# Progress in the Knowledge, Application and Influence of Extremely Low Frequency Signals
## 3 Results and Discussion
### Analysis by Documents That Investigate in ELF  
Figure 15 shows the temporal evolution of the documents published in Scopus, considering all the documents found with the search keywords (documents) as well as the documents that maintain relations in communities and are presented in Figure 4 (filter documents). The first documents were published in 1956, with an upward trend until 2018, at which point a decline occurs, as shown in 2019. In the evolution of the total documents there is an abrupt change, going from 362 documents in 1999, in which 159 are a source title, to 565 in the year 2000 (160 source titles). Subsequently the number of publications decreased to 297 in 2001 (160 source titles). This fact does not correspond to the number of source titles published in these years. If the major contributions in Scopus for 2000, Table 2, are carefully analyzed, it is verified that there is a substantial increase in contributions in 2000 and that three of them disappear in 2001, which contributed to 93 the previous year. The main difference between the evolution of the total documents and the evolution of the documents considered by the communities studied is the year from which production exceeds 100 documents. In the total documents, this trend begins in 1989, while in the documents that are related by communities, this occurs seven years later in 1996. The evolution of the documents considered in the communities also presents a temporary increase, although with less variability than that shown in the global document data. Until 1969, the number of publications on this subject was sporadic and less than 10 per year, the increase in research may coincide with the first investigations on geomagnetic pulsations. The most important node of the \begin{table}
\begin{tabular}{l c c c} \hline \hline \multirow{2}{*}{**Source Title**} & \multicolumn{3}{c}{**Year**} \\ \cline{2-4}  & **1999** & **2000** & **2001** \\ \hline Petroleum intelligence weekly & 6 & 53 & 2 \\ Middle East Economic survey & 0 & 40 & 0 \\ European chemical news & 2 & 30 & 0 \\ Petro strategies & 2 & 29 & 0 \\ Bioelectromagnetics & 9 & 20 & 23 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Main contributions in 2000 of total ELF documents. Figure 15: Representation of the temporal evolution of the documents published in Scopus on ELF. geomagnetic pulsations community [104], was published in that year and lays the foundation for extensive further work. The number of publications does not increase to more than 100 until 1996. In this year several works located among the most important nodes of the natural ELF phenomena community have been published [98] and the lightning ELF phenomena community [73]. From that year on, the number of publications increases almost every year, reaching a maximum of 223 documents in 2014, in that year the publication of several of the works that represent the main nodes of the biological effects community takes place, such as [36]. This general increase may be an indicator that, once the relationships between authors have been established, these collaborations can be maintained throughout the professional life of the researchers or of the research groups consolidated in the subject. Thus, for the novel authors on this subject to acquire a high H-index value, they need to inherit the relationships of the research group to which they belong (Table 1). The first publications of ELF waves date from 1956 and more than 66% of the publications have been published since 2000. One of the reason for this increase may be due to the increasing exposure of society to ELF electromagnetic fields and the need for determination of the effects that such exposure can have on human health, since from that time on, research with this perspective increases and more than half of the publications representing the most important nodes of health-related communities are developed such as ELF in health [28; 59] and the electrophysiological effects community [33; 75].