On September 12, 1986, when the largest magnetic storm (Dst=-157 nT) during Viking operations took place, the satellite observed unusual energy modulation of sub-keV ring current ions. The modulation is seen in both the pitch angle domain (conic-like distribution) and the space-time domain (wavy variation of ion flux with a period of a few minutes). The wavy variation is synchronized with a clear and distinct magnetic field-line oscillation which is probably a standing Alfv¿n wave. This is observed simultaneously on the ground as a Pc5 pulsation over a wider range of latitude than the satellite observation. ¿ American Geophysical Union 1996