# Progress in the Knowledge, Application and Influence of Extremely Low Frequency Signals
## 3 Results and Discussion
### Community Analysis
This study is not exempted from inconveniences, such as the exact determination of when these types of effects occur, the existence of interferences, etc., therefore subsequent work is necessary to verify the results obtained. An interesting job is [33]: it presents the effects of ELF magnetic fields on the nervous system and its functional changes. One of these changes is the perception of human pain, with a significant increase in pain sensitivity (hyperalgesia) after exposure to oscillating magnetic fields [85],but without affecting the sensory threshold. The effects on the neurogenesis of the hippocampus suggest that exposure to ELF fields could be useful to improve the function of this area of the brain, which is mainly involved in learning and memory, as well as to facilitate functional compensation of age. Another aspect to consider is if the exposure to EMF-ELF produces harmful cardiovascular effects [32]. The analysis of electrocardiogram data collected in studies such as [86] indicate that exposure to these fields exerts its primary influence during repolarization in the interval during which a flow of calcium ions occurs. These changes in the flow of ions can be a mechanism of regulation of the organism in the presence of ELF fields. During the last decades, several organizations have issued guidelines to limit occupational and public exposure to EMF-ELF, trying to prevent the acute effects they can cause. An example of such works is [87], where computational methods are used to estimate the dosimetry quantities of these fields. It is appreciated that the possible effects of these fields do not only depend on the duration of the exposure, but also on their intensity [86]. In all the works the same conclusion is reached, the results are quite variable and the mechanisms of action of the ELF fields are still poorly defined, so more experiments are needed to confirm the results. The ELF wave generation community consists of works based on the generation of ELF waves through different mechanisms or sources, published from 1963 to 2019. Figure 10a shows the isolated distribution of this community, where the two most important are observed by size. The Scopus references of the first three nodes by size are:  
1. \"Sensitive Broadband ELF/VLF Radio Reception with The AWESOME Instrument\" with 126 references [88], (VLF: very low frequency, AWESOME: atmospheric weather electromagnetic system for observation, modeling, and education). 2. \"ELF and VLF Wave Generation by Modulated HF Heating Of The Current-Carrying Lower Ionosphere\" with 95 references [4]. 3. \"ELF and VLF Radiation From The Polar Electrojet Antenna\" with 67 references [1]. The most common keyword found in this community's documents is the search word \"extremely low frequency\" with 15 repeats, as shown in the keyword representation in Figure 10b. The following words: \"ionosphere\" and \"ionospheric heating\", with 11 and 10 repetitions respectfully, are both words that are typical of the theme developing the community. ELF communications have been very important in recent years and continue to be researched today. These communications are very long-distance, as they involve large wavelengths and have been applied in very diverse fields, among which military communications stand out. An example of this is the application of this type of wave for communication between fixed stations and submerged submarines [89]. Many works have been developed and published based on the study of the generation,  
Figure 10: Representation of the ELF wave generation community: (**a**) isolated distribution of publications, (**b**) keywords. propagation, and reception of ELF waves, since their discovery in the middle of the 20 th century. Recently these communications have been used for emergency situations after natural or mining disasters [90], where other usual communication systems would not work. For an antenna to be an efficient radiator it needs to have dimensions of the order of the wavelength being irradiated, so ELF antennas need large dimensions.