Summary
The Galactic cosmic ray spectrum manifests pronounced variations over the 11-year sunspot cycle and more subtle variations over the 22-year solar magnetic cycle. An important tool to study these variations is repeated latitude surveys with neutron monitors onboard icebreakers in conjunction with land-based references. We revisit 13 annual latitude surveys from 1994 to 2007 using reference data from Mawson instead of McMurdo (closed in 2017). We then consider two more latitude surveys (2018 and 2019) with a monitor similar to the 3NM64 in the previous surveys but without lead rings around the central tube, a so-called "semi-leaded neutron monitor.'' The new surveys extend the linear relationship among data taken at different cutoff rigidity. They also confirm the "crossover'' in spectra measured near solar minima during epochs of opposite solar magnetic polarity and the absence of a crossover for epochs having the same solar magnetic polarity. This research was supported by the Development and Promotion of Science and Technology Talents Project (DPST) and NSRF via the Program Management Unit for Human Resources & Institutional Development, Research, and Innovation [grant number B39G660028]. The 2018 and 2019 surveys were conducted in cooperation with the Polar Research Institute of China (PRIC), We gratefully acknowledge the logistical support provided by Australia's Antarctic Program for operating the Mawson neutron monitor. We also extend our appreciation to the Northern Science Park (Chiang Mai) for providing laboratory space that enabled our research team to function smoothly.