Ionospheric sensors

Incoherent Scatter RADAR (ISR) - Informative videos

Informative videos

An informative video on RADAR in general and on the Incoherent Scatter RADAR (ISR) in particular, which is capable of detecting electrons "100 km Away".

https://www.youtube.com/watch?v=OnlYZdRZ_pw

Another informative video.

https://www.youtube.com/watch?v=9F4Oqwa30J4&t=3878s

Noting that, typically, Incoherent Scatter RADARs work between 50 MHz (HF)  and 1200 MHz (1.2 GHz) (UHF).

Note: "The Jicamarca Radio Observatory (JRO) is the equatorial anchor of the Western Hemisphere chain of Incoherent Scatter Radar (ISR) observatories extending from Lima, Peru to Søndre Strømfjord, Greenland)."

https://en.wikipedia.org/wiki/Jicamarca_Radio_Observatory

Figure 1

The Incoherent Scatter RADAR (ISR) - Theory

Measurement of electron parameters, H+(proton) and other ion signals based on spectroscopy of the RADAR signals scattered off electrons

The ionosphere is the ionized section of the Earth's upper atmosphere consisting of molecules and atoms being ionized or having electrons extracted due to solar radiation (UV photoionization). For instance, molecular oxygen and molecular nitrogen, atomic oxygen, hydrogen and helium may be ionized, thereby producing the corresponding positively charged molecules or atoms i.e., ions and a large content of electrons. Ions and electrons constitute the plasma. The total electron content (TEC) is one of the most significant ionospheric parameters, as electromagnetic signals transmitted in the ionosphere e.g., from GPS satellites, may bounce off the electrons and be deflected causing signal distortion and delay.

It is therefore important to continuously measure the total electron content in order to perform corrections on the signal (e.g. improve geolocalisation). When an electromagnetic wave propagates into the ionosphere, the electrons acting as an antenna are excited by the incoming wave and re-radiate it. In other terms, they do not absorb the wave but they reflect it or scatter it. Given that the electrons are moving at varying velocities due to ionospheric dynamics and random thermal motion, the reflection or re-radiated signal from each electron will have a Doppler-shifted frequency. The higher the temperature, the more variable the thermal motion of the electrons becomes. The re-radiated or scattered wave/signal does not consist of one frequency but of a distribution of frequencies which becomes larger with higher temperatures. The ground receiver stations receive a signal composed of the superposition of the re-radiated/scattered waves from all the electrons in the path of the incoming wave. As a result, the scatter signal is not coherent but is termed incoherent. The positively-charged ions having a much larger mass are not excited by the incoming electromagnetic wave in the way electrons are, and therefore do not re-radiate the signal. However, the electrons tend to remain close to the positively charged ions which by their large mass and small velocity influence restrictively the motion of the electrons. As a result, the distribution function of the electrons is modified by the positive ions. By analysing the incoherent scatter signal received, we can establish relationships between electron content/density, electron temperature, ion temperature, ion composition and electron/ion velocities.

In Figure 2, the transmission frequency of the RADAR of 430 MHz is represented with the vertical line and the frequencies scattered off the electrons are represented with the line of the curve. We have different frequencies or a spectrum of frequencies, each of which is associated with a certain power. We can understand that some electrons have a scattered frequency that is higher than 430 MHz and some electrons have one that is lower than 430 MHz. The electrons that are moving in the direction of the RADAR wave appear to provide a frequency that is higher and those that are moving in the opposite direction appear to provide a frequency that is lower.

The more electrons there are, the more power we receive. Therefore, the sum of all the power underneath the spectrum, represented by the grey area, is a measure of the number of electrons in the ionosphere.

The higher the temperature, the more intense the random thermal motion and therefore the more variable will be the frequencies that are scattered. The larger the variability, the greater the spectral width represented by the line shown in the picture. A larger variability is linked to an increased temperature. Thus, the spectral width reflects the electron temperature.

In the presence of ions, which do not themselves reflect/scatter the signal, the scattering of the frequency off the electrons is modified and different spectrum widths may be present which would be indicative of the temperature of the ions which modified the signal. Using modelling approaches for spectrum analysis, the temperature and ion identity/composition can be determined.

References: 

[1] https://www.naic.edu/ao/scientist-user-portal/atmospheric/guides/Radar-Reflections

[2] https://en.wikipedia.org/wiki/Incoherent_scatter

Figure 2

How HAARP works

As HAARP heats the ionosphere, electron clouds expand creating electron-density enhancements or depressions that align with the magnetic field lines. These are called field-aligned (density) irregularities.

Text based on transcript from video: https://www.youtube.com/watch?v=ZssJ0InqBcw&t=667s

HAARP transmits powerful radio waves into a localized area of the ionosphere above the facility. The altitudes of interest are from 70 to 350 km, i.e., in the D region of the ionosphere up to the F region. The frequencies that are used are those termed here "important characteristic frequencies" (to be described) or their harmonics.

The radio waves that are being transmitted into the ionosphere transfer energy to the electrons there, raising their temperature. Then, the clouds of electrons expand; however they can't expand randomly, but instead they are forced to expand along magnetic field lines. As a result, we have electrons that are lined up with the magnetic field lines and that bunch together creating "field-aligned-irregularities (FAI)". These can be used for many different applications including radio wave scattering, propagation effects and so on.

It is noted that at HAARP, the magnetic field lines are quite steep at about a 76-degree-angle. (https://youtu.be/ZssJ0InqBcw?si=jvF0R7Vo6yBKeVlo&t=733).

Figure 3 Slide from video https://www.youtube.com/watch?v=ZssJ0InqBcw&t=667s

When a satellite transmits through field-aligned (density) irregularities., its signal may be amplified by 40 times.

Text based on transcript from video: https://www.youtube.com/watch?v=ZssJ0InqBcw&t=1022s

Figure 2 shows the HAARP instrument at the bottom and its radiation envelop directly overhead. When it heats in the ionosphere at the area shown, roughly between 200 and 300 kilometers altitude, we will have the formation of field-aligned irregularities in the region along the magnetic field lines*. 

If a satellite is overhead and transmits through these field-aligned irregularities (as shown in Figure 2**), its signal will show scintillation, meaning it will twinkle or flicker, as it will be refracted to a certain degree by the structures on which it will bump; however, quite surprisingly its signal may be characterized by a 16 dB gain, meaning an amplification by 40 times. Usually, the gain is smaller, such as 10 dB, meaning that the signal will be amplified by 10 times, which is still considered as a significant number.

Satellites can use these irregularities as reflection points: these can reflect down to a receiver on the ground. We can also transmit from the ground up into these irregularities; the radio waves will be reflected and can be received on the ground.

*It is noted that the magnetic field lines represent the slope lines of the magnetic field. 

** UHF satellite - signal at 255 MHz

Figure 4: Signal amplification during transmission through field-aligned-irregularities (from https://www.youtube.com/watch?v=ZssJ0InqBcw&t=1022s).

How HAARP generates ULF, ELF and VLF waves.

If HAARP transmits a wave of 5 MHz modulated at 1 kHz, we can obtain an ELF wave of 1 kHz.

https://www.youtube.com/watch?v=ZssJ0InqBcw&t=1135s

Text based on transcript: In Figure 3, we are transmitting a modulated HF signal up into the heated region. If this HF is modulated at some kilohertz frequency, then this gets demodulated by non-linearities in the heated region. As a result, we have propagating out of there the demodulated radio waves from that process. Those are received on the ground.

Figure 5: Generation of ULF, ELF and VLF waves by HAARP (from https://www.youtube.com/watch?v=ZssJ0InqBcw&t=1135s).

HAARP very long-distance propagation

https://www.youtube.com/watch?v=ZssJ0InqBcw&t=1350s

Text based on transcript: HAARP can create artificial ionospheric turbulence or AIT. This is related to non-linear phenomena and takes advantage of a type of wave guide that exists between the E and F regions of the ionosphere. There were some experiments done with this a few years ago for very long distance propagation, from HAARP to Antarctica i.e. a distance of approximately 15 000 kilometers. They used the HAARP to couple a signal into this wave-guide; and then they decoupled it through natural refraction at Antarctica and other locations that are far away. They found that the best propagation was along the solar terminator, also called the "gray line" (shown on the right).

Figure 6: HAARP very long distance propagation (from https://www.youtube.com/watch?v=ZssJ0InqBcw&t=1350s and https://sos.noaa.gov/catalog/datasets/daynight-terminator-daily/)

Polar ionosphere: gateway to the outer magnetosphere

Where generated ULF/ELF/VLF waves access the outer magnetosphere

The magnetosphere encompasses the ionosphere at its base, the plasmasphere (or inner magnetosphere), and the outer magnetosphere. The plasmasphere is found above the ionosphere. When discussing near-Earth space, we often refer to the ionosphere and the overlying magnetosphere as a coupled system.

1. Polar field lines are special

🔹Near the magnetic poles, field lines do not loop back to Earth like they do at lower latitudes.

🔹Instead, they either stretch outward into space, or form very large loops back to Earth, thus connecting the ionosphere to the outer magnetosphere.

Please note on the image on the left, the field lines in the region "cusp".

This is why the polar ionosphere is often called the "gateway" to the outer magnetosphere.

At high‑latitude sites such as HAARP, the magnetic field is extremely steep (about 76°), which means that the field lines above the facility rise almost vertically. This steep inclination enhances the direct connection between the ionosphere and the outer magnetosphere, making the polar region an especially efficient access point for energy and particle exchange.

Related video: HAARP High-Frequency Active Auroral Research - Whit Reeve’

https://youtu.be/ZssJ0InqBcw?si=y5LkDMcuum7iGi2D&t=667

2. Ionospheric outflows follow these field lines

🔹The image shows ionospheric outflows — streams of charged particles rising from the ionosphere. (Cf. region "cusp")

🔹These particles travel along the field lines into the outer magnetosphere.

🔹 This is a real phenomenon. It’s how Earth’s atmosphere slowly leaks into space.

3. ELF/VLF waves can also propagate along these paths

🔹When facilities like HAARP or EISCAT generate ELF/VLF waves, those waves can couple into the same field lines.

🔹They then travel in whistler mode along the magnetic field, sometimes reaching the conjugate hemisphere or bouncing between mirror points.

So the field lines shown in the image represent functional pathways for both particles and waves.

Figure 1: Main regions of the Earth's magnetosphere

Figure 2: 5th image from https://lasp.colorado.edu/mop/resources/graphics/

Modification of the ionosphere with radio waves - the pioneering ionospheric heater/phased array of Platteville (Boulder, Colorado), predecessor of HAARP 

Modifying the ionosphere with radio waves” by W.F. Utlaut

 New Scientist 55, No 808, p.288-290 

Reading notes

The Platteville Atmospheric Observatory near Boulder, Colorado was one of the first major ionospheric heaters in the world. Linked to the Institute of Telecommunication Sciences (ITS) and the U.S. Department of Commerce and Telecommunications, it operated from 1968 to 1984 on ionospheric processes. It is still operational performing wind profile studies. The transmitting aerial array consisted of 10 elements forming a ring of 110 cm in diameter. Using an effective radiating power of 100 MW, the upgoing power would be distributed over a circular area in the ionosphere of 100 Km in diameter with an approximate power flux density of less than 50μW/m2. The installation was designed to perform ionospheric modification with frequencies from 5 MHz to 10 MHz using right or left circular polarization. It was found that depending on the polarization, there were different profiles of velocity and paths transversed during propagation. Right polarization was associated to “ordinary waves” and O-mode, while left polarization to “extraordinary waves” and X-mode. 

When modification was performed with X-mode excitation, electron temperature would increase by 35% in the F-region area attained by the beam. Detection of electron heating was quantified by the attendant effect on the rate of dissociation-recombination of electrons and molecular oxygen ions which leads to the 630 nm emission of oxygen (red line) and the generation of air glow.  For this process, the reaction rate is inversely proportional to electron temperature so that the emission intensity decreases compared to background when the temperature is rapidly increased, and increases after the heater is off. Experiments showed that the increase and decrease of electron temperature occurs within tens of seconds.

When modification was performed with O-mode excitation, there was an unexpected and nearly opposite result; there was an increase in the 630 nm (red) oxygen line after power-on and decrease after power-off. The generation of airglow (enhancement of natural airglow) implied that electrons excited oxygen by collisions. For this process, significant numbers of electrons with energies greater than 2 electron volts (eV) are required, when the ambient level is approximately 0.5 eV. Enhancement of other emission lines indicated that some electrons obtained energies equal to perhaps 10 eV. These processes appear to require generation of plasma "parametric" instabilities. The term "parametric" refers to the periodic modulation of a certain parameter of an oscillating system with sufficient amplitude at a certain frequency to cause the oscillation to become unstable.

Figure 7:  Transmission and reception of a signal using ionospheric-related mechanisms (Source).

Very large transmitters illuminating the ionosphere at 85km altitude with VLF waves - Antennas called "poor-man's RADAR" acting as receptors

Professor Craig Rodger refers to his work at Scott Base, Antarctica, on the occasion of the 150-year anniversary of scientific partnership between New Zealand and the USA.

Introductory excerpt where Professor Craig Rogers refers to a triangle antenna he installed in Antarctica, which is used as he mentioned as a "poor-man's RADAR": https://www.youtube.com/watch?v=dN8Opakal3Q&t=2298s

https://www.youtube.com/watch?v=dN8Opakal3Q&t=2340s

"We also use them as a form of "poor man's radar" where somebody else pays for a very large transmitter, launches a very low frequency (VLF) radio wave out into the environment and that is trapped between the Earth (where the Earth could be the conducting ground or seawater) and the lowest part of the ionosphere. The ionosphere is the charged part of Earth's atmosphere. At night, it's going to be about 85 km.' 

'The wave will be trapped inside this region. Somebody else pays for the transmitter; it's normally nations who are using them for military purposes. We don't know what they're saying, but we can pick up the radio waves thousands and thousands of kilometres away. And we use it as a poor man's radar to monitor energy coming in to the upper atmosphere.' 

"So, if you have energy in the form of hot particles coming from space or solar x-rays from a solar flare zapping the ionosphere, it will change that radio wave as it bounces between the Earth and the ionosphere. And thereby, we can monitor heights like 85 km very cheaply from the ground. And 85 km is a very difficult altitude to monitor otherwise. It's too low for satellites; a satellite would just fall out of the sky. It's too high for an airplane or balloon. The only things that can monitor those sort of altitudes are rockets, but a rocket goes through very quickly, and so it's a very short measurement. So, we use this technique.' 

'The nearest of these large military transmitters to New Zealand is located on the West Coast of Australia. It has a call sign: NWC. It's actually one of the most powerful radio transmitters in the world operating at 19.8 kHz. So, very roughly it's got a 20 km wavelength, broadcasting 1 million watts of power. And we monitor it in Dunedin (New Zealand), and we also monitor it in Scott base."

"Let me zoom in on that transmitter. And as you come in from space you can see it looks more and more like some sort of alien landing field to me. It's absolutely huge. (...)"

Reference: https://fulbright.org.nz/fulbright-good-works-presents-going-south-to-the-ice-with-professor-craig-rodger/

Figure 8: VLF transmitter illuminating the ionosphere at 85 km altitude. An antenna called "poor-man's" RADAR acts as a receiver.

Basic processes associated with high-power electromagnetic interactions with plasma in the ionosphere

Reference: Large ionospheric disturbances produced by the HAARP HF facility (abstract and Figure 1)

1. Electron temperature elevation 

2. Plasma irregularity formation

3. Electrostatic wave generation

4. Parametric decay and strong turbulence

Amplification of electromagnetic waves by magnetospheric injection in high latitudes   (Magnetospheric Amplification)

The electromagnetic signal from a BBC radio shortwave transmitter arriving 5000 km away is billions to a trillion times weaker than when it left the transmitter.

By contrast, when a VLF signal transmitted from Antarctica is injected into the overhead magnetosphere and travels roughly 15 000 km along geomagnetic field lines to a receiver in Canada, it can arrive amplified by 100 to 4000 times 📈.

Reference: "VLF wave-injection experiments from Siple Station, Antarctica" https://vlfstanford.ku.edu.tr/biblio/vlf-wave-injection-experiments-siple-station-antarctica-1

In ordinary long‑distance radio propagation, attenuation is the rule. A BBC shortwave transmitter operating from Woofferton in the UK can direct a powerful 250 kW beam toward Africa, yet by the time that signal has travelled roughly 5000 km and reached radio receivers in its target region, it has weakened by around 100 to 120 dB. In practical terms, that means the listener receives a signal billions to a trillion times weaker than what was transmitted, often in the picowatt or femtowatt range, but still perfectly intelligible because HF receivers are designed for such tiny field strengths. This enormous decrease is simply the natural consequence of geometric spreading, ionospheric absorption, and the general inefficiency of long‑distance HF propagation.

By contrast, VLF propagation in the magnetosphere behaves according to a completely different paradigm. When a VLF signal is transmitted from Antarctica—as in the classic Siple Station experiments—and injected into the magnetosphere at high latitudes, it does not simply radiate outward and fade. Instead, the wave couples into whistler‑mode propagation and travels along geomagnetic field lines, guided through the magnetosphere rather than dispersed by it. Over the course of its roughly 15 000 km journey to a receiver in Canada, the signal can undergo wave–particle interactions that actually increase its amplitude. Instead of losing energy, the wave can draw energy from resonant electrons in the magnetosphere, emerging at the far end 100 to 4000 times stronger than simple geometric spreading would predict.

This juxtaposition—HF signals that fade by a factor of a trillion over 5000 km, versus VLF signals that can grow by orders of magnitude over 15 000 km—captures the extraordinary nature of magnetospheric amplification. It is one of the few natural environments in which a transmitted electromagnetic wave can arrive not weaker, but significantly stronger, than classical propagation models would ever allow.

(Note: AI-assisted text generation)

Figure 9: Magnetospheric amplification of VLF waves transmitted from Antarctica and arrriving in Canada. Reference: https://vlfstanford.ku.edu.tr/biblio/vlf-wave-injection-experiments-siple-station-antarctica-1

Tropospheric ducting and troposcatter

Tropospheric ducting

https://www.facebook.com/agencenationaledesfrequences/posts/pfbid02cr9ky3Uabz2eHQPzPzyxA2V6i7Fpax3uyoTbrixzTDcePRjmk234XZajgqU88sVKl

Normal propagation: A radio wave travels in a straight line (line‑of‑sight) until it reaches the radio horizon, which is slightly farther than the geometric horizon because of standard atmospheric refraction — the atmosphere bends the waves slightly.

Radio horizon: The limit beyond which a ground‑based receiver cannot normally “see” the transmitter because Earth curves away.

Tropospheric waveguide / ducting propagation: Under certain atmospheric conditions (especially temperature inversions), layers in the troposphere can act like a waveguide (duct) that traps and guides VHF/UHF radio waves over very long distances beyond the normal radio horizon.

Troposcatter

Troposcatter does not trap the wave. Instead, tiny fluctuations in humidity, temperature, and air density in the troposphere scatter a small fraction of the radio energy in many directions. A receiver beyond the horizon captures some of that scattered energy.

Characteristics

• Signal is weak and noisy

• Requires high power and directional antennas

• Was historically used for military and long-distance telecom links before satellites/fiber

• Works reliably beyond line-of-sight without requiring special ducting weather