Mesospheric radar echoes in polar and middle latitudes
Climatology of polar mesosphere winter echoes (PMWE)
During the months from September until April of the years 2001 until 2005 mesospheric radar echoes have been observed with the ALWIN VHF radar, which are essentially smaller than the polar mesosphere summer echoes (PMSE) and occur at heights between 55 km and 85 km. The height of the maximum occurrence rate is with 71.5 km markedly below the PMSE maximum (85-86 km). As to be seen in the first figure PMWE are preferentially observed during day-time at solar zenith angles χ<98°. During night-time PMWE occur normally only during disturbed geomagnetic conditions. The mean PMWE occurrence rate is 2.9%, during day-time with 6.1% markedly stronger than during night-time with 0.8%.
The influence of strong geomagnetic disturbances upon the PMWE occurrence is demonstrated in the second figure. Basing on a superposed epoch analysis for the 5 strongest geomagnetic storms during the years 2001 until 2005 the variations are shown of the mean PMWE occurrence rate and the geomagnetic activity (left part) as well as of solar proton fluxes and the absorption of cosmic radio noise (CNA: cosmic noise absorption). The CNA values are a good indicator of the ionisation of the lower ionosphere. Thereafter the PMWE occurrence is markedly enhanced during the precipitation of high energetic particles (30% instead of the mean 2.9%) and demonstrates the importance of an enhanced ionisation in the mesosphere for the observation of PMWE.
Atmospheric turbulence is important for the creation of PMWE. An indication of this statement can be found in the third figure, where in the lower part the seasonal variation of the PMWE occurrence rate is presented after elimination of those parts caused by precipitation of high energetic particles. In the upper part the seasonal variation of the energy dissipation rate is presented for an altitude of 70 km due to observations with the Saura MF Radar (9/2004-4/2005). The similar seasonal variations are an indication that turbulence is important for the creation of PMWE. Further details concerning the climatology of PMWE can be found in Zeller et al. (2006).
Results of complex experiments for the investigations of PMWE with simultaneous rocket and radar observations support the statement that PMWE can be explained by atmospheric turbulence in connection with enhanced electron densities. Details can be found in Lübken et al. (2006), Brattli et al. (2006), and Lübken et al. (2007).