Solar Physics最新文献

High spectral resolution imaging spectroscopy plays a crucial role in solar observation, regularly serving as a backend instrument for solar telescopes. These instruments find direct application in deriving Doppler velocity from hyperspectral images, offering insights into the dynamic motion of matter on the solar surface. In this study, we present the development of a Fabry–Pérot interferometer (FPI) based imaging spectrometer operating at the Fe I (617.3 nm) wavelength for precise Doppler velocity measurements. The spectrometer features a moderate spectral resolution of (lambda/Deltalambdaapprox60{,}000), aiming to balance the imaging signal-to-noise ratio (SNR). The instrument underwent successful observational experiments on the 65-cm Educational Adaptive-Optics Solar Telescope (EAST) at the Shanghai Astronomy Museum. Obtained Doppler velocities were compared with data from the Helioseismic and Magnetic Imager (HMI), the maximum column and row correlation coefficients are 0.9261 and 0.9603, respectively. The estimated cut-off normalized frequency of the power spectral density (PSD) curve for velocity map is approximately 0.4/0.21 times higher than that observed in the HMI data, with potentially higher spatial resolution achievable under better seeing conditions. Based on the estimated imaging SNR levels, the accuracy of velocity measurements is approximately 50 m s⁻¹.

To estimate the hemispheric flux generation rate of the large-scale radial magnetic field in Solar Cycles 23 and 24, we use the photospheric observations of the solar magnetic fields and results of the mean-field dynamo models. Results of the dynamo model show the strong impact of the radial turbulent diffusion on the surface evolution of the large-scale poloidal magnetic field and on the hemispheric magnetic flux generation rate. To process the observational data set, we employ the parameters of the meridional circulation and turbulent diffusion from the Surface Flux-Transport (SFT) models. We find that the observed evolution of the axisymmetric vector potential contains the time–latitude patterns which can result from the effect of turbulent diffusion of the large-scale poloidal magnetic field in the radial direction. We think that the SFT models can reconcile the observed rate of hemispheric magnetic flux generation by considering radial turbulent diffusion and lower values of the diffusion coefficient.

Improvement of the model providing the boundary condition of the solar-wind speed near the Sun is essential for gaining a better forecast of space weather. We optimized the parameters of the distance from the coronal hole boundary (DCHB) model and the Wang–Sheeley (WS) model, which enabled the determination of solar-wind speed from observations of the Sun’s magnetic field. In this study, we used solar-wind speed data derived from interplanetary scintillation (IPS) observations at the Institute for Space-Earth Environmental Research (ISEE) for six Carrington rotations in Solar Cycle 23 as reference data. A comparison of IPS observations and optimized DCHB models demonstrated strong-to-moderate positive correlations and small deviations, except for solar maximum data. The degraded correlation at the solar maximum is ascribed to the effect of the rapid structural evolution of the solar wind and coronal magnetic field. The performance of the optimized DCHB model was better than that of the optimized WS model. To solve a limitation of the DCHB model in reproducing slow-wind speeds, we propose a modified version of the DCHB model and optimize it for IPS observations. The optimized solutions for the modified DCHB model demonstrate performance comparable to that of the original model. The results obtained in this study suggest that the DCHB acts better as a controlling parameter for the solar-wind speed than the expansion factor and that both the optimized DCHB model and its modified version are useful for improving the estimation of the solar-wind speed at the source surface from magnetograph observations.

On 21 April 2023, a significant M1.7 solar flare erupted from Active Region 13283, accompanied by a filament eruption and a full-halo Coronal Mass Ejection, which reached Earth on 23 April, triggering a severe geomagnetic storm, with Kp reaching 8 (G4) and Dst plummeting to −212 nT together with a sharply distinguished long-lasting negative double-dip behavior of the (z)-component of the interplanetary magnetic field. This event led to remarkable auroral displays, even at mid-latitudes in Europe. The flare-induced filament eruption caused distinct intensity dimming in the solar corona, observed in specific EUV wavelengths. We observed the dimming region growing at its fastest rate before the flare reached its peak of intensity. Notably, the proximity of the flare to a large southern coronal hole influenced the expansion and propagation of the coronal mass ejection toward Earth, probably impacting the solar wind speed and density. Additionally, we observed a sudden expansion of the coronal hole during the flare, leading us to speculating that the adjacent flare may have further stimulated the flow of solar-wind particles along the open magnetic-field lines. In accordance with the severe Dst-index disturbance, we also report changes in the potential of the pipeline of an Italian energy infrastructure company with respect to the surrounding soil as well as double-dip variation in the H-component of the terrestial magnetic field observed locally (reminiscent to what reported in Dst-index and IMF B_z) temporal profiles, confirming the effects of the geomagnetic storm at Italy mid-latitudes. Several solar radio events have been observed too. Therefore this study provides insights into the dynamic solar phenomena and their potential geomagnetic implications.

Low and Lou (Astrophys. J. 352, 343, 1990) presented a family of nonlinear force-free magnetic fields that have established themselves as the gold standard for extrapolating force-free magnetic fields in solar physics. Building upon this important work, our study introduces a novel grid-free machine-learning-based method to effectively solve the equilibria proposed by Low and Lou. Through extensive numerical experiments, our results unequivocally demonstrate the efficient capability of the machine-learning algorithm in deriving numerical solutions for Low and Lou’s equilibria. Furthermore, we explore the opportunities and challenges of applying artificial-intelligence technology to real observed solar active regions.

The Sun is a major source of energy for the planetary system in our solar system. The solar output shows variations in timescales from a few days (Bartel’s 27-day solar rotation cycle) to several years (the 11-year solar cycle and longer timescales). This variability can be seen in the magnetic field, particle flux, and electromagnetic radiation flux behavior. Several indicators, such as the sunspot number and the Mg II index, have been used as solar activity proxies. Further, direct measurements in radio at centimeter wavelengths have been conducted since 1947 (the F10.7 index). This work uses multiscale techniques to study the relations between these solar indexes and their long-term variations through multiscale techniques. The monthly averages of these indexes from 1979 to 2022 are analyzed using wavelet scalogram, global wavelet spectrum, wavelet cross-correlation, and wavelet entropy techniques. As a result, some nonlinear multiscale aspects in the long-term variations of these solar indexes are identified. The major scales at which these indexes vary are found to be, in order of decreasing energy: sunspots (130.1, 253.9, 11.7, 5.0, and 2.0 months); F10.7 (130.1, 253.9, 39.1, 10.9, 9.9, and 5.4 months), and Mg II (132.9, 39.0, and 10.3 months). Thus, all three indexes present the nearly 11-year solar cycle period as the strongest signal. The three indexes are correlated with a coefficient higher than 0.85 and vary in phase for scales near the 11-year solar cycle, with slight and large deviations from it for longer and shorter scales, respectively. The wavelet entropy analysis shows that the F10.7 and sunspot number values are comparable, while Mg II entropy values are much lower. The entropy also indicates that the minimum values for all the indexes occur close to the solar minimum. However, after the last solar maximum in 2014, the entropy increased even in the declining phase of the cycle, during the 2015 – 2020.

The solar wind, representing one of the most impacting phenomena in the circum-terrestrial space, constitutes one of the several manifestations of the magnetic activity of the Sun. With the aim of shedding light on the scales beyond the rotational period of the Sun (i.e., Space Climate scales), this study investigates the phase relationship of a solar activity physical proxy, the Ca II K index, with solar wind properties measured near the Earth, over the whole space era (last five solar cycles). Using a powerful tool such as the Hilbert–Huang transform, we investigate the dependence of their phase coherence on the obtained time scale components. Phase coherence at the same time scales is found between all the components and is also preserved between adjacent components with time scales ≳ 2 yrs. Finally, given the availability of the intrinsic modes of oscillation, we explore how the relationship of Ca II K index with solar wind parameters depends on the time scale considered. According to our results, we hypothesize the presence of a bifurcation in the phase-space Ca II K index vs. solar wind speed (dynamic pressure), where the time scale seems to act as a bifurcation parameter. This concept may be pivotal for unraveling the complex interplay between solar activity and solar wind, bearing implications from the prediction and the interpretation point of view in Space Climate studies.

One of the most exciting benefits of solar small-scale brightening is their oscillations. This study investigated the properties of small-scale brightening (SSBs) in different regions of the Sun and found that there are differences and similarities in the properties of oscillated and non-oscillated SSBs in different regions of the Sun, including quiet Sun (QS), the adjacent to active regions (AAR), and coronal hole (CH).

The damping per period (Q-factor) and maximum Doppler velocity of SSBs varied depending on the region, with the less bright internetwork SSBs in QS having lower damping time (120 seconds) and higher maximum Doppler velocities (47 km s⁻¹) compared to the brighter network SSBs (with 216 seconds & 37 km s⁻¹, respectively), while in AAR, internetwork SSBs tend to have higher damping time (about of 220 seconds) and wider maximum Doppler velocity (10 to 140 km s⁻¹) ranges compared to network SSBs (130 seconds and 10 to 85 km s⁻¹). In CH, both types of SSBs show similar damping time (120 seconds), but internetwork SSBs tend to have higher maximum Doppler velocities (100 km s⁻¹) compared to network SSBs (85 km s⁻¹).

It was also pointed out that the majority of network SSBs in AARs are in the overdamping mode, while in QS, internetwork SSBs demonstrate overdamping behavior and oscillated network SSBs exhibit critical damping behavior. However, it is important to remember that the physical mechanisms underlying the damping of SSBs may vary depending on the local plasma conditions and magnetic environment.

The asymmetry in hard X-ray (HXR) emission at the footpoints (FPs) of flare loops is a ubiquitous feature closely associated with nonthermal electron transport. In this study, we analyze the asymmetric HXR radiation at two flare ribbons, which is thermal-dominated during a long-duration C4.4 flare that occurred on March 20, 2023, combining multi-view and multi-waveband observations from the Advanced Space-based Solar Observatory (ASO-S), Solar Orbiter (SolO), and Solar Dynamics Observatory (SDO) spacecraft. We find that the H i Lyman-alpha (Ly(alpha )) emission presents similar features to the He ii (lambda)304 emission, both in the light curve and spatio-temporal evolution of a pair of conjugate flare ribbons. The spectra and imaging analysis of the HXR emission, detected by the Spectrometer Telescope for Imaging X-rays (STIX) in 4-18 keV, reveal that the two-ribbon flare radiation is thermal dominated by over 95%, and the radiation source mainly concentrates on the northern ribbon, leading to an asymmetric distribution. To understand the underlying reasons for the HXR radiation asymmetry, we extrapolate the magnetic field within the active region using the nonlinear force-free field (NLFFF) model. For 78% of the magnetic field lines starting from the northern flare ribbon, their lengths from the loop-tops (LTs) to the northern FPs are shorter than those to the southern FPs. For 62% of the field lines, their magnetic-field strengths at the southern FPs exceed those at the northern FPs. In addition, considering the larger density, (approx1.0times10^{10} {mathrm{cm^{-3}}}), of the low-lying flare loops ((< 32 {mathrm{Mm}})), we find that the shorter path from the LT to the northern FP enables more electrons to reach the northern FP more easily after collisions with the surrounding plasma. Therefore, in this thermal-dominated C-class flare, the asymmetric location of the flare LT relative to its two FPs plays a dominant role in the HXR radiation asymmetry, while such asymmetry is also slightly influenced by the magnetic mirror effect, resulting in larger HXR radiation at the FPs with weaker magnetic strength. Our study enriches the understanding of particle transport processes during solar flares.

The meridional circulation of the solar magnetic fields in Solar Cycles 21 – 24 was considered. Data from both ground-based and space observatories were used. Three types of time–latitude distributions of photospheric magnetic fields and their meridional circulations were identified depending on the magnetic-field intensity. (i) Low-strength magnetic fields. Positive- and negative-polarity magnetic fields were distributed evenly across latitude and they weakly depended on the magnetic fields of active regions and their cycle variation. (ii) Medium-strength magnetic fields. For these positive- and negative-polarity magnetic fields a sinusoidal wave-like, pole-to-pole, antiphase meridional circulation with a period of ≈22 yr was revealed. The velocities of meridional flows were slower at the minima of solar activity, when they were at high latitudes in the opposite hemispheres, and maximal at the solar maxima, when the centers of positive- and negative-polarity flows crossed the equator. The time–latitude dynamics of these fields coincides with that of coronal holes and reflects the solar global magnetic-field dynamics including the solar polar-field reversals. (iii) High-strength (local, active-region) magnetic fields. They were distributed symmetrically in the northern and southern hemispheres. The magnetic fields of active regions were formed only during the periods when the medium-strength positive- and negative-polarity magnetic fields approached at low latitudes. Magnetic fields of both leading and following sunspot polarity migrated from high to low latitudes. The meridional-flow velocities of high-strength magnetic fields were higher in the rising and maximum than in the declining phases. Some of the high-latitude active-region magnetic fields were captured by the second type of meridional circulation flows and transported along with them to the appropriate pole. However, the magnetic fields of active regions are not the main ones in the solar polar-field reversals. The results indicate that high-strength magnetic fields were not the main source of weak and medium-strength ones. The butterfly diagram is the result of a superposition of these three types of magnetic-field time–latitude distributions and their cycle evolution. The results suggest that different strength magnetic fields have different sources of their generation and cycle evolution.