By making use of multiple ground‐based and spaceborne instruments, we study ionospheric and thermospheric behavior during the moderately intense geomagnetic storm of 29–31 August 2004 (minimum Dst excursion of −128 nT). Although this storm was far from the strongest in solar cycle 23, it provoked quite interesting effects in the ionosphere, such as opposite hemispheric asymmetries in the ionospheric F layer and in the topside ionosphere and a development of the ionospheric superfountain effect in the postsunset sector. Data from ground‐based GPS receivers and ionosondes revealed large increase in total electron content (TEC) and in NmF2 in the southern hemisphere, whereas in the northern hemisphere, very weak or no effect was observed. On the contrary, the topside measurements indicated the occurrence of a positive storm in the northern hemisphere. Overall, the strongest storm ...