Solar Physics最新文献

We investigate the long-term dynamic behavior of the sunspot penumbra to umbra area ratio by analyzing the Debrecen Photoheliographic Data (DPD) of sunspot groups during the period 1976–2017 (Solar Cycles 21–24). We consider all types of spots and find that the average penumbra–umbra ratio does not exhibit any significant variation with spot latitudes, solar-cycle phases as well as sunspot-cycle strengths. However, the behavior of this ratio is different when we consider the latitudinal distribution of the northern and southern hemispheres separately. Our analysis indicates that for daily total sunspot area the average spot ratio varies from 5.5 to 6.5 and for very large sunspots (> 5000 (mu)Hem; one (mu)Hem is (10^{-6}) the area of visual solar hemisphere) its value rises to about 8.3. In the case of the group-sunspot area, the average spot ratio is ∼6.76. Furthermore, we found that this ratio exhibits a trend for both smaller (area <100 (mu)Hem) and large (area > 100 (mu)Hem) sunspots. Finally, we report the periodic and quasiperiodic variations present in this ratio time series after applying the multitaper method (MTM) and Morlet-wavelet technique. We found that along with the ∼11-year solar-cycle period, the penumbra to umbra area ratio also shows several midterm variations, specifically, Rieger-type and quasibiennial periodicities. We also found that Rieger-type periods occur in all cycles, but the temporal evolution and the modulation of these types of periodicities are different in different solar cycles.

Abstract

The Atacama Large Millimeter/submillimeter Array (ALMA) is a general purpose telescope that performs a broad program of astrophysical observations. Beginning in late 2016, solar observations with ALMA became available, thereby opening a new window onto solar physics. Since then, the number of solar observing capabilities has increased substantially but polarimetric observations, a community priority, have not been available. Weakly circularly polarized emission is expected from the chromosphere where magnetic fields are strong. Hence, maps of Stokes V provide critical new constraints on the longitudinal component of the chromospheric magnetic field. Between 2019 and 2022, an ALMA solar development effort dedicated to making solar polarimetry at millimeter wavelengths a reality was carried out. Here, we discuss the development effort to enable solar polarimetry in the 3 mm band (ALMA Band 3) in detail and present a number of results that emerge from the development program. These include tests that validate polarization calibration, including evaluation of instrumental polarization: both antenna-based “leakage” terms and off-axis effects (termed “beam squint” for Stokes V). We also present test polarimetric observations of a magnetized source on the Sun, the following sunspot in a solar active region, which shows a significant Stokes V signature in line with expectations. Finally, we provide some cautions and guidance to users contemplating the use of polarization observations with ALMA.

Abstract

Since the launch of the Fermi mission in 2008, it has become possible to study high-energy solar (gamma) -rays with an unprecedented imaging capability. In particular, the position of the (>100text{ MeV}) (gamma) -ray source can shed light on the origin of high-energy protons that is still controversial. However, the imaging of solar (gamma) -ray sources with the Fermi Large Area Telescope (LAT) is a complex multi-stage process influenced by a number of factors and instrumental effects, which is difficult to fully comprehend a priori. The SOL2014-09-01 behind-the-limb event was significant, for which the (gamma) -ray source position was not firmly established at once. Following the methodology outlined by the Fermi/LAT team, we estimated the proton power-law indices and (gamma) -ray centroid positions at two temporal intervals of this event, separated by one hour. Our estimates for the first interval are comparable to estimates recently updated by the Fermi/LAT team, thereby confirming the consistency of the analysis applied. Although, in the second interval, corresponding to the decay phase of the flare, the proton power-law index clearly hardened, the presumable position of the fading (gamma) -ray source remained unchanged. Its constancy in both temporal intervals and its proximity to the bases of long coronal loops connected to the flare site support the flare origin of high-energy protons injected into these loops along with electrons and trapped there for a long time. Our experience analyzing Fermi/LAT data clarifies their complex handling and will hopefully benefit the solar community in their wider use.

Abstract

The Full-disk MagnetoGraph (FMG) onboard the Advanced Space based Solar Observatory has obtained a series of line-of-sight magnetic-field measurements since its launch in October 2022. It is important to compare its observational data with other existing solar telescopes. In this paper, we make a detailed comparison of four active regions and a pore region simultaneously observed by FMG, the Helioseismic and Magnetic Imager (HMI) onboard the Solar Dynamic Observatory, and the Solar Magnetism and Activity Telescope (SMAT) at Huairou Solar Observing Station. We find that the magnetic-field structure and spatial distribution are basically consistent among these three instruments. The initial correlation coefficient of magnetic field is approximately 0.90. The coefficient slightly increases when sunspot umbra regions are excluded, and it increases significantly up to 0.98 for the magnetic field in penumbra regions. The magnetic field observed by FMG tends to be weaker than the HMI in strength in sunspot umbra due to saturation effect, whereas larger outside sunspot. The differences are probably due to different noise levels, seeing conditions (SMAT is affected by the Earth’s atmosphere) and observational and calibration methods.

We show using superposed epoch analysis (SEA) that the most energetic protons ((>60text{ MeV})) in the near-Earth interplanetary magnetic field (IMF) have a peak almost immediately (less than a day) after the peak in solar-flare index (SFI), while protons greater than 10 MeV peak one day after the SFI and protons greater than 1 MeV peak two days after the SFI.

The geomagnetic indices AU, -AL, PC, Ap, and -Dst peak after two to three days in SEAs after the peak in SFI. The auroral electrojet indices AU and -AL, however, have only low peaks. In particular, the response of the eastward electrojet, AU, to SFI is negligible compared to other geomagnetic indices.

The SEAs of the SFI and cosmic-ray counts (CR) show that the deepest decline in the CR intensity also follows with a 2 – 3-day lag the maximum of the SFI for Solar Cycles 20 – 24. The depths of the declines are related to the SFI strength of each cycle, i.e., the average decline is about 5% for Cycles 21 and 22, but only 3% for Cycle 24. The strongest Cycle 19, however, differs from the other cycles such that it has a double-peaked decline and lasts longer than the decline of the other cycles.

The double-superposed epoch analyses show that the response of IMF Bv², which is about two days, and CR to SFI are quite simultaneous, but sometimes Bv² may peak somewhat earlier than the decline existing in CR.

Abstract

The maximum slope of the sunspot number during the rising phase of a sunspot cycle has an excellent correlation with the maximum value of the sunspot number during that cycle. This is demonstrated using a Savitzky–Golay filter to both smooth and calculate the derivative of the sunspot-number data. Version 2 of the International Sunspot Number ( (S) ) is used to represent solar activity. The maximum of the slope during the rising phase of each cycle was correlated against the peaks of solar activity. Using three different correlation fits, the average predicted amplitude for Solar Cycle 25 is 130.7 ± 0.5, among the best correlations in solar predictions. A possible explanation for this correlation is given by the similar behavior of a shape function representing the time variation of the sunspot number. This universal function also provides the timing of the solar maximum by the time from the slope maximum to the peak in the function as late 2023 or early 2024. A Hilbert transform gives similar results, which are caused by the dominance of the 11-yr sunspot-cycle period in a Fourier fit of the sunspot number.

Self-organization in continuous systems is associated with dissipative processes. In particular, for magnetized plasmas, it is known as magnetic relaxation, where the magnetic energy is converted into heat and kinetic energy of flow through the process of magnetic reconnection. An example of such a system is the solar corona, where reconnection manifests as solar transients like flares and jets. Consequently, toward investigation of plasma relaxation in solar transients, we utilize a novel approach of data-constrained MHD simulation for an observed solar flare. The selected active region NOAA 12253 hosts a GOES M1.3 class flare. The investigation of extrapolated coronal magnetic field in conjunction with the spatiotemporal evolution of the flare reveals a hyperbolic flux tube (HFT), overlying the observed brightenings. MHD simulation is carried out with the EULAG-MHD numerical model to explore the corresponding reconnection dynamics. The overall simulation shows signatures of relaxation. For a detailed analysis, we consider three distinct subvolumes. We analyze the magnetic field line dynamics along with time evolution of physically relevant quantities like magnetic energy, current density, twist, and gradients in magnetic field. In the terminal state, none of the subvolumes is seen to reach a force-free state, thus remaining in nonequilibrium, suggesting the possibility of further relaxation. We conclude that the extent of relaxation depends on the efficacy and duration of reconnection, and hence on the energetics and time span of the flare.

Abstract

Self-organized critical avalanche models are a class of cellular automata that, despite their simplicity, can be applied to the modeling of solar (and stellar) flares and generate robust power-law distributions in event size measures. However, bridging the conceptual gap to both magnetohydrodynamics and real flare observations continues to prove challenging. In this paper, we focus on a specific, key aspect of this endeavor: the definition of magnetic energy and its consequences for the model’s internal dynamics and energy release statistics. We show that the dual requirement of releasing energy and restoring local stability demands that the instability criterion and boundary conditions be set in a manner internally consistent with a given energy definition; otherwise, unphysical behavior ensues, e.g., negative energy release. Working with three energy definitions previously used in the literature, we construct such internally consistent avalanche models and compare/contrast their energy release statistics. Using the same set of models, we also explore a recent proposal by Farhang et al. (2018, 2019), namely, that avalanches/flares should maximize the amount of energy released by the lattice when instabilities are triggered. This tends to produce avalanches of shorter duration but higher peak energy release, adding to a similar total energy release. For the three energy definitions we tested, these avalanche models exhibit almost identical distributions of event size measures. Our results indicate that the key to reproduce solar-like power-law slopes in these size measures is lattice configurations in which most nodes remain relatively far from the instability threshold.

Abstract

We investigated correlations between cosmic-ray intensity and 14 solar and interplanetary parameters, which were classified into four cases. We used the modulation of cosmic-ray intensity observed at six distinct stations with different latitudes and cut-off rigidities. We used the partial least-squares (PLS) method to rank the parameters. In the first case, we employed 11 parameters without considering halo-type coronal mass ejections (CMEs) and solar proton events (SPEs). In addition, we considered energetic phenomena associated with halo CMEs for the second case and SPEs in the third case. In the fourth case, we combined all of the parameters. The results based on the magnitude of the first principal component show that the sunspot number (SN), interplanetary magnetic field (IMF), heliospheric current sheet (HCS), and plasma velocity are the parameters with the strongest influence on the modulation of the cosmic-ray intensity at all six stations and in the first case we considered. For a halo-type CME (second case), SN, IMF, HCS, CME speed, and proton density were identified as the most significant parameters, which is identical to the results obtained in the fourth case. During an SPE (third case), the most significant parameters were SN, IMF, HCS, SPEs, and plasma velocity. The INVK and OULU stations, with nearly the same latitude and altitude, exhibit similar results. Our analysis of the results from the low-latitude stations (PSNM and TSMB) yielded different results from the other three stations at higher latitude. For the PSNM and TSMB stations, (B_{y}) , (B_{x}) , and the cone angle are the parameters that most strongly influence the modulation of the cosmic-ray intensity. This occurs because the influence of these parameters on cosmic-ray modulation depends on the latitude.

Solar white-light flares (WLFs) are those accompanied by brightenings in the optical continuum or integrated light. The White-light Solar Telescope (WST), as an instrument of the Lyman-alpha Solar Telescope (LST) on the Advanced Space-based Solar Observatory (ASO-S), provides continuous solar full-disk images at 360 nm, which can be used to study WLFs. We analyze 205 major flares above M1.0 from October 2022 to May 2023 and identify 49 WLFs at 360 nm from WST observations, i.e. with an occurrence rate of 23.9%. The percentages of WLFs for M1 – M4 (31 out of 180), M5 – M9 (11 out of 18), and above X1 (7 for all) flares are 17.2%, 61.1%, and 100%, respectively, namely the larger the flares, the more likely they are WLFs at 360 nm. We further analyze 39 WLFs among the identified WLFs and investigate their properties such as white-light enhancement, duration, and brightening area. It is found that the relative enhancement of the white-light emission at 360 nm is mostly (>90%) less than 30% and the mean enhancement is 19.4%. The WLFs’ duration at 360 nm is mostly (>80%) less than 20 minutes and its mean is 10.3 minutes. The brightening area at 360 nm is mostly (>75%) less than 500 arcsecond² and the median value is 225. We find that there exist good correlations between the white-light enhancement/duration/area and the peak soft X-ray (SXR) flux of the flare, with correlation coefficients of 0.68, 0.58, and 0.80, respectively. In addition, the white-light emission in most WLFs peaks around the same time as the temporal derivative of SXR flux as well as the hard X-ray emission at 20 – 50 keV, indicative of the Neupert effect. It is also found that the limb WLFs are more likely to have a greater enhancement, which is consistent with numerical simulations.