Electron velocity distribution and lion roars in the magnetosheath
Whistler waves which are termed "lion roars" in the magnetosheath are studied using data obtained by the Spectrum Analyser (SA) of the Spatio-Temporal Analysis of Field Fluctuations (STAFF) experiment aboard Cluster. Kinetic theory is then employed to obtain the theoretical expression for the whistler wave with electron temperature anisotropy which is believed to trigger lion roars in the magnetosheath. This allows us to compare theory and data. This paper for the first time studies the details of the electron velocity distribution function as measured by the Plasma Electron And Current Experiment (PEACE) in order to investigate the underlying causes for the different types of lion roars found in the data. Our results show that while some instances of lion roars could be locally generated, the source of others must be more remote regions of the magnetosheath.