Abstract
Abstract. On 30 October 2004 during a strong solar proton event, layers of enhanced backscatter from altitudes between 55 and 75km have been observed by both ESRAD (52MHz) and the EISCAT VHF (224MHz) radars. These echoes have earlier been termed Polar Mesosphere Winter Echoes, PMWE. After considering the morphology of the layers and their relation to observed atmospheric waves, we conclude that the radars have likely seen the same phenomenon even though the radars' scattering volumes are located about 220km apart and that the most long-lasting layer is likely associated with wind-shear in an inertio-gravity wave. An ion-chemistry model is used to determine parameters necessary to relate wind-shear induced turbulent energy dissipation rates to radar backscatter. The model is verified by comparison with electron density profiles measured by the EISCAT VHF radar. Observed radar signal strengths are found to be 2-3 orders of magnitude stronger than the maximum which can be expected from neutral turbulence alone, assuming that previously published results relating radar signal scatter to turbulence parameters, and turbulence parameters to wind shear, are correct. The possibility remains that some additional or alternative mechanism may be involved in producing PMWE, such as layers of charged dust/smoke particles or large cluster ions.
Highlights
Polar Mesosphere Summer Echoes (PMSE), which are very strong radar returns from 80–90 km altitude, seen in the summertime at polar latitudes, have been studied intensively since their discovery more than 20 years ago
After considering the morphology of the layers and their relation to observed atmospheric waves, we conclude that the radars have likely seen the same phenomenon even though the radars’ scattering volumes are located about 220 km apart and that the most long-lasting layer is likely associated with wind-shear in an inertio-gravity wave
Observed radar signal strengths are found to be 2–3 orders of magnitude stronger than the maximum which can be expected from neutral turbulence alone, assuming that previously published results relating radar signal scatter to turbulence parameters, and turbulence parameters to wind shear, are correct
Summary
Polar Mesosphere Summer Echoes (PMSE), which are very strong radar returns from 80–90 km altitude, seen in the summertime at polar latitudes, have been studied intensively since their discovery more than 20 years ago No matter what underlying structure may be present in the neutral atmosphere at mesospheric heights, it will not have caused measureable scatter of the VHF radio waves used by the radars unless it is coupled to structures in electron density This means that the majority of PMWE observations reported from the relatively low-sensitivity ESRAD radar have been made during solar proton events, which are the occasions when the electron density at the relevant heights is most enhanced. It should be noted that the ESRAD radar has operated during a major solar proton event during the summer of July, without any detectable scattering layers other than those normally seen throughout the summer months between 75 and 95 km altitudes (Barabash et al, 2004) These considerations lead us to believe that the primary cause of PMWE lies in the background neutral atmosphere, and not in the nature of the agent causing the enhanced electron density. Assuming that the wind is primarily horizontal, the beam-aligned component corresponds to half of the meridional wind speed (the beam is directed at an elevation of 60◦ above horizontal)
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Free) 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
