Abstract
Abstract. 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)
Highlights
Polar Mesospheric Summer Echoes (PMSE) have been observed at VHF and UHF frequencies (Cho and Rottger, 1997) as strong radar echoes from near the altitude of the Northern Hemisphere polar summer mesopause, located at ∼85 km altitude
Observations have been mainly confined to the North Polar regions, though PMSE have been reported in Antarctica and at mid-latitudes
Since PMSE and related phenomena are critically dependent on the extremely low temperatures prevailing at the polar summer mesopause, the occurrence of PMSE may be a critical indicator of climate change and global warming (Bremer et al, 2003)
Summary
Polar Mesospheric Summer Echoes (PMSE) have been observed at VHF and UHF frequencies (Cho and Rottger, 1997) as strong radar echoes from near the altitude of the Northern Hemisphere polar summer mesopause, located at ∼85 km altitude. We will show that significant phase and amplitude changes in VLF propagation may be observed when large-scale biteouts are present, and that it should be possible to detect such changes when ambient electron density concentration levels at 85 km are below ∼500 el/cc, under which circumstances PMSE itself is not directly detectable by radar. Typical “night-time” values for β and h at high latitudes during the summer months are likely to be 0.30 km−1 and 76 km, respectively, which give 500 el/cc at 85 km altitude (Thomson, 1993; McRae and Thomson, 2000; Friedrich et al, 2002) This coincides with the minimum 85 km background electron concentration for PMSE VHF/UHF radar observations. As shown below, VLF propagation characteristics should be even more sensitive to “biteouts” during periods of lower electron density background levels, these would be unlikely to occur for much of the time during wellilluminated polar summer conditions
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