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When solar energetic particle (SEP) events are observed at the ground by at least two sea- level neutron monitors (NMs) at different locations, they are called Ground Level Enhancements (GLEs). Very rarely, SEP-associated increases are observed exclusively at polar high-altitude NMs, which are the most sensitive NMs on Earth due to their reduced geomagnetic and atmospheric shielding. These events are called sub-GLEs. For this reason, sub-GLEs impose a sensitivity threshold on the NM network to SEP events, as they are characterized by increases that are not seen by any other NM at the ground. In this work, we employ the most recent NM yield function (Mishev et al., 2020) to quantitatively estimate the sensitivity of high-altitude (atmospheric depth ≈ 650 g/cm2) and sea-level (1033 g/cm2) polar NMs to SEPs. The sensitivity is defined as the minimum particle fluence above a given energy needed to observe a statistically significant increase in the NM count-rate. We denote this minimum observable fluence as the fluence threshold $F_{thres}$ (> $E_{eff}$) and refer to the corresponding reference energy as the effective energy $E_{eff}$.
Mishev, A. L., Koldobskiy, S. A., Kovaltsov, G. A., Gil, A., & Usoskin, I. G. (2020). Updated neutron‐monitor yield function: Bridging between in situ and ground‐based cosmic ray measurements. Journal of Geophysical Research: Space Physics, 125(2), e2019JA027433.