The FengYun-3E satellite (FY-3E) is the world’s first civil morning-twilight orbiting meteorological satellite. It can overcome the spatial limitations of Ground-based Global Navigation Satellite System (GNSS) observations alone and provide critical data support for global ionospheric research. It is necessary to make full use of this data source. In this paper, we first used a deep learning model to correct the nonlinear biases in FY-3E radio occultation (RO) total electron content (TEC). Then, we integrated GNSS observations and FY-3E RO ionospheric data to model the global ionospheric TEC. Before correction, the root mean square error (RMSE) and mean absolute error (MAE) between the FY-3E RO TEC and GIM-TEC in 2025 were 5.92 TECU and 4.88 TECU, respectively. We used the 2024 data as the dataset to build a multi-channel Transformer bias correction model for FY-3E RO TEC, which is driven by temporal, spatial, solar, and geomagnetic parameters. After correction, the RMSE and MAE between the FY-3E RO TEC and GIM-TEC in 2025 are reduced to 4.26 TECU and 2.94 TECU, corresponding to improvements of 28
This study develops a long short-term memory (LSTM)-based framework for global three-dimensional electron density (Ne) modeling, using long-term (2001–2019) multi-GNSS radio occultation observations with solar and geomagnetic activity indices. The model’s extrapolation capability is evaluated for 2020 using vertical electron density profiles from eight globally distributed ionosondes, while consistency with Swarm-A in situ observations is assessed under both geomagnetically quiet and disturbed conditions. The performance of the LSTM model is compared with gated recurrent unit (GRU) and standard recurrent neural network (RNN) models. Results show that LSTM outperforms both GRU and RNN, with correlation coefficients exceeding 0.9 on the test set. The root mean square error (RMSE) exhibits an altitude-dependent pattern, peaking at 250–350 km and increasing at low latitudes. During extrapolation, the deep learning models generally outperform the International Reference Ionosphere (IRI) model, with LSTM yielding the lowest overall error. Under geomagnetically quiet conditions, Swarm-A Ne observations show strong correlations (R > 0.8) with all three models, with RMSEs below 0.04× 10^6 el/cm3. During disturbed periods, the correlations remain comparable, while RMSEs increase to above 0.06× 10^6 el/cm3.
Molecular absorption features constitute essential diagnostics of late-type stellar atmospheres, with titanium oxide (TiO) bands serving as sensitive tracers of effective temperature and magnetic activity. While near-infrared TiO indices are well established, visible-band diagnostics provide complementary constraints for stellar classification and parameter estimation. This study investigates the metallicity dependence of the B-index, a TiO absorption measure centered at 567 nm, through synthetic spectra generated with ATLAS9 and high-resolution observations from the HARPS spectrograph. The synthetic grid spans effective temperatures between 3500–4000 K and metallicities from = -4 to +0.2, while the observational sample comprises 23 MK spectral types stars with effective temperatures between 3500–4000 K. Both theoretical and empirical analyses demonstrate that the B-index exhibits negligible sensitivity to stellar metallicity, with only marginal correlations detected. Therefore, there is no requirement to include this feature in the calculation of the B-index. The B-index’s low sensitivity to metallicity reinforces its utility as an effective tool for spectral classification, stellar atmosphere modeling, and the study of magnetically active stars.
A novel and analytically simplifiable parametrization for the equation of state (EoS) of the dark energy, which elegantly captures the universe’s transition from a decelerated matter-dominated era ( ω≈ 0 at high redshift) to the current phase of accelerated expansion ( ω→ -1 as z→ 0 ) is introduced. The primary focus lies in a comprehensive theoretical formulation of this model within the standard Friedmann framework, deriving key cosmological parameters and exploring the model’s behaviour across cosmic history. This parametrization provides a smooth and physically consistent evolution of the EoS, naturally reproducing the observed transition from deceleration to acceleration around z≈ 0.5 , without invoking exotic fields or higher-order corrections. In contrast to conventional parametrized dark energy models such as Chevallier-Polarski-Linder (CPL), Jassal-Bagla-Padmanabhan (JBP), and Barboza-Alcaniz (BA) expansions, the present model stands out for its simplicity, stability, and freedom from divergences across the full redshift range. Using observed Hubble data, best fit values of model parameters are obtained and are consistent with current cosmological parameter values. The state-finder diagnostics and a phase space analysis well support the dynamic dark energy parametrization.
Ultracool and brown dwarf candidates (d_⊙≲ 125 pc, |b|>8^∘) were identified via multiband and astrometric data from CatWISE (W_1W_2), 2MASS (JHK_s), and Gaia DR3 (G, G_RP, π, μ_α, μ_δ). N≈ 11.8 × 10^3 candidates emerged once simultaneously constrained by color and absolute magnitude criteria (e.g., G-W_2 ≥ 1.75(G-J)-2.25), whereby ≃ 350 sources are absent from the Gaia Ultracool Dwarf (UCD) catalog. Spectral types were approximated using a hybrid M_G-M_W_1 sigmoid that offers additional temperature coverage in certain cases. Subsequent efforts may focus on extending sampling to the deeper NIR VVVX footprint that partly encompasses the Galactic plane, and over the long-term spectroscopically (in)validating candidates missing from the Gaia UCD database.
We investigate the global m=1 bending mode instability in radially hot, Milky Way-like galactic discs using a linear perturbation framework. Our analysis shows that radial heating plays a key role in driving the instability: as the Toomre parameter Q increases, the disc becomes progressively more susceptible to bending instability, accompanied by a systematic decrease in the growth timescale of the unstable m=1 mode. This behaviour reflects the enhanced in-plane random motions that weaken the effective restoring forces and facilitate the growth of bending modes. We further demonstrate that the dark matter halo significantly influences the stability properties of the disc, with low-mass halo systems supporting a larger number of unstable modes, while more massive halos enhance the vertical restoring force and suppress bending instabilities, leading to slower growth of the m=1 mode. Our results suggest that radially hot discs are inherently prone to global m=1 bending instabilities even in isolation, whereas massive dark matter halos act to stabilize the disc against such vertical distortions.
In this paper I will describe the basic principles of chemical evolution of galaxies, its main ingredients and uncertainties. By means of chemical evolution we can perform the so-called galactic archaeology, which consists in reconstructing the history of star formation of galaxies and in particular of the Milky Way, starting from the observed stellar and gas abundances. Galactic archaeology is a powerful tool to predict also the behavior of galaxies at high redshift. In particular, we adopt the “time-delay model” which is a way of interpreting the [X/Fe] vs. [Fe/H] relations (X is the abundance of a specific chemical element) in terms of different timescales of the stellar progenitors of the chemical elements relative to Fe, which is the indicator of “metallicity”. I will then describe how we can reconstruct the star formation histories of galaxies of different morphological type (spirals, ellipticals) starting from the available observations. Particular attention will be paid to the study of the Milky Way which is the best studied galaxy at the moment. I will start describing the first chemical evolution models and then the most recent ones, the main difference between the old and new models being the observational data to compare with. Finally, I will foresee which could be the future improvements to chemical models and what constraints can we derive to better understand galaxy evolution.
Optical colour variability and inter-band timing probe synchrotron evolution in blazar jets, but survey cadence can bias colour–magnitude trends and apparent lags when multi-band sampling is non-simultaneous and interrupted by seasonal gaps. We analyse Zwicky Transient Facility (ZTF) g - and r -band photometry of the high-synchrotron-peaked blazar 4FGL J0540.5+5823. Quasi-simultaneous colours are constructed by nearest-neighbour pairing within Δ t_max=0.03-0.05 d, and two-band activity episodes are defined objectively via Bayesian Blocks restricted to the g/r overlap. Colour–magnitude slopes are measured with robust regression, and their significance is assessed with a season-wise block-permutation test to account for clustered cadence. Inter-band delays are estimated with the z-transformed discrete correlation function (ZDCF) and, for the three best-sampled episodes, with flare peak-time offsets from asymmetric envelope fits. We find a persistent bluer-when-brighter trend ( b=0.063± 0.013 for Δ t_max=0.05 d) with sampling-aware significance p_block=6.3× 10^-3 . The global ZDCF prefers a positive lag of order 10 d but with broad uncertainty, while episode-level ZDCF lags are consistent with zero; envelope fits reveal one episode with an r -leading peak offset of 8.2± 2.2 d. The coexistence of a stable baseline colour trend with episode-dependent timing behaviour favours band-dependent profile evolution rather than a rigid inter-band delay, motivating denser multi-band and polarimetric follow-up.
This study presents the orbital dynamics of a spacecraft moving along a planar trajectory in the shape of a generalized parabola. The analysis method used is inspired by Siacci’s theorem, since the acceleration is decomposed into a central component and a tangential component. This allows us to obtain the expression for the continuous thrusts of the spacecraft as a function of the orbital distance, in cases where the thrust is parallel or perpendicular to the trajectory. Numerous original solutions are presented, which are then applied, for example, to the organization of orbital rendezvous or to suggest alternatives to the classical Hohmann transfer. In the latter case, Δ v and flight time comparisons are carried out, in particular with simulations of Earth-Saturn transfers.
Advances in Very Long Baseline Interferometry (VLBI) have enabled, for the first time, horizon-scale images of the two most accessible supermassive black hole candidates: M87* at the centre of the elliptical galaxy Messier 87 and Sgr A* at the centre of the Milky Way. These images reveal ring-like emission surrounding central brightness depressions whose angular sizes are consistent with the black-hole signatures predicted by general relativity. The theoretical framework and early predictions of observable black hole signatures, developed from the 1970s onward, established the scientific motivation and defined the instrumental requirements for this endeavour. Achieving the required angular resolution and sensitivity demanded transformational enhancements to existing VLBI arrays, pursued along two complementary paths: space VLBI, extending baselines beyond the Earth, and mm-VLBI, observing at the shortest accessible radio wavelengths from the ground. The latter strategy, realised through the addition of the Atacama Large Millimeter Array to a global network of mm-wavelength telescopes, advances in receiver technology, data processing, and image reconstruction methods, and the formation of an international collaboration, yielded the first images of the predicted black hole shadows. This established the observational foundation for a broad programme of follow-up studies.
A physically viable, effective anisotropic double-layer structure is constructed, consistent with the description of an effective stellar double-layer structure. To achieve this, Einstein’s field equations of gravity for a fluid in the anisotropic pressure regime were used, along with a linear equation of state related to quark matter and a quadratic equation of state related to exotic matter, to model the core and envelope, respectively. Our results reveal that this structure satisfies the dominant and strong energy conditions for an acceptable gravitational source and also exhibits high stability with respect to the Harrison-Zeldovich-Novikov criterion, convective motion, gravitational cracking, and dynamical stability assessed by radial pulsation analysis. In this respect, this model can serve as a basis for subsequent theoretical extensions, such as the inclusion of electric charge or even the influence of the cosmological constant, which could modify the internal structure, boundary conditions, and global properties of the system.
The energy deposition of fast electrons in interstellar molecular gas is considered. We use the rotationally resolved cross sections for electron-impact excitation of H2 molecule that were calculated using the adiabatic-nuclei molecular convergent close-coupling method. The initial electron energy distribution is assumed mono-energetic, and the differential equation for electron energy distribution is solved. We compare calculated energy deposition parameters with the results of similar studies in which the Monte Carlo approach was used. It is shown that about 11 per cent of the initial energy of fast electrons goes into direct ro-vibrational excitation of energy levels of H2 molecule including pure rotational excitation in neutral molecular gas. About 7 per cent of initial electron energy goes into the excitation to v=1 vibrational state of H2 molecule, most of this energy eventually converts into emission of transitions at near-infrared wavelengths. For ro-vibrational levels with v ≥ 3 , the electron-impact excitation to electronic states followed by downward radiative transitions to the ground electronic state is the dominant mechanism of excitation. The yields for excitation to vibrational states via radiative cascading from excited electronic states are found to be 1.5-2 times higher than were obtained in previous studies.
Testing the rigidity-dependent modulation of cosmic rays (CRs) through superposed epoch or quantitative regression analysis requires precise FD event timing and accurate FD magnitude of real Forbush events. However, superposition tendencies in raw CR data make analysis of CR data a non-trivial exercise. The current work demonstrates that analytical transformation of observational data, such as harmonic analysis, is required to separate the smoothed signal frequency components and other superposed spurious signals from the high-frequency component, which contains the real FD events. Given the comparative detailed and rigorous analyses performed, we speculate that some existing articles in the field, which selected FDs from raw CR data, may contain FD catalogs whose magnitude, number, and timing might possibly be biased by unwanted or spurious signals, such as 11-year solar cycle variations and CR anisotropies. For the first time, rigidity dependence modulation of CRs is tested using FD lists selected through detailed and rigorous analyses. The magnitude and timing of the FDs are not biased by spurious signals. Rather than the commonly employed two-variable regression model, we used multivariate regression analysis, taking into account contributions from other NM characteristics. We find that rigidity, altitude, latitude, and longitude make significant contributions to CR intensity variations.
Reaction rate determination is an essential problem in the theories of cosmological and stellar nucleosynthesis. I investigate the closed-form analytic evaluation of thermonuclear reaction rates, which are necessary for both cosmic and celestial nucleosynthesis. An alternate velocity distribution to the Maxwell-Boltzmann distribution can be considered in nuclear fusion processes occurring in the interior of stars when there is a diversion from hydrostatic equilibrium. The extension of non-resonant reaction rate integrals in standard, cut-off, depletion, and screening instances is examined in this study, utilising the MacDonald distribution. This study is primarily on the use of generalized special functions that represent the extended non-resonant thermonuclear functions in mathematical physics. Moreover, an attempt is made to analyse the reaction probability integrals corresponding to the several forms of the slowly varying cross-section factor S(E).
Particles in space and astrophysical plasmas can be efficiently accelerated through interactions with compressive magnetohydrodynamic (MHD) turbulence. In this work, we investigate the coupled transport and acceleration of suprathermal electrons in corotating interaction regions (CIRs) of the solar wind based on the transit-time damping (TTD) mechanism. Using reconstructed Parker spiral magnetic fields from solar wind observations, we estimate the transverse magnetic field gradients that characterize compressive turbulence in CIRs. From resonant interactions with fast-mode MHD waves, we derive the momentum and pitch-angle diffusion coefficients and solve the Fokker–Planck equation to follow the evolution of the electron momentum distribution. Our results show that TTD-driven stochastic acceleration naturally produces suprathermal electron populations from an initial Maxwellian distribution. The resulting suprathermal fraction can reach values of order 10^-3 - 10^-2 , depending on the plasma conditions and heliocentric distance. As the transverse magnetic field gradient decreases with radial distance, the overall acceleration efficiency is reduced while the range of resonant interactions broadens. We further find that lower plasma beta enhances the acceleration efficiency due to stronger magnetic compressions. These results suggest that compressive turbulence in CIRs can play an important role in generating suprathermal electron populations in the heliosphere.
The lack of a consensus on the spiral pattern rotation speed of the Milky Way Galaxy in the Lin-Shu density wave theory frame proposed in the 1960s shows the urgency of making use of the large volume and high-quality observational data. We study the kinematics of the spiral density wave pattern in the 3 kpc solar vicinity, using main-sequence OB, A, and F stars with Gaia DR3. Small nonaxisymmetric perturbations of the background axially symmetric gravitational potential that generate the spiral tightly wound waves are assumed. Systematic variations of stellar velocities, induced by these perturbations, are examined numerically utilizing a nonlinear least-squares fit. A calculation leads to the pattern rotation speed of Ω _p = 41– 49 km s^-1 kpc^-1 . This Ω _p differs from the angular velocity of mean circular rotation near the Sun of Ω _0≈ 27 km s^-1 kpc^-1 and sets the corotation circle well inside of the solar circle, at Galactocentric distances r_cor = 4–6 kpc (for the solar distance r_0 = 8.2 kpc). In agreement with the concept of slowly growing waves, the estimated amplitude of the perturbed spiral potential is small, i.e. ≲ 7% of the background potential |Φ _0| ∼ r_0^2Ω _0^2 . The system’s central bar and density wave driven spirals might be dynamically coupled because the bar rotates almost with the same angular velocity, with the value of the bar rotation speed of Ω _bar∼ 40 km s^-1 kpc^-1 drawn from the literature on bar dynamics.
Despite proximity of SN 2023ixf and a wealth of observational data, the released hydrodynamic models leave too broad range of the derived explosion energy and the ejected mass. We revisit the hydrodynamic modeling based on a broader set of observables than have been previously used. Among those of top priority is the early maximum ejecta velocity that is crucial in removing parameter degeneracy. The inferred parameters of SN 2023ixf are the explosion energy of 2.8× 10^51 erg, ejecta mass of 13.2 M_⊙ , presupernova radius of 1540 R_⊙ , and 56Ni mass of 0.07 M_⊙ . The circumstellar matter is composed by the dense circumstellar shell with the mass of 0.01 M_⊙ and radius of 5× 10^14 cm , as well as the external rarefied wind. Both circumstellar components are consistent with the early H α broad wings caused by the Thomson scattering and the intrinsic column density provided by X-ray data. Based on the radiation hydrodynamics we, for the first time, simulate the SN 2023ixf phenomenon from the explosion to the emergence of the hard X-ray radiation.
