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Ann. Geophys., 23, 1807-1820, 2005
www.ann-geophys.net/23/1807/2005/
© European Geosciences Union 2005
HF doppler sounder measurements of the ionospheric signatures of small scale ULF waves
L. J. Baddeley, T. K. Yeoman, and D. M. Wright
Department of Physics and Astronomy, University of Leicester, University Road, Leicester LE1 7RH, UK
Abstract. An HF Doppler sounder, DOPE (DOppler Pulsation Experiment) with three
azimuthally-separated propagation paths is used to provide the first
statistical examination of small scale-sized, high m waves where a direct
measurement of the azimuthal wavenumber m, is made in the ionosphere. The
study presents 27 events, predominantly in the post-noon sector. The
majority of events are Pc4 waves with azimuthal m numbers ranging from –100
to –200, representing some of the smallest scale waves ever observed in the
ionosphere. 4 Pc5 waves are observed in the post-noon sector. The fact that
measurements for the wave azimuthal m number and the wave angular frequency
are available allows the drift-bounce resonance condition to be used to
hypothesise potential particle populations which could drive the waves
through either a drift or drift-bounce resonance interaction mechanism.
These results are compared with the statistical study presented by Baddeley et al. (2004)
which investigated the statistical likelihood of such driving particle
populations occurring in the magnetospheric ring current. The combination of
these two studies indicates that any wave which requires a possible drift
resonance interaction with particles of energies >60 keV, is
statistically unlikely to be generated by such a mechanism. The evidence
presented in this paper therefore suggests that in the pre-noon sector
the drift-bounce resonance mechanism is statistically more likely
implying an anti-symmetric standing wave structure while in the post-noon sector both a drift or drift-bounce resonance interaction is
statistically possible, indicating both symmetric and anti-symmetric
standing mode structures. A case study is also presented investigating
simultaneous observations of a ULF wave in ground magnetometer and DOPE
data. The event is in the lower m range of the statistical study and displays
giant pulsation (Pg) characteristics.
Keywords. Ionosphere (Ionosphere-magnetosphere interactions)
– Magnetospheric physics (MHD waves and instabilities)
– Space plasma physics (Wave-particle interactions)
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