Papers by Judy Stephenson
Annales Geophysicae, 2014
A case study of a magnetohydrodynamic (MHD) wave in the solar wind that is strongly correlated wi... more A case study of a magnetohydrodynamic (MHD) wave in the solar wind that is strongly correlated with a magnetospheric field line resonance observed by the SuperDARN (Super Dual Auroral Radar Network) radar at Sanae, Antarctica is presented. The data from the ACE (Advanced Composition Explorer) satellite at the solar libration point are analysed. The data time series are bandpass filtered at the pulsation frequency and the analytic signal deduced. From these data the partition of energy between the field components is computed. It is shown that energy is equally partitioned between the kinetic energy and transverse magnetic potential energy densities. The energy flux vector is closely aligned with the background magnetic field. The transverse magnetic and velocity components are in antiphase. This is the first identification of the triggering wave as a transverse Alfvén wave which originates upstream from the space craft and is propagated to the magnetosphere to trigger the pulsation.
Annales Geophysicae, 2010
Field line resonances have been observed for decades by ground-based and in situ instruments. The... more Field line resonances have been observed for decades by ground-based and in situ instruments. The driving mechanism(s) are still unclear, although previous work has provided strong grounds that coherent waves in the solar wind may be a source. Here we present further evidence, with the use of multitaper analysis, a sophisticated spectrum estimation technique. A set of windows (dpss tapers) is chosen with characteristics that best suit the width of the narrowband peaks to be identified. The orthogonality of the windows allows for a confidence level (of say 95%) against a null hypothesis of a noisy spectrum, so that significant peaks can be identified. Employing multitaper analysis we can determine the phase and amplitude coherence at the sampling rate of the data sets and, over their entire duration. These characteristics make this technique superior to single windowing or wavelet analysis. A high degree of phase and amplitude (greater then 95%) coherence is demonstrated between a 2.1 mHz field line resonance observed by the SHARE radar at Sanae, Antarctica and the solar wind oscillation detected by WIND and ACE satellites.
Advances in Space Research, 2006
The SHARE high frequency (HF) radar in Antarctica is used to compare ionospheric plasma flow osci... more The SHARE high frequency (HF) radar in Antarctica is used to compare ionospheric plasma flow oscillations in the Pc5 frequency range with low-frequency oscillations in the solar wind pressure measured by the ACE spacecraft. Ten different days in 2000 and 2001 are analysed with respect to different frequencies and geomagnetic latitudes. Both data sets are bandpass filtered and a complex demodulation technique is used to calculate the correlation in each band. On a number of occasions the wave packet structure of the Pc5 pulsations is in good or excellent agreement with the wave packet structure of the solar wind pressure oscillations. This strongly suggests that the oscillations were directly driven by the solar wind. Particularly good correlation is found in the frequency band 0.8-1.2 mHz. Pulsations in this frequency range are hard to reconcile with the magnetospheric cavity mode model. We conclude that, at least on some occasions, Pc5 pulsations may be directly driven and the magnetosphere cavity/waveguide then assumes a more passive role.
Geophysical Research Letters, 2002
We present HF radar observations of Pc5 field line resonances in the post-midnight Antarctic iono... more We present HF radar observations of Pc5 field line resonances in the post-midnight Antarctic ionosphere on April 28/29 1997. Simultaneous oscillations were seen in the solar wind parameters observed by WIND. The spectra of both sets of data show peaks near 1.3 mHz, 1.9 mHz, 2.7 mHz and 3.3 mHz. The data were bandpass filtered and a complex demodulation technique applied to calculate the instantaneous energy flux in the solar wind in each band and hence the power incident on the front of the magnetosphere 70 minutes later. The magnitude of the instantaneous power deposited in the ionosphere at each frequency through Joule heating followed this closely and was an order of magnitude smaller. We conclude that, at least on this occasion, the field line resonances could have been directly driven by the solar wind oscillations.
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Papers by Judy Stephenson