Solar X-ray Flares Research Articles

An Improved Method to Derive the Lower Energy Cutoff of non-Thermal Electrons From Hard x-Rays of Solar Flares

By changing a dimensionless calculation to a dimensional one, introducing a more accurate bremsstrahlung cross section, and using a more reasonable fitting energy range, we have recalculated the hard X-ray bremsstrahlung produced by a beam of power-law electrons with a lower energy cutoff (Ec). The method to deduce Ec from the hard X-ray spectral observations has therefore been refined in comparison with our previous one. The universality of this method has been clarified and discussed. We have applied this improved method to the 54 BATSE/Compton Gamma Ray Observatory (CGRO) hard X-ray events. It was found that about 44% of sample hard X-ray spectra can be directly explained by a beam of power-law electrons with a lower energy cutoff. The value of Ec, varying from 45 keV to 97 keV, is on average 60 keV. Another 44% of sample hard X-ray spectra might be explained by a beam of power-law electrons with the energy cutoff lower than 45 keV, which is however beyond the availability of BATSE/CGRO. Still another 11% sample hard X-ray spectra cannot be explained by a beam of power-law electrons with a lower energy cutoff. These results, based on the lower energy resolution data, however, should be compared in the future with that based on a higher energy resolution data, like the data from HESSI.

Understanding Solar Flare Waiting-Time Distributions

The observed distribution of waiting times Δt between X-ray solar flares of greater than C1 class listed in the Geostationary Operational Environmental Satellite (GOES) catalog exhibits a power-law tail ∼(Δt)γ for large waiting times (Δt>10 hours). It is shown that the power-law index γ varies with the solar cycle. For the minimum phase of the cycle the index is γ=−1.4±0.1, and for the maximum phase of the cycle the index is −3.2±0.2. For all years 1975–2001, the index is −2.2±0.1. We present a simple theory to account for the observed waiting-time distributions in terms of a Poisson process with a time-varying rate λ(t). A common approximation of slow variation of the rate with respect to a waiting time is examined, and found to be valid for the GOES catalog events. Subject to this approximation the observed waiting-time distribution is determined by f(λ), the time distribution of the rate λ. If f(λ) has a power-law form ∼λα for low rates, the waiting time-distribution is predicted to have a power-law tail ∼(Δt)−(3+α) (α>−3). Distributions f(λ) are constructed from the GOES data. For the entire catalog a power-law index α=−0.9±0.1 is found in the time distribution of rates for low rates (λ<0.1 hours −1). For the maximum and minimum phases power-law indices α=−0.1±0.5 and α=−1.7±0.2, respectively, are observed. Hence, the Poisson theory together with the observed time distributions of the rate predict power-law tails in the waiting-time distributions with indices −2.2±0.1 (1975–2001), −2.9±0.5 (maximum phase) and −1.3±0.2 (minimum phase), consistent with the observations. These results suggest that the flaring rate varies in an intrinsically different way at solar maximum by comparison with solar minimum. The implications of these results for a recent model for flare statistics (Craig, 2001) and more generally for our understanding of the flare process are discussed.

Coronal Mass Ejections Associated with Impulsive Solar Energetic Particle Events

An impulsive solar energetic particle (SEP) event observed on the Wind spacecraft on 2000 May 1 was associated with an impulsive solar active region M1 X-ray flare. The timing and position of a fast (v = 960 km s-1), narrow CME observed in the LASCO coronagraph on SOHO make clear the connection between the CME and the flare and SEP event. Impulsive SEP events have long been associated with impulsive flares, but only gradual SEP events have thus far been found to be associated with CMEs. A comparison of impulsive SEP events with CME observations from the Solwind and LASCO coronagraphs revealed further good cases of narrow (10°-40°) CMEs associated with impulsive SEP events. A recent model of impulsive flares includes jets or plasmoids that are ejected upward from magnetic reconnection sites over active regions and might therefore be expected to appear in exceptional cases as faint and narrow CMEs in coronagraphs. We suggest that this model allows us to understand better SEP production and propagation in impulsive flares.

On the Fast Fluctuations in Solar Flare Hα Blue Wing Emission

Fine temporal structures in hard X-ray and microwave emissions of solar flares have been known for many years. Recent observations with high time and spatial resolution revealed that emissions in the wings of Hα could also exhibit fast (subsecond) fluctuations. We argue that such fluctuations are physically related to the small-scale injection of high-energy electrons. We explore this through numerical calculations. The energy equation and the equations for energy-level populations in hydrogen, in particular including the nonthermal collisional excitation and ionization rates, are solved simultaneously for an atmosphere impacted by a short-lived electron beam. We determine the temporal evolution of the atmospheric temperature, the atomic level populations, and the Hα line intensity. We find that although the background Hα wing emission is mainly formed in the photosphere, the fast fluctuations are probably produced in the chromosphere, which is penetrated by ~20 keV electrons. To yield Hα wing fluctuations of amplitude comparable to the observations, a mean energy flux of ~ × 1011 ergs cm-2 s-1 is required for the electron beam, if one adopts a Gaussian macrovelocity of 25 km s-1. Such a burst contains a total energy of 1025-1026 ergs. These parameters are compatible with elementary flare bursts.

Absorption events associated with solar flares

During the upward period of solar cycle 23, the Imaging Riometer at Zhongshan, Antarctica (geomag. lat. 74.5 S) was used to study the solar proton events and the Xray solar flares which are associated with the absorption events. In our study, the relationship between the absorption intensity and X-ray flux is found in a power form which is consistent with the theoretical result. The imaging riometer absorption data at Ny-Alesund, Svalbard reconfirm the above relationship. We also argue that only M-class flares can generate a significant daytime absorption.

A comparison between statistical properties of solar X-ray flares and avalanche predictions in cellular automata statistical flare models

We perform a tentative comparison between the statistical properties of cellular automata statistical flare models including a highly variable non-linear external driver and the respective properties of the WATCH flare data base, constructed during the maximum of solar cycle 21. The model is based on the concept of SelfOrganized Criticality (SOC). The frequency distributions built on the measured X-ray flare parameters show the following characteristics: (1) The measured parameters (total counts, peak count-rates and, to a lesser extent, total duration) are found to be correlated to each other. Overall distribution functions of the first two parameters are robust power laws extending over several decades. The total duration distribution function is represented by either two power laws or a power law with an exponential roll-over. (2) By sub-grouping the peak count-rate and the total counts as functions of duration and constructing frequency distributions on these sub-groups, it is found that the slope systematically decreases with increasing duration. (3) No correlation is found between the elapsed time interval between successive bursts arising from the same active region and the peak intensity of the flare. Despite the inherent weaknesses of the SOC models to simulate realistically a number of physical processes thought to be at work in solar active regions and in flares’ energy release, we show that the model is able to reproduce the bulk of the above statistical properties. We thus underline two main conclusions: (i) A global, statistical approach for the study of rapid energy dissipation and magnetic field line annihilation in complex, magnetized plasmas may be of equal importance with the localized, small-scale Magnetohydrodynamic (MHD) simulations, and (ii) refined SOC models are needed to establish a more physical connection between the cellular automata evolution rules and the observations.

The Role of Long-Duration X-ray Solar Flares in the Generation of Interplanetary Disturbances

At present, it is widely believed that coronal mass ejections (CMEs) rather than solar flares, as assumed previously, are the sources of sporadic interplanetary disturbances. CMEs are an integral part of the powerful nonstationary processes that in many cases give rise to long-decay flares (LDFs). We numerically simulate the energy balance in a giant loop that forms during a LDF. For geoefficient disk flares, we show that maintaining the observed X-ray flux requires a prolonged input of a substantial amount of energy into this loop from above, from the region of primary energy release (probably, from a vertical current sheet). Part of the energy from this region propagates outward both at the onset of the process, with plasmoid ejections, and during the prolonged dynamic phase, thereby enhancing the CME. Using the series of LDFs in March 1993 as an example, we consider the role of flares in producing the corresponding interplanetary disturbances. The large amplitude of the Forbush effect and the strong interplanetary disturbance on March 8–10 near the Earth and on March 15 at a heliocentric distance of about 5 AU (Ulysses) are shown to have been associated with the long-duration flare of March 6, 1993. The March 1993 events give a typical example of CME and magnetic-configuration opening that result in post-CME energy release. This is accompanied by the appearance of new arch systems inside the active region and/or by the development of giant loop systems outside. Such a process enhances CME and increases its geoefficiency.

SOHO/Energetic and Relativistic Nucleon and Electron Experiment Measurements of Energetic H, He, O, and Fe Fluxes during the 1997 November 6 Solar Event

A brilliant solar X-ray and Hα flare and a coronal mass ejection (CME) on 1997 November 6 were associated with high particle fluxes in interplanetary space at energies above MeV. The CME had an exceptionally high leading edge velocity (1560 km s-1) as observed by the Large Angle and Spectrometric Coronagraph (LASCO) on board the Solar and Heliospheric Observatory (SOHO). The Energetic and Relativistic Nucleon and Electron experiment (ERNE), also on SOHO, measured high H, He, O, and Fe fluxes in several energy channels from 3 to 200 MeV nucleon-1. The oxygen energy spectrum in energy range 3-200 MeV nucleon-1 was of a broken-power-law form with a break around 50 MeV nucleon-1. In addition, ERNE observed abrupt changes in the intensity, elemental composition, and anisotropy of high-energy particles, which may indicate two energetic particle sources during several hours after the solar flare eruption. The observational results lead us to conclude that, most of the time, the O and Fe nuclei were injected by the interplanetary shock associated with the coronal mass ejection emitted around 12 UT on November 6. However, during two time periods the injection source might have been different or complementary. The first period was in the very beginning of the event, 13:20-13:40 UT, when the particle streaming showed very strong anisotropy with the maximum intensity from the direction of the Sun. The second was between 17 UT November 6 and 5 UT November 7, when particle fluxes were dominated by a particle population with a different elemental composition and a different spectral shape of O as compared with the particle population prevailing in the beginning and during the long decay phase of the event. We propose that the source of these particles was associated with a coronal shock wave traveling in the low solar atmosphere.

Sympathetic flaring with BATSE, GOES, and EIT data

Sympathetic flaring is defined as the initiation of a solar flare as a result of a transient phenomenon occurring elsewhere on the Sun. Discovery of sympathetic flaring or lack thereof, may lead to a greater understanding of the physics of flare initiation. Knowledge of a mechanism for initiating solar flares would also aid in predicting at least some solar flares. Two studies of sympathetic flaring are presented in this paper. The first part of the paper presents a test for sympathetic flaring in flares observed with the Burst and Transient Source Experiment. A Monte Carlo simulation is used to compare the distribution of solar X-ray flares in time to that expected from a time-varying, Poisson distribution. No evidence for sympathetic flaring is found, though it cannot be ruled out. The X-ray flare data also do not allow discovery of sympathetic flares occurring within 2 min of the initial flare. Because the observations do allow for at least some flares to occur sympathetically, the second part of the paper examines one possible mechanism for initiating flares. The mechanism examined is large-scale coronal transients observed by the SOHO/Extreme Ultraviolet Imaging Telescope: EIT waves. A comparison of the rate of flaring in the interval prior to an EIT wave to the rate of flaring while the wave traverses the solar disk shows no increase in the number of flares due to the EIT wave.

Insights into the Damage Mechanism of Teflon® FEP from the Hubble Space Telescope

Metallized Teflon® FEP (fluorinated ethylene propylene) thermal control material on the Hubble Space Telescope (HST) has been found to be degrading in the space environment. Teflon® FEP thermal control blankets (space-facing FEP) retrieved during the first servicing mission (SM1) were found to be embrittled on solar-facing surfaces and contained microscopic cracks. During the second servicing mission (SM2) astronauts noticed that the FEP outer layer of the multi-layer insulation (MLI) covering the telescope was cracked in many locations around the telescope. Large cracks were observed on the light shield, forward shell and equipment bays. A tightly curled piece of cracked FEP from the light shield was retrieved during SM2 and was severely embrittled, as witnessed by ground testing. A failure review board was organized to determine the mechanism causing the MLI degradation. Density, x-ray crystallinity and solid-state nuclear magnetic resonance (NMR) analyses of the FEP retrieved during SM1 were inconsistent with results of FEP retrieved during SM2. Because the retrieved SM2 material was curled while in space, it experienced a higher temperature extreme during thermal cycling, estimated at 200°C, than the SM1 material, estimated at 50°C. An investigation on the effects of heating pristine FEP and FEP retrieved from the HST was therefore conducted. Samples of pristine, SM1 and SM2 FEP were heated to 200°C and evaluated for changes in density and morphology. Elevated-temperature exposure was found to have a major impact on the density of the retrieved materials. The characterization of the polymer morphology of the as-received and heated FEP by NMR provided results that were consistent with the density results. Differential scanning calorimetry (DSC) was conducted on pristine, SM1 and SM2 FEP. DSC results provided evidence of chain scission and increased crystallinity in the space exposed FEP, which supported the density and NMR results. Samples exposed to simulated solar flare x-rays, thermal cycling and long-term thermal exposure provided information on the environmental contributions to degradation. These findings have provided insight into the damage mechanisms of FEP in the space environment.

What are the bright loop-top kernels in soft X-ray flares?

Abstract A major puzzle in the study of solar X-ray flares is the presence of a bright, persistent ‘kernel’ at or near the tops of flaring loops in soft X-ray images from Yohkoh. Here we argue that they can be explained by a tangled magnetic field geometry with unresolved current sheets continually being formed. Hot (20 MK) plasma is formed near the current sheets and is surrounded by plasma which has cooled to ∼ 10 MK plasma so that the two temperatures appear to co-exist.

Physical Requirements for Flares in Stars

AbstractA wide range of stellar analogs of solar X-ray flares has been observed. During the maximum of the solar activity cycle, one or two Mclass flares peaking at Lx ~ 1026 erg s−1 in the GOES 1−8 Å passband take place on average every day on the Sun. Such run-of-the-mill events have been measured by ASCA on our nearest neighbor, Proxima Centauri, a dM5.5e flare star (Haisch, Antunes &amp; Schmitt 1995). At the other extreme, RS CVn systems and T Tauri stars have been observed to flare with peak luminosities of Lx ~ ×1032 ergs s−1 (Haisch &amp; Schmitt 1996 and references therein). Current wisdom has it that this wide range of flares spanning at least six order of magnitude (and another factor of 10–100 if one counts even lower level solar flares) on the young T Tauri stars, main sequence G, K and M stars, and in evolved RS CVn subgiants and giants can all be understood as versions of solar flares originating ultimately in a convectively-driven magnetic dynamo. There is evidence pointing to two distinct types of dynamo perhaps even in the Sun (Durney, Young and Roxburgh 1993): the α − ω dynamo generated in a rather thin boundary layer near — in some interpretations just below — the interface between a star’s radiative core and convective envelope; and a turbulent distributed dynamo operative more or less throughout a convection zone. But what is one to make of the evidence for flares on Be stars (Smith, Robinson and Corbet 1998) where there is no subsurface convection to drive a dynamo? (Convective envelopes are thought to begin among the late-A spectral type stars, and steadily deepen for cooler stars.) This paper does not attempt to provide an answer, but it does attempt to outline the characteristics and requirements of magnetically-powered flaring.

Time dependent ionization balance in solar flares

We have analyzed the time evolution of solar flare X-ray spectra from He-like Fe, Ca and S ions measured for a 1992 September 6 flare by the Bragg Crystal Spectrometer (BCS) on the Yohkoh satellite. We derived time-dependent electron temperature, ion temperature, electron density as well as the ion density ratios such as n(Li-like)/n(He-like), n(Be-like)/n(He-like), n(B-like)/n(He-like) for Fe ions and n(He-like)/n(H) for Fe and Ca ions. The results show a deviation from ionization equilibrium. In this paper we investigate the origin of the deviation and consider time dependent ionization models assuming material flow in solar flares or multiple loops occurring with a short time period.

What Affects the Power‐Law Distribution of the X‐Ray Solar Flares? A Theoretical Study Based on a Model of Uniform Normal Field

The power-law distribution of the X-ray solar flares has long been observed, supporting the idea that the solar corona is in a state of self-organized criticality. In this paper, we used a model that simulates the complex relaxation of the coronal magnetic field to study the conditions that affect (1) the very existence of the power-law distribution; and (2) the exponent of the power law. The model consists of a large number of flux tubes arranged in a two-dimensional, square-lattice fashion that undergoes relaxation as any local current exceeds a threshold. We have found that a necessary condition for our model to exhibit a power-law distribution is the conservation of the relative magnetic helicity. The exponent of the power-law distribution is slightly affected by the rule of relaxation chosen, but it is quite insensitive to the pattern of boundary perturbation and the variation of the boundary helicity injection. The power-law exponent is found to be in the range 1.25-1.35.

The Electron Temperature and Fine Structure of Soft X‐Ray Solar Flares

I discuss the determination of the electron temperature of soft X-ray solar flares using data obtained from the iron-line Bragg Crystal Spectrometer (BCS) and Soft X-ray Telescope (SXT) instruments on the Yohkoh spacecraft. I find that there is a substantial difference in the derived temperatures obtained from the two instruments, with the highest temperatures obtained from the BCS. However, in some regions of certain flares, the SXT temperatures equal the temperatures expected from the iron-line spectra. These hot regions are almost always considerably weaker in intensity than the brightest regions in the SXT flare images. I discuss the relationship of the SXT hot regions to the brightest regions in the flare and show that, at least in some cases, the hot SXT regions appear to be related to the hard X-ray flare component. I also show that the temperature data from the two instruments can be reconciled by concluding that flare loops that appear to be single loops are actually collections of loops with different temperatures on arcsecond or less spatial scales. This result was obtained indirectly by Doschek, Strong, & Tsuneta in 1995 from an analysis of SXT flare images and is substantiated so far by flare images from TRACE. The appearance of a soft X-ray flare depends critically on the electron temperature response of the imaging instrumentation.

Benchmarking the MEKAL spectral code with solar X-ray spectra

With the likelihood that high-resolution soft X-ray spectra of non-solar astronomical sources will soon become available, it is desirable to examine the accuracy of spectral synthesis codes. In this paper, a benchmark study of the MEKAL code, extensively used in the past for spectra from , , SAX , and other spacecraft, is presented using high-resolution solar flare X-ray spectra obtained with the Bragg Flat Crystal Spectrometer (FCS) on SMM . Lines in the range 5-20 A are used to adjust the wavelengths in the MEKAL code. Many of the lines are due to Fe ions, and arise from 3-2 transitions for spectra obtained during the decay phase of one of the flares, while others arise from higher-excitation (4-2, 5-2 etc.) transitions for spectra obtained near the peak of a second flare. Laboratory measurements of the wavelengths of these lines were also used to confirm the SMM values as well as published identifications from the HULLAC atomic code. The adjustments needed were up to 35 mA for line wavelengths above 13 A but much less at shorter wavelengths. Some of these adjustments will be perceptible for spectra from the forthcoming XMM and Chandra spacecraft.

The Solar Flare Soft X‐Ray Differential Emission Measure and the Neupert Effect at Different Temperatures

Data from the Yohkoh Soft X-ray Telescope (SXT) and Bragg Crystal Spectrometer (BCS) are used to obtain the differential emission measure (DEM) of solar flare soft X-ray plasma. The maximum entropy method is used to obtain the DEM for 80 flares observed by SXT and BCS from 1991 October to 1992 November. We find that it is often difficult to describe the DEM using a monotonically decreasing function; functions with a hump at high temperature are occasionally more useful. From the time history of the DEM, we show that higher temperature plasma peaks earlier and cools faster. We also show that high-temperature plasma (>16.5 MK) is more likely than low-temperature plasma to exhibit the so-called Neupert effect, in which the time derivative of the soft X-ray emission measure is similar to the light curve of the impulsive hard X-ray emission for the flare.

Influence of X-Ray Solar Flare Radiation on Degradation of Teflon in Space

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Effects of Radiation and Thermal Cycling on Teflon ® FEP

Surfaces of the aluminized Teflon® FEP (fluorinated ethylene propylene) multilayer thermal insulation on the Hubble Space Telescope (HST) were found to be cracked and curled in some areas at the time of the second servicing mission (SM2) in February 1997, 6.8 years after HST was deployed in low Earth orbit (LEO). In an effort to understand what elements of the space environment might cause such damage, pristine second-surface aluminized Teflon® FEP was tested for durability to various types of radiation, to thermal cycling and to radiation followed by thermal cycling. Types of radiation included synchrotron vacuum ultraviolet and soft x-ray radiation, simulated solar flare x-ray radiation, electrons and protons. Thermal cycling was conducted in various temperature ranges to simulate HST orbital conditions for Teflon® FEP. Resultsoftensiletestingoftheexposedspecimensshowedthatexposuretohighfluencesofradiation caused degradation in tensile properties of FEP. However, exposure to radiation alone in exposures comparable to those experienced by HST did not produce reduction in ultimate tensile strength and elongation of Teflon® similar to that observed for HST-retrieved aluminized Teflon®. Synergism of radiation exposure and thermal cycling was evident in the results of three experiments: thermal cycling following electron and proton irradiation, thermal cycling following x-ray exposure, and additional thermal cycling of a sample retrieved from HST. However, irradiation and thermal cycling with comparable HST SM2 exposure conditions did not produce the degradation observed in the FEP material retrieved during HST SM2.

Interaction of solar flare X-rays with the atmosphere of Titan

During solar flares an intense flux of X-rays is emitted into space. When it reaches Saturns orbit, it is still strong enough to significantly disturb Titans dense atmosphere. The main effect is to ionize the neutral component of the atmosphere and to increase, therefore, the electron and ion density at altitudes of 400–900 km. Especially near a maximum of the solar cycle, when flare activity is high, the average plasma density can, for long intervals, remain at a level of a few tens of particles per cm 3. As a result, the atmospheric photochemistry will be affected, but also an increase of production rate of small haze particles might be expected. Ionized atoms are usually excited ; when they relax to the ground level in the process of resonant fluorescence, Titans albedo at specific wavelengths increases. It is estimated that at Earth orbit the flux of photons emitted from Titan in the 3 KeV argon line during a strong X1-type flare is equal to 1.6×10 −20 J m −2 s −1 for 10% Ar abundance in Titans atmosphere.