# Progress in the Knowledge, Application and Influence of Extremely Low Frequency Signals
## 3 Results and Discussion
### Community Analysis
A statistical study of the ionic events presents in the plasma sphere, detected by the fast auroral snapshot explorer (FAST), indicates that 99% of the events found are associated with broadly extremely low frequency (BBELF) broadcasts or with electromagnetic ion cyclotron (EMIC) [100]. ELF events are the most numerous and cover 84% of the total, occur at all local times, with a peak near noon and a minimum near sunset. EMIC events are concentrated in the evening and at midnight, occurring at lower latitudes than the others. Another type of event widely studied in this community are choir waves, or magnetospheric choir emissions, which occur in the ELF/VLF band (0.3-12.5 kHz) [27]. These types of emissions are the most intense of all those generated within the Earth's magnetosphere, occur during episodes of anisotropy of the distribution of energy electrons (10-100 keV), and contain information related to the mechanism generating them. In [101] the data obtained by satellite and those of a ground station are compared and a mechanism for generating these waves based on the interrelation of ELF noise emissions and discrete emissions under cyclotron wave-particle interactions is discussed. This model opens new possibilities for the explanation of the characteristics of the generation of these chorus waves such as their high growth rates or their relationship with the ELF hiss. This phenomenon is widely studied in [97]. It is a broadband electromagnetic emission in the frequency range from a few hundred Hz to about 2 kHz. It has been observed for more than 4 decades by terrestrial systems and satellite observations, and normally occurs at high geomagnetic latitudes. It should not be confused with the plasmaspheric whistle, which remains enclosed in the plasmasphere and never penetrates at low altitudes or to the ground, unlike the ELF hiss. The pattern of occurrence of the low-altitude ELF hiss has some similar properties to those of the choir emissions in whistling mode, both emissions have a local time distribution and similar frequency range. Chorus emissions occur predominantly at dawn and during the day, being considered a possible source of highly accelerated electrons in the Van Allen outer radiation belt. This emission can be propagated as an unconducted emission and it has been shown that subject to the action of the reflection of the magnetosphere and the degradation of its structure can lead to ELF hisses. The outer electrons of the radiation belts can undergo a gyrosonant interaction resulting in various modes of magnetospheric waves, including chorus waves outside the plasmasphere, ELF hiss, or electromagnetic waves of ion cyclotron within the plasmasphere [102]. The electrons of the radiation belts are subject to precipitation into the atmosphere due to the dispersion of the resonant passage angle by plasma waves during the recovery phase of a storm or during periods of prolonged activity of substorms. One of the publications among the 10 most-important by its nodes is [102], where a timescale of loss of electron precipitation is established due to the dispersion by each of the EMIC wave, chorus, hiss and wave modes, which can be 1 day or less. Other communities: the group of other communities makes up less than 2% of the total work found, for this reason these communities are not studied in detail individually (Figure 12). This group includes the ELF community in crop effects (0.30%), the ELF in power lines community (0.11%), the geomagnetic pulsations community (0.21%), and the under extremely low frequency (UELF) waves community (0.09%). The ELF community in crop effects is constituted by the publications that evaluate the biological effects of ELF electromagnetic fields on crops such as soybeans, corn, or seeds, among other grains. The most-represented keywords in the documents of this community are represented in Figure 12b, with the highest number of repetitions being \"pulsed electric fields\" with 8 repeats.