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Annales Geophysicae An interactive open-access journal of the European Geosciences Union
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Volume 20, issue 2
Ann. Geophys., 20, 139-150, 2002
https://doi.org/10.5194/angeo-20-139-2002
© Author(s) 2002. This work is distributed under
the Creative Commons Attribution 3.0 License.
Ann. Geophys., 20, 139-150, 2002
https://doi.org/10.5194/angeo-20-139-2002
© Author(s) 2002. This work is distributed under
the Creative Commons Attribution 3.0 License.

  28 Feb 2002

28 Feb 2002

Electromagnetic waves and bursty electron acceleration: implications from Freja

L. Andersson4,1, J.-E. Wahlund2, J. Clemmons3, B. Gustavsson1, and L. Eliasson1 L. Andersson et al.
  • 1Swedish Institute of Space Physics, Kiruna, Sweden
  • 2Swedish Institute of Space Physics, Uppsala, Sweden
  • 3NASA Goddard Space Flight Center, Greenbelt, USA
  • 4Now at Laboratory of Atmospheric and Space Physics, University of Colorado, USA
  • Correspondence to: L. Andersson
  • (laila.andersson@lasp.colorado.edu)

Abstract. Dispersive Alfvén wave activity is identified in four dayside auroral oval events measured by the Freja satellite. The events are characterized by ion injection, bursty electron precipitation below about 1 keV, transverse ion heating and broadband extremely low frequency (ELF) emissions below the lower hybrid cutoff frequency (a few kHz). Large-scale density depletions/cavities, as determined by the Langmuir probe measurements, and strong electrostatic emissions are often observed simultaneously. A correlation study has been carried out between the E and B field fluctuations below 64 Hz and 10 Hz, respectively, (the DC instruments upper threshold) and the characteristics of the precipitating electrons. This study revealed that the energisation of electrons is indeed related to the broadband ELF emissions and that the electrostatic component plays a predominant role during very active magnetospheric conditions. Furthermore, the effect of the ELF electromagnetic emissions on the larger scale field-aligned current systems has been investigated, and it is found that such an effect cannot be detected. Instead, the Alfvénic activity creates a local region of field-aligned currents. It is suggested that dispersive Alfvén waves set up these local field-aligned current regions and, in turn, trigger more electrostatic emissions during certain conditions. In these regions, ions are transversely heated, and large-scale density depletions/cavities may be created during especially active periods.

Key words. Ionosphere (particle acceleraton; wave-particle interactions) Magnetospheric physics (auroral phenomena)

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