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Wednesday, July 29, 2009

who stole the sunshine?



Turned cloudy today, rained last night but stayed fine today. Some promising breaks in the cloud hint at sunny weather later. No radar for me today but exciting ground breaking UK experiments are currently underway. The heating facility is being used to probe the lower ionosphere using partial reflection. Magnetospheric radar, mesospheric radar, ionospheric heating - is there anything that field of antennas cannot do?


My radar run yesterday was not so hot. Things were very quiet and all I really got was a nice quiet, solar illuminated ionosphere. Nothing to help me with my work but a couple of cool things showed up in the data anyway. The plot to the right shows the estimated electron density provided by the VHF radar.
You can see how the electron concentration is highest at about 200 km (F-layer) and decreases with descending altitude until it disappears below ~90 km (D-layer); the radar is just not sensitive enough to pick out the much lower electron density below there.

If you look closely from ~12:45 UT you can see an undulation in the data, noticeable in the bit coloured green. This looks like gravity waves to me (not to be confused with gravitational waves), which are generated in the troposphere and are important for transferring energy and momentum to the mesosphere. Caused by airflow over mountains or weather fronts, these disturbances are called gravity waves since gravity acts as the restoring force that causes the oscillation.

Another interesting feature is the appearance of PMSE layers - polar mesosphere summer echoes). The most obvious is around 14:00 UT where there are strong radar echoes. These are not true estimates of the electron density, rather they are caused by coherent scatter where more power is returned to the radar giving the false impression of much higher electron density. The coherent scatter is actually from irregularities with spacing of half the radar wavelength (in this case about 70cm). PMSE are caused by the presence of electrically charged ice particles - the mesopause is very cold so water vapour freezes. These same ice particles cause noctilucent clouds.

So there may be something useful in that data even if not for me.

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