The geomagnetic field exhibits variations due to different phenomena. It has regular variations (the clearest example is the daily variation) and irregular variations (such as geomagnetic storms), considered in this sense as magnetic disturbances. Often, the latter have a sudden onset, so we call them rapid sudden commencements and they form part of what we call rapid magnetic variations. Other rapid variations that have aroused interest in the scientific community are geomagnetic bays -now known as sub-storms -, geomagnetic pulsations or solar flare effects. The study of rapid variations has been difficult, especially when it comes down to establishing a definition of the phenomenon and, overall, producing a physical explanation. Given this interest, many years ago, the IAGA (International Association of Geomagnetism and Aeronomy) created a specific committee for a deep study of these phenomena. This committee