Dipole tilt modulation of hemispheric response asymmetry in the storm-time polar thermospheric density

Previous studies of storm-time hemispheric asymmetry in thermospheric density have focused on direct drivers such as Joule heating, but overlooked the influence of Earth’s magnetic field topology. This study first links the Earth’s dipole tilt angle, which influences magnetic field geometry and its solar wind interaction, to the hemispheric density response asymmetry. Using in situ neutral density data from China’s TianMu-1 constellation, CHAMP, Grace FO and Swarm C, we analyzed 62 intense geomagnetic storm events (Kp ≥ 7, 2000–2025), yielding 171 satellite samples, to quantify the correlation between dipole tilt angle and storm-time hemispheric density response asymmetry. Statistical results show that asymmetry increases linearly with the dipole tilt angle at storm onset (R = 0.66; slope ≈ 2.29% per degree), with a bias that hemispheric responses are nearly identical when the dipole tilt angle is − 10°. This dependence may be related to the difference in energy input to southern and northern polar regions due to tilt-driven geometric variations in magnetotail reconnection. In this statistical sample, the hemispheric asymmetry exhibits no systematic dependence on storm intensity in terms of relative density perturbations. Our findings provide a reference for incorporating the dipole tilt angle into thermospheric density modeling. During intense geomagnetic storms, the thermosphere expands and its density increases, especially over the polar regions. These changes affect the orbits of low-Earth-orbit satellites. Scientists have long known that such density increases are not the same in the Northern and Southern Hemispheres, but most previous studies focused on direct local drivers, such as Joule heating. In this study, we analyze whether the dipole tilt angle affecting the Earth’s magnetic field topology plays a role in forming this north–south difference. By combining neutral density measurements from multiple satellites, including CHAMP, Grace FO, Swarm C, and China’s TianMu-1 constellation, we analyzed 62 intense geomagnetic storm events including 170 satellite samples from 2000 to 2025. Our results show a clear positive linear relationship between the dipole tilt angle and the hemispheric asymmetry in thermospheric density response. Interestingly, when the dipole tilt angle is approximately −10° (the Northern Hemisphere is tilted away from the Sun), the two hemispheres respond almost equally. Notably, this linear relationship is independent of geomagnetic storm intensity. These findings suggest that the dipole tilt angle may influence the distribution of energy injected into the polar regions during geomagnetic storms by affecting Earth’s magnetic field topology. Incorporating the dipole tilt angle into thermospheric density models could improve predictions of upper atmosphere conditions.