In southern India, in the city of Madurai, from 2018 to 2022, an unusual 'catcher' of cosmic particles stood — the mini-ICAL prototype, weighing 85 tons. It is a 600-times scaled-down version of the future neutrino detector (those elusive ghost particles) that will be built deep underground. But while the big facility waits its turn, the small copy worked diligently: it registered muons — heavy relatives of electrons, born in the atmosphere when cosmic rays bombard it.
The site's key feature is its location. Madurai sits almost on Earth's magnetic equator, where the planet's magnetic field most strongly filters out charged particles from space. It's like a sieve with very fine mesh: low-energy particles simply cannot break through the magnetic armor. The cutoff threshold here is 17 gigavolts (for comparison: at the poles it's nearly zero). So the muon flux was, as expected, lower than at other observatories around the world.
The measurements covered a momentum range of roughly 1 to 5 gigaelectronvolts over the speed of light. Comparison with predictions from different models (built into the CORSIKA program) showed that the data sit slightly below the calculated curves, but the overall picture matches. This is good news for the future full-scale experiment: it means the models can be tuned more precisely.
So a small prototype at the magnetic equator became a calibration tool for big science. It confirmed that Earth's magnetic field acts as a natural filter, and this must be taken into account when 'hunting' for neutrinos.
Earth's magnetic field at the equator blocks about three times more cosmic particles from reaching us than at the latitude of Moscow or Paris. So tropical residents get less radiation from space — at least from charged particles.