The electrical conductivity of the upper mantle is estimated from low-latitude magnetic field variations (magnetic storms) caused by large fluctuations in the equatorial ring current. The data base is derived from magnetic field magnitude data measured by satellites OGO 2, 4, and 6, which offer better global coverage than land-based observatories. The procedure of analysis consist of 1) separation of the disturbance field into internal and external parts relative to the surface of the earth, 2) estimation of an electromagnetic response function Q(ω) which relates the internally generated magnetic field variations to the external variations due to the ring current, and 3) interpretation of the estimated response function using theoretical response functions for assumed conductivity profiles. Special consideration is given to possible oceanic and ionospheric effects. Best estimates of the geomagnetic response function Q(ω) for 0.2 to 2.0 cpd indicate an upper mantle conductivity of the order of 10-2 S/m. This conductivity is reasonable, especially when laboratory measurements of conductivity versus temperature for olivine are considered.