
Great example above of a Tropo Ducting
path potentially open between Spain and South America on Tuesday 25th June
2013. That evening lots of
144 MHz DX being worked between mainland Spain
and the Canary islands, mistakenly reported on the DXclusters as via
Sporadic-E,
as even on
50 MHz
the
Es
had been extremely poor throughout Europe all day long.
VHF
144 MHz Tropo propagation openings map
(Click on historical image below for current map)

Above a VHF Tropo propagation map using
144 MHz
APRS signals, by Jon Harder NG0E, shown on the morning of Wednesday 26th
June 2013 indicating pathways. Interestingly the Tropo ducting paths are
still open from Portugal and the Azores to the Canary Islands.
Historical image shown courtesy of
John Harder NG0E
Sporadic Es and
Ionospheric chart for Rome
Showing daily height of
Sporadic-E
clouds (h'Es)
and MUF.
For the Northern hemisphere,
between April to August MUF (3000)F2 frequencies greater than
15 MHz
generally indicate potentially good 50 MHz
or higher
Sporadic-E
(Es)
conditions. It is possible for the MUF to exceed
30 MHz
in Winter at the peak of the solar cycle every 11 years, but this does not guarantee any
Es
propagation.
Also shown, if present, is the height of the
Sporadic-E
reflecting atmospheric layer usually between
90-131 km
(h'Es),
with the higher the height the better, as it may indicate stronger ionised
metal bearing layers which will support higher VHF frequencies and also last longer than
Es layers at lower altitudes.
The
live data Ionogram shown below from Rome, Italy is a very good guide to
whether or not
Sporadic-E exists over
central Europe on any given day, it is one of several Ionogram generating sites, the
others can be found from the RSGB website
here or the DK5YA VHF page.
It is interesting to observe that the height of the
Sporadic-E
layers is not uniform across all of the locations in Europe at the
same time of measurement, for
example at 08:00 UTC on 13.07.2018 the heights ranged from
108 km
in Rome, 97 km
in Athens, 108 km
in Svalbard, 105 km
in Dourbes, Belgium,
115 km in Juliusruh,
Denmark, 103 km
in Tromso, Norway, 113
km in Gibilmanna and
115 km
in Pruhonice.
Also the
Sporadic-E
layer
heights vary by time of day, with Rome descending from
108 km
at 08:00 to
99 km
by 11:00 UTC, it is thought all
sporadic-E
clouds descend over time after reaching their peak altitude.

Image above courtesy of the Rome Observatory of the
Instituto Nazionale di Geofisica e Vulcanologia (INGV)
Sporadic-E and theories
Sporadic-E
(abbreviation Es)
enhanced VHF radio propagation is just that,
sporadic yet present most days for several hours at a time during daylight hours on
50 MHz
usually most years
between
May to August
in the Northern hemisphere, peaking in
June
sometimes with rare short duration openings supporting radio signal
reflections on frequencies as high as
144 MHz,
and with daily timings
over several years showing the greatest chance of
Es
being present between
11:00-12:00 UTC
and
16:00-18:00 UTC.
Sporadic-E
is observed on 144 MHz
less than 10% as often as on 50 MHz.
For
many years there was also recorded a much weaker and shorter
Sporadic-E
season around the Winter solstice (21st December) when the intensity of the
sun's solar radiation is at its maximum over the winter months. I had only
ever worked it once on 28th December 2018.
However as of 2020/2021 things have changed quite noticeably from previously
recorded years, the usual Es season didn't end in August but carried on
throughout September, October, November, December and now January 2021 with
at least 3 large European wide
Sporadic-E
openings each month on
50 MHz
workable even from here in the far North of England. I don't know if this is
because many more Radio Amateurs are using the weak signal data mode FT8 and
are able now to detect3and work Es openings in a way that wasn't previously
possible with CW and SSB and/or is it because so many people are at home
monitoring the VHF bands due to Covid-19 lockdowns. Or is some unexplained
physical change in the atmosphere.
Excellent
Sporadic-E
144 MHz
events occurred in 1989, 2006, 2009, 2010, 2011, 2017 and 2020 which have been at
both maximum and minimum points in the 11 year sun solar cycle, which
demonstrates that is not a factor required for
Sporadic-E
to occur. For an excellent evaluation summary of
144 MHz Sporadic-E
from 2001 to the present day I highly recommend viewing the
mmmonvhf page here
which shows you all the data collated and broken down into time, day, month,
year, quantity and Es
cloud positions.
Another characteristic supported by my own live monitoring over 35 years and
extensive collected data from DXcluster spots since 2001, in the Northern hemisphere for
European propagation, is that the vast majority of
Sporadic-E
reflection areas or clouds seem to occur mostly over the
Bay of Biscay, Switzerland and the Balkans.
Here in the UK it is probable on most days in Summer to work Italy and Spain
easily on
50 MHz, it is much rarer perhaps 5-10%
of Es
days for the
reflecting Es
cloud to be situated over the North Sea allowing communication from the UK to
Scandinavia, however when this does occur it is in this very marked direction.
Even rarer openings occur in the direction of Iceland from the UK, but many
more multi hop
Sporadic-Es
clouds do reasonably often open paths from the UK to the USA.
Sporadic-E
(Es) occurs in the
Ionosphere at heights of between
90-131 km
(110 km average)
altitude and appears to concern strong areas of non-uniform and patchy plasma metallic ion
and electron density irregularities that cause VHF radio waves to be
reflected back to Earth by forward scatter. The metallic ions necessary are
deposited in the Ionosphere by daily meteor activity (heating and ablation)
even in the absence of major shower or meteor storm activity, the primary
metal types being iron (Fe) and Magnesium (Mg). The total metal ion density
determining whether or not the
Es
layer can support VHF signal forward scatter or not. The winds and electric
fields at these altitudes act to compress the ions into thin layers of
around
4 km
in depth.
There
are minimum distances for each band for propagation via
Sporadic-E,
lesser distances would be impossible by this mode as the required MUF would
be simply too high and must therefore be by another mechanism such as
Aircraft Scatter or Tropo scatter etc. The minimum distances by VHF band are
shown below.
50
MHz minimum
Es
distance 400 km
70
MHz minimum
Es
distance 400 km
144
MHz minimum
Es
distance 1400 km
Research and published papers indicate that it is the daily ablation of
thousands of metallic meteors from all directions that are required, rather
than intense meteor showers from single radiants. During the Summer months
there is approximately 3 times more metallic meteorite deposition than in
the winter months.
There have been several theories proposed over the years
for Sporadic-E
formation, some are being substantiated and some have limited correlation to observed events,
and
the required ingredients required to produce
Es
layers, to support forward scatter at VHF frequencies, appear to differ between the
50 MHz
and
144 MHz
frequency bands.
Sporadic-E occurs most notably on
the VHF
28 MHz, 50 MHz, 70 MHz and
144 MHz amateur radio bands
where the ionized E layer of the atmosphere at around
110 km altitude reflects
forward scatter VHF radio signals back
to Earth, rather than them normally travelling straight through the
atmosphere into space,
with received radio signals being extremely strong.
Low power levels of 3 Watts have allowed many QSOs between Europe and North America on the
50 MHz
band via multiple reflections.
Monitor the VHF amateur radio bands and beacons and if
Es
signals are
exceptionally strong on a lower band such as
50 MHz
and at the lower end of the single hop distance range
(500 km)
this can be a good indicator that
Es
will be supporting even higher frequencies, so consider listening up on the
70 MHz
or
144 MHz
bands too. If travelling in your car away from your shack try monitoring
87.6 MHz
FM on your analogue vehicle radio to see if you can hear
Es
broadcast station signals from outside the UK
(Tip from Dave Edwards G7RAU, thanks!)

Long distances
on
50 MHz can be
worked via
Sporadic-E layers from
around 400-7000 km,
with single hop 400-2350 km
where the path is
Earth-Cloud-Earth, double-hop up to 4700 km
where the path is
Earth-Cloud-Earth-Cloud-Earth and triple-hop
up to
7000 km
where the path is
Earth-Cloud-Earth-Cloud-Earth-Cloud-Earth.
With the advent of special weak signal data modes even these distances may
be exceeded in the future.
An
absolutely exceptional partial Sporadic-E
opening occurred on the morning of 24th July 2018 between 0700-0800 UTC when
the station of VK8AW
(PH57)
Darwin, Australia worked and was heard by stations in Europe on
50 MHz
with the furthest station being G3TXF
(IO71)
in England at a distance of 14,118
km!
Previous theory and long established observations
prior to 2018 stated there should be no
F2 or
TEP propagation except at
solar maximum, but having communicated via e-mail with
Gary Ashdown
VK8AW
it appears that his signals were reaching Southern Europe and the Middle
East via Trans Equatorial Propagation (TEP) and were being further extended by widespread European
Sporadic-E
linking to it. Apparently 48-50 MHz
signals are observed by
VK8AW
regularly from the Middle East and China and the
new weak signal data mode FT8 is allowing two way radio communications,
where they were previously unworkable or undetectable except at solar maximum.
This
mechanism of Sporadic-E plus TEP
propagation has been seen again on 50
MHz in Summer 2019 with many Japanese
stations working Southern Europe and even UK stations on the South coast of
England.
Also
in June 2019
Sporadic-E plus marine Tropo Ducting
allowed signals on the 144 MHz
band to be received from D41CV
in the Cape Verde Islands off the West Coast of Africa to be received by
OE3NFC
in Austria at a distance of 5107 km,
the marine
tropo ducting
path being from Cape Verde to Southern Spain where it joined the
Sporadic-E
path between Southern Spain and Austria.
On
144 MHz
the distances for single hop Es
contacts appear to all be in the range of between
1400-2350 km
with rarer multi hop exceeding this to a
recorded maximum of
3600 km.
The exact multi hop mechanism
for each signal may not be readily identifiable and
may be signals reflecting off a single
Es
cloud layer or multiple separate
Sporadic-E clouds and/or
various mixtures of Cloud and the Earth with reflections from water offering
significantly less attenuation than from land.
It is also theorised
and pretty much correlated that the
Es cloud
layers are not uniform and flat, but may parts tilted at times at up to 45
degrees where they often have an irregular wavy surface, like corrugated metal
sheeting (see image below). This can slightly
complicate understanding the precise Sporadic-E
hop
mechanism being observed on
50 MHz
with the theory being of many instances where two cloud
Es formations exist, the
Es layers
being
tilted towards each other.
This two cloud
Es
formation appears to account for
50 MHz signals in the
distance range of 2400-3200 km
i.e. more than a single hop maximum
(2350 km), but much less than for
double-hop (4700 km).
The signal path being
Earth-Cloud-Cloud-Earth.

Noctilucent clouds at around
85km altitude - Photo by Jan Koeman,
Kloetinge, the Netherlands, July 2009
In
the two Es cloud formation path (not double-hop)
live example shown below, captured at 18:12 UTC on 17th July 2018,
I was hearing the station of SV1NZX
in KM17
square via
Sporadic-E
on 50 MHz
at a very strong signal level, my station
G0ISW
in IO84
square being 2741 km
in distance away and therefore well beyond the maximum single hop distance
of 2350 km.
Looking at the live MUF mapping software
display there were two significant Es
cloud areas in place (circled in red) on the path, both at a similar distance from our stations,
therefore it appears that the signal was being reflected from these separate
Es
clouds at the same time, with both
Es
clouds layers probably being slightly tilted
towards each other with wavy irregular cloud surfaces, the path being
Earth-Cloud-Cloud-Earth.

The exact conditions necessary for the appearance of
Sporadic-E layers have remained
mostly a
mystery until recent years where a gradual consensus is being formed, if likened to a recipe the key ingredient always required
is
the
Sun strongly illuminating the ionosphere
and causing significant and intense solar ionising radiation, with the peak
ionisation being at local midday
during the Summer months (May to August) in the Northern hemisphere.
Additional ingredients required for the formation of
Sporadic-E
may
include some of the following: High Meteoric deposition rates, Ionospheric
Wind shear, thunder storms, Atmospheric Gravity Waves (AGS), Noctilucent
Clouds and Solar cycle
activity.
Below
is a table of theorised
Sporadic-E required ingredients and
observed strong correlation in the Northern hemisphere.
Es
theorised ingredients
|
50 MHz |
144 MHz |
|
|
|
Peak Summer Season from May to August |
Not
always needed |
Yes |
Intense
Solar Radiation |
Yes |
Yes |
High
Meteoric metal deposition rate |
Yes |
Yes |
Major
Meteor Shower |
Not
needed |
Not
needed |
Atmospheric Ground Waves |
Not
needed |
Possibly |
Ionospheric Wind shear |
Possibly |
Possibly |
Thunder
storms/Sprites |
Not
needed |
May
possibly assist |
Noctilucent Clouds |
Possibly |
Probably |
Frustratingly however, sometimes the
Sporadic-E on
144 MHz
defies any correlation with AGW, wind shear and thunder storms. In the
example below at 17:53 UTC on 23rd July 2018 there is a pronounced
Es
reflecting layer shown over the Balkans and their mountain ranges, but
checks on live wind charts show no jet stream or other significant wind
activity anywhere near and also no lightning whatsoever.

After
sunset the ionised
Es
layer usually fades until it can no longer support VHF signals being
reflected from it, probably due to no longer receiving solar radiation,
however on rare occasions
Sporadic-E
VHF
propagation has been known to occur even up to local midnight.

Image from US Naval Research Laboratory
From observations in Europe, over the past 30 years, these are randomly
generated signal
reflecting areas, which vary in size depending on the frequency band they
can support i.e. 50
MHz Sporadic-E
clouds are significantly larger and can often be around
500 x 500 km (5 x 5 grid
squares) in size, whereas on 144 MHz
they may often only be 50-100 km
in size i.e. a single grid square, unless a very significant and much rarer
large opening event. The usual small reflecting area on
144 MHz
means fewer stations will be ideally located to benefit from using it.
In
the images below white lines
show the paths on 50 MHz,
grey lines
are 70 MHz
and red lines
are 144 MHz.
 
On
144 MHz
the
Sporadic-E
layers in Summer have been observed to move slowly from East to West and sometimes
correlate with
intense thunder storms, but not always. Note the correlation
between this very significant
144 MHz
Sporadic-E
opening reflection zone, shown below on 12.07.2018, and the live map of
lightning strikes at the same time in the same area. The
144 MHz
opening on the border of Poland with Ukraine shown below lasted just over 1
hour and despite much similar lightning visible in Norway no such
Sporadic-E
opening occurred there. However later in the week further extensive thunder
storms were present and no
Sporadic-E
was observed anywhere in Europe, all the necessary combination of
ingredients not being present.

In addition whilst
50 MHz
Sporadic-E
band openings can last most of the day,
144 MHz
openings can be very short lived and occur much less frequently, only being
available for a few days each year and for much less time, sometimes only as
little as a few minutes or an hour.
From my own observations 144 MHz
Sporadic-E reflecting
areas do often coincide with intense thunder storms and due to the height of the
Es reflecting layer being between 90-131 km
I can only theorise a link with the little known electrical Sprite phenomenon perhaps,
possibly generating Atmospheric Gravity Waves (AGW) and causing wind shear
at
90 km
altitude and higher.
No such correlation with thunder storms and
Sporadic-E
is seen anywhere near as often on
the other lower VHF bands.

Image credit
https://en.wikipedia.org/w/index.php?curid=4262250
Abestrobi Own work CC BY-SA
3.0
The higher the frequency the more
intense the Sporadic-E
ionised
layer has to be to support forward scatter propagation via it, at 144 MHz
in particular it can be frustrating to see the small reflecting area favour
radio amateurs in a particular geographic area reasonably close, but in your
own location nothing is heard and vice versa. Sometimes the reflecting areas
at the centre of the lines connecting the QSOs converge over a particular
area each year, the Bay of Biscay near France is one such area. Often the
reflecting layer/cloud moves over hours in a East to West direction. Using
the Live-MUF mapping software it is very easy in real time to observe these
events.
Sometimes
on
144 MHz
there may be correlation with
Es
reflecting areas and thunder storms generating extensive lightning activity
(another theory), but by no means do they coincide every time. On the
morning of 3rd May 2016 there was a clear
50 MHz
Es reflecting
layer centred over the Baltic states, but no wind shear or AGW from
mountains, nor any thunder
storm lightning activity found using live data, so the required ingredients
were much simpler.
In June 1989, during a
particularly intense radio
Es event on
144 MHz, I observed the distant DX stations
heard to
slowly all shift SW over the course of two hours, in a direction not associated
with the jet stream, but consistent with the Earth's rotation in
relation to the Sun.

If we
consider the 'wind shear' theory formulated in the 1960's by Whitehead
(1961) and Axford (1963) in which vertical shears in the horizontal wind
form thin layers (several kilometres) thick from metallic ionisation through
ion-neutral collisional coupling and acting in the presence of the Earths
magnetic field and through the Lorentz force. The original metallic ions
thought to be present from meteoric origin.
With
the wind shear effects being required
at the
Sporadic-E height of at least
80-90 km we should look first at evidence
of observed wind effects or clouds near that altitude with the highest
visible clouds in the Earth's atmosphere being the rarely seen
Noctilucent clouds
at
80-90 km
in altitude, just below the Mesopause. These clouds are also seasonal May to
August in the Northern hemisphere and are bluish-white in colour and are
only visible just after sunset when still illuminated by the sun and have
undulations, corrugations and rich veins. These clouds do prove that there
are wind effects present at these very high altitudes and surprisingly also
water vapour too, their non uniform composition can be likened to the
theorised irregularity of Sporadic-E
clouds. Apparently research 25 years ago found that these Noctilucent clouds
exhibit good radio reflection properties due to metallic meteor deposition
being attracted to and coating the tiny water droplets which have turned to
ice.
In
2020 I have come to the conclusion that there is a definite link between
Sporadic-E
propagation and Noctilucent clouds
at around 80-90 km
altitude. These clouds only form in exactly the same Summer months as Sporadic-E and
historic research has shown they exhibit good radio reflecting properties,
they are thin and wispy, non regular and vary considerably all factors which
would support the variability of
Sporadic-E reflecting areas. Also in
the Summer months the predominant wind direction at these altitudes is from
East to West just the same as observed
Sporadic-E
reflection areas movement and the wind speeds can apparently reach up to 300
km/Hour.
The
noctilucent clouds are formed from ice crystals being attracted to metallic
meteorite particles and gathering around them acting as a reflector to radio
waves if sufficiently dense.
Wind shear
from the Jet Stream at altitudes of between 9-16 km above ground level over
mountains has been suggested at
144 MHz to have an association with
Sporadic Es, part of the so
called 'wind shear' theory. Using the live wind chart below and setting the
wind height altitude to 500 hPA (11 km)
should allow observations and comparisons to be conducted in real-time with
Sporadic Es
cluster maps to support or discount any such correlation, with wind effects.
(Click on historical image below for current live
jet stream map)

For
European
Sporadic-E
formed via AGW effects we need to look also at a map of European mountain
ranges to see if there is any correlation with observed
Es
clouds, it does not however explain Es
clouds on
50 MHz
often seen formed over the Bay of Biscay, North Sea or the Atlantic
Ocean. Sporadic-E
has been observed to regularly form over the
Balkans, Pyrenees and the Alps, does this coincide with Jet stream wind
direction on the same days?

Noctilucent
clouds
do prove
that there are wind effects present at these very high altitudes caused by
Atmospheric Gravity Waves (AGW), their non uniform composition can be
likened to the theorised irregularity of
Sporadic-E
clouds.

Noctilucent clouds at around
85km altitude - Photo by Jan Koeman,
Kloetinge, the Netherlands, July 2009
How
are there wind effects and potential 'wind shear' at
90 km
altitude required for Sporadic-E?
Well, Atmospheric Gravity Waves (AGW) required to form
Noctilucent clouds
and other wind effects at these high altitudes can be generated by either
violent intense thunderstorms or wind flow over high mountain ranges, which
cause vertical displacement of the air flow with the AGWs forming when
buoyancy pushes air up and gravity later pulls it back down.

The
AGW can be likened to a corrugated tin roof effect being placed on the
original horizontal air flow and these uneven waves of air then travel to
high altitudes and new waves form underneath so a sequence of waves will be
formed. AGW are medium scale waves with the horizontal wavelength ranging
from several tens to several thousands of kilometres and a vertical
wavelength of several kilometres. Using radar measurement AGW have been
observed to reach heights of around
85-88 km, the same height as
Noctilucent clouds. The AGW can even penetrate up to the Ionosphere where
they trigger ionospheric irregularities and add to the recipe for
Sporadic-E
likelihood.
The generally accepted height of the
Sporadic Es reflecting
layer is around
90-131 km, which is very
significantly higher than the jet stream altitude of
11 km.
Although annual the intensity of Es
events varies by year and in recent years has been very poor compared with
the 1980's and 1990's, despite peaks and troughs of the solar cycle. In 2018
it has been much better, than for some time.
There is a
definite connection with the radiation intensity of the Sun
due to the seasonal Summer nature of the event in the Northern hemisphere.
Also the
Es
events tend to mostly occur during daylight hours at VHF. A 3-6 day cycle
pattern of building up to a
Es
peak also seems quite prevalent, possibly an electric ion charge/discharge cycle period
perhaps, it does seem strange to see very active
Sporadic-E
one day and absolutely nothing the next day despite weather and Solar
conditions appearing unchanged.
The
Earth has a Global Electric Circuit (click on image below for video
explanation) with lots of variables daily impacting
upon it at Ionospheric altitudes, many of these factors will contribute to
whether or not VHF radio signals can be propagated over vastly longer
distances than normal on any given day.

When I participated in the fantastic Leonids November 2002 meteor
storm event, with over 700+ meteors per minute, the intensity was such that
the whole
144 MHz band was wide open
for many hours with the high signal strengths associated with
Sporadic-E
signals. I have also observed other meteor showers coinciding with a
Sporadic-E like opening, as happened most recently in April 2017
during the peak of the Lyrids meteor shower, no other mention of
Sporadic-E by stations
until May, but the signal strengths I observed that day were very high and
prolonged again, for an hour or more.
The
long standing mystery of what exactly causes
Sporadic Es propagation continues,
since it was first observed in the 1930's.
Tropo and Air pressure charts
(Click on historical image below for current chart)

Above an Air Pressure chart from the UK Met Office for Wednesday 26th June
2013 showing the very large system of High pressure between Africa and South
America leading to good Tropo ducting opportunities.
Historical image shown courtesy of
UK Met Office
|