The idea of listening an emission originated not from a radio operator, not from a modem, not from any device finds us a bit surprised at once.  May be because we would expected to find noise, electric discharge or something like that. But when we find tone, whistlers and other, superimposed as a human voices chorus, we are astonished thinking nobody has generated them. When we discover the links between these emissions and the magnetic field of the earth, the atmospherics storms and physical atomic reactions which happen several thousands kilometers from earth, we are stupefied.
   This article goes into the description of phenomena occurring in the lower part of VLF that is from 0 to 15 kHz, also named “Nature Radio”. Radio signals with wave lenght of tenth or hundreds kilometers, together with the electromagnetic component, take with them the earth voice and the space around it.

   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.

   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 magnetism

   The planet 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.
   The true cause of the terrestrial magnetism is unknown yet: ancient theories explained the inside of the earth as done of magnetic material. Now, this sounds impossible, because of the high temperature in the fluid center of the earth. The magnetic field is originated by electric currents produced from the flux of the fluid center of the earth.
   The plane show the geomagnetich chart of the Italy. Click here to see full screen detailed charth.

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 discharge.
C) The atmospheric electric discharge.

  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.
   Potential grows with altitude by adding, in the upper parts  of the atmosphere, about 400.000 V.

    The 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 volts.
    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.

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.
   This empty isn't really  empty, in fact inside it we are inside the magnetosphere with plenty of charges: under its influence the radio signal begin its path  driven by the magnetic field flux, an hypothetical  duct where it can travel without any smoothing. 
   Figure show in a "S" point the lightning (the origin static) and in a "W" point the arrive of the whistler.

 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

   The very 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 it.
   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.

MICHEL ANDRE' for translation.
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