Sky & Telescope - April 2024 - 63

Altitude (km)
ILLUSTRATION: BEATRIZ INGLESSIS / S&T; GRAPH: DIETER
BILITZA ET AL / JOURNAL OF GEODESY (2011)
dial, when the announcer identifies the station as WBBM
in Chicago. This might seem odd if you are listening from,
say, Albany, New York, more than 1,100 kilometers (700
miles) from the Windy City. Yet this happens every night.
A total solar eclipse produces a broad, round area of
darkness that greatly reduces sunlight as it sweeps across
Earth's surface in a relatively narrow path. Its effect on the
local intensity of sunlight is remarkably similar to what
happens at night. Distant radio stations along and near
to the path of totality might briefly experience enhanced
propagation, thus making long-distance reception possible
during a solar eclipse. Why is this so?
We can thank Earth's ionosphere for this extended
long-distance radio reception. Radio waves are merely one
part of the electromagnetic spectrum and therefore are
affected by electrical charges. Since ions are electrically
charged, radio waves and ions interact.
The ionosphere is an upper atmospheric zone composed
of tenuous, electrically conductive layers consisting of both
neutral and charged particles. It extends from an altitude
of approximately 50 km to more than 400 km. The ions
present in this region interact with radio waves in two
ways. They can either absorb (or deplete) radio signals,
thus reducing their intensity and signal strength, or they
can reflect (or refract) the waves, changing their direction.
Conceptually, this is akin to a radio-wave " mirror "
analogous to a telescope mirror.
The main reflecting layer in the ionosphere, called
the F2 layer, is about 290 km above Earth's surface
and is present both day and night. But at a height
of about 50 to 100 km lies the D or depletion
layer, which absorbs energy from radio signals
passing through it to the F2 layer, weakening
them and limiting their transmission range.
100
150
200
250
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400
450
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50
0 1
2
3
4
5
6
Electron density (x 1011 per m3)
EXTENDED RANGE The density of electrons in the ionosphere varies
dramatically between day and night, greatly extending the range
of AM radio transmissions. Note that at altitudes lower than about 170
km (105 miles), the count essentially drops to zero.
But as sunset approaches, the D layer rapidly loses its
ionization and essentially vanishes, allowing the F2 layer
to reflect signals over much greater distances. In other
words, radio waves are absorbed only during daylight
hours and at night easily reach the F2 layer, where they
are reflected back toward the ground. Likewise, on the
way back down, the D layer's usual obstruction is gone,
so the waves reach the ground in a well-preserved state
- often over far greater distances from the transmitting
station than is possible during daylight hours (as illustrated
on page 64).
7
8
Local noon
Local midnight
Electrons in
the Ionosphere
sk yand tele scope .o r g * APRIL 2024 63
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