Solar Flare Loops Research Articles

The Temperature of the Bright Knots at the Tops of Solar Flare Loops

The Temperature of the Bright Knots at the Tops of Solar Flare Loops

Relationship between Cold and Hot Post--Solar Flare Loops and the Impact on the Reconnection Flare Model

The relationship between cold and hot postflare loops has been studied using the 1973 September 7 and 1974 January 15 flare images recorded by the Skylab S082 spectroheliograph. The images are of intense emission lines in the extreme ultraviolet region that span the 1 × 10<SUP>4</SUP> (He I) to 3.2 × 10<SUP>6</SUP> K (Ni XVIII) temperature range. The analysis of the images does not support the widely held notion that cold loops are always smaller than hot loops, lie below hot loops, and are similar in shape. It is found that the coldest and hottest loops often differ significantly in size and shape. Based on the analysis of a time sequence of Skylab images of cold and hot loops, there is no evidence that the loop system expands in a discontinuous manner, as would be the case if higher loops were sequentially formed and activated by reconnection of the magnetic field. It is found that the individual coronal loops expand in a gradual manner. These conclusions are consistent with images of postflare loop systems in the 10<SUP>7</SUP> K range that were recorded by the soft X-ray telescope on the Yohkoh spacecraft. Previous observations and interpretations of postflare loop systems, which led to the formulation of the reconnection flare model, are reexamined. In light of recent high-quality imagery that spans the temperature range from 10<SUP>4</SUP> to 10<SUP>7</SUP> K, it is concluded that the observations do not support all aspects of the reconnection flare model as presently articulated. Elements of the reconnection flare model that are inconsistent with the modern observations should be reconsidered.

The role of magnetic loops in solar flares

X -ray and ultraviolet observations of flares have provided much important information on their spatial structure and magnetic topology. The early observations fromSkylabemphasized the role of simple loops and loop arcades, but later observations from theSolar Maximum Missionhave greatly complicated this picture. Flares appear in a multitude of loops with complex spatial and temporal interrelations. In many cases, interactions between different loops appear to play a crucial role. The inferred magnetic topology of solar flares will be reviewed with emphasis on the implications for processes of energy release and transfer. It will be shown that the spatial resolution of the observations obtained so far is still inadequate for solving many basic questions of solar flare research.

A Numerical Simulation for the Cooling Process of the Solar Flare Loop

Extended abstractWe have simulated numerically the hydrodynamic cooling process after the maximum phase of a solar flare 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. The main results are as follows: 1.At the early stage of the gradual phase, thermal conduction maintains chromospheric evaporation, but with the cooling of the atmosphere, chromosphere evaporation decreases gradually.2.In most of the gradual phase, the velocity is smaller than 40km s−1 in the corona and 4km s−1 in the chromosphere.3.From the middle stage of the gradual phase, the coronal atmosphere appears to have a quasi-periodic oscillation. The period is about two minutes, and the amplitude of velocity is within ±20km s−1.4.The transition region continues to move downward at first and then changes very slowly for quite a long time. The upward motion of the transition region takes place only at the latest stage, when the atmosphere cools below the quiet-Sun case.5.In contrast to the changes of temperature, the density of the corona does not seem to vary until the violent descent of coronal material takes place at the end of the gradual phase.6.The coronal part cools mainly by thermal conduction, while the chromospheric part cools by radiative loss. With our initial model, it takes about 25 minutes for cooling from the maximum phase to nearly the quiet-Sun case.7.The soft X-ray heating of the chromosphere seems to be of negligible importance in our calculations, but if the coronal density is greater than 1011 cm−3 at the maximum phase of the flare, the soft X-ray heating may play some role in the gradual phase.8.At the latest stage of the gradual phase, the atmosphere remains dense and at low temperature. As a further consequence, it would evolve into a post-flare loop.

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.

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 |

On the stability of proton beams against resonant scattering by Alfven waves in solar flare loops

The growth of Alfven waves in magnetized hydrogen plasma at flare-loop densities and magnetic field strengths driven by a dilute population of streaming protons in the energy range 10-1000 keV is investigated. The streaming distribution induces wave growth principally through resonant interactions between forward-moving protons and forward-propagating waves. All other factors constant, the maximum growth rate increases with mean beam energy and with increased narrowness of the beam distribution. The frequency of the most rapidly growing waves is inversely related to the mean beam energy. Under typical flare conditions, thermal damping when mean energy equals the Alfven speed effectively stabilizes proton beams of moderate collimation. At beam energies only one order of magnitude larger, thermal damping under flare loop conditions is insufficient to prevent highly restrictive upper limits on the beam-to-background proton density ratio for even mildly collimated proton beams.

A Dynamic Scaling Law for Solar and Stellar Flare Loops

AbstractWe discuss an ordinary differential equation which describes how the pressure in a coronal loop may evolve in time under the influence of a uniform, but time varying heating rate.

Electric fields and plasma structure in coronal magnetic loops

Solar flare loop spectra are analyzed by means of recently developed density diagnostics to determine if the Stark effect observed in the Balmer emission lines is due to pressure broadening or the presence of macroscopic electric fields. The database covers five postflare loops and three active prominences. The diagnostics include the density sensitivity of the intensity ratios of strong Na I D, Mg I b, and the Mg I lines relative to the Sr II lines. A total of 43 spectra were obtained in the 3500-6000 A interval, with most lines falling in the 3600-4500 A region. A prism at the entrance to the instrument permitted extracting additional data on the structure of Stark-broadener Balmer lines. A dc electric field was detected, with an energy level of 100 V/cm, oriented transverse to the loop magnetic field vector. Electron densities of 1-10 trillion/cu cm were estimated for the cool structures within loops and 100 billion/cu cm in active prominences. The densities indicated that a pressure deficit may be present in the loop structures in the corona, suggesting an inward directed gas pressure force in cool loop structures. 24 references

X-ray flaring activity of flare stars and RS CVn binaries

In recent times, X-ray flares have been observed in several flare stars and RS CVn binaries. From an analysis of the X-ray flare observations we find a correlation between the average energy emitted in the X-ray band in flares and the bolometric luminosity of the stars. This relation is similar to the relation obtained in the optical band specifically for the flare stars and hence we suggest a similarity in the flaring nature of the flare stars and the RS CVn binaries. Further, a correlation is also found between the X-ray flare decay time and the peak X-ray luminosity for the flare stars and the RS CVn binaries. We show that this relation can be explained by the solar flare loop models.

Numerical simulations of loops heated to solar flare temperatures. III - Asymmetrical heating

A numerical model is defined for asymmetric full solar flare loop heating and comparisons are made with observational data. The Dynamic Flux Tube Model is used to describe the heating process in terms of one-dimensional, two fluid conservation equations of mass, energy and momentum. An adaptive grid allows for the downward movement of the transition region caused by an advancing conduction front. A loop 20,000 km long is considered, along with a flare heating system and the hydrodynamic evolution of the loop. The model was applied to generating line profiles and spatial X-ray and UV line distributions, which were compared with SMM, P78-1 and Hintori data for Fe, Ca and Mg spectra. Little agreement was obtained, and it is suggested that flares be treated as multi-loop phenomena. Finally, it is concluded that chromospheric evaporation is not an effective mechanism for generating the soft X-ray bursts associated with flares.

Chromospheric evaporation velocities in solar flares

view Abstract Citations (49) References (11) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Chromospheric evaporation velocities in solar flares Fisher, G. H. ; Canfield, R. C. ; McClymont, A. N. Abstract It is shown that the upper limit to chromospheric evaporation velocities in solar flare loops is 2.35 Cs, where Cs is the sound speed in the evaporated material. Upward velocities greater than this limit would imply the existence of additional nonhydrodynamic forces. Several hydrodynamic calculations are made of collisional heat produced by nonthermal energetic electrons, and upward velocities are found which are at the upper limit of large (less than 3 x 10 to the 10th ergs/sec/sq cm) energy flux values. At more modest fluxes (10 to the 10th or 10 to the 9th ergs/sec/sq cm) both thermal and thick target models yield upward velocities which are within 10 to 20 percent of the upper limit. When both temperature and velocity of the evaporated plasma are considered, the thermal model is found to be the most consistent with recent observational data. Publication: The Astrophysical Journal Pub Date: June 1984 DOI: 10.1086/184290 Bibcode: 1984ApJ...281L..79F Keywords: Chromosphere; Evaporation Rate; Solar Flares; Solar X-Rays; Constraints; Hydrodynamics; Solar Physics full text sources ADS |

Thermal and nonthermal phenomena in solar flare loops at 20 cm wavelength and in X-rays

We present X-ray images from the P78-1 satellite for a long-lasting burst at 20 cm wavelength mapped with the Very Large Array on 19 May, 1979 by Velusamy and Kundu (1981). The decimeter wave observations were originally interpreted in terms of two models, one invoking thermal electrons radiating at low harmonics of the gyrofrequency, and the other invoking mildly relativistic electrons emitting gyrosynchrotron radiation. If indeed the 20 cm source is thermal, it should also be visible in soft X-rays, while if it is nonthermal, the soft X-ray emission should be weak or spatially or temporally distinct from the 20 cm burst. We find that only one of the three 20 cm sources was approximately co-spatial with the soft X-ray source, and that it was only partially thermal. The 20 cm burst is therefore primarily decimeter type IV emission from mildly relativistic electrons of the post-flare phase. The long lifetime (≳ 2h) and smooth temporal variation of the burst belie its nonthermal nature and suggest continuous acceleration as well as long term storage of energetic electrons.

A model of hot loops associated with solar flares

A dynamical model is proposed for the formation of soft X-ray emitting hot loops in solar flares. It is examined by numerical simulations how a solar model atmosphere in a magnetic loop changes its state and forms a hot loop when the flare energy is released in the form of heat liberation either at the top part or around the transition region in the loop.

Radiation signatures from a locally energized flaring loop

The radiation signatures from a locally energized solar flare loop based on the physical properties of the energy release mechanisms were consistent with hard X-ray, microwave, and EUV observations for plausible source parameters. It was found that a suprathermal tail of high energy electrons is produced by the primary energy release, and that the number of energetic charged particles ejected into the interplanetary medium in the model is consistent with observations. The radiation signature model predicts that the intrinsic polarization of the hard X-ray burst should increase over the photon energy range of 20 to 100 keV.