ULF Doppler oscillations of L = 2.5 flux tubes
Article first published online: 26 JUL 2006
Copyright 2006 by the American Geophysical Union.
Journal of Geophysical Research: Space Physics (1978–2012)
Volume 111, Issue A7, July 2006
How to Cite
2006), ULF Doppler oscillations of L = 2.5 flux tubes, J. Geophys. Res., 111, A07205, doi:10.1029/2005JA011192., , , , , and (
- Issue published online: 26 JUL 2006
- Article first published online: 26 JUL 2006
- Manuscript Accepted: 17 FEB 2006
- Manuscript Revised: 10 FEB 2006
- Manuscript Received: 14 APR 2005
- ULF waves;
- VLF pulsations
 Plasmaspheric ducts may execute Doppler oscillations driven by propagating ultra-low frequency (ULF) waves. We examined about 100 such events recorded over 1 year under magnetically quiet conditions at L = 2.5 using artificially generated whistler-mode VLF signals and ground magnetometers. Joint peaks in the VLF Doppler and magnetometer spectra occurred at the frequency expected for ULF waves generated by the ion-cyclotron instability in the upstream solar wind. The VLF Doppler shifts are most likely due to radial motion of flux tubes of a few kilometers, driven by the east-west electric field of propagating ULF waves. When the frequencies match, the incoming wave energy also couples to standing poloidal and azimuthal field line oscillations, producing field line resonance signatures in both the D and H components on ground-based magnetometers. The phases of the VLF and ULF oscillations are consistent with ionospheric rotation of the downgoing ULF wave field. Since the scale size of VLF flux tubes is significantly smaller than for ULF flux tubes, VLF Doppler observations can provide more precise spatial information on ULF wave fields in the plasmasphere. Furthermore, it should be possible to use ULF oscillations to monitor the formation of quarter wavelength mode standing field line oscillations when the conjugate ionospheres have different conductivities.