IN THE ENGLISH
TRANSLATION SOME PARTS WERE PUT APART BECAUSE AT THE PRESENT THEY ARE TECHNICALLY
OVERTAKEN. THE PRESENT TREATMENT DOESN'T WANT TO BE A COMPLETE ONE:
SOMEWHERE ELSE THIS TREATMENT IS MUCH MORE DETAILED: LIKE IN A WEB SITES
OF THE LONG WAVE CLUB OF AMERICA, THE
INSPIRE PROJECT OR STEVE
MC. GREEVE SITE. THIS ONLY WANTS TO BE
A GENERAL INTRODUCTION ADDRESSED TO THE BEGINNERS.
OF NATURAL ORIGIN, by IK1QFK
We don't know who was the first one to discover the presence of these signals; but, given their maximal intensity around 5 kHz, the discover was connected to the use of these frequencies. Telephonic societies noticed that first, but they couldn't explain the origin of these interference (the long air wires which unawares were used as antennas). First telegraphists noticed by experimenting with low frequencies (thinking that to cover long distances were necessary long waves). First military spy electronic department noticed that by using triodes done to amplificate currents between two earth connections (this in order to intercept communications inserted in the ground). Maybe even us, we noticed that when unawares we have detached the ground in a microphone connected to an audio amplifier and we thought at that moment to hear our neighbor doing works at his home with his electric drill.
Despite the many occasional meetings with those signals their origin was finally discovered in 1957 after works done during the international geophysical year: during that period, when scientists of different countries did together a scientific research program just in order to study this phenomenon, 66 nations participated to the study of geomagnetic phenomenon, of meteorology of atmosphere, of cosmic rays and of many other phenomenas.
Begun on the 1st of July 1957 in correspondence with the maximum sun activity period, ended up december the 31st 1958 a lot of money was spent and many devices as observatory astronomic, probe balloons , meteorological missiles and satellites for the study of high layer of the atmosphere. After this study an answer has been given to the many questions up to that moment unanswered about the provenance of natural origin signals.
ORIGIN OF THE
The explanation of this phenomenon concerns few physical fields as the terrestrial magnetism , electrostatic energy in the atmosphere, and many others: we will examine them one by one in the part dedicated to the genesis of these signals in order to understand which device connects between then different fields.
| A) The terrestrial
on which we live is as a big magnet: with the two poles north and south
different and opposite, they are not the same as the geographic poles and
their positions changes from time to time. This makes the north measured
with the compass is different from the geographic one called magnetic
declination. Lines that connect points with the same magnetic declination,
are called isomagnetics and they are representative of the not regular
distribution of the magnetic field.
B) The magnetosphere
The strength line of the earth magnetic field extend around our planet in the space so creating the magnetosphere: the place where generate part of the radio listened signals of natural origin. This region is 200 and further Km of altitude where the movement of charged electrical particle is controlled basically by the earth magnetic field and is not conditioned by the collision with themselves. The magnetosphere in its part in front of the sun, extends over 57000 Km about ten times the earth radius; in the other side of the sun it must probably be of hundreds of earth radius. The long shaped look is originated by the solar wind influence, or solar plasma which is formed by electrons and proton swarms, emitted by the sun, that strongly compress the magnetosphere in its sun oriented part.
During the international geophysical
year was also discovered the presence of huge radiation zones inside the
magnetosphere: these regions called "Van Allen's band", contain protons
and electrons full of energy cough by the earth magnetic field. Even the
sun is object of research for the magnetic field: it has two poles which
sign invert during the solar activity during the 11 years cycle. Some of
the space missions as Ulisse one, did fly over starting from 1996 the two
magnetic poles of the sun; the measuring from a point of view never
tested before and studying the intensity and the orientation of the
interplanetary magnetic field, allowed to have data confirmed and sure
and not simply supposed by theories. Now, done this long but useful
introduction, we can now start by describe the phenomenon which is
at the head of part of these signals, that is the atmospheric electric
|C) The atmospheric electric
The earth crust
is composed by minerals and salty liquid (the oceans) and can so be considered
very good conductor. Measuring done around it show in a clean sky day the
atmosphere is home of a electric field with intensity of 100 V/m,
produced of a superficial negative discharge.
effect of the earth magnetic field is to put on movement the atmospheric
charges originated by the ionization produced by the cosmic ray
and by the solar wind taking down the positive ones. The density of this
current is very low ( few amperes by square meter) but, given
the big earth surface the total current that goes
on the ground has an intensity of about 1800 A. this kind of
“dark discharged” would be able to neutralize the earth charge in
few hours if there were no about 300 storms per day on the earth (hundred
lightning hit the earth every second).
| When a
storm is approaching the discharged are distributed as shown in picture:
the clouds base is strongly negative, so, given its closeness to the ground
of 2-3 Km., under it is it formed a en electric field of about 1000 V/m
with a potential difference between clouds and ground of 2 – 3 millions
This field versus is opposite to the existent one in a clean sky day and, given its intensity, does intense ionization processes of molecule in the zone under the could making it conductor: this allows the making of the lightning.
KIND OF SIGNALS
|A) The spherics
The discharge is very violent and can easily arrive to 10.000 Amper, taking to the earth something like ten coulomb.
Sound and light effects during the lightning are not everything; there also is a big electromagnetic component which is radiated end which is named spheric or static.
It's a electromagnetic signal of wide
spectrum, composed by dumped waves of relatively low frequencies,
rich of harmonic components: it spreads in a long distance because of the
earth wave; this has big influence in order to spread in low frequencies.
Even Shumann resonances are involved: they happen because of the
natural wave's guide formed by the earth and by the ionosphere (freq.
7.8 Hz and harmonic frequencies 15.6, 23.4, 31.2 Hz ).
As in a wave's guide the right frequency signals can travel inside this natural channel with very low attenuation values. The spheric decametrics components spread by ionosphere reflection the same way as the radioamatorial signals. A long time ago it was supposed that the distance a spheric up to 15 kHz could run was about 2000 Km, but coordinated listening in VLF denied this data: done on the occasion of the Shuttle fly (Sepac mission) they allowed the verification the spherics presence even at a distance of 7000 Km denying a theory which seemed a good one. This episode confirms that VLF are a unexplored field yet, so with plenty of satisfactions for who study it.
Tweeks are spherics in the atmosphere below the ionosphere. They sounds like a bird's sing, in a frequency range of 1-7 kHz. In a spectrogram they result like a descending tone on the order of 25 to 150 mS. Tweeks are normally heard in the evening, after the sunset.
|B) The whistlers
One part of the
spheric spreads inside the ionosphere, and we as can notice (if we are
listening in medium waves) the approaching of a storm even if it's still
very far. The other part of the spheric escapes the ionospheric barrier,
it overtakes it and keeps running its path in the empty.
If the attenuation is low , also because of
the many kilometers done, it's not possible to say the same thing
about the dispersion that is under that influence: this dispersion
is caused by the interaction between the spheric end the magneto
plasma which is in the ionosphere, and its intensity distorts
the original signal.
The result is a whistler lasting few seconds with a frequency going down from 5 - 6 Hz down to 1 Hz; they also exist whistlers with a growing frequency but they are less frequent compared to the other ones. The effect is very suggestive and whistlers modulated and places upon make think the noise made by the dentist's drill, or the sounds of science fiction movies as Star Trek..
This kind of propagation that we can call "magnetospherics", happens in VLF only, and we haven't noticed similar behaviors in higher frequencies: practically it happen in a North South direction connecting between them geomagnetic conjugate point. This doesn't mean that we can calculate a whistler propagation path on a geographic map.
C) Discrete Emissions
We can also find in this weird world
discreet emissions which last less then whistlers, but linked to
them for a common origin: in the closest zones to equator they interact
with themselves whistler waves and electron plasma feeds energetic
shaped: like in a particle accelerators (cyclotron), the electrons which
are present under the effect of the terrestrial magnetic field and
for the action of the variable electromagnetic field coming from
the whistler wave , are under the effect of an acceleration, which
generates falling or rising tones; when these two signals follow at the
same time, they origin a hook which lasts not more then 2-3 seconds.
When the natural radioactivity is strong two or more of these signals can lay on, in this case they are called chorus. It's known that these discreet phenomena happen not just with spherics but also with telegraphic signals under 20 kHz; these artificial emissions, strong as intensity, are obviously of military origin. The terminology of these signals takes its origin from their spectrum analysis, done by calculator help with special FFT programs.
D) Signals linked to seismic phenomena
long waves are studied by seismic observatories. The score of the
study of earth currents and of VLF and ULF emissions occurring together
within seismic phenomena, is establishing the relationship between
those signals and and the earthquake: the score is to be able to
forecast, one day, those terrible events.
Research done up to the present days are divided in three frequencies bands:
- Between 0,01 and 12 Hz: particularly between 3.1 and 12 Hz; the presence of weird noises would be connected to those phenomena.
- Between 30 and 150 Hz, where are controlled the level of background noise and the level of the carrier of LW Broadcast station .
The origin of these signals is in the compression of the rock layer that contain quartz and the following electromagnetic emission before and during the earthquake. These episodes, which are quite rare, are difiucultly observed; since a real monitoring is deficient, the probability of meeting these signals is random and not frequent.
There are two systems to receive natural radio emissions in VLF: by frequency conversion or direct reception.
A) Frequency conversion
Usually done with a IC mixer for HF
(SO42P), consists of amplitude modulation of a carrier (generated by quartz)
by VLF signal deriving from the antenna. VLF converters on the market the
frequency carrier is chosen on not crowded HAM band (28 MHz) or lower frequencies
(2-4 MHz). To the carrier (at example of 28 MHz) are hooked, up and down,
all frequencies from few Hz to a certain value which arrives in commercial
converters to around 1 MHz (changing from model to model), quite as side
bands. Converted as said, the signal can be normally listened with a common
HAM receiver, using all tool characteristics and performances.
Carefully: the coupled condenser at the SO42P input, that allows the passage of the signal to the converter, decides also the lowest cut frequency.
It's better to get sure that the frequency is not to high because in that case are not included frequencies lower than 10 kHz. A dynamic microphone connected instead of the antenna to the converter, could be easily used by tuning the signal at 28 MHz in AM and talking in the microphone you can hear your voice: if this doesn't happen it's impossible to hear the natural origin radio signals of maximum intensity of 5 kHz which is just a little bit upper the human voice band (1 kHz): in this case the coupled condenser must be replaced by another one more powerful.
Two are the benefits of this systems: the first one is the possibility of using the side bands and even the CW tight filter to select portions of signal otherwise lost confuses between many other signals in a wide band listening. The second benefit consists of approaching even the frequencies near to few Hz in CW mode: the DC becomes in fact tone at 28 MHz and eventual signals, even a few hundreds Hz, are around this carrier and they are so perfectly receivable.
B) Direct reception
The reception without frequency conversion
is possible because of the these emission are inside the band listenable
from the human ear. The problem is to receive this signal with a long wire
antenna, with a whip or between two ground connections and to amplifier
The simplicity of this operation made possible the diffusion of electric field portable receivers at a very low price.
They consists of a stilus antenna and of a little audio amplifier which amplifies the stilus signal and sends it to an auricular; the only existent control is the amplification gain, and in more complicated models, the frequencies choice of cut between changer. The signal listened is wide band type and frequency limits are decided from our ear.
Since these receivers are extremely easy to carry, it's possible to do listenings in open country far from electrical noises, trying to use at the best dB available. Even if the listening doesn't give computerized analysis, we can have the beauty of the signals different from any others.
Since the radioamatorial experience has begun, we keep discovering some new applications about the big possibilities radio offers us every day in every way. VLF are just a little part of all of this but they take us closer to human life of other fields: the presence of magnetite in some migrant birds nostrils, and in some parts of the human brain connects the existence of these signals to our life.
It is possible to suppose a link between these signals and the perception called "sixth sense" like the nervousness in some animals just before the earthquake; in fact electroencephalogram shows that our brain uses electric impulses in ULF band. About this matter we don't have many contributions and this could be a stimulus in order to make a careful study of the subject.
ANDREA BERTOCCHI and
MICHEL ANDRE' for translation.
Scienze tecniche, De Agostini editions
Fisica e laboratorio, Atlas editions
Fisica moderna, Cremonese editions
Dizionario delle tecniche e delle scienze, EP/SAIE editions
Stelle e pianeti, Arnoldo Mondadori editions
Top secret radio 1 and 2, CD editions
Tuttoscienze, LA STAMPA daily paper insert n°573
Radiorama, Medicea editions
CQ elettronica, CD editions
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