A Study On The Morphology Of The Ionospheric Disturbances Over Asia-Australia Sector

Accompanying the occurrence of geomagnetic storms, the behaviors of the ionosphere are significantly deviated from its background level, which is named as the ionospheric storms. As the key of ionospheric weather study, the ionospheric storms represent an extreme form of the'space weather'and can induce significant, adverse effects on increasingly ground- and space-based technological systems. However, systemic studies on the morphology of ionospheric storms at middle and low latitudes in the Chinese sector are still seldom found. In this paper, utilizing the critical frequency of the F₂ layer (f_oF₂) observed at the Asia-Australia Sector for nearly fifty years, we statistically investigate the dependence of the variations of the storm-time N_mF₂ on the geomagnetic activity, solar activity, season and local time, respectively.The present result shows that there are generally more positive ionospheric storm effects than negative ones presented at Kokubunji. In summer, negative storm effects prevail during high solar activity, whereas positive storm effects are usually observed during low solar activity; in winter, positive storm effects dominate regardless of solar activity conditions; and in equinox, negative storm effects with smaller amplitude are often observed during high solar activity, and positive storm effects often occur during low solar activity conditions. The correlation analysis between the disturbance of N_mF₂ and ap index also illustrates the above-mentioned feature. Furthermore, our results show that there is a time delay of 12?15 hours between the ionospheric maximum negative disturbances and the maximum geomagnetic perturbation, and the time delay of the ionospheric positive disturbances are 3 and 10 hours, respectively. In addition, under high geomagnetic activity conditions, positive disturbances are often seen in the evening, and negative disturbances in the morning.As far as effects of geomagnetic storms on the ionosphere are concerned, there is prominent north-south symmetry about the geomagnetic equator in Asia-Australia sector. In summer, negative storm effects prevail during high solar activity, whereas in winter positive storm effects are usually observed during low solar activity. Beyond the equatorial abnormal region, the lower the geomagnetic latitude is, the later negative storm arrives. However, the response time of positive storm is regardless of geomagnetic latitude. Negative storm often appears at the middle-high latitude, and positive storm often appears at the low latitude, the response sign changes at the geomagnetic latitude of about±30°. Under high geomagnetic activity conditions, negative disturbances are often seen around midnight at middle-high latitude, and shift to the morning sector with decreasing latitude; positive disturbances are often seen in the evening at middle-low and low latitudes.