Storm‐time magnetic configurations at geosynchronous orbit: Comparison between the main and recovery phases

The present study statistically examines storm‐time geosynchronous magnetic configurations in terms of storm phase, storm intensity, and magnetic local time. It is found that the nightside geosynchronous magnetic field is more stretched with increasing ∣ Sym ‐ H ∣irrespective of the storm phase, but for a given value of Sym ‐ H the magnetic field tends to be more stretched during the main phase than during the recovery phase. This result suggests that the relative contribution of the tail current to Sym ‐ H is larger during the main phase than during the recovery phase. Interestingly, the tendency is just the opposite in the midday sector. That is, the geosynchronous magnetic field is less stretched during the main phase than during the recovery phase. It is suggested from a storm‐time ring current simulation that the ring current is indeed weakened in the midday sector during the main phase as enhanced magnetospheric convection sweeps ions on previously closed trajectories to the dayside magnetopause. Reflecting the opposite storm‐phase dependences of the dayside and nightside magnetic field, the geosynchronous magnetic configuration is significantly more asymmetric during the main phase than during the recovery phase, and the degree of the asymmetry increases with ∣ Sym ‐ H ∣. Most importantly, the asymmetry is predominantly between day and night rather than between dawn and dusk as expected from the conventional model of the partial ring current. This result suggests that the dawn‐dusk asymmetry of the low‐ and midlatitude ground magnetic depression, which the partial ring current model sought to explain, can be attributed mostly to field‐aligned currents.