Hot Flare Plasma Research Articles

The collisional relaxation of electrons in hot flaring plasma and inferring the properties of solar flare accelerated electrons from X-ray observations.

X-ray observations are a direct diagnostic of fast electrons produced in solar flares, energized during the energy release process and directed towards the Sun. Since the properties of accelerated electrons can be substantially changed during their transport and interaction with the background plasma, a model must ultimately be applied to X-ray observations in order to understand the mechanism responsible for their acceleration. A cold thick target model is ubiquitously used for this task, since it provides a simple analytic relationship between the accelerated electron spectrum and the emitting electron spectrum in the X-ray source, with the latter quantity readily obtained from X-ray observations. However, such a model is inappropriate for the majority of solar flares in which the electrons propagate in a hot megaKelvin plasma, because it does not take into account the physics of thermalization of fast electrons. The use of a more realistic model, properly accounting for the properties of the background plasma, and the collisional diffusion and thermalization of electrons, can alleviate or even remove many of the traditional problems associated with the cold thick target model and the deduction of the accelerated electron spectrum from X-ray spectroscopy, such as the number problem and the need to impose an ad hoc low energy cut-off.

Multitemperature analysis of solar flare observed on 2003 March 29

AbstractWe present results of multitemperature analysis of GOES C7.2 class flare SOL2003-03-29T10:15. This event occurred close to the centre of the solar disk and had two maxima in soft X-rays. We have performed analysis of physical parameters characterizing evolution of conditions in the flaring plasma. The temperature diagnostics have been carried out using the differential emission measure (DEM) approach based on the soft X-ray spectra collected by RESIK Bragg spectrometer. Analysis of data obtained by RHESSI provided opportunity to estimate the volume and thus calculating the density and thermal energy content of hot flaring plasma.

LARGE-SCALE CONTRACTION AND SUBSEQUENT DISRUPTION OF CORONAL LOOPS DURING VARIOUS PHASES OF THE M6.2 FLARE ASSOCIATED WITH THE CONFINED FLUX ROPE ERUPTION

We present a detailed multi-wavelength study of the M6.2 flare which was associated with a confined eruption of a prominence using TRACE, RHESSI, and NoRH observations. The pre-flare phase of this event is characterized by spectacular large-scale contraction of overlying extreme ultraviolet (EUV) coronal loops during which the loop system was subjected to an altitude decrease of ~20 Mm for an extended span of ~30 min. This contraction phase is accompanied by sequential EUV brightenings associated with hard X-ray (HXR) (up to 25 keV) and microwave (MW) sources from low-lying loops in the core of the flaring region which together with X-ray spectra indicate strong localized heating in the source region before the filament activation and associated M-class flare. With the onset of the impulsive phase of the M6.2 flare, we detect HXR and MW sources that exhibit intricate temporal and spatial evolution in relation with the fast rise of the prominence. Following the flare maximum, the filament eruption slowed down and subsequently confined within the large overlying active region loops; the event did not lead to a coronal mass ejection (CME). During the confinement process of the erupting prominence, we detect MW emission from the extended coronal region with multiple emission centroids which likely represent emission from hot blobs of plasma formed after the collapse of the expanding flux rope and entailing prominence material. RHESSI observations reveal high plasma temperature (~30 MK) and substantial non-thermal characteristics with electron spectral index (~5) during the impulsive phase of the flare. The time-evolution of thermal energy exhibits a good correspondence with the variations in cumulative non-thermal energy which suggest that the energy of accelerated particles efficiently converted to hot flare plasma implying an effective validation of the Neupert effect.

Soft X-ray emission in kink-unstable coronal loops

Context. Solar flares are associated with intense soft X-ray emission generated by the hot flaring plasma in coronal magnetic loops. Kink-unstable twisted flux-ropes provide a source of magnetic energy that can be released impulsively and may account for the heating of the plasma in flares.

The solar photospheric-to-coronal Fe abundance ratio from X-ray fluorescence lines

The ratio of the Fe abundance in the photosphere to that in coronal flare plasmas is determined by X-ray lines within the complex at 6.7~keV (1.9~\AA) emitted during flares. The line complex includes the He-like Fe (\fexxv) resonance line $w$ (6.70~keV) and Fe K$\alpha$ lines (6.39, 6.40~keV), the latter being primarily formed by the fluorescence of photospheric material by X-rays from the hot flare plasma. The ratio of the Fe K$\alpha$ lines to the \fexxv\ $w$ depends on the ratio of the photospheric-to-flare Fe abundance, heliocentric angle $\theta$ of the flare, and the temperature $T_e$ of the flaring plasma. Using high-resolution spectra from X-ray spectrometers on the {\em P78-1} and {\em Solar Maximum Mission} spacecraft, the Fe abundance in flares is estimated to be $1.6\pm 0.5$ and $2.0 \pm 0.3$ times the photospheric Fe abundance, the {\em P78-1} value being preferred as it is more directly determined. This enhancement is consistent with results from X-ray spectra from the {\em RHESSI} spacecraft, but is significantly less than a factor 4 as in previous work.

EARLY CHROMOSPHERIC RESPONSE DURING A SOLAR MICROFLARE OBSERVED WITHSOHO's CDS ANDRHESSI

We observed a solar microflare with RHESSI and SOHO's Coronal Diagnostic Spectrometer (CDS) on 2009 July 5. With CDS we obtained rapid cadence (7 s) stare spectra within a narrow field of view toward the center of AR 11024. The spectra contain emission lines from ions that cover a wide range of temperature, including He I (< 0.025 MK), O V (0.25 MK), Si XII (2 MK), and Fe XIX (8 MK). The start of a precursor burst of He I and O V line emission preceded the steady increase of Fe XIX line emission by about 1 minute, and the emergence of 3-12 keV X-ray emission by about 4 minutes. Thus the onset of the microflare was observed in upper chromospheric (He I) and transition region (O V) line emission before it was detected in high temperature flare plasma emission. Redshifted O V emission during the precursor suggests explosive chromospheric evaporation, but no corresponding blueshifts were found with either Fe XIX (which was very weak) or Si XII. Similarly, in subsequent microflare brightenings the O V and He I intensities increased (between 49 s and almost 2 minutes) before emissions from the hot flare plasma. Although these time differences likely indicate heating by a nonthermal particle beam, the RHESSI spectra provide no additional evidence for such a beam. In intervals lasting up to about 3 minutes during several bursts, the He I and O V emission line profiles showed secondary, highly blueshifted ( approximately 200 km/s) components; during intervals lasting nearly 1 minute the velocities of the primary and secondary components were oppositely directed. Combined with no corresponding blueshifts in either Fe XIX or Si XII, this indicates that explosive chromospheric evaporation occurred predominantly at either comparatively cool temperatures (< 2 MK) or within a hot temperature range to which our observations were not sensitive (e.g., between 2 and 8 MK).

Bright X‐Ray Flares in Orion Young Stars from COUP: Evidence for Star‐Disk Magnetic Fields?

We have analyzed a number of intense X-ray flares observed in the Chandra Orion Ultradeep Project (COUP), a 13 day observation of the Orion Nebula Cluster (ONC), concentrating on the events with the highest statistics (in terms of photon flux and event duration). Analysis of the flare decay allows to determine the physical parameters of the flaring structure, particularly its size and (using the peak temperature and emission measure of the event) the peak density, pressure, and minimum confining magnetic field. A total of 32 events, representing the most powerful 1% of COUP flares, have sufficient statistics and are sufficiently well resolved to grant a detailed analysis. A broad range of decay times are present in the sample of flares, with τlc (the 1/e decay time) ranging from 10 to 400 ks. Peak flare temperatures are often very high, with half of the flares in the sample showing temperatures in excess of 100 MK. Significant sustained heating is present in the majority of the flares. The magnetic structures that are found, from the analysis of the flare's decay, to confine the plasma are in a number of cases very long, with semilengths up to 1012 cm, implying the presence of magnetic fields of hundreds of G (necessary to confine the hot flaring plasma) extending to comparable distance from the stellar photosphere. These very large sizes for the flaring structures (length L R*) are not found in more evolved stars, where, almost invariably, the same type of analysis results in structures with L ≤ R*. As the majority of young stars in the ONC are surrounded by disks, we speculate that the large magnetic structures that confine the flaring plasma are actually the same type of structures that channel the plasma in the magnetospheric accretion paradigm, connecting the star's photosphere with the accretion disk.

Thermal and non-thermal energies of solar flares

The energy of the thermal flare plasma and the kinetic energy of the non-thermal electrons in 14 hard X-ray peaks from 9 medium-sized solar flares have been determined from RHESSI observations. The emissions have been carefully separated in the spectrum. The turnover or cutoff in the low-energy distribution of electrons has been studied by simulation and fitting, yielding a reliable lower limit to the non-thermal energy. It remains the largest contribution to the error budget. Other effects, such as albedo, non-uniform target ionization, hot target, and cross-sections on the spectrum have been studied. The errors of the thermal energy are about equally as large. They are due to the estimate of the flare volume, the assumption of the filling factor, and energy losses. Within a flare, the non-thermal/thermal ratio increases with accumulation time, as expected from loss of thermal energy due to radiative cooling or heat conduction. Our analysis suggests that the thermal and non-thermal energies are of the same magnitude. This surprising result may be interpreted by an efficient conversion of non-thermal energy to hot flare plasma.

Energy release in solar flares

One of the basic features of solar flares is the very fast heating of large plasma volumes. A significant fraction of the hot flare plasma is contained in X-ray kernels which are situated near the tops of flaring loops (loop-top flare kernels). In the present paper it is argued that in the flare kernels a chain (cascade) of violent reconnections develops, as a result of which the kernels are filled with many transient reconnection sites, so that the flare energy release occurs over a large plasma volume. Formation of the kernels is briefly discussed. It is suggested how such turbulent model of the flare energy release can also explain basic features of the flare impulsive phase, i.e. acceleration of a large number of electrons in very short time.

High Spatial Resolution Observations of a Hot Region in a Solar Flare with the [ITAL]Transition Region and Coronal Explorer[/ITAL

The Transition Region and Coronal Explorer (TRACE) provides some of the highest spatial resolution images ever taken of hot solar flare plasma. The TRACE 195 A channel is particularly sensitive to high-temperature flare plasma because of the presence of the Fe XXIV λ192 resonance line in this bandpass. The TRACE 171 A channel observes emission from thermal bremsstrahlung during a flare. Since this emission is generally weak, it is usually not possible to derive electron temperatures for flare plasma from TRACE observations. In this Letter, we present analysis of the 2000 March 24 X1.8 limb flare that produced high count rates in both the TRACE 195 and 171 A channels. We find evidence for a small, high-temperature region near the top of the flare arcade. This hot region appears to lie at the base of the cusp-shaped structure that extends above the arcade. The TRACE observations are consistent with a strong enrichment of Fe over its photospheric value in the hot region that suggests in situ heating of this plasma. We also find that multithermal simulations of flare evolution reproduce the observations much better than an isothermal model does.

Sawtooth oscillations in solar flare radio emission

For the first time, we found a spectral fine structure of solar meter wave radio burst emission which can be due to sawtooth oscillations in the hot flare plasma. This finding newly underlines an analogy between coronal and laboratory plasma processes. The sawteeth occur during the impulsive flare phase hard X-ray emission and consist of a sequence of almost identical narrow band ($\delta f/f \simeq$ 1% ) drift bursts. All cases of our sample were associated with a radio emitting coronal shock wave (type II burst). Similar oscillations are familiar in tokamak plasmas and understood as signature of the kink instability of the toroidal current. We argue that the radio sawteeth are nonthermal plasma emission due to 2-4% density fluctuations of the flare plasma. The fluctuations can be excited by a current instability in a coronal flare loop or in a vertical flaring current sheet e.g. occuring behind a rising magnetic flux rope. This is in analogy to kink instability effects observed in laboratory plasmas.

Iron K beta line emission in solar flares observed by YOHKOH and the solar abundance of iron

view Abstract Citations (29) References (41) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Iron K beta Line Emission in Solar Flares Observed by YOHKOH and the Solar Abundance of Iron Phillips, K. J. H. ; Pike, C. D. ; Lang, J. ; Watanbe, T. ; Takahashi, M. Abstract A weak line at 1.757 A in solar flare X-ray spectra from the Bragg Crystal Spectrometer (BCS) on Yohkoh is identified as the K beta or inner-shell 1s-3p transition in photospheric iron. The observed line intensity and light curve during several flares suggest that, as with the iron K alpha lines at 1.936 and 1.940 A, excitation occurs principally by fluorescence, with X-ray emission from the hot flare plasma in the low corona exciting photospheric iron atoms. The intensity ratio of the K beta line to the Fe XXV resonance line, which is near in wavelength (1.851 A) and simultaneously observed by the BCS, depends on the photospheric and coronal Fe/H abundance ratio as well as on heliocentric distance and height of the X-ray source. These observations indicate that the coronal Fe/H abundance ratio is not more than a factor of 2 higher than the photospheric. Significant variations from flare to flare cannot be confirmed. Publication: The Astrophysical Journal Pub Date: November 1994 DOI: 10.1086/174870 Bibcode: 1994ApJ...435..888P Keywords: Abundance; Iron; K Lines; Solar Flares; X Ray Spectra; Photosphere; Scientific Satellites; Solar Corona; Statistical Distributions; Solar Physics; SUN: ABUNDANCES; SUN: FLARES; SUN: X-RAYS; GAMMA RAYS full text sources ADS |

Intercomparison of flare observations with two SMM spectrometers: BCS and HXIS

The temperature diagnostics of hot flare plasma, obtained from two Solar Maximum Mission instruments (HXIS and BCS), is compared. A good general agreement between the HXIS and BCS-Fe temperature scales has been found. However, for the growth phase of some flares a systematic difference, T HXIS>T Fe, has been found, which is not likely to be due to the typical non-thermal electron beams. Possible explanation of this effect is briefly discussed.

The dependence of solar hard X-ray burst intensity on magnetic field strength

The measurements of peak intensities of hard X-ray bursts from hot flare plasma electrons as a function of peak frequency of associated microwave radio bursts are discussed. The latter is proportional to the magnetic field strength. The results suggest that the flare hard X-rays are emitted when the electron plasma frequency is comparable to the electron gyrofrequency. Thus, the hard X-ray peak intensity varies as B 4.2, where B is the magnetic field strength.

Investigations of turbulent motions and particle acceleration in solar flares

Investigations of X-ray spectra of solar flares show that intense random (turbulent) motions are present in hot flare plasma. Here we argue that the turbulent motions are of great importance for flare development. They can efficiently enhance flare energy release and accelerate particles to high energies.

Differential emission measure analysis of hot-flare plasma from solar-maximum mission X-ray data

We have investigated differential emission measure (DEM) distribution of hot flare plasma ( T>10 MK) using SMM X-ray data from Bent Crystal Spectrometer (BCS) and Hard X-ray Imaging Spectrometer (HXIS). We have found that the analysis provide a very sensitive test of consistency of observational data coming from different instruments or different channels of the same instrument. This has allowed to eliminate some systematic differences contained in the analysed data. Typical examples of the DEM distribution are discussed. It is stressed that these improvements in the multitemperature flare diagnostics are very important for the discussion of flare energetics.

Size of the X-ray kernel of the large 1979 August 20 solar flare

view Abstract Citations (6) References (9) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Size of the X-ray kernel of the large 1979 August 20 solar flare Landecker, P. B. ; McKenzie, D. L. Abstract At the time of the intense class X5 solar flare on August 20, 1979, monochromatic small raster maps near 7 A were recorded of the flaring active region with 20 arc sec resolution (FWHM) by the SOLEX Solar X-ray Spectrometer/Spectroheliograph. From these maps, the spatial extent (FWHM) of the X-ray kernel during the rise of this flare at about 0912 UT was determined to be less than 28 arc sec. The electron density in the hot flare plasma was estimated to be above 10 to the 11th per cu cm. This is the first soft X-ray measurement of the angular size of such an intense flare. Publication: The Astrophysical Journal Pub Date: November 1980 DOI: 10.1086/183385 Bibcode: 1980ApJ...241L.175L Keywords: Solar Flares; Solar X-Rays; X Ray Spectra; Cosmic Plasma; Electron Flux Density; Solar Corona; Spectroheliographs; Solar Physics full text sources ADS |

Thermal X-rays from stellar flares - Reevaluation of scaling from solar flares

view Abstract Citations (39) References (20) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Thermal X-rays from stellar flares: reevaluation of scaling from solar flares. Mullan, D. J. Abstract A simple method of estimating the ratio of thermal X-ray luminosity to optical luminosity in stellar flares is presented which is valid at times close to and following peak flare luminosity. It is proposed that at such times, the X-ray and optical luminosities are, respectively, the energy radiated by the plasma and the energy conducted out of the plasma. If the conductive energy directed downward toward the photosphere and lower chromosphere is regarded as the energy source for optical flares, then the luminosity ratio is equal to the ratio of conductive to radiative cooling times in the hot flare plasma. A self-consistent model for red-dwarf flares is discussed in which the decay time of the optical light curve is controlled by the time scale for thermal conduction from the hot flare plasma. The condition that the two times must be approximately equal leads to estimates of electron densities in flares, provided the flare temperatures are known; it is argued that stellar flare temperatures are 1 to 4 times higher than the mean temperature of solar flares. Numerical results are given for the luminosity ratio in flares of seven UV Ceti stars. It is concluded that if the solar luminosity ratio is used in attempting to predict thermal X-ray fluxes from stellar flares, the predicted X-ray flux accompanying an optical flare of given amplitude will provide only an upper limit on the actual thermal X-ray flux. Publication: The Astrophysical Journal Pub Date: July 1976 DOI: 10.1086/154492 Bibcode: 1976ApJ...207..289M Keywords: Energy Dissipation; High Temperature Plasmas; Light Curve; Solar Flares; Solar X-Rays; Stellar Flares; Stellar Luminosity; Convective Heat Transfer; Dwarf Stars; Numerical Analysis; Radiant Cooling; Stellar Models; Stellar Radiation; Stellar Temperature; X Ray Astronomy; Space Radiation full text sources ADS |

Mass motions in a heated flare filament

Energy transport in a hot flare plasma is examined with particular reference to the influence of fluid motion. On the basis of dimensional considerations the dynamical timescale of the flare plasma is shown to be comparable to the timescale for energy loss by conduction and radiation. It is argued that mass motion is likely to have a profound influence on the evolution of the flare.

Spatial distribution of XUV emission in solar flares

The spatial distribution and time evolution of flare plasmas are studied with observations obtained with the NRL XUV spectroheliograph aboard Skylab. The hot 10--20 million degree plasma (Fe xxiii--xxiv) is located at the position of a gap in emission of the cooler plasmas (He ii to Fe xiv-xvi) which often take the form of two ribbons. As the flare cools, the Fe xxiii-xxiv plasma disappears and the region between the two ribbons is filled gradually with emissions from lower stage ions. Comparison between spectroheliograms and magetograms shows that the basic magnetic field configuration associated with the flares observed is a low- lying (4000 to 13,000 km) loop or arch structure. The flare apparently results from a plasma instability in the loop which produces localized heating in a small volume, usually near the top of the loop. Geometrical and physical properties of the hot flare plasma are presented, and implications of the observational results are briefly discussed. (AIP)