Flare Heating Research Articles

Unsteady magnetic reconnection in non-periodic multiple current sheets

The process of magnetic reconnection in non-periodic three-layer current sheets is studied numerically by using two-dimensional magnetohydrodynamic simulation. The results show that unlike periodic current sheets, it is complex unsteady magnetic reconnection. It may be important for solar flare and corona heating.

Asymmetric X-ray line broadening in solar flares

The symmetry and time development of X-ray spectral lines are examined for many flares using Yohkoh Bragg Crystal Spectrometer (BCS) observations. We examine the degree of line blueshift and asymmetric broadening as a function of flare impulsiveness. The results of the analysis present a consistent observational picture for the 16 flares that were studied. The blueshift of the total flare spectrum increases with increasing fractional rate of change of flux. This result supports models that predict stronger heating in flares results in more blueshifted plasma. It also suggests that most flares will exhibit very weak or no blueshifts if the peak fractional energy release rate remains relatively low. This will be the case if stationary plasma builds up quickly by early ‘gentle’ evaporation or rapid slowing of moving plasma, even when most of the hot plasma is generated by explosive chromospheric evaporation.

The analysis of energy release in solar flares based on X-ray observations

Extended review of selected papers which deal with the problem of flare heating in solar coronal loops is presented. Discussed methods of the analysis of flare heating based on the X-ray observations have been worked out using the Palermo-Harvard hydrodynamic code. The case is presented when the assumption of the uniform heating across the loop is made. The existence of multiple elementary heating episodes is postulated as well. Next the possibility of the non-uniform heating across the loop is assumed and its manifestation in the X-ray observations is investigated. The application of proposed methods of the analysis to the observations of solar flares in X-rays is presented.

Simultaneous Extreme-Ultraviolet Explorer and Optical Observations of AD Leonis: Evidence for Large Coronal Loops and the Neupert Effect in Stellar Flares

We report on the first simultaneous Extreme-Ultraviolet Explorer (EUVE) and optical observations of flares on the dMe flare star AD Leonis. The data show the following features: (1) Two flares (one large and one of moderate size) of several hours duration were observed in the EUV wavelength range; (2) Flare emission observed in the optical precedes the emission seen with EUVE; and (3) Several diminutions (DIMs) in the optical continuum were observed during the period of optical flare activity. To interpret these data, we develop a technique for deriving the coronal loop length from the observed rise and decay behavior of the EUV flare. The technique is generally applicable to existing and future coronal observations of stellar flares. We also determine the pressure, column depth, emission measure, loop cross-sectional area, and peak thermal energy during the two EUV flares, and the temperature, area coverage, and energy of the optical continuum emission. When the optical and coronal data are combined, we find convincing evidence of a stellar 'Neupert effect' which is a strong signature of chromospheric evaporation models. We then argue that the known spatial correlation of white-light emission with hard X-ray emission in solar flares, and the identification of the hard X-ray emission with nonthermal bremsstrahlung produced by accelerated electrons, provides evidence that flare heating on dMe stars is produced by the same electron precipitation mechanism that is inferred to occur on the Sun. We provide a thorough picture of the physical processes that are operative during the largest EUV flare, compare and contrast this picture with the canonical solar flare model, and conclude that the coronal loop length may be the most important factor in determining the flare rise time and energetics.

Atmospheric heating in solar flares by chromospheric condensation

More and more observational and theoretical evidence has shown the existence of a chromospheric condensation during solar flares. Can such a chromospheric condensation play a role in the energy process of the chromospheric flare? In this paper, we have empirically studied the influence of the chromospheric condensation on the distribution of the atmospheric radiative loss. The main results are as follows: when the temperature of the condensation is lower than that below it, the radiative loss changes only slightly; but if the temperature of the condensation is higher than that below it, the radiative loss may change greatly both in the chromosphere and in the photosphere; the backwarming of the atmosphere due to the condensation may be very effective; the temperature increase in the temperature minimum region (TMR) may be as much as 500 K; and the backwarming may be effective as deep as h = 0 km, resulting in an enhancement of the continuum emission.

Solar flare model atmospheres

Solar flare model atmospheres computed under the assumption of energetic equilibrium in the chromosphere are presented. The models use a static, one-dimensional plane parallel geometry and are designed within a physically self-consistent coronal loop. Assumed flare heating mechanisms include collisions from a flux of non-thermal electrons and x-ray heating of the chromosphere by the corona. The heating by energetic electrons accounts explicitly for variations of the ionized fraction with depth in the atmosphere. X-ray heating of the chromosphere by the corona incorporates a flare loop geometry by approximating distant portions of the loop with a series of point sources, while treating the loop leg closest to the chromospheric footpoint in the plane-parallel approximation. Coronal flare heating leads to increased heat conduction, chromospheric evaporation and subsequent changes in coronal pressure; these effects are included self-consistently in the models. Cooling in the chromosphere is computed in detail for the important optically thick HI, CaII and MgII transitions using the non-LTE prescription in the program MULTI. Hydrogen ionization rates from x-ray photo-ionization and collisional ionization by non-thermal electrons are included explicitly in the rate equations. The models are computed in the 'impulsive' and 'equilibrium' limits, and in a set of intermediate 'evolving' states. The impulsive atmospheres have the density distribution frozen in pre-flare configuration, while the equilibrium models assume the entire atmosphere is in hydrostatic and energetic equilibrium. The evolving atmospheres represent intermediate stages where hydrostatic equilibrium has been established in the chromosphere and corona, but the corona is not yet in energetic equilibrium with the flare heating source. Thus, for example, chromospheric evaporation is still in the process of occurring.

Measurements of Electric Fields in Coronal Magnetic Structures

AbstractWe review the use of Stark effect to measure d.c. and wave-related electric fields predicted in models of coronal and flare heating, prominence support, and related phenomena. We describe the Mk II limb electrograph now in routine operation at NSO/Sac Peak, using high Paschen series lines around 8500 Å, and its testing (using the Zeeman effect) as a transverse magnetograph, on penumbral magnetic fields. We summarize our new upper limits on the d.c. fields in quiescent prominences and of the Alfvén wave flux in the corona, and a possible detection of 15 – 20 V cm−1electric fields in a bright surge at the limb. We also outline our ongoing observations at Brackettγ(2.16μ) and our plans to observe in the very sensitive 15-9 transition of H I at 11.539μ.

Manifestation of Multiple Energy Release on the Evolution of Flares in the Diagnostic Diagrams

AbstractIn the present contribution we advance the investigation of evolution of flares in the emission measure-temperature diagrams. Models are considered in which the flare heating consists of a pair of elementary heating episodes (EHE) taking place in separate loops within flaring region. The parameters of the modelling are: the time profile of heating within individual episode, the time elapsed between individual energy releases and the amount of energy released. Earlier, we have shown that some of the X-ray flares observed by theBCSonSMMdisplay evolutionary patterns in the emission measure-temperature diagrams which are hard to interpret in the framework of model with single energy episode. Present analysis offers a possibility to interpret some of these patterns.

Quasi-periodic Particle Injection into Coronal Loops

view Abstract Citations (40) References (55) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Quasi-periodic Particle Injection into Coronal Loops Aschwanden, Markus J. ; Benz, Arnold O. ; Dennis, Brian R. ; Gaizauskas, Victor Abstract We present observations of the flare of 1989 June 22, 1445 UT (in active region NOAA 5555), obtained with the Hard X-Ray Burst Spectrometer (HXRBS) on SMM at energies >52 keV, and with the broad-band radio spectrometer PHOENIX at ETH/Zurich in the frequency range of 100-2800 MHz. The radio emission is dominated by a ≲100% polarized decimetric continuum at 400-1400 MHz, peaking at 750 MHz. The decimetric radio flux is highly correlated with the 50-150 keV hard X-ray flux but is delayed by 3.5-5.4 s with respect to the hard X-rays. The HXR emission shows an excess of ≳10 fast (≳100 ms) spikes (according to Poisson statistics). The radio emission exhibits weak fine structure, consisting of ≍45 quasi-periodic pulses with a mean period of 1.6 s. The frequency-time drift pattern of this fine structure is found to be consistent with segments of inverted-U type bursts, suggesting quasi-periodic injection of electron beams into a loop system. The loop system has an average height of 68,000 km and expands with a velocity of 200 km s-1 due to flare heating. Chromospheric evaporation enhances the electron density near the footpoints. The type III- exciting electrons have a mean velocity of υ/c = 0.30±0.10 (22 keV) and propagate along inverted-U burst trajectories with a mean duration of 2.5 s. For those electrons which reach the mirror point near the opposite footpoint of the loop system, we calculate (from the density and loop length) a low energy cutoff of ≥ 8 keV due to collisional deflection, yielding a propagation velocity of v/c = 0.18 and a propagation delay of 5.1±1.0 s, which agrees well with the observed delay of 5.16 s between the cross-correlated HXR and radio flux. The ≥ 8 keV electrons provide free energy for a loss cone instability near the secondary footpoint, which is observed as decimetric continuum polarized in the same sense of circular polarization as the type III bursts. The constraints from the Hα flare position and the magnetic potential field extrapolation indicate that the loss cone emission is produced in the diverging field region above the umbra of the leading sunspot, which has a photospheric field strength of -1600 G. This flare allows us to deconvolve quasi-periodic particle injection and subsequently triggered coherent radio emission from trapped particles in flare-associated loops. It demonstrates that quasi-periodic modes of particle acceleration, particle dynamics in mirror loops, and the resulting plasma instabilities can be efficiently diagnosed from correlated hard X-ray and radio signatures. Publication: The Astrophysical Journal Pub Date: October 1993 DOI: 10.1086/173283 Bibcode: 1993ApJ...416..857A Keywords: SUN: CORONA; SUN: FLARES; SUN: PARTICLE EMISSION; SUN: RADIO RADIATION; SUN: X-RAYS; GAMMA RAYS full text sources ADS |

Electron acceleration and heating in solar flares: Interpretation of H? signatures

Vector magnetogram, Hα, and hard X-ray observations of flares are reviewed which show that nonthermal electron signatures in Hα are never cospatial with regions of maximum current density for the small number of flares analyzed, but lie to the sides of these regions. By considering electron acceleration and transport requirements, four conditions are found that must be fulfilled to observe nonthermal electron signatures in Hα: (1) The plasma betaβ ≤ 0.3 in the acceleration region. (2) The energy flux of electrons above 20 keV is greater than 1010 erg cm−2 s−1. (3) The column densityN ≤ 1020 cm−2 between the electron source and the chromosphere. (4) The coronal pressure in the flux tube connecting to the Hα layerp ≤ 100 dyne cm−2. Condition 2 can be most easily met in the initial stages of flares. In contrast, the only condition for a high-pressure Hα signature isp ≥ 1000 dyne cm−2, which is most easily met in a region of maximum current density or heating and far enough into the flare for significant heating to have occurred. Thus, high-pressure signatures should be expected to occur more frequently than nonthermal electron signatures and to occur generally later in time.

Inferring chromospheric flare heating from hydrogen-line wings

Strong flare heating that penetrates deeply into the solar chromosphere is examined, and a simple model that is supported by a numerical radiative transfer simulation is applied to study how elevated free electron densities produce markedly increased Stark emission in the wings of hydrogen Lyman and Balmer lines. The proportionality that exists between the observed Stark wing intensity and the total energy deposition in the partially ionized layer is calculated for various Lyman and Balmer lines. It is concluded that when the effects of background opacity sources on the wing enhancements are not important, the magnitude of the nonthermal heating that penetrates the chromosphere may be inferred from the observed line wings. It is predicted that Ly-alpha extended wings will exhibit enhancement of Stark shape, even when inherent damping wing opacity exceeds Stark wing opacity over the entire absorption profile.

Flare energy release: observational consequences and signatures

It is generally accepted, but not yet compellingly demonstrated, that the energy released in solar flares is stored in stressed magnetic fields. Little is known, at present, about how the most obvious manifestations of flare energy release — heating, mass motion, magnetic field reconfiguration and particle acceleration — are related to the spatial distribution of free energy within those fields. To address this issue we have underway at Mees Solar Observatory a programme of simultaneous polarimetric and spectroscopic observations that allow us to explore the spatial relation between active region currents, flare particle acceleration and flare heating. In this paper we discuss several days observations of two flare-productive active regions. By using the Haleakala Stokes polarimeter, we observed the spatial distribution of the Stokes profiles of two photospheric Fe 1 lines, from which we inferred the spatial distribution of the vector m agnetic field and the vertical current density. In flares that were observed on the same days, we then compared the locations of vertical currents to the sites of non-thermal electron precipitation and high coronal pressure inferred from Hα line profiles and spectroheliograms obtained with the Mees charge coupled device imaging spectrograph. Without exception we found that the sites of significant energetic electron precipitation into the chromosphere were at the edges of regions of vertical current, not within them. In contrast, we found that the footpoints of high-pressure flare plasmas during the main phase of the observed flares all coincided very well with such currents.

Energetic particles in solar flares: theory and diagnostics

Recent progress and future prospects in diagnostics of energetic electrons and ions in the flares are reviewed, together with the roles they play in the flare as a whole. Most of the discussion centres on hard X-ray and gamma-ray and thermal plasma emission data, rather than on radio sources. SinceSolar Maximum MissionandHinotorithere has been major progress in all areas of flare electron diagnostics. Electron spectra are now recoverable with some precision, electrons with energies above 10 MeV are known to be highly anisotropic, and indications are available of the spatial distribution of electrons at 20 keV. Timescales of electron acceleration are now known to be shorter than 0.1 s. Energetic electrons are believed to carry much of the flare power. Ion diagnostics are more limited. For greater than 1 MeV ions the flux, spectrum and acceleration timescale are now quite well known. Low energy ions are hard to diagnose but have been invoked as a flare heating mechanism alternative to electron beams. The problems with beam heating models are discussed with special attention to the problems of the low energy proton model and its only direct diagnostic, Hα impact polarization. Finally, theoretical problems associated with return currents and with accelerator requirements are discussed and attention is drawn to the possible importance of entropy as well as energy considerations.

Temperature-dependent saturation of hydrogen lines in solar flares

The standard two-level picture of Lyα thermalization does not formally apply in the solar case because at temperatures typical for Lyα formation (T ≥ 15000 K), the collisional rate from level 2 to level 3 greatly exceeds the collisional rate from level 2 to level 1. For electron densities typical of quiescent solar line formation, this collisional interlocking is not a significant consideration owing to the greater importance of radiative interlocking effects. However, when strong flare heating enhances the free electron density to the order of 10 13 cm −3 , it is found that the important collisional rate from level 2 to level 3 supports an upward series of collisions leading ultimately to the continuum

Flare heating and ionization of the low solar chromosphere. II - Observations of five solar flares

Two neutral Mg spectral lines formed in the temperature-minimum region and the low chromosphere, at 4571 and 5173 A, are used to quantify the changes in the atmospheric structure as a function of time during five solar flares. Eight proposed flare heating and ionization mechanisms and predictions of the effects of each on the temperature minimum region are discussed. Two Mg spectral observations made at the National Solar Observatory (Sacramento Peak), along with observations of hard and soft X-rays from the SMM and GOES satellites, are compared to the predictions of the eight proposed mechanisms. The initial effects in all five flares are consistent with backwarming by enhanced Balmer- and Paschen-continuum radiation originating in the upper chromosphere. Extended heating observed in two of the flares is most likely due to UV irradiation. In all cases heating by the dissipation of nonreversed electric currents, collisions with an electron or proton beam, irradiation by soft X-rays, and dissipation of Alfven waves are eliminated.

A hydrodynamic model of the gradual phase of the solar flare loop

view Abstract Citations (58) References (41) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS A Hydrodynamic Model of the Gradual Phase of the Solar Flare Loop Gan, W. Q. ; Fang, C. Abstract The hydrodynamic cooling process after the maximum phase of a solar flare has been simulated numerically, with improvements on the chromospheric radiative loss and the resolution of the transition region, together with the introduction of the mechanism of chromospheric heating by coronal soft X-rays. At the early stage of the gradual phase, chromospheric evaporation continues to take place and the transition region moves further downward. At the middle stage of the gradual phase, the corona atmosphere appears to have a quasi-periodic oscillation, and the transition region remains at the same position for a long time. At the later stage to of the gradual phase, the descent of coronal material does not happen, and the atmosphere evolves into a state with high density and low temperature. Finally, the atmosphere does not reach the quiet-sun conditions directly, and it would perhaps undergo a process of instability and develop into the stage of postflare loop. The efficiency of flare soft X-ray heating of the chromosphere is also discussed. Publication: The Astrophysical Journal Pub Date: July 1990 DOI: 10.1086/168989 Bibcode: 1990ApJ...358..328G Keywords: Chromosphere; Coronal Loops; Hydrodynamics; Solar Flares; Stellar Models; Atmospheric Heating; Atmospheric Temperature; Cooling; Evaporation; Solar X-Rays; Solar Physics; HYDRODYNAMICS; SUN: CHROMOSPHERE; SUN: CORONA; SUN: FLARES; SUN: TRANSITION REGION full text sources ADS |

An equation for the evolution of solar and stellar flare loops

An ordinary differential equation describing the evolution of a coronal loop subjected to a spatially uniform but time-varying heating rate is discussed. It is assumed that the duration of heating is long compared to the sound transit time through the loop, which is assumed to have uniform cross section area. The form of the equation changes as the loop evolves through three states: 'strong evaporation', 'scaling law behavior', and 'strong condensation'. Solutions to the equation may be used to compute the time dependence of the average coronal temperature and emission measure for an assumed temporal variation of the flare heating rate. The results computed from the model agree reasonably well with recent published numerical simulations and may be obtained with far less computational effort. The model is then used to study the May 21, 1980, solar flare observed by SMM and the giant April 12, 1985, flare observed on the star AD Leo.

Workshop on Impulsive Solar Flares, 2nd, University of New Hampshire, Durham, Sept. 26-28, 1988, Proceedings

view Abstract Citations References Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Workshop on Impulsive Solar Flares, 2nd, University of New Hampshire, Durham, Sept. 26-28, 1988, Proceedings Chupp, E. Abstract Papers on impulsive solar flares are presented, covering topics such as the optical properties of impulsive flares, soft X-ray spectroscopy of solar flares, the energy release process in impulsive bursts, high-velocity evaporation and a high-speed shock wave during the impulsive phase of the April 24, 1984 flare, nonpotential magnetic fields at sites of gamma-ray flares, and meter-decimeter and microwave radio observations of solar flares. Other topics include rise times in solar radio bursts, removal of the gradual component in analyses of solar impulsive bursts, ion and relativistic electron transport in solar flares, neutrons and gamma-ray emission on June 3, 1982, emission characteristics of three intense solar flares in cycle 21, and solar flare gamma-ray observations with the Hinotori satellite. Additional topics include spectra of relativistic solar proton ground-level events recorded in Antarctica, a 153-d periodicity in the occurrence of solar flares producing energetic interplanetary electrons, a search for solar neutron response in neutron monitor data, neutral beams in two-ribbon flares and in the geomagnetic tail, beam heating in solar flares, and solar flare gamma-ray line shapes. Publication: The Astrophysical Journal Supplement Series Pub Date: June 1990 DOI: 10.1086/191437 Bibcode: 1990ApJS...73.....C Keywords: Conferences; Solar Flares; Emission Spectra; Gamma Ray Bursts; Interplanetary Space; Ion Beams; Microwaves; Optical Properties; Relativistic Electron Beams; Shock Waves; Solar Heating; Solar Magnetic Field; Solar Neutrons; Solar Protons; Solar Radio Bursts; Transport Properties; X Ray Spectroscopy; Solar Physics full text sources Publisher |

Beam heating in solar flares - Electrons or protons?

The current status of electron and proton beam models as candidates for the impulsive phase heating of solar flares is discussed in relation to observational constants and theoretical difficulties. It is concluded that, while the electron beam model for flare heating still faces theoretical and observational problems, the problems faced by low and high energy proton beam models are no less serious, and there are facets of proton models which have not yet been studied. At the present, the electron beam model remains the most viable and best developed of heating model candidates. 58 refs.

Flare heating and ionization of the low solar chromosphere. I - Inversion methods for MG I 4571 and 5173

Various methods of inverting solar Mg I 4571 and 5173 spectral line observations are examined to find the best method of using these lines to calculate the vertical temperature and electron density structure around the temperature minimum region. Following a perturbation analysis by Mein (1971), a Fredholm integral equation of the first kind is obtained which can be inverted to yield these temperature and density structures as a function of time. Several inversion methods are tested and compared. The methods are used to test data as well as to a subset of observations of these absorption lines taken on February 3, 1986 before and during a solar flare. A small but significant increase is found in the temperature and a relatively large increase in the electron density during this flare. The observations are inconsistent with heating and ionization by an intense beam of electrons and with ionization by UV photoionization of Si I.