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Annales Geophysicae An interactive open-access journal of the European Geosciences Union
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Volume 28, issue 9 | Copyright
Ann. Geophys., 28, 1723-1735, 2010
https://doi.org/10.5194/angeo-28-1723-2010
© Author(s) 2010. This work is distributed under
the Creative Commons Attribution 3.0 License.

  22 Sep 2010

22 Sep 2010

Observations and modelling of the wave mode evolution of an impulse-driven 3 mHz ULF wave

J. D. Borderick1, T. K. Yeoman1, C. L. Waters2, and D. M. Wright1 J. D. Borderick et al.
  • 1Dept. of Physics and Astronomy, University of Leicester, University Road, Leicester, LE1 7RH, UK
  • 2School of Mathematical and Physical Sciences, The University of Newcastle, Callaghan, 2308, New South Wales, Australia

Abstract. A combination of an HF Doppler sounder, a network of ground magnetometers, upstream solar wind monitors and a numerical model is used to examine the temporal evolution of an Ultra Low Frequency (ULF) wave. The event occurred on 16 April 1998 and followed a solar wind density and pressure increase seen in the upstream ACE spacecraft data. The magnetometer and HF Doppler sounder data show that the event develops into a low-m (−6) field line resonance. HF signals that propagate via the ionosphere exhibit Doppler shifts due to a number of processes that give rise to a time-dependent phase path. The ULF electric and magnetic fields are calculated by a one-dimensional model which calculates the wave propagation from the magnetosphere, through the ionosphere to the ground with an oblique magnetic field. These values are then used to determine a model HF Doppler shift which is subsequently compared to HF Doppler observations. The ULF magnetic field at the ground and Doppler observations are then used to provide model inputs at various points throughout the event. We find evidence that the wave mode evolved from a mixture of fast and Alfvén modes at the beginning of the event to an almost purely shear Alfvénic mode after 6 wavecycles (33 min).

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