In this section we are going to dive in to the details of Very Low Frequency (VLF) ionospheric monitor technology, theory of operation, design and construction notes. We will also cover some hints on reading our plotter graphs, typical events and how to interpret them.
To start with, we briefly fly through the fundamentals. As most of us aware, our planet, the Earth, all its inhabitants, flora and fauna, every live creature including ourselves are directly dependant and rely on our brightest star in the sky, the Sun. Solar energy is the essential element in maintaining life on Earth, giving us light and heat, allowing plants to grow and produce oxigen.
However, carried by the solar wind, the power of solar energy can also bring some significant dangers, such as charged particles' radiation hitting the Earth's atmosphere, high level of UV radiation and so on. Fortunately, our planet has a natural protection from the harmful emissions. First of all, that is the Earth's athmosphere, which extends to around 10 000 km in to space from the Earth's surface and acts as a primary shield, absorbing about 23% of solar energy. Another protection layer is the Earth's magnetic field or magnetosphere which adds more guard from solar particles and cosmic rays.
For the purpose of this section, we will review the athmosphere, its structure and layers, with the focus on the ionosphere, its interraction with solar wind and the Earth.
Most planets in our Solar System have atmospheres, although with very different properties. Generally speaking, an atmosphere is a gaseous structure surrounding a planet and kept in place by a planet's gravity. The Earth atmosphere is composed mainly of nitrogen (78%) and oxigen (21%), in comparison, on Mars, the atmosphere is made mostly of carbon dioxide (96%)
Our atmosphere is a composition of overlapping layers with different gas, temperature and pressure properties:
- Troposphere - the lowest layer, extending up to about 10-20 km above the surface. Most of the clouds and weather conditions are occuring in this layer.
- Stratosphere - the layer above, stretching to about 50 km. That is where the UV absorbing ozone layer resides in between 20-30 km.
- Mesosphere - extends from about 50 to 85 km. So called Noctilucent Clouds are observable in this layer during summer, as well as most meteors burns out there. The top of this layer is known as being the coldest in the Earth's atmosphere with temperatures as low as about -90° C
- Thermosphere - starts from around 85 km and extends to between 500 - 1000 km. International Space Station is travelling in this layer having an average altitude of about 400 km.
- Exosphere - the uppermost layer, starting from the top of the thermosphere and gradually fading out to the space at around 10 000 km
Although the part of the atmosphere, Ionosphere is unlike the above layers, is itself represented by several regions (or layers) and spread widely across mesosphere, thermosphere and sometimes exosphere. Ionosphere contains large number of electrically charged particles or ions, which are formed by solar radiation, hence the name. Ionosphere is a very active part of the atmosphere, it reacts quickly to solar wind and radiation, solar flares and Coronal Mass Ejections. Ionosphere can also respond to Earth based events, such as volcanic eruptions and major earthquakes. Ionospheric regions are able to change its characteristics rapidly, some regions are so dependant on solar radiation that can only exist during daylight and dissapear at night.
The main regions of ionosphere comes as follows:
- D layer - the lowermost region, starting at around 60 km and extending to about 90 km. The layer exist during daylight only and dissapears at night.
- E layer - the layer above D, it is extending as high as around 150 km. The layer normally stays 24h, however ions' density weakens during a night.
- F layer - the uppermost region. It is stretches as high as 500 km or sometimes higher. During the day the layer splits to F1 and F2 regions.
Ionosphere's ability to respond to various disruption factors makes it a great target for Earth based studies. In the next few chapters we will briefly outline how ionosphere's features were discovered and importance of that discovery to our modern world.
We are all get used and take for granted many technological advances available today to general population, some of them would be unheard of just a decade or two ago. Take mobile phones, high speed broadband, WiFi connectivity everywhere, portable satellite internet and navigation and many, many more. But how did everything start, what is behind of all those technologies that makes our lives so much different?
1800's was the period where many technological breakthroughs where achieved and many discoveries supported by theoretical background were made. Electricity was one of them, many scientists and engineers made their input in practical and theoretical works. That was followed closely by discovery of the electro magnetic waves in the second part of 19th century and then the world has changed to as never before.
Electromagnetic spectum is covering wide range of wavelenghts, starting from sub-radio Extra Low Frequencies (ELF) to Ionizing Radiation Gamma rays. For the purpose of this section we will concentrate on Radio spectrum' portion and quickly glance through the range of waves (frequencies) belonging to it.
The full electromagnetic spectrum is shown on the NASA Science image above. The traditional electromagnetic Radio Waves portion is considered to be from 3kHz to 300GHz and that is where all the possible wireless communications are taking place today (we omit here some very specific communications below 3kHz).
To move forward, we will briefly explain here electromagnetic waves' measurement units and how to find your way around them.
Electromagnetic waves' measuring unit base is Hertz (Hz), Radio Waves, depending on a band, are typically measured with adding kilo, Mega or Giga prefix. Such as WiFi frequencies are laying within 2.4GHz and 5GHz (Gigahertz) bands, it is equally possible to quote them as 2400MHz and 5000MHz (Megahertz) bands. FM radio works in the 87-108MHz (Megahertz) band and AM radios are working within 153-270kHz and 521-1620kHz (kilohertz) ranges.
An older way, however still very much in use, especially on higher frequencies, is to quote a meter as a base unit, hence the term wavelength. That is, FM radio frequency of 100MHz is equal to 3 meters, BBC4 AM radio broadcasting on 198kHz wavelength is 1515 meters, MSF time signal station broadcasting from Anthorn, UK on 60kHz has a wavelength of 5000 meters (or 5km). Very short frequency wavelengths are often measured in nanometers (nm), that is in use in fibre optics and visual light measurements.
A simple math hints to figure out wavelength of a given frequency:
- if frequency is in MHz: 300 divide by frequency, eg 300 divide by 100MHz = 3 meters
- if Frequency is in kHz: 300000 divide by frequency, eg 300000 divide by 10000kHz = 30 meters
To calculate a frequency of a given wavelength:
- if frequency is in MHz: 300 divide by wavelength, eg 300 divide by 3 meters = 100MHz
- if Frequency is in kHz: 300000 divide by wavelength, eg 300000 divide by 30 meters = 10000kHz
The calculations are based of a fact that radio waves are propagating at the speed of light of 300000 km/s
The following table is a quick reference to the Radio Waves (or Radio Frequency) spectrum. We have put together wavelengths vs frequencies, formal International Telecommunication Union's (ITU) spectrum allocation abbreviations and most common usage.
General characterization of the above allocations:
Very Low Frequency (VLF), 3kHz - 30kHz
Long range coverage (hundreds to several thousand kilometers, depending on application) in some cases global coverage. Penetrating sea water up to 40 meters deep, hence actively used communicating with submarines. Very large antennas and transmitt power required. Narrow channels' bandwidth (<200Hz). Radio propagation of this band depends on the state of D and E ionospheric regions.
Low Frequency (LF), 30kHz - 300kHz
Similar characteristics to VLF, especially at the lower end. Smaller coverage (up to 2000 km), but slightly better channels' bandwidth, allowing LF radio broadcast, which is now legacy in most regions considering high energy consumption and audience move to other platforms. Very large antennas and transmitt power required. Radio propagation of this band depends on the state of D and E ionospheric regions.
Medium Frequency (MF), 300kHz - 3MHz
Known mostly for its AM radio broadcast band, which is still in good use in many countries, also carries navigational and amateur stations' signals. Ranges differs dramatically between day and night, also between summer and winter months. Daytime ranges can extend up to several hundred kilometers, however during a night propagation enhaces, allowing longer distances, up to several thousand kilometers. Large antennas and high power are still required, however on much smaller scale than LF and VLF bands. Radio propagation of this band depends on the state of all three D, E and F ionospheric regions.
High Frequency (HF), 3MHz - 30MHz
Probably most well known band historically, accomodating many sorts of radio communications - navigation, government, military, amateur, broadcast etc. The HF band is very sensitive to the state of ionosfere and solar radiation. The band is known having full or partial radio blackouts (or loss of communication) during geomagnetic storms. Radio coverage is heavily depending on the state of ionosphere, solar activity and chosen operational sub-band. Coverage is ranging from several dozen of kilometers to a global coverage. Typical antennas are of much smaller sizes than lower frequencies bands and transmit power can be as low as several dozen of watts or less. HF band relies mostly on the F layer of the ionosphere and can be used for ionospheric studies.
Very High Frequency (VHF), 30MHz - 300MHz
Mostly local, near line of sight, <100km communication, however under favorable conditions coverage can extend to hundreds and (rarely, on the lower band section) thousands of kilometers. VHF is hosting the FM radio broadcast band, aeronautical navigation, satellite, amateur and land mobile communication. Lower section of the band (30 - 70MHz) can be used for ionospheric studies. The higher band section is rarely depending on the state of ionosphere and mostly penetrates to the outer space, however can be affected by major solar and geomagnetic events, such as aurorae.
Ultra High Frequency (UHF), 300MHz - 3GHz
Near line of sight, local coverage. Used mostly for land mobile, amateur, cellular, satellite, GPS and WiFi communication. Penetrates ionosphere to the outer space. Not generally affected by ionosphere's state, however satellite and GPS communication can be disturbed during major solar events. Higher band section is capable for broadband data applications.
Super High Frequency (SHF), 3GHz - 30GHz
Line of sight communication, typical applications are longer range microwave radio links (eg. between cellular base stations), WiFi and satellite. Not generally affected by ionosphere's state, however satellite and GPS communication can be disturbed during major solar events. Longer range channels can be affected by weather conditions (rain, snow). High bandwidth channels, capable of broadband data applications.
Extra High Frequency (EHF), 30GHz - 300GHz
Line of sight communication, typical applications are short range microwave radio links and radio astronomy. Heavily affected by weather conditions. High bandwidth channels, capable of broadband date applications.
As we see, the ionosphere plays an ultimate role in most of the radio communication bands, generally in the range of 3kHz - 70MHz, with some relatively rare partial effects on higher frequencies. Before moving further, let's come back in time and see how the ionosphere was discovered and how it affects radio propagation.
In the early years of radio communicaton it was widely believed long distance communication is only possible using the long wavelengths or frequencies shorter than 1.5MHz. In 1901 the first cross Atlantic radio transmission used 300kHz frequency, as well as in 1912 the legendary Titanic's radio station used 500kHz and 1000kHz frequencies for all of her radio transmissions. Although there were numerous suggestions of a sort of reflective layer it the atmosphere, it wasn't until 1920's when multiple reports from amateur radio experimenters started to appear.
As frequencies shorter that 1.5MHz were considered worthless for professional use, it was decided to grant a license for amateurs to conduct their experiments in certain portions of 1.5 - 30MHz spectrum. Quite soon it appeared that long distance communication is well possible on the HF band, using modest amateur level equipment and relatively small antennas. Further studies has indeed confirmed the existence of a reflective layer up in the atmosphere, which later became known as the ionosphere. Going further, different ionospheric regions were discovered and named as E, D and F
In simple terms, ionospheric radio propagation works by reflecting, refracting, absorbing or passing through the radio waves. That is caused by ionisation level of ionospheric regions or their parts and is directly dependent on the solar activity. Some frequencies, such as VHF/UHF and higher, are in general passing through into the outer space. HF frequencies are reflecting/refracting off the F layer, VLF/LF frequencies are reflecting/refracting off D or E layer and not reaching F layer at all.
As an example, take a radio transmitter transmitting on 15MHz frequency. The signal will travel into the atmosphere, through D and E layers of the ionosphere and reflecting back to Earth off the F layer (so called a single skip) . If ionization levels are high (solar activity is high) then multiple skips can occur providing long distance communication. Similar physics works on the lower frequencies, however there D and E layers are the reflecting layers.
Here we conclude this section of basic theoretical background. In the next section we will review several ways of ionospheric research and look closer at Very Low Frequency ionosphere monitoring.