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International smoothed sunspot numbers

Sunspot number

PREDICTED

SILSO graphics - Royal Observatory of Belgium April 2022
Figure 28-3
Figure 28-3 - History of Sunspot Cycles
SPORADIC-E

Sporadic-E layers are only a few kilometres thick and can have peak electron
density comparable to the F regions' maximum density. Sporadic-E layers, like the
higher F-layer, are very efficient refractors of HF signals. Sporadic-E is formed
mainly by the concentration of winds of ions and electrons into a thin layer. At mid-
latitudes, sporadic-E is most prevalent during summer and at midday. In the
equatorial zone, it is nearly always present at midday.

Communication distances of 800-2200 km can occur using a single sporadic-E
region. This variability in distance depends on several factors, including ion cloud
height and density. The maximum usable frequency (MUF) also varies widely, but
most commonly falls in the 25-150MHz range, which includes the FM broadcast
band (87.5-108MHz), band 1 VHF television (American channels 2-6, Russian
channels 1-3, and European channels 2-4, the latter no longer used in Western
Europe), CB radio (27MHz) and the amateur radio 2-meter, 4-meter, 6-meter, and
10-meter bands. Strong events have allowed propagation at frequencies as high as
250MHz.

As its name suggests, sporadic-E is an abnormal event, but can happen at almost
any time; it does, however, display seasonal patterns. Sporadic-E activity peaks
predictably in the summer in both hemispheres. In North America, the peak is most
noticeable in mid-to-late June, trailing off through July and into August. A much
smaller peak is seen around the winter solstice. Activity usually begins in mid-
December in the southern hemisphere, with the days immediately after Christmas
being the most active period.
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