Regions of primary energy release of solar flares associated with singularities in the magnetic field

The locations of sites of primary energy release of solar flares are studied. Magnetic singularities revealed earlier—self-intersections (reconnections) of F = 0 surfaces, where F is a differential factor determining the structural singularity in a potential magnetic field—are considered as possible sites of energy release. Six flare events demonstrating paired sources of non-thermal hard X-rays emission observed on March 17, 2002, July 17, 2002, April 6, 2004, November 4, 2004, November 6, 2004, and December 1, 2004 are analyzed for probable singularities. In each event analyzed, each source of non-thermal hard X-rays emission can be associated with an individual magnetic singularity; in other words, there is a magnetic-field line passing near the singularity and ending near (i.e. within about 10″) the source located on the photosphere (in the chromosphere). For the homologous flares observed on November 4 and 6, 2004, the same magnetic singularity is responsible for the source of non-thermal hard X-rays emission observed in the eastern sector of the flare region on November 4 and the source observed in the western part on November 6. A proposed interpretation associates these observations with a reversal of the electric field generated in the magnetic singularity on November 6, compared with the electric field generated on November 4, attributed to corresponding changes occurring in the photospheric magnetic field.     

Common features in the development of powerful long-duration solar X-ray flares

We have begun an investigation of the possible origins of considerable of powerful solar flares. This effect is manifest, first and foremost, in the existence of high-temperature plasma in flare loops over many hours. Analysis of the soft X-ray emission in two energy bands detected by the GOES satellites for about 20 powerful solar flares reveals long time intervals during the decay phase when the source temperature decreases, in general, exponentially. The characteristic time t _i for a decrease in the temperature by a factor of ten is 3–10 hours for most powerful events. In addition, another interval of very slow decrease with a characteristic time t _i of tens of hours can be identified in some cases. We found a gradual change in the dependence of the temperature on the square root of the emission measure for the source as a whole, which characterizes the transition from purely coronal processes to powerful flares with a prolonged inflow of plasma from the chromosphere. Modeling the energy balance in a loop can yield the requirements for the source of plasma heating in a long-lived arch system. A necessary condition for the development of prolonged flares seems to be a powerful coronal mass ejection, which initiates the formation of a source of plasma heating at coronal heights. Our analysis shows that a considerable fraction of the energy is often released in the region of the cusp, and that systems of giant coronal arches rising to heights of about 100 000 km above the limb are formed in most prolonged events (called dynamical flares in the terminology of Svestka).     

Cyclotron instability in solar flares

The properties of the cyclotron-maser radiation generated by anisotropic distributions of fast electrons with power-law momentum distributions are studied. The momentum and pitch-angle distributions of the electrons are chosen with regard to available observational constraints. The results obtained for these model distributions are compared with those in the literature for other distributions. The dependence of the properties of the cyclotron instability on the parameters of the distribution of fast electrons is analyzed. These properties are in good quantitative agreement with observational data on millisecond solar radio spikes.     

Gamma-ray radiation with energy 2.223 MeV and the density distribution in the solar atmosphere during flares

Interactions of particles accelerated in solar flares with matter in the solar atmosphere give rise to neutrons, which are efficiently captured on hydrogen nuclei as they are slowed to thermal velocities. This capture is accompanied by the emission of a gamma-ray with energy 2.223 MeV. Observational data for the temporal profiles of the gamma-ray fluxes in this line are used to study the plasma-density distribution in the solar atmosphere during the flares of December 16, 1988, March 22, 1991, and November 6, 1997. This analysis is based on comparisons between the observations and profiles computed taking into account a number of parameters describing the generation and transport of the flare neutrons in atmospheric layers of various densities. In three cases studied, the density of the material in the photosphere below the flare region is enhanced compared to the density in an unperturbed part of the solar atmosphere at the same height. In the case of the December 16, 1988 flare, we are able for the first time to relate the profile of the 2.223 MeV line with the shape of the accelerated particle (proton) spectrum. This opens new possibilities for studies of particle acceleration on the Sun based on observations of flare gamma-ray emission.     

Some structural features of the convective-velocity field in the solar photosphere

An algorithm for measuring horizontal photospheric velocities previously employed to process aerospace images is adapted for problems in solar physics and realized in a computational code. It differs from the standard procedure of local correlation tracking in a special choice of trial areas (“targets”), whose displacements are determined bymaximizing the correlation between the original and various shifted positions of the target. Specifically, an area is chosen as a target in a certain neighborhood of each node of a predefined grid if either the contrast or the entropy of the brightness distribution reaches its maximum in this area. The horizontal velocities obtained are then interpolated to the positions of imaginary “corks” using the Delaunay triangulation and affine transformations specified by the deformation of the obtained triangles at the time step considered. The motion of the corks is represented by their trajectories. A superposition of flows on different scales, from mesogranular to supergranular, can clearly be seen. “Large mesogranules” with sizes of order 15 Mm are revealed. In many cases, these are stellate in shape. Areas of strong convergence of the horizontal flows are detected; this convergence is sometimes accompanied by swirling. Evidence is found for the possible coexistence of convection cells with different circulation directions, so-called l-type and g-type cells.     

Ionospheric response to X-class solar flares in the ascending half of the subdued solar cycle 24

The signature of 11 X-class solar flares that occurred during the ascending half of the present subdued solar cycle 24 from 2009 to 2013 on the ionosphere over the low- and mid-latitude station, Dibrugarh (27.5^°N, 95^°E; magnetic latitude 17.6^°N), are examined. Total electron content (TEC) data derived from Global Positioning System satellite transmissions are used to study the effect of the flares on the ionosphere. A nonlinear significant correlation (R² = 0.86) has been observed between EUV enhancement (ΔEUV) and corresponding enhancement in TEC (ΔTEC). This nonlinearity is triggered by a rapid increase in ΔTEC beyond the threshold value ～1.5 (×10¹⁰ ph cm^?2 s^?1) in ΔEUV. It is also found that this nonlinear relationship between TEC and EUV flux is driven by a similar nonlinear relationship between flare induced enhancement in X-ray and EUV fluxes. The local time of occurrence of the flares determines the magnitude of enhancement in TEC for flares originating from nearly similar longitudes on the solar disc, and hence proximity to the central meridian alone may not play the dominating role. Further, the X-ray peak flux, when corrected for the earth zenith angle effect, did not improve the correlation between ΔX-ray and ΔTEC.     

Spectral-temporal evolution of low-frequency pulsations in the microwave radiation of solar flares

Low-frequency pulsations of 22 and 37 GHz microwave radiation detected during solar flares are analyzed. Several microwave bursts observed at the Metsähovi Radio Observatory are studied with time resolutions of 100 and 50 ms. A fast Fourier transformation with a sliding window and the Wigner-Ville method are used to obtain frequency-time diagrams for the low-frequency pulsations, which are interpreted as natural oscillations of coronal magnetic loops; the dynamical spectra of the pulsations are synthesized for the first time. Three types of low-frequency fluctuations modulating the flare microwave radiation can be distinguished in the observations. First, there are fast and slow magneto-acoustic oscillations with periods of 0.5–0.8 s and 200–280 s, respectively. The fast magneto-acoustic oscillations appear as trains of narrow-band signals with durations of 100–200 s, a positive frequency drift dν/dt=0.25 MHz/min, and frequency splitting δν=0.01–0.05 Hz. Second, there are natural oscillations of the coronal magnetic loops as equivalent electrical circuits. These oscillations have periods of 0.5–10 s and positive or negative frequency drift rates dν/dt=8×10^?3 Hz/min or dν/dt=?1.3×10^?2 Hz/min, depending on the phase of the radio outburst. Third, there are modulations of the microwave radiation by short periodic pulses with a period of 20 s. The dynamical spectra of the low-frequency pulsations supply important information about the parameters of the magnetic loops: the ratio of the loop radius to its length r/L≈0.1, the plasma parameter β≈10^?3, the ratio of the plasma densities outside and inside the loop ρ_e/ρ_i≈10^?2, and the electrical current flowing along the loop I≈10¹² A.     

Relationship between impulsive and post-eruptive processes in solar X-ray flares

Powerful solar flares contain one or more impulsive events, plasma ejection, and the subsequent development of gigant post-eruptive loops. In the middle of the 1980s, Jakimiec proposed an analysis of the flare loops based on log T-1/2log EM diagrams constructed from the observed soft X-rays (the so-called Jakimiec model). We have used this method to construct and analyze these diagrams not for various arbitrary events, but instead for similar flares within a single center of activity; in other words, for homological flares (two-ribbon flares observed in November 2000, powerful prolonged events observed in October–November 2003, etc.). This eliminated the effect of differences in the magnetic configurations, enabling us to find a new relationship: the slope (tan α) of the logT-1/2log EM line during the flare decay depends on the maximum temperature T _max at the source of the soft X-rays. The dependence of tan α on T _max gradually evolves from a series of short flares to a series of powerful, prolonged, nonstationary processes. Our results support the idea that the development of post-eruptive loops depends on the energy of the impulsive events for the phenomenon as a whole. Explosive evaporation simultaneously increases both the temperature and the density of the plasma at the loop top. The subsequent evolution of the post-eruptive formations depends on the difference in the initial conditions and on the degree of opening of the magnetic configuration. The importance of our analysis for the duration of flares and differences between dimmings is briefly discussed.     

Discontinuous plasma flows near reconnecting current layers in solar flares

A model for magnetic reconnection in high-conductivity plasma in the solar corona is analyzed in a strong-magnetic-field approximation. The model includes a Syrovatskii current layer and magnetohydrodynamic (MHD) discontinuities attached to the ends of the layer. A two-dimensional analytical solution for the magnetic field is used to compute the distributions of the plasma flow velocity and plasma density in the vicinity of the corresponding current configuration. The properties of jumps in the density and velocity along the attached discontinuities are studied. Based on the character of the variations of the magnetic field and plasma flows at the MHD discontinuities, it is shown that, with the parameter values considered, an MHDdiscontinuity can include regions of trans-Alfvénic, fast, and slowshocks. The results obtained could be useful to explain the presence of “super-hot” (with effective electron temperatures exceeding 10 keV) plasma in solar flares. Other possible applications of the theory of discontinuous flows near regions of magnetic reconnection to analogous non-stationary phenomena in astrophysical plasmas are noted.     

Acceleration of charged particles in collapsing magnetic traps during solar flares

This paper examines the mechanisms for the acceleration of electrons, protons, and ions during solar flares. The acceleration is assumed to occur in two steps. The particles are first pre-accelerated by the electric field of a high-temperature current sheet undergoing magnetic reconnection. A collapsing magnetic trap in the solar corona provides further acceleration. It is shown that the Fermi mechanism accelerates trapped protons and ions even more efficiently than it does electrons. The particles escaping from the trap have energies reaching several GeV. The energy distribution of the accelerated ions is essentially independent of their mass and degree of ionization.     

Some features of the streamer belt in the solar corona and at the Earth’s orbit

The rays of enhanced brightness making up the structure of the coronal-streamer belt can be traced to the lowest atmospheric layers in the Sun, with the angular size remaining nearly constant, d ≈ 2.5° ± 0.5°. This suggests that the physical mechanism generating the slow solar wind in the rays of the streamer belt differs from the mechanism giving rise to the fast solar wind from coronal holes. At distances of R < (4–5) R _⊙, the rays of the streamer belt are not radial in the plane of the sky and show deviations toward the corresponding pole. They then become essentially radial at R > (4–5) R _⊙. A transverse cross section of streamers in the corona and its continuation into the heliosphere—a plasma sheet—can be represented as two radially oriented, closely spaced rays (d ≈ 2.0°–2.5°) with enhanced density and an angular size of d. We also show that the ray structure of the streamer belt is involved in the development of coronal mass ejections (CMEs). The motion of a small-scale CME occurs within a magnetic flux tube (ray of enhanced brightness) and leads to an explosive increase in its angular size (rapid expansion of the tube). It seems likely that large-scale CMEs are the result of the simultaneous expansion of several magnetic tubes. We suggest that a small-scale CME corresponds to a “plasmoid” (clump of plasma of limited size with its own magnetic field) ejected into the base of a magnetic tube, which subsequently moves away from the Sun along the tube.     

The role of plasma ejections in the development of large solar flares of various durations

Recent observations indicate that relatively strong plasma ejections are accompanied by the formation of systems of coronal loops with two glowing ribbons near their footpoints. However, while two-ribbon flares can sometimes last for many hours, for example, soft X rays, they sometimes decay within tens of minutes. We study here factors affecting the durations of flares using four major flares occurring in July 15–18, 2002, as examples. Various ground-based and satellite observations are used to show that short-duration events involved collimated (narrow) plasma ejections directed to the north and the subsequent formation of compact loops in the leading part of the active region. During one event, a powerful eastward ejection in a wide solid angle was followed by the formation of an extended arch system in the trailing part, which determined the long duration of the flare. It is proposed that in events involving collimated jets and corresponding narrow features in coronal mass ejections (CMEs), systems of coronal loops do form, but post-eruptive energy release either does not occur or is expressed very faintly. So the energy does not go downward from this region, and the plasma is emitted free in the coronal loops. In contrast to such rapid flares, wide ejections and bright, large-scale CMEs are accompanied by the formation and prolonged existence of an extended arch system. Thus, powerful nonstationary solar processes involve a large-scale CME and the flare itself, with the pattern of a particular event determined by the reconnection scenario and the evolution of the ejected plasma.     

Magnetic flux in an active solar region and its correlation with flares

The correlation between the magnetic flux in an active solar region and associated powerful solar flares is studied. The behavior of the active regions AR 10486 and AR 10365 is considered. These regions produced a series of class X flares as they crossed the solar disk. The flares appeared when the magnetic flux exceeded 10²² Mx. The magnetic flux remained constant during all the flares except for one. During this flare, the flux decreased by about 10%; this impulsive decrease of the flux was also recorded in the absence of flares. No energy flux from the photosphere to the corona at the time of the flare was observed. The behavior of the photospheric field in AR 10486 and AR 10365 is consistent with a slow accumulation of energy in the corona and the explosive release of energy stored in the magnetic field of a current sheet above an active region during the flare.     

Stellar sources of the short-lived radionuclides in the early solar system

We discuss the possible stellar sources of short-lived radionuclides (SLRs) known to have been present in the early solar system (²⁶Al, ³⁶Cl, ⁴¹Ca, ⁵³Mn, ⁶⁰Fe, ¹⁰⁷Pd, ¹²⁹I, ¹⁸²Hf, ²⁴⁴Pu). SLRs produced primarily by irradiation (⁷Be, ¹⁰Be) are not discussed in this paper. We evaluate the role of the galactic background in explaining the inventory of SLRs in the early solar system. We review the nucleosynthetic processes that produce the different SLRs and place the processes in the context of stellar evolution of stars from 1 to 120 M_⊙. The ejection of newly synthesized SLRs from these stars is also discussed. We then examine the extent to which each stellar source can, by itself, explain the relative abundances of the different SLRs in the early solar system, and the probability that each source would have been in the right place at the right time to provide the SLRs. We conclude that intermediate-mass AGB stars and massive stars in the range from ∼20 to ∼60 M_⊙ are the most plausible sources. Low-mass AGB stars fail to produce enough ⁶⁰Fe. Core-collapse Type II supernovae from stars with initial masses of <20 M_⊙ produce too much ⁶⁰Fe and ⁵³Mn. Sources such as novae, Type Ia supernovae, and core-collapse supernovae of O-Ne-Mg white dwarfs do not appear to provide the SLRs in the correct proportions. However, intermediate-mass AGB stars cannot provide ⁵³Mn or the r-process elements, so if an AGB star provided the ⁴¹Ca, ³⁶Cl, ²⁶Al, ⁶⁰Fe, and ¹⁰⁷Pd, and if a late stellar source is required for ⁵³Mn and the r-process elements, then two types of sources would be required. A separate discussion of the production of r-process elements highlights the difficulties in modeling their production. There appear to be two sources of r-process elements, one that produces the heavy r-process elements, including the actinides, and one that produces the elements from N to Ge and the elements ∼110 < A < ∼130. These can be assigned to SNII explosions of stars of ?11 M_⊙ and stars of 12-25 M_⊙, respectively. More-massive stars, which leave black holes as supernova remnants, apparently do not produce r-process elements.     

The generation of hard X-rays and relativistic protons observed during solar flares

The flare source of thermal X-rays above a magnetic arch in the corona arises from the dissipation of the magnetic energy of the current sheet formed at the reconnection of magnetic-field lines. The sources of hard X-rays emitted from the footpoints of the magnetic arch are beams of electrons accelerated in field-aligned currents induced by the Hall electric field generated in the current sheet. Both the hard X-rays detected above the active region and the type III radio emission are radiated by electrons accelerated in the field-aligned currents induced by Alfven waves. The solar cosmic rays are emitted promptly at the instant of the flare. It is important that the Lorentz electric field accelerates protons along the singular magnetic X line. The relativistic protons propagate along the interplanetary magnetic field. These protons have exponential spectra, typical for acceleration occurring in current sheets. A mechanism that is relevant for the generation of delayed cosmic rays, which demonstrate significant anisotropy and a power-law spectrum with γ ∼5, is also discussed.     

Some aspects of release and transport of gases in deep granitic rocks: possible implications for nuclear waste repositories

Radon, radium and helium data from three sites in granitic rock in Sweden (Forsmark and Laxemar) and Finland (Olkiluoto) from depths greater than 1,000 m were interpreted by a model that describes how daughter nuclides, including helium from uranium and thorium decay, escape from minerals, enter the pore water in the rock matrix and diffuse to the seeping water in the fractures in the rock. The radium concentrations are on the order of <30 Bq/l of water that has recently infiltrated and then emerged from the rock. Radon concentrations are mostly several orders of magnitude larger. The model predicts values in the same range. The fair agreement between model results, field data and laboratory data over a scale spanning micrometres over meters to kilometres, and time scales of days to millions of years, shows that the micropores of the rock matrix are connected even at depths down to and beyond a kilometre. Molecular diffusion in the matrix pore water is a key migration mechanism. Laboratory-derived sorption coefficients for radium are of the same magnitude as those needed in the modelling of the in situ data to give good agreement.     

Some properties of the joining of solar filaments

Some possibilities for the reconnection of magnetic-field lines of solar filaments that approach when the photospheric polarity inversion lines change their positions, are discussed. The interaction between filaments depends on their internal properties, which are determined by the filament chirality, or the sign of the helicity of the filament magnetic field. In quadrupolar magnetic configurations, filaments with the same chirality can exchange their halves. Filaments with opposite chirality rupture after the reconnection of the polarity inversion lines, since the two fragments of the different filaments cannot be connected continuously. The morphology and connectivity of the filaments are analyzed using daily Hα filtergrams obtained over the period of maximum activity of the 23rd solar cycle. Examples of alterations of the filament connectivity occuring during the evolution of photospheric fields are presented.     

Gold mobilizing fluids in the Witwatersrand Basin: composition and possible sources

Summary Crush-leach data were obtained, using High Performance Gradient Ion-Chromatography and Capillary Electrophoresis, on individual generations of aqueous fluid inclusions in hydrothermal quartz from three different auriferous conglomerate horizons (reefs) in the late Archaean Witwatersrand Basin, South Africa. These data, supplemented by oxygen isotope analyses of hydrothermal quartz and in combination with microthermometric analyses, help to constrain the chemical composition, pH, temperature of formation and the possible source of the mineralizing fluid which, in places, was capable of mobilizing some of the primarily detrital gold in the fluvial Witwatersrand sediments. The dominant cations in the aqueous fluid inclusions are Na⁺ and Ca²⁺, with Cl^– or HCO ₃ ^– being the dominant anion, whereas K⁺, Mg²⁺, and SO ₄ ^2– are subordinate. Most fluid inclusions have elevated NH ₄ ⁺ concentrations which are directly correlated with those of N03. In a number of samples small amounts of organic acids (formate, propionate, and acetate) were also detected.A largely meteoric source is inferred for the gold-mobilizing fluids in the Witwatersrand reefs because of a lack of Br in the fluid, a composition distinctly different from that of seawater, the presence of organic acids, and ¹⁸O_fluid values around O%o. The fluids are ascribed to hydrothermal infiltration triggered by the 2020 Ma Vredefort impact which also created a secondary permeability in the form of a dense network of micro-fractures preferentially in the conglomerate beds of the already metamorphosed Witwatersrand rock sequence. This fluid differs from the regional metamorphic fluid in the basin by having a considerably higher pH (5.7–7.2). The difference in pH might explain why the older, fairly acidic metamorphic fluid was apparently less capable of mobilizing the gold as gold solubility reaches its peak at the pH calculated for the fluid ascribed to the impact.

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Gold-mobilisierende Fluide inn Witwatersrand Becken: Zusammensetzung und mögiche Herkunft

Zusammenfassung Einzelne Generationen von wäßrigen Flüssigkeitseinschlüssen in hydrothermalem Quarz von drei verschiedenen Gold-haltigen Konglomerathorizonten (Reefs) im spät-Archaischen Witwatersrand Becken, Südafrika, wurden mittels Ionenchromatographie und Kapillarelektrophorese untersucht. Weiters bestimmten wir die Sauerstoffisotopen-Verhältnisse hydrothermaler, zum Teil Gold-führender Quarze. Zusammen mit mikrothermometrischen Ergebnissen helfen die neu gewonnenen Daten, die chemische Zusammensetzung, pH-Wert, Bildungstemperatur und mögliche Herkunft der Gold-mobilisierenden Fluide zu rekonstruieren. Die dominierenden Kationen und Anionen in den wäßrigen Einschlüssen sind Na⁺ und Ca²⁺ beziehungsweise Cl^– und HCO ₃ ^– . In geringeren Mengen fanden sich K^-, Mg²⁺ und SO ₄ ^2– . In den meisten Einschlüssen wurden erhöhte Konzentrationen von NH ₊ ⁴ festgestellt, welche direkt mit denen von NO _- ³ korrelieren. Weiters konnten in etlichen Einschlüssen auch organische Säuren (Formfiat, Propionat und Azetat) nachgewiesen werden.Auf Grund des Mangels an Br^–, einer Zusammensetzung, die stark von Meerwasser abweicht, der Anwesenheit organischer Säuren und ¹⁸O_fluid-Werten um 0%o ist eine meteorische Herkunft der Gold-mobilisierenden Fluide im Witwatersrand Becken wahrscheinlich. Die Zirkulation dieser Fluide wird durch das 2020 Ma Vredefort Impakt-Ereignis erklärt, welches durch Bildung einer sekundären Permeabilität in Form eines feinen Netzwerkes von Mikrospalten, vor allem in den Konglomeratlagen, die bereits metamorph geprägten Gesteine des Witwatersrandes für die hydrothermale Infiltration vorbereitet hat. Diese Fluide unterscheiden sich von den älteren, regionalmetamorphen Fluiden durch einen wesentlich höheren pH-Wert (5.7–7.2), bei dem Gold seine maximale Löslichkeit unter den gegebenen physikalisch-chemischen Bedingungen besitzt. Dieser Unterschied im pH-Wert mag erklären, warum die relativ sauren regionalmetamorphen Fluide im Vergleich zu den Impakt-Fluiden kaum in der Lage waren, die ursprünglich detritischen Goldkörner zu mobilisieren.

     

岩爆破坏过程能量释放的数值模拟

岩爆是地下岩石开挖中的一种工程灾害现象,是岩体结构发生破坏时,由于内部储存的弹性能释放并转换为动能而造成动力形式的破坏。岩爆破坏过程中的能量释放与岩体在应力峰值前后的应力-应变特性紧密相关。另外,施工中开挖速度引起的加载速率的变化也会对岩爆的产生有明显影响。以岩体全过程应力-应变曲线试验为基础分析岩爆破坏过程。分析中采用的模型考虑了岩石峰值后应力-应变特性及加载速率的影响。运用数值方法对岩石洞室的开挖过程进行了模拟,在模拟中对岩体破坏的发生及弹性能释放过程进行了分析。数值分析结果显示,岩体洞室开挖过程中岩石破坏由岩体表面向岩体内部发展,岩石的弹性能释放率也随着破坏的发展而不断增加。分析结果还显示,岩体破坏时的弹性能释放速率会随着开挖速率的提高而明显增加。