Neutrino portals, terrestrial upscattering, and atmospheric neutrinos

We consider the upscattering of atmospheric neutrinos in the interior of Earth producing heavy neutral leptons (HNLs) which subsequently decay inside large volume detectors (e.g., Super-Kamiokande or DUNE). We compute the flux of upscattered HNLs arriving at a detector and the resultant event rate of visible decay products. Using Super-Kamiokande's atmospheric neutrino dataset we find new leading constraints for dipole couplings to any flavor with HNL masses between roughly 10 and 100 MeV. For mass mixing with tau neutrinos, we probe new parameter space near HNL masses of similar to 20 MeV with prospects for substantial future improvements. We also discuss prospects at future experiments such as DUNE, JUNO, and Hyper-Kamiokande.