Regarding this story, I would like to report on some travels I did over the last few years that have allowed me to refine the analysis. Since the starting point was an observation in Santa Cruz Island, in the Galapagos Archipelago, and even when I had already identified the factors I consider most relevant for the subject (the latitude and the moment of the year), I wanted to rule out:
1) That the phenomenon was specially related to the situation of its scene in an oceanic island and that the significant distance from the coast had a role to play. My visit to wild areas in the South of Florianopolis and to Campeche Island (close to it and inhabited) and to the Ballestas Islands, in Peru, plus the distant Fernando de Noronha, in the Brazilian Atlantic (350 kilometers far from the coast) and Rapa Nui (Easter Island), 3800 kms. far, gave me some elements in this sense.
2) That it was something related properly to the Pacific Ocean with all its particularities (its faults, trenches, currents, etc.). In addition to my visits to Rapa Nui and the Ballestas Islands, I got info from a tour to the Paracas Peninsula, in the Peruvian coast, and from a visit, in the same trip, to the city of Lima.
3) That I wasn't in front of something exclusive of the 0º latitude but before a phenomenon that could be appreciated also in close zones. The experiences of Fernando de Noronha, situated at 3º 50' and visited in its two seasons and the cities of Natal (5º 47') and Recife (8º 03') were useful in this sense. All the places visited from 2018 to 2024 are situated at 27º or less.
4) That it wasn't required to proceed to the observations during equinoxes. With this goal on mind, and also for some other factors (tickets availability, prices, convenience of traveling on certain dates for particularities of the destinations) I traveled in different moments of the year, mostly very close to equinoxes but also randomly or close to the summer solstice.
5) That the type of terrain with its peculiarities, flora, etc. (particularly the cactuses) had a special relevance for the subject. In order to study this, my visits to the Atacama area, in Chile, the Uyuni Salt Flats, in Bolivia, the Peruvian city of Nazca and the Paracas Peninsula were instrumental. I was also interested in the vastness of the open spaces in these areas to compare it with the situation of the Galapagos, isolated in the Pacific.
6) That the distribution of the flora had a relevant role, since the surroundings of Tortuga Bay are densely covered, with the sole exception of the access pathway. The South of Florianopolis, Campeche Island and Fernando de Noronha offered me the best opportunities in this sense, although their flora is different, closer to the so-called Atlantic forest.
7) That the altitude could have an influence on the subject. The Uyuni and Talar Salt Flats, Copacabana and Isla del Sol, in the Titicaca Lake, and the city of La Paz, all situated between 3500 and 4000 meters, gave me the opportunity to study this, relevant for the bulge produced in the equator by the Earth's rotation.
I wasn't able to hear, in any of these occasions, the sounds that gave origin to this research. In that same trip to Galapagos I visited various similar settings in Isabela and San Cristobal Islands without finding myself before the phenomenon again. This could be the result of the dates, which were already leaving behind the equinox, or of the latitude, closer to 0º in Santa Cruz that in the populated areas of Isabela and San Cristobal, which are almost at 1º. With this elements in hand I could conclude that, excluding any other possible factor present only in the area where the sounds were audible, it was all about the conjunction of the accurate moment and place.
I wanted to know also if, on the date of the observation, solar storms of particular intensity had been registered. The information I could obtain until now shows that it wasn't the case, although, curiously, an AI application had forecasted six considerable storms for that day. We must say, anyway, that the autumnal equinox of that year in the Southern hemisphere, occurred days earlier, on 20th March, had been preceeded by a total solar eclipse, the first of a series of three that will have this peculiarity (the coincidence of day with the equinox), separated by lapses of nineteen years. The influence this could have had on the phenomenon I was able to appreciate is still to be determined.
References
Lijo Jose, C. Vineeth et al. (2020). Response of the Equatorial Ionosphere to the Annular Solar Eclipse of 15 January 2010. JGR Space Physics Vol. 125, Nº 8.
López Soto, Ignacio (2022). Aplicación de técnicas de aprendizaje automático para evaluar y predecir la actividad geomagnética solar en las comunicaciones. Trabajo de fin de grado en Ingeniería en Tecnologías de Telecomunicación, Escuela Técnica Superior de Ingeniería, Universidad Pontificia Comillas, Madrid. Director de proyecto: Miguel Ángel Sanz Bobi.
Stankov, Stanimir et al. (2017). Multi-instrument observations of the solar eclipse on 20 March 2015 and its effects on the solar ionosphere over Belgium and Europe. EDP Sciences/ Journal of Space Weather and Space Climate Vol. 7.
Photo: Ahu Tongariki from the quarry of Rano Raraku (Rapa Nui).
