| Electromagnetic ion cyclotron (EMIC) triggered emissions have been recently observed onboard the Cluster spacecraft close to the plasmapause in the equatorial region of the magnetosphere (Pickett et al., 2010).
The non linear mechanism of the wave amplification is the same as for the wellknown whistler-mode chorus emissions (Omura et al., 2010). The EMIC triggered emissions appear as risers: electromagnetic structures that have a positive frequency drift with time. They can thus be considered as the EMIC analogue of rising frequency whistler-mode chorus emissions. In addition, they propagate away from the magnetic equator.
Although these EMIC risers are not common in Cluster data, the observed properties vary from one case to the other: left or right hand polarization, trigger frequency close to one half of the H+ or He+ local gyrofrequency. In addition to the time frequency dispersion, the best observational parameter to identify such triggered emission is the high level of coherence of the structure, specially compared to the one of the surrounding Pc1 waves.
The observations in Cluster are made around 22 MLT at the edge of the plasmapause. This dependence is interpreted in terms of an orbit effect for one part and a physical cause for the other. A comparison between Cluster results and observations performed by other spacecraft will also be presented. As the EMIC triggered emissions observed in situ have the same aspect as the Hydromagnetic Chorus emissions observed from the ground, the possibility that both terms name the same emissions will be discussed. |