Investigating Magnetosphere‐Ionosphere‐Thermosphere Coupling During Geomagnetic Storms With a Novel Thermospheric Disturbance Index

This study introduces a novel thermospheric disturbance index, , designed to describe density variations during geomagnetic storms. The index is constructed by applying the spectral whitening method to thermospheric density data derived from precise orbit determination of satellites from 2015 to 2019. We systematically investigate the response characteristics, time lags, and correlations between this thermospheric index and its geomagnetic () and ionospheric () counterparts, which are derived using the same methodology for storm events (Dst ≤ −50 nT). Case studies reveal a strong correspondence among the three indices during storm evolution. Notably, not only effectively captures the propagation of large‐scale Traveling Atmospheric Disturbances but also identifies minor density perturbations during periods of quiet Dst. Statistical results indicate that while the time lag varies between events, the maximum correlation is predominantly found when lags by less than 1 hr. At zero lag, the correlation coefficient between and is ≤−0.60 in ∼90% of events, while it is ≥0.60 for and in ∼63% of cases. Based on this significant correlation, we establish a linear fitting relationship, demonstrating the feasibility of using the geomagnetic index to estimate the thermospheric index . This provides a useful empirical reference for thermospheric disturbance assessment and may contribute to future forecasting frameworks subject to validation over longer solar cycle conditions.