Artist's impression of the terrestrial plasmasphere (Credit:www.windows.ucar.edu)
Science Nuggets: Magnetospheric Physics
Determining the source of EM waves in the magnetosphere
NTC (Non-thermal continuum radiation) are electromagnetic waves observed in the environment of all magnetised planets and generated in regions of plasma density gradients. Terrestrial NTC is widely believed to be generated at the equatorial plasmapause by conversion of electrostatic to electromagnetic waves. The generation of the electrostatic wave and the conversion from electrostatic to electromagnetic wave happens on very small scale sizes, such that it is impossible to study with directly due to the resolution of the instruments available. However, by remotely studying the NTC, it is possible to access to these processes and help to understand the evolution of the plasmasphere.
NTC sources located at medium magnetic latitude associated with Cluster 3 WHISPER's spectrogram. Image adapted from Grimald et al. (2008)
In a recent paper, Grimald et al. (2008) presented an event with an unusual spectral signature. In the spectrograms, the NTC appears as wide bands instead of the usually observed narrow band and is observed at medium magnetic latitude, close to the plasmapause. A quick survey of the data shows this spectral signature is rarely seen but is not unique. By studying the direction of propagtion of the wave and the maximum of intensity of the band, Grimald et al. demonstrate that, for this event, the sources are located at about -20 degrees magnetic latitude and that the wave is emitted at a different frequency than expected. This result doesn't question the standard interpretation of NTC, but shows that several mechanisms could lead to its generation.
For more details, see:
Grimald, S.; Décréau, P. M. E; Canu, P.; Rochel, A.; Vallières, X.
Medium latitude sources of plasmaspheric nonthermal continuum radiations observed close to harmonics of the electrons cyclotron frequency
J. Geophys. Res., 113, A11216, doi:10.1029/2008JA013290 (2008), [Online]