# Progress in the Knowledge, Application and Influence of Extremely Low Frequency Signals
## 3 Results and Discussion
### Community Analysis
This is what has driven the development of different types of ELF antennas, considering local and even spatial geographic characteristics, demonstrating that the D region of the ionosphere when illuminated by a powerful source can also act like an ELF antenna [91]. In recent decades, ELF waves have been shown to be generated through high frequency (HF) modulated heating (3-10 MHz region), which takes place in the D and E regions of the lower ionosphere (60-100 km). The modulated heating of the electron gas of these regions of the ionosphere results in the modulation of the corresponding conductivity tensor, based on the conductivity of the ionosphere depending on the temperature, and radiation is generated in the ELF band. This generation mechanism is the most common, with much research to prove it, such as [4], that represents the second-most-important node of this community. Another example from the literature of this type is [92], where the calibrated measurements of ELF waves generated by the HF-modulated heating of the ionosphere by the HAARP (high frequency active auroral research program) transmitter located in Gakona (Alaska) are presented. The generated signals were detected after spreading more than 4400 km in the Earth-ionosphere waveguide. [2] establishes the possibility of increasing the efficiency of the ELF generation by ionospheric modulation considering among other factors the ionospheric temperature and the existing modulated currents. In addition, characteristics of the ionospheric cavity where they are generated, such as the characteristic conductivity times of their region D, can be deduced from the ELF signals generated by the HF heating [91]. An alternative to the power modulation developed in the previous works is the geometric modulation, in which the ionospheric heating beam is geometrically directed at the desired frequency [93]. The waves generated in this way have improved amplitudes of 7 to 11 dB, and a directional dependence  
The most important publication in this community for its existing relationships with other publications is [88], which presents the reception of natural and artificial ELF waves through the AWESOME receiver (atmospheric weather electromagnetic system for observation, modeling, and education), the sensitivity characteristics achieved, their frequency, as well as the precision in time and their cross modulation. The main author of this publication, [PERSON], develops other works that are also located among the 10 most-important nodes of this community. One of them deals with the generation of ELF waves by striking two sources of ionospheric heating [3]. The resulting radiation pattern shows a strong directional dependence (up to 15 dB) that depends on the physical separation of the two sources. In [94] a 100-day record of ELF wave generation experiments is presented at the HAARP (High Frequency Advanced Auroral Research Project) facilities near Gakona, Alaska. It is observed that the generation during the local night is on average weaker but more variable, with a small number of very strong generation periods. The amplitudes of the signal from one day to the next can vary by up to 20-30 db. The third-most-important publication by its nodes is [1], which presents the characteristics of ELF radiation established by the polar electrojet antenna. In another work developed by the same authors almost a decade later, the results of the radiation produced by the Norwegian Tronse heating antennas are exposed and the efficiency differences achieved in these cases are indicated, as well as their possible improvements [95]. The natural ELF phenomena community is formed by works based on the determination and study of the characteristic of natural ELF phenomena existing in the terrestrial magnetosphere. Figure 11a shows the isolated distribution of this community, the Scopus references of the three most-important nodes by size are:  
1. \"Broadband ELF Plasma Emission During Auroral Energization 1. Slow Ion Acoustic Waves\" with 97 references [96]. 2.