ANGEO Annales Geophysicae ANGEO Ann. Geophys. 1432-0576 Copernicus Publications Göttingen, Germany 10.5194/angeo-23-1807-2005 HF doppler sounder measurements of the ionospheric signatures of small scale ULF waves Baddeley L. J. 1 Yeoman T. K. 1 Wright D. M. 1 Department of Physics and Astronomy, University of Leicester, University Road, Leicester LE1 7RH, UK 28 07 2005 23 5 1807 1820 Copyright: © 2005 L. J. Baddeley et al. 2005 This work is licensed under the Creative Commons Attribution 3.0 Unported License. To view a copy of this licence, visit https://creativecommons.org/licenses/by/3.0/ This article is available from https://angeo.copernicus.org/articles/23/1807/2005/angeo-23-1807-2005.html The full text article is available as a PDF file from https://angeo.copernicus.org/articles/23/1807/2005/angeo-23-1807-2005.pdf

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 <i>m</i> waves where a direct measurement of the azimuthal wavenumber <i>m</i>, 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 <i>m</i> 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 <i>m</i> 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.&nbsp;(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 &gt;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 <i>m</i> range of the statistical study and displays giant pulsation (Pg) characteristics.<br><br> <b>Keywords.</b> Ionosphere (Ionosphere-magnetosphere interactions) – Magnetospheric physics (MHD waves and instabilities) – Space plasma physics (Wave-particle interactions)