Observations of Discrete Harmonics Emerging from Equatorial Noise



A number of modes of oscillations of particles and fields can exist in space plasmas. Since the early 1970s, space missions have observed noise-like plasma waves near the geomagnetic equator known as 'equatorial noise'. Several theories were suggested, but clear observational evidence supported by realistic modelling has not been provided. Here we report on observations by the Cluster mission that clearly show the highly structured and periodic pattern of these waves. Very narrow-banded emissions at frequencies corresponding to exact multiples of the proton gyrofrequency (frequency of gyration around the field line) from the 17th up to the 30th harmonic are observed, indicating that these waves are generated by the proton distributions. Simultaneously with these coherent periodic structures in waves, the Cluster spacecraft observes 'ring' distributions of protons in velocity space that provide the free energy for the waves. Calculated wave growth based on ion distributions shows a very similar pattern to the observations.


Includes supplement


Plasma waves, Magnetosphere, Protons, Space plasmas, Harmonics (Electric waves)


The Presidential Early Career Award for Scientists and Engineers through NASA grant NNX10AK99G; NSF GEM AGS-1203747; UC Lab Fee grant 12-LR- 235337. Praemium Academiae grants LH12231, and LH14010; EPSRC grant EP/H00453X/1; NSF grant AGS-1405041.


CC BY 4.0 (Attribution), ©2015 Macmillan Publishers Ltd.