Abstract
Abstract. Broadband waves are common on auroral field lines. We use two different methods to study the polarization of the waves at 10 to 180 Hz observed by the Cluster spacecraft at altitudes of about 4 Earth radii in the nightside auroral region. Observations of electric and magnetic wave fields, together with electron and ion data, are used as input to the methods. We find that much of the wave emissions are consistent with linear waves in homogeneous plasma. Observed waves with a large electric field perpendicular to the geomagnetic field are more common (electrostatic ion cyclotron waves), while ion acoustic waves with a large parallel electric field appear in smaller regions without suprathermal (tens of eV) plasma. The regions void of suprathermal plasma are interpreted as parallel potential drops of a few hundred volts.
Highlights
Broadband waves in the magnetosphere are important since they can efficiently redistribute energy between particle populations
Previous studies of broadband emissions by sounding rockets and the Freja satellite at altitudes up to 1700 km suggest that the broadband emissions are sometimes Electrostatic Ion Cyclotron (EIC), Ion Acoustic (IA), slow ion acoustic waves (Bonnell et al, 1996; Kintner et al, 1996; Wahlund et al, 1998) or Doppler-shifted low frequency dispersive Alfven waves (Stasiewicz et al, 2000; Lund et al, 2001; Stasiewicz and Khotyaintsev, 2001)
This study indicates that broadband emissions observed by the four Cluster spacecraft at altitudes of 4–5 Earth radii (RE) are often well described as electrostatic linear waves in a homogeneous plasma, and are a mixture of wave modes labeled with different names
Summary
Broadband waves in the magnetosphere are important since they can efficiently redistribute energy between particle populations. This observation is consistent with textbook plasma physics, stating that even low density cold Maxwellian electron distributions can damp ion acoustic waves Using a realistic plasma model, including several particle distributions, we find that in the cases we have studied, field-aligned electron beams with energies of about hundred eV can generate the observed waves. This is compared to the temporal and spatial growth rates provided by WHAMP. It is interesting to note that the solution seem to reflect the growth profiles even though the reconstruction tries to distribute energy as evenly as possible
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