Galactic cosmic-ray recurrent and transient short-term variations: advanced data analysis and modeling

A particle detector (PD) on board the European Space Agency LISA Pathfinder (LPF) mission allowed for the detection of galactic-cosmic rays (GCRs) above 70 MeV/n. The LPF spacecraft (S/ C) orbited around the L1 Lagrangian point from February 2016 through July 2017. Proton and helium nuclei (constituting roughly 98% of the GCR bulk in the inner heliosphere) were sampled at 0.067 Hz and hourly-averaged data allowed for the study of long-term and short-term GCR flux variations with a statistical uncertainty of 1%. Since LPF was in orbit for one year and a half during the descending phase of the solar cycle N. 24 in a positive polarity period, a corresponding long-term increase in the mean GCR count-rate during the mission elapsed time was observed. Short-term GCR flux variations observed by LPF are studied in detail. Some of those variations are found to be recurrent in time since they are associated with solar wind disturbances originated from long-living structure on the Sun surface. The quasi-periodicities related to Sun rotation are investigated through the Hilbert spectral analysis, using the Hilbert-Huang transform. The association of GCR recurrent short-term variations with solar wind disturbances is discussed. Another class of short-term variations consists of depressions or peaks in the GCR count rate associated with transient solar wind disturbances. The most intense transient phenomena of solar origin are represented by the interplanetary counterparts of coronal mass ejections (ICMEs). The typical signature on GCR observations of an ICME transit is a sudden intensity decrease and a gradual recovery, called Forbush decrease (FD). Three FDs were observed during the LPF mission elapsed time. The second part of this work focuses on the characterization of a FD observed on board LPF on August 2, 2016. A near-Earth ICME transit was observed in correspondence of this FD and a smooth rotating magnetic field accompanied by low values of temperature and plasma-beta allowed for the identification of a clear magnetic cloud (MC) structure. The Grad-Shafranov (GS) reconstruction is applied in order to retrieve the large-scale magnetic field configuration within the ICME. The GS reconstruction is an advanced data-analysis technique aiming to recover the configuration of a magnetic closed structure starting from single S/C in a quasi-3-D geometry. After performing the GS reconstruction, we combined this technique with a suited Monte Carlo simulation, in the test-particle approximation, in order to investigate the effect of the MC passage in terms of cosmic-ray modulation. In this simulation, the only physical process able to modulate the isotropic particle flux is the particle drift associated with gradient and curvature of the magnetic field. The simulation results show an excellent agreement with LPF data during the FD and a study of the energy dependence of the simulated decrease is found to be in agreement with ground-based observations. Our result suggests that for the analyzed MC a fraction of low-energy particles do leak into the MC, remaining trapped and/or pushed out by external closed field lines. Moreover, the small magnetic field fluctuations observed inside the MC does not favor a large-scale cross-field transport through diffusion process or random walk. Hence, we suggest that gradient and curvature drifts are mainly responsible for GCR FDs originated by large-scale closed MC structure.