Why is the study of the ionosphere important?
In this era where technology is a fundamental part of everyday life, the study of so-called space weather is of absolute importance. The influence of geomagnetic and solar storms affects every area of human civilization. From aspects related to civilian life to military ones and, last but not least, the possible impact that these events could have on people’s health and the repercussions on the environment.
In short: just as it is important to know the weather, it has become equally important to know space meteorology.
The ionosphere is a particular shell of ionized particles, i.e. electrically charged, composed of ions and electrons. This layer is partially ionized. Still, the neutral gases make up the majority of this layer, and the presence of ionized components forces the ionosphere to react to any variations of electromagnetic fields.
Its discovery dates back to the dawn of radio communication. Thanks to the ionosphere, Marconi was able to turn on the lights of the city of Sydney in Australia from the Italian city of Genoa. All this simply because radio waves, bouncing in the ionosphere, were able to travel thousands of km.
The terrestrial ionosphere is located at altitudes between 60 and 2000 km and has a very complex structure. It is divided into layers with different plasma concentrations and chemical compositions and reacts to variations in solar activity, the magnetic field, and the succession of day and night in the various seasons of the year. Simply put, the ionosphere breathes, rising and falling mainly due to the variation in the solar ultraviolet radiation within it.
There are many ways, direct and indirect, to study the ionosphere. In addition to sounding rockets, the first experiments with satellites relating to the observation of the ionosphere date back to 1962 with the Canadian-American satellite Alouette.
The USSR launched its first ionoprobe in 1970, with the Kosmos-381 satellite. In 1979, a further evolution occurred with Interkosmos-19 and then, in 1987, the more advanced Kosmos-1809.
Sergei Pulinets, interview with an expert
We had the opportunity to interview Sergei Pulinets, responsible for the ionoprobes on board the Ionosfera-M satellites.
After graduating from Lomonosov University in Moscow, he worked for thirty years at the Institute for Terrestrial Magnetism, Ionosphere, and Radio Waves Propagation located in Troitsk, now part of Moscow. Subsequently, he carried out research for six years in Mexico at the state institute UNAM for earthquake prediction.
Having returned to Russia, after various positions in the aerospace industry, in 2009 he was appointed responsible for the main instrument of the satellites of the Ionosphere constellation, the LAERT probe, thanks to the experience he acquired in the past years with previous Soviet Ionoprobes such as the Interkosmos 19 and the Kosmos 1809.
Due to the particular activity carried out, he is also supervisor of the construction processes of satellites and instruments at the state industry VNIIEM
How to study the ionosphere
Compared to other methods of investigating the ionosphere, such as observations from the ground, a dedicated satellite network has undoubted advantages. In this regard, Doctor Pulinets states:
“We can use ground-based instruments to measure the ionosphere, for example from fixed stations or by launching sounding rockets.
Radio pulses are emitted in a wide range of frequencies and the signal reflected by the ionosphere is recorded. Depending on the emitted frequency, it is reflected at different altitudes, which allows getting the vertical distribution of electron concentration by scanning frequency.
A reflection is obtained when the transmitted frequency resonates with the vibrations of free electrons in the ionosphere.
The delay between sending the signal and its reflection and the retransmitted frequency is detected. In this way, we have an Ionogram which, however, is only related to the slice of sky that we have scanned from the ground or during the flight trajectory of the sounding rocket.
In all of the world, there is a network of ionosondes and, in Russia, they are managed by the Federal Hydrometeorological Service. These are partial surveys, which is why, with the advent of satellites, it was decided to apply these techniques in space. But here too there are limitations.
These are always ionograms relating to what is between the satellite and the ground, along the satellite orbit, but the area between the neighbor orbits cannot be detected except with a certain approximation.
Sometimes GPS and GLONASS satellites are used, in high-altitude orbits (nearly 20,000 km), together with satellites positioned in low Earth orbit, using a technique called occultation.
In essence, the satellite in the lower orbit, during its revolution, is hidden by the Earth, and on its occultation, the signal sent by the high-orbit GNSS satellite crosses the atmosphere at different altitudes, and then the altitude profile is reconstructed.
But even in this way, we will always have a very partial and approximate representation of a slice of the sky. Not enough to obtain a model of the ionosphere on a larger scale.
Satellites dedicated to the study of the ionosphere are found in so-called sun-synchronous orbits, i.e., orbits in which the satellite transits over an area at the same local time. We thus have the condition of constant solar illumination during the satellite’s passages. But we know well that the ionosphere varies in height and density during the 24 hours, so even the investigation with just one satellite in the SSO, the sun-synchronous orbit, will give us very partial information.
The Ionosphere inherits the experience of the Soviet satellites Kosmos-1809 which, at the time, should have constituted the first example of a network of satellites capable of representing a model of the ionosphere, and not just a precise representation of it.”
The Ionosphere-M satellites
The Ionosphere constellation will comprise four satellites plus an additional satellite, called Zond which will be placed in an almost circular heliocentric orbit. The Ionosphere-M satellites will be launched in pairs. The first two satellites will already be able to provide their measurements independently and on this point Doctor Pulinets says:
“The ionosphere constellation will already be able to provide appreciable data with the first two satellites, which will be launched in May 2024. The full operation of the four Ionosphere-M satellites will be fundamental for the development of the ionosphere model as the particular configuration of the orbits of the four satellites will be able to obtain more data from a greater area of investigation. Each pair of satellites will be placed in the same orbit but staggered by 180°.
Each satellite travels one orbit in 90′; during this time, the earth rotates 25°. If we use a pair of satellites offset by 180°, the rotation will be only 12.5° between one reading and another. With two pairs of satellites, then positioned on two different orbital planes, staggered by 45°, greater precision will be achieved in the measurements carried out.”
The four satellites will not communicate autonomously with each other but will exclusively transmit their data to the Earth stations as Sergey Pulinets specifies:
“The four Ionosphere-M satellites plus the Zond satellite are not expected to communicate with each other. Instead, it is envisaged that data will be transmitted from them exclusively with the ground stations positioned in the three regions of Russia: European, Siberian, and Eastern.”
The Zond satellite
The fifth satellite of the constellation will be called Zond like the glorious interplanetary probes of the Soviet era. Doctor Pulinets explains its operational peculiarities in detail.
“The Zond satellite will be placed in a near-circular sun-synchronous orbit. It will have its instruments, including magnetometers, coronagraphs, spectral telescopes, spectrophotometers, and X-ray and UV photometers, ozone meters, constantly pointed at the Sun. From its constant flow of data, we will be able to interpret the interactions of solar activity on the Ionogram.”
The Ionosphere satellites inherit some of the characteristics of the probes used in the past but will also present new functions as Sergey Pulinets explains to us:
“The Ionosphere satellites are essentially traditional; the only instrument that, for example, was not present in the old Kosmos-1809, is an ozonometer. With today’s technologies, what already existed and was cutting edge for its time has been optimized. Western probes of the same type used analog frequency scanning.
Furthermore, Western Ionoprobes had very long scanning times: many tenths of a second, even, and in some cases, minutes. This means that at a speed of 7 km/s, the measurement could never have been punctual and, consequently, the resulting Ionogram was very long.
The Interkosmos-19 satellite was already equipped with a digital synthesizer (IS 338), capable of emitting 338 frequencies from 0.3 to 16 MHz. Ionosphere-M will be equipped with a digital synthesizer capable of emitting 400 frequencies between 0.1 and 20 MHz, with a frequency of one scan every 8 seconds.
The launch, initially planned for December 10, 2023, has been moved to May 2024 due to further checks we are carrying out on the ground to minimize the drawback common to this type of Ionoprobes: interference. Although the new Ionoprobes have lower powers than, for example, those mounted on the Priroda module of the MIR space station, the problem of interference with other equipment cannot yet be said to have been resolved.”
We therefore thank Doctor Sergey Pulinets for the great friendliness and availability shown in granting this interesting interview which allowed us to discover a field of study of great importance with practical implications in all areas of the life of humanity in the 21st century.