Space Environment Laboratory
325 Broadway, Boulder, CO 80303–3326
(303)–497–5127 SE–10

Radio Wave Propagation

he Sun’s electromagnetic radiation The ionosphere occasionally becomes dis-
is a continuum that spans radio turbed as it reacts to certain types of solar activ-
wavelengths through the infrared, ity. Solar flares are an example; these distur-
visible, ultraviolet, x-ray, and be- bances can affect radio communication in all
yond. Ultraviolet radiation, through a process latitudes. Frequencies between 2 MHz and
termed photo ionization, interacts with upper 30 MHz are adversely affected by increased
atmospheric constituents to form an ionized absorption, whereas on higher frequencies
layer called the ionosphere. (e.g., 30–100 MHz) unexpected radio reflec-
The ionosphere affects radio signals in different tions can result in radio interference.
ways depending on their frequencies (see Fig- Scattering of radio power by ionospheric irreg-
ure 1), which range from extremely low (ELF) ularities produces fluctuating signals (scintilla-
to extremely high (EHF). On frequencies be- tion), and propagation may take unexpected
low about 30 MHz the ionosphere may act as an paths. TV and FM (on VHF) radio stations are
efficient reflector, allowing radio communica- affected little by solar activity, whereas HF
tion to distances of many thousands of kilome- ground-to-air, ship-to-shore, Voice of Ameri-
ters. Radio signals on frequencies above 30 ca, Radio Free Europe, and amateur radio are
MHz usually penetrate the ionosphere and, affected frequently. Figure 2 illustrates various
therefore, are useful for ground-to-space com- ionospheric radio wave propagation effects.
munications. Some satellite systems, which employ linear
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Frequencies (hertz)

Figure 1. The electromagnetic spectrum includes x-rays, visible light, and radio waves.
 2

A Figure 2. Radio waves that reach the iono-

sphere can go astray.
A. Wave penetrates the ionospheric layer.
B. Wave is absorbed by the layer.
Ionosphere C
B C. Wave is scattered in random directions
by irregularities in the layer.
D. Wave is reflected normally by the layer.

polarization on frequencies up to 1 GHz, are af- the lower regions of the ionosphere on the sunlit
fected by Faraday rotation of the plane of polar- side of Earth. A sudden ionospheric distur-
ization. bance (SID) of radio signals can ensue. An SID
can affect very low frequencies (e.g., OMEGA)
Solar Flare Effects
as a sudden phase anomaly (SPA) or a sudden
A solar flare is a sudden energy release in the enhancement of signal (SES). At HF, and
solar atmosphere from which electromagnetic sometimes at VHF, an SID may appear as a
radiation and, sometimes, energetic particles short-wave fade (SWF). This disturbance may
and bulk plasma are emitted (Figure 3). A sud- last from minutes to hours, depending upon the
den increase of x-ray emissions resulting from magnitude and duration of the flare.
a flare causes a large increase in ionization in
Solar flares also create a wide spectrum of radio
noise; at VHF (and under unusual conditions at
HF) this noise may interfere directly with a
wanted signal. The frequency with which a ra-
dio operator experiences solar flare effects will
vary with the approximately 11-year sunspot
cycle; more effects occur during solar maxi-
mum (when flare occurrence is high) than dur-
ing solar minimum (when flare occurrence is
very low). A radio operator can experience
great difficulty in transmitting or receiving sig-
nals during solar flares.
Energetic Particle Effects
On rare occasions a solar flare will be accompa-
nied by a stream of energetic particles (mostly
Figure 3. A eruption on the limb of the Sun. This protons and electrons). The more energetic pro-
picture was taken in Hydrogen-
light (656.3 nm). tons, traveling at speeds approaching that of
 3

light, can reach Earth in as little as 30 minutes. gation often becomes impossible during these
These protons reach the upper atmosphere near events.
the magnetic poles (Figure 4). The lower re-
Geomagnetic Storm Effects
Sufficiently large or long-lived solar flares and
disappearing filaments (DSF) are sometimes
accompanied by the ejection of large clouds of
plasma (ionized gases) into interplanetary
space. These plasma clouds are called coronal
mass ejections (CME). A CME travels through
the solar wind in interplanetary space and
sometimes reaches Earth (Figure 5).This re-
sults in a world-wide disturbance of Earth’s
magnetic field, called a geomagnetic storm.
Another type of solar activity, known as a coro-
nal hole (CH), produces high-speed solar wind
streams that buffet Earth’s magnetic field (Fig-
Figure 4. Solar energetic particles following Earth’s ure 6); geomagnetic storms that may be accom-
magnetic field lines can penetrate the upper atmo-
sphere near the magnetic poles, resulting in ioniza- panied by ionospheric disturbances can result.
tion and creating a polar cap absorption event.
These ionospheric disturbances can have ad-
gions of the polar ionosphere then become verse effects on radio signals over the entire fre-
heavily ionized, and severe HF and VHF signal quency spectrum, especially in auroral lati-
absorption may occur. This is called a polar cap tudes. In particular, HF radio operators
absorption (PCA) event. PCA events may last attempting to communicate through the auroral
from days to weeks, depending upon the size of zones (the regions of visible aurora, or “North-
the flare and how well the flare site is magneti- ern Lights”) during storms can experience rap-
cally connected to Earth. Polar HF radio propa- id and deep signal fading due to the ionospheric

Figure 5. An ejection from the Sun travels to Earth and distorts Earth’s magnetic field, resulting in geomagnetic
activity.
 4

irregularities that scatter the radio signal. Auro-

ral absorption, multipathing, and non-great-
circle propagation effects combine to disrupt
radio communication during ionospheric storm
conditions. During large storms the auroral
irregularity zone moves equatorward. These
irregularities can produce scintillations that ad-
versely impact phase-sensitive systems on fre-
quencies above 1 GHz (e.g., the Global Posi-
tioning System). Geomagnetic storms may last
several days, and ionospheric effects may last
a day or two longer.

Figure 6. The Sun as seen in x-rays. The darkest

areas are coronal holes; bright areas overlie active
regions.
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Depressed Maximum Usable Frequencies (MUF)
Increased Lowest Usable Frequencies (LUF) The Space Environment Laboratory
Increased fading and flutter monitors and forecasts these phenomena:
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Faraday rotation Full descriptions of these and other products and services are
Scintillation available from the Space Environment Laboratory:
Loss of phase lock
Radio Frequency Interferences (RFI) Space Environment Laboratory
0%#.%*) 2-.!(- NOAA R/E/SE
325 Broadway
Position errors Boulder, CO 80303–3328 USA
References
Telephone: (303) 497–5127
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 Duty Forecaster: (303) 497–3171

available 24 hours/day
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Telex number: 888776 NOAA BLDR

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E-mail address: sesc@sel.noaa.gov
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___________________________________________________________
Written by Norm Cohen and Kenneth Davies, 1994