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Solar eruptions are well-known drivers of extreme space weather, which can greatly disturb the Earth’s magnetosphere and ionosphere. The triggering process and initial dynamics of these eruptions are still an area of intense study. Here we perform a magnetohydrodynamic simulation taking into account the observed photospheric magnetic field to reveal the dynamics of a solar eruption in a real magnetic environment. In our simulation, we confirmed that tether-cutting reconnection occurring locally above the polarity inversion line creates a twisted flux tube, which is lifted into a toroidal unstable area where it loses equilibrium, destroying the force-free state, and driving the eruption. Consequently, a more highly twisted flux tube is built up during this initial phase, which can be further accelerated even when it returns to a stable area. We suggest that a nonlinear positive feedback process between the flux tube evolution and reconnection is the key to ensure this extra acceleration. Solar eruptions are large explosions occurring in the solar atmosphere. Here, the authors perform magnetohydrodynamic simulations to unveil the dynamics of a solar eruption, and find that these are dominated by nonlinear processes involving flux tube evolution and reconnection.

Solar flares and coronal mass ejections, the primary space weather disturbances affecting the entire heliosphere and near-Earth environment, mainly emanate from sunspot regions harbouring high degrees of magnetic twist. However, it is not clear how magnetic helicity, the quantity for measuring the magnetic twist, is supplied to the upper solar atmosphere via the emergence of magnetic flux from the turbulent convection zone. Here, we report state-of-the-art numerical simulations of magnetic flux emergence from the deep convection zone. By controlling the twist of emerging flux, we find that with the support of convective upflow, the untwisted emerging flux can reach the solar surface without collapsing, in contrast to previous theoretical predictions, and eventually create sunspots. Because of the turbulent twisting of magnetic flux, the produced sunspots exhibit rotation and inject magnetic helicity into the upper atmosphere, amounting to a substantial fraction of injected helicity in the twisted cases that is sufficient to produce flare eruptions. This result indicates that the turbulent convection is responsible for supplying a non-negligible amount of magnetic helicity and potentially contributes to solar flares.

This paper explores the characteristics of 42 solar X-class flares that were observed between February 2011 and November 2014, with data from the Solar Dynamics Observatory (SDO) and other sources. This flare list includes nine X-class flares that had no associated CMEs. In particular our aim was to determine whether a clear signature could be identified to differentiate powerful flares that have coronal mass ejections (CMEs) from those that do not. Part of the motivation for this study is the characterization of the solar paradigm for flare/CME occurrence as a possible guide to the stellar observations; hence we emphasize spectroscopic signatures. To do this we ask the following questions: Do all eruptive flares have long durations? Do CME-related flares stand out in terms of active-region size vs. flare duration? Do flare magnitudes correlate with sunspot areas, and, if so, are eruptive events distinguished? Is the occurrence of CMEs related to the fraction of the active-region area involved? Do X-class flares with no eruptions have weaker non-thermal signatures? Is the temperature dependence of evaporation different in eruptive and non-eruptive flares? Is EUV dimming only seen in eruptive flares? We find only one feature consistently associated with CME-related flares specifically: coronal dimming in lines characteristic of the quiet-Sun corona, i.e. 1 – 2 MK. We do not find a correlation between flare magnitude and sunspot areas. Although challenging, it will be of importance to model dimming for stellar cases and make suitable future plans for observations in the appropriate wavelength range in order to identify stellar CMEs consistently.

It is of great interest and importance to study the variabilities of solar EUV, UV and X-ray irradiance in heliophysics, in Earth’s climate, and space weather applications. A careful study is required to identify, track, monitor and segment the different coronal features such as active regions (ARs), coronal holes (CHs), the background regions (BGs) and the X-ray bright points (XBPs) from spatially resolved full-disk images of the Sun. Variability of solar soft X-ray irradiance is studied for a period of 13 years (February 2007–March 2020, covers Solar Cycle 24), using the X-Ray Telescope on board the Hinode (Hinode/XRT) and GOES (1 – 8 Å). The full-disk X-ray images observed in Al_mesh filter from XRT are used, for the first time , to understand the solar X-ray irradiance variability measured, Sun as a star, by GOES instrument. An algorithm in Python has been developed and applied to identify and segment coronal X-ray features (ARs, CHs, BGs, and XBPs) from the full-disk soft X-ray observations of Hinode/XRT. The segmentation process has been carried out automatically based on the intensity level, morphology and sizes of the X-ray features. The total intensity, area, and contribution of ARs/CHs/BGs/XBPs features were estimated and compared with the full-disk integrated intensity (FDI) and GOES (1 – 8 Å) X-ray irradiance measurements. The XBPs have been identified and counted automatically over the full disk to investigate their relation to solar magnetic cycle. The total intensity of ARs/CHs/BGs/XBPs/FD regions are compared with the GOES (1 – 8 Å) X-ray irradiance variations. We present the results obtained from Hinode/XRT full-disk images (in Al_mesh filter) and compare the resulting integrated full-disk intensity (FDI) with GOES X-ray irradiance. The X-ray intensity measured over ARs/CHs/BGs/XBPs/FD is well correlated with GOES X-ray flux. The contributions of the segmented X-ray features to FDI and X-ray irradiance variations are determined. It is found that the background and active regions have a greater impact on the X-ray irradiance fluctuations. The mean contribution estimated for the whole observed period of the background regions (BGs) will be around 65 ± 10.97 % , whereas the ARs, XBPs and CHs are 30 ± 11.82 % , 4 ± 1.18 % and 1 ± 0.52 % , respectively, to total solar X-ray flux. We observed that the area and contribution of ARs and CHs varies with the phase of the solar cycle, whereas the BGs and XBPs show an anti-correlation. We find that the area of the coronal features is highly variable suggesting that their area has to be taken into account in irradiance models, in addition to their intensity variations. The time series results of XBPs suggest for an existence of anti-correlation between the number of XBPs and the sunspot numbers. It is also important to consider both the number variation and the contribution of XBPs in the reconstruction of total solar X-ray irradiance variability.

Solar flares are generally believed to be powered by free magnetic energy stored in the corona 1 , but the build up of coronal energy alone may be insufficient to trigger the flare to occur 2 . The flare onset mechanism is a critical but poorly understood problem, insights into which could be gained from small-scale energy releases known as precursors. These precursors are observed as small pre-flare brightenings in various wavelengths 3 – 13 and also from certain small-scale magnetic configurations such as opposite-polarity fluxes 14 – 16 , where the magnetic orientation of small bipoles is opposite to that of the ambient main polarities. However, high-resolution observations of flare precursors together with the associated photospheric magnetic field dynamics are lacking. Here we study precursors of a flare using the unprecedented spatiotemporal resolution of the 1.6-m New Solar Telescope, complemented by new microwave data. Two episodes of precursor brightenings are initiated at a small-scale magnetic channel 17 – 20 (a form of opposite-polarity flux) with multiple polarity inversions and enhanced magnetic fluxes and currents, lying near the footpoints of sheared magnetic loops. Microwave spectra corroborate that these precursor emissions originate in the atmosphere. These results provide evidence of low-atmospheric small-scale energy release, possibly linked to the onset of the main flare. Magnetic energy powers explosive flares on the Sun. Now, observations of unprecedented resolution identify the precursors of such flares in the lower solar atmosphere. These findings will help to constrain theoretical models of flare formation.

Although solar activity may significantly impact the global environment and socioeconomic systems, the mechanisms for solar eruptions and the subsequent processes have not yet been fully understood. Thus, modern society supported by advanced information systems is at risk from severe space weather disturbances. Project for solar–terrestrial environment prediction (PSTEP) was launched to improve this situation through synergy between basic science research and operational forecast. The PSTEP is a nationwide research collaboration in Japan and was conducted from April 2015 to March 2020, supported by a Grant-in-Aid for Scientific Research on Innovative Areas from the Ministry of Education, Culture, Sports, Science and Technology of Japan. By this project, we sought to answer the fundamental questions concerning the solar–terrestrial environment and aimed to build a next-generation space weather forecast system to prepare for severe space weather disasters. The PSTEP consists of four research groups and proposal-based research units. It has made a significant progress in space weather research and operational forecasts, publishing over 500 refereed journal papers and organizing four international symposiums, various workshops and seminars, and summer school for graduate students at Rikubetsu in 2017. This paper is a summary report of the PSTEP and describes the major research achievements it produced.

The temperature variations of the corona and its individual surface features as a function of the solar cycle are an interesting and important aspect of understanding the physics of the Sun. To study the temperature variations, we have used the full-disk soft X-ray images of the corona obtained from Hinode/X-Ray Telescope (XRT) in different filters. A sophisticated algorithm has been developed in Python to segment the different coronal features such as the active regions (ARs), coronal holes (CHs), background regions (BGs), and X-ray bright points (XBPs), derived the total intensity of all the features, and generated the temperature maps of the corona using the filter ratio method. Due to the XRT straylight issue in some filters and unavailability of a good pair of images, we used for our analysis the filter combinations of Ti-poly and Al-mesh for the period from February 01, 2008 to May 08, 2012 and Al-poly and Al-mesh for the period from May 09, 2012 to June 30, 2021, in total for 14 years which covers Solar Cycle 24. The first analysis in using the XRT intensity values of the coronal features from segmented solar disk and their relation to solar activity is presented. We discuss the temperature variations of a full-disk corona and all features (ARs, CHs, BGs, and XBPs). Our time series plots of the average temperature of the full-disk and all the features show temperature fluctuations synchronized with the solar cycle (sunspot number). Although the temperature of all features varies, but the mean temperature estimated for the whole observed period of the full-disk is around 1.29 ± 0.16 MK and active regions (ARs) are around 1.76 ± 0.32 MK, whereas BGs, CHs, and XBPs are 1.27 ± 0.15 MK, 1.23 ± 0.14 MK, and 1.37 ± 0.18 MK, respectively. In addition, we found that the mean temperature contribution estimated of the background regions (BGs) is around 93.2%, whereas ARs, CHs, and XBPs are 3.1%, 1.6% and 2.1%, respectively, to the average coronal temperature of the full-disk. The temperature values and their variations of all the features suggest that the features show a high variability in their temperature and that the heating rate of the emission features may be highly variable on solar cycle timescales. It is evident from the analysis that the filter-ratio method can be directly used for temperature analysis of coronal features and to study their surface temperature variability as a function of solar magnetic activity.

We surveyed the relationship between the scale of space weather events and their occurrence rate in Japan, and we discussed the social impact of these phenomena during the Project for Solar–Terrestrial Environment Prediction (PSTEP) in 2015–2019. The information was compiled for domestic users of space weather forecasts for appropriate preparedness against space weather disasters. This paper gives a comprehensive summary of the survey, focusing on the fields of electricity, satellite operations, communication and broadcasting, satellite positioning usage, aviation, human space activity, and daily life on the Earth’s surface, using the cutting-edge knowledge of space weather. Quantitative estimations of the economic impact of space weather events on electricity supply and aviation are also given. Some topics requiring future research, which were identified during the survey are also described.Graphic Abstract

The magnetic field changes the radiative output of the Sun and is the main source for all the solar surface features. To study the role of the underlying photospheric magnetic field in relation to emission features observed in the solar corona, we have used the full-disk soft X-ray images from Hinode/X-Ray Telescope ( Hinode/XRT ) and the magnetograms obtained from the Helioseismic and Magnetic Imager (HMI) on board the Solar Dynamics Observatory (SDO) for a period of about 13 years (May 2010 – June 2023), which covers Solar Cycle 24 and the ascending phase of Solar Cycle 25. A sophisticated and established algorithm developed in Python is applied to the X-ray observations from Hinode/XRT to segment the different coronal features by creating segmentation maps of the active regions (ARs), coronal holes (CHs), background regions (BGs), and X-ray bright points (XBPs). Further, these maps have been applied to the full-disk (FD) line-of-sight (LOS) magnetograms from HMI to isolate the X-ray coronal features and photospheric magnetic counterparts, respectively. We computed full-disk and featurewise averages of X-ray intensity and LOS magnetic field (MF) over ARs, CHs, BGs, XBPs, and FD regions. Variations in the quantities resulting from the segmentation, namely the mean intensity, temperature from the filter ratio method, and the unsigned magnetic field of ARs, CHs, BGs, XBPs, and FD regions, are intercompared and compared with the sunspot number (SSN). We find that the X-ray intensity and temperature over ARs, CHs, BGs, XBPs, and FD regions are well correlated with the underlying magnetic field. We discuss the intensity, temperature, and magnetic field variations of the full-disk corona and of all the features. The time series plots of the unsigned magnetic field of the full disk and all the features show magnetic field fluctuations synchronized with the solar cycle (sunspot number). Although the magnetic field of all features varies, the mean, spatially smoothed magnitude of the magnetic field values estimated for the whole observed period of the full disk is around 8.9 ± 2.60 G, active regions (ARs) are around 34.4 ± 18.42 G, whereas BGs, CHs, and XBPs are 7.7 ± 1.72 G, 6.6 ± 1.04 G, and 15.62 ± 8.76 G, respectively. In addition, we find that the mean magnetic field contribution of the background regions (BGs) is around 85 % , whereas ARs, CHs, and XBPs are 11 % , 2 % , and 2 % , respectively, to the average magnetic field of the full disk. The magnetic field time series of all the features suggest that the features show a high variability in their magnetic field and the fluctuations in magnetic field are correlated to fluctuations in intensity and temperature, suggesting that the magnetic field is important in producing different emission features, which are associated with different intensity and temperature values. The magnetic field is responsible for the heating rate of the emission features, which are highly variable on solar cycle timescales. We conclude from the full-disk intensity-temperature-magnetogram analysis that the magnetic field plays a crucial role in driving the different brightenings, emissions, and temperature and heating of the corona at the sites of these magnetic features. In this study, we demonstrate that the segmented coronal features observed in the soft X-ray wavelength can be used as proxies to isolate the corresponding underlying magnetic structures.