Satgen 633  Summer Sporadic E  by GM4IHJ  (BID SGEN633)  2001-05-12

It is that time of year again, when daylight hours at stations in
northern temperate latitudes experience the joy or pain of Sporadic
Ionisation of the E layer of the lower ionosphere (SpE). Located at
between 80 and 100 kms altitude , discrete patches of the E layer
become ionised strongly enough to provide propagation of radio waves
up to 200 MHz in frequency on just one or two days, with much more
frequent weaker events providing excellent propagation between 50MHz
terrestrial stations during the daylight and early evening hours
roughly one day in every three.

Unfortunately there is a down side to this for satellite users. Peak
levels of ionisation in the E layer , can seriously attenuate and
sometimes completely block uplink and/or downlink signals on modes K,
A, B, and J. With the hardest hit usually being mode J 145 MHz up 435
MHz down, where the 145 MHz up, simply does not get up through the
ionised E layer to reach the much higher satellite. Fortunately the
interuption rarely affects the whole of the satellites in range path.
Disturbance is usually present when the satellite is on southerly
bearings , then as is travels northwards its path to the satellite
suddenly moves clear of the SpE, and normal communication becomes
possible.

Just what causes SpE is still hotly debated. Weather forecasters tend
to suggest it is connected with thunderstorms. But while this view
seemed to gain some support when it recently became clear that these
storms do affect the atmosphere/ionosphere at altitudes far above the
visible storms themselves. There are certain features of SpE that do
not fit this scenario :-

a. We get a weak SpE season in December/January every year, and a
strong SpE season May to August . But thunderstorms do not fit this
pattern.

b. Long term studies of SpE suggest that the Spe cloudlets tend to
locate over the same places day after day . season after season. A
pattern which closely fits the position of eddies in the high
altitude jet stream winds.

c. SpE is often preceded or followed by a brief episode of Field
aligned irregularity propagation, in locations where the SpE is just
beginning to form , or , more often in places where SpE has recently
been strongly present.

d. Several studies over the past 30 years have confirmed the presence
in SpE clouds of dense patches of meteoric/cometary dust. It is
suggested that this dust is collected in jet stream eddies. A
suggestion which is further supported when it is noted that the
seasonal nature of SpE coincides directly with those times of year
when the Earths inclined orbit around the Sun takes it up or 6 months
later down, through the dense tracks of cometary dust particles
trapped in orbits where they and their parent comets have orbital
resonance with either Jupiter or Saturn.