Global Ionospheric Response to the Geomagnetic Storm Event of April 6-7, 2000

A great geomagnetic storm occurred during April 6-7, 2000, with the Dst index reaching a minimum value of about-320 nT. Using the ionosonde and digisonde data from about 25 stations distributed globally, the morphology of the ionospheric response to this geomagnetic storm event is studied by comparing the storm-time disturbed value of f₀F₂ with its quiet time reference value. To identify the mechanism responsible for the ionospheric disturbances during this event, a comparison is made between the ratio of disturbed to undisturbed values of N_mF₂ and of the neutral composition concentration ratio (n_o/n_N2). The results of data analysis showed that during the main and early recovery phases of the geomagnetic storm, a negative ionospheric F₂ layer storm occurred globally. The maximum negative deviation of f₀F₂ from its quiet time reference value occurred during the early recovery phase of the geomagnetic storm. During the late recovery phase of the geomagnetic storm, positive ionospheric disturbances occurred in some stations, particularly in those high latitude stations. It is showed that the main characteristics of the ionospheric disturbances during this geomagnetic storm event are consistent with that expected from the storm effects resulted from storm-induced thermospheric circulation system.