A gravity field acting on a collisionless, magnetized space plasma causes electrons and ions to drift into opposite directions, in addition to gyrating around the magnetic field lines. This sets up an electric current which flows perpendicular to the gravity and magnetic fields in the eastward direction. Here we present the first observational evidence for such a gravity-driven current system in the Earth's low-latitude ionosphere. Its magnetic field signal, although 10,000 times smaller than the ambient Earth's magnetic field, is clearly visible in CHAMP satellite magnetic measurements. We find a current ribbon of more than 50 kA, about 66¿ wide in latitude, which moves with the sun northward in summer and southward in winter. Correcting magnetic measurements for this current's signature should lead to a better agreement between low-orbiting satellite and ground-based observations.