Properties and relationship between solar eruptive flares and Coronal Mass Ejections during rising phase of Solar Cycles 23 and 24

Statistical relationship between major flares and the associated CMEs during rising phases of Solar Cycles 23 and 24 are studied. Totally more than 6000 and 10,000 CMEs were observed by SOHO/LASCO (Solar and Heliospheric Observatory/Large Angle Spectrometric Coronagraph) during 23rd [May 1996–June 2002] and 24th [December 2008–December 2014] solar cycles, respectively. In particular, we studied the relationship between properties of flares and CMEs using the limb events (longitude 70–85°) to avoid projection effects of CMEs and partial occultation of flares that occurred near 90°. After selecting a sample of limb flares, we used certain spatial and temporal constraints to find the flare-CME pairs. Using these constraints, we compiled 129 events in Solar Cycle 23 and 92 events in Solar Cycle 24. We compared the flare-CME relationship in the two solar cycles and no significant differences are found between the two cycles. We only found out that the CME mean width was slightly larger and the CME mean acceleration was slightly higher in cycle 24, and that there was somewhat a better relation between flare flux and CME deceleration in cycle 24 than in cycle 23.     

Energy-transfer processes in flares

This paper deals with Solar Maximum Year observations that can shed light on the roles of energetic electron beams and thermal conduction in solar flares. The emphasis is on X-ray and UV images and on the interpretation of chromospheric spectra. The format is that of a one-sided debate advocating the view that most of the flare energy that reaches the chromosphere is transferred by thermal conduction rather than by energetic electron beams. Reference is made to papers offering opposing points of view on this still controversial question.     

High-temperature phenomena in flares

High temperature phenomena occurring in solar flares are reviewed based on hard X-ray images and spectral analyses of highly ionized iron lines observed aboard the Hinotori spacecraft.Five basic flare components are proposed, i.e., impulsive (I), gradual hard (GH), thermal (T), quasi thermal (QT) and hot thermal (HT) components. A flare shows some combination of the five components. Energy release and transport for each component would give a lot of variety to the hard X-ray image, spectrum and time history of X-rays.     

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.     

Study of energy release in flares

Recent advances in the study of energy release in Flares are reviewed. Progress has been made in modelling coronal X-ray emission and the chromospheric response to energy imput. These advances are based on theoretical studies and on the comparison of complementary data obtained from spacecraft and ground-based observatories. We first review the modelling of the coronal flare derived from radio, X-ray and XUV observations. Then we summarize results on the chromospheric response to various energy imput. Observations of X-ray continuum intensity and polarization, transition zone lines and chromospheric lines do not show evidence of particle trapping by a turbulent front. Although they might be in agreement with trapping and partial precipitation. White light flares appear to result from energy deposited above the photosphere. They are probably due to electron bombardment. The implication of these results on the primary energy release process are discussed and prospects for new research are presented.     

Modeling large solar flares

The basic ideas to model the large solar flares are reviewed and illustrated. Some fundamental properties of potential and non-potential fields in the solar atmosphere are recalled. In particular, we consider a classification of the non-potential fields or, more exactly, related electric currents, including reconnecting current layers. The so-called ‘rainbow reconnection’ model is presented with its properties and predictions. This model allows us to understand main features of large flares in terms of reconnection. We assume that in the two-ribbon flares, like the Bastille-day flare, the magnetic separatrices are involved in a large-scale shear photospheric flow in the presence of reconnecting current layers generated by a converging flow.     

The magnetohydrodynamic development of two-ribbon flares or a five-finger theory for solar flares

A semi-analytical model for the electrodynamic development of two-ribbon flares is presented. A current filament above a bipolar active region starts rising according to the model of Van Tend and Kuperus. Due to this motion large induced electric fields arise at a magnetic neutral line far below the filament, resulting in and associated with magnetic reconnection and the formation of a current sheet. The interaction of this current sheet with the original current filament, the background magnetic field and the boundary layer of the photosphere determine the further electrodynamic development of the flare. The model predicts the energy release, the time of maximum, the height of the energy source and other quantities reasonably well.     

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.     

On classification of RHESSI flares

Based on the light curves and images of RHESSI flares, we tried to make a preliminary classification of solar flares. Three basic types of flares seem to be existed: accordantly gradual flares, accordantly impulsive flares, and early impulsive flares. The proportion for each type is given. The possible physical meaning related to different types is discussed.     

Energy release and transport in arcade flares

This work is based on hard and soft X-ray observations from the YOHKOH satellite. We investigate an example of an arcade flare, for which the arcade channel is seen in soft X-rays as a long bright filament. We have found that:

1. (1) Energy can efficiently flow along the arcade channel from the very beginning of a flare.

2. (2) During flare evolution a few kernels of hard X-ray emission develop along the arcade channel. Clearly, they are new, additional sources of the flare energy release. A probable scheme of formation of such hard X-ray kernels is briefly discussed.

     

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.     

Chromospheric line emission in seismically active flares

Some flares are known to drive seismic transients into the solar interior. The effects of these seismic transients are seen in helioseismic observations of the Sun’s surface thousands of km from their sources in the hour succeeding the impulsive phase of the flare. Energetic particles impinging from the corona into the chromosphere are known to drive strong, downward-propagating shocks in active region chromospheres during the impulsive phases of flares. H observations have served as an important diagnostic of these shocks, showing intense emission with characteristic transient redshifts. In most flares no detectable transients penetrate beneath the active region photosphere. In those that do, there is a strong correlation between compact white-light emission and the signature of seismic emission. This study introduces the first known H observations of acoustically active flares, centered in the core of the line. The morphology of line-core emission H in the impulsive phase of the flare is similar to that of co-spatial line-core emission in NaD₁, encompassing the site of seismic emission but more extended. The latter shows a compact red shift in the region of seismic emission, but a similar feature is known to appear in a conjugate magnetic footpoint from which no seismic emission emanates. Radiative MHD modelling based on the profiles of chromospheric line emission during the impulsive phases of flares can contribute significantly to our understanding of the mechanics of flare acoustic emission penetrating into the solar interior and the conditions under which it occurs.     

Analysis of long duration flares

Yohkoh has observed many long duration events permitting a statistical study of the properties of these interesting events. We have selected ten flares for analysis which have durations between 5 and 20 hours, and size ranging from C to X GOES class. Employing the Soft X-ray Telescope, the Bragg Crystal Spectrometer, GOES spacecraft, and ground-based H data, we examine the morphology, temperature, emission measure, location of the hard X-ray source, non-thermal velocities and upflows of the plasma at different stages in the flare development. Our results are used to address the question of the energy source that maintains the hot plasma at temperatures of several million degrees for many hours.     

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.     

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.     

Origin and location of chromospheric evaporation in flares

Observation of two flares obtained with the Solar Maximum Mission spectrometers indicate that at flare onset the emission in soft (3.5 – 8 keV) and hard (16 – 30 keV) X-rays is predominant at the footpoints of the flaring loops. Since, at the same time, blue-shifts are observed in the soft X-ray spectra from the plasma at temperature of 10⁷ K, we infer that material is injected at high velocity into the coronal loops from the footpoints. These areas are also the sites of energy deposition, since their emission in hard X-rays is due to non-thermal electrons penetrating in the denser atmosphere. Hence, chromospheric evaporation occurs where energy is deposited. During the impulsive phase, the configuration of the flare region changes indicating that the flaring loop is progressively filled by hot plasma.     

Energetic electrons in solar flares as viewed in X-rays

Hard X-ray observations provide the most direct diagnostic we have of the suprathermal electrons and the hottest thermal plasma present in solar flares. The Ramaty High Energy Solar Spectroscopic Imager (RHESSI) is obtaining the most comprehensive observations of individual solar flares ever available in hard X-rays. For the first time, high-resolution spectra are available for a large number of flares that accurately display the spectral shape and its evolution and, in many cases, allow us to identify the transition from the bremsstrahlung X-rays produced by suprathermal electrons to the bremsstrahlung at lower energies emitted by thermal plasma. Also, for the first time, images can be produced in arbitrary energy bands above 3–4 keV, and spectra of distinct imaged components can be obtained.I review what we have learned from RHESSI observations about flare suprathermal electron distributions and their evolution. Next, I present computations of the energy deposited by these suprathermal electrons in individual flares and compare this with the energy contained in the hot thermal plasma. I point out unsolved problems in deducing both suprathermal electron distributions and the energy content of the thermal plasma, and discuss possible solutions. Finally, I present evidence that electron acceleration is associated with magnetic reconnection in the corona.     

The flares of April, 1980

A brief report is given on the characteristics of solar flares from the active region Boulder 2372. It is interesting to note the morphological similarity (homology) between many of these events, most likely caused by the overall constancy of the magnetic field configuration. Under closer examination this first-order homology breaks down, a fact that may help us understand the basis of the flare mechanism.