Abstract SSEs are closely linked to megathrust earthquakes, as they occur near seismically locked zones and can perturb stress conditions along plate boundaries. Understanding their timing is, therefore, crucial for assessing seismic hazards. This study investigates the recurrence intervals of slow slip events (SSEs) at four regions in Japan: Kii peninsula, Yaeyama islands, Boso peninsula, and Bungo Channel. Using renewal processes based on the Brownian Passage Time (BPT) model and the Gamma model as well as the moment method, the study estimates the mean recurrence intervals and their coefficients of variation (the standard deviation divided by the mean) along with the estimation uncertainties in each region. Final estimated mean recurrence intervals are 0.96 years (Kii), 0.51 years (Yaeyama), 3.97 years (Boso), and 3.16 years (Bungo) for all methods. Coefficients of variation reflecting event periodicity in the four regions (0.42−0.46, 0.25−0.40, 0.44−0.54, 0.26−0.28) suggest that SSEs are quasi-periodic, being neither completely deterministic nor highly random. Compared to earthquakes with recurrent patterns of occurrence, which are typically less sensitive to small stress changes, SSEs generally exhibit higher variability in the recurrence, suggesting greater sensitivity to external perturbations. However, Bungo SSEs, with higher stress drops, exhibit lower variability, indicating more stable recurrence. The bootstrap analysis also confirmed regional characteristics. In Kii, SSEs occurring at the intermediate area, with the distance from the Nankai Trough axis to the fault center is approximately 25–36 km, show greater timing variability. In Yaeyama, recurrence appears linked to slip size, suggesting time-predictable behavior. In Boso, the 2011 Tohoku–Oki earthquake likely influenced SSE timing. In Bungo, the unique 2014 SSE, possibly affected by the Tohoku–Oki event, was highlighted. In addition, the study performs predictive model comparisons for the baseline renewal process and a retrospective predictability analysis for all the regions, and provides forecasts of the next SSE in the Kii region along with prediction intervals for its occurrence time.

Abstract We present candidate models for the 14th generation of the International Geomagnetic Reference Field (IGRF-14), specifically the Definitive Geomagnetic Reference Field (DGRF) for epoch 2020.0 and the IGRF for epoch 2025.0. These models were developed using high-quality magnetic measurements from the European Space Agency’s Swarm satellite mission, including data from three low-Earth orbiting satellites equipped with Absolute Scalar Magnetometers and Vector Field Magnetometers. These models are first GC RAS candidates in the IGRF series, where we aim on methodological clarity and incremental innovation, and are part of the CAMPUS project, which is devoted to the construction of a series of Earth’s magnetic field models. Our methodology employs rigorous data selection criteria to minimize contamination from external sources and with strict scalar-vector consistency constraints. The internal magnetic field is parameterized using spherical harmonic functions up to degree 13, with coefficients determined through a two-stage inversion process involving linear inversion of vector measurements followed by nonlinear refinement including absolute scalar intensity data. For the DGRF 2020 model, we utilized data collected between January 2019 and December 2020, while the IGRF 2025 model incorporates measurements from January 2023 to September 2024. Comparison between the models indicates changes in field structure over the 5-year interval, particularly in the South Atlantic Anomaly (SAA) region. These candidate models effectively capture the main features of Earth’s core-generated magnetic field and are consistent with long-term trends observed in previous IGRF generations, contributing to the collective scientific effort to maintain accurate descriptions of Earth’s magnetic field. Graphical Abstract

Abstract The spatial distribution of electron density in the ionosphere exhibits notable variability and undergoes considerable changes during storms and substorms driven by solar wind disturbances. Electron density variations and irregularities can cause total signal blackouts of broadcast waves during strong scintillation periods and enhance satellite positioning errors. We analyzed Global Navigation Satellite System (GNSS)—total electron content (TEC) and Arase satellite observation data to elucidate the characteristics of the electron density variation in the plasmasphere and ionosphere during the May 2024 super storm. To identify the electron density variation in the ionosphere, we calculated the ratio of the TEC difference (rTEC), which is defined as the difference from the 10-quiet-day average TEC divided by the average value. Additionally, we estimated the electron density in the plasmasphere and inner magnetosphere from the upper frequency limit of the upper hybrid resonance (UHR) waves observed by the Arase satellite. Consequently, an L–t plot of the electron density showed that the plasmasphere contracted from L = 7.0 to L = 1.5 within 9 h after a sudden commencement. During the storm recovery phase, the plasmapause gradually shifted to a higher L-shell. The electron density in the plasmasphere recovered to the geomagnetically quiet-time level on a 4-day scale. The timescale of the plasmaspheric refilling was much longer than that of other coronal mass ejection (CME)-driven storms during the Arase era. The rTEC in the Northern Hemisphere showed that an enhancement in the rTEC value occurred at high latitudes [60°–70° in magnetic latitude (MLAT)] in the daytime [10–14 in magnetic local time (MLT)], approximately 1 h after the storm onset. Subsequently, a tongue of ionization (TOI) formed in the polar cap owing to the effect of the solar wind and magnetosphere in driving horizontal flows in the ionosphere. The rTEC was globally depleted during the storm recovery phase. The depletion indicates the occurrence of a negative storm owing to a neutral composition (O/N 2 ) change driven by the energy input from the magnetosphere in the high-latitude thermosphere. The coincidence of the long refilling timescale of the plasmasphere and the depletion of the rTEC suggests that a strong negative storm impedes plasmaspheric refilling. Graphical Abstract

Abstract Measurements from low Earth orbit satellites play an important role in modern geomagnetic field modelling. In this study, we present two geomagnetic field models, WHUEMM-S, derived by sequential inversion, and WHUEMM-C, derived by comprehensive inversion. Both models are constructed from calibrated Swarm A/B, GRACE-FO 1, and CryoSat-2 observations collected between January 2019 and July 2024. Both models represent the core field with degree 15 spherical harmonics and temporal sixth order B-splines. This study assesses the impact of these inversion strategies and evaluates the value of non-dedicated satellites in geomagnetic field modelling. Power spectral analysis shows that both models produce a temporally stable main field (MF) energy and secular variation (SV) energy, with differences from CHAOS-7.18 of about 1 nT 2 and 1 (nT/year) 2 for spherical harmonic degrees below 6. Stronger regularization damping in WHUEMM causes a sharp decrease in secular acceleration (SA) at degrees above 7. WHUEMM-C departs from CHAOS-7.18 mainly in the axial dipole and a few low-order sectoral terms, whereas the high-degree misfits in WHUEMM-S are probably driven by spectral truncation and residual external signals. Global MF maps confirm that both models reproduce mid- and low-latitude features well; however, at high latitudes WHUEMM-S deviates more from CHAOS-7.18 than WHUEMM-C does. SV derived from observatory records confirm that each model maintains smooth temporal end points and reliably captures long-term trends. This demonstrates that carefully calibrated, non-dedicated data from GRACE-FO 1 and CryoSat-2 can be used to build global geomagnetic models without compromising robustness. Finally, using WHUEMM-S as the parent model, we produced and submitted three IGRF-14 candidate models. Graphical Abstract

Abstract The strong nonlinearity of shallow-water deformation and tsunami run-up makes the application of the conventional inversion method to onshore tsunami records challenging. Additionally, many iterations of nonlinear tsunami simulations are required to capture the nonlinear characteristics of trace heights, which can be unfeasible. This study proposed a new approach for the nonlinear inversion of earthquake fault slips using tsunami trace height data. Instead of demanding simulations, a surrogate model was employed to reach a global optimum, which enabled the instantaneous estimation of the desired model output. The proposed method, which uses both tsunami waveforms and trace heights, was then applied to the 2024 Noto Peninsula earthquake in Japan. The results revealed a large slip of over 6 m offshore of the eastern Noto Peninsula. Our results demonstrate the potential of the new approach to complement coastal tsunami observations challenged by uncertainty and scarcity. Graphical Abstract

The magnetic reconnection environment around the dayside geomagnetopause under long-lasting southward interplanetary magnetic field (IMF) conditions is investigated using the in-situ observation by GEOTAIL satellite from 1994 to 2019. We focus on the degrees of asymmetry in the plasma density, ion temperature and the magnetic field strength between both sides of the magnetopause, that is, the ratio of the value in the magnetosphere to that in the magnetosheath in order to compute the much more realistic current sheet systems in numerical simulations. To exclude all of the interplanetary disturbance events such as Corotating Interaction Regions (CIRs) and Coronal Mass Ejections (CMEs), we investigate magnetopause crossings under long-lasting southward IMF conditions. GEOTAIL satellite sometimes repeatedly across the magnetopause during each pass due to the oscillating of the magnetopause. The degrees of asymmetry vary even during a single pass. This variation depends on the locus of the observation point, particularly the GSM Y-position, not on the time. The degrees of asymmetry in the plasma density, ion temperature and the magnetic field strength have significant variation in the data. The data points of the degree of asymmetry in the plasma density significantly spread in the duskside, while that in the magnetic field strength further widely spread in the dawnside. The degree of asymmetry in the plasma density and that in the magnetic field strength have fixed negative correlation on a log–log scale.Graphical

Accurate dating of young eruptions from explosive volcanoes is essential for forecasting future eruptions and for defining the hazardscape of volcanic fields. However, precise dating of Quaternary eruptions is often challenging due to limited number of applicable dating methods or lack of datable eruptive phases. Moreover, small volume eruptions (e.g., monogenetic type), despite their significance on regional scale, have traditionally deserved less attention than their large volume counterparts. Puketerata is a maar-lava dome complex in the central Taupō Volcanic Zone (New Zealand), encompassing mafic and silicic phreatomagmatic eruptions with well-preserved pyroclastic deposits sourced from closely spaced vents. Its most recent activity is estimated to ca. 16 ka based on medial and distal stratigraphic surveys. Here, we carried out two independent age determinations and an additional paleomagnetic analysis on the volcanic succession of the Puketerata maar-lava dome complex with an aim to unravel the timing of volcanic activity. Combined U-Th disequilibrium and (U-Th)/He dating of zircon from two lava domes yielded eruption ages of 11.3 ± 2.6 ka and 11.3 ± 1.7 ka, which are concordant with the radiocarbon ages of 11.3–11.7 ka obtained on charcoal from the base of the pyroclastic sequence. Paleomagnetic data on the lavas from the two lava domes suggest at least ~ 100 years difference between their emplacements. Our geochronological results and new stratigraphic observations suggest that the volcanic/magmatic history of the Puketerata is complex with multiple eruptions within a small, confined area, where the most recent eruptions occurred only at ca. 11.5 ka, which is significantly younger than previously thought. This provides an additional datum for volcanic hazards assessment and stratigraphic correlations in New Zealand.Graphical

Abstract The density of the solar wind plasma near the Earth’s magnetosphere sometimes decreases to only several per cent of the usual value, and such density extrema result in a significant reduction of the dynamic pressure and Alfvén Mach number ($$M_A$$ M A ) of the solar wind flow. While a symmetric expansion of the Earth’s magnetosphere by the low dynamic pressure was assumed in previous studies, a global magnetohydrodynamic (MHD) simulation study predicted a remarkable dawn-dusk asymmetry of the magnetospheric shape under low-density solar wind and Parker-spiral interplanetary magnetic field (IMF) configuration. Here, we present observations consistent with the asymmetric deformation of the magnetosphere under low-$$M_A$$ M A solar wind and Parker-spiral IMF conditions, focusing on the significant expansion of the dawn-flank magnetosphere detected by the Geotail spacecraft. A global MHD simulation reproduced the dawnward expansion of the near-Earth magnetosphere, which was consistent with the observation by Geotail. The solar wind flow had a non-negligible dusk-to-dawn component and partly affected the dawnward expansion of the magnetosphere. Local, roughly Alfvénic sunward acceleration of magnetosheath ions at the dawn flank magnetopause suggests magnetosheath plasma entry into the magnetosphere through open field lines generated by magnetic reconnection at the dayside magnetopause. At the same time, Cluster 1 and 3, located near the southern polar cusp, also detected continuous antisunward ion jets and occasional sunward jets, which are consistent with the occurrence of magnetic reconnection near the southern cusp. These observations suggest that enhanced plasma acceleration at the dayside magnetopause operates under the low-$$M_A$$ M A solar wind and Parker spiral IMF conditions and that plasma influx across the dawnside magnetopause is at work under such a low-$$M_A$$ M A condition. These results can be helpful in understanding interactions between low-$$M_A$$ M A solar/stellar winds and celestial objects, such as inner planets and exoplanets. Graphic Abstract

Geological and geodetic observations of the Moon from spacecraft revealed that it expanded by a few km for the first several hundred million years and then contracted later. The period when the planet expanded most coincides with that when the mare volcanism of the Moon was active. Given the high initial temperature of the deep mantle inferred from the giant impact and mantle overturn hypotheses of the Moon, the observed early expansion is difficult to account for by thermal expansion only. To understand the observed radial change of the Moon, we numerically calculated the thermal evolution of a one-dimensional spherically symmetric mantle caused by transport of heat, mass, and incompatible heat-producing elements (HPEs) by migration of magma that is generated by internal heating. The mantle is assumed to be enriched in HPEs at its base in the initial condition. The calculated mantle expands for the first several hundred million years by melting of the deep mantle and upward migration of the generated magma to the uppermost mantle; the top of the partially molten region rises to the depth level of around 300 km, which is shallow enough to generate mare basalts of the Moon. The migrating magma, however, extracts HPEs from the deep interior, and the planet then contracts gradually by cooling and solidification of the partially molten mantle. We obtained a thermal history model that is consistent with the observed history of radial change of the Moon when the initial mid-mantle temperature T_{{text{M}}} approx 1600 ,{text{K}} and the initial ratio of the concentration of HPEs in the crust to that of the mantle F_{{{text{crst}}}}^{*} le 12. This model suggests that melting of the deep mantle and upward migration of the generated magma strongly affect the thermal history of the Moon. The model we developed here is a good starting point for constructing more realistic models of the thermal history of the Moon where the effects of heat and mass transport by mantle convection are also considered.Graphical