This space physics research project concerns studies of the interaction between radio waves (or, as we say, electromagnetic radiation) and the outer, tenuous part of the Earth's atmosphere at hundreds of kilometres above the surface of the Earth. Due to the strong, natural ultra-violet (UV) and X-ray radiation from the Sun, a few of the atoms and molecules in this gas are broken up into electrons, which are elementary particles with negative electric charge, and ions of positive electric charge. Since the behaviour of electric and magnetic fields and waves is strongly affected by this tenuous ionised gas, which constitutes what we call a plasma, this region of the outer planetary atmospheres is called the ionosphere.

Study of the Earth's ionospheric plasma - a space plasma which has existed in our own "space backyard" for millions, if not billions of years - is very important for a complete understanding of the interplay between Earth and its space environment (also known as Earthspace) since it constitutes an important link in the biosphere-atmosphere-ionosphere-magnetosphere-heliosphere chain that influences life on Earth.

Also, since over 99% of all known matter in universe is in the plasma state and information from it comes in the form of electromagnetic radiation (radio, infrared, optical, ultraviolet, X-ray, and gamma radiation) a good knowledge of the intricate physics which governs the interaction between electromagnetic radiation and plasmas anywhere, is essential for the understanding of Universe as a whole. In view of this, we now use the near-Earth space plasma (ionosphere, lower magnetosphere) as a giant natural laboratory where we perform model experiments, mimicking conditions in other space plasma environments. This allows us to use a powerful stimulus-response methodology for investigating, in a systematic manner, phenomena which would otherwise be impossible to study experimentally.

Open questions which we address in this research project include:

• How do the large variations in the Sun's activity, mediated to the Earth via the natural solar radiation and the solar wind, affect the state of the ionosphere and/or the upper atmosphere?

• How can we understand the mechanisms responsible for the generation of the copious amounts of natural electromagnetic radiation in the Earth's magnetosphere that continuously "bombard" the ionosphere and have done so for eons?

• How can we understand and predict the detailed mechanism behind the huge short-time and long-time natural variations in the plasma content of the ionosphere, causing the depletion of this plasma over vast volumes (often referred to as ionospheric irregularities, "holes", "cavities", or "bubbles") and observed ever since the early days of ionospheric research?

• Can we learn anything new about our own space habitat from the electromagnetic radiation observed? Can we apply such knowledge to help interpret the physical processes that generate the electromagnetic radiation received from remote sources such as radio astronomical objects? If so, does this radiation contain any signatures which can help us understand the creation and the development of Universe and our own planet?

• Do the natural physical processes that take place in the Earth's magnetosphere and ionosphere, located at hundreds to thousands of km altitude, have any measurable influence on the atmosphere or the ozone layer at much lower altitudes, or perhaps even on the Earth's weather system and climate?

• Does the electromagnetic radiation from the tens of thousands of broadcast, TV, utility, radar and research stations deployed on the ground by man over the past century, or from the many satellites put into orbit over the past half century, have any effect on the ionosphere? If so, is this an adverse or a positive effect? Or, if measurable at all, is it harmless?

Radio waves are a combination of oscillating electric and magnetic fields which, in the short-wave range, have an oscillation rate, or as we prefer to say, a frequency, which is in the order of a few million times per second (Megahertz, MHz). We call these short-wave radio frequencies, from 3 to 30 MHz, the High Frequency (HF) range. This is the short-wave radio band used by international short-wave/HF broadcasters such as the BBC World Service, Voice of America, and Radio Sweden. The frequency range in which our FM radio stations operate is tenfold higher and is called the Very High Frequency (VHF) range. Still a tenfold higher is the Ultra High Frequency (UHF) band where we find, for instance, TV and radar transmitters. Satellite TV and mobile radio communication systems operate in the Super High Frequency (SHF) range a thousand times higher than HF. So despite its name, given in the early days of radio communication to discriminate it from the long-wave (Low Frequency/LF) and medium-wave (Medium Frequency/MF) bands used by many broadcasters at the time, the short-wave (High Frequency/HF) band of frequencies are nowadays considered to be low-frequency (long wavelength) in nature.

The ionospheric plasma has, at each given altitude, its own natural resonance frequency called the plasma frequency. The exact value of this plasma frequency depends on how many electrons per cubic metre there happen to be at the altitude in question, and this, in turn, is determined by the solar activity and the local density of the atmosphere. At about 100 km altitude where the atmosphere is relatively dense but the solar radiation is weak the ionospheric plasma frequency lies typically in the long-wave/LF or medium-wave/MF range. At altitudes of 200-400 km it lies in the short-wave/HF range. At still higher altitudes of 500-1000 km the solar radiation is quite strong, but the atmosphere so very dilute that the plasma frequency is again lower. At the particular altitude where the frequency of a vertically propagating radio wave coincides with the local ionospheric plasma frequency, the radio wave will interact with the ionosphere in such a way that the wave will be reflected. It is this reflecting property of the ionosphere, discovered already about a hundred years ago, which makes the everyday high-frequency radio communication over large distances possible.

Recently we have demonstrated experimentally that radio waves from modern radio stations are strong enough that the interaction in the reflecting region of the ionosphere is such that the plasma no longer can be thought of as a passive, reflecting mirror. A more accurate description of the ionospheric plasma is that it self-modifies weakly its reflecting properties more or less in synchronism with the radio waves that propagate through and reflect from it. The fact that the "mirror" so to speak takes an active part in the reflection process is due to weak, complicated, but important non-linear (non-proportional) properties which can lead to turbulence and perhaps even chaos in the plasma. We call this phenomenon ionospheric modification and its manifestation shows that the old, simplistic picture, based on linear models of the physical processes in plasma, is no longer adequate. Like all plasmas in nature, the ionosphere is a complex physical system and not just a passive mirror of radio signals. We have only just begun to understand this complexity and much more basic research is needed before we have an adequate knowledge of the true behaviour of the ionosphere and other space plasma.

For hundreds of millions of years the Earth's ionosphere and upper atmosphere have been subjected to perturbations from above in the form of enormous radiation and particle streams of solar and other cosmic origin. This is manifested in many ways, for instance by the Aurora Borealis ("northern lights") and the Aurora Australis ("southern lights") which occur almost daily in the upper parts of the polar zone atmospheres. Other examples are the large and small "ionospheric holes" or "cavities" which appear regularly when the Earth rotates away from the Sun so that the ionosphere receives less solar UV and X-ray radiation. The same thing happens also during solar eclipses and during magnetospheric storms. Hence, "ionospheric holes" have been an integral and most natural part of our space environment for as long as life has existed on Earth, and are entirely different from and totally independent of the holes which have appeared over the last decades in the ozone layer at much lower altitudes. "Ionospheric holes" are created mainly due to the absence of electromagnetic radiation and can therefore not be created by radio waves, for instance. And in contrast to ozone holes, "ionospheric holes" provide (a slightly) increased protection from cosmic radiation.

Other perturbations of the ionosphere are caused by the extremely powerful radio emissions which occur naturally in space. One example is the Auroral Kilometric Radiation (AKR) which can release up to hundreds of billions of watts of electromagnetic radiation in the ELF-MF range. This powerful radiation impinges upon the ionosphere from above. Other examples are the radio emissions from the plasma around the Sun, called Soloar Corona Radio Emissions which were observed on the Earth for the first time in the 1940s. They are caused by giant eruptions on the Sun and are accompanied by an intense particle radiation which can easily destroy communication satellites and electric power grids on the ground. In addition, meteors and other "space dust" which we can observe as "shooting stars" deposit substantial amounts of various chemical substances in the ionosphere and the upper atmosphere.

From below, the ionosphere is continually being irradiated with thousands of megawatts of radio waves generated by lightning strokes associated with the thunderstorms which occur in the Earth's lower atmosphere at a rate of about one hundred per second. It is also irradiated by the electromagnetic waves from the tens of thousands of broadcast, TV, utility and radar stations that are in use on the surface on the Earth. The radiated powers from the more powerful of these stations range from a few hundred kilowatts to a few megawatts. The handful of research radio facilities that use radio waves for studying the environment use the same type of transmitters with the same powers, but are so few that they contribute negligibly to the total man-made radiation. However, they are absolutely indispensible when it comes to monitoring the atmosphere and space surroundings of our planet, and for developing better tools for such investigations.

Our research has demonstrated that not only natural perturbations but also radio waves from modern high-power radio stations can have such a "modifying" effect. As a consequence, the ionospheric plasma produces its own radio emissions of a more or less noise-like character at frequencies which are offset from that of the original radio wave. However, this secondary radiation is extremely faint, only about one billionth of the power of the primary radio transmission, and is very difficult to detect. In a way, it is the same as trying to detect, in the midst of a roaring gale, the small ripples of about one tenth of a millimetre caused by dropping a small grain of sand on the surface of an ocean, while the natural ocean waves are tens of metres high. Such precision measurements of extremely diminutive waves are possible only by using modern signal analysers and powerful signal processing methods, allowing us to "dig noise out of noise" and they allow us to develop better radio methods for studying the Earth's space environment. This is in line with the long-standing tradition in space physics where space has been investigated with radio methods for almost a century.

The basis for the development of a new diagnostic tool of this type was laid in 1981 when during ionospheric radio experiments using the EISCAT/Heating facility in Tromsø in Norway, Bo Thidé from IRFU was the first to detect, on the ground, weak secondary electromagnetic radiation from the ionospheric plasma, a phenomenon now known as stimulated electromagnetic emission (SEE). Since then we have learned how to utilise this SEE radiation as a method for studying many new physical properties of plasma both in the proximity of the Earth and further out in the Universe. By using our experiments to learn how to "read off" the properties of the secondary radio emission, we obtain a "fingerprint" of the local conditions in the region where this radiation appears. Since such secondary radio emissions are excited in almost all plasma and this emission can propagate over vast distances, through even extreme environments before they reach us on Earth, this "fingerprinting technique" has provided completely new possibilities for exploring physical conditions in those regions of our universe where we can never expect to make direct measurements.

As is well known, there exists around the Earth a magnetic field which does not vary very much with time or space, and which we use when we navigate with the help of a compass. This magnetic field penetrates the ionospheric plasma to make it magnetised. A magnetised plasma is birefringent ("double refracting") for radio waves in a way similar to that in which certain crystals are birefringent for light waves (a light ray is split up into two parts, or polarisation modes, which propagate and refract independently and differently in the crystal).

The birefringence of the magnetised ionospheric plasma is due to the fact that the Earth's magnetic field strongly affects the motion of the charged particles (electrons, ions) of the ionosphere. Among other things, it means that the electrons and ions can move fairly easily along the direction of the Earth's magnetic field, while the motion across the magnetic field is restricted to a circular motion perpendicular to this direction.

This dependence of the motion of the plasma particles on the direction of the external magnetic field causes what is called symmetry breaking. As a consequence of this, a magnetised plasma will, when being is subjected to external perturbations such as powerful radio waves, structure itself into long, "cigar shaped" structures orientated along the Earth's magnetic field from which a few percent of the plasma will be "snow-ploughed" away. It is of great interest to study the properties of such self-organised irregularities, created under controlled experimental conditions, since similar structures appear spontaneously in nature but are often difficult to analyse there. Our experiments therefore help us understand better non-linear phenomena which exhibit self-structuring and chaos, something which occurs in many different physical systems both on Earth and of the Universe as a whole.

The plasma structures elongated along the Earth's magnetic field which are formed under the action of external radio wave irradiation, also have the property that they contribute to the transformation of the plasma turbulence into the secondary radio emission mentioned above.