Study for Nanosatellites of Multiscale Temporal Evolution of Quasi-Periodic Pulsations in X-Class Solar Flares

Solar flares are among the most energetic phenomena in the heliosphere, releasing up to 1032 erg of magnetic energy within minutes to hours. The temporal structure of this energy release is highly complex, often exhibiting quasi-periodic pulsations (QPPs) across timescales ranging from fractions of a second to several minutes. Despite decades of research, the physical origin of these pulsations remains unresolved. This thesis investigates the multiscale temporal evolution of QPPs in X-class solar flares using high-cadence X-ray observations from the Spectrometer/Telescope for Imaging X-rays (STIX) onboard Solar Orbiter. The Hilbert-Huang Transform—combining Empirical Mode Decomposition (EMD) with Hilbert spectral analysis—is employed to extract instantaneous frequency and energy information from five X-class flare light curves in the 16–28 keV energy band. This adaptive method overcomes the limitations of traditional Fourier-based approaches for non-stationary flare signals. The reliability of the EMD-Hilbert pipeline is assessed through synthetic-data experiments with known oscillatory components and correlated noise. The analysis reveals recurrent frequency concentrations spanning low (∼6–10mHz), intermediate (∼18–30 mHz), and higher (∼45–70 mHz) ranges across the analyzed events. The low and intermediate components are consistent with standing Alfvénic modes in coronal loops. The high-frequency components, predominantly observed during the decay phase, point to nonlinear energy transfer. A key finding is the transition from resonance-dominated impulsive phase to turbulence-dominated decay phase, with power-law spectral scaling β ≈ 1.8±0.1 during the decay phase. This scaling agrees with theoretical predictions for reduced magnetohydrodynamic (RMHD) turbulence in coronal loops (Nigro et al., 2008). The results demonstrate that QPPs manifest the interplay between resonant wave excitation, fragmented reconnection, and turbulent cascade processes. This unified interpretation provides new observational constraints on flare energy release. Additionally, this thesis supports the Sun CubE OnE (SEE) mission, a proposed 12U CubeSat for high-cadence solar monitoring. Laboratory characterization of Silicon Photomultiplier (SiPM) detectors validates compact detector systems for nanosatellite based flare observations, establishing key requirements for future space weather missions.