# Progress in the Knowledge, Application and Influence of Extremely Low Frequency Signals
## 3 Results and Discussion
### Community Analysis
The representation of the most-used words in the documents of this community is shown in Figure 8b, indicating that the most repeated is \"Schumann resonance\" with 96 repeats, being one of the most common resonance phenomena in the studied cavity. The next word is \"lightning\" with 89 repeats--this appearance is justified as the rays are the main causes of resonance phenomena in the studied cavity. \"sprites\", \"ionosphere\" and \"elves\", with 51, 41, and 39 repeats respectively are also among the 10 most-repeated keywords. The work with the greatest relationships in this community is [64], a book dedicated to Schumann resonance (SR) that occurs in the cavity delimited between the Earth's surface and the lower edge of the ionosphere. This phenomenon was predicted by [PERSON] in 1952 [6] and comprises signals from 7 Hz to 3 KHz. SR is the most-studied ELF resonant phenomenon, since it is a tool that permits establishing varied geophysical information. This information includes the climatic diagnosis [66], the study of global and continental thunderstorms [67], the determination of solar properties [68], and the global location of \"red sprites\" (optical phenomena in the mesosphere) that are associated with extremely powerful rays [16] or the relation with phenomenon's seismic [69] activity, with other possible applications. The work [64] is of interest for the treatment of the main properties of SR, its detection and the existing observatories, as well as the interpretation of the data obtained and the possible information that can be derived from them. Several works located among the most important nodes of this community are based on the study of ELF waveguide of the cavity formed by the Earth-ionosphere, as well as the determination of the propagation parameters of the waves from the electrodynamic properties of this cavity. One of them is [70], where a simple and approximate expression is obtained for the eigenvalues of the propagation of the ELF waves in the waveguide of the Earth's ionosphere, without considering the anisotropy due to the Earth's magnetic field. This publication forms the basis of others such as [71], which presents predictions for modal frequencies, wave phase speeds and quality factors that reasonably coincide with observations in the frequency range of SR (5-40 Hz). Among the 10 most-important publications are several works where transient \"sprite\" events, or high-altitude luminous glows are studied [15]. The generation mechanisms proposed for these phenomena include the heating of environmental electrons through quasi-electrostatic thunder cloud fields, runaway electron processes driven by the same fields, heating by means of electromagnetic lightning, and others. These phenomena often coincide with lightning bolts of great amplitude and with transient excitations of SR [16]. The sprites sometimes appear to follow the horizontal progression of lightning in clouds and are not rare compared to typical lightning rates, as indicated in [65]. This work indicates the presence of sprites at distances of up to 1000 km, through the observations of the background RS established at the Wet Greenwich station (Rhode Island) and at the Yucca Ridge station (Colorado). The data of these stations were used in [72] for the detection of transient light events (sprites, blue jets, elves) of the large convective mesoscale systems (CMS) over US High Plains. The analysis of the detected sprites supported the hypothesis that these phenomena are almost exclusively associated with the positive lightning that occurs from Could Ground (CG) and that have peak currents substantially larger than those of a storm system. Later in [15], it was determined that sprites are related to intense discharges of lightning clouds from the ground (CG) and that they radiate electromagnetic energy at the ELF frequency (<1.5 kHz). In recent decades, the study of ELF signals has allowed us to understand transient luminous events (TLE) and the correlations between these phenomena.