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1.
We analyze particle acceleration processes in large solar flares, using observations of the August, 1972, series of large events. The energetic particle populations are estimated from the hard X-ray and γ-ray emission, and from direct interplanetary particle observations. The collisional energy losses of these particles are computed as a function of height, assuming that the particles are accelerated high in the solar atmosphere and then precipitate down into denser layers. We compare the computed energy input with the flare energy output in radiation, heating, and mass ejection, and find for large proton event flares that:
- The ～10–102 keV electrons accelerated during the flash phase constitute the bulk of the total flare energy.
- The flare can be divided into two regions depending on whether the electron energy input goes into radiation or explosive heating. The computed energy input to the radiative quasi-equilibrium region agrees with the observed flare energy output in optical, UV, and EUV radiation.
- The electron energy input to the explosive heating region can produce evaporation of the upper chromosphere needed to form the soft X-ray flare plasma.
- Very intense energetic electron fluxes can provide the energy and mass for interplanetary shock wave by heating the atmospheric gas to energies sufficient to escape the solar gravitational and magnetic fields. The threshold for shock formation appears to be ～1031 ergs total energy in >20 keV electrons, and all of the shock energy can be supplied by electrons if their spectrum extends down to 5–10 keV.
- High energy protons are accelerated later than the 10–102 keV electrons and most of them escape to the interplanetary medium. The energetic protons are not a significant contributor to the energization of flare phenomena. The observations are consistent with shock-wave acceleration of the protons and other nuclei, and also of electrons to relativistic energies.
- The flare white-light continuum emission is consistent with a model of free-bound transitions in a plasma with strong non-thermal ionization produced in the lower solar chromosphere by energetic electrons. The white-light continuum is inconsistent with models of photospheric heating by the energetic particles. A threshold energy of ～5×1030 ergs in >20 keV electrons is required for detectable white-light emission.
2.
We consider the plasma mechanism of sub-terahertz emission from solar flares and determine the conditions for its realization in the solar atmosphere. The source is assumed to be localized at the chromospheric footpoints of coronal magnetic loops, where the electron density should reach n ≈ 1015 cm?3. This requires chromospheric heating at heights h ? 500 km to coronal temperatures, which provides a high degree of ionization needed for Langmuir frequencies ν p ≈ 200–400 GHz and reduces the bremsstrahlung absorption of the sub-THz emission as it escapes from the source. The plasma wave excitation threshold for electron-ion collisions imposes a constraint on the lower density limit for energetic electrons in the source, n 1 > 4 × 109 cm?3. The generation of emission at the plasma frequency harmonic ν ≈ 2ν p rather than the fundamental tone turns out to be preferred. We show that the electron acceleration and plasma heating in the sub-THz emission source can be realized when the ballooning mode of the flute instability develops at the chromospheric footpoints of a flare loop. The flute instability leads to the penetration of external chromospheric plasma into the loop and causes the generation of an inductive electric field that efficiently accelerates the electrons and heats the chromosphere in situ. We show that the ultraviolet radiation from the heated chromosphere emerging in this case does not exceed the level observed during flares. 相似文献
3.
A. Gordon Emslie 《Solar physics》1985,98(2):281-291
We solve the energy equation for the high-temperature (coronal) component of flare plasma for two models of energy input: (i) direct collisional heating by a beam of suprathermal electrons, and (ii) ohmic heating by the beam-neutralizing reverse current. We discuss the regimes where each case is applicable, and solve for the differential emission measure distribution of the coronal plasma in each case. Scaling laws between loop temperatures and injected electron fluxes are derived for both models; these are testable observationally through coordinated soft X-ray and hard X-ray observations, thus providing a method of discriminating between the two cases. We also readdress the question of the energetic importance of a return current which is below the instability threshold for generation of ion-acoustic plasma turbulence. We find that unless the ambient coronal density is very low ( 109 cm –3), collisional heating will always dominate there, in agreement with the findings of previous authors. However, in the chromosphere/corona transition region, the relatively low temperature and correspondingly high plasma resistivity imply that reverse current ohmic heating can predominate the flare energetics, by up to an order of magnitude.Presidential Young Investigator. 相似文献
4.
J. G. Doyle P. B. Byrne B. R. Dennis A. G. Emslie A. I. Poland G. M. Simnett 《Solar physics》1985,98(1):141-158
Here we complete an energy balance analysis of a double impulsive hard X-ray flare. From spatial observations, we deduce both flares probably occur in the same loop within the resolution of the data. For the first flare, the energy in the fast electrons (assuming a thick-target model) is comparable to the convective up-flow energy, suggesting that these are related successive modes of energy storage and transfer. The total energy lost through radiation and conduction, 2.0 × 1028 erg, is comparable to the energy in fast electrons 2.5 × 1028 erg. For the second flare, the energy in the fast electrons is more than one order of magnitude greater than the energy of the convective up-flow. Total energy losses are within a factor of two lower than the calculated fast electron energy. We interpret the observations as showing that the first flare occurred in a small loop with fast electrons heating the chromosphere and resulting in chromospheric evaporation increasing the density in the loop. For the second flare most of the heating occurred at the electron acceleration site. The two symmetrical components of the Ca xix resonance line and a high velocity down-flow of 115 km s –1 observed at the end of the second hard X-ray burst are consistent with the flare eruption (reconnection) region being high in the flare loop. The estimated altitude of the acceleration site is 5500 km above the photosphere. 相似文献
5.
For the November 5, 1980 flare it is investigated how the plasma in a large flaring loop responds to the injection of energetic electrons. Observations are compared with the results of a one-dimensional numerical simulation. For the simulation it is assumed that at the time the injection is started, the plasma is in an equilibrium state with a constant pressure along the loop and conductive heating compensated by radiative losses. Especially important for the evolution of the impulsively heated plasma is the penetration depth of the fast electrons compared to the depth of the transition layer. Both parameters are known from the observations. The injected energy is 2.6 × 1011 ergs cm ?2 in 30 s (as derived from the hard X-ray observations) and computations show that the high temperature plasma of the loop responds to it with upward motions of about 50 km s?1, i.e. with velocities much smaller than the ion sound speed (≈ 500km s?1). The heating of the plasma due to the absorption of beam energy can be understood using a constant density approximation. After the heating phase the plasma returns in about 5 min to its initial state by conductive cooling. The downward conducted energy is radiated away in the transition zone. The numerical simulation shows that impulsive heating by non-thermal electrons only does not explain the observed large increase in the density of the loop during the flare. It is therefore required that continuous energy and/or mass input occur after the impulsive phase. 相似文献
6.
We analyse four solar flares which have energetic hard X-ray emissions, but unusually low soft X-ray flux and GOES class (C1.0–C5.5). These are compared with two other flares that have soft and hard X-ray emission consistent with a generally observed correlation that shows increasing hard X-ray accompanied by increasing soft X-ray flux. We find that in the four small flares only a small percentage of the nonthermal electron beam energy is deposited in a location where the heating rate of the electron beam exceeds the radiative cooling rate of the ambient plasma. Most of the beam energy is subsequently radiated away into the cool chromosphere and so cannot power chromospheric evaporation thus reducing the soft X-ray emission. We also demonstrate that in the four small flares the nonthermal electron beam energy is insufficient to power the soft X-ray emitting plasma. We deduce that an additional energy source is required, and this could be provided by a DC-electric field (where quasi-static electric field channels in the coronal loops accelerate electrons, and those electrons with velocity below a critical velocity will heat the ambient plasma via Joule heating) in preference to a loop-top thermal source (where heat flux deposited in the corona is conducted along magnetic field lines to the chromosphere, heating the coronal plasma and giving rise to further chromospheric evaporation). 相似文献
7.
J.-C. Henoux G. Chambe D. Heristchi M. Semel B. Woodgate D. Shine J. Beckers 《Solar physics》1983,86(1-2):115-122
Linear polarization in two chromospheric lines (Hα and SI 1437 A) was observed in the gradual phase of solar flares. The polarized electric vector is directed towards disk center. This polarization could be due to collisional excitation of hydrogen and SI by energetic electrons beamed in the vertical direction. Direct excitation by a highly energetic beam of electrons of order 10–100 keV. is doubtful. The heat flux in the region connecting the transition zone to the high chromosphere during the gradual phase of a flare could lead to an anisotropic excitation. Selecting a function which represents the velocity distribution of electrons carrying heat flux, the relationship between conductive heat flux and linear line polarization has been computed. The application of the relationship between linear polarization and heat flux to the observed degree of polarization leads to the determination of the conductive heat flux in the high chromosphere. This conductive flux is of the order of magnitude of the total radiation loss in the chromosphere and below, which is also of the order of magnitude of the conductive flux in the transition zone. 相似文献
8.
We examine empirical atmospheric structures that are consistent with enhanced white-light continuum emission in solar flares. This continuum can be produced either by hydrogen bound-free emission in an enhanced region in the upper chromosphere, or by H- emission in an enhanced region around the temperature minimum. In the former case, weak Paschen jumps in the spectrum will be present, with the spectrum being dominated by a strong Balmer continuum, while in the latter case the spectrum exhibits a weaker, flat enhancement over the entire visible spectrum.We find that when proper account is taken of radiative backwarming processes, the two enhanced atmospheric regions above are not independent, in that irradiation by Balmer continuum photons from the upper chromosphere creates sufficient heating around the temperature minimum to account for the temperature enhancements there. Thus the problem of main phase white-light flare production reduces to one of creating temperature enhancements of order 104 K in the upper chromosphere; radiative backwarming then naturally accounts for the enhancements of order 100 K around the temperature minimum.Heating by electron and proton bombardment, and by XUV irradiation from above, are then considered as candidates for creating the necessary enhancements in the upper chromosphere. We find that electron bombardment can be ruled out, whereas bombardment by protons in the few-MeV energy range is a viable candidate, but one without strong observational support. The XUV irradiation hypothesis is examined by incorporating it self-consistently into the PANDORA radiative transfer algorithm used to construct the empirical model atmospheres; we find that the introduction of XUV radiation, with flux and spectrum appropriate to white-light flare events, does indeed produce sufficient radiative heating in the upper chromosphere to balance the radiative losses associated with the required temperature enhancements.In summary, we find that the radiative coupling of (i) the upper chromosphere and temperature minimum regions (through Balmer continuum photons) and (ii) the transition region and upper chromosphere (through XUV photons) can account for white-light emission in solar flares.Presidential Young Investigator. 相似文献
9.
Impulsive heating of the upper chromosphere by a very powerful thermal flux is studied as the cause of hard X-rays during a solar flare. The electron temperature at the boundary between the corona and chromosphere is assumed to change in accordance with the hard X-ray intensity in an elementary flare burst (EFB). A maximum value of about 108 K is reached after 5 s, after which the boundary temperature decreases. These high-temperature changes lead to fast propagation of heat into the chromosphere. Numerical solution of the hydrodynamic equations, which take into account all essential dissipative processes, shows that classical heat conduction is not valid due to heat flux saturation in the case of impulsive heating from a high-temperature source. The saturation effect and hydrodynamic flow along a magnetic field lead to electron temperature and density distributions such that the thermal X-ray spectrum of a high-temperature plasma can be well enough approximated by an exponential law or by two power-law spectra. According to this dissipative thermal model for the source of hard X-rays, the emission measure of the high-temperature plasma increases monotonously during the whole EFB even after the temperature maximum. Some results for the low-temperature region are discussed in connection with short-lived chromospheric bright points. 相似文献
10.
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. 相似文献
11.
The thermal balance of the plasma in the day-time equatorial F region is examined. Steady-state solutions of electron and ion temperatures are obtained, assuming the ions are O+ and H+. The theoretical concentrations of O+ and H+ and the field-aligned velocity were obtained following Moffett and Hanson (1973), while theoretical photoelectron heating rates of the electron gas were taken from Swartz et al. (1975).The results demonstrate the gross features in the electron and ion temperatures as observed at the Jicamarca Observatory and in the ion temperatures observed on the OGO-6 satellite. The rapid increase in electron temperature above 500 km at the magnetic equator is due to heating by photoelectrons created at higher latitudes and travelling up along the field lines. The rapid increase in ion temperature is due to good thermal contact with the electrons rather than the neutrals. It is shown that field-aligned interhemispheric thermal plasma flows appreciably affect these temperatures, and that, with a net plasma flow from the summer hemisphere to the winter hemisphere, the temperatures are higher in the winter hemisphere. These effects are related to the character of the ion temperature minimum observed by OGO-6 near the magnetic equator. 相似文献
12.
The 850 K exospheric temperature inferred for Jupiter from the radio-occultation experiments on Pioneers 10 and 11 is shown to imply a heat input of 0.25–0.5 erg cm?2s?1. One possible source of this energy is precipitation of electrons from a warm plasma (temperature corresponding to energies of the order of 30–500 eV). A mechanism is suggested wherein the presence of this plasma can be accounted for by centrifugal acceleration and adiabatic compression of ionospheric electrons and protons. Present ideas of the source strength of ionospheric plasma, however, give heating rates that are too small by 1–2 orders of magnitude, although inferences from direct plasma measurements suggest that the required plasma is indeed present. 相似文献
13.
T. Takakura 《Solar physics》1992,142(2):327-339
Numerical simulation is made of the impulsive loop flare caused by transient heat conduction along the loop with an applied axial electric current.It is assumed that a segment near the top of the coronal loop is heated to above 107 K by a heat input that is small compared with the total flare energy, which is given by the magnetic energy of the initial current. Due to the heat conduction, a hump appears in the velocity distribution of electrons, which may excite electron plasma waves with a sufficiently high intensity to cause an anomalous resistivity, as shown theoretically in a previous paper. In that paper, an effect of the plasma waves on the dynamics of electrons was taken into account consistently, but an anomalous heating due to an ohmic dissipation of the initial current under the anomalous resistivity was not taken into account.The aim of the present study is to study the subsequent dynamics of the heated gas caused by the anomalous heating, but in order to avoid an unpractically long computation time, the energy density of the plasma waves is estimated by the energy density of electrons in the velocity hump, without taking into account the effect of the plasma waves consistently in the dynamics of the electrons.The initial current starts to decay gradually by an ohmic dissipation under the anomalous resistivity occurring near the top of the loop to heat this region more. The enhanced heat conduction causes the velocity humps in a wider location. Consequently, the anomalous heating continues and spreads in a self-generating way even after the end of the initial minor heating. Thus the temperature near the loop top becomes above 108 K and the high-temperature region spreads in both directions along the loop with such a high speed as (2–3) × 104 km s–1, which is nearly equal to the speed of flux-limited heat conduction. On the other hand, induced electric field estimated from the anomalous resistivity is 3.3 × 107 V at the termination of the present simulation, under the modest initial current of 1.5 A m–2.X-ray emissions expected from the present model loop, show three sources, two footpoints with unequal brightness and a coronal source expanding along the loop in both directions. 相似文献
14.
We study the temperature of electrons advected with the solar wind to large solar distances far beyond 1 AU. Almost nothing is known about the thermodynamics of these electrons from in-situ plasma observations at these distances, and usually it is tacitly assumed that electrons, due to adiabatic behaviour and vanishing heat conduction, rapidly cool off to very low temperatures at larger distances. In this article we show, however, that electrons on their way to large distances undergo non-adiabatic interactions with travelling shocks and solar-wind bulk-velocity jumps and thereby are appreciably heated. Examining this heating process on an average statistical basis, we find that solar-wind electrons first cool down to a temperature minimum, which depending on the occurrence frequency of bulk velocity jumps is located between 3 and 6 AU, but beyond this the lowest electron temperature again starts to increase with increasing solar distance, finally achieving temperatures of about 7×104 K to 7×105 K at the location of the termination shock. Hence these electrons are unexpectedly shown to play an important dynamical role in structuring this shock and in determining the downstream plasma properties. 相似文献
15.
Giannina Poletto 《Solar physics》1981,73(2):233-256
Following previous order of magnitude estimates (Poletto, 1980), the possibility that hot downflowing motions in the solar transition region could be ascribed to spicular matter returning to the chromosphere after being heated by compression, is more thoroughly investigated. The equations describing the one-dimensional non stationary motion of the spicular plasma during the heating process are analytically solved, and the temporal profiles of temperature, density and velocity are given for a set of representative situations. The results are finally compared with available data. 相似文献
16.
T. Takakura 《Solar physics》1991,136(2):303-316
Numerical simulation is made of the transient heat conduction during local heating in a model coronal magnetic loop with an axial electric current. It is assumed that a segment near the top of the normal coronal loop is heated to above 107 K by a sufficiently small heat input as compared with the total flare energy. A hump appears in the velocity distribution of electrons moving down the temperature gradient with speeds slightly below the thermal one. Consequently, electron plasma waves are excited. The high intensity of the waves persists in the upper region of the loop for more than a second until the termination of the simulation. The energy density of the plasma waves normalized with respect to thermal density is 10–3.5 at maximum. A theoretical estimate gives an anomalous resistivity 5 orders of magnitude greater than an initial value. Based on the above result, we propose a model for impulsive loop flares. 相似文献
17.
Richard J. Defouw 《Solar physics》1970,14(1):42-61
The convective stability of a simple model chromosphere is investigated. The model chromosphere consists of protons, electrons, and hydrogen atoms in the ground state; ionization is collisional and recombination is radiative. The analysis indicates stability when the kinetic temperature (T) is less than 17 500K (assuming T increases with height). However, for T > 17 500K, the model chromosphere is overstable in the absence of magnetic fields provided the temperature inversion is sufficiently steep. For smaller values of the temperature gradient, field-free regions are stable if the density is small and monotonically unstable if it is large. In the presence of a magnetic field, the model chromosphere is monotonically unstable for T > 17 500K, regardless of the temperature gradient.The convective instability of the model chromosphere results from the fact that the plasma is thermally unstable for T > 17 500K. Thermally unstable regions of the solar atmosphere, although not represented in detail by the model, should behave in a similar fashion.Field-free regions of the solar chromosphere are probably not monotonically unstable, but overstability is possible and may explain the origin of chromospheric oscillations with periods less than 200 sec. It is suggested that spicules result from the monotonic instability of magnetic regions. A similar instability in the corona may be responsible for the large Doppler spreading of radar echoes.Elementary considerations of thermal balance predict that the temperature gradient should diverge at levels of marginal stability. The chromospheric region of spicule formation and the corona should therefore both be bounded below by abrupt temperature jumps. 相似文献
18.
The processes by which energetic electrons lose energy in a weakly ionized gas of carbon dioxide are discussed and a consistent set of electron impact cross-sections is compiled. Calculations of the excitations, ionizations and neutral particle heating produced by the absorption of electrons in carbon dioxide gas are carried out for fractional ionizations ranging from 10?2 to 10?6. 相似文献
19.
The thermal balance of the plasma in the night-time mid-latitude F2-region is examined using solutions of the steady-state O+ and electron heat balance equations. The required concentrations and field-aligned velocities are obtained from a simultaneous solution of the time-dependent O+ continuity and momentum equations.The results demonstrate the systematic trend for the O+ temperature to be 10–20 K greater than the electron temperature during the night at around 300 km, as observed at St. Santin by Bauer and Mazaudier. It is shown that frictional heating between the O+ and neutral gases is the cause of the O+ temperature being greater than the electron temperature; the greater the importance of frictional heating in the thermal balance the greater is the difference in the O+ and electron temperatures. A study is made of the roles played in the thermal balance of the plasma by the thermal conductivity of the O+ and electron gases; collisional heat transfer between O+ electrons and neutrals; frictional heating between the O+ and neutral gases; and advection and convection due to field-aligned O+ and electron motions. The results of the study show that, at around 300 km, electron cooling by excitation of the fine structure of the ground state of atomic oxygen plays a major role in the thermal balance of the electrons and, since the temperature of the ions is little affected by this electron cooling process, in determining the difference between the ion and electron temperatures. 相似文献
20.
K. Ohki 《Solar physics》1975,45(2):435-452
Interferometric radio observations together with soft X-ray observations are presented here to show that during the growth phase of soft X-ray flares, a large mass increase occurs simultaneously with the creation of an X-ray hot region in the corona. The lack of an increase of radio flux from pre-flare active regions absolutely excludes the possibility of the coronal accumulation of low-temperature matter just prior to flare onset. Therefore we suggest a hypothesis that a large amount of hot matter, which contains almost the entire energy in the flare, is supplied from the chromosphere into the corona during each flare. Since even small flares produce coronal hot regions radiating thermal soft X-rays and microwaves, the formation of the hot region may be a basic process in most flares. Energy, created by some instability in the corona, travels by thermal conduction to the chromosphere where the dense matter is heated and subsequently expands into the corona, producing the observed hot region. Impulsive heating of the chromosphere by nonthermal electrons which simultaneously emit hard X-rays is not sufficient to be the energy source in our model. Slower heating, which supplies the flare more energy than that supplied in the impulsive phase, is required. If the temperature of the energy source in the corona exceeds 2 × 107 K, the conductive energy flux becomes sufficient to exceed the radiation loss from the chromosphere-corona transition region. This excess energy may cause the chromospheric gas expansion. 相似文献