Speaker
Description
Galactic cosmic rays (GCRs) exhibit a small anisotropy around Earth, which presents as diurnal variation (DV) in the count rates of ground-based neutron monitors (NMs). This fluctuation has a typical amplitude of around 0.3 %. Although the properties of DV have been extensively studied, previous literature still lacks a generalized DV model. Such a model could be used, for example, to separate DV from other fluctuations or to incorporate it into models of cosmic ray variability. In this work, as part of my Master's thesis research,present the first steps towards empirically modelling DV using 1-hour measurement data from the Oulu NM. Five different DV models were derived using several approaches including Fourier transform, wavelet transform and superposed epoch analysis. The validity of the models was tested by subtracting them from the NM data and examining their effect on the diurnal signal in the multitaper power spectrum, as well as on the shape of the average DV derived using superposed epoch analysis. The NM count rate data was also examined after model subtraction during both high-amplitude and low-amplitude DV periods. The testing showed that the wavelet transform provides a useful indicator for the amplitude of DV across different time periods. For simpler applications, Fourier transform and inverse Fourier transform may provide a straightforward way to extract the DV signal from measurements. From Oulu NM data, I also investigated the properties of DV after Forbush decreases (FDs), ground-level enhancements (GLEs) and during different solar magnetic polarity. DV shifts by 2-3 hours to earlier hours after FDs in agreement with previous literature, although no amplitude change was observed. Following GLEs, a possible and interesting shift of about 1 hour to later times was observed. During periods of negative solar magnetic field polarity, the maximum of DV is seen about 1-2 hours later than during positive polarity, also consistent with earlier findings.