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Re: (meteorobs) sporadic E
At 12:05 AM 1999-06-21 +0100, you wrote:
>I often see references to 'sporadic E' in the meteor literature. What is
>the cause of this phenomenon and why is it of concern to radio observers? I
>can appreciate that it's an upper atmospheric effect and is undesirable to
>such observers.
Sporadic E is an atmospheric phenomenon which takes place between about 85
and 115 km above the earth's surface, in what's called the E layer of the
atmosphere. The ions in the E layer can, at times, become
strongly-concentrated and aligned such that they cause very strong
reflection of radio waves, particularly noticeable from about 20 MHz on up
to at least 250 MHz. The more strongly the E-layer reflects, the higher the
frequency that is reflected. The reflecting layer is not contiguous across
the entire earth, however: most often, a small area of the E layer,
commonly called an E-cloud, forms and causes these reflections. The clouds
are not visible to the eye, being ionic in nature. Radio wave research
indicates a sporadic E cloud may be only a few hundred feet to a few
hundred kilometers in size. E clouds appear to occur most often where there
is severe weather, particularly severe thunderstorms with very high
cumulonimbus clouds ("anvil" tops) stretching up to 50,000 feet or more,
although sporadic E propagation often appears where there are no such
thunderstorms, too. Sporadic E is most common in the late Spring through
early summer, and also throughout the month of December, in either the
northern or southern hemispheres. Sporadic E propagation can occur at any
time of the day or night, but seems to favor daylight hours in particular,
and especially early morning and late day hours. Often, you can tell when
sporadic E propagation is occurring within range of your location by
watching your television set or listening to your FM broadcast radio for
stations located hundreds or even a thousand miles away.
So sporadic-E propagation takes place due to reflection areas in the
atmosphere which happen to be at the same altitude range where meteors burn
and reflect radio waves. It is usually easy to tell whether a radio signal
is being reflected by sporadic E propagation or by meteors: meteor
reflections are very short-lived, beginning very suddenly, building to a
strong signal very rapidly, then deteriorating rapidly within a matter of a
few seconds or much less (although the lower the frequency, the longer a
meteor trail may reflect radio waves). Sporadic E propagation usually
occurs for just a few minutes but can last for hours at times. When it does
occur, it masks meteor reflections, often severely contaminating radio
meteor observations.
Clear skies,
SteveH
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