High-altitude polar neutron monitors (NM) have exceptionally high sensitivity to solar energetic particles (SEP) due to the reduced atmospheric cutoff compared to sea-level stations and negligible geomagnetic cutoff in their locations. Along with the classic Ground-Level Enhancement (GLE) events caused by SEP, high-altitude polar NMs are able to register so-called sub-GLE events still significantly strong but not detectable by instruments at sea level. There are only two relatively short periods when at least two such instruments, which is the minimum requirement to detect SEP, were/are available for the research: from 2016 onwards with the neutron monitors Dome C and South Pole and from 1964 to 1972 with the South Pole and Vostok NMs.

In this work, we carefully investigated the period from 1964 to 1972 using the data from South Pole (2820 m a.s.l.) and Vostok (3488 m a.s.l.) NMs, as well as from sea-level McMurdo and Thule NMs. We searched for SEP event signatures in the data with several detailed SEP catalogues. Along with the well-known GLEs #15–25, we found two previously unknown sub-GLE candidates: on 09 June 1968 and 27 January 1969, seen as statistically significant increases in South Pole and Vostok NM data, but not observed by sea-level McMurdo and Thule NMs.