# Progress in the Knowledge, Application and Influence of Extremely Low Frequency Signals
## 3 Results and Discussion
### Community Analysis
A more precise interpretation of these events and the type of lightning that originates them has been established: in [73] these events are studied, determining that their duration is less than 1 ms and that they occur in the lower ionosphere between 75 and 105 km of altitude, just after  
Figure 8: Representation of the lightning ELF phenomena community: (**a**) publications, (**b**) keywords. the beginning of the cloud. A work to highlight is [74], where a global distribution of the rate of occurrence of these transient light events is established, observed by the FORMOSAT-2 satellite for 3 years. During the observation period, 80% of the registered TLE were elves (electromagnetic pulses generated by lightning strikes), and the remaining 20% sprites. Elves concentrate on the Caribbean Sea, the South China Sea, the eastern Indian Ocean, the central Pacific Ocean, the western Atlantic Ocean and the southwest Pacific Ocean; while sprites congregate over central Africa, the Sea of Japan and the western Atlantic Ocean. So, sprites are located mainly on the ground, as are lightning bolts, while elves are mostly found in the oceans and on the coasts. The electrophysiological effects community presents 7.44% of the total studied works and its isolated distribution is shown in Figure 9a. In all publications of this community the discussed topics are the following: topics related to electrophysiological and cognitive effects on the human brain, the cardiac system and the nervous system, derived from exposure to ELF electromagnetic fields. According to Scopus, the references of the documents that represent the most important nodes considering their size are:  
1. \"Human Electrophysiological and Cognitive Effects of Exposure to ELF Magnetic And ELF Modulated RF And Microwave Fields: A Review Of Recent Studies\" with 108 references [75]. 2. \"A Replication Study of Human Exposure To 60 Hz Fields: Effects on Neurobehavioral Measures\" with 109 references [76]. 3. \"A Consensus Panel Review of Central Nervous System Effects of The Exposure to Low-Intensity Extremely Low-Frequency Magnetic Fields\" with a total of 59 references [33]. Although it was not developed until 10 years later, it should be noted that the number of references of the most important article is almost the same. The most-used keywords in the documents of this community are represented in Figure 9b, the search word \"extremely low frequency\" is the most-found in the documents (73 times). The following words by repetition number are \"magnetic field\" and \"electromagnetic fields\", with 54 and 36 repeats respectively. Both are of a general nature and are found in all communities. There is also a very characteristic word of this community: \"electroencephalogram\" with 24 repetitions. Until the 1970s there was no awareness about how ELF fields affect the biological behaviour of human beings, developing some research in which the effects on animals were considered [77]. The concepts developed in works of this type allowed us to establish a simplified description of the interaction between an animal (or a conductive animal model) and an ELF electric field, as well as to develop qualitative estimates of different exposure situations and to be able to interpolate the results in human beings. The document with the major node in this study is [76], which complemented and reinforced previous studies on exposure to low-frequency electric and magnetic fields and neurological  
Figure 9: Representation of the electrophysiological effects community: (**a**) isolated distribution of publications, (**b**) keywords. outcomes [78]. In [76] a combination of electric fields (60 Hz and 9 kV/m) and magnetic fields (20 pT) was used, and the cardiac constants by electrocardiogram (ECG) and the activity by electroencephalogram (EEG) of the study subjects were studied. The results supported the hypothesis that ELF electric and magnetic fields interact and influence human functions, with a decrease in cardiac constants and alterations in measures of brain activity.