Radar measurements of turbulence, electron densities, and absolute reflectivities during polar mesosphere winter echoes (PMWE)

On January 21, 2005, strong radar echoes known as 'polar mesosphere winter echoes' (PMWE) were observed by the ALWIN VHF radar at 69°N. Peak reflectivites of ∼10−14/m were observed at approximately 13 UT. At the same time the Saura MF radar which is only ∼15 km away from ALWIN measured electron densities (Ne) and turbulence energy dissipation rates (ϵ). This combination is rather seldom because PMWE are rare (occurrence rate is 1-2%) and electron densities are critical: too low values do not give a PMWE, and too large values prohibit to derive Ne due to signal absorption. At PMWE altitudes (∼62 km) typical energy dissipation rates before, during, and after the PMWE are ϵ∼100 mW/kg. The electron density during the PMWE is typically 109/m3 and much smaller prior to PMWE. We have applied a theoretical model based on turbulence theory to derive absolute reflectivities for these ϵ and Ne values and arrive at ∼2 × 10−15/m during PMWE (in nice agreement with measurements) and ∼3 × 10−17/m outside PMWE. The latter value is indeed below the detection limit of the ALWIN radar. The nice quantitative agreement between measured and calculated absolute volume reflectivities confirms earlier conclusions that neutral air turbulence is the main cause for PMWE. Furthermore, we have analyzed the autocorrelation function of the EISCAT 224 MHz PMWE observations from November 10, 2004, and find that the spectral form is different inside the PMWE compared to outside. The shape is Gaussian inside PMWE (compatible with turbulent scatter), whereas it is Lorentzian above the PMWE, indicating non-turbulent, incoherent scatter.