Solar Physics最新文献

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.

Unraveling the intricate interplay between the solar photosphere’s magnetic field and the dynamics of the upper solar atmosphere is paramount to understanding the organization of solar magnetic fields and their influence on space weather events. This study delves into the organization of photospheric magnetic fields particularly in the context of coronal holes (CHs), as they are believed to harbor the sources of fast solar wind. We employed the signed measure technique on synthetic images that depict various arrangements of magnetic fields, encompassing imbalances in the sign of the magnetic field (inward and outward) and spatial organization.

This study provides compelling evidence that the cancellation functions of simulated regions with imbalanced magnetic fields along the boundaries of supergranular cells align with cancellation function trends of observed photospheric magnetic regions associated with CHs. Thus the analysis serves as a significant proof that CHs arise from the formation of imbalanced magnetic patterns on the edges of supergranular cells.

Using the magnetic power spectrum approach, we explore the magnetic energy changes at different spatial scales in four moderate-size decaying active regions (ARs). We find the dominant energy variations to take place at large spatial scales while the energy at low scales changes insignificantly. The analysis of the energy transfer function allows us to conclude that the direct turbulent cascade might occur occasionally and does not significantly contribute to the flux budget. Instead, we confirm the turbulent erosion, along with turbulent diffusion, to be the dominant mechanisms of the AR decay. We also reveal a gradual monotonous convergence of two coherent sunspots of opposite magnetic polarities as the decay proceeds. The sunspots exhibit magnetic connection seen as plasma loops in UV images. We suppose that the convergence is a result of an AR-size (Omega )-loop submergence beneath the photosphere.

The iodine-cell technique, which is known to be efficient in precisely establishing Doppler velocity shifts, was once applied by the author to measuring the solar differential rotation based on full-disk spectroscopic observations (Takeda and Ueno 2011). However, the data reduction procedure (in simple analogy with the stellar case) adopted therein was not necessarily adequate, because a specific characteristic involved with the disk-resolved Sun (i.e., center–limb variation of line strengths) was not properly taken into consideration. Therefore this problem is revisited based on the same data but with an application to theoretical spectrum fitting, which can yield absolute heliocentric radial velocities ((v_{mathrm{obs}})) in a consistent manner as shown in the study of solar gravitational redshift (Takeda and Ueno 2012). Likewise, instead of converting (v_{mathrm{obs}}) into (omega ) (angular velocity) at each disk point, which suffers considerable errors especially near the central meridian, (omega ) is derived this time by applying the least squares analysis to a dataset comprising (v_{mathrm{obs}}) values at many points. This new analysis resulted in (omega ) (deg day⁻¹) = (13.92 (pm 0.03) -1.69(pm 0.34)sin ^{2}psi -2.37(pm 0.62) sin ^{4}psi ) ((psi ): the heliographic latitude) along with the gravitational redshift of 675 m s⁻¹, which are favorably compared with previous publications. In addition, how the distribution of observing points on the disk affects the result is also examined, which reveals that rotation parameters may suffer appreciable errors depending on cases.