Solar Physics最新文献

This study investigates the properties of waves that propagate along a density interface in partially ionised plasmas, separating two regions of different properties, including ionisation degree. Our analysis covers frequencies that are much smaller than the collisional frequency of particles, so we are using a single-fluid approximation, where the partial ionisation aspect of the plasma appears through the ambipolar diffusion in the generalised Ohm’s law. The derived dispersion relation is solved numerically. Our results show that guided waves along a density interface undergo very little change in their propagation speed (frequency); however, their damping rate shows variation with the ionisation degree and plasma-(beta ) parameter. We find that waves can only propagate when plasma-(beta >1.2), indicating pressure-driven dynamics relevant to photospheric structures with moderate magnetic fields. The damping rate increases with higher neutral particle content but decreases with higher plasma-(beta ) values. For ionisation degrees close to fully ionised plasma, the damping is minimal but becomes more significant as the neutral particle concentration increases. These findings provide important insights into wave behaviour in partially ionised plasma interfaces and lay the groundwork for future studies of wave propagation in partially ionised plasma slab waveguides.

Joy’s law is a well-established statistical property of solar active regions that any theory of active region emergence must explain. This tilt angle of the active region away from an east-west alignment is a critical component for converting the toroidal magnetic field to poloidal magnetic field in some leading dynamo theories, and observations show its importance for the reversal of the sign of the global solar magnetic dipole. This review aims to synthesise observational results related to the onset of Joy’s law, placing them within the broader context that describes active region emergence as a largely passive process occurring near the surface of the Sun.

During Solar Cycle 24, several groups independently noticed that a sharp transition in coronal activity occurred early in 2011. The transition took the form of a sudden jump in the intensity of (“hot”) EUV lines formed at temperatures above about log(T) = 6.1, whereas (“cool”) lines formed below log(T) = 6.0 showed little change. This led to the suggestion of bimodal behavior in the corona, and has been linked to the timing of the “terminator” of the previous solar cycle. An obvious question is whether this behavior is typical of solar cycle onsets in the corona. In this brief article we investigate whether the corona showed similar behavior at the onset of Solar Cycle 25, using data from the EUV Variability Experiment (EVE) on the Solar Dynamics Observatory (SDO) satellite. Previous observations have shown that hot coronal lines vary by orders of magnitude over the solar cycle while cool lines show very little variation. EVE has measurements of a number of strong coronal lines, and here we compare the onsets to Cycles 24 and 25 in the hot Si xii 499 Å line and the cool Ne viii 770 Å line. We find that, in contrast to Cycle 24, the onset of emission in the higher temperature lines during Cycle 25 is relatively gradual, with no clear indication of bimodal behavior, suggesting that sharp onsets of coronal activity are not a recurrent feature of the solar cycle.

The Sun’s differential rotation is a significant phenomenon that sets off the twisting of the magnetic field in loops, which in turn results in the formation of different solar activity indicators, such as sunspots (SSNs), flares, coronal mass ejections (CMEs). In this work, the differential rotation of the solar coronal region is investigated utilizing Solar Dynamics Observatory (SDO)/Atmospheric Imaging Assembly (AIA) full-disk images during 2011 and 2021 (including a portion of Solar Cycle 24 and Cycle 25 ascending phase) at the 21.1 nm wavelength. The equatorial region exhibits the highest average sidereal rotation rate (14.6°/day) decreasing to 13.6°/day towards the poles of both hemispheres. This study reveals that the average and equatorial rotation rates of the coronal region show patterns similar to solar activity during Solar Cycles 24 and 25. Abrupt variations in these rotation rates seem to correspond with the phases of the solar activity cycle. This indicates that abrupt variations in these rotation rates might be driven by fluctuations in solar activity. The analysis reveals that the 21.1 nm EUV corona exhibits a slight change in equatorial rotation rate and rotational gradient as compared to the 19.3 nm line. We also noticed that the present work shows negligible north-south asymmetry from 2011 to 2021. Furthermore, the rotational gradient is lower than that of the solar photosphere, suggesting it decreases with increasing altitude/temperature from the photosphere to the corona. We believe that the study of rotational parameters may be essential to map the magnetic behavior of the solar atmosphere. Furthermore, rotational parameters may help train AI models that will eventually be helpful in forecasting solar activity indicators.

Solar flares are energetic phenomena that influence coronal plasma dynamics through the magnetic reconnection-driven large-scale reconfiguration, heating and particle acceleration. Even though the energy release is usually strongly localised, it is well known that the flaring can impact a large part of the solar atmosphere through e.g. fast MHD shocks and particle acceleration. Coronal rain is a well known product of strongly stratified heating, seen in quiescent (non-flaring) and flaring conditions. This study investigates quiescent rain showers neighboring a flare site, focusing on their temporal evolution across the pre-flare, impulsive, and gradual phases. Using high-resolution imaging from the Interface Region Imaging Spectrograph (IRIS) and the Atmospheric Imaging Assembly (AIA) on the Solar Dynamics Observatory (SDO), we perform a quantitative comparison of rain quantity, intensity, and velocity before and after a C7.5 flare. Our results reveal an increase of approximately 27% in the average number of rain events from pre-flare to impulsive phases, suggesting a possible causal link with the flare perturbations. Besides, a significant increase in both average intensity and downflow velocity by 17% and 18%, respectively, from pre-flare to the gradual phases, suggesting a possible flare-induced density enhancement in the neighbouring coronal rain. These findings highlight the potential of using rain as a sensitive indicator of magnetic or thermodynamic changes, primarily governed by internal loop dynamics, but potentially influenced by external, flare-related perturbations.

Understanding and predicting solar-cycle variability requires accounting for nonlinear feedbacks that regulate the buildup of the Sun’s polar magnetic field. We present a simplified but physically grounded algebraic approach that models the dipole contribution of active regions (ARs) while incorporating two key nonlinearities: tilt quenching (TQ) and latitude quenching (LQ). Using ensembles of synthetic cycles across the dynamo effectivity range (lambda _{R}), we quantify how these mechanisms suppress the axial dipole and impose self-limiting feedback.

Our results show that (i) both TQ and LQ reduce the polar field, and together they generate a clear saturation (“ceiling”) of dipole growth with increasing cycle amplitude; (ii) the balance between LQ and TQ, expressed as (R(lambda _{R}) = mathrm{dev(LQ)}/mathrm{dev(TQ)}), transitions near (lambda _{R} approx 12^{circ }), with LQ dominating at low (lambda _{R}) and TQ at high (lambda _{R}); (iii) over (8^{circ }leq lambda _{R} leq 20^{circ }), the ratio follows a shallow offset power law with exponent (n approx 0.36 pm 0.04), significantly flatter than the (n=2) scaling assumed in many surface flux–transport (SFT) models; and (iv) symmetric, tilt-asymmetric, and morphology-asymmetric AR prescriptions yield nearly identical (R(lambda _{R})) curves, indicating weak sensitivity to AR geometry for fixed transport.

These findings demonstrate that nonlinear saturation of the solar cycle can be captured efficiently with algebraic formulations, providing a transparent complement to full SFT simulations. The method highlights that the LQ–TQ balance is primarily controlled by transport ((lambda _{R})), not by active-region configuration, and statistically disfavors the SFT-based (1/lambda _{R}^{2}) dependence.

Our study investigates the properties of Alfvén waves in partially ionised solar plasmas in the presence of steady, field-aligned, flows of charged and neutral particles. Our work aims to understand how such flows modify wave propagation and damping in environments where ion-neutral collisions are significant. We employ a two-fluid model that treats ions and neutrals as separate colliding fluids and incorporates background steady flows for both species. Using a combination of analytical dispersion analysis and numerical solutions, we examine the impact of these flows on the behaviour of Alfvén waves. Our results show that steady flows lead to substantial modifications of wave properties, including Doppler shifts, propagation direction reversal, flow-dependent changes in damping rates, and the appearance of a new mode associated with neutral flow and collisional coupling. We also identify conditions under which flow-driven mode conversion can arise. Our results offer new insights into the interplay between plasma flows and particle collisions in the regions of the solar atmosphere where partial ionisation is relevant.

Recently, a novel blast wave solution based on shock dynamics had been proposed. This study adopts a series of improvements and optimization strategies to develop this solution for the purpose of real forecasting capabilities, which leads to a Blast Wave Model (BWM). Firstly, an empirical formula was used to derive the initial shock velocity from the linear speed of the associated coronal mass ejections (CME) observed by the Large Angle and Spectrometric Coronagraph (LASCO) onboard the Solar and Heliospheric Observatory (SOHO). Secondly, a correction relation was introduced to account for the effect of the shock’s main propagation direction on its arrival time. Finally, an appropriate judgment index was established to allow the BWM model to determine whether a shock would reach the Earth. The BWM model was used to predict the arrival times of 337 shock events associated with CMEs from January 2013 to July 2023, and the prediction results demonstrated that the success rate for the shock’s arrival and non-arrival is as high as 64%. For those Earth-reaching events, the model had an averaged absolute forecast error of 9.1 hours for the arrival time, and a relative error of less than 15% for 61% cases. Compared with other models of the same kind (three versions of the Shock Propagation Model, the Shock Time of Arrival model), the BWM model shows a higher level of forecast accuracy and smaller prediction errors of the shock arrival time.

To model the structure and dynamics of the heliosphere well enough for high-quality forecasting, it is essential to accurately estimate the global solar magnetic field used as inner boundary condition in solar wind models. However, our understanding of the photospheric magnetic field topology is inherently constrained by the limitation of systematically observing the Sun from only one vantage point, Earth. To address this challenge, we introduce global magnetic field maps that assimilate far-side active regions derived from helioseismology into solar wind modeling. Through a comparative analysis between the combined surface flux transport and helioseismic Far-side Active Region Model (FARM) and the base Surface Flux Transport Model without far-side active regions (SFTM), we assess the feasibility and efficacy of incorporating helioseismic far-side information in space weather forecasting. We are employing the Wang-Sheeley-Arge solar wind (WSA) model for statistical evaluation and leveraging the EUropean Heliospheric FOrecasting Information Asset (EUHFORIA), a three-dimensional heliospheric MHD model, to analyze a case study. Using the WSA model, we show that including far-side magnetic data improves solar wind forecasts for 2013 – 2014 by up to (50%) in correlation and (3%) in root mean square error and mean absolute error, especially near the Earth and Solar TErrestrial RElations Observatory – Ahead (STEREO-A). Additionally, our 3D modeling shows significant localized differences in heliospheric structure that can be attributed to the presence or absence of active regions in the magnetic maps used as input boundaries. This highlights the importance of including far-side information to more accurately model and predict space weather effects caused by solar wind, solar transients, and geomagnetic disturbances.