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Atmospheric Electricity:
By
DR DAVID MADDISON
Nature’s Spectacular fireworks!
If you think about nature’s electrical fireworks, lightning and
the auroras at the poles immediately come to mind. But now
we can see that what happens out in space is vastly more
spectacular and in fact, much larger than planet Earth.
This is the story of Nature’s planet-wide spectacle which
has been running ever since the Earth was formed.
E
arth’s atmosphere is highly electrically active, most
visibly manifested by phenomena such as spectacular
lightning displays and, for those living in the higher
latitudes, the Aurora Australis and Aurora Borealis.
Then we have the invisible ionosphere, which plays a vital role in the propagation of radio signals for long distances.
Other electrical and related phenomena in the atmosphere and near space include the magnetosphere, coronal
mass ejections from the Sun impacting the magnetosphere,
the solar wind, the plasmosphere, the ring current, the Van
Allen Belts, electrical charging of clouds and transient
luminous events such as sprites, elves and blue jets. Plus
there are controversial unexplained phenomena such as
ball lightning.
History
Early experimenters noticed the similarity between
sparks and discharges from electrical machines, Leyden
jars and lightning, and concluded that they were the same
phenomena.
It was William Wall in 1708 that first noted that spark
discharges resembled miniature forms of lightning. In 1750
Benjamin Franklin thought that electrical charge could
be drawn from clouds with a tall enough metal aerial but
he was beaten to the experiment in 1752 by Frenchman
16 Silicon Chip
Thomas-François Dalibard who managed to draw sparks
from a passing cloud with a 12m aerial.
In 1752 Franklin is said to have performed his famous
kite experiment (however, there is some dispute now as to
whether Franklin ever did that experiment!). It was done
by Romas and Cavallo, both of whom drew long sparks
from a metal string.
G. Beccaria repeated Le Monnier’s work and determined
that the atmosphere had a positive charge. Other later workers found seasonal variations in the Earth’s electric field
and variations with altitude.
L.G. Le Monnier was another that repeated the kite experiment and discovered the electrical field within the
atmosphere which came to be known as the “fair weather
condition”.
Vertical electric field in the atmosphere
There is a high strength vertical electric field in the atmosphere. On a clear day, over flat land or the ocean at the
equator, this is around 120V/m so there might be a potential
difference almost 200V between your nose and the ground.
We do not notice this because the body is a relatively
good conductor and so the potential difference is “shorted
out”, bringing the potential between your nose and the
Earth close to zero.
siliconchip.com.au
FINE
WEATHER
FINE
WEATHER
50km
300,000 VOLTS
ELECTROSPHERE
RAIN
CORONA
EARTH
NET ELECTRIC CURRENT
Atmospheric electrical circuit showing the relationship
between fair weather current flow and charge transport in
thunderstorm conditions.
The field is in a direction that gives the surface of the
Earth a negative charge under normal circumstances, the
“fair weather condition”.
The electric field continues to increase with height (although not in a linear fashion) until an altitude of around
50,000 metres (that’s more than three times higher than jet
aircraft fly), at which point the atmosphere is sufficiently
conductive, due mainly to ionisation by ultraviolet light,
that no further increase in potential occurs.
The potential difference between that altitude and Earth
has a mean value of 280kV, varying from 150kV to 600kV.
Even though the electric field gradient extends to 50km
altitude, most of the charge is located within the first 10,000
metres, since the net positive charge contained between the
ground and 10,000 metres is nearly equal to the net negative charge of the Earth. At 10,000 metres, the electric field
gradient has reduced from around 120V/m to around 5V/m.
One might ask the question, “Why not use this potential
difference to generate power?”
It has been asked many times before! But since air is a
good insulator, the current available is very small, so little
useful power could be generated.
Although it is possible to use this field to drive a small
electrostatic motor you can build yourself (see box), little
useful work can be done. In fact, the current density of
the atmosphere is around 1pA (picoamp) per square meter in built-up areas, to 2.4pA per square meter in most
other areas.
An often-cited figure for the total current over the whole
of the Earth’s surface is around 1800A; the range is 750 to
2000A.
Using typical figures of 1800A total current and 280kV
mean potential, the total power for the entire Earth is 504
siliconchip.com.au
Electrical equivalent circuit of atmospheric electrical flow
with thunderstorms. Note that some values are slightly
different to those mentioned in text as different values are
used by different researchers. (After www.slac.stanford.
edu/cgi-wrap/getdoc/slac-wp-020-ch11g-Kirkby.pdf).
megawatts. Using the voltage and current figures in Ohm’s
Law also allows us to calculate the effective resistance of
the atmosphere as 155.
The air of the atmosphere is generally regarded as a good
insulator unless it is heavily ionised, as is the ionosphere,
in which case it becomes an electrical conductor.
The fact that some small current does exist from the atmosphere to the Earth does mean that the atmosphere is
somewhat conductive. The main source of the lower atmosphere’s limited electrical conductivity is ions which
are air molecules which have either gained or lost an electron and are no longer electrically neutral and therefore
able to carry charge.
How are the ions generated?
In 1912 Hess, with a balloon-borne electrometer, established that the amount of ionisation of the air increased
with height, meaning that whatever was causing it was
stronger at high altitudes. This was a mystery because it
was previously thought that ground-based radioactivity
was causing electrons to be stripped from atoms and molecules, turning them into ions.
It was eventually proved that cosmic rays from outer
space were responsible for the generation of a majority of
atmospheric ions, constantly replenishing them as older
ions are carried to Earth in the charge transfer process.
Apart from ions, charge can also be carried on dust and
water droplets. Ions can also be generated by radioactivity at ground level but that is not the major contributor to
ions in the atmosphere as originally thought.
Interestingly, the potential gradient of 120V/m at the
equator increases to 300-400V/m in industrial areas with
a lot of dust. (The figure is typically 155V/m at 60° latiMay 2016 17
tude and 71V/m at the South Pole, although these figures
do vary with time).
How does the Earth become charged?
How is this charge maintained? If it were not replenished
there would soon be no charge on the Earth. It is estimated
that the negative charge of the Earth would dissipate in 1060 minutes, so how the charge was replenished remained
a great mystery in the early days of research into atmospheric electricity.
It is now known that thunderstorms, lightning and electrified (but non-thunder) clouds are the “battery” responsible for maintaining a permanent potential difference
between the upper atmosphere and the surface. Bolts of
lightning do not discharge the entire atmosphere as might
be thought but a majority of lightning strikes bring a large
amount of negative charge to Earth’s surface. The corresponding return part of this circuit occurs elsewhere, in
areas of fine weather.
As mentioned above, the Earth has a net negative charge
and the electric field is directed downward. Current flows
from sky to Earth in areas of good weather. Beneath thunderstorms however, a positive charge builds up on the
Earth’s surface. In that case the field is directed upwards
and current flows from the Earth to the sky.
The two areas of flow together complete an electrical
circuit which keeps the charge on the Earth replenished
as illustrated in the diagram. At any given time about 1%
of the Earth’s surface that is beneath thunderstorms carries
a positive rather than negative charge.
All around the world, 24 hours a day, there are around
40 to 50 lightning flashes per second so there is certainly enough activity to keep the Earth-atmosphere system
charged.
What causes lightning?
Lightning is an extremely complex process, even today
not fully understood.
Essentially what happens within a thundercloud is that
due to convection currents, lots of ice particles and water
droplets rub across each other, causing them to become
charged. This is similar to what happens when you rub
two insulating items together such as wool and a balloon.
Pure ice crystals develop a positive charge while “graupel”, a water and ice mixture with a slushy consistency,
becomes negatively charged. The lighter ice crystals are
carried by updrafts in the cloud while the heavier graupel
stays near the cloud base. This results in charge separation within the cloud, with positive charge near the top
and negative charge near the bottom.
The positive charge at the top of the cloud and the negative charge at the bottom of the cloud form a “battery” that
is of the correct sign to drive the Earth beneath it negative.
Typically the positive charges are 6 to 7km high at a temperature of around -20°C and the negative charges are 3 to
4km high with a temperature of around -10°C.
This charge builds and builds – and when a sufficient
charge has developed between oppositely charged areas
such as between a cloud and Earth, within a cloud or between clouds, the normally insulating air breaks down and
becomes conducting.
The exact processes are complex and not well understood
but for a cloud-to-ground strike the process is as follows:
1) A downward “leader” is formed and descends from
the cloud.
2) When the leader approaches the ground an upward
streamer can form which is much like a leader but in the
opposite direction.
3) “Attachment” occurs where the leader and streamer
connect, establishing a low-resistance electrical pathway.
4) “Discharge” or the “return stroke” occurs which is
the most powerful part of the lightning strike, resulting a
discharge of energy from the cloud to the ground with a
typical current of 30,000A and up to several hundred mil-
Global distribution of lightning strikes in units of flashes per square kilometre per year as detected by a NASA satellite
carrying the Optical Transient Detector (OTD) and the Lightning Imaging Sensor (LIS). The place that receives the most
lightning is near a village in the Democratic Republic of the Congo with around 160 strikes per square km per year.
18 Silicon Chip
siliconchip.com.au
lion volts. A number of return strokes can happen in quick
succession and the return stoke neutralises the positive
ground charge caused by the storm. You can see a video of
these phenomena at http://youtu.be/dukk07c2eUE
A conventional cloud-to-ground to discharge is called a
negative stroke but there are rare positive strokes as well,
which originate at the tops of clouds and are much more
powerful. While aircraft are designed to withstand negative strokes, positive strokes were unknown when aircraft
lightning safety standards were first established and it is
unclear how well they would withstand a hit with such a
lightning strike (see www.damninteresting.com/the-powerof-positive-lightning/ for a report on the first aircraft known
to have been destroyed by positive lightning).
There are several different types of lightning. Some
variations include “superbolts” which are about one hundred times more powerful than normal lightning and “ball
lightning”, the actual existence of which is unproven but
subject to intense speculation.
Ball lightning is in the form of luminous free floating
spherical objects up to a metre in size, is usually associated
with cloud to ground discharges, lasts from a second to a
minute, often moves horizontally and has been claimed
to go through solid objects and has even been claimed to
appear inside an aircraft cabin.
St Elmo’s Fire is a coronal discharge that can occur in
areas of high electric field such as might appear beneath
thunderstorms. Throughout history, it has occurred as an
eerie glow on the spars of sailing ships – a portent of doom
according to sailors.
An effect similar to St Elmo’s Fire was the glow which
surrounded flight BA9 after it flew through a cloud of volcanic ash over Indonesia on 24 June 1982, nearly causing
the plane to crash (https://en.wikipedia.org/wiki/British_
Airways_Flight_9)
An eerie glow, visible from inside the plane, occurred
when a British Airways flight passed through the ash of an
erupting volcano. At the time, all four engines had failed.
The global electric circuit
The continuous electric current between the lower layers of the ionosphere and the Earth’s surface is known as
the global electrical circuit (GEC).
In recent times it has come to be understood that this
system is vast, extremely complicated and variable and has
many factors that influence it.
The Sun has a major influence on the GEC and atmospheric electrical phenomena. Apart from bathing the world
in life-giving energy, the Sun emits a constant stream of
charged particles which hit the Earth’s protective magnetosphere. These eventually pass down magnetic field lines
which are concentrated at the magnetic poles where they
cause the Aurora Borealis in the northern hemisphere and
the Aurora Australis in the southern hemisphere.
The aurora has associated large electrical currents or
“electrojets” flowing in the direction from the day side to
the night side.
Occasional explosions of huge amounts of material from
The various elements of earth's Global Electrical Circuit. Image credit: University of Colorado, “Electrical Connections and
Consequences Within the Earth System” (ECCWES) project.
siliconchip.com.au
May 2016 19
Nikola Tesla
Despite the claims of many
conspiracy theorists (and, we
must warn, modern-day internet scams), Nikola Tesla did not
invent a method for harvesting
large amounts of “free energy”
from the atmosphere.
He did however have a lot of
interest in wireless transmission
of electrical energy and radio,
something that is firmly based
in science.
In 1900, Tesla had asked J.P. Morgan to invest in his Wardenclyffe tower project to transmit a wireless radio signal across the
Atlantic Ocean. Morgan agreed to this and funded the project to
the extent of US$150,000. Telsa had also planned experiments
in wireless power transmission but did not initially tell Morgan
about this.
On the 12th December 1901, Marconi managed to send radio
signals across the Atlantic thus beating Tesla to this goal as Tesla’s project was still under construction. In 1903, Telsa finally
revealed his intention to Morgan of wireless power transmission
and Morgan declined further funding.
Conspiracy theorists claim this was because Morgan saw that
wireless power transmission could not be metered but the reason
is that Morgan did not find the idea attractive and also he had
already told Tesla he would not invest more than the originally
agreed amount. In addition, Tesla’s radio system was more expensive than Marconi’s. Further, the “Rich Man’s Panic” of 1903
made it difficult for Tesla to get funding from others and also his
own patents for his previous inventions expired, leading to very
difficult times for Tesla.
A detailed rebuttal of the conspiracy theorists’ claims can
be found at http://thelibertarianrepublic.com/evil-capitalistsprevent-nikola-tesla-creating-free-energy/ See also https://
en.wikipedia.org/wiki/World_Wireless_System
It turns out that short range wireless power transmission is
coming into use for applications such as charging mobile phones
but even medium and long range wireless power transmission is
possible. Techniques involve inductive, capacitive and magnetodynamic coupling and microwaves or lasers.
An artist’s 1925 image of what Tesla’s wireless
power transmission system might have looked like
in the “future”. The tower is transmitting electrical
power to aircraft and the city in the background.
20 Silicon Chip
Magnetosphere of Earth showing various structures. Image
credit: NASA.
the Sun occur during events known as coronal mass ejections that can impact the Earth causing disruption of communications and electrical grids and damage to satellites.
These events can also lead to spectacular aurorae, often
extending much further toward the equator than normal.
The Sun is responsible for providing most of the radiation that ionises the ionosphere and the properties of the
ionosphere vary with the 11-year sunspot cycle.
The Magnetosphere
The magnetosphere is a region around Earth with a magnetic field in which charged particles from the Sun (or
elsewhere in space) are influenced by the field.
Near the Earth, the field is shaped much like that of a
bar magnet but further away it is greatly distorted by the
continual flow of particles from the Sun, the solar wind.
The magnetosphere protects the Earth from harmful
charged particles by deflecting them around the Earth or into
the polar regions. Without the magnetosphere, life would
be very challenging or it may not even have evolved at all.
The solar wind would also eventually strip away much
of the atmosphere, as is believed to have happened on
Mars (which has neither a protective magnetic field nor
significant atmosphere – it’s less than 1% of Earth’s and is
95% carbon dioxide).
There are a number of structures within the magnetosphere. The first one is the bow shock which is the leading
edge of the magnetosphere that particles first encounter, setting up a shock wave as they are moving at around 400km/s.
The magnetopause is an area of balance between the
planet’s magnetic field and the solar wind.
The magnetosheath is a magnetically turbulent region between the bow shock and the magnetopause. The particles
in this region are mainly solar wind from the bow shock.
The magnetotail is the opposite structure to that of the
compressed magnetic field on the side of the Earth facing
the Sun. It extends far out into space and between the upper and lower structures there is a sheet of plasma.
The plasmasphere is located above the ionosphere and
is also known as the inner magnetosphere. It contains low
energy or cool plasma. It was discovered due to the analysis of VLF Whistler data, which will be discussed later in
this article.
siliconchip.com.au
Van Allen radiation belts
Within the inner part of the magnetosphere exist the two
Van Allen radiation belts, discovered in 1958. These trap
energetic charged particles within the magnetic field and
can cause damage to spacecraft if those are not protected.
The Atmosphere
Earth’s atmosphere is the medium in which an electrical
gradient and associated current is maintained and also the
medium in which thunderstorms develop, thus forming a
vital part of the global electrical circuit.
Ionosphere
The ionosphere exists as a layer from about 65km altitude
to about 600km. One of its most important properties is that
it reflects radio waves. It can do this because it contains a
significant proportion of charged particles in the form of
atmospheric atoms which have had electrons removed by
high energy radiation from the Sun, such as UV and Xrays as well as, to a lesser extent, cosmic rays from space.
These particles are said to be “ionised”, hence the name
of this layer. These particles form a plasma that is electrically conductive and hence capable of reflecting radio waves
under the right circumstances and conducting electrical
currents. The properties of the ionosphere vary from day
to night and also according to geomagnetic activity such
as solar outbursts.
For further information about the ionosphere see the article on HAARP, SILICON CHIP, October 2012.
Cosmic Rays
Cosmic rays generate most of the ions which are the cur-
rent carriers for the atmospheric electrical current. These
are mostly atomic nuclei originating from somewhere out
in space which strike the Earth’s atmosphere at near-light
speeds.
Geomagnetically induced currents
Geomagnetically induced currents are currents induced
into artificial electrical conductors (such as long power
lines or telephone lines) on Earth, as a result of electric
fields produced by rapid magnetic field variation during
geomagnetic storms. These currents can cause damage or
destruction of transformers and can even bring down entire
electrical grids. Electrical grids are much more vulnerable
when they are heavily loaded with little excess capacity.
A severe geomagnetic storm, which struck the Earth on
13 March 1989, brought down Canada’s Hydro-Quebec
735kV power grid; the blackout lasted nine hours and was
ended only when the utility company implemented many
temporary bypasses and “fixes”.
Schumann resonance
The area between the Earth and the densest, most conducting part of the ionosphere forms a wave-guide that has
a certain resonant frequency and conducts electromagnetic
waves that are generated by lightning discharges around
the planet.
It was named after physicist Winfried Otto Schumann
who in 1952 predicted that this cavity between the Earth and
ionosphere would have a characteristic resonant frequency.
The fundamental Schumann resonant frequency is
7.83Hz, with peaks at intervals of around 6.5Hz: 14.3. 20.8,
27.3 and 33.8Hz.
A “Static Motor” from the 1920s
The video “Free Atmospheric Electricity Powers Small Motor – Tesla Radiant Energy” (https://youtu.be/do4IO_
U3B5o) shows the operation of a small motor powered by the electric field gradient in the atmosphere. A hexacopter
was used to hoist the antenna wire around 30 metres into the air. The video title implies a connection with Nikola
Tesla’s ideas. Tesla never proposed harvesting “free energy” but did have ideas of wirelessly transmitting electrical
energy. This screen grab from that video comes from the book “Homemade Lightning: Creative Experiments in
Electricity” 3rd Edition but originally comes from the publication “Practical Electrics” from 1924.
siliconchip.com.au
May 2016 21
Listening to the sounds of the ionosphere. . .
It is relatively easy to hear natural signals from the ionosphere
and elsewhere, “Earth songs”, such as from lightning, tweeks,
whistlers, the auroral chorus, hisses and other sounds, although
they won’t happen all the time so patience will be required.
One way to listen to to these signals is to connect via the internet to a number of VLF receivers that are on-line at all times. One
site with links to a number of these receivers is at http://abelian.
org/vlf/ The received signal at Florida seems particularly active.
Many people find these sounds quite relaxing to listen to and they
may even help you go to sleep!
A selection of
WR-3 receivers
(row at bottom)
being field tested
at a radio quiet
location in
California before
shipping to
customers. The
author owns one
of these units.
Live streaming of a VLF signal can also be heard at www.vlf.it/
cumiana/livedata.html along with a number of regularly updated
spectrograms representing the outputs of different VLF station
setups.
Another method to listen to Earth songs is to purchase a broadband VLF receiver such as the fully assembled WR-3 receiver from
www.auroralchorus.com/wr3order.htm These are made as a cottage industry by pioneering hobbyist Stephen P. McGreevy rather
than a commercial organisation. Examples of auroral recordings
made by Stephen McGreevy can be heard at links on www.auroralchorus.com/aucho.htm The device sells for US$170, including
shipping to Australia. The WR-3 is also available without a housing or antenna and costs US$85 shipped to Australia. The user
needs to supply appropriate standard headphones in both cases.
Stephen McGreevy also has free plans for his BBB-4 “bare bones
basic” receiver at www.auroralchorus.com/bbb4rx3.htm
A different VLF receiver is available in kit form, called the Inspire
VLF-3. This is part of the NASA-inspired educational project for
The circuit
of Stephen
McGreevy’s
BBB-4 VLF
receiver.
22 Silicon Chip
natural VLF signals. An Inspire VLF-3 kit costs US$155, including shipping to Australia. The order page is at http://theinspireproject.org/default.asp?contentID=27
Note that unlike the WR-3 above, this is a kit and does require
assembly. The theory of operation is described at http://theinspireproject.org/downloads/pdf/inspire%20Theory_of_Operations.
pdf Assembly instructions are at http://theinspireproject.org/
downloads/pdf/VLF_Instructions_Apr2011.pdf It is suggested
that you look at those first before deciding if you are comfortable
building this kit. A simple telescopic antenna is also needed, or
a 2m length of wire.
If you want to try to build your own device to receive broadband
VLF signals, SILICON CHIP described a simple VLF preamplifier, designed to plug in to your PC sound card input, in the April 2011
issue. The circuit is shown below (this was part of our feature on
detecting earthquakes via VLF radio but is just as valid for listening to VLF from space!).
(See www.siliconchip.com.au/Issue/2011/April/Can+Earthquakes+Be+Predicted+By+VLF+Radio+Signals%3F).
+9V SUPPLY
LONG WIRE
OR VERTICAL
WHIP ANTENNA
(Id = 4mA)
1k
10k
100nF
G
K
A
A
D
S
ZD1
3.3V
100nF
Q1
MPF102
SHIELDED
CABLE
TO PC SOUND CARD
MIC INPUT (PINK)
3.5mm STEREO PLUG
MPF102
10M
220
ZD2
3.3V
S
G
D
K
SC
2011
VLF PREAMP
ZD1, ZD2
A
K
A method for listening to Schumann Resonances is described
at www.vlf.it/poggi1/schumann.html Also see www.backyardastronomy.net/schumann_resonance.html
Spectrum Lab (www.qsl.net/dl4yhf/spectra1.html) is a popular free spectrum analyser software program with which natural
sounds from the ionosphere can be analysed (as well as it having
many other uses). Using this program along with a PC sound card
and an active antenna as a front end is described at www.qsl.
net/dl4yhf/speclab/natradio.htm The author of that article also
notes that even a long wire can be connected to a sound card as
a temporary antenna but surge protection is very important or
your computer could be destroyed.
Note that receiving the Earth’s natural radio signals requires
broadband receivers (as opposed to narrow band receivers that
are required for receiving many man-made signals). Such receivers
have to be used as far away as possible from power lines and so
A WR-3 receiver it is best not to use them in cities as the signal will be dominated
without housing by 50Hz or 60Hz hum from power lines (frequency depends on
or antenna – a
what country you are in). A minimum distance from power lines
cheaper way to of 500m is suggested but possibly far more is needed for high
purchase this
voltage lines.
unit but you will
VLF falls within the audio frequencies and the receivers are eshave to provide
sentially
low noise, high gain amplifiers that have their output fed
the housing
directly into an audio amplifier. No RF conversion is necessary
and antenna
as in a normal radio receiver. The antenna can be a simple long
yourself.
length of wire or magnetic loop antenna.
siliconchip.com.au
Schumann
resonance around
the earth between
the surface and
ionosphere (not to
scale) showing the
fundamental mode (a
single standing mode
wave around the entire planet)
and the second and third order
harmonics. Image credit: Neotesla at japansk.
The fundamental resonance corresponds to a wavelength
of around 38,300km (using the speed of light in a vacuum)
which is approximately the circumference of the Earth, explaining why this cavity resonates at that frequency, with
a single standing wave around the entire planet.
The wavelength is a little less than the actual circumference of the Earth (around 40,075km at the equator) due to
considerations of spherical geometry and due to other effects such as the slight conductivity of air due to the presence of ions. The frequencies vary a little with the changing
state of the ionosphere due to day and night variations and
the impact of solar storms on the ionosphere.
When a lightning strike occurs, the discharge acts as a
huge transmitting antenna and emits electromagnetic radiation over a range of frequencies. This energy is discharged
into the wave-guide where components of that energy are
reinforced at the resonant frequencies. There is a constant
supply of this energy from lightning as there are, at any
given time, 40-50 lightning strikes per second worldwide.
The measurement of Schumann resonances has various
applications in lightning detection and analysis, monitoring of the ionosphere and space weather and monitoring
of climate via global thunderstorm activity.
(The phenomenon of Schumann resonances has been
hijacked by “alternative” medicine practitioners and New
Age people who attribute all manner of healing properties to these electromagnetic waves which simply cannot
interact with the human body because of their extraordinarily long wavelengths. Unfortunately, a vast majority of
on-line material to do with Schumann resonance is of a
non-scientific nature).
Natural VLF radio signals from lightning and
elsewhere
Lightning discharges generate a rich variety of electromagnetic radiation as a broadband pulse, including light,
radio waves, X-rays and gamma rays, although much of
the energy of the emissions from lightning is in the VLF
frequency range (3kHz-30kHz) with some in the ELF range
(3Hz-3kHz, the atmospheric science definition of ELF being different to the conventional ITU one).
VLF and ELF waves can propagate over long distances by travelling in the natural ELF and VLF waveguide
formed between the Earth and ionosphere. In addition to
propagating beneath the ionosphere, some ELF and VLF
siliconchip.com.au
signals can exit the ionosphere where they will follow the
magnetic field lines of the magnetosphere. They can reach
10,000km or more above the Earth before re-entering at a
different location.
There are several different types of emissions possible,
which are characterised as static, tweeks, whistlers, the
chorus and hiss. Note that tweeks, whistlers and the chorus start out as static but by the time they are received they
have changed because of the complex path they have taken
to the receiver.
Lightning strike “static”
Lightning strike static,
sometimes incorrectly called
sferics (derived from “atmospherics”) are the signals from Spectrogram of static
lightning that most people signals. Audio of this signal
will be familiar with, as they can be heard at www.
are the same sounds as re- spaceweather.com/audio/
ceived on an AM broadcast inspire/1lowdenssfer.mp3
band radio during an electrical storm. They sound like constant crackling and popping,
somewhat like the noises made when someone eats potato
chips. The static signals are from lightning strikes within
about 1,000km. They are characterised on a spectrogram
(frequency on vertical axis, time on horizontal axis) as vertical lines indicating that all frequency components in the
signal arrived at the same time.
NOTE: The term sferic should probably apply to all
electromagnetic signals that come from lightning, not just
the “static” described here; however this static is often incorrectly called a sferic. While that is true, there are other
types of sferics as well.
Tweeks
Tweeks are lightning radio
emissions that have travelled from around 2,000km
or more distant, within the
wave-guide formed between Spectrogram of tweek
the Earth and the ionosphere. signals. Audio of this signal
The ionosphere varies in its can be heard at www.
properties through its thick- spaceweather.com/audio/
ness so some frequency com- inspire/3tweeks.mp3
ponents will travel faster
than others. This is akin to how light travels slower in water
or glass than in free space and explains how the colours
of a rainbow are generated with a glass prism – some frequencies emerge sooner than others.
Tweeks have a vertical line at high frequencies but if
you follow the line down you will see it curves off to the
right, indicating that the
lower frequencies have
been delayed in their arrival compared with higher
frequencies. This results in
a somewhat musical quality
to the signal.
Spectrogram of whistler
signals. Audio of this signal
Like tweeks, whistlers can be heard at www.
have a musical quality due spaceweather.com/audio/
to the propagation of dif- inspire/7purewhist.mp3
Whistlers
May 2016 23
ferent frequency components of the signal at different velocities leading to different frequency components of the
signal becoming offset in time. It is the interaction of the
signal with the plasma environments of the ionosphere and
magnetosphere that cause the time delay for the different
frequency components. Whistler signals travel along the
magnetic field lines of the Earth and can go to the opposite side of Earth and return. Whistlers were discovered
in 1886 when a previously unknown noise was heard on
phone lines.
Chorus
Two types of “choruses” can be occasionally be listened
to on VLF radio, the dawn chorus and the auroral chorus.
As the name implies, the dawn chorus is best listened to
at dawn and can resemble the dawn chorus of birds, although it has also been described as having a sound like
dogs barking or squawks from flocks of birds. It consists
of a wide variety of overlapping sounds.
Its presence is dependent upon geomagnetic activity such
as the emission of a solar flare from the Sun. The auroral
chorus is generated within the aurora and can be heard
in areas close to where the aurora occurs and of course is
also strongest during geomagnetic activity. A recording of
a “VLF auroral chorus” can be heard at https://youtu.be/
FQdrcDyYRiQ
Hiss
Hissing sounds are typically emitted via the aurora and
are high-pitched when converted to audio. Hiss can also
originate in the magnetosphere, including the bow shock
region where the incoming solar wind impacts with the
magnetosphere.
Above thunderstorm electrical phenomena
There is currently intense interest in the relatively recently-discovered electrical phenomena that occur above
thunderstorms, usually at altitudes of 50 to 100km. These
electrical discharges are triggered by conventional lightning
and have a variety of types that have come to be known as
sprites, elves, blue jets, halos, trolls and several other types.
Collectively they are known as transient luminous events.
1973 over the South China Sea but when he reported it he
was not believed as “everyone knew lightning went down
not up”. Other sightings were later made and reported by
other pilots. There is also an anecdotal report from 1730
by Johann Georg Estor of optical activity above a thunderstorm which is interpreted as a sighting of a sprite. It was
not until 1989 that Jack Winkler of the University of Minnesota recorded such a discharge by accident when he was
looking for something else.
The phenomenon became known as a sprite after the
elusive mythical creature with magical powers. Only very
powerful lightning causes them and they are much weaker electrically and of shorter duration than conventional
lightning.
The first images of sprites from space were taken incidentally as part of thunderstorm videos taken from the Space
shuttle during the period 1989-91. Israeli astronaut Ilan
Ramon was on the ill-fated Columbia space shuttle crash
in 2003 and specifically sought to capture sprite images
from space. He managed to transmit several images back to
Earth. Remarkably, his camera was found among the crash
debris and more images were recovered.
Japanese astronaut Satoshi Furukawa was moved to
continue Ramon’s legacy and attempted to capture more
sprite images from the International Space Station in 2011.
He captured images of six sprites and Ramon’s work now
continues.
Only one in 10,000 lightning events leads to a sprite.
They are believed to be caused when the intense electrical
field created by stronger positive lightning bolts causes a
sympathetic electrical breakdown of the upper atmosphere.
Sprites can reach all the way to the top of the ionosphere
but start at around 50km altitude and last around 17 milliseconds. A halo occasionally precedes a sprite (see diagram) and lasts around one millisecond.
Elves
Discovered in 1994, ELVES is an acronym standing for
Emissions of Light and Very low frequency perturbations
due to Electromagnetic pulse Sources. Triggered by lightning, they are a flat, disc-like discharge around 400km in
diameter and last for about a millisecond.
Sprites
Jets
An electrical discharge above a thunderstorm was first
observed by a US Air Force pilot named Ronald Williams in
A variety of “jet” phenomena have been observed such as
blue jets and gigantic jets. They were discovered in 1994.
An elve over the South Pacific and two sprites over
Australia, captured by astronaut Ramon in 2003 from the
Columbia which sadly later crashed killing all seven crew.
A montage of a variety of forms of sprites as seen from
jet aircraft on specialised missions to photograph sprites.
Image source: NHK.
24 Silicon Chip
siliconchip.com.au
Lightning detection and tracking
Every time there is a lightning
strike, an enormous amount of RF
energy is produced. The frequency
ranges from the proverbial “DC to
Daylight” but the low-frequency energy is attenuated less
than high frequencies, therefore is easier to detect.
You’ll hear this yourself
as crackles and crashes on
any AM radio station that’s
not tuned to a local station
– in fact, even a strong, local
station can be all but blanked
out by a local thunderstorm with
lots of lightning. Having said that, the interference from a thunderstorm can be many hundreds of kilometres
away if the lightning strikes are big enough!
SILICON CHIP published a build-it-yourself hand-held lightning detector in the July 2011 issue which relied on detecting this RF energy (siliconchip.com.au/Issue/2011/July/
A+Portable+Lightning+Detector). All parts are readily available
and the PCB can be obtained from the SILICON CHIP on-line shop.
Looking for that burst of “static” is exactly the same process
which commercial lightning tracking services use, although they
use multiple detectors around the world and measure the exact time
the lightning strike is detected. By triangulation, they can pinpoint
the spot where the lightning struck to within a few hundred metres.
We published an article in the November 1996 describing the
(then) new LPATS lightning detection system, which used this exact
approach. We understand that the company behind this system has
now been incorporated into the Weatherzone group (www.weatherzone.com.au) but the theory of operation is much the same.
Weatherzone is a commercial operation (although it has a lot
of free weather data, including radar and lightning). On the other
hand, Blitzortung.org is a community of volunteer lighting detector
station operators, software developers and other system support
personnel who run a website which plots lightning strikes around
the world in real time.
A related site is LightningMaps.org who take the data from
Blitzortung.org and visualise it in various ways. Some links for
some third party Apps for mobile devices can be seen at www.
System Blue lightning detector kit for use with the
Blitzortung.org project. This hardware is not intended
for stand-alone use; it is designed to be connected to
the Blitzortung.org servers via the Internet. Detailed
information on this model is not currently available
on the web site but extensive documentation on the
previous System Red model is available for perusal at
www.blitzortung.org/Documents/TOA_Blitzortung_RED.pdf
siliconchip.com.au
Lightning strikes during a storm in south east Australia
on 29 January 2016. The lighter colours are the most
recent strikes (less than 20 minutes old) and the darker
colours represent strikes that occurred in the past (100
to 120 minutes ago). Map from www.blitzortung.org/en/
page_0/index.php You do not need to own the Blitzortung
hardware to view a lightning strike map.
lightningmaps.org/apps; a real-time map can also be displayed
in a browser on a mobile device without any app, www.lightningmaps.org/realtime
Currently Blitzortung.org has 500 lightning detector stations
around the world which are connected via the Internet and there
are also numerous servers to process the received data. Each receiver records the arrival of a lightning strike with microsecond
precision and the connected VLF receivers locate the position of
the strike based on algorithms which use time of arrival and time
of group arrival methods.
Anyone can join Blitzortung.org and set up their own lightning
detector station which must be made from the kit they supply. The
kit of hardware, which must be assembled, can be purchased for
under 300 Euros. “System Blue” is the current hardware model of
the detector.
Here are some links to some other DIY lightning detector circuits: www.lissproductions.org/wuhu_manual/2011/11/29/diylightning-detectors/ and www.techlib.com/electronics/lightning.html This circuit is said to be very easy to build (see picture):
www.techlib.com/electronics/lightningnew.htm
Another DIY lightning detector. See a video of operation
at www.techlib.com/files/ld1.avi
May 2016 25
Various above-thunderstorm electrical phenomena. As a rough guide of horizontal scale, this diagram could represent
around 150km or even more in extent. From http://la.climatologie.free.fr/orage/tle-english.jpg
These start at the cloud tops and rise to about 40 to 50km
altitude. Jets seem to be associated with hail activity rather
than lightning. You can see one over Darwin, NT in one
of the videos referenced below.
Others
Other transient luminous event phenomena include
trolls, gnomes and pixies.
Videos of transient luminous events
The video at http://esamultimedia.esa.int/images/
ISS/2005-09-27_Lighting_story/ISS-lyn.avi is an animation of a variety of transient luminous events.
“Rare Lightning: Giant Red Sprites, Elves and Sprite
Halo’s Real Time Video” https://youtu.be/D7mqs6fng7o
“Extreme Rate Lightning Phenomena - Sprite Documentary - World History” https://youtu.be/Vzz0QoCkAlU
“Lightning Sprites phenomena” https://youtu.be/FGMU89tcqeA
“Rare Lightning: Giant Red Sprites, Elves and Sprite
Halo’s Real Time Video” https://youtu.be/D7mqs6fng7o
“Blue Jet (Transient Luminous Event) - Thunderstorm
Darwin 8th Dec 2015” https://youtu.be/zLYPKuoxH1c
“Upper Atmospheric Lightning: Gigantic Jet” https://
youtu.be/gk9ju2WUY5Q
Conclusion
There is a wide variety of interesting electrical phenomena in the Earth’s environment and these can be responsi26 Silicon Chip
ble for both spectacular visual displays and can also affect
radio listening. Among these are the electric field of the
Earth, lightning, the aurora and VLF radio. The amateur
can make many interesting observations themselves with
simple equipment.
NOTES: SILICON CHIP has no commercial association
with the manufacturers of any of the devices mentioned
in this article. Also, take all appropriate safety precautions
when undertaking any possible observations described
herein. Circuits referenced that were not designed by us
have not been tested by SILICON CHIP.
Auroral photography from central Victoria taken at Little
Desert National Park by Mark Sansom. It was taken with a
Sigma 17-70 lens set to 17mm, f/2.8 and ISO3200 with a 30
second exposure on 13th October 2012.
siliconchip.com.au
Viewing the Aurora Australis from southern Australia
Kp lines for a part of the Southern Hemisphere showing the likelihood of visibility of the aurora for a certain Kp
index. Note that the Kp is just an indicator, and just because a certain Kp index is achieved it does not mean that
the aurora will be visible. Conversely, the aurora may be visible at a Kp of less than what appears on the map for a
certain location.
While either of the polar auroras are best viewed close to the
not indicate that the aurora will definitely be visible. Rather, Kp inpoles, it is still possible to view them from southern Australia
dicates that the aurora is likely to be visible in a given region for a
under certain circumstances with a camera and the Aurora Auscertain index based on past experience. Zero represents little or no
tralis can sometimes even be viewed with the naked eye. The
auroral activity while nine represents a major geomagnetic storm.
viewing can be surprisingly good, even if not as good as if you
One website written by a Melbourne-based person to assist
were further south (eg, Antartica!).
mainly Southern Hemisphere auroral viewers is https://auroIt is important to choose a night time viewing location which
ras.live It consolidates the main numbers of interest into a user
is far from light pollution sources, such as cities and towns and
friendly format and the web site is also mobile friendly so can be
with a clear view of the southern horizon (make sure you really
used on a smart phone in the field.
are looking south – check with a compass or smart phone App).
There are many other websites and Apps covering data related
For personal safety it is best to have someone with you and also
to the aurora and “space weather” in general. One App which you
tell others where you are going and when you are expected back.
might want to look at is “Aurora Forecast” which is free for iOS
The best time for viewing can be determined by a number of
and Android but there are many others. Just search the App Store
smart phone Apps and web resources. There are certain “space
or Google Play with the term “aurora”.
weather” numbers to look for that should give the best viewSome popular websites for forecasting are www.spaceweather.
ing. Numbers to be considered are as follows:
com/, www.aurora-service.eu/, www.auroBz, the strength of the interplanetary magra-service.net (Southern Hemisphere) and
netic field in the direction of the rotational axis
www.sws.bom.gov.au/ (Australia’s Bureau
of the Earth. It is created by waves and other
of Meterology). Maps of predicted aurora
disturbances in the solar wind when it interacts
locations for both hemispheres can be seen
with the Earth’s magnetosphere. The more negat www.swpc.noaa.gov/products/auroraative the number the better the aurora will be.
30-minute-forecast
Speed and Density of the solar wind. In
Some videos of auroras from southern
both cases the higher the number the better,
Australia and New Zealand are as follows:
as a greater speed and density of the solar
“Aurora Australis (G4) on 17-18 March
wind will see a stronger interaction with the
2015 at Geelong, VIC” https://youtu.be/
magnetosphere.
esAUdlIUnFM, “Aurora Australis (Southern
Kp is the planetary index and is represented
Lights) Point Addis, AUSTRALIA 21 Dec
by a scale from 0 to 9 and is a measure of the Predicted location of auroras
2015 ” https://youtu.be/_4PGJwqC_90,
level of geomagnetic activity. The higher the updated every 30 minutes at
“Aurora Australis : Camera VS Naked Eyes”
Kp index the further from the poles the aurora www.swpc.noaa.gov/products/
https://youtu.be/oq6GO-i7t4Y
SC
should, in principle, be visible however it does aurora-30-minute-forecast
siliconchip.com.au
May 2016 27
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