Are interesting research on ionospheric disturbances induced by astronomical events (solar disturbances, X-SIDs, meteor bombardment, etc...). and natural phenomena due to the activity tropospheric radio.

The frequency bands of ELF (Extremely Low Frequency), VLF (Very Low Frequency) and LF (Low Frequency) cover, respectively, the frequency ranges from 300 Hz to 3 kHz, 3 kHz to 30 kHz, 30 kHz to 300 kHz.


Nature Radio Band


In this region of the electromagnetic spectrum is not admissible cosmic radiation due to the shielding effects of the ionosphere.
It is possible, however, interesting experiments related with local, natural and astronomical events that affect the Earth's upper atmosphere and magnetosphere.
Our planet is a natural source of radio waves at radio frequencies emitted by the atmosphere when it encounters particles from deep space, as a result of interactions within the same atmosphere as a result of geophysical phenomena related settling of the Earth's crust.
This experiences, achievable with relatively simple and cheap equipment (see pictures below), are within the reach of any electronic experimenter with a minimum of experience in the assembly of circuits.

There is a large and interesting literature available on the web regarding the experiments that can be conducted in ELF-VLF band to monitor the natural electromagnetic phenomena (activities often called "Radio Nature"), some of which are sponsored by corporations and famous universities (such as the NASA).

Also interesting are the studies on the possible correlation between the increase of the background noise picked up by the antenna (in this frequency band are mainly used wire antennas, whip, ferrite antennas or large loop) and the occurrence of telluric phenomena (the so-called seismic precursors) or meteorological events. In the latter case affects record and correlate possible changes in background noise (along with their spectral information) with the increase in certain periods of the year, the "bombing" had the most intense meteor showers that intersect the Earth's orbit .

The following images show some examples of projects related to amateur monitoring tools in ELF-VLF band particularly suited to gain experience with the reception, recording and interpretation of low-frequency electromagnetic phenomena related to natural terrestrial and astronomical events.

Prototype of an ELF-VLF correlation receiver (bandwidth from 1.5 to 7.5 kHz) tested by RadioAstroLab and used for about a year of continuous observations and automatic natural phenomena related to ionization of the atmosphere and the "meteor bombardment".


Prototype ELF-VLF receiver

The image show the ferrite antenna test and the internal circuitry of the VLF prototype receiver.


The system was placed in an isolated mountain village 100 meters away from a rural house served by electricity (for the laptop for the data recording). The installation site was checked for the effective and quiet "cleaning" by electromagnetic interference mainly due to the distribution network at 50 Hz able to significantly disturb the reception of weak natural radio signals. The receiver is powered by rechargeable batteries that provide autonomous operation for approximately a month of continuous operation.

The following spectrograms show the frequency response of the system and its excellent immunity to radiated and conducted from the environment test room (which is very noisy due to the presence of computers, switching power supplies, fluorescent tubes, electrical appliances). The tests were carried out before placing the instrument in the planned site for observations.

It is obtained the first recording ("radio silence") by connecting the receiver input to a resistor of value equal to the DC resistance of the antenna. When you connect the antenna spectrogram shows the information contained in the second graph.
It recognize signals of artificial origin, such as those intermittent characteristic of three stations of the Russian network ALPHA used for connection with world wide military submarines (now switched off), signals "doppler" produced by the passage of cars along a road post to about 300 meters away from the laboratory and the typical harmonic distribution lines for electricity to 230 V – 50 Hz (lower graph). Are visible signals of natural origin caused by atmospheric electrical discharge sand other local signals of unknown origin.


spectrograms on environment disordersspectrograms on environment disorders


The tools used in these frequency bands are relatively simple when compared to traditional radio devices, easily achievable by amateurs with a minimum of practice in the assembly of electronic circuits.

It is basically audio preamps (some with wider bandwidth of the limit audible) in low noise, input circuit adapted and optimized for different types of antennas (whip, rows, ferrite magnetic loops of different sizes).

The output signal can be a voltage directly proportional to the power associated with the incident radiation (detector stage quadratic amplitude) or the signal is not revealed that it is directly sent (if the bandwidth allows) to an audio magnetic recorder or applied to the input of a PC sound card, so you can analyze, in real time, the basic characteristics (intensity and spectrum) of the incident radiation with the help of one of the various (and excellent) programs available for free on the web.
The choice between one or the other (preferably both) possibility is linked to the type of research that you want to lead and to the characteristics of the received signal that should be noted.

To obtain meaningful results (and scientifically interesting) it is important that different experimenters are organized to coordinate observations using apparatuses with similar characteristics positioned in different places (diversity reception space) so as to detect any correlation between the response to the same events, making "safest" and documented recordings.


Study of SIDs (Sudden Ionosphere Disturbance).

An indirect method widely used to study solar flares, then reveal the so-called ionospheric effects, provides for the permanent monitoring, in the VLF band (typically at 150 kHz frequencies below), of a strong and stable signal from a fairly distant radio station, recording the intensity emission variations in time.

When experience intense solar flares are emitted beams of X-rays that hit the Earth's ionosphere (the layer of electrically charged particles of the upper atmosphere), "disruption" to its ability to reflect radio waves and causing a marked fading, with progressive and slow decay of intensity of the received signal. This system is considered enough reliable for the monitoring of the flares. To ensure a correct reception of the signal is essential to use a stable receiver in frequency and with the circuit of the AGC (automatic gain control) disabled: only in this way there is the certainty that the variations of intensity of the signal are not caused by instability of the receiving system.

The solar flares also emit charged particles at high speed that in a couple of days reach the surface of the Earth (while the radio waves and X-rays takes about 9 minutes) interacting with the Earth's magnetic field and deforming it. Some particles are channeled and "guided" along the lines of force of the field through the poles, producing the characteristic phenomenon of the polar aurora. The distortion of the Earth's magnetic field produces the phenomenon called "geomagnetic storm" (geomagnetic storm) observed with the magnetometers.

Any body radiates electromagnetic waves in proportion to its physical temperature: this mechanism is called thermal radiation since it is closely linked to the physical temperature of the object body (Planck's law).
The radio component of the "quiet Sun" is a thermal emission from the hot ionized gas, corresponding to a surface temperature of the photosphere equal to about 6000 K, easily detectable in the SHF band (where the mechanism of emission is more intense) also by amateur installations, using.


Sudden Ionosphere Disturbance


Schematization of the procedure of the study of SID phenomena that uses a continuous monitoring of certain radio stations operating in the VLF band with the aim of highlighting variations of signals reflected from the ionosphere caused by fluctuations of its ionization characteristics induced by transient phenomena of solar origin.


Below we see a block diagram of a typical VLF receiver suitable for monitoring of SID events.


VLF receiver for SID events


 Typical recording of SID events


Activities strongly supported by a group of enthusiasts, mostly amateurs, belonging to the Solar Division of the American Association of Variable Stars Observers (AAVSO). Worldwide group consisting of little more than 100 observers (among them some Italians).

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Material on the study of electromagnetic phenomena induced by "bombing" meteorite on Earth's upper atmosphere with both passive (receiving ionospheric disturbances caused by the ionization produced by the traces rain) and active (excitation tracks meteor radio signals and focused study of related radio-echo) can be found on the following sites:


"Radio Nature" is a fascinating and easily accessible area of study and research, largely "beaten" by amateur radio astronomers and amateur radio confront issues related to the reception of the natural electromagnetic phenomena that occur in the lower part of the spectrum.

The phenomenology of emissions focus on many and varied aspects, ranging from noise due to weather phenomena to those ionosphere caused by interactions of Earth's upper atmosphere with the solar wind or with swarms of meteors and fireballs, to issues related events and earthquakes and another events.
In the connections shown (real points of reference for lovers of the sector) are widely developed the theoretical reasons that justify and encourage this type of research, illustrated and described easily (and economically) several projects of antennas and ELF-VLF receivers feasible by fans equipped with a minimum of practice in the construction and development of electronic circuits.

The following site is edited by Renato Romero: recommended for anyone involved in the study and observation of the natural electromagnetic phenomena (not necessarily correlate with astronomical events) in ULF-ELF-VLF bands:


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