Anomalous occurrence features of the preliminary impulse of geomagnetic sudden commencement in the South Atlantic Anomaly region

Occurrence features of the preliminary impulse (PI) of geomagnetic sudden commencement (SC) both in the Pacific Ocean and South Atlantic Anomaly (SAA) regions were investigated using the long‐term magnetic field data obtained from the Circum‐pan Pacific Magnetometer Network (CPMN) and NICT Space Weather Monitoring (NSWM) magnetometer networks. The low‐latitude preliminary reverse impulse (PRI) at Okinawa (OKI: dip latitude = 37.97°) in the Pacific Ocean region appeared in all the magnetic local time (MLT) sectors with the peak occurrence rate of 40% around noon. On the other hand, the PRI occurrence rate at Santa Maria (SMA: dip latitude = −34.35°) near the center of the SAA region showed a significant enhancement in the daytime sector (0800–1600 h, MLT) with the peak value of 80%, which resembles the occurrence feature of the equatorial PRI. Moreover, the PRI amplitude around noon at SMA was about 3.0 times larger than that at OKI. From the calculation of the ionospheric conductivity derived from the IRI‐2007 and NRLMSISE‐00 models, it is shown that the height‐integrated conductivity was more enhanced in the SAA region (SMA), where the ambient magnetic field intensity is weak, compared with that in the Pacific Ocean region (OKI). Therefore, the anomalous increase of the PRI occurrence and amplitude is caused by the significant enhancement of the ionospheric conductivity due to the weakness of the ambient magnetic field intensity in the SAA region.