Two New Sub-GLEs Found in Data of Neutron Monitors at South Pole and Vostok: On 09 June 1968 and 27 February 1969

Intense solar energetic particle (SEP) events can be observed by neutron monitors (NMs) as so-called ground-level enhancements (GLEs). High-altitude polar NMs have high sensitivity for SEP due to the reduced atmospheric energy cutoff and very low geomagnetic rigidity cutoff compared to other NMs. There is a special class of sub-GLE events, viz. events that are weaker than standard GLEs and can only be observed by high-altitude polar NMs. So far, only one sub-GLE and three candidates are known, all in the period 2012 – 2015. In this work, we inspected the period from March 1964 to December 1969 in the data of the South Pole and Vostok high-altitude polar NMs on the Antarctic Plateau in search of possible sub-GLEs. We found two previously unknown events from 09 June 1968 and 27 February 1969 that formally match the definition of sub-GLE. They were associated with significant enhancements in the integral SEP intensity $J(>60\text{ MeV})$ $J(>60\text{ MeV})$ measured by space-borne particle detectors, as well as with strong X-class solar flares from the western part of the solar disk. The identified sub-GLEs were analyzed and the corresponding SEP characteristics were estimated.