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Ann. Geophys., 17, 1528-1534, 1999
www.ann-geophys.net/17/1528/1999/
© European Geosciences Union 1999
Waveform and packet structure of lion roars
W. Baumjohann1, R. A. Treumann1, E. Georgescu1,2, G. Haerendel1, K.-H. Fornacon3, and U. Auster3
1Max-Planck-Institut für extraterrestrische Physik, Garching, Germany
2Institut for Space Sciences, Bucharest, Romania
3Institut für Geophysik und Meteorologie, TU Braunschweig, Germany
Abstract. The Equator-S magnetometer is very sensitive
and has a sampling rate of normally 128 Hz. The high sampling rate allows for
the first time fluxgate magnetometer measurements of ELF waves between the ion
cyclotron and the lower hybrid frequencies in the equatorial dayside
magnetosheath. The so-called lion roars, typically seen by the Equator-S
magnetometer at the bottom of the magnetic troughs of magnetosheath mirror
waves, are near-monochromatic packets of electron whistler waves lasting for a
few wave cycles only, typically 0.25 s. They are right-hand circularly polarized
waves with typical amplitudes of 0.5–1 nT at around one tenth of the electron
gyrofrequency. The cone angle between wave vector and ambient field is usually
smaller than 1.5°.
Key words. Interplanetary physics (MHD waves and
turbulence; plasma waves and turbulence)
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