Signatures of atmospheric electric discharges are revealed in a larger range of the electromagnetic spectrum than just the optical. All thunderstorm discharges, intra-cloud and cloud-to-ground, generate radiation over a wide spectrum. The maximum is at extremely low frequencies, and this signature may be observed at large distances from the original source. Here we divide the range in two: Ultra Low Frequencies (ULF) and Extremely Low Frequencies (ELF) - though sometimes all the range is called ULF or ELF or the limits may be different:
During the second half of the 20th century, the last decades in particular, scientist became more interested in these frequency ranges. Firstly, because the existence of different natural electromagnetic resonance phenomena have been predicted in these ranges and secondly, because people learnt how to measure naturally generated fields of very small frequency and very small amplitude compared to man-made disturbances. The development of electronics and computer technology and sciences was also very helpful. At present such measurements can be used to study atmospheric thunderstorm activity. Researchers create methods of identification and evaluation of the atmospheric electric discharges by means of electromagnetic field observations. These ideas could be also used in the studies of Transient Luminous Events, such as sprites or elves.
The respective lengths of the ELF waves in vacuum (and let us consider the air as vacuum as the first approximation) range from 100 - 100000 km. These waves propagate in the area between the Earth and the ionosphere and interfere with each other. For some waves the interference is constructive and the waves resonate. A cavity - the Earth-ionosphere cavity is created (the same area acts as a waveguide for VLF waves - the Earth-ionosphere waveguide). The resonances are called the Schumann Resonances and they are observed by measurements of the magnetic or electric field in the ELF range: the first three resonances are observed at the frequencies about 8, 14, and 20 Hz. More information on this topic could be found in Oulu Space Physics Textbook for example.
The Schumann Resonances are widely believed to be excited mainly by thunderstorm activity. The main contribution is provided by negative cloud-to-ground discharges, which are the most likely to occur and thus constituting the background level. But sometimes very strong, transient signals occur and then the amplitudes of the Schumann resonances usually grow significantly. These are thought to be related to stronger, positive cloud-to-ground discharges. Sprites, which are some kind of high-altitude discharges, usually follow such a positive cloud-to-ground discharge (their parent-lightnings), and moreover, they are also believed to radiate in the ELF range.
The measurements in the ELF range and observations of the Schumann resonances are made in observational stations in Europe and all over the world. The European stations are situated in Hungary, Israel, Slovakia, Sweden, and Poland.
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Until the 1980s this range was connected to different kinds of geomagnetic pulsations that have magnetospheric origin. In the frequency range of 1 to 5 Hz (sometimes even higher) - part of the ULF range and the beginning of the ELF range, other electromagnetic phenomena were observed on the Earth. Unlike the Schumann Resonances, that are present all the time and everywhere, the occurrence of these resonances is different in different parts of the globe. They are the Ionospheric Alfven Resonances, and, as the name suggests, take place in the the densest part of the ionosphere, called the F region. This region which is filled with ionospheric plasma acts as a resonator for Alfven waves, propagating along geomagnetic field lines and is created under special conditions in the ionosphere. Oulu Space Physics Textbook gives more information and some references to this topic.
With the results of optical observations, the correlations between identified sprite events, the ULF magnetic signals and IAR-SRS events in particular, can be studied. This will teach us how the atmosphere and the ionosphere can be coupled, what role the TLEs play, and also how certain ULF events can be recognised. The measurements of natural fields in the ULF range require the same conditions as the Schumann Resonance studies - electromagnetically quiet place and special low-noise equipment. Such measurements are made in several regions such as Russia (several stations over the Asian and European part of it), Finland (chain of stations), Crete, Svalbard, and Poland. The Spectral Resonance Structures are observed there and studied in particular.
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