Study of some unusual VLF emissions

The study of very low frequency (VLF) emissions has been conducted for many years, and has resulted in numerous publications. A few emission types, such as whistlers (electromagnetic signals generated by lightning, and dispersed on their path through the magnetosphere) and chorus (the result of Doppler shifted cyclotron resonance interactions in the magnetosphere), are already well understood. However, many other emissions are not well explained, mainly due to their infrequent occurrence. These emissions typically occur at frequencies below 4 kHz, can be periodic (with periods mostly in the range 2 s to 5 s), and are believed to be generated by some form of instability. These extraordinary emissions, as well as geomagnetic conditions at the time of their occurrence, are discussed.