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 υ_d= 0.4 (F/ϱ_ch)^1/3, where F is that part of the flare energy flux driving chromospheric evaporation, and ϱ_ch is the mass density in the preflare chromosphere just below the preflare transition region. This implies that chromospheric downflows as measured by Hα asymmetries may be a powerful probe of flare energetics.

     

Chromospheric and coronal explosions in solar flares

The phenomenon described as a coronal explosion results directly from a chromospheric explosion. These two phenomena always occur together. They are the manifestations of the impulsive phase explosions in solar flares. These explosive processes occur during and immediately after the onset of the impulsive phase of flares. A previously presented model, describing the relation between the two kinds of explosions, appears to be able to explain qualitatively, and in many cases also quantitatively, the observations relevant to these explosive processes.     

Multiple downflow velocities above sunspotss

Supersonic transition zone downflow appears to be commonly occurring above sunspots. The downflow has been observed in 29 emission lines in the ultra-violet spectrum with the High Resolution Telescope and Spectrograph (HRTS) on two rocket flights and on 5 consecutive days during the Spacelab-2 mission. Spectra from the second rocket flight, contain the most extreme example of downflow, showing speeds up to 180 km s^–1 and extending over 60 are sec along the slit. The observations demonstrate the existence of several different distinctive flow speeds within the 1 arc sec resolution element throughout the temperature range 7000–240 000 K.Paper presented at the 11 th European Regional Astronomical Meetings of the IAU on New Windows to the Universe, held 3–8 July, 1989, Tenerife, Canary Islands, Spain     

Rayleigh-taylor instability in solar prominences as a mechanism for dXouble-ribbon flares

In this paper, the energy storage for a spotless two-ribbon flare is discussed with reference to the morphology of the chromospheric fibrils surrounding a filament prior to the flare. Also, on the basis of the Kippenhahn-Schluter model of filaments, we discuss the instability of magnetic structure in these filaments. We found that once the gradient of the magnetic field or the curvature of the magnetic “trough” exceeds certain critical value, the Rayleigh-Taylor instability will be triggered off, leading to the sudden disappearance (Disparition Brusque) of the filament. At the same time, a neutral current sheet will be formed in the field with magnetic flux concentrated on both sides of the filament. Rapid reconnection of the field lines then lead to the onset of a two-ribbon flare.     

Coronal explosions as a signature of current loop coalescence in solar flares

The coronal explosions, discovered by De Jager and Boelee (1984), and interpreted by them as manifestations of plasma streaming out of the flare kernels, can also be interpreted as signatures of current loop coalescence in the flaring region.     

Electron beam as origin of white-light solar flares

We study the effect of chromospheric bombardment by an electron beam during solar flares. Using a semi-empirical flare model, we investigate energy balance at temperature minimum level and in the upper photosphere. We show that non-thermal hydrogen ionization (i.e., due to the electrons of the beam) leads to an increase of chromospheric hydrogen continuum emission, H^– population, and absorption of photospheric and chromospheric continuum radiation. So, the upper photosphere is radiatively heated by chromospheric continuum radiation produced by the beam. The effect of hydrogen ionization is an enhanced white-light emission both at chromospheric and photospheric level, due to Paschen and H^– continua emission, respectively. We then obtain white-light contrasts compatible with observations, obviously showing the link between white-light flares and atmospheric bombardment by electron beams.     

Energetic electrons as an energy transport mechanism in solar flares

We review the observations and theory relating to the role of energetic electrons in the solar flare, with particular emphasis on discriminating between thermal and nonthermal origins of these electrons. We discuss diagnostics in hard X-rays, especially those relating to the recent observations of the SMM and HINOTORI satellites. We also briefly address the response of the atmosphere to energy input in the form of high energy electrons, in particular through the diagnostics of both the Fe K feature and optically thin transition region lines such as 0V. Finally, we discuss the relative roles of electron and proton heating in -ray flare events.     

Further evidence for downflow in the solar atmosphere

Downflow velocities of the higher atmospheric layers of the Sun are derived from wavelength measurements of the Mg ii core absorption.     

Kink instability of solar coronal loops as the cause of solar flares

Solar coronal loops are observed to be remarkably stable structures. A magnetohydrodynamic stability analysis of a model loop by the energy method suggests that the main reason for stability is the fact that the ends of the loop are anchored in the dense photosphere. In addition to such line-tying, the effect of a radial pressure gradient is incorporated in the analysis.Two-ribbon flares follow the eruption of an active region filament, which may lie along a magnetic flux tube. It is suggested that the eruption is caused by the kink instability, which sets in when the amount of magnetic twist in the flux tube exceeds a critical value. This value depends on the aspect ratio of the loop, the ratio of the plasma to magnetic pressure and the detailed transverse magnetic structure. For a force-free field of uniform twist the critical twist is 3.3, and for other fields it is typically between 2 and 6. Occasionally active region loops may become unstable and give rise to small loop flares, which may also be a result of the kink instability.     

Electron acceleration in solar flares

We present observations of an intense solar flare hard X-ray burst on 1980 June 27, made with a balloon-borne array of liquid nitrogen-cooled germanium detectors which provided unprecedented spectral resolution (1 keV FWHM). The hard X-ray spectra throughout the impulsive phase burst fitted well to a double power-law form, and emission from an isothermal 10⁸–10⁹K plasma can be specifically excluded. The temporal variations of the spectrum indicate that the hard X-ray burst is made up of two superposed components: individual spikes lasting 3–15 s, whch have a hard spectrum and a break energy of 30–65 keV; and a slowly varying component characterized by a soft spectrum with a constant low-energy slope and a break energy which increases from 25 keV to 100 keV through the event. The double power-law shape indicates that acceleration by DC electric fields parallel to the magnetic field, similar to that occurring in the Earth's auroral zone, may be the source of the energetic electrons which produce the hard X-ray emission. The total potential drop required for flares is typically 10² kV compared to 10 kV for auroral substorms.     

Energy release in solar flares

We examine observational evidence concerning energy release in solar flares. We propose that different processes may be operative on four different time scales: (a) on the sub-second time scale of sub-bursts which are a prominent feature of mm-wave microwave records; (b) on the few-seconds time scale of elementary bursts which are a prominent feature of hard X-ray records; (c) on the few-minutes time scale of the impulsive phase; and (d) on the tens-of-minutes or longer time scale of the gradual phase.We propose that the concentration of magnetic field into magnetic knots at the photosphere has important consequences for the coronal magnetic-field structure such that the magnetic field in this region may be viewed as an array of elementary flux tubes. The release of the free energy of one such tube may produce an elementary burst. The development of magnetic islands during this process may be responsible for the sub-bursts. The impulsive phase may be simply the composite effect of many elementary bursts.We propose that the gradual phase of energy release, with which flares typically begin and with which many flares end, involves a steady process of reconnection, whereas the impulsive phase involves a more rapid stochastic process of reconnection which is a consequence of mode interaction.In the case of two-ribbon flares, the late part of the gradual phase may be attributed to reconnection of a large current sheet which is being produced as a result of filament eruption. A similar process may be operative in smaller flares.Also, Department of Applied Physics, Stanford University.     

Energy release in solar flares

We summarize key problems in our understanding of energy release in solar flares, as addressed by participants in a recent workshop. These problems fall into three broad areas: (i) Transport and thermalization of energy, (ii) acceleration of particles, and (iii) origin and effects of mass motions. We then describe how suitably coordinated collaborative observing sequences during the forthcoming Solar Maximum Year are potentially capable of resolving some of these issues.     

Heat transfer in solar flares

Radiative cooling and heat conduction determine the temperature structure of flare plasmas along magnetic field. It is shown that both in the case of slow heating and of impulsive heating, temperatures are distributed in such a way that classical collisional heat conduction is valid.     

Particle acceleration in solar flares

Particle acceleration during solar flares is a complex process where the main actors (Direct (D.C.) or turbulent electric fields) are hidden from us. It is easy to construct a successful particle accelertion model if we are allowed to impose on the flaring region arbitrary conditions (e.g., strength and scale length of the D.C. or turbulent electric fields), but then we have not solved the acceleration problem; we have simply re-defined it. We outline in this review three recent observations which indicate that the following physical processes may happen during solar flares: (1) Release of energy in a large number of microflares; (2) short time-scales; (3) small length scales; and (4) coherent radiation and acceleration sources. We propose that these new findings force us to reformulate the acceleration process inside a flaring active region assuming that a large number of reconnection sites will burst almost simultaneously. All the well-known acceleration mechanisms (electric fields, turbulent fields, shock waves, etc.) reviewed briefly here, can be used in a statistical model where each particle is gaining energy through its interaction with many small reconnection sites.     

Electron acceleration in solar flares

We present observations of an intense solar flare hard X-ray burst on 1980 June 27, made with a balloon-borne array of liquid nitrogen-cooled germanium detectors which provided unprecedented spectral resolution (≲1 keV FWHM). The hard X-ray spectra throughout the impulsive phase burst fitted well to a double power-law form, and emission from an isothermal 10⁸–10⁹K plasma can be specifically excluded. The temporal variations of the spectrum indicate that the hard X-ray burst is made up of two superposed components: individual spikes lasting ∼3–15 s, whch have a hard spectrum and a break energy of 30–65 keV; and a slowly varying component characterized by a soft spectrum with a constant low-energy slope and a break energy which increases from 25 keV to ≳100 keV through the event. The double power-law shape indicates that acceleration by DC electric fields parallel to the magnetic field, similar to that occurring in the Earth's auroral zone, may be the source of the energetic electrons which produce the hard X-ray emission. The total potential drop required for flares is typically ∼10² kV compared to ∼10 kV for auroral substorms.

     

Electron acceleration in solar flares

For the period September 1978 to December 1982 we have identified 55 solar flare particle events for which our instruments on board the ISEE-3 (ICE) spacecraft detected electrons above 10 MeV. Combining our data with those from the ULEWAT spectrometer (MPI Garching and University of Maryland) electron spectra in the range from 0.1 to 100 MeV were obtained. The observed spectral shapes can be divided into two classes. The spectra of the one class can be fit by a single power law in rigidity over the entire observed range. The spectra of the other class deviate from a power law, instead exhibiting a steepening at low rigidities and a flattening at high rigidities. Events with power-law spectra are associated with impulsive (<1 hr duration) soft X-ray emission, whereas events with hardening spectra are associated with long-duration (<1 hr) soft X-ray emission. The characteristics of long-duration events are consistent with diffusive shock acceleration taking place high in the corona. Electron spectra of short-duration flares are well reproduced by the distribution functions derived from a model assuming simultaneous second-order Fermi acceleration and Coulomb losses operating in closed flare loops.     

Energy release in solar flares

Team 2 of the Ottawa FLARES 22 Workshop dealt with observational and theoretical aspects of the characteristics and processes of energy release in flares. Main results summarized in this article stress the global character of the flaring phenomenon in active regions, the importance of discontinuities in magnetic connectivity, the role of field-aligned currents in free energy storage, and the fragmentation of energy release in time and space.Report of Team 2, Flares 22 Workshop, Ottawa, May 25–28, 1993.