High-latitude HF Doppler observations of ULF waves: 2. Waves with small spatial scale sizes

摘要

The DOPE (Doppler Pulsation Experiment) HFDoppler sounder located near Troms?, Norway (geographic: 69.6°N 19.2°E; L= 6.3) is deployed to observe signatures, in the high-latitude ionosphere, ofmagnetospheric ULF waves. A type of wave has been identified which exhibits nosimultaneous ground magnetic signature. They can be subdivided into two classeswhich occur in the dawn and dusk local time sectors respectively. They generallyhave frequencies greater than the resonance fundamentals of local field lines.It is suggested that these may be the signatures of high-m ULF waveswhere the ground magnetic signature has been strongly attenuated as a result ofthe scale size of the waves. The dawn population demonstrate similarities to atype of magnetospheric wave known as giant (Pg) pulsations which tend to beresonant at higher harmonics on magnetic field lines. In contrast, the wavesoccurring in the dusk sector are believed to be related to the storm-time Pc5spreviously reported in VHF radar data. Dst measurements support theseobservations by indicating that the dawn and dusk classes of waves occurrespectively during geomagnetically quiet and more active intervals.Key words. Ionosphere (auroral ionosphere;ionosphere-magnetosphere interactions) · Magnetospheric physics (MHD waves andinstabilities)