Gravity Waves and Es

My blogging colleague John, EI7GL, yesterday enquired about the modulation of E-layer ionisation, with a focus on internal atmospheric gravity waves.

Gravity waves (GW), first of all, should not be confused with gravitational waves.  The latter are a distortion of spacetime, whereas atmospheric GWs are simply waves whose restoring force is gravity that occur throughout the atmosphere.

A GW starts its life as a fairly small amplitude disturbance near or at ground level.  Mountains are a particular cause of GWs, notably when strong winds blow over them, launching waves into the atmosphere.

Once caused, GWs grow in amplitude as they propagate upwards through the atmosphere.  They do so quickly, also extending out to very large distances, well away from their source.  Wind velocity at ~90km is typically 100-400m/s.  Watch the video of GW modelling below, noting particularly the scales involved for the 'upper atmosphere' bit (which is actually more properly called the middle atmosphere), and the little time indicator - see how quickly the waves spread:


Here's a real example of gravity waves off Western Australia modulating cloud condensation, though this is obviously only at a few km up, in the troposphere:


Once GWs reach the top of the mesosphere - the kind of height used in the second part of the video, they start to break.  That is, just like water waves, the stuff coming up from behind overruns the stuff in front, becoming unstable and breaking, where they dump their energy.

Upwelling due to GWs is the mechanism by which the mesosphere becomes the coldest part of the atmosphere - roughly minus 135 degrees Celsius.  It does this in midsummer, which seems a bit counterintuitive, until you remember that a rising parcel of air, all other things being equal, will expand and cool.  The expansion is large, and so therefore is the cooling.


What does all this have to do with Es and radio?  Well, the GWs are propagating in an area where metallic meteoric debris is deposited and floating around.  By 'debris', I'm talking very small - nanometres only.  In summer, this accumulates a coating of ice, which doesn't make them very much bigger; they may get to a couple of micrometres, still only the size of a typical bacterium.

The exact height of the E and D layer varies, but Es happens round about the upper D and lower E regions.  It is usually misleading to think of fixed boundaries of the kind that atmospheric layer diagrams present.

All this metal and ice is highly charged, and certainly strongly reflects frequencies around 50MHz, which is where middle atmosphere radars operate.  Indeed, there are accounts from WW2, when the Chain Home radar system around the UK was operating (at around 30MHz), of enormous squadrons of German aircraft being detected, but never appearing over Britain.  The cause was almost certainly strong Es, though it would not have been understood as such then.

So, gravity waves and the charged particles they 'push around' present a rapidly and ever-changing landscape of modulated charged particles to radio waves, which can and do reflect off these wave fronts and their associated, complex structures arising from other effects such as windshear.  We can often hear these effects in sudden periods of very strong signals (much longer than meteor scatter, for example), and then sudden drops to nothing.

Thunderstorms produce GWs through their very strong convective nature.  So much air is going upwards, and so quickly, that they produce GWs in the process.  These again travel into the E region, where they modulate the charged particles.

Summer echoes from the mesosphere.

The phenomena of PMSE (Polar Mesospheric Summer Echoes), Es and NLC (noctilucent clouds) all show much the same seasonal variations, because they are all, in essence, caused by the same processes.  Of course, PMSE can also occur in winter, possibly due to infrasound from rough seas, as can Es.  NLC only occur in summer, because that is the only time when water and temperature levels are correct.  A link between the occurrence of NLC and 50MHz polar propagation from Europe to Japan has often been offered up for some time, and there is certainly cause to consider this possible. 

Well, John, I hope that gives a quick flavour of gravity waves and the things they can cause.  Once you start studying these things, a bit like radio, you only ever have more and more things to find out about - and questions to ask! 

And just a reminder that, from around May 20th until early August, noctilcuent cloud season will be underway.  It's usually the first week in June before they become very active and more easily spotted.

Gravity waves modulating a noctilucent cloud layer.  Wavelength is of the order of tens of kilometres.

Complexity of NLC.