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1.
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
2.
We have investigated numerically how a temperature difference between electrons and protons is produced in a flaring loop by adopting a one-fluid, two-temperature model instead of a single-temperature model. We have treated a case in which flare energy is released in the form of heating of electrons located in the top part of the loop.In this case, a large temperature difference (T
e/T
p 10) appears in the corona in the energy-input phase of the flare. When the material evaporated from the chromosphere fills the corona, the temperature difference in the loop begins to shrink rapidly from below. Eventually, in the loop apex, the proton temperature exceeds the electron temperature mainly due to cooling of the electrons by conduction down the loop and heating of the protons by compression of the ascending material. In the late phase of the flare (t 15 min from the flare onset), the temperature difference becomes less than 2% of the mean temperature of electrons and protons at every point in the loop. 相似文献
3.
The 20 August 1992 flare around 14:28 UT was observed in H, H and Ca ii H with the imaging spectrographs at Locarno-Monti, Switzerland, with the radiotelescopes in Bern, and in soft and hard X-rays by the Yohkoh satellite. In this paper we discuss the analysis of the temporal and spatial evolution of this flare, well observed at chromospheric and coronal layers. We find that the chromospheric electron density shows well-correlated rises with the hard X-rays emphasizing the direct response of the chromosphere to the energy deposition. Although both footpoints of the loops show simultaneous rises of the electron density, non-thermal electron injection is only observed in one of the footpoints, while an additional heating mechanism, like thermal conduction, must be assumed for the other footpoint. However, it is puzzling that all the chromospheric observations in both footpoints are delayed by 3 s compared to the hard X-ray light curve. Although this would be compatible with the thermal heating of one footpoint, it is in contradiction to the non-thermal heating of the other one. Finally, we observed evidence that during the first part of the flare a thermal conduction front propagates at a speed of 2000 km s-1 into a second loop, in which the energy release occurs in the second part of the flare. 相似文献
4.
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. 相似文献
5.
Slow-mode shocks produced by reconnection in the corona can provide the thermal energy necessary to sustain flare loops for many hours. These slow shocks have a complex structure because strong thermal conduction along field lines dissociates the shocks into conduction fronts and isothermal subshocks. Heat conducted along field lines mapping from the subshocks to the chromosphere ablates chromospheric plasma and thereby creates the hot flare loops and associated flare ribbons. Here we combine a non-coplanar compressible reconnection theory with simple scaling arguments for ablation and radiative cooling, and predict average properties of hot and cool flare loops as a function of the coronal vector magnetic field. For a coronal field strength of 100 G the temperature of the hot flare loops decreases from 1.2 × 107 K to 4.0 × 106 K as the component of the coronal magnetic field perpendicular to the plane of the loops increases from 0% to 86% of the total field. When the perpendicular component exceeds 86% of the total field or when the altitude of the reconnection site exceeds 106km, flare loops no longer occur. Shock enhanced radiative cooling triggers the formation of cool H flare loops with predicted densities of 1013 cm–3, and a small gap of 103 km is predicted to exist between the footpoints of the cool flare loops and the inner edges of the flare ribbons. 相似文献
6.
P. Foukal 《Solar physics》1975,43(2):327-336
EUV observations show many active region loops in lines formed at temperatures between 104K and 2×l06K. The brightest loops are associated with flux tubes leading to the umbrae of sunspots. It is shown that the high visibility of certain loops in transition region lines is due principallly to a sharp radial decrease of temperature to chromospheric values toward the loop axis. The plasma density of these cool loops is not significantly greater than in the hot gas immediately surrounding it. Consequently, the internal gas pressure of the cool material is clearly lower. The hot material immediately surrounding the cool loops is generally denser than the external corona by a factor 3–4. When the active region is examined in coronal lines, this hot high pressure plasma shows up as loops that are generally parallel to the cool loops but significantly displaced laterally. In general the loop phenomenon in an active region is the result of temperature variations by two orders of magnitude and density variations of around a factor five between adjacent flux tubes in the corona. 相似文献
7.
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
8.
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
9.
Steven G. Wallenhorst 《Solar physics》1982,79(2):333-341
Models of the solar corona which include the effects of hot downflowing material are considered. Temperature-height profiles of the quiet and flaring corona are derived, under the assumptions of hydrostatic equilibrium and that the dominant cause of transition region heating is due to the enthalpy of the downflowing matter. In addition, scaling laws for the lengths of coronal loops are derived. It is found that inclusion of the downward enthalpy flux leads to a loop scaling law for quiet Sun loops which does not differ appreciably from that of Rosner et al. (1978). However, inclusion of the effects of enthalpy flux lead to a scaling law for compact flare loops of L = (3.6 × 109)T
infc
sup0.55
cm, which predicts much smaller loop sizes than expected from the quiet Sun loop law; these predicted lengths, however, are in agreement with the observed small sizes of compact flare loops. 相似文献
10.
When analyzing YOHKOH/SXT, HXT (soft and hard X-ray) images of solar flares against the background of plasma with a temperature T?6 MK, we detected localized (with minimum observed sizes of ≈2000 km) high-temperature structures (HTSs) with T≈(20–50) MK with a complex spatial-temporal dynamics. Quasi-stationary, stable HTSs form a chain of hot cores that encircles the flare region and coincides with the magnetic loop. No structures are seen in the emission measure. We reached conclusions about the reduced heat conductivity (a factor of ～103 lower than the classical isotropic one) and high thermal insulation of HTSs. The flare plasma becomes collisionless in the hottest HTSs (T>20 MK). We confirm the previously investigated idea of spatial heat localization in the solar atmosphere in the form of HTSs during flare heating with a volume nonlocalized source. Based on localized soliton solutions of a nonlinear heat conduction equation with a generalized flare-heating source of a potential form including radiative cooling, we discuss the nature of HTSs. 相似文献
11.
Z. Švestka 《Solar physics》1973,31(2):389-400
The assumption that the flare originates in the corona or transition layer, is confronted with the known properties of chromospheric flares. It is concluded that the basic mode of the energy transport into chromosphere is heat conduction. Only in some flares non-thermal particles contribute to the brightening in lower atmospheric layers: electrons with energy close to 100 keV produce chromospheric bright patches, and protons above 20 MeV cause the photospheric enhancements. The particle-produced brightenings are superposed on the basic quasi-thermal flare and involve only small areas as compared with the extensive regions heated through conduction.The most probable height of the flare origin appears to be close to the transition layer, between some 4000 and 7000 km above the photosphere. The non-thermal acceleration (when present) occurs probably higher than where the flare originates. There is no obvious reason why the high electron density in chromospheric flares could not be explained as simply due to increased ionization in the existing plasma, without any flare-induced mass condensations.Though there are several facts supporting the flare origin in the corona (or transition layer), one cannot exclude the alternative that the flare instability involves simultaneously a wide (and in different cases different) range of altitudes. Energy considerations give some support to such a supposition.Mitteilungen aus dem Fraunhofer Institut Nr. 121.Visiting scientist at the Fraunhofer Institute, grant of Stifterverband für die Deutsche Wissenschaft. 相似文献
12.
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
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 investigate the structure of slow-mode MHD shocks in a plasma where both radiation and thermal conduction are important. In such a plasma a slow shock dissociates into an extended foreshock, an isothermal subshock, and a downstream radiative cooling region. Our analysis, which is both numerical and analytical, focuses on the nearly switch-off shocks which are generated by magnetic reconnection in a strong magnetic field. These shocks convert magnetic energy into kinetic energy and heat, and we find that for typical flare conditions about f of the conversion occurs in the subshock while the remaining 1/3 occurs in the foreshock. We also find that no stable, steady-state solutions exist for radiative slow shocks unless the temperature in the radiative region downstream of the subshock falls below 105 K. These results suggest that about 2/3 of the magnetic energy released in flare loops is released at the top of the loop, while the remaining 1/3 is released in the legs of the loop. 相似文献
15.
The H analysis of the development of the strong impulsive and faint gradual phase of the June 26, 1983 flare indicates the following: (1) The flare originated from two microprominences on the southeast border of NOAA 4227. Several similar events are summarized in Table II. (2) The main flare structure was a flare cone, which consisted of a bright surge-like stream, elevated above two flare ribbons (located in the cone's base). The flare cone had a height of about 40 × 103 km and lasted 4 min in H. The upper part of the cone was terminated by a very fine loop, which was bent to the west, where later a chromospheric brightening occurred at the footpoint of a flaring arch. A 300 keV burst and radio spikes were observed during the maximum flare phase. (3) The flaring arch system, with its apex at a height of about 48 × 103 km, formed the skeleton for the coronal helmet structure (Figure 7(c)). The velocity of the plasma moving along the flaring arch was between 3500 km s–1} and 6900 km s–1} during the first brightening (14:07 UT). 相似文献
16.
The role of heat flux limitation in soft X-ray emitting solar flare plasmas is considered. Simple analytic arguments suggest that flux limitation is likely to be important during the explosive heating phase, even for relatively modest coronal energy fluxes (say 109 erg cm-2 s-1). This conclusion is reinforced by a detailed flare loop simulation of the heating phase. Since flux saturation effectively bottles up the coronal heat flux, mass motions now assume a dominant role in transferring energy from the coronal flare source to the lower transition region. The mass-energy exchange between the corona and chromosphere produces dramatic changes in the thermal structure of the plasma which are reflected in the differential emission measure profile of the flaring loop. 相似文献
17.
We calculate the spatial structure of hard X-ray emission during the impulsive phase of electron-heated solar flares. Both direct non-thermal bremsstrahlung and the thermal bremsstrahlung arising from the heated plasma are considered. Our results indicate that the spread of non-thermal emission into the upper parts of the loop, through evaporation of the chromospheric target, may be more important than the appearance of a hot thermal source in the corona. The effects of varying the viewing angle to the flare loop, and of finite-size resolution element, are also considered, and we compare our results with observations from the Solar Maximum Mission Hard X-Ray Imaging Spectrometer. We also contrast the predicted structures with those predicted by other models of flare energy release, and it is found that the electron-heated model provides the most satisfactory agreement with the observations.On leave from: Department of Physics and Astronomy, The University, Glasgow G12 8QQ, Scotland, U.K.Presidential Young Investigator. 相似文献
18.
19.
P. Macneice R. Pallavicini H. E. Mason G. M. Simnett E. Antonucci R. A. Shine D. M. Rust C. Jordan B. R. Dennis 《Solar physics》1985,99(1-2):167-188
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. 相似文献
20.
It has been shown that the main problems of the circuit theory of solar flares - unlikely huge current growth time and the origin of the current interruption - have been resolved considering the case of magnetic loop emergence and the correct application of Ohm's law. The generalized Ohm's law for solar flares is obtained. The conditions for flare energy release are as follows: large current value, > 1011 A, nonsteady-state character of the process, and the existence of a neutral component in a flare plasma. As an example, the coalescence of a flare loop and a filament is considered. It has been shown that the current dissipation has increased drastically as compared with that in a completely ionized plasma. The current dissipation provides effective Joule heating of the plasma and particle acceleration in a solar flare. The ion-atom collisions play the decisive role in the energy release process. As a result the flare loop resistance can grow by 8–10 orders of magnitude. For this we do not need the anomalous resistivity driven by small-scale plasma turbulence. The energy release emerging from the upper part of a flare loop stimulates powerful energy release from the chromospheric level. 相似文献