X-ray plasma ejections and jets from solar compact flares observed with the Yohkoh soft X-ray telescope

Yohkoh soft X-ray observations have revealed coronal X-ray plasma ejections and jets associated with solar flares. We have studied an X-ray plasma ejection on 1993 November 11 in detail, as a typical example of X-ray plasma ejections (possibly plasmoids expected from the reconnection model). The results are as follows: (1) The shape of the ejected material is a loop before it begins to rise. (2) The ejecta are already heated to 5 – 16 MK before rising. (3) The kinetic energy of the ejecta is smaller than the thermal energy content of the ejecta. (4) The thermal energy of the ejecta is smaller than that of the flare regions. (5) The acceleration occurs during the impulsive phase. These results are compared with the characteristics of X-ray jets, and a possible interpretation (for both plasmoids and jets) based on the magnetic reconnection model is briefly discussed.     

Yohkoh observations of the solar corona

The Yohkoh soft X-ray telescope obtains several images every 90 minutes. Data from the declining phase of the solar cycle have been used to compare the X-ray signal with other indicators of activity and to study coronal heating. X-ray emission from a north polar coronal hole is found broadly consistent with results of previous EUV observations. In diffuse emission regions, temperature rises to around 2.2 MK and levels off in the height range 1.5 – 1.9 R_O. Such emission underlies streamers and may be the source of the low-speed solar wind. X-ray signatures for Coronal Mass Ejection (CME) events which involve the detection of reduced X-ray intensities in the corona, have been developed with Yohkoh data. CME observations are described     

Study of combined soft and hard X-ray images of solar flares

We have studied soft and hard X-ray images of 13 solar flares from six active regions observed by the Hard X-ray Imaging Spectrometer (HXIS). Our results indicate the presence of pre-hard X-ray burst excesses in the 11.5–30.0 keV range, indicating a slow buildup of the acceleration process or a strong preheating. During the impulsive phase, all of the events show the simultaneous energization of neighboring field structures, which, in the case we show in some detail, share about equal amounts of the released energy. This association seems to be indicative of strong acceleration and energy release triggered by the interaction between magnetic loops.     

On upward motions of coronal hard X-ray sources in solar flares

Coronal hard X-ray (HXR) sources were discovered by the Yohkoh HXT telescope in about two dozen limb flares: Impulsive and gradual ones. On the basis of HXT data, we investigated the spatial evolution of coronal sources. Slow ascending motions of sources are seen in several flares. In five events, it was possible to estimate the velocity of the upward motion with values between 10 and 30 km/s. We present these observational results and conclude that coronal source motions should be studied statistically using the RHESSI high-resolution HXR imaging data. We discuss the possibility that coronal HXR emission is generated as bremsstrahlung of the fast electrons accelerated in collapsing magnetic traps due to joint action of the Fermi-type first-order mechanism and betatron acceleration.     

Coronal spectroscopy: Probing sources of slow solar wind in active regions,and the early phases of solar flares

Coronal spectroscopy has pushed forward the understanding of physical processes in all phenomena on the Sun. In this review we concentrate specifically on plasma parameters measured in sources of the slow solar wind in active regions and the early phases of solar flares. These topics are a key part of the science goals of the Solar Orbiter mission (Müller et al., 2020) which has been designed to probe what drives the solar wind and solar transients that fill the heliosphere.Active regions, outside of flaring, have general characteristics that include closed loops showing red-shifted (down-flowing plasma), and the edges of the active regions showing blue-shifted (upflowing plasma). Constraining and understanding the evolution, behaviour and cause of the flows has been developed in the past years and are summarised. Of particular importance is the upflowing plasma which, in some cases, can contribute to the slow solar wind, and this review concentrates on recent results on this topic.The early phases of solar flares and their energy sources are not yet fully understood. For decades, there has been a huge interest in pin-pointing the trigger of a solar flare. Coronal spectroscopy has revealed small-scale dynamics that occurs tens of minutes before the flare begins. The understanding of the trigger is key to improving flare predictions in the future, as well as understanding the physical processes.Finally we look to the future of coronal spectroscopy, with new instruments and methodologies being developed that build on the current knowledge, and will improve significantly our physical understanding of processes at all scales on the Sun.     

The automated prediction of solar flares from SDO images using deep learning

In the last few years, there has been growing interest in near-real-time solar data processing, especially for space weather applications. This is due to space weather impacts on both space-borne and ground-based systems, and industries, which subsequently impacts our lives. In the current study, the deep learning approach is used to establish an automated hybrid computer system for a short-term forecast; it is achieved by using the complexity level of the sunspot group on SDO/HMI Intensitygram images. Furthermore, this suggested system can generate the forecast for solar flare occurrences within the following 24 h. The input data for the proposed system are SDO/HMI full-disk Intensitygram images and SDO/HMI full-disk magnetogram images. System outputs are the “Flare or Non-Flare” of daily flare occurrences (C, M, and X classes). This system integrates an image processing system to automatically detect sunspot groups on SDO/HMI Intensitygram images using active-region data extracted from SDO/HMI magnetogram images (presented by Colak and Qahwaji, 2008) and deep learning to generate these forecasts. Our deep learning-based system is designed to analyze sunspot groups on the solar disk to predict whether this sunspot group is capable of releasing a significant flare or not. Our system introduced in this work is called ASAP_Deep. The deep learning model used in our system is based on the integration of the Convolutional Neural Network (CNN) and Softmax classifier to extract special features from the sunspot group images detected from SDO/HMI (Intensitygram and magnetogram) images. Furthermore, a CNN training scheme based on the integration of a back-propagation algorithm and a mini-batch AdaGrad optimization method is suggested for weight updates and to modify learning rates, respectively. The images of the sunspot regions are cropped automatically by the imaging system and processed using deep learning rules to provide near real-time predictions. The major results of this study are as follows. Firstly, the ASAP_Deep system builds on the ASAP system introduced in Colak and Qahwaji (2009) but improves the system with an updated deep learning-based prediction capability. Secondly, we successfully apply CNN to the sunspot group image without any pre-processing or feature extraction. Thirdly, our system results are considerably better, especially for the false alarm ratio (FAR); this reduces the losses resulting from the protection measures applied by companies. Also, the proposed system achieves a relatively high scores for True Skill Statistics (TSS) and Heidke Skill Score (HSS).     

Energy transfer in solar flares

We review the recent advances in the field of energy transfer and dissipation in solar flares. New observations and theoretical results have been obtained during the SMY and discussed in several workshops. Important new results have been provided by imaging hard X-ray and radio observations, high resolution spectra in the soft X-ray range, polarization measurements and combined optical, gamma- and X-ray data. We summarize results on the following topics: a) interpretation of hard X-ray bursts; b) heating and cooling of X-ray flare plasmas; c) chromospheric heating and evaporation; d) white-light flares. An overall picture of the importance of transfer processes is given, together with prospects for development of future research topics.     

Reconnection during substorms and solar flares

The patterns of reconnection in the Earth magnetotail and in the solar corona above the active region are presented. The electric field and field-aligned currents (FAC) generation in the current sheet are discussed.     

Energetic electrons in impulsive solar flares: Radio diagnostics

Radio emissions during and outside solar flares are tracers of energetic electrons from the bottom of the corona to the interplanetary space. This review focusses on impulsive flares, where joint analyses of radio, hard X-ray and γ-ray observations proved to be powerful probes of the properties of accelerated electrons and of the sites in the corona where they are accelerated. Evidence of electron acceleration and transport in the corona from microwave imaging and decimetre wave spectroscopy is reviewed and compared, and recent work on the interpretation of microwave spectra in terms of energetic electron spectra is discussed. The two directions for future instrumentation are the extension to shorter wavelengths, with the aim of probing relativistic electrons, and solar dedicated spectral imaging from centimetric to metric waves to provide a unified view of the acceleration signatures that stem so far from different instruments with either spectroscopic or imaging capabilities.     

Driving major solar flares and eruptions: A review

This review focuses on the processes that energize and trigger M- and X-class solar flares and associated flux-rope destabilizations. Numerical modeling of specific solar regions is hampered by uncertain coronal-field reconstructions and by poorly understood magnetic reconnection; these limitations result in uncertain estimates of field topology, energy, and helicity. The primary advances in understanding field destabilizations therefore come from the combination of generic numerical experiments with interpretation of sets of observations. These suggest a critical role for the emergence of twisted flux ropes into pre-existing strong field for many, if not all, of the active regions that produce M- or X-class flares. The flux and internal twist of the emerging ropes appear to play as important a role in determining whether an eruption will develop predominantly as flare, confined eruption, or CME, as do the properties of the embedding field. Based on reviewed literature, I outline a scenario for major flares and eruptions that combines flux-rope emergence, mass draining, near-surface reconnection, and the interaction with the surrounding field. Whether deterministic forecasting is in principle possible remains to be seen: to date no reliable such forecasts can be made. Large-sample studies based on long-duration, comprehensive observations of active regions from their emergence through their flaring phase are needed to help us better understand these complex phenomena.     

Energy input in solar flares and coronal explosions

A coronal explosion is a density wave observed in X-ray images of solar flares. The wave occurs at the end of the impulsive phase, which is the time at which the flare's thermal energy content has reached its maximum value. It starts in a small area from where it spreads out, mainly into one hemisphere, with velocities that tend to rapidly decrease with time, and which are between ～ 10³ and a few tens of km s^?1. We interpret them as magneto-hydrodynamic waves that (mainly) move downward from the low corona into denser regions.     

Physics of the impulsive phase of solar flares

The physics of the impulsive phase of solar flares is discussed in relation to high resolution microwave, hard X-ray and ultraviolet observations. High spatial resolution observations of the structure of microwave flaring loops and their interpretation in terms of arcades of loops as the sites of primary energy release are presented. Theoretical interpretation of the confinement of microwave producing energetic electrons in the coronal part of loops is discussed. High temporal and spatial resolution measurements in hard X-rays, as well as observations of the spectral evolution of the hard X-ray emission are presented. Observations of the relative locations of microwave and hard X-ray emitting regions are presented and their significance with respect to the energy release site and electron acceleration is discussed. The relative timing of the peaks of impulsive hard X-ray and microwave burst is discussed. The significance of ultraviolet measurements in obtaining the density of flaring regions is discussed. Possible diagnostics of impulsive phase onsets from cm-λ polarization data are presented, and the role of the emergence of new flux and of the current sheet formed between closed loops in producing impulsive energy release at centimeter wavelengths are analyzed. Decimeter and meter wave manifestations of preflash phase and millisecond pulsations at centimeter and decimeter wavelengths and the relevant physical processes involved are discussed.     

Turbulence,complexity, and solar flares

The issue of predicting solar flares is one of the most fundamental in physics, addressing issues of plasma physics, high-energy physics, and modelling of complex systems. It also poses societal consequences, with our ever-increasing need for accurate space weather forecasts. Solar flares arise naturally as a competition between an input (flux emergence and rearrangement) in the photosphere and an output (electrical current build up and resistive dissipation) in the corona. Although initially localised, this redistribution affects neighbouring regions and an avalanche occurs resulting in large scale eruptions of plasma, particles, and magnetic field. As flares are powered from the stressed field rooted in the photosphere, a study of the photospheric magnetic complexity can be used to both predict activity and understand the physics of the magnetic field. The magnetic energy spectrum and multifractal spectrum are highlighted as two possible approaches to this.     

Proton versus electron heating in solar flares

Proton and electron heating of a flaring atmosphere is compared in a kinetic approach for the particles ejected from a non-neutral reconnecting current sheet (RCS) located above the top of reconnected flaring loops in a two-ribbon flare. Two kinds of high-energy particles are considered: particles accelerated by a super-Dreicer electric field and those ejected from the reconnection region as neutral outflows, or separatrix jets. The beam electrons are assumed to deposit their energy in Coulomb collisions and Ohmic heating of the ambient plasma particles by the electric field induced by the precipitating beams. The protons are assumed to deposit their energy in generation of kinetic Alfvén waves (KAWs), which, in turn, dissipate due to Cherenkov resonant scattering on the ambient plasma electrons. The beam electrons are found to provide a fast (within a few tenth of a second) heating of the atmosphere that is well spread in depth from the corona to the lower chromosphere. The protons are shown to precipitate to the lower atmosphere much slower (up to few seconds for beam and up to 10–20 s for slow jets). Slow jet protons provide heating of the two compact regions: the first located at the top of a flaring loop just below the RCS, and the second one appearing at the transition region (TR) and upper chromosphere; fast beam protons deposit their energy in the TR and chromosphere only.     

First-order Fermi shock acceleration in solar flares

First order Fermi shock acceleration of electrons, protons and alpha particles is compared to observations of energetic particle events. For each event, a unique shock compression ratio produces spectra in good agreement with observation. The simple model predicts that the acceleration time to a given energy will be approximately equal for electrons and protons and, for reasonable solar parameters, can be less than 1 second to ～ 100 MeV.     

Magnetic reconnection configurations and particle acceleration in solar flares

Numerical simulations of two types of flares indicate that magnetic reconnection can provide environments favorable for various particle acceleration mechanisms to work. This paper reviews recent test particle simulations of DC electric field mechanism, and discusses how the flare particles can escape into the interplanetary space under different magnetic configurations.     

Magnetic field structure changes in the vicinity of solar flares

Changes in the structure of the sunspot group and its magnetic field are studied in Hale Region 17644 (May 1981) in connection with the May 16 3B/X1 flare. The characteristic changes, also found in HR 16850 (May 1980) and HR 17098 (September 1980), are the following: Rapid motions of umbrae of opposite polarity in the vicinity of the magnetic zero line, parallel to this line, but in opposite direction. Appearence of new small spots before the flare, leading to a more complicated field structure. Simplification of the magnetic structure after the flare in some days, i.e. decrease of spot areas in the affected territory and the straightening of the magnetic zero line.     

Magnetic reconnection in solar flares and corresponding radio bursts

The 2D MHD model of the flare magnetic reconnection shows that a reconnection activity, changes of the magnetic field topology and generation of waves are connected. It is found that after the phase of a quasi-stationary reconnection in the extended current sheet above the flare arcade the tearing mode instability produces the plasmoids which then can interact and generate MHD waves. Results of particle-in-cell simulations of the tearing processes, which accelerate electrons, are mentioned. Then all these processes are discussed from the point of view of possible radio emissions. While shocks can contribute to the type II radio burst, the superthermal electrons trapped in plasmoids can generate so called drifting pulsating structures. Furthermore, regions with the MHD turbulence may manifest themselves as the lace or dm-spike bursts.     

Differential emission measure distributions in X-ray solar flares

X-ray spectrometer RESIK has observed spectra in the four wavelength bands from 3.3 Å to 6.1 Å. This spectral range contains many emission lines of H- and He-like ions for Si, S, Ar and K. These lines are formed in plasma of coronal temperatures (T > 3 MK). Analysis of their intensities allows studying differential emission measure distributions (DEM) in temperature range roughly between 3 MK and 30 MK. The aim of present study was to check whether any relationship exists between the character of DEM distribution, the event phase and the X-ray flare class. To do this we have calculated and analyzed the DEM distributions for a set of flares belonging to different GOES classes from the range B5.6–X1. The DEM distributions have been calculated using “Withbroe–Sylwester” multiplicative, maximum likelihood iterative algorithm. As the input data we have used absolute fluxes observed by RESIK in several spectral bands (lines + continuum). Respective emission functions have been calculated using the CHIANTI v. 5.2 atomic data package.     

Magnetic reconnection in a high-temperature plasma of solar flares

In the frame of a simple self-consistent model for high-temperature turbulent current sheet (HTCS) /1/, three effects are considered. (i) Gradient instabilities create anamalous plasma diffusion across magnetic field and increase the power of energy release in HTCS. (ii) Penetration of a small transverse component of magnetic field into HTCS also can significantly increase an energy output of HTCS. (iii) There appears electric current circulating around a current sheet at a compression of longitudinal magnetic field. This current induces a Joule heat; however, a total flux of the longitudinal field remains constant.