The dynamical background of polar mesosphere winter echoes from simultaneous EISCAT and ESRAD observations

Abstract

Abstract.On 30 October 2004 during a strong solar proton event, layers of enhanced backscatter from altitudes between 55 and 75 km have been observed by both ESRAD (52 MHz) and the EISCAT VHF (224 MHz) 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 220 km 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.

Isolated lower mesospheric echoes seen by medium frequency radar at 70° N, 19° E

“…Comparing with proton flux data from GOES spacecraft, we ascertain that ILMEs of at least 1-2 h day −1 duration are almost invariably associated with winter enhanced proton precipitation. These conditions are consistent with the background ionisation conditions for PMWE, deduced by Belova et al (2005) and Lübken et al (2006), although any relation to background turbulence (Lübken, 1996) has yet to be investigated. The review by Zeller et al (2006) is arguably the best illustration of the degree of agreement between PMWE and ILME occurrence.…”

“…It should come as no surprise therefore that phenomena described as Polar Mesospheric Winter Echoes (PMWE) (e.g. Kirkwood et al, 2002;Belova et al, 2005;Lübken et al, 2006;Zeller et al, 2006) and perennial equatorial mesospheric echoes (e.g. R. Woodman and J. Chau, private communication) should exist.…”

Polar mesosphere winter echoes during MaCWAVE

“…The relationship between PMWE and PMSE is illustrated by Fig. 1, which shows typical strengths of summertime PMSE (uppermost panel) and compares with typical PMWE as previously reported in association with solar proton events (middle panel, see also Belova et al, 2005). The lowermost panel shows a comparison with PMWE observed without a concurrent solar proton event, which are the subject of this paper.…”

“…Correspondence to: S. Kirkwood (sheila.kirkwood@irf.se) (Kirkwood et al, 2002a(Kirkwood et al, , 2002bStebel et al, 2004;Belova et al, 2005). It has long been known that layers of relatively weakly enhanced radar echo can be seen in the mesosphere at many latitudes, and strongly enhanced echoes are also known to occur at high-latitudes in summer.…”

“…Top panel shows the results for summer, heights 80-90 km, for the whole of July 2003. Middle panel shows the results for winter, heights 50-75 km, during a major solar proton event in October 2003 (Belova et al, 2005). Lowest panel shows the results for winter, heights 50-75 km, during the MacWAVE campaign in January 2003.…”