUICE mission. A preliminary extrapolation of the terrestrial case to the conditions of Jovian atmosphere suggests that H I emissions in the 650 nm and 1870 nm spectral ranges are the most promising for identifying lightning on Jupiter with the MAJIS instrument.