Within our solar system, space weather is influenced by the solar wind and the interplanetary magnetic field carried by the solar wind plasma. Severe solar storms can expose people to increased radiation levels, shut down electrical systems, and interfere with radio signals. Space weather probably affects the weather and climate on our planet, but we do not yet have a precise understanding of the influence. Some aspects of space weather can be monitored at ground level with a detector, such as a “neutron monitor,” that measures cosmic rays from outer space. We have developed a portable “Changvan” neutron monitor to investigate the energy spectrum of cosmic rays and its solar modulation, i.e., variations over the typically 11-year sunspot cycle. The Changvan monitor located inside the insulated shipping container comprises three proportional counters in the standard-design neutron monitors, but the central counter is deficient in the lead producer. This monitor records counting rates during passage through a range of geomagnetic latitudes, which this technique is called “latitude survey.” The ultimate goal of the latitude surveys is to help accurately interpret data from every neutron monitor in a space environment that is always changing. In this work, we show a preliminary analysis of Changvan monitor data collected from two latitude surveys during 2019-2020 on the Chinese icebreaker MV Xue Long from Shanghai, China to Zhongshan station, Antarctica and back. A strong anti-correlation can be observed between the Changvan monitor count rate and barometric pressure.

We have developed a portable neutron monitor ("Changvan") with three sections to investigate cosmic ray spectral variations via latitude surveys. The Changvan uses the NM64 design for two sections, but the third lacks the lead producer, so we call this a "semi-leaded" neutron monitor. The Changvan was operated on two voyages on the Chinese icebreaker Xuelong between Shanghai and Antarctica during 2019 and 2020. Repeated measurements with the same detector over different phases of the solar cycle provide precise information about cosmic ray spectral variation. In addition, we tested two techniques to track spectral variations, which can be implemented at fixed stations. The count rate ratio of unleaded vs. leaded counters varies with geomagnetic cutoff rigidity, indicating sensitivity to the cosmic ray spectrum. Histograms of the time delay between successive neutron counts determine the leader fraction, also used to monitor short-term and solar-cycle spectral variations. We report measurements of the response functions of the count rates and leader rates of the unleaded and leaded counters during these two latitude surveys.

Cosmic rays are highly energetic particles from space. When cosmic rays hit the Earth's atmosphere, they produce a cascade of subatomic particles. A large portion of these particles are neutrons which can be detected using a neutron monitor either at a fixed location or shipborne. Shipborne neutron monitors have an advantage in that they can measure particles over a wide range of rigidity by conducting a latitude survey. In Thailand, we assembled the Changvan neutron monitor, a mobile NM64-type monitor consisting of three units of 10BF3 gas-filled proportional counters -only two of which are surrounded by lead. We made two expeditions (2018 and 2019 survey years) by sailing the Changvan from Shanghai to Antarctica and back. To study the energy-dependent effective area (yield function) of the Changvan leaded and unleaded counters, we perform a Monte-Carlo simulation in two steps. The first step simulates the interaction of the cosmic rays in the atmosphere and records the secondary particles that reach sea level. The second step injects these particles into a model of the detector with surroundings. Multiple atmospheric profiles are used to probe how changes affect the differential response function (𝐷 𝑅𝐹). We compare the simulated 𝐷 𝑅𝐹 with data taken during the survey year 2019. We investigate and discuss the differences in the 𝐷 𝑅𝐹 between the two leaded edge counters and the unleaded middle counter, all of which are inside the same reflector.

Figure 1: The count rate recorded by a neutron monitor is an indicator of the Galactic cosmic ray flux, which undergoes "solar modulation" related to solar activity [1-3]. As solar activity increases (shown in the top panel, Source: WDC-SILSO Royal Observatory of Belgium, Brussels), the pressure-corrected count rate recorded by the neutron monitor in Thule decreases (bottom panel, Source: Bartol Research Institute, University of Delaware, USA). The solar magnetic polarity reversal can be observed as the polarity shifts between positive (represented by A > 0) and negative (represented by A < 0) values. This work presents observations for the periods 1994-2007 and 2019-2020, as indicated by horizontal bars between the two panels.