On March 24th, 2015, at about 15:30 hrs. I was in the Galapagos islands, more precisely in Santa Cruz. This group of islands is divided by the Equator: a majority of them is situated under it, while five (Darwin and Wolf, among others) are above. Santa Cruz is one of the southern islands, so at this date had just passed (on March 20th) its autumnal equinox. During the equinoxes the Sun is exactly above the Equator, so the solar wind affects Earth more intensely and it gets attracted by its magnetism. This is what makes the auroral activity stronger on these dates, because it is originated precisely in that interaction.
Some time after having entered the path that conducts to Tortuga Bay, the most famous beach on the island, I started hearing strong radio sounds. Having worked on radio stations for several years I was used to them, but I noted something quite strange on these ones in particular: It was not clear that human voices were involved, even when at some moments they seemed to arise. Everything sounded like an AM or shortwave transmission, but the apparent voices were gutural, as if there was a deteriorated recording tape somewhere. And that sound that suddenly came and then went away through the cactuses forest that surrounds the beach lasted for the half hour that takes getting there and also for my return. It was a bit late for that part of the world where the sun comes down all in a sudden at 6 PM, so the visitors were scarce and they were concentrated on the beach. I went over the whole path in both directions almost alone, and maybe because of it I was aware of any sound that could arise. There was no one in the forest, in which the opuntias tall as trees looked astonishing and, I think now, worked as antennas in that occasion. I say this because the sound could be heard inside the forest but not on the beach: it is also possible that there it had been masked by the sea sounds.
Now, why I connect this with the auroras, that occur so far away, in the zones around the polar circles? I do this for several reasons. The first of them is that they sound exactly like this, something that I would discover later. This sound is called auroral kilometric radiation and it coincides with what I could appreciate at that moment; but the existing recordings have been taken in space vehicles. The reason why auroras cannot be heard where they are seen is the ionosphere, the layer of the atmosphere fundamental for radio transmissions and technologies like the GPS, and also home of beautiful phenomena like the so-called falling stars (meteors) and auroras themselves. But the electric density of the ionosphere (which is composed by plasma, that is, by electrically charged gases) is not equal on the polar circles and on the Equator. An anomaly present in this zone along with the presence of bubbles in the plasma (disminutions in the electric density of the layer) could permit the pass of these sounds that in other zones of the planet cannot be heard. It seems that the bubbles would be the reason for this sound filtration, because the anomaly, on the contrary, is related with a higher electric density.
Other factors could have collaborated in this story that intrigued me for so long. We have to remember that the Galapagos are in the middle of the Pacific Ocean, almost one thousand kilometers far from the coast. Most of them are, in fact, the summits of submarine volcanoes. Maybe the equinoxial closeness of the Sun to the Equator and the isolation of the archipelago were the relevant factors here, the same way that a space vehicle starts to appreciate the auroral radiation when it gets far from Earth. It is possible that not only the ionosphere blocks the pass of the sound, but also some other factor that in the Galapagos, for their peculiar situation, is not present. We have to remember, for example, the existence of an equatorial bulge originated by the Earth's rotation that determines that the sea level is dozens of kilometers higher in the Equator that in the poles of the planet. Even considering that the same phenomenon generates bulking of the ocean and the atmosphere, we should not reject the idea that it could have been played a role in the interesting situation I experienced.
All this should be analyzed by whoever has the knowledge and the technology to do so, if they find the subject as interesting as I do. What I can say is that, if around the next equinox someone dares to walk by the narrow path that leads to Tortuga Bay, maybe they will hear strange and imponent voices from which all the time it seems that it could arise any intelligible word. But they will not be human voices, but a dialogue between the Earth and the Sun in a language that only they can comprehend.
Sources:
Adhikari, Binod, y Narayan P. Chapagain, "Polar Cap Potential and Merging Electric Field during High Intensity Long Duration Continuous Auroral Activity". Journal of Nepal Physical Society, Nº 1, vol.3, agosto 2015.
Asimov, Isaac. El planeta doble (The Double Planet). Compañía General de Ediciones, México, 1971. Traducción basada en el original de 1960 y la revisión de 1966.
Bronzato Nogueira, Paulo Alexandre, "Estudo da anomalia da ionização equatorial e dos ventos termosféricos meridionais durante períodos calmos e perturbados na região de baixas latitudes brasileira". Dissertação de mestrado do curso de pos-graduação en Geofísica Espacial. Instituto Nacional de Pesquisas Espaciais. Sao José dos Campos, Brasil, 2009.
Muon Ray, "The Strange Sounds of Earth's Aurora. Auroral Kilometric Radiation". Video basado en una grabación de la misión Cluster de la Agencia Espacial Europea y texto explicativo. https://www.youtube.com/watch?v=btDLs7OX2LA. 2015.
Stern, David. P. y Mauricio Peredo, "The Exploration of the Earth's Magnetosphere". Sitio web educativo de la NASA. https://www-spof.gsfc.nasa.gov/Education/Menpart2.html. 2000.
Picture: Isabela, Galápagos.
