| 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 ~85km. 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
homogeneous biteout layer along the Great Circle Path (GCP) corridor, for
transmitter receiver path lengths in the range of 500–6000km.
For profiles during the 10h about midnight and under quiet geomagnetic
conditions, where the electron density at 85km would normally be less than
500el/cc, it was found that received signal perturbations were significant,
of the order of 1–4dB and 5–40° 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
85km is under 500el/cc, under which circumstances PMSE are not directly
detectable by VHF/UHF radars.
Key words. Electromagnetism (wave propagation) – Ionosphere
(polar ionosphere) – Radioscience (ionospheric propagation) |