Solar Physics最新文献

In situ measurements of kinetic scale current sheets in the solar wind show that they are often approximately force-free although the plasma (beta) is of order one. They frequently display systematic asymmetric and anti-correlated spatial variations of their particle density and temperature across the current sheet, leaving the plasma pressure essentially uniform. These observations of asymmetries have previously been modelled theoretically by adding additional terms to both the ion and electron distribution functions of self-consistent force-free collisionless current sheet models with constant density and temperature profiles. In this article we present the results of a modification of these models in which only the electron distribution function has an additional term, whereas the ion distribution function is kept as a thermal (Maxwellian) distribution function. In this case the nonlinear quasineutrality condition no longer has a simple analytical solution and therefore has to be solved alongside Ampère’s law. We find that while the magnetic field remains approximately force-free, the non-zero quasineutral electric field gives rise to an additional spatial substructure of the plasma density inside the current sheet. We briefly discuss the potential relation between our theoretical findings and current sheet observations.

Solar flares exhibit precursor soft X-ray emission at high temperatures (the Hot Onset Precursor Event, here HOPE for short). This phenomenon is readily seen for events at or above the GOES C-class, but at present we do not know whether or not a “pure” filament-eruption event, such as one coming from the polar-crown filament zone, also exhibits this property. We study this question using both older and more modern GOES/XRS soft X-ray data. The currently operating GOES-R spacecraft (GOES-16 through GOES-19) have different sensor technology and also higher cadence than in the earlier satellites in the series, and so we devote an Appendix to describing these data from the point of view of a user interested in the detection of faint sources; most GOES/XRS users focus on flare observations, rather than the quiet Sun. Because the HOPE signatures appear at the very beginning of the development of a flare, they require study at the lowest flux levels. Our searches both with GOES-R and earlier data do not detect HOPE in purely filament-eruption events. At a representative HOPE temperature of 10 MK, the emission measure for any HOPE source must be less than about (10^{46}text{ cm}^{-3}).

HEL1OS ((mathbf{H})igh (mathbf{E})nergy L1 (mathbf{O})rbiting X-ray (mathbf{S})pectrometer) is one of the remote sensing payloads on board Aditya-L1 mission designed to continuously monitor and measure the time-resolved spectra of solar flares between 8 keV and 150 keV. This broad energy range has been covered by using compound semiconductor detectors: cadmium telluride (CdTe: 8 – 70 keV) and cadmium zinc telluride (CZT: 20 – 150 keV) with geometric areas of 0.5 cm² and 32 cm², respectively. A stainless steel collimator provides a field-of-view of 6° × 6° optimized to limit the off-axis response while keeping the design within the instrument mass constraints. The in-house designed low-noise digital pulse processing-based front-end electronics has achieved a spectral resolution of ≈ 1 keV at 14 keV (CdTe) and ≈ 7 keV at 60 keV (CZT). The instrument is also equipped with processing and power electronics to process the signal, drive the electronics, bias the detectors with required low and high voltages for optimal performance of the overall system. In this article, we present design aspects of the instrument, results from the pre-launch ground-based tests, and the in-orbit operations, which have indicated optimal performance in line with that expected.

Over the past three decades, models of solar flares and eruptions based on quasi-separatrix layers (QSLs) have made several important, observationally verified predictions regarding how the magnetic reconnection happens in 3D. Thus, they have become the best available theory of how and where solar flares and eruptions happen. We review the properties of QSLs, the close correspondence between QSL traces in the lower atmosphere and flare ribbons, together with their association to electric current enhancements, both modelled and observed ones. Furthermore, we review the slipping and slip-running nature of the magnetic reconnection in QSLs, and the associated apparent footpoint motions of the reconnecting structures, both modelled and observed. In addition, the purely 3D reconnection geometries involving the erupting magnetic flux rope are reviewed as well, along with the observational evidence for these processes. Finally, we discuss the indications that dynamics within the QSLs could play a role in heating the solar corona.

In this work, we present an extensive review and detailed analysis of sunspot measurements, drawings, and engravings made by John Flamsteed and, mainly, by Philippe de La Hire during the Maunder minimum. All available information and contemporary knowledge about the sunspot nature are shown. The coordinates, areas, and numbers of sunspots and sunspot groups are reconstructed. Based on these observations, La Hire, Jean-Dominique Cassini, and his son Jacques Cassini regularly published results that shed light on the purpose of sunspot measurements and the scientific paradigm of that time. In particular, astronomers believed that sunspots were recurrent over decades. We compare the reconstructed time-latitude diagram with those obtained by Spoerer (Ueber die periodicitat der sonnenflecken seit dem Jahre 1618..., 1889) and Ribes and Nesme-Ribes (Astron. Astrophys. 276, 549, 1993). The sidereal differential rotation rate is estimated, and its latitudinal profile is reconstructed. We also evaluate the fraction of sunspot groups that obey or violate Joy’s law.

Imaging observations of the solar lower atmosphere by the Atmospheric Imaging Assembly (AIA) have been mostly used as the context, and their quantitative information has been much less explored. The chromosphere responds rapidly to energy release by magnetic reconnection during flares. Furthermore, a flare is a collection of multiple energy release events that can be identified in spatially resolved chromosphere observations. In this paper, we conduct a statistical and semi-quantitative study of the relative photometry in the UV 1600 Å and EUV 304 Å passbands for 18 flares observed by AIA. In each flare, we have identified thousands of flare ribbon pixels in the UV 1600 Å images, and measured their brightness (counts per second) and the rise and decay timescales, which are indicative of heating properties in flare loops. The analysis shows that bright flare pixels, characterized by peak brightness larger than ten times the quiescent brightness, exhibit sharplight curves with the half rise time below 2 min, followed by a two-phase decay with a rapid decay on timescales comparable to the rise time and then a more gradual decay. Flare ribbon pixels identified in both UV 1600 Å and EUV 304 Å images exhibit similar time profiles during the rise, and their peak brightness appear to be related by a power law. Our analysis shows that AIA observed flare brightness in UV 1600 Å relative to the quiescent brightness is a meaningful measurement of the flare chromosphere photometry. AIA observations for over a decade thus provide a unique and extensive database for systematic and semi-quantitative study of flaring chromosphere, either in the context of the Sun as a star, or in spatially resolved manner that helps to probe the nature of flare energy release on elementary scales.

Magnetic switchbacks, the localized and abrupt reversals in the magnetic field direction, are prominent features in the solar wind. We present the results of a study of switchbacks and the solar energetic particle (SEP) events associated with a twin-CME scenario – described as sequential eruptions of two coronal mass ejections (CMEs) from the same active region occurring within a short time interval – observed by Parker Solar Probe during August 18 – 19, 2022. The two consecutive CMEs, originating from active region (AR) 13078, displayed overlapping trajectories, and the primary CME traversed the wake of the pre-CME within its predicted turbulence duration window, leading to the formation of a significant solar energetic particle (SEP) event. The interaction of these CMEs was further complicated by their embedding within a high-speed stream emanating from a nearby coronal hole which led to increased solar wind density, plasma temperature, and intensified magnetic field strength. The interaction between the twin-CME event and the high-speed stream results in a compression and subsequent process which forced magnetic reconnection. This reconnection produced a distinct chain of magnetic switchbacks downstream of the CME wake, characterized by sharp directional changes in the magnetic field, enhanced transverse ion current, and suprathermal alpha particle flux. The orientation of the magnetic field of the high-speed stream as it surrounded the twin-CME suggests that interchange reconnection facilitated the emergence of switchback structures in the turbulent CME sheath, aligning with predictions from the model by Zank et al. (2020). In addition to the supportive evidences of interchange reconnection as a plausible explanation for switchbacks, our findings are expected to provide deeper insights into CME evolution in high-speed stream environments and have implications for understanding turbulent plasma processes that contribute to solar wind structuring.

We analyze historical Ca ii K images from the Kodaikanal Observatory (KO) spanning 1907 to 1996, encompassing Solar Cycles 14 through 22. These digitized images were processed using the Equal Contrast Technique (ECT) to ensure uniform data quality for studying long and short-term variations. From these standardized images, we identify and compute the areas of both plages and network regions in both solar hemispheres in every image. We then utilizy this revised, uniform Ca ii K plage area time series for Solar Cycles 14 to 22. Our primary objective is to investigate the presence of short, Rieger-type periods and quasi-biennial oscillations (QBOs), specifically those near ≈ 1.3 years. To achieve this, we employ both Lomb-Scargle periodograms and Morlet wavelet maps. Our power spectrum analysis consistently shows that Rieger-type periods are significant across all solar cycles, in both the northern and southern hemispheres and in the whole disk data. However, the wavelet analysis reveals that both Rieger-type and QBO periodicities are intermittent, exhibiting varying periods in different cycles and hemispheres. This indicates that plages and network areas demonstrate asymmetric behavior between the two hemispheres. We have also discussed the potential reasons behind these observed periodicities.

For over a century, solar images have been captured across different spectral ranges. Initially, these images were taken on photographic plates, and with the development of CCD cameras, the images transitioned from analogue to digital formats. Analyzing digital images enables us to identify and analyze trends and features on the solar disk more efficiently. However, complications due to instrument malfunction or environmental factors can result in suboptimal images. Traditionally, several statistical parameters are used to check image quality, but these measures do not always yield satisfactory results. In this article, we describe a convolutional classification neural network for near-real time image quality assessment of GONG Dopplergrams. We also present a case study where this approach significantly improved the quality of science data products in an automated data reduction pipeline without any human intervention.

Fast ((mathrm{V}_{mathrm{CME}} > 1000text{ km},text{s}^{-1})) coronal mass ejections (CMEs) capable of accelerating protons beyond 300 MeV are thought to trigger hours-long sustained (gamma )-ray emission (SGRE) after the impulsive flare phase. Meanwhile, CME-CME interactions can cause enhanced proton acceleration, increasing the fluxes of solar energetic particles. This study explores the role of fast CME interactions in SGRE production during CME clusters, which we define as a series of CMEs linked to > C-class flares with waiting times < 1 day from the same active region (AR). We focus on clusters in major CME-productive ARs (major ARs), by defining a major AR as one that produced > 1 CME associated to a major (> M-class) flare. The study identified 76 major ARs between 2011 and 2019, of which 12 produced all SGRE events. SGRE-producing ARs exhibit higher median values for the speed of their fastest CMEs (2013 vs. 775 km s⁻¹) and the class of their strongest flares (X1.8 vs. M5.8), compared to SGRE-lacking ARs. They also produced relatively faster CMEs (median speed: 1418 vs. 1206.5 km s⁻¹), with the SGRE-associated CMEs occurring during periods of higher CME rates than typical fast CME epochs. Twelve of 22 (54.5%) SGRE events and 5 of 7 (71.4%) long-duration (> 10 h) SGRE events occurred during CME clusters, with high chances of CME-CME interactions. A case study on very active major ARs showed that all SGRE-associated CMEs with (mathrm{V}_{mathrm{CME}} lesssim 2000text{ km},text{s}^{-1}) underwent CME-CME interactions within ≲ 10 ({mathrm{R_{odot }}}), while SGRE-associated CMEs faster than 3000 km s⁻¹ did not undergo interactions.