# Progress in the Knowledge, Application and Influence of Extremely Low Frequency Signals
## 1 Introduction  
The exposure to electromagnetic fields of natural or artificial origin is common in our society, where continuous technological developments increase the knowledge and evolution of these fields. Among the existing signals, the study and determination of extremely low frequency (ELF) signals, located in the range of 0.3 Hz to 0.3 kHz, is increasingly important and demanded, and scientific contributions based on this topic date from 1957 to the present. Although all the investigations carried out on ELF signals have a common basis such as the generation-reception of waves, they cover different areas of application. For an antenna to generate or receive a very-low-frequency wave, it requires dimensions of the order of the wavelength it is radiating so this process is not trivial. ELF signals have wavelengths of the order of the Earth's diameter, so to transmit these signals, the Earth itself should be used as an antenna [1]. The need to establish large antennas, together with the wide applications ofthis type of signal in ionospheric systems, magnetospheric systems, remote sensing, and long-distance communications, among others, implies the multidisciplinary development of scientists based on the establishment of efficient ELF signal radiation [2]. Different mechanisms are considered to generate these signals [3], among which the high frequency modulated ionospheric heating (HF) predominates bibliographically [4]. The determination of ELF signals derived from natural electromagnetic phenomena is a topic of special interest, since it involves multidisciplinary branches among which geophysics, astrophysics, engineering, climatology, earth sciences, etc. are distinguished. The most relevant ELF electromagnetic waves in the planet's tropospheric-ionospheric system include Schumann resonance (SR), geomagnetic pulsations, and waves related to the Alfven resonator mode [5]. SR are the resonance modes that exist in the cavity established between the Earth's surface and the lower edge of the ionosphere [6]. These signals were initially observed in the 1960s [7] and remain a topic of study today [8]. The geomagnetic pulses are hydro-magnetic waves that arise from resonant processes and propagate in the magnetosphere [9]. They are a valuable tool for investigating the connections between the Sun and the Earth, the interaction between the solar wind and the magnetosphere, the dynamics of the magnetosphere, and the auroral processes [10]. On the other hand, Alfven waves are oscillations that occur in an ionized fluid permeated by a magnetic field, such as the Earth's ionosphere or the magnetosphere [11; 12]. They play an important role for the knowledge of the Sun, the interstellar medium and the Earth's magnetosphere [13; 14]. Observations of natural ELF radiation have formed the cornerstone in the measurement and interpretation of the effects of lightning in the upper atmospheric regions, as well as in the space near the Earth [15], where the most used tool is the signals derived from SR. SR has been used to investigate the electrical processes that occur in the atmosphere [16], the characteristics of the surface and the lower ionosphere, as well as the tropospheric-ionospheric coupling [5] applied to the Earth and Mars [17], to Saturn [18], or Venus [19], among other celestial bodies. These signals have also been studied as precursor elements of seismic events, due to the variability of SR records in the presence of earthquakes [20], being a topic widely researched in recent decades [21]. The diverse applicability of these signals, together with the complexity of their detection [22] and the need to develop specific equipment for the measurements [23], have been widely developed research topics in recent years and even today they are considered topics of interest as they are not efficiently resolved [24]. Magnetospheric ELF emissions are also important and have aroused great scientific interest since 1969 [25].