Cluster Observations of Non Time Continuous Magnetosonic Waves

Observations of magnetosonic waves in the inner magnetosphere have always shown that the waves are time continuous when viewed in a time-frequency spectrogram. Recently, examples of magnetosonic waves with modulated, rising tone elements have been reported. New observations from Cluster also show the existence of other, intermittent occurrences of magnetosonic waves. In this paper we investigate the properties of these wave emissions to determine the causes of these non-time continuous emissions. We report on a statistical study of the rate of frequency change for the rising tone emission features and show that they are modulated by fluctuations in the magnetic field. The more intermittent emissions appear to be controlled by the plasma density.