This paper presents a case study of ionospheric response to an interplanetary magnetic field (IMF) southward turning. It is based on a comprehensive set of observations, including a global network of ground magnetometers, global auroral images, and a SuperDARN HF radar. There is a clear evidence for a two-stage ionospheric response to the IMF southward turning, namely, fast initial onset and slow final reconfiguration. The fast onset is manifested by nearly simultaneous (within 2 min) rise of ground magnetic perturbations at all local times, corroborated by a sudden change in the direction of line-of-sight velocity near local midnight and by the simultaneous equatorward shift of the auroral oval. The slow reconfiguration is characterized by the different rising rate of magnetic perturbations with latitudes: faster at high latitude than at lower latitudes. Furthermore, a cross-correlation analysis of the magnetometer data shows that the maximum magnetic perturbation is reached first near local noon, and then spread toward the nightside, corresponding to a dayside-to-nightside propagation speed of ~5 km/s along the auroral oval. Global ionospheric convection patterns are derived based on ground magnetometer data along with auroral conductances inferred from the Polar UV images, using the assimilative mapping of ionospheric electrodynamics (AMIE) procedure. The AMIE patterns, especially the residual convection patterns, clearly show a globally coherent development of two-cell convection configuration following the IMF southward turning. While the foci of the convection patterns remain nearly steady, the convection flow does intensify with time and the cross-polar-cap potential drop increases. The overall changes as shown in the AMIE convection patterns therefore are fully consistent with the two-stage ionospheric response to the IMF southward turning.