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Ionospheric irregularities can induce Global Navigation Satellite System (GNSS) scintillation and carrier-phase loss of lock, thereby degrading the reliability of high-precision positioning and related space-based services. Using observations from ∼250 GNSS stations of the Crustal Movement Observation Network of China (CMONOC) spanning February 2011 to December 2023, the rate of TEC index (ROTI) was derived to characterize the solar-cycle climatology of ionospheric irregularities as a function of latitude, local time (daytime/nighttime), and season, and to quantify their responses to solar and geomagnetic forcing. A clear latitudinal─diurnal regime structure emerges. At low latitudes (<25°N), irregularities occur most frequently at nighttime and exhibit the strongest dependence on F10.7. In the mid-latitude belt (25°─35°N), the daytime─nighttime contrast weakens, indicating a transition regime more strongly modulated by regional processes such as gravity waves and medium-scale traveling ionospheric disturbances (MSTIDs). North of 40°N, irregularity activity becomes more pronounced during daytime, and the correlation with F10.7 strengthens again, consistent with the increasing influence of storm-time energy input toward subauroral latitudes. Pronounced seasonal asymmetries are also observed. Daytime irregularity activity is enhanced in spring─summer and minimized in winter, whereas nighttime activity peaks in spring─autumn and is weakest in summer. During the rising phase of Solar Cycle 25, autumn irregularity activity intensifies markedly in both daytime and nighttime sectors, suggesting a shift in seasonal dominance from spring toward autumn. Finally, low-latitude nighttime irregularities show significant correlations with F10.7 and geomagnetic indices (Ap, Kp, AE, and Dst), while corresponding daytime correlations are generally weaker.

Evidence of a gravitational wave (GW) signal has emerged in pulsar timing array (PTA) data, opening a new window into the nanohertz GW Universe. We explore the physics of GW signals that may explain the data, with a focus on GW backgrounds (GWBs) considering both astrophysical and cosmological origins. We describe how: ▪ An astrophysical nanohertz GWB emerges as the superposition of individual signals from inspiraling massive black hole binaries. ▪ Environment coupling, eccentricity, and sparse sampling cause great uncertainty in the theoretical prediction of the supermassive black hole signal but also offer a way to determine the origin of the signal. ▪ PTA data offer unprecedented opportunities to constrain high-energy physics beyond the Standard Model by probing early Universe GWBs that originated during or after inflation. ▪ Different early Universe GWBs, typically created by nonlinear and out-of-equilibrium dynamics, can explain the PTA data (e.g., those from inflation scenarios, first-order phase transitions, or topological defects). ▪ The PTA detection of GWs opens a new window to explore the Universe, with profound implications for astrophysics and particle physics (probing, e.g., the equation of state of the early Universe, the origin of cosmological perturbations, the nature of dark matter, or whether exotic objects like primordial black holes or cosmic strings exist). An astrophysical nanohertz GWB emerges as the superposition of individual signals from inspiraling massive black hole binaries. Environment coupling, eccentricity, and sparse sampling cause great uncertainty in the theoretical prediction of the supermassive black hole signal but also offer a way to determine the origin of the signal. PTA data offer unprecedented opportunities to constrain high-energy physics beyond the Standard Model by probing early Universe GWBs that originated during or after inflation. Different early Universe GWBs, typically created by nonlinear and out-of-equilibrium dynamics, can explain the PTA data (e.g., those from inflation scenarios, first-order phase transitions, or topological defects). The PTA detection of GWs opens a new window to explore the Universe, with profound implications for astrophysics and particle physics (probing, e.g., the equation of state of the early Universe, the origin of cosmological perturbations, the nature of dark matter, or whether exotic objects like primordial black holes or cosmic strings exist).