# Progress in the Knowledge, Application and Influence of Extremely Low Frequency Signals
## 3 Results and Discussion
### Community Analysis
\"Propagation of Whistler Mode Chorus To Low Altitudes: Spacecraft Observations of Structured ELF Hiss\" with 71 references [97]. 3. \"Interferometric Determination of Broadband ELF Wave Phase Velocity Within A Region of Transverse Auroral Ion Acceleration\" with 84 references [98]. Figure 11b shows the 10 most present keywords in the works that make up this community, the first three are \"wave particle interaction\", \"radiation belt\", and \"chorus\", repeated 18, 12, and 11 times, respectively. All these words are characteristic of the theme developed in the community, as are most of those located among the 10 most-repeated. The oldest publication of this community dates from 1969, and presents the magnetic noise existing in the magnetosphere, in the frequency range of 10 to 800 Hz, measured by the American satellite OGO 3 [25]. This document also establishes a statistical study of the spatial extent and frequency of occurrence of these signals, drawing conclusions such as that constant noise and noise bursts constitute two distinct populations. The terrestrial magnetosphere is considered one of the sources of ELF radiation par excellence, mainly due to the presence of energy phenomena such as lightning. A detailed analysis of the trajectories of the rays present in the magnetosphere, establishes its division into four different categories, considering: (1) the rays that were damped in their first jump from the equator to high latitudes, or that were trapped in the limit of the plasma pause for 1 s or less; (2) the rays that entered the plasmasphere, and that evolve to become plasmaspheric whistles, lasting tens of seconds; (3) the rays that are magnetospherically reflected at higher long latitudes at high latitudes and that evolve towards the emission of ELF waves, holding times close to 1 s; and (4) rays that are approximately aligned with the field, carrying most of the power of the chorus wave, and are damped at mid-latitudes within a period of 1 s. Each of these categories of rays behaves morphologically differently, and in most cases corresponds to different emissions [26]. In addition to the rays reflected in the magnetosphere, ELF emissions are formed by transverse acceleration ions (TAI) and explosions of the suprathermal electron burst (STEB), both taking place at the top of the auroral ionospheric regions. In the third-most-important publication due to the size of the nodes, the characteristics of the TAI-derived ELF waves are established, and their possible generation mechanisms are discussed [98]. These emissions of magnetospheric plasma have been observed by instruments located in spacecraft that orbit the Earth in all lines of the auroral field, and at all local times, from ionospheric altitudes. An example of this is [96], which represents the most important node in this community and where measurements are carried out by instruments located in the Freja spacecraft. These measures allowed to verify that ELF emissions are formed by several wave modes, the frequency range of 30-400 Hz is mainly dominated by slow ion acoustic waves (SIA), characterized by an almost perpendicular polarization and a small magnetic component. This emission is associated with large mass ions (O +) of temperatures up to 30 eV and low temperature electrons (1-2 eV). Other components of wave mode, more difficult to identify are the alphanic waves or electric fields of great amplitude. Figure 11: Representation of natural ELF phenomena community: (**a**) isolated distribution of publications, (**b**) keywords. In [99], observations with this same satellite of 0-20 eV ions in the auroral ionosphere are presented. The results show a correlation between electronic precipitation and ionic heating for elements with energies below several hundred eV (energies associated with suprathermal electron explosions). So, the most intense heating events are strongly correlated with ELF waves and suprathermal electron bursts, the correlation being stronger for frequencies close to 500 Hz and electrons with energies of 500 eV.