Multi-frequency Radar Studies of the High Latitude Mesosphere
The radar signature of Polar Mesosphere Summer Echoes (PMSE), which are associated with Noctilucent Clouds (NLC) (the highest clouds over the Earth), has been studied using Medium-Frequency (MF), High-Frequency (HF), and Very-High-Frequency (VHF) radars deployed over the central Alaskan region. The echo morphology at the different frequencies is described in case studies wherein PMSE events were observed concurrently using at least two radar systems. The identity of MF and HF radar echoes as PMSE is resolved for the first time by means of simultaneous measurements made with VHF radars, the reference sensors employed traditionally for PMSE studies. Radar reflectivity estimates, derived from in-situ rocket measurements, suggest that HF radars are optimal for the observation of PMSE edge-dominated type of scatter. MF radars, on the other hand, show comparable reflectivity values for edge and turbulent scattering components, as may be expected for wider antenna beam systems that are exposed to other echo sources. The VHF scattering calculations validate previous research on PMSE, suggesting an increase of the Schmidt number to maintain irregularities of scale sizes in the order of a few meters or less. A large Schmidt number is not needed at MF/HF frequencies since the wavelengths are larger than the Kolmogorov micro-scale and mesospheric layers can be sustained at low charging levels. Rocket measurements of mesospheric dust content and simultaneous analyzed MF radar backscattered power profiles show a similar type of structure. Dust particles are produced most likely by meteor trails reaching to the upper mesosphere region and may be related to some non-summer Mesospheric-radar Echoes (ME). On the basis of echo duration and signal strength, we suggest that HF radars are most favorable for PMSE monitoring. MF radars show highly organized PMSE layers quite often but are more susceptible to ionospheric absorption and higher altitude returns associated with geomagnetic activity. However, since a number of MF stations are located at polar or near polar latitudes, including Antarctica, it may be possible to use the PMSE signature studied here to investigate its long-term variability as well as its low latitude boundary. The latter could be an indicator of global change.
Radiophysics; Atmospheric Science; Remote Sensing
dissertation or thesis