The impact of PMSE and NLC particles on VLF propagation
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 similar to85 km. Noctilucent clouds and electron density biteouts are two phenomena that sometimes occur together with PMSE. Electron density biteouts 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. The code assumes a homogeneous background ionosphere and a homo-geneous biteout layer along the Great Circle Path (GCP) corridor, for transmitter receiver path lengths in the range of 500-6000 km.For profiles during the 10 h about midnight and under quiet 0 Geornagnetic conditions, where the electron density at 85 km would normally be less than 500el/cc, it was found that received signal perturbations were significant, of the order of 1-4 dB and 5-40degrees of phase. Perturbation amplitudes increase roughly as the square root of frequency. At short range perturbations are rather erratic, but more consistent at large ranges, readily interpretable in terms of the shifts in excitation factor, attenuation factor and v/c ratios for Wait's modes. Under these conditions such shifts should be detectable by a well constituted experiment involving multiple paths and multiple frequencies in the north polar region in summer. It is anticipated that VLF propagation could be a valuable diagnostic for biteout/PMSE when electron density at 85 km is under 500 el/cc, under which circumstances PMSE are not directly detectable by VHF/UHF radars.
Authors: Nunn, D., Clilverd, M.A., Rodger, C.J., Thomson, N.R.