Runaway Acceleration Research Articles

Testing the DC-Electric Field Model in a Solar Flare Observed by YOHKOH and the Compton Gamma-Ray Observatory

We apply a DC-electric field model to the analysis of soft and hard X-ray observations of a solar flare observed by Yohkoh and the Compton Gamma Ray Observatory (CGRO) on 6 September 1992. The flare was observed simultaneously in the soft X-ray Ca XIX line by the Yohkoh Bragg Crystal Spectrometer (BCS) and in hard X-rays (greater than 50 keV) by the CGRO Burst and Transient Spectrometer Experiment (BATSE). A strong stationary component of Ca XIX emission was present at the start of impulsive hard X-ray emission indicating an extended phase of heating prior to the production of energetic nonthermal electrons. We interpret the preflare Ca XIX emission as a signature of Joule heating by field-aligned currents. We relate the temporal variation of impulsive hard X-ray emission to the rate of runaway electron acceleration by the DC-electric field associated with the current. We find that the initial rise in hard X-ray emission is consistent with electron acceleration by a DC-electric field that increased from a preflare value of less than approximately 10(exp -5) V/cm to approximately (9 +/- 1) x 10(exp -5) V/cm at the time of the first hard X-ray peak and then remained constant during the rest of the impulsive phase. We attribute the increase in electric field strength to the formation of a current sheet at the reconnection point of two loop structures. The decrease in hard X-ray emission after flare maximum is consistent with a reduction in the number of runaway electrons due to an increase in coronal density produced by chromospheric evaporation. The increased density quenches the runaway process by enhancing collisional thermalization of electrons. To avoid the generation of an unrealistically large magnetic field, the flaring region must be highly filamented into greater than approximately 10(exp 6) oppositely directed current channels of approximately 30 cm width with an initial preflare current of approximately 3 x 10(exp 10) A per channel.

A thermal/nonthermal model for solar hard X-ray bursts

A formalism is developed for analyzing high-resolution hard X-ray spectra, incorporating the coexistence of thermal and nonthermal bremsstrahlung. The two processes are physically linked by the presence of electric currents, which both heat the surrounding plasma via Joule dissipation and accelerate electrons via the runaway process. We use this formalism to analyze the flare of 1980 June 27 and find that both the gradual and spike components of the hard X-ray emission are consistent with runaway acceleration. We also find that significant heating is observed only in the gradual component. The electric field is always sub-Dreicer, the maximum total potential drop in the acceleration region is found to be approximately 100 kV in two of the spikes, and the average accelerated electron flux is approximately 10<SUP>34</SUP> electrons/s. We argue that classical resistivity is a valid assumption for this event and find the density in the current channels (approximately 10<SUP>11</SUP>/cc) and a lower limit on the volume of the heated plasma. We find that the ratio of the electric field to the Dreicer field (epsilon = E/E<SUB>D</SUB> varies systematically, whereas the value of E alone does not. We also find that the acceleration region fragmented into many current/return current pairs, and that the fragmentation varied systematically. We also discuss further implications of this model.

Turbulent acceleration in solar flares

Turbulent acceleration in the impulsive phase of solar flares is reviewed, with the emphasis on bulk energization of nonrelativistic electrons and prompt acceleration of the gamma-ray emitting nonrelativistic ions and relativistic electrons. The primary energy release in a flare cannot be due to collisional dissipation. Anomalous resistivity requires that the current flows in many narrow channels with the current density above threshold for current instability. The bulk energization of the electrons is due to the damping of low-frequency electrostatic turbulence generated by the current instability. This turbulence also limits the rate a nonthermal electron tail forms due to runaway acceleration. Stochastic and gyroresonant acceleration by MHD turbulence are discussed briefly, emphasizing the need for preacceleration. Stochastic acceleration is favorable for the gamma-ray emitting particles only if an adequated source of the MHD turbulence can be identified. Subject headings: acceleration of particles — MHD — Sun: flares — turbulence

Turbulent Acceleration in Solar Flares

AbstractTurbulent acceleration in the impulsive phase of solar flares is reviewed, with the emphasis on bulk energization of nonrelativistic electrons and prompt acceleration of the gamma-ray emitting nonrelativistic ions and relativistic electrons. The primary energy release in a flare cannot be due to collisional dissipation. Anomalous resistivity requires that the current flows in many narrow channels with the current density above threshold for current instability. The bulk energization of the electrons is due to the damping of low-frequency electrostatic turbulence generated by the current instability. This turbulence also limits the rate a nonthermal electron tail forms due to runaway acceleration. Stochastic and gyroresonant acceleration by MHD turbulence are discussed briefly, emphasizing the need for preacceleration. Stochastic acceleration is favorable for the gamma-ray emitting particles only if an adequated source of the MHD turbulence can be identified.Subject headings: acceleration of particles — MHD — Sun: flares — turbulence

A hybrid thermal/nonthermal model for the energetic emissions from solar flares

We present an alternative interpretation of the high-resolution solar flare spectra obtained in 1981 by Lin et al. In our interpretation electron heating and acceleration are simultaneous and physically linked, rather than heating being a secondary effect of particle acceleration. We show that the observed properties of solar flare X-ray emission can in general be explained through the Joule heating and electric field acceleration of runaway electrons in current channels. We have fitted a hybrid thermal/nonthermal electron distribution, consisting of hot isothermal electrons with a nonthermal tail of runaway electrons, to representative spectra obtained by Lin et al. The hybrid model relaxes the electron number and energy flux requirements for the hard X-ray emission over those of a purely nonthermal model. The low-energy 'cutoff' to the nonthermal part of the X-ray spectrum is determined by the critical velocity in the electron distribution above which runaway acceleration occurs. We relate the fit parameters to the physical properties (such as the electric field strength in the current channels) of the acceleration region.

On the production of hard X-rays in solar flares

The problem of producing the hard X-ray burst at the onset of solar flares may be thought of in terms of the problem of producing the non-thermal electrons which emit the X-rays via bremsstrahlung. Electron acceleration to relativistic energies without similar ion acceleration is difficult to achieve, even in an ad hoc theoretical model. Yet from global energetic considerations, it is not feasible to accelerate the electrons as a minor constituent of the total energetic particle population. Therefore, it is necessary to invoke a more sophisticated process for the electron acceleration. In this paper we describe a mechanism for achieving this via an initial acceleration of a neutralized ion beam. When such a beam impacts the chromosphere, the electrons start to scatter while the ions continue downwards, rapidly setting up an electric field which is either cancelled by the inflow of background chromospheric electrons or results in the runaway acceleration of beam electrons. In the former case the result is simply heating, whereas in the latter case much of the ion kinetic energy is transferred into electron kinetic energy. The final electron energy may be similar to the typical energy of the ions. The electrons that are accelerated are those in the neutral beam that experience an electric field greater than the critical Dreicer field. Thus there will be a low-energy cut-off to the electron spectrum which overcomes the well-known energetics problem at low energies with certain other spectral forms.

Acceleration and energization by currents and electric fields

The acceleration of the influential ≲ 100 keV electrons in flares observed in hard X-rays and several radio emissions is unknown. Shock-waves and MHD turbulence, successfully applied to interprete interplanetary energetic particles, have recently been called in question concerning energetic flare electrons and ions. Other possible mechanisms are considered which are closely related to the primary flare energy release. In particular, runaway acceleration by the electric field of the reconnection current sheet, bulk heating by microturbulence, and cross-field ion currents due to bulk motion as a primary result of reconnection are reviewed. All three are likely to occur in some way. Their relative importance cannot be definitively assessed due to the lack of information on non-thermal, low energy protons.

The role of O‐type neutral lines in magnetic merging during substorms and solar flares

Observational evidence suggests that magnetospheric substorms may be associated with the formation of a pair of neutral points or lines in the geomagnetic plasma sheet, containing an X‐type point (or line) and an O‐type one. While magnetic merging theory has concentrated almost entirely on X‐type neutral configurations (points, lines, or sheets), here the role of O‐type configurations is examined, with special attention to three points: (1) How does the X‐O configuration extend in three dimensions? To this end, an analytical model of the configuration was derived, useful for visualizing the geometry and for numerical treatment of plasma flows in it. (2) What modifications are needed in the MHD condition E = v × B near the O‐type line, where it tends to make v grow without limit? By analyzing equations of motion for charged particles near an O‐type neutral line and their solutions in limiting cases, it was found that at a certain distance from the neutral line the mean particle motion became decoupled from that of magnetic field lines (which obeys the MHD condition). The decoupling distance depended on initial conditions in momentum space, suggesting that the MHD approximation which averages out such conditions may not suffice for describing plasma dynamics near the neutral line. Similar problems arise with merging flows near X‐type neutral lines, and although the treatment there is more difficult and requires more approximations, it appears that the same qualitative conclusions apply there as well. (3) What is the role of O‐type neutral lines in particle acceleration? It was found that after inflowing particles are decoupled from the field line motion, they go over to a mode of runaway acceleration along the neutral line. This process is much more efficient along an O‐type line than along an X‐type line, and it is concluded that if merging occurs at an X‐O pair, two particle populations may be expected: low‐energy particles accelerated adiabatically by earthward convection past the X‐type line, dependent mainly on the total amount of flux which has been merged, and high‐energy particles converted toward the O‐type line and undergoing there runaway acceleration. The second acceleration process depends critically on the rapidity of merging and is therefore expected to vary considerably from event to event. All this agrees with observations, and similar processes may also be important in solar flares, where a ‘Y‐type neutral point’ has been proposed, which actually represents the limiting form of an X‐O configuration.

Dynamics of runaway electrons in toroidal systems

A self-consistent model of the runaway electron acceleration process that includes electron drift orbits has been developed. Two new effects obtained by the analysis are a shrinkage of the beam radius with time and a limitation on the maximum attainable γ, γmax ≲ (Wce/Wpe)2, in such a beam. For the ORMAK runaway regime data, these results are minor corrections to a previously developed model which did not include drift orbits. However, for higher-energy-density runaway beams, the drift orbit effects may become more significant.

A New Model of Solar Flares

view Abstract Citations (50) References (37) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS A New Model of Solar Flares Sturrock, P. A. ; Coppi, B. Abstract Observational data concerning solar flares are reviewed briefly in order to list the salient requirements of a model of solar flares. The requirements of a theory of the flash phase are discussed in more detail, and previous flare theories are reviewed to determine the extent to which they meet these requirements. It is submitted that no theory meets enough of the requirements to be acceptable. A new model is proposed in which the preflare state consists of a mass of gas supported by a sheared magnetic field. This field configuration is likely to be met in the neighborhood of a fault line, across which there has been a shear displacement of the photosphere, since such a fault line tends to coincide with the dividing line between regions of opposite magnetic polarity. Such a configuration of gas and magnetic field is believed to be responsible for quiescent prominences, but the energy released in a flare is believed to come from lower parts of such a system, involving stronger fields and greater mass. This model there- fore explains the close relationship between flares and quiescent filaments. The flash phase of a solar flare is identified with a gravitational resistive instability of the above plasma-field configuration. It is shown that the rise time of this instability is sufficiently rapid to explain the very rapid development of the flash phase. The instability tends to break up the plasma into filaments with dimensions of a kilometer or less, in agreement with certain inferences which have been drawn from flare spectra. The development of fine-scale structure provides conditions for subsequent slower release of stored energy; this subsequent release is identified with the main or decay phase of flares. The flash-phase instability develops high current parallel to the magnetic field, which must be driven by an electric field parallel to the magnetic field. This permits one to identify the acceleration process in solar flares as runaway acceleration, and it is shown that this interpretation enables one to explain the observed energies of protons produced by solar flares. There is a brief discussion of the possibility of interpreting certain secondary flare phenomena, such as surges and loop prominences, by means of the proposed model. Publication: The Astrophysical Journal Pub Date: January 1966 DOI: 10.1086/148472 Bibcode: 1966ApJ...143....3S full text sources ADS |