For those who
would like to know a little more about how their radio can reach
more than 300km without the need for satellites and other infrastructure
1. The ionosphere
1.1 The regions
of the ionosphere
a region extending from a height of about 50 km to over 500 km,
some of the molecules of the atmosphere are ionised by radiation
from the Sun to produce an ionised gas. This region is called
the ionosphere, figure 1.1.
is the process in which electrons, which are negatively charged,
are removed from (or attached to) neutral atoms or molecules to
form positively (or negatively) charged ions and free electrons.
It is the ions that give their name to the ionosphere, but it
is the much lighter and more freely moving electrons which are
important in terms of high frequency (HF: 3 to 30 MHz) radio propagation.
Generally, the greater the number of electrons, the higher the
frequencies that can be used.
the day there may be four regions present called the D, E, F1
and F2 regions. Their approximate height ranges are:
region 50 to 90 km;
region 90 to 140 km;
region 140 to 210 km;
region over 210 km.
the daytime, sporadic E (section 1.6) is sometimes observed in
the E region, and at certain times during the solar cycle the
F1 region may not be distinct from the F2 region but merge to
form an F region. At night the D, E and F1 regions become very
much depleted of free electrons, leaving only the F2 region available
for communications; however it is not uncommon for sporadic E
to occur at night.
the E, F1, sporadic E when present, and F2 regions refract HF
waves. The D region is important though, because while it does
not refract HF radio waves, it does absorb or attenuate them (section
F2 region is the most important region for high frequency radio
is present 24 hours of the day;
high altitude allows the longest communication paths;
usually refracts the highest frequencies in the HF range.
lifetime of electrons is greatest in the F2 region which is one
reason why it is present at night. Typical lifetimes of electrons
in the E, F1 and F2 regions are 20 seconds, 1 minute and 20 minutes,
1.1 Day and night structure of the ionosphere.
and loss of electrons in the ionosphere
from the Sun causes ionisation in the ionosphere. Electrons are
produced when this radiation collides with uncharged atoms and
molecules, figure 1.2. Since this process requires solar radiation,
production of electrons only occurs in the daylight hemisphere
of the ionosphere.
1.2 Production (top) and loss (bottom).
a free electron combines with a charged ion a neutral particle
is usually formed, figure 1.2. Essentially, loss is the opposite
process to production. Loss of electrons occurs continually, both
day and night.
most important feature of the ionosphere in terms of radio communications
is its ability to refract radio waves. However, only those waves
within a certain frequency range will be refracted. The range
of frequencies refracted depends on a number of factors (section
1.4). Various methods have been used to investigate the ionosphere
and the most widely used instrument for this purpose is the ionosonde,
figure 1.3. Note that many references to ionospheric communications
speak of reflection of the wave. It is, however, a refraction
ionosonde is a high frequency radar which sends very short pulses
of radio energy vertically into the ionosphere. If the radio frequency
is not too high, the pulses are refracted back towards the ground.
The ionosonde records the time delay between transmission and
reception of the pulses. By varying the oscillation frequency
of the pulses, a record is obtained of the time delay at different
1.3 Ionosonde operation.
less than about 1.6 MHz are interfered with by AM broadcast stations.
As the frequency is increased, echoes appear first from the lower
E region and subsequently, with greater time delay, from the F1
and F2 regions. Of course, at night echoes are returned only from
the F2 region and possibly sporadic E since the other regions
have lost most of their free electrons.
the ionosphere is "sounded" not only by signals sent up at vertical
incidence. Oblique sounders send pulses of radio energy obliquely
into the ionosphere (the transmitter and receiver are separated
by some distance). This type of sounder can monitor propagation
on a particular circuit and observations of the various modes
being supported by the ionosphere can be made. Backscatter ionosondes
rely on echoes reflected from the ground and returned to the receiver,
which may or may not be at the same site as the transmitter. This
type of sounder is used for over-the-horizon radar.
ionosphere is not a stable medium which allows the use of one
frequency over the year, or even over 24 hours. The ionosphere
varies with the solar cycle, the seasons, the circuit and during
any given day. So, a frequency which may provide successful propagation
now, may not do so an hour later.
due to the solar cycle
Sun goes through a periodic rise and fall in activity which
affects HF communications; solar cycles vary in length from
9 to 14 years. At solar minimum, only the lower frequencies
of the HF band will be supported by the ionosphere, while at
solar maximum the higher frequencies will successfully propagate,
figure 1.4. This is because there is more radiation being emitted
from the Sun at solar maximum, producing more electrons in the
ionosphere which allows the use of higher frequencies.
1.4 Solar cycle and seasonal dependence of E and F region frequencies
for a near vertical incidence sky wave (NVIS) circuit in the
are other consequences of the solar cycle. Around solar maximum
there is a greater likelihood of large solar flares occurring.
Flares are huge explosions on the Sun which emit radiation that
ionises the D region causing increased absorption of HF waves.
Since the D region is present only during the day, only those
communication paths which pass through daylight will be affected.
The absorption of HF waves travelling via the ionosphere after
a flare has occurred is called a short wave fade-out (section
3.1). Fade-outs occur instantaneously and affect lower frequencies
the most. Lower frequencies are also the last to recover. If
it is suspected or confirmed that a fade-out has occurred, it
may help to try using a higher frequency. However, if a flare
is very large, the whole of the HF spectrum may be rendered
unusable. The duration of fade-outs can vary between about 10
minutes to over an hour depending on the intensity and duration
of the flare.
region frequencies are greater in summer than winter, figure
1.4. However, the variation in F region frequencies is more
complicated. In both hemispheres, F region noon frequencies
generally peak around the equinoxes (March and September). Around
solar minimum the summer noon frequencies are, as expected,
generally greater than those in winter, but around solar maximum,
winter frequencies at certain locations, can be higher than
those in summer. In addition, frequencies around the equinoxes
(March and September) are higher than those in summer or winter
for both solar maximum and minimum. The observation of noon,
winter frequencies often being greater than those in summer
is called the seasonal anomaly (this is not observed in figure
1.5 indicates the variations in E and F region frequencies at
noon and midnight from the poles to the geomagnetic equator.
During the day and with increasing latitude, solar radiation
strikes the atmosphere more obliquely, so the intensity of radiation
and the electron density production decreases towards the poles.
1.5 Representation of latitudinal variations.
in figure 1.5 how the daytime F region frequencies peak not
at the magnetic equator, but around 15 to 20 degrees north and
south of it. This is called the equatorial anomaly. At night,
frequencies reach a minimum around 60 degrees latitude north
and south of the geomagnetic equator. This is called the mid-latitude
trough. Large tilts can occur in the vicinity of these phenomena
which may lead to variations in the range of sky waves that
have reflection points nearby.
frequencies are normally higher during the day and lower at
night, figure 1.6. With dawn, solar radiation causes electrons
to be produced in the ionosphere and frequencies increase reaching
their maximum around noon. During the afternoon, frequencies
begin decreasing due to electron loss and with evening, the
D, E and F1 regions become insignificant. HF sky wave communication
during the night is therefore by the F2 region and absorption
of radio waves is lower because of the lack of the D region.
Through the night, frequencies decrease reaching their minimum
just before dawn.
1.6 E and F layer frequencies for a Singapore to Ho Chi Minh
circuit some time in a solar cycle.
D region, which becomes insignificant at night, attenuates waves
as they pass through it. Absorption was discussed in section 1.4.1
when describing how solar flares can cause disruptions or degradations
to communication paths which pass through daylight. Absorption
in the D region also varies with the solar cycle, being greatest
around solar maximum. Signal absorption is greater in summer and
during the middle of the day, figure 1.7. There is a variation
in absorption with latitude, with more absorption occurring near
the equator and decreasing towards the poles, although certain
solar events will significantly increase absorption at the poles.
Lower frequencies are absorbed to a greater extent, so it is advisable
to use as high a frequency as possible.
1.7 Example of diurnal and seasonal variations in absorption at
Sydney, 2.2 MHz.
the polar regions absorption can affect communications quite dramatically
at times. Sometimes high energy protons ejected from the Sun during
large solar flares will move down the Earth's magnetic field lines
and into the polar regions. These protons can cause increased
absorption of HF radio waves as they pass through the D region.
This increased absorption may last for a number of days and is
called a Polar Cap Absorption event (PCA), section 3.2.
E may form at any time. It occurs at altitudes between 90 to 140
km (in the E region), and may be spread over a large area or be
confined to a small region. It is difficult to know where and
when it will occur and how long it will persist. Sporadic E can
have a comparable electron density to the F region, implying that
it can refract comparable frequencies to the F region. Sporadic
E can therefore be used for HF communications on higher frequencies
than would be used for normal E layer communications at times.
Sometimes a sporadic E layer is transparent and allows most of
the radio wave to pass through it to the F region, however, at
other times the sporadic E layer obscures the F region totally
and the signal does not reach the receiver (sporadic E blanketing).
If the sporadic E layer is partially transparent, the radio wave
is likely to be refracted at times from the F region and at other
times from the sporadic E. This may lead to partial transmission
of the signal or fading, figure 1.8.
1.8 Some possible paths when sporadic E is present.
E in the low and mid-latitudes occurs mostly during the daytime
and early evening, and is more prevalent during the summer months.
At high latitudes, sporadic E tends to form at night.
F occurs when the F region becomes diffuse due to irregularities
in that region which scatter the radio wave. The received signal
is the superposition of a number of waves refracted from different
heights and locations in the ionosphere at slightly different
times. At low latitudes, spread F occurs mostly during the night
hours and around the equinoxes. At mid-latitudes, spread F is
less likely to occur than at low and high latitudes. Here it is
more likely to occur at night and in winter. At latitudes greater
than about 40 degrees, spread F tends to be a night time phenomenon,
appearing mostly around the equinoxes, while around the magnetic
poles, spread F is often observed both day and night. At all latitudes
there is a tendency for spread F to occur when there is a decrease
in F region frequencies. That is, spread F is often associated
with ionospheric storms (section 3.3).
2. HF communications
of HF propagation
Frequency (3 to 30 MHz) radio signals can propagate to a distant
receiver, figure 2.1, via the:
wave: near the ground for short distances, about 100 km
over land and 300 km over sea. The range of the wave depends
on antenna height, polarisation, frequency, ground types,
vegetation, terrain and/or sea state;
or line-of-sight wave: this wave may interact with the earth-reflected
wave depending on terminal separation, frequency and polarisation;
wave: refracted by the ionosphere, all distances.
2.1 Types of HF propagation.
limits of sky waves
all HF waves are refracted by the ionosphere, there are upper
and lower frequency bounds for communications between two terminals.
If the frequency is too high, the wave will penetrate the ionosphere,
if it is too low, the strength of the signal will be lowered due
to absorption in the D region. The range of usable frequencies
the solar cycle;
place to place;
on the ionospheric region used for communications.
the upper limit of frequencies varies mostly with these factors,
the lower limit is also dependent on receiver site noise, antenna
efficiency, transmitter power, E layer screening (section 2.6)
and absorption by the ionosphere.
2.3 The usable
any circuit there is a Maximum Usable Frequency (MUF) which is
determined by the state of the ionosphere in the vicinity of the
refraction area(s) and the length of the circuit. The MUF is refracted
from the area of maximum electron density of a region. Therefore,
frequencies higher than the MUF for a particular region will penetrate
that region. During the day it is possible to communicate via
both the E and F layers using different frequencies. The highest
frequency supported by the E layer is the EMUF, while that supported
by the F layer is the FMUF.
F region MUF in particular varies during the day, seasonally and
with the solar cycle. Long term data displays a range of frequencies
observed and some of the IPS predictions mirror this. A range
of F region MUFs is provided in the predictions and this range
extends from the lower decile MUF (called the Optimum Working
Frequency, OWF), through the median MUF to the upper decile MUF.
These MUFs have a 90%, 50% and 10% chance of being supported by
the ionosphere, respectively. IPS predictions usually cover a
period of one month, so the OWF should provide successful propagation
90% of the time or 27 days of the month. The median MUF should
provide communications 50% or 15 days of the month and the upper
decile MUF 10% or 3 days of the month. The upper decile MUF is
the highest frequency of the range of MUFs and is most likely
to penetrate the ionosphere, figure 2.2.
2.2 Range of usable frequencies. If the frequency, f, is close
to the ALF then the wave may suffer absorption in the D region.
If the frequency is above the EMUF then propagation is via the
F region. Above the FMUF the wave is likely to penetrate the ionosphere.
chances of successful propagation discussed above rely on the
monthly prediction of solar activity being correct. Sometimes
unforeseen events occur on the Sun resulting in the monthly predictions
being inaccurate. One of the roles of the Australian Space Forecast
Centre (ASFC) at IPS is to provide corrections to the monthly
predictions, warning customers of changes in communication conditions.
D region does not allow all frequencies to be used since the lower
the frequency the more likely it is to be absorbed. The Absorption
Limiting Frequency (ALF) is provided as a guide to the lower limit
of the usable frequency band. The ALF is significant only for
circuits with refraction points in the sunlit hemisphere. At night,
the ALF is zero, allowing frequencies which are not usable during
the day to successfully propagate.
2.4 Hop length
hop length is the ground distance covered by a radio signal after
it has been refracted once from the ionosphere and returned to
Earth, figure 2.3. The upper limit of the hop length is set by
the height of the ionosphere and the curvature of the Earth. For
E and F region heights of 100 km and 300 km, the maximum hop lengths
with an elevation angle of 4 degrees, are 1800 km and 3200 km,
respectively. Distances greater than these will require more than
one hop. For example, a distance of 6100 km would require a minimum
of 4 hops by the E region and 2 hops via the F region with such
an elevation angle. More hops may be required with larger antenna
2.3 Hop lengths based upon an antenna elevation angle of 4 degrees
and heights for the E and F layers of 100 km and 300 km, respectively.
are many paths or modes by which a sky wave may travel from a
transmitter to a receiver. The mode by a particular layer which
requires the least number of hops between the transmitter and
receiver is called the first order mode. The mode that requires
one extra hop is called the second order mode. For a circuit with
a path length of 5000 km, the first order F mode would require
at least two hops (2F), while the second order F mode would then
require three hops (3F). The first order E mode has the same number
of hops as the first order F mode. If this results in a hop length
of greater than 2050 km, which corresponds to an elevation angle
of 0 degrees, the E mode is not possible. This also applies to
the second order E mode. Of course, the E region modes will only
be available on daylight circuits.
modes are those propagated by one region, say the F region, figure
2.4. More complicated modes consisting of combinations of refractions
from the E and F regions, ducting and chordal modes are also possible,
2.4 Examples of simple propagation modes.
modes and ducting involve a number of refractions from the ionosphere
without intermediate reflections from the ground. There is a tendency
to think of the regions of the ionosphere as being smooth, however,
the ionosphere undulates and moves, with waves passing through
it which may affect the refraction of the signal. The ionospheric
regions may tilt and when this happens chordal and ducted modes
may occur. Ionospheric tilting is more likely near the equatorial
anomaly, the mid-latitude trough and in the sunrise and sunset
sectors. When these types of modes do occur, signals can be strong
since the wave spends less time traversing the D region and being
attenuated during ground reflections.
2.5 Some other propagation modes.
of the high electron density of the daytime ionosphere in the
vicinity of 15 degrees of the magnetic equator (near the equatorial
anomaly), transequatorial paths can use these enhancements to
propagate on higher frequencies. Any tilting of the ionosphere
may result in chordal modes, producing good signal strength over
may result if tilting occurs and the wave becomes trapped between
refracting regions of the ionosphere. This is most likely to occur
in the equatorial ionosphere, near the auroral zone and mid-latitude
trough. Disturbances to the ionosphere, such as travelling ionospheric
disturbances (section 2.9), may also account for ducting and chordal
2.6 E layer
daytime communications via the F region, the lowest usable frequency
via the one hop F mode (1F) is dependent upon the presence of
the E region. If the operating frequency for the 1F mode is below
the two hop EMUF, then the signal is unlikely to propagate via
the F region due to screening by the E region, figure 2.6. This
is because the antenna elevation angles of the 1F and 2E modes
2.6 E layer screening occurs if communications are required by
the 1F mode and the operating frequency is close to or below the
EMUF for the 2E mode. Note the paths through the absorbing D region.
sporadic E layer may also screen a wave from the F region. Sometimes
sporadic E can be quite transparent, allowing most of the wave
to pass through it. At other times it will partially screen the
F region leading to a weak or fading signal, while at other times
sporadic E can totally obscure the F region with the possible
result that the signal does not arrive at the receiver, figure
1.9 (section 1.6).
range and elevation angle
oblique propagation, there are three dependent variables:
or path length;
diagrams below illustrate the changes to the ray paths when each
of these is fixed in turn.
2.7: Elevation angle fixed
- as the
frequency is increased toward the MUF, the wave is refracted
higher in the ionosphere and the range increases, paths 1
- at the
MUF for that elevation angle, the maximum range is reached,
the MUF, the wave penetrates the ionosphere, path 4.
2.7 Elevation angle fixed.
2.8: Path length fixed (point-to-point circuit)
- as the
frequency is increased towards the MUF, the wave is refracted
from higher in the ionosphere. To maintain a circuit of fixed
length, the elevation angle must therefore be increased, paths
1 and 2;
- at the
MUF, the critical elevation angle is reached, path 3. The
critical elevation angle is the elevation angle for a particular
frequency, which if increased, would cause penetration of
the MUF, the ray penetrates the ionosphere, path 4.
2.8 Path length fixed.
2.9: Frequency fixed
- at low
elevation angles the path length (ground range) is greatest,
- as the
elevation angle is increased, the path length decreases and
the ray is refracted from higher in the ionosphere, paths
2 and 3;
- if the
frequency will return when sent vertically up into the ionosphere,
then there is no skip distance. However, if this is not the
case, then as the elevation angle is increased beyond the
critical elevation angle for that frequency then the wave
penetrates the ionosphere and there is an area around the
transmitter within which no sky wave communications can be
received, path 4. To communicate via the sky wave within the
skip zone, the frequency must be lowered.
2.9 Frequency fixed.
skip zone is an area around a transmitter in which neither the
ground wave nor the sky wave propagate. Skip zones can often be
used to advantage if it is desired that communications are not
heard by a particular receiver. Selecting a different frequency
will alter the size of the skip zone and if the receiver is within
the skip zone and out of reach of the ground wave, then it is
unlikely that it will receive the communications. However, factors
such as sidescatter, where reflection from terrain outside the
skip zone results in the wave transmitting into the zone, may
affect the reliability of this technique.
zones vary in size during the day, with the seasons, and with
solar activity. During the day, solar maximum and around the equinoxes,
skip zones generally are smaller in area. The ionosphere during
these times has increased electron density and so is able to support
fading results from dispersion of the signal by the transmitting
antenna. A number of modes propagate which have variations in
phase and amplitude. These waves may interfere with each other
if they reach the receiver, figure 2.10.
2.10 Multipath fading. The signal may travel by a number of paths
which, if they arrive at the receiver and are of similar amplitude
with time delay, may interfere and cause fading.
known as Travelling Ionospheric Disturbances, TIDs, may cause
a region to be tilted, resulting in the signal being focused or
defocused, figure 2.11. Fading periods of the order of 10 minutes
or more can be associated with these structures. TIDs travel horizontally
at 5 to 10 km/minute with a well defined direction of travel.
Some originate in auroral zones following an event on the Sun
and these may travel large distances. Others originate in weather
disturbances. TIDs may cause variations in phase, amplitude, polarisation
and angle of arrival of a wave.
fading results from changes to the polarisation of the wave along
the propagation path. The receiving antenna is unable to receive
components of the signal; this type of fading can last for a fraction
of a second to a few seconds.
fading can be observed around sunrise and sunset particularly,
when the operating frequency is close to the MUF, or when the
receive antenna is positioned close to the boundary of the skip
zone. At these times of the day, the ionosphere is unstable and
the frequency may oscillate above and below the MUF causing the
signal to fade in and out. If the receiver site is close to the
skip zone boundary, as the ionosphere fluctuates, the skip zone
boundary also fluctuates.
2.11 Focusing and defocusing effects caused by tilting and travelling
ionospheric disturbances (TIDs).
noise arises from internal and external origins. Internal or thermal
noise is generated in the receiving system and is usually negligible
when compared to external sources. External radio noise originates
from natural (atmospheric and galactic) and man-made (environmental)
noise, caused by thunderstorms, is normally the major contributor
to radio noise in the HF band and will especially degrade circuits
passing through the day-night terminator. Atmospheric noise is
greatest in the equatorial regions of the world and decreases
with increasing latitude. Its effect is also greater on lower
frequencies, hence it is usually more of a problem around solar
minimum and at night when lower frequencies are used.
noise arises from within our galaxy. Receive antennas with high
angle lobes are more likely to be affected by this type of noise.
noise emanates from ignition systems, neon signs, electrical cables,
power transmission lines and welding machines. This type of noise
depends on the technological advancement of the society and the
size of the population.
from other users on the same frequency may be intentional, such
as jamming or due to propagation conditions.
noise tends to be vertically polarised, so selecting a horizontally
polarised antenna may help in reducing noise. Using a narrower
bandwidth, or a directional receiving antenna (with a lobe in
the direction of the transmitting source and a null in the direction
of the unwanted noise source), will also aid in reducing the effects
of noise. Selecting a site with a low noise level and determining
the major noise sources are important factors in establishing
a successful communications system.
and 27 MHz propagation
and 27 MHz are used for line-of-sight or direct wave communication,
for example, ship-to-ship or ship-to-shore. The frequency bands
are divided into channels and one channel is usually as good as
the next. This is in contrast to medium frequency (MF: 300 kHz
to 3 MHz) and HF where the choice of a frequency channel may be
crucial for good communications.
VHF and 27 MHz operate mainly by line-of-sight, it is important
to mount the antenna as high as possible and free from obstructions.
Shore stations are usually on the tops of hills to provide maximum
range, but even the highest hills do not provide coverage much
beyond about 45 nautical miles (80 km) because of the Earth's
for VHF and 27 MHz should concentrate radiation at low angles
(towards the horizon) as radiation directed at high angles will
usually pass over the receiving antenna, except when communicating
with aircraft. VHF and 27 MHz do not usually suffer from noise
except during severe electrical storms. Interference results from
many users wishing to use the limited number of channels, and
this can be a significant problem in densely populated areas.
MHz and the lower frequencies in the VHF band can, at times, propagate
over large distances, well beyond the normal line-of-sight limitations.
There are three ways that this can take place:
solar maximum and during the day, the ionospheric F region
will often support long range sky wave communications on 27
MHz and above;
E layers can sometimes support 27 MHz and lower frequency
VHF propagation over circuits of about 500 to 1000 nautical
miles (1000 to 2000 km) in length. This kind of propagation
is most likely to occur at mid-latitudes, during the daytime
- 27 MHz
and VHF can also propagate by means of temperature inversions
(ducting) at altitudes of a few kilometres. Under these conditions,
the waves are gradually bent by the temperature inversion
to follow the curvature of the Earth. Distances of several
hundred nautical miles can be covered in this way.
frequency (MF) sky wave propagation
the MF (300 kHz to 3 MHz) and HF bands can be used for long distance
sky wave communications at night. During the night the D region
disappears, so absorption falls to very low levels. This is why
radio broadcast stations operating in the MF and 4 MHz bands can
be heard over long distances at night.
wave MF and HF propagation
is possible to communicate up to distances of several hundred
nautical miles on MF/HF bands at sea by using ground wave propagation.
ground wave follows the curvature of the Earth and its range does
not depend upon the height of the antenna. However, the range
does depend upon the transmitter power and also upon the operating
frequency. Low frequencies travel further than high frequencies.
Thus under ideal low noise conditions (noon, during winter), it
is possible to communicate over distances of about 500 nautical
miles at 2 MHz by using a 100 W transmitter. At 8 MHz, under the
same conditions and using the same transmitter power, the maximum
range is reduced to about 150 nautical miles.
that ground wave propagation is much less efficient over land
than it is over sea because of the much lower conductivity of
the ground and other factors. Consequently, ranges over land are
wave communications vary daily and with the seasons. Greatest
communication ranges are achieved during the daytime in winter
because background noise levels are lowest during these hours.
ground wave communications over hundreds of nautical miles can
only be achieved if the transmitting and receiving antennas are
chosen to direct and receive radiation at low angles. Tall whips
are ideal for this purpose.
3. The effects
of solar disturbances
wave fade-outs (SWFs)
called daylight fade-outs or sudden ionospheric disturbances (SIDs).
Radiation from the Sun during large solar flares causes increased
ionisation in the D region which results in greater absorption
of HF radio waves, figure 3.1. If the flare is large enough, the
whole of the HF spectrum can be rendered unusable for a period
of time. Fade-outs are more likely to occur around solar maximum
and in the first part of the decline to solar minimum.
3.1 Fade-outs affect only those circuits where the wave passes
through the D region. That is, circuits with daytime sectors.
Night circuits are unaffected by fade-outs.
main features of SWFs are:
circuits with daylight sectors will be affected;
usually last from a few minutes to sometimes two hours,
with a fast onset and a slower recovery. The duration of
the fade-out will depend on the intensity and duration of
magnitude of the fade-out will depend on the size of the
flare and the position of the Sun relative to the point
where the radio wave passes through the D region. The higher
the Sun with respect to that point, the greater the amount
is greatest at lower frequencies, which are the first to
be affected and the last to recover. Higher frequencies
are normally less affected and may still be usable, figure
3.2 Fade-outs affect lower frequencies first and these are the
last to recover. Higher frequencies are least affected and with
many fade-outs will be unaffected.
cap absorption events (PCAs)
are attributed to high energy protons which escape from the Sun
when large flares occur and move along the Earth's magnetic field
lines to the polar regions. There they ionise the D region, causing
attenuation of HF waves passing through the polar D region. PCAs
are most likely to occur around solar maximum, however, they are
not as frequent as fade-outs.
may commence as soon as 10 minutes after the flare and last
for up to 10 days;
effects of PCAs can sometimes be overcome by relaying messages
on circuits which do not require polar refraction points;
the winter polar zone (a region of darkness) can suffer
the effects of PCAs. The particles from the Sun may actually
produce a night D region.
to events on the Sun, sometimes the Earth's magnetic field becomes
disturbed. The geomagnetic field and the ionosphere are linked
in complex ways and a disturbance in the geomagnetic field can
often cause a disturbance in the F region of the ionosphere.
ionospheric storms sometimes begin with increased electron density
allowing higher frequencies to be supported, followed by a decrease
in the electron density leading to the successful use of only
lower than normal frequencies of the F region. An enhancement
will not usually concern the HF communicator, but the depression
may cause frequencies normally used for communication to be too
high with the result that the wave penetrates the ionosphere.
storms may last for a number of days and mid and high latitudes
are affected moreso than the lower latitudes, generally. Unlike
fade-outs, higher frequencies are most affected by ionospheric
storms. To reduce storm effects, a lower frequency should be used
storms can occur throughout the solar cycle and are related to
coronal mass ejections (CMEs) and coronal holes on the Sun. Figure
3.3 shows how an ionospheric storm has caused frequencies in the
main to be depressed at Canberra, Australia (a mid-latitude station)
from 24 to 28th. Higher frequencies would probably have been unsuccessful
over this time.
3.3 Canberra, Australia observed and median vertical MUFs for
latter part September 1998. Significant depressions in F region
frequencies occurred between 24 to 28 September due solar activity.