Sporadic E Visualised (2016 Updated Edition)

The 6m sporadic E season is firmly upon us, with good hints of short-lived Es clouds over the past few days.

Whilst I watch the WSPR screen carefully, it's often seen that signals sometimes follow strange deviations by Doppler shift.  Most times, this is down to reflections from passing aircraft.  But not always.

If you've ever made an SSB QSO on 6m, or followed WSPR traces during the peak of the season, you will know that Es propagation can come and go in seconds, but also last for a few hours.  Oftentimes, the signal comes and goes in gentle wave-like patterns.  Overall, the Es paths are quite complex, even if the underlying mechanism is relatively simple.

Polar mesospheric summer echoes at 46.5MHz - loose links with NLC and Es.

Now, the ionisation that leads to Es is at a height of 90-120km.  This isn't visible, other than by radio reflections.  Noctilucent clouds, on the other hand - which also occur in the summer months (but do not appear at all in winter, unlike Es) - are readily visible from mid-high latitudes (~40-65 N and S.)  Like Es, NLC also drift east to west, or NE to SW in the summer.  NLC occur somewhat lower in the atmosphere, at about 80-90km, but are subject to pretty much the same dynamics - mostly driven by gravity waves (not gravitational waves!)

We also see reflections at 46.5MHz from polar mesospheric summer echoes - PMSE - at the same height as NLC.  PMSE is caused by ionisaton, and NLC form around charged, metallic meteor debris.  Clearly, all these phenomena are linked to a greater or lesser degree.

Noctilucent cloud over the Irish Sea, 2009.

Whilst NLC are not always linked to Es occurrence, there is a loose correspondence.  The point of this post is to highlight the likely similarity in the form of Es clouds with the visible NLC.  From there, we can start to appreciate how these strange changes in propagation conditions occur, and what kinds of structures cause them.  No doubt Es clouds are modulated pretty much in the same way as NLC - dominated by the breaking of gravity waves propagating up from the lower atmosphere.

The atmospheric radar at Aberystwyth, Wales, captures significant ionisation in the mesosphere in June 2016.

It's interesting to think about how the fading heard on 6m, for example, might be linked to the passing of structures along the path between sender and receiver.  Here's a screen grab of how a JT65A signal changed over the course of just 48 seconds.  Remember that the wavy output is a result of signal strength, not modulation.

The variation in QSB periodicity can be explained by the essentially infinite range of gravity wave wavelengths, and also their velocity, which ranges typically from 40-80m/s.

Gravity wave-induced structure in an extensive NLC display from 2015.  These move across the field of view with considerable speed, partly explaining QSB periodicity.