# Progress in the Knowledge, Application and Influence of Extremely Low Frequency Signals
## 3 Results and Discussion
### Analysis by Documents That Investigate in ELF
Another aspect of the last decades is the use of natural ELF signals for the diagnosis of the environment. This topic is of special interest due to its great applicability for the determination of global properties such as thunderstorms [15], among other things. In 2019 the number of publications decreased by almost 40% of those established in 2018--this may be because in that year 50% of the publications that make up the UELF waves community were developed [109] and they were not carried out years later. In Figure 16, the distribution of the different publication formats of the documents found in the search for Scopus is represented. Ninety-one percent of the total documents are found as an article (77.86%) or conference paper (13.14%). These results are like those found in other works [37]. Other contributions can be found in review format (3.76%) and book chapter (1.78%), and publications in the rest of the formats form 2.23% of the total. In book format there are only 37 contributions (0.3%), although in each of the two largest communities (ELF in health and lightning ELF phenomena), the most-important node by relationships is a book ([56] and [64] respectively), with a high-references number as less than 41 in most of the cases. English is the most-used language (92%) of the total publications, followed by Chinese (1.97%), French (1.74%), and German (1.7%). The rest of the languages have a representation below 0.5%, these being languages such as Japanese, Italian, Russian, Spanish, etc. It is common for the language by excellence in scientific publications to be English, since many scientific journals establish this in the  
Figure 16: Representation of the type of documents published in Scopus about ELF. norms of their editions, in order to have maximum dissemination regardless of the country of origin or the headquarters of the publisher. Only China produces many scientific publications in its own language, such as Gaodianya Jishu/High Voltage Engineering, Nongye Gongcheng Xuebao/Transactions of the Chinese Society of Agricultural Engineering, and Shengwuyixue Gong Chengxuezzhi/journal of Biomedical Engineering, among others. A community where the publication in Chinese journals is more than 85% of the total documents is in the ELF in crop effect community, where the effect of ELF electromagnetic fields on corn [36] and soybean crops is investigated [103], among others. The increase of the population in China is a problem due to the lack of resources that can be derived, so the study of the exposure of crops to these fields can produce an increase in their growth to ensure the existence of food in continuous growth for the population. The thematic areas most used for scientific dissemination are mainly medicine (14.76%), followed by biochemistry, genetics and molecular biology (12.66%), and engineering (12.38%). Areas of special interest are physics and astronomy (8.16%) and earth and planetary sciences (7.62%). In the rest of the areas, the contribution is less than 5%, as is the case of environmental science (4.24%) or agricultural and biological Sciences (3.76%). The large number of researchers in different disciplines such as medicine, engineering, physics, and astronomy allows us to state that the subject studied is a multidisciplinary issue of great impact and is currently being investigated [110; 111].