VLF Workshop, Sodankylš, Finland, 27th September - 1st October 2004

The Impact of PMSE and NLC Regions on VLF Propagation

Mark Clilverd1, David Nunn2, Craig Rodger3, Neil Thomson3, Th. Ulich4, Jyrki Manninen4, Timo Rantala4

1British Antarctic Survey (NERC), Cambridge, U.K.
2School of Electronics and Computer Science, University of Southampton, Southampton, U.K.
3Dept of Physics, University of Otago, Dunedin, New Zealand
4Sodankylä Geophysical Observatory, Sodankylä, Finland


PMSE or Polar Mesosphere Summer Echoes are a well known phenomenon in the summer northern polar regions, in which anomalous VHF/UHF radar echoes are returned from heights of 85 km. Associated phenomena at these altitudes are noctilucent clouds and electron density biteouts. The latter are electron density depletion layers of up to 90%, which may be several kms thick. Using the NOSC Modefndr code based on Wait's modal theory for subionospheric propagation, we calculate the shifts in received VLF amplitude and phase that occur as a result of electron density biteouts. Where the electron density at 85 km would normally be less than 500 el/cc, it was found that received signal perturbations were significant, of the order of 1-4 dB and 5-40 degrees of phase. Perturbation amplitudes increase roughly as the square root of frequency. Data from an experiment involving multiple paths and multiple frequencies in the north polar region in summer is presented, and interpreted in terms of PMSE effects.