Magnetizing force, which acts in a magnetic field of steep gradient, was applied to air in a cube heated from above and cooled from below, and with the four vertical walls thermally insulated. A four-poles magnet was installed to apply the cusp-shaped magnetic field to air in the cubic enclosure. A simple model equation was derived for magnetizing force and numerically computed for the system. Without a magnetic field, the conduction was stable, but under the magnetizing force a strong downward flow occurred from the center of the top heated plate and the average Nusselt number attained Nu=1.17 at $Ra=105$ and γ=0.5, which is equivalent to a temperature difference of 4 [°C] between the top and bottom walls under a maximum magnetic induction of 0.9 [T] inside a cube of $0.0643 [m3]$ heated from above. The flow visualization experiment with hot incense smoke proved the downward flow from the top hot plate.

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