First Observational Results of the Neutral Gas Mass Spectrometer Onboard the China's TianMu‐1 Constellation

Abstract In situ observations of thermospheric composition have long been scarce, limiting our understanding of the upper atmosphere and its response to space weather. This study focuses on the Neutral Gas Mass Spectrometer (NGMS) onboard the TM19 and TM20 satellites of China's TianMu‐1 constellation, presenting the first systematic elaboration of its core technical characteristics and novel in situ observational results of O and N 2 number densities. During the 10–12 October 2024 geomagnetic storm, the ratio of O to N 2 number densities (O/N 2 ) decreased significantly at mid‐high latitudes, owing to the stronger enhancement of N 2 compared with O. N 2 perturbations were confined to latitudes above ∼±30° in both hemispheres, whereas O perturbations exhibited a global latitudinal distribution, likely reflecting that heavy N 2 is locally enhanced by upwelling and decays equatorward via diffusion, while O is transported globally by storm‐time circulation and waves. Additionally, the O/N 2 observations from TM19 and TM20 showed high cross‐satellite response consistency with a correlation coefficient of 0.89, while the MSIS 2.0 model systematically overestimated the O/N 2 under geomagnetically quiet conditions. The NGMS onboard TianMu‐1 provides crucial data support for optimizing atmospheric models.