EMIC Waves Observed During the July 2009 Geomagnetic Storm

The most intense geomagnetic storm of the first two thirds of 2009, with minimum Dst = -80 nT, occurred on July 22, 2009, and was characterized by unusually strong and widespread wave activity in the Pc 1-2 frequency band. These waves, understood to be electromagnetic ion cyclotron (EMIC) waves generated near the equatorial magnetosphere, are excited by anisotropies in ring current ion distributions, and, via parasitic interactions, can cause depletions of relativistic electrons in Earth’s radiation belts. We have used data from a global array of search coil magnetometers and highly sensitive, rapidly sampled fluxgate magnetometers from L ~ 3 to L ~ 15 to characterize the latitudinal and longitudinal extent of Pc 1-2 waves observed before, during, and after this storm interval. Simultaneous data from the low altitude satellite CHAMP, orbiting in the noon/midnight plane, provide direct information on the latitudinal distribution of these waves in the upper ionosphere. Although as expected the most common and most intense wave activity occurred near local noon, intense Pc 1 waves were observed immediately before and during the beginning of the short main phase of this storm in the American sector (during nighttime hours), and wave occurrence spanned a broad region of local times during the recovery phase.