## 3 Results and Discussions
### Comparison with Model Calculation  
The relationships between the neutron-dose rates calculated by using the analytical model (PARMA) and those measured with the portable neutron-monitoring system are plotted in Figure 5. Overall good agreements of the calculated and measured neutron-dose rates were produced for both routes covering a wide range of altitudes. Since the model calculations did not consider the shielding effects of the structure of the snow vehicle and onboard equipment, it was assumed that the slightly smaller measured values in the range of low dose rates (i.e., at lower altitudes) were due to the attenuation of the lower-energy secondary cosmic radiation inside the vehicle.  
On the other hand, some discrepancies (\(\sim\)10%) were seen at the high dose rates, i.e., at high-altitude areas. Since no major space weather events were reported in the respective period (i.e., from late December 2018 to January 2019) [52], this discrepancy is attributable to short-term changes of environmental factors such as atmospheric parameters (other than barometric pressure), heliospheric modulation by the solar wind, and geomagnetic cutoff rigidity at the monitoring location.  
It can be pointed out that the chronological variations of meteorological conditions such as clouds and snowfall affected the energy spectra of the cosmic neutrons, and those changes altered the responses of the neutron detector as the efficiency of WENDI-II varied depending on the neutron energy [44; 45]. However, this possibility is low since it is known that the shapes of cosmic-neutron-energy spectra in the atmosphere do not notably change with altitude. In a previous monitoring performed at Mt. Fuji, Japan, covering  
Figure 5: Relationship between the calculated neutron-dose rates (X-axis) using PARMA [18; 19] and the measured neutron-dose rates (Y-axis) along the routes shown in Figure 1 for the period from November 2018 to January 2019. The standard deviation of each plot was less than 3%.  
similar altitudes (40 to 3700 m) [53], the energy spectra of cosmic neutrons measured by using a Bonner multi-sphere neutron spectrometer were quite identical in their shapes, irrelevant to the altitude. This finding has been well-reflected in our model calculations. For example, the neutron-energy spectra calculated by PARMA at three locations along the routes (indicated with red points in Figure 1) are shown in Figure 6. It is seen that the shapes of neutron-energy spectra are nearly the same while the overall level of neutron flux decreases with increasing atmospheric pressure, i.e., decreasing altitude. Thus, the relative response of WENDI-II would be stable, regardless of the atmospheric pressure.  
Regarding the effect of geomagnetic-field variation surrounding the Earth, it was confirmed that the count rates of the ground-based neutron monitors (South Pole: SOPO and South Pole Bare: SOPB) placed at 90.0\({}^{\circ}\) S reduced by a few percent during the respective period [52]. As PARMA simply described the azimuth and zenith dependences of the cut-off rigidities assuming a dipole magnet [19], it can be assumed that, as a result of such simplification of the model structure, the observed count reductions of ground-based neutron monitors were overly reflected in evaluation of solar activities expressed as the values of heliocentric or force field potential, i.e., they caused the smaller cosmic-neutron intensities at the high-latitude and high-altitude area. This possibility should thoroughly be investigated with more comprehensive data in further research.  
These findings indicate the need for further studies for improving the reliability of model calculations of cosmic-radiation intensities in the atmosphere and imply that the portable neutron-monitoring system employed in the present study would be useful for achieving this task.