It was exciting to find the traces of the explosion of the Hunga-Tonga volcano in the data collected by sensors of the detectors of the PolarquEEEst experiment, located more than 13500 km far away. We had mounted and put into operations those detectors back in 2018, in one of the more hostile locations in the world, the Spitzbergen Island, in the Svalbard archipelago. There, less than one thousand kilometers from the North Pole, the Ny-Alesund international Research Station hosts research projects and long term observations of teams from all over the world.

The explosion we detected took place on January 15, 2022, at 17:14:45 local time, corresponding to 04:14:45 UTC, destroying part of the Hunga Tonga–Hunga Ha'apai island, in the south Pacific Ocean, sending clouds of ash 40 km high into the atmosphere, causing tsunamis in Tonga, Fiji, American Samoa, Vanuatu, and along the Pacific rim. It caused also an atmospheric shock-wave which propagated around the globe, whose associated pressure disturbances were measured by weather stations at many locations. For instance, in New Zealand the maximum amplitude of about 7 hPa was recorded. In Europe, the shock-wave was measured at various locations, with a typical amplitude in the 1÷3 hPa range.

Very interestingly, the shock-wave circled the Earth two or even more times, while it is relatively rare for a volcano to generate a shock-wave strong enough to circle many times the globe. Another famous eruption giving rise to similar effects was the one of the Krakatoa volcano in Indonesia, which occurred in 1836 and was recorded by barometers all around the world, and is now considered a pioneering event in infra-sound monitoring studies. Consequently, there is an intrinsic interest for all the data about the Hunga Tonga eruption, and its associated atmospheric shock-wave.

We detected the shock-wave with the sensors hosted on the PolarquEEEst detectors, developed and put in place by the Extreme Energy Events collaboration, whose scientific goal is to measure the cosmic rays rate at the northernmost latitude. Three identical detectors are hosted at the research infrastructures managed by the Consiglio Nazionale delle Ricerche (CNR), and precisely at the Arctic Station "Dirigibile Italia", the Amundsen-Nobile Climate Change Tower, and the Gruvebadet Aerosol laboratory, respectively. They have been almost continuously taking data since June 2019.

The passage of the shock-wave over Ny Alesund manifested itself in the real time pressure and rate sensor readouts measurements from the POLA sensors, that we take in time steps of 30 seconds, and that are shown, for a period of 6 days starting from January 15, 2022, in the figure below. Indeed, some sharp pressure variations are well visible by eye, corresponding to the various passages of the atmospheric shock wave through the Ny-Alesund site.

In principle, the cosmic particle rate measure at the ground is expected to be influenced by atmospheric pressure variations, since these change the average height of the primary interaction on top of the atmosphere and the density of the medium the subsequent Extensive Atmospheric Shower propagates through. This so called "barometric" effect predicts an inverse correlation between cosmic particle rate measured on the ground and atmospheric pressure. It is a novelty for the field, and its existance has been experimentally verified, in a different occasion, by another group, exploiting essentially the same techniques used here. However, due to the limited acceptance of the POLA detectors, we were not able to put in evidence any fluctuation in the cosmic rays rate associated with the shock-wave generated by the Hunga-Tonga event. Simply, it was too small to be detected by our instrumentation, limited by the space and logistic constraints due to being in such a peculiar location.

Nevertheless, collected data were used to extract information about the shock-wave amplitude and its times of arrival at Ny Alesund, and, from these, about its group velocity, which turned out to be compatible both with theoretical predictions and other experimental measurements, performed at various locations in the world. Pressure data were also analyzed by means of techniques well beyond the usual frequency analysis traditionally performed in this field, namely exploiting algorithms that are employed to extract tiny signals over smooth background in multiple fields, from particle physics to neurology. This helped to put in evidence that the shock-wave was still travelling in the Earth atmosphere more than one week later of the Hunga-Tonga eruption, with a progressively decreasing amplitude.

This was a nice example of cross-contamination across different fields of the human knowledge, and, above all, an extremely exciting adventure!