Chromospheric downflow velocities as a diagnostic in solar flares

We explore the dynamics of chromospheric condensations driven by evaporation during the impulsive phase of solar flares. Specifically, we find that the maximum chromospheric downflow speed obeys the approximate relation _d = 0.4 (F/ _ch )^1/3, where F is that part of the flare energy flux driving chromospheric evaporation, and _ch is the mass density in the preflare chromosphere just below the preflare transition region. This implies that chromospheric downflows as measured by H asymmetries may be a powerful probe of flare energetics.     

X-ray observations of filament eruption in the 1980 May 21 flare

X-ray and H observations of an erupting filament, discussed herein, and other observations of the associated flare on 1980 May 21, suggest that an erupting filament played a major role in the X-ray flare. While Antonucci et al. (1985) analyzed the May 21 flare as one of the best cases of chromospheric evaporation, the possible contribution from X-ray emitting erupting plasma has been ignored. We show that pre-heated plasma existed and may have contributed part of the blue-shifted X-ray emission observed in the Caxix line, which was formerly attributed solely to chromospheric evaporation. Thus it remains an open question - in two-ribbon flares in particular - just how important chromospheric evaporation is in flare dynamics.     

The Evaporation Regime in a Confined Flare

We studied the evolution of a small eruptive flare (GOES class C1) from its onset phase using multi-wavelength observations that sample the flare atmosphere from the chromosphere to the corona. The main instruments involved were the Coronal Diagnostic Spectrometer (CDS) aboard SOHO and facilities at the Dunn Solar Tower of the National Solar Observatory/Sacramento Peak. Transition Region and Coronal Explorer (TRACE) together with Ramaty High-Energy Spectroscopic Imager (RHESSI) also provided images and spectra for this flare. Hα and TRACE images display two loop systems that outline the pre-reconnection and post-reconnection magnetic field lines and their topological changes revealing that we are dealing with an eruptive confined flare. RHESSI data do not record any detectable emission at energies ≥25 keV, and the observed count spectrum can be well fitted with a thermal plus a non-thermal model of the photon spectrum. A non-thermal electron flux F ≈ 5 × 10¹⁰ erg cm⁻² s⁻¹ is determined. The reconstructed images show a very compact source whose peak emission moves along the photospheric magnetic inversion line during the flare. This is probably related to the motion of the reconnection site, hinting at an arcade of small loops that brightens successively. The analysis of the chromospheric spectra (Ca II K, He I D₃ and Hγ, acquired with a four-second temporal cadence) shows the presence of a downward velocity (between 10 and 20 km s⁻¹) in a small region intersected by the spectrograph slit. The region is included in an area that, at the time of the maximum X-ray emission, shows upward motions at transition region (TR) and coronal levels. For the He I 58.4 and O v 62.97 lines, we determine a velocity of ≈−40 km s⁻¹ while for the Fe XIX 59.22 line a velocity of ≈−80 km s⁻¹ is determined with a two-component fitting. The observations are discussed in the framework of available hydrodynamic simulations and they are consistent with the scenario outlined by Fisher (1989). No explosive evaporation is expected for a non-thermal electron beam of the observed characteristics, and no gentle evaporation is allowed without upward chromospheric motion. It is suggested that the energy of non-thermal electrons can be dissipated to heat the high-density plasma, where possibly the reconnection occurs. The consequent conductive flux drives the evaporation process in a regime that we can call sub-explosive.     

Dynamical processes in the June 26, 1999 flare

The evolutionary and spatial characteristics of the motions in the flaring chromosphere of a 2B/M2.3 flare are investigated by analyzing the asymmetry in the H_α profiles. The possibility of reconciling the results of observations with the theory of chromospheric evaporation is considered. The spectroscopic H_α observations of the flare performed with the KG-2 CrAO coronagraph with a temporal resolution of 5–10 s and a spatial resolution as high as 1 arcsec cover all stages of flare development. The following results have been obtained: (1) The H_α profile asymmetry is a general characteristic of the flare emission irrespective of its intensity and its belonging to different structural features and phases of flare development. (2) Most of the H_α emission profiles in flare regions exhibit a red asymmetry. However, a blue asymmetry was observed in small local regions at all stages of flare development. (3) A red asymmetry that appeared before the onset of the impulsive phase and persisted after its end was observed at the sites of main energy release, i.e., the energy source responsible for the dynamical processes in the flare came into operation earlier and existed longer than the HXR emission. (4) The asymmetry pattern changed with flare phase: the red wing intensity dominated in the pre-impulsive phase and at the onset of the impulsive and gradual phases (while the line core was unshifted or slightly shifted). At the maximum of the impulsive phase, the nearly symmetric profiles with extended wings were redshifted as a whole, i.e., the entire emitting volume moved down with a velocity of several tens of km/s. This type of asymmetry cannot be explained by the dynamical model of chromospheric condensation (Canfield and Gayley 1987). (5) The H_α profiles show no evidence of chromospheric heating by a beam of nonthermal electrons during the impulsive phase (Canfield et al. 1984). (6) The lifetime of the downflows and the change in their velocities with time are inconsistent with the dynamical model of chromospheric condensation (Fisher 1989). (7) The morphological features of the velocity field are also inconsistent with the theory of chromospheric evaporation, because the highest differently directed velocities were detected at the flare loop tops, not at the sites of main energy release. We conclude that the investigated flare shows spectral features that are inconsistent with the standard chromospheric evaporation model.     

Spatial relations between preflares and flares

We have conducted an initial search for discrete preflare brightenings as observed in soft X-radiation by Yohkoh. The Yohkoh images allow us to identify, to within a few arc seconds, the location of a preflare event relative to the succeeding flare. Our initial motivation in this study was to search for early coronal brightenings leading to flare effects, as had been suggested by earlier studies; thus we concentrated on Yohkoh limb events. We find no evidence for such early coronal brightenings. Between 15% and 41% of the 131 suitable events matched our criteria for preflare brightening: the same active region; brightening within one hour of the flare peak; preflare brightness less than 30% of the flare peak. In the great majority of the preflare cases, we found that physically separate nearby structures brightened initially. Often these structures appeared to share a common footpoint location with the flare brightening itself. In a few cases the preflare could have occurred in exactly the same structure as the flare.     

Flare stars: Postflare behaviour of emission lines

The problem of the postflare behaviour of chromospheric emission lines in UV Cet-type flare stars is considered. It is shown that the postflare behaviours of different emission lines differ essentially from one another. In particular, the intensity of hydrogen and helium lines must be greatest at flare peak in continuum, while the intensities of 2800 Mg_II and H and K Ca_II lines reach their maximum magnitudes in the intermediate period between two flares. Theoretical postflare light curves are derived for the most important chromospheric lines: helium, hydrogen ionized magnesium and calcium (Figures 8, 9, 11, and 16). The definite regularities in sequence of these light curves are established (Figure 17). Methods of obtaining the mass concentration,n _*, in the chromospheres of flare stars based on the analysis of hydrogen and calcium light curves are elaborated. Values ofn _* obtained for a group of UV Cet-type flare stars are listed in Table VII. The conditions of the intensification of emission lines during the flare are examined, as is the possibility of the existence of an empirical relationship between chromospheric concentration,n _* and flare frequency,f _U .     

Analysis of X-ray observations of the 15 June 1973 flare in active region NOAA 131

Observations and analyses of the 1B/M3 flare of 15 June, 1973 in active region NOAA 131 (McMath 12379) are presented. The X-ray observations, consisting of broadband photographs and proportional counter data from the Skylab/ATM NASA-MSFC/Aerospace S-056 experiment, are used to infer temperatures, emission measures, and densities for the flaring plasma. The peak temperature from the spatially resolved photographs is 25 × 10⁶ K, while the temperature from the full-disk proportional counter data is 15 × 10⁶ K. The density is 3 × 10¹⁰cm^–3. The X-ray flare emission appears to come primarily from two low-lying curvilinear features lying perpendicular to and centered on the line where the photospheric longitudinal magnetic field is zero. Similarities in the preflare and postflare X-ray emission patterns indicate that no large-scale relaxation of the coronal magnetic configuration was observed. Also discussed are H and magnetic field observations of the flare and the active region. Finally, results of numerical calculations, including thermal conduction, radiative loss and chromospheric evaporation, are in qualitative agreement with the decay phase observations.Presently at NASA/Marshall Space Flight Center.     

Chromospheric Height and Density Measurements in a Solar Flare Observed with RHESSI – II. Data Analysis

We present an analysis of hard X-ray imaging observations from one of the first solar flares observed with the Reuven Ramaty High-Energy Solar Spectroscopic Imager (RHESSI) spacecraft, launched on 5 February 2002. The data were obtained from the 22 February 2002, 11:06 UT flare, which occurred close to the northwest limb. Thanks to the high energy resolution of the germanium-cooled hard X-ray detectors on RHESSI we can measure the flare source positions with a high accuracy as a function of energy. Using a forward-fitting algorithm for image reconstruction, we find a systematic decrease in the altitudes of the source centroids z(ε) as a function of increasing hard X-ray energy ε, as expected in the thick-target bremsstrahlung model of Brown. The altitude of hard X-ray emission as a function of photon energy ε can be characterized by a power-law function in the ε=15–50 keV energy range, viz., z(ε)≈2.3(ε/20 keV)^−1.3 Mm. Based on a purely collisional 1-D thick-target model, this height dependence can be inverted into a chromospheric density model n(z), as derived in Paper I, which follows the power-law function n _e(z)=1.25×10¹³(z/1 Mm)^−2.5 cm⁻³. This density is comparable with models based on optical/UV spectrometry in the chromospheric height range of h≲1000 km, suggesting that the collisional thick-target model is a reasonable first approximation to hard X-ray footpoint sources. At h≈1000–2500 km, the hard X-ray based density model, however, is more consistent with the `spicular extended-chromosphere model' inferred from radio sub-mm observations, than with standard models based on hydrostatic equilibrium. At coronal heights, h≈2.5–12.4 Mm, the average flare loop density inferred from RHESSI is comparable with values from hydrodynamic simulations of flare chromospheric evaporation, soft X-ray, and radio-based measurements, but below the upper limits set by filling-factor insensitive iron line pairs.     

Do EUV Nanoflares Account for Coronal Heating?

Recent observations with EUV imaging instruments such as SOHO/EIT and TRACE have shown evidence for flare-like processes at the bottom end of the energy scale, in the range of E _th≈10²⁴–10²⁷ erg. Here we compare these EUV nanoflares with soft X-ray microflares and hard X-ray flares across the entire energy range. From the observations we establish empirical scaling laws for the flare loop length, L(T)∼T, the electron density, n _e(T)∼T ², from which we derive scaling laws for the loop pressure, p(T)∼T ³, and the thermal energy, E _th∼T ⁶. Extrapolating these scaling laws into the picoflare regime we find that the pressure conditions in the chromosphere constrain a height level for flare loop footpoints, which scales with h _eq(T)∼T ^−0.5. Based on this chromospheric pressure limit we predict a lower cutoff of flare loop sizes at L _∖min≲5 Mm and flare energies E _∖min≲10²⁴ erg. We show evidence for such a rollover in the flare energy size distribution from recent TRACE EUV data. Based on this energy cutoff imposed by the chromospheric boundary condition we find that the energy content of the heated plasma observed in EUV, SXR, and HXR flares is insufficient (by 2–3 orders of magnitude) to account for coronal heating.     

Initial phase of chromospheric evaporation in a solar flare

In this paper we discuss the initial phase of chromospheric evaporation during a solar flare observed with instruments on the Solar Maximum Mission on May 21, 1980 at 20:53 UT. Images of the flaring region taken with the Hard X-Ray Imaging Spectrometer in the energy bands from 3.5 to 8 keV and from 16 to 30 keV show that early in the event both the soft and hard X-ray emissions are localized near the footpoints, while they are weaker from the rest of the flaring loop system. This implies that there is no evidence for heating taking place at the top of the loops, but energy is deposited mainly at their base. The spectral analysis of the soft X-ray emission detected with the Bent Crystal Spectrometer evidences an initial phase of the flare, before the impulsive increase in hard X-ray emission, during which most of the thermal plasma at 10⁷ K was moving toward the observer with a mean velocity of about 80 km s^-1. At this time the plasma was highly turbulent. In a second phase, in coincidence with the impulsive rise in hard X-ray emission during the major burst, high-velocity (370 km s^-1) upward motions were observed. At this time, soft X-rays were still predominantly emitted near the loop footpoints. The energy deposition in the chromosphere by electrons accelerated in the flare region to energies above 25 keV, at the onset of the high-velocity upflows, was of the order of 4 × 10¹⁰ erg s^-1 cm^-2. These observations provide further support for interpreting the plasma upflows as the mechanism responsible for the formation of the soft X-ray flare, identified with chromospheric evaporation. Early in the flare soft X-rays are mainly from evaporating material close to the footpoints, while the magnetically confined coronal region is at lower density. The site where upflows originate is identified with the base of the loop system. Moreover, we can conclude that evaporation occurred in two regimes: an initial slow evaporation, observed as a motion of most of the thermal plasma, followed by a high-speed evaporation lasting as long as the soft X-ray emission of the flare was increasing, that is as long as plasma accumulation was observed in corona.     

Multiwavelength analysis of a well observed flare from SMM

We describe and analyse observations of an M1.4 flare which began at 17: 00 UT on 12 November, 1980. Ground based H and magnetogram data have been combined with EUV, soft and hard X-ray observations made with instruments on-board the Solar Maximum Mission (SMM) satellite. The preflare phase was marked by a gradual brightening of the flare site in Ov and the disappearance of an H filament. Filament ejecta were seen in Ov moving southward at a speed of about 60 km s^–1, before the impulsive phase. The flare loop footpoints brightened in H and the Caxix resonance line broadened dramatically 2 min before the impulsive phase. Non-thermal hard X-ray emission was detected from the loop footpoints during the impulsive phase while during the same period blue-shifts corresponding to upflows of 200–250 km s^–1 were seen in Ca xix. Evidence was found for energy deposition in both the chromosphere and corona at a number of stages during the flare. We consider two widely studied mechanisms for the production of the high temperature soft X-ray flare plasma in the corona, i.e. chromospheric evaporation, and a model in which the heating and transfer of material occurs between flux tubes during reconnection.     

Preflare energy build-up in a filament circuit

The two-dimensional Van Tend and Kuperus (1978) scenario for pre-flare energy build-up is extended to a fully three-dimensional model and applied to the 16 May, 1981 flare observed at Debrecen. It is shown that there is plenty of free energy (10³³ erg) available to explain the ensuing large two-ribbon flare. This estimate is an order of magnitude larger than the simple estimate made by Van Tend, as a result of the three-dimensional character of the present model. It is further confirmed that the global form of the preflare circuit is decisive for determining the amount of energy stored in the preflare configuration, while the internal structure of the filament is of little importance. This is in accordance with the similar claims of Alfvén and Van Tend and Kuperus.Order of magnitude estimates are derived for all the lumped circuit parameters of the preflare filament-return current circuit; self-inductance, resistance, current strength, and applied voltage. It is found that the model gives correct predictions for the independently observed photospheric flow velocity and current strength in filaments.NAS/NRC Resident Research Associate.     

Hβ Doppler-velocity fields within sites of flares in a solar active region

In this paper, we analyze the relationship between photospheric magnetic fields and chromospheric velocity fields in a solar active region, especially evolving features of the chromospheric velocity field at preflare sites. It seems that flares are related to unusually distributed velocity field structures, and initial bright kernels and ribbons of the flares appear in the red-shifted areas (i.e., downward flow areas) close to the inversion line of H Dopplergrams with steep gradients of the velocity fields, no matter whether the areas have simple magnetic structure or a weak magnetic field, or strong magnetic shear and complex structure of the magnetic fields. The data show that during several hours prior to the flares, while the velocity field evolves, the sites of the flare kernels (or ribbons) with red-shifted features come close to the inversion line of the velocity field. This result holds regardless of whether or not the flare sites are wholly located in blue-shifted areas (i.e., upward flow areas), or are far from the inversion line of the Doppler velocity field (V = 0 line), or are partly within red-shifted areas. There are two cases favourable for the occurrence of flares, one is that the gulf-like neutral lines of the magnetic field (B = 0 line) occur in the H red-shifted areas, the other is that the gulf-like inversion lines of the H Doppler velocity field (V = 0 line) occur in the unipolar magnetic areas. These observational facts indicate that the velocity field and magnetic field have the same effect on the process of flare energy accumulation and release.     

Coronal X-ray activity preceding solar flares

A comprehensive survey of Skylab S-054 soft X-ray images was performed to investigate the characteristics of coronal enhancements preceding solar flares. A search interval of 30 min before flare onset was used. A control sample was developed and tests of the statistical results performed. X-ray images with preflare enhancements were compared with high resolution H images and photospheric magnetograms.The results are as follows: preflare X-ray enhancements were found in a statistically significant number of the preflare intervals, and consisted of one to three loops, kernels or sinuous features per interval. Typically, the preflare feature was not at the flare site and did not reach flare brightness. There was no systematically observed time within the preflare interval for the preflare events to appear and no correlation of preflare event characteristics with the subsequent flare energy. Gas pressures of several preflare features were calculated to be on the order of several dyne cm^–2, typical of active region loops, not flares. These results suggest that observations with both high spatial resolution and low coronal temperature sensitivity are required to detect these small, low pressure enhancements that preceded the smaller flares typical of the Skylab epoch. H brightenings were associated with nearly all of the preflare X-ray enhancements. Changing H absorption features in the form of surges or filament activations were observed in about half of the cases. These results do not provide observational support for models which involve preheating of the flare loop, but they are consistent with some current sheet models which invoke the brightening of structures displaced from the flare site tens of min before onset.     

Investigation of X-Ray Flares with Long Rising Phases

We have used Yohkoh and GOES X-ray observations to investigate flares with a long rising phase. We have found that a characteristic feature of such flares is a long time interval, Δ t ≥ 20 min, between the temperature maximum and the maximum of the emission measure. We have carried out detailed analysis for 10 limb flares of this type. Time variation of the heating function, E_H(t), has been determined for their loop-top X-ray kernels. The time variation of E_H(t), together with the temperature–density diagnostic diagrams, have been used to explain the large value of the time interval, Δ t. The main point is that for these flares the heating function E_H(t) decreases so slowly after the temperature maximum, that for the long time, Δ t, the energy flux reaching flare foot points is sufficient to maintain significant chromospheric evaporation. Investigation of the flare evolution in the temperature–density diagnostic diagrams allowed us to work out a new method of determination of the density for flare kernels. This method can be applied to all the kernels for which their altitudes can be estimated. The advantage of this method is that for the density determination it is not necessary to assume what is the extension of the emitting plasma along the line of sight.     

Structures of chromospheric magnetic field in the vicinity of the filament of active region 5669

In this paper, the chromospheric magnetic structures and their relation to the photospheric vector magnetic field in the vicinity of a dark filament in active region 5669 have been demonstrated. Structural variations are shown in chromospheric magnetograms after a solar flare. Filament-like structures in the chromospheric magnetograms occurred after a solar flare. They correspond to the reformation of the chromospheric dark filament, but there is no obvious variation of the photospheric magnetic field. We conclude that (a) some of the obvious changes of the chromospheric magnetic fields occurred after the flare, and (b) a part of these changes is perhaps due to flare brightening in the chromospheric H line.During the reforming process of the dark filament, a part of its chromospheric velocity field shows downward flow, and it later shows upward flow.     

Spatial and temporal evolution of soft and hard X-ray emission in a solar flare

We study the spatial and temporal characteristics of the 3.5 to 30.0 keV emission in a solar flare on April 10, 1980. The data were obtained by the Hard X-ray Imaging Spectrometer aboard the Solar Maximum Mission Satellite. It is complemented in our analysis with data from other instruments on the same spacecraft, in particular that of the Hard X-ray Burst Spectrometer.Key results of our investigation are: (a) Continuous energy release is needed to substain the increase of the emission through the rising phase of the flare, before and after the impulsive phase in hard X-rays. The energy release is characterized by the production of hot (5 × 10⁷ T 1.5 × 10⁸ K) thermal regions within the flare loop structures. (b) The observational parameters characterizing the impulsive burst show that it is most likely associated with non-thermal processes (particle acceleration). (c) The continuous energy release is associated with strong chromospheric evaporation, as evidenced in the spectral line behavior determined from the Bent Crystal Spectrometer data. Both processes seem to stop just before flare maximum, and the subsequent evolution is most likely governed by the radiative cooling of the flare plasma.     

Formation Mechanism of Soft X-Ray Transient Trans-Equatorial Loop System

The Soft X-ray Telescope (SXT) onboard Yohkoh often observed large-scale coronal loops connecting two active regions situated in opposite hemispheres. These are the trans-equatorial loop systems (TLSs). The formation mechanism of TLSs is not yet known. We analyzed a TLS observed simultaneously with Yohkoh/SXT and a coronagraph (SOHO/LASCO-C1). SOHO/LASCO-C1 observed loop expansion and eruption at the west solar limb. Yohkoh/SXT observed a rising motion (chromospheric evaporation) of hot and dense plasmas from the active regions located at the footpoints of the loop. Important results of our analyses are that (1) the loop eruption and the rising motion of the plasmas were simultaneous, (2) the TLS had a cusp-like appearance, and (3) the highest temperature region of the TLS was located above the bright loop seen in soft X rays. These observational results (loop expansion, eruption, and chromospheric evaporation) suggest that this bright (high-density) TLS was created by the same mechanism by which a solar flare occurs, namely, magnetic reconnection. In this paper, we propose a formation mechanism of the TLS that forms between two independent active regions.     

Clear Detection of Chromospheric Evaporation Upflows with High Spatial/Temporal Resolution by Hinode XRT

We find clear evidence for typical chromospheric evaporation associated with small transient brightenings, using the data from the X-ray Telescope (XRT) onboard Hinode. We found 13 events, each having a pair of evaporation upflows arising almost symmetrically from both foot points of a magnetic loop. These facts strongly support the standard flare model based on the magnetic reconnection. The apparent upflow velocities of three of the events are ≈?500?km?s^?1, while those of the other events are ≈?100?km?s^?1. This is the first clear direct detection of evaporating upflow motion in soft X-ray images from Hinode/XRT; such images were obtained with high cadence (≈?60?s) and high spatial resolution (1?arcsec).     

A qualitative interpretation of 7 August 1972 impulsive phase flare Hα line profiles

Tanaka's (1977) unique H profiles of the kernels of the 7 August 1972 flare were quantitatively interpreted by Brown et al. (1978; henceforth BCR) in terms of a thick target electron beam model. They found that this interpretation required beam inhomogeneity and/or partial precipation and large (60–100 km s^–1) macroturbulence. The latter requirement is somewhat suspect, since the only independent evidence also comes from efforts to understand the profiles of optically thick chromospheric lines. Relationships between model atmosphere parameters and line profile parameters calculated by Dinh (1980) show that these requirements could be considerably reduced, if not totally eliminated, if the actual chromospheric flare heating mechanism were simultaneously capable of pushing the flare transition region to greater column density and causing less heating of the residual chromosphere than the BCR models. This then implies that the chromosphere is heated primarily by a mechanism through which the heating effects do not penetrate as far below the flare transition region as is the case for a power-law spectrum of non-thermal electrons whose parameters are chosen appropriate to the nonthermal thick target interpretation of hard X-rays. Thermal conduction and optically thick radiation are examples of such a mechanism.