... Energetic magnetospheric ions are often unstable to the electromagnetic ion cyclotron (EMIC) instability . ... generation of EMIC waves below the proton gyrofrequency , and . ... Depending on the local proton gyrofrequency, the frequency of the EMIC waves varies: in the magnetosphere we deal with the ULF range (more precisely 0.1-5 Hz), while in the auroral field lines we have ELF waves (tens or hundreds of Hz; see radio frequencies and flickering aurora ). ... Space Physics Text Book of Oulu ...