Sporadic E-Layer Ionisation On 80m

During the QRP Hours Contest held on the night of 14 April 2012, conditions on the 80m amateur radio band were not ideal for QRP operation (ie low power operation where the transmitter output is 5W or less).  A number of stations reported in their contest logs that received signals exhibited deep fading, and receive noise levels were high.  Ionospheric soundings taken at the time in the Australasian region suggest that these adverse conditions were possibly due to sporadic E-layer ionisation.

This page examines the ionospheric conditions prevailing during the contest period.  As an introduction, it also examines the means by which ionograms are generated, and identifies the components shown on an ionogram taken during the QRP Hours Contest.  The effects of sporadic E on radio communications are discussed, and the extent of sporadic E reported in the Australasian region during the contest period is examined.  Finally; the page looks at an example of a phenomenon known as "E-layer screening", followed by references for further reading.

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Ionospheric conditions during the QRP Hours Contest

The ionogram in Figure 1 below shows the ionospheric conditions present above Hobart at 1105 UTC on Saturday 14 April 2012, during the QRP Hours Contest conducted on the 80m amateur radio band.  The discussion which follows examines how an ionosonde operates and how it generates an ionogram, and what this particular ionogram can tell us about the ionosphere and resulting propagation conditions prevailing on that night.
Hobart ionogram @ 1105
              UTC on 14 Apr 2012 Figure 1 - Hobart Ionogram for 1105 UTC on 14 April 2012.  This ionogram was obtained from the Ionospheric Prediction Service (IPS) websiteLike many of IPS's ionosondes, the one in Hobart generates an ionogram every five minutes.  A means of generating an ionogram like this one, and how to understand the information it contains, is explained below.
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The Ionosonde

An ionogram like that shown in Figure 1 above is a record generated by a Vertical Incidence (VI) Ionosonde, which is a vertical-sounding radar sweeping in frequency from about 1.7 MHz up to around 17 MHz.  The radar emits a pulse of energy upwards and measures the time taken for it to reflect (actually refract) off an ionospheric layer and return to the radar receiver.  Figure 2 below shows the concept - for ease of depiction, the radar is shown having separate transmit and receive antennas.
Basic
                ionosonde operation Figure 2 - The Vertical Incidence Ionosonde.  This simplified depiction of a VI ionosonde shows the transmitter (Tx) sending a signal upwards into the ionosphere, which is reflected from the ionospheric layer and returned to the receiver (Rx).  If the frequency is too high, the radar pulse will penetrate the ionospheric layer and continue into space - the frequency at which this just occurs is termed the "critical frequency" for that ionospheric layer.
Using this technique to determine the height of the ionospheric layer is relatively simple.  The time taken from when the ionosonde transmits the pulse and subsequently receives it is called the Round-Trip Time (RTT).  The RTT is quite short - in the order of milliseconds.  The Round-Trip Distance (RTD) - the distance from the ground up to the ionospheric layer and back down to the ground - can be calculated using the RTT and the speed of light in the atmosphere.  In radio terms, much of the atmosphere traversed by the radio wave (the troposphere, stratosphere, and neutral or non-ionised regions of the ionosphere) is considered "free space", so the value of 300 000 km per second is used.  The RTD, and resulting Virtual Height (H) of the ionospheric layer, are calculated as follows:
RTD & virtual
        height calculation
The height is "virtual" because it is somewhat different from the physical height of the ionospheric layer.  As stated above; i
n radio terms, much of the atmosphere traversed by the radio wave is "free space". *  However; the radio wave propagates more slowly in an ionospheric layer (that is, its speed is less than 300 000 km/s) than it does in the rest of the Earth's atmosphere; therefore the time taken by the radar pulse from leaving the transmitter to arriving at the receiver will be slightly longer than if the radar pulse did not traverse an ionospheric layer.  The ionogram is a graph of virtual height versus frequency - it plots the virtual height for each pulse frequency transmitted by the ionosonde as it sweeps across its frequency range (approximately 1.7 to 17 MHz).  This form of ionogram is known as a Virtual Incidence Ionogram, abbreviated to VI ionogram.

* The term "free space" applied to radio waves means that the matter within a given region does not change the speed, direction, polarisation, amplitude or other characteristics of the wave.  For high frequency (HF) radio, the troposphere and stratosphere can be considered to be "free space".  An ionospheric layer is not free space because it consists of free electrons and positive ions - also known as a "plasma" - which affect the characteristics of the radio wave.  For example; an electron can be excited by a radio wave, and as a result it may collide and recombine with a positive ion to form a neutral atom or molecule - the energy imparted to the electron is lost from the radio wave, effectively resulting in attenuation of the radio wave, albeit by a small amount.  However, if enough electrons collide with enough ions due to excitation, then the attenuation of the wave may be significant.  Also, the Earth's magnetic field is present in an ionospheric layer, which means the plasma is "anisotropic" - this means the speed at which the radio wave propagates in the plasma will depend on the direction in which the wave is travelling.

Sometimes the ionisation density in an ionospheric layer is strong enough to reflect the radar pulse back towards the ground with sufficient energy that it reflects off the ground and back up to the ionospheric layer.  If the pulse survives a second reflection from the ionospheric layer then it can be received by the ionosonde's receiver.  In this case, the ionosonde does not distinguish between single and double hops, so it reports the virtual height as being double the virtual height of the actual layer.  Triple hops and even higher are possible.  The figures below illustrate the concept.
Double hop sounding Figure 3 - Double Hop Reflection.  The signal transmitted by the VI ionosonde can reflect from the ionospheric layer, return to Earth, reflect from the Earth and ascend again towards the ionosphere, reflect again from the ionospheric layer, and return to the receiver.  In such a case, the calculated virtual height of the ionospheric layer is twice the single hop virtual height because the round-trip time is twice that of the single hop.
Triple hop sounding Figure 4 - Triple Hop Reflection.  This is similar to the double-hop reflection case, only an additional hop including an Earth reflection and ionospheric layer reflection occurs.  In this case, the calculated virtual height of the ionospheric layer is three times the single hop virtual height because the round-trip time is triple that of the single hop.
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Deciphering the ionogram

Armed with the basic knowledge of VI ionosonde operation, and how the ionosonde does not distinguish between pulse returns based on the number of hops, we can now look again at the ionogram shown in Figure 1.  At first glance, it still looks like a somewhat confused mess of traces.  The first step to deciphering the information presented is to identify what are actual ionospheric layers, and what are "phantoms" ** due to multi-hop reflections like those shown in Figures 3 and 4.  This section of the discussion does exactly that, and then looks at some very interesting reflections involving both layers.

** The traces arising from these reflections are referred to here as "phantoms" because they do not represent actual ionospheric layers - they represent apparent layers, as shown in the discussion below.

Will the real ionospheric layers please stand up?

There are only two actual ionospheric layers shown in the ionogram: a typical night-time F-layer, and a sporadic E-layer.  To illustrate these, the ionogram shown in Figure 1 above is overlaid with a mark-up showing these layers in Figure 5.
Ionogram
                showing only Es- & F-layers Figure 5 - Hobart Ionogram for 1105 UTC on 14 April 2012 with sporadic E-layer (Es) and typical night-time F-layer (F) highlighted.  The highlighted traces represent the actual ionospheric layers depicted on this ionogram - the other traces are "phantoms" arising from multi-hop reflections, which are described below.

Multi-hop reflections from individual layers

Now that the actual layers are identified, it is quite easy to identify the multi-hop reflections arising from those layers individually (which occur as shown in Figures 3 and 4 above).  These are shown for the Es- and F-layers respectively in Figures 6 and 7 below - these figures are shown side-by-side to allow ready comparison between multi-hop Es- and F-layer returns.
Ionogram showing
              multi-hop Es
Ionogram showing
              multi-hop F
Figure 6 - Sporadic E-layer Multi-hop Reflections.  The actual sporadic E-layer, and the double and triple hop phantoms (designated respectively as "2Es" and "3Es"), are shown highlighted in this mark-up of the Hobart ionogram for 1105 UTC on 14 April 2012.
Figure 7 - F-layer Multi-hop Reflections.  The actual F-layer and its double hop phantom (designated as "2F") are shown highlighted in this mark-up of the Hobart ionogram for 1105 UTC on 14 April 2012.

Multi-hop reflections involving both layers

Figures 6 and 7 together show two traces on the ionogram as actual ionospheric layers, and a further three traces as phantoms of the individual Es- and F-layers arising from multi-hop propagation between the ionosonde and the ionospheric layers.  This leaves two traces unaccounted for - both are situated at heights between the F-layer trace and its double hop phantom.  The lower (fainter) of these traces looks like a weak replica of the F-layer trace; while the higher of these traces looks very similar to the F-layer double hop phantom.  This description gives a clue as to how these traces arise.  The lower of these two traces will be dealt with first.

Sometimes the Es-layer is so strongly ionised that the F-layer above it is not visible - this phenomenon is known as E-layer screening.  However; this is not the case for the ionogram being examined here - the Es- and F-layers can be clearly discerned.  Obviously the radar signal is penetrating the Es-layer to reach the F-layer - this raises the question of whether it is possible for the radar signal to undergo a double hop involving the two layers and the Earth's surface before returning to the ionosonde receiver.  Figure 8 below examines two such propagation mechanisms, while Figure 9 highlights the result in the ionogram - these figures are shown side-by-side to allow ready comparison between the propagation mechanism and the corresponding result on the ionogram.
Double hops between Es-
              and F-layers
Ionogram showing E+F double hop
Figure 8 - Double Hop Reflection From Es- and F-layers.  As shown in the left-hand side of the diagram, the signal transmitted by the ionosonde can reflect from the Es-layer, return to Earth, reflect from the Earth and ascend again towards the ionosphere, reflect again from the F-layer, and return to the receiver.  The right-hand side of the diagram shows the converse case.  In both cases, the calculated virtual height of the phantom ionospheric layer is the sum of the virtual heights of the two layers. Figure 9 - Es- and F-layer Multi-hop Reflections.  The actual Es- and F-layer traces, and the double hop phantom arising from the combination of single hop reflections from the Es- and F-layer (designated as "Es+F"), are shown highlighted in this mark-up of the Hobart ionogram for 1105 UTC on 14 April 2012.  The height of the "Es+F" phantom trace is equal to the virtual height of the Es-layer trace shifted upwards by the virtual height of the F-layer trace; or conversely as the virtual height of the F-layer trace shifted upwards by the virtual height of the Es-layer trace.

The explanations of the multi-hop phenomenon in Figures 8 and 9 leaves only the
higher remaining phantom trace to account for.  As stated above, this phantom trace looks very similar to the F-layer double hop phantom, only its virtual height does not match a multi-hop phantom of that layer.  The mechanism by which this trace arises is shown in Figure 10 below, while Figure 11 shows the result on the ionogram - these figures are shown side-by-side to allow ready comparison between the propagation mechanism and the corresponding result on the ionogram.
Hop between Es- and F-layer
Ionogram showing F-Es-F hop
Figure 10 - Reflection Between Es- and F-layers.  The signal transmitted by the ionosonde can pass through the Es-layer, reflect from the F-layer, return to Es-layer and reflect from that, ascend again to the F-layer and reflect from that, descend through the Es-layer and return to the receiver.  In this case, the calculated virtual height of the phantom ionospheric layer is the virtual height of the F-layer summed with the difference of the virtual heights of the F- and Es-layers.  This virtual height is equivalent to twice the virtual height of the F-layer minus the virtual height of the Es-layer.
Figure 11 - Multi-hop Reflections between Es- and F-layers.  The actual Es- and F-layer traces, and the multi-hop phantom arising from the reflections from the F-layer to Es-layer and back to the F-layer (designated as "2F-Es"), are shown highlighted in this mark-up of the Hobart ionogram for 1105 UTC on 14 April 2012.  The "2F-Es" phantom trace height is equal to the height of the F-layer double hop phantom trace shifted downwards by the virtual height of the Es-layer trace.

Putting it all together

Having examined the traces for the actual ionospheric layers - with their individual multi-hop reflections and the reflections involving both layers as shown in Figure 12 below - all of the traces can be identified on the ionogram in Figure 13 below.
All hops Ionogram showing all traces
Figure 12 - Composite of all Reflections.  The signal transmitted by the ionosonde follows many different propagation paths between the ground and the ionospheric layers, and between ionospheric layers.  The horizontal distance between the transmitter and receiver is exaggerated in order to show the various propagation paths.  The colours for each path correspond to the resulting traces shown on the ionogram mark-up in Figure 13.
Figure 13 - Hobart Ionogram for 1105 UTC on 14 April 2012 with all traces highlighted.  The highlighted traces are presented using the same colours and designations as shown in the figures above; and the trace order shown in the legend on the right-hand side is the same as the height order of the traces shown in the ionogram.
Figure 14 below contains the virtual height data for the Es- and F-layers measured directly from the ionogram shown in Figure 1 (this is easily obtained by using the Ionogram Viewer on the IPS website); and the computed height data for all other traces depicted in Figure 13 above.
Height data
                for ionogram traces
Figure 14 - Tabulated virtual height data for the Hobart Ionogram at 1105 UTC on 14 April 2012.  The Es- and F-layer data was measured directly from the ionogram at the approximate vertical centre of each layer for each of the frequencies shown in the figure.  All other trace data, ie for the 2Es, 3Es, Es+F, 2F-Es and 2F phantoms, was computed from the Es- and F-layer data.  The grey areas represent frequencies at which the data was not present on the ionogram for the actual ionospheric layers, or not computed for the phantom traces.  The measured Es- and F-layer data was entered into a Microsoft Excel workbook; this data and the resulting calculated virtual heights for the phantom traces were plotted on charts overlaying the ionograms, thus generating the marked-up ionograms shown on this page.  The computed trace data shows extremely good correlation with the phantom traces shown on the ionograms, as evidenced by the mark-ups shown in the various figures.  The colour coding for the tabulated virtual height data is the same as the corresponding trace highlight colour used in the marked-up ionograms.
The following are example calculations for the phantom trace virtual heights at 2.0 MHz, using the Es- and F-layer virtual heights of 106 km and 238 km respectively, as shown in Figure 14:
These results, as well as the other phantom trace virtual heights shown in Figure 14, can be compared with the respective heights plotted in Figures 6, 7, 9 & 11; or Figure 13.

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The impact of sporadic E on night-time radio communications in the 80m band

In this section we will examine the effect of sporadic E on some typical amateur radio communications paths used on 80m at night, and also examine the effect of the sporadic E-layer critical frequency (abbreviated to foEs).

The simplistic view

The ionogram in Figure 1 shows that radio signals propagating from the ground can reach both the Es- and F-layers, because E-layer screening is not occurring.  For a given radio communications circuit operating during the ionospheric conditions recorded in the ionogram; signals emitted by the transmitter can be reflected by the Es-layer; and also can pass through that layer to be reflected by the F-layer, where they can pass through the Es-layer again to reach the ground.  This has implications for signal propagation over multiple paths simultaneously, which in turn means signals received via multiple paths can reinforce or cancel depending on the electrical lengths of those paths. 
 

To help understand the impact of sporadic-E ionisation, we can consider some radio communications circuits which are representative of those routinely used by amateur radio operators for "local" communications at night on the 80m band.  Figures 15, 16, and 17 below show 1500 km and 500 km circuits using antennas which provide low, medium and high take-off angles.  Each figure shows a typical propagation path under normal conditions (ie, when sporadic-E ionisation is not present), and possible propagation paths which are open when sporadic-E ionisation is present.  Note that there are literally infinite possible propagation paths - the actual path or paths taken depend on factors such as the antenna gain patterns at the transmitter and receiver, the ionisation densities in the Es- and F-layers, the distribution of ionisation densities in the layers (ie do the densities and therefore the critical frequencies vary along the paths), the presence or absence of layer "tilts", and so on.  Accordingly, the propagation paths presented in Figures 15, 16 and 17 are merely a subset of such paths selected to illustrate some effects of this form of ionisation.  Each of these figures is a scale drawing using the ionospheric layer height data for 3.6 MHz as determined from Figure 1 above.
Single hop 1500 km path with Es &
              F
Figure 15 - 1500 km circuit at 3.6 MHz using low take-off angle.  The yellow trace shows a single hop propagation path under normal conditions, when only the F-layer is present, serving the 1500 km circuit between the transmitter (Tx) and receiver (Rx).  The low take-off angle for this path is typical of that exhibited by a vertically polarised antenna (eg a 1/4 wavelength monopole, also known as a "groundplane" or "vertical" antenna in amateur radio parlance).  The red trace shows a possible double hop propagation path when sporadic E ionisation is present.
Double hop 1500 km path with Es &
              F Figure 16 - 1500 km circuit at 3.6 MHz using moderate take-off angle.  The yellow trace shows a double hop propagation path under normal conditions, when only the F-layer is present, serving the 1500 km circuit between the transmitter (Tx) and receiver (Rx).  The moderate take-off angle for this path is typical of that exhibited by a horizontally-polarised antenna (eg a 1/2 wavelength dipole) installed at a height less than 1/2 wavelength above the ground.  The red trace shows a possible double hop propagation path when sporadic E ionisation is present.
Single hop 500 km path with Es &
              F
Figure 17 - 500 km circuit at 3.6 MHz using high take-off angle.  The yellow trace shows a single hop propagation path under normal conditions, when only the F-layer is present, serving the 500 km circuit between the transmitter (Tx) and receiver (Rx).  The high take-off angle for this path is typical of that exhibited by a horizontally-polarised antenna (eg a 1/2 wavelength dipole) installed at a height of approximately 1/4 wavelength or less above the ground.  The red and orange traces show two separate, possible double hop propagation paths when sporadic E ionisation is present.
The combined effect of signals propagating via paths supported by the Es- and F-layers is of particular interest.  In Figures 15, 16 and 17 above; note that the take-off angles for the Es- and F-layer reflections are similar, so the transmitting and receiving antennas cannot distinguish between these propagation paths.  If the signal arriving at the receiver via the F-layer path is in-phase with the signal (or signals) arriving via the Es-layer path (or paths), then the received signals will reinforce - at the receiver, the radio operator will hear a relatively strong signal.  Conversely; if the signal arriving at the receiver via the F-layer path is anti-phase with the signal (or signals) arriving via the Es-layer path (or paths), then the signals will cancel - at the receiver, the radio operator will hear no signal.  Slight changes in the virtual heights and ionisation densities of the two layers result in changes in the electrical lengths of the Es- and F-layer signal paths, which in turn means a set of paths resulting in signal reinforcement may change to producing signal cancellation, and vice versa.  This means the received signals will reinforce then cancel then reinforce, and so on, over a period of time - this manifests itself as the deep fading (QSB) with periods of minutes which can be experienced on the 80m amateur radio band at night when sporadic E ionisation is present.

The complicated view (the effects of angle of incidence and critical frequency)

It was stated earlier that the highest frequency at which the radar pulse from the vertical-sounding ionosonde is reflected from an ionospheric layer is termed the "critical frequency" for that ionospheric layer.  A simplistic view of the effect of sporadic E ionisation on communications using the 80m band could consider this phenomenon not to be a problem if the critical frequency foEs is below 3.5 MHz (the lowest frequency in the 80m amateur radio band).  However; this is not necessarily the case.  It is possible that sporadic E-layer ionisation having a critical frequency less than the frequencies in the 80m amateur radio band can cause problems for communications circuits using that frequency band.  To understand why, it is useful to examine the relationship between Maximum Usable Frequency (MUF) and the critical frequency (fc) for an ionospheric layer as shown in Figure 18.  This approach may seem counter-intuitive at first, as we're trying NOT to use the sporadic E layer ionisation in this case, so why would we be interested in its MUF?  It turns out that this same relation can be used to determine the maximum frequency at which a radio signal will be reflected by sporadic E layer ionisation.
Path
                geometry Figure 18 - Simplified path geometry for an ionospheric hop.  The maximum usable frequency (MUF) is dependent on both the critical frequency (fc) and the angle of incidence (designated by the Greek letter "theta").  The angle of incidence is the angle between the incident ray path and a line perpendicular to the ionospheric layer and passing through the highest point that the radio signal reaches into the ionosphere, and is larger for longer distances between the transmitter (Tx) and receiver (Rx).  For a given critical frequency, the MUF increases as the angle of incidence increases; or, for a given angle of incidence, the MUF changes in direct proportion to the critical frequency.
Using the relationship between MUF, critical frequency (fc), and the angle of incidence ('theta') presented in Figure 18 above, we can examine what minimum critical frequency, or foEs, the sporadic E-layer must have at the ionospheric reflection point in order to cause possible problems for communications using the 80m band.

In Figure 15; the angle of incidence for the E-layer reflection point is approximately 71°; the cosine is 0.321, and the secant ('sec') of this angle is 3.1.  For an E-layer MUF (Emuf) of 3.6 MHz, foEs is only approximately 1.2 MHz!  This will not appear on an ionogram, because the ionosonde does not transmit at such a low frequency.

For Figure 16, where the E-layer reflection point is a "topside" reflection point, the angle of incidence is approximately 50°; and the secant of this angle is 1.6.  For an Emuf of 3.6 MHz, foEs is only approximately 2.3 MHz.  This will appear on an ionogram (strictly speaking, one would need to refer to a topside ionogram produced by a topside sounder carried on a satellite, assuming that the sounding signal could penetrate the F-layer), but is obviously lower than the frequencies in the 80m band.

In Figure 17, there are two angles of incidence at the E-layer - the traditional bottomside reflection point and a topside reflection point.  The respective angles of incidence are approximately 46° and 21°; resulting in secants of 1.4 and 1.1.  The respective foEs values are 2.5 and 3.4 MHz for an Emuf of 3.6 MHz.

It is worth noting that as the range between the transmitter and receiver shortens, the angle of incidence decreases and the secant of that angle decreases.  This means the foEs increases towards the operating frequency; with the limiting case being NVIS propagation where the distance between transmitter and receiver is so short that the angle of incidence is approximately 0° (this would be the case where the transmitter and receiver are separated by a mountain range or similar geographical feature preventing ground wave and/or direct line-of-sight propagation between the two locations).

These calculations show that the maximum frequency at which the sporadic E-layer returns a vertical incidence signal to an ionosonde located under the reflection point for a propagation path on the 80m band can be less than the 80m operating frequency while still resulting in E-layer reflections.  The calculation for the longer distance path involving fewer hops shows that in some cases the presence of an E-layer capable of supporting 80m reflections may not show on the ionogram because the foEs is below the minimum frequency used by the ionosonde.

Other effects

The author's observations during weekly, early evening 80m CW and SSB nets over approximately five years to May 2012 suggests that the presence of sporadic E-layer ionisation affecting the 80m amateur radio band is characterised by "sizzling" noise (QRN) up to strength 9 as observed with some radio receivers, and the deep and sudden fading (QSB) mentioned above.  Also, it appears that there have been more instances of sporadic E-layer ionisation occurring in the south-eastern corner of Australia during the 2011/12 (Southern hemisphere) spring / summer season than in the previous years since 2006/07.  This increased incidence appears to be extending into the 2012 autumn season.

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Distribution of sporadic E-layer ionisation during the QRP Hours Contest 2012

An idea of the geographical spread of sporadic E-layer ionisation during the QRP Hours Contest over the period from 1000 UTC to 1159 UTC on 14 April 2012 can be gained by examining the map shown in Figure 19 below.  The following are summarised observations of ionograms for each of the ionosondes shown in Figure 19, starting at the most westward location and moving East:

As can be deduced from the observations summarised above, and the map showing the ionosonde locations below, the geographical distribution of this sporadic E incidence was quite extensive.  Also, this incident occurred over a number of hours, of which the contest period was merely two of them.
Stations and ionosondes
              during QRP Hours 2012
Figure 19 - Map showing the distribution of stations participating in the QRP Hours Contest 2012, and some of the ionosondes in the IPS network.  This is a "Great Circle" projection centred on Australia.  The area encompassing the ionosondes shown on this map at which sporadic E-layer ionisation was observed during the contest period of 1000 - 1159 UTC covers approximately 93° of longitude and 42° of latitude, centred at 143°26'E 33°21'S marked with a red-coloured cross (in south-western NSW).
An hour or so after the QRP Hours Contest finished (at 1159 UTC); the sporadic E-layer ionisation lived up to its "sporadic" name - the propagation conditions experienced at Gundaroo, NSW, returned to what would normally be expected for the 80m band at this time of the year, and the author was able to work VK6 (Western Australia) on SSB using 100W PEP output fed to the same antenna as that used during the contest.

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An example of E-layer screening

It was mentioned above that occasionally the sporadic E-layer ionisation is dense enough to render the F-layer above invisible on the ionogram; and this phenomenon is known as E-layer screening.  Figure 20 below shows a somewhat spectacular example of this.
E-layer
                screening over Canberra Figure 20 - Canberra Ionogram for 0858 UTC on 16 February 2012 showing E-layer screening.  This VI ionogram recorded by the IPS ionosonde in Canberra, just before 7 pm Australian Eastern Standard Time (AEST), shows the sporadic E-layer ionisation at a virtual height of approximately 115 km, reflecting vertical incidence signals at frequencies ranging from approximately 1.7 MHz to 9.4 MHz.  Only the lowest trace represents an actual ionospheric layer - all other horizontal traces are phantoms due to multi-hop reflections between the Es-layer and ground.  The ionisation density is sufficient for the ionosonde to detect the quintuple hop reflection (ie the radar return due to the pulse bouncing five round-trips between the Earth and E-layer and back) giving rise to an approximate virtual height of 575 km.  The typical early evening / night-time F-layer cannot be seen on the ionogram, even though it actually exists - it is completely screened from ground-based radar signals by the densely-ionised E-layer.
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Further Reading

The references listed below are recommended for readers wanting to learn more about the ionosphere and how it supports high frequency (HF) radio wave propagation.
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This page was created by Mike Dower VK2IG: 30 May 2012; last updated: 31 May 2012.  Material may be copied for personal or non-profit use only.