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1.
We studied the morphology and spatial distribution of loops in an active region, using coordinated observations obtained with both the S082A XUV spectroheliograph and the S056 grazingincidence X-ray telescope on Skylab. The active region loops in the temperature range 5 × 105 –3 × 106 K fall basically into two distinctive groups: the hot loops with temperatures 2–3 × 106 K as observed in coronal lines and X-rays, and the relatively cool loops with temperature 5 × 105 –1 × 106 K as observed in transition-zone lines (Ne vii, Mg ix). The brightest hot coronal loops in the active region are mostly low-lying, compact, closely-packed, and show greater stability than the transition-zone loops, which are fewer in number, large, and slender. The observed aspect ratio of the hot coronal loops is in the range of 0.1 and 0.2, which are almost two orders of magnitude larger than those for the Ne vii loops. Brief discussion of the MHD stability of the loops in terms of the aspect ratio is presented. 相似文献
2.
An emission measure analysis is performed for the Prominence-Corona Transition Region (PCTR) under the assumption that the cool matter of quiescent filaments is contained in long, thin magnetic flux loops imbedded in hot coronal cavity gas. Consequently, there is a transition region around each thread.Comparison of the model and observations implies that the temperature gradient is perpendicular to the magnetic lines of force in the lower part of the PCTR (T < 105 K). It is shown that in this layer the heating given by the divergence of the transverse conduction fails to account for the observed UV and EUV emission by several orders of magnitude. It is, therefore, suggested that the heating of these layers could be due to dissipation of Alfvén waves.In the high-temperature layers (T 105 K), where the plasma 1, the temperature gradient is governed by radiative cooling balancing conductive heating from the surrounding hot coronal gas. Also in these outer layers the presence of magnetic fields reduces notably the thermal conduction relative to the ideal field-free case. Numerical modelling gives good agreement with observed DEM; the inferred value of the flux carried by Alfvén waves, as well as that of the damping length, greatly support the suggested form of heating. The model assumes that about 1/3 of the volume is occupied by threads and the rest by hot coronal cavity matter.The brightness of the EUV emission will depend on the angle between the thread structure and the line of sight, which may lead to a difference in brightness from observations at the limb and on the disk. 相似文献
3.
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
4.
A coronal magnetic arcade can be thought of as consisting of an assembly of coronal loops. By solving equations of isobaric thermal equilibrium along each loop and assuming a base temperature of 2 × 104 K, the thermal structure of the arcade can be found. The possible thermal equilibria can be shown to depend on two parameters L
*
p
* and h
*/p
* representing the ratios of cooling (radiation) to condu and heating to cooling, respectively. Arcades can contain four types of loops: hot loops with summits hotter than 400000 K; cool loops at temperatures less than 80000 K along their lengths; hot-cool loops with cool summits and cool footpoints but hotter intermediate portions; and warm loops, cooler than 80000 K along most of their lengths but with summits as hot as 400000 K. Two possibilities for coronal heating are considered, namely a heating that is independent of magnetic field and a heating that is proportional to the square of the local magnetic field. When the arcade is sheared the thermal structure of the arcade may change, leading in some cases to non-equilibrium or in other cases to the formation of a cool core. 相似文献
5.
Using measurements of EUV and X-ray spectral lines we derive the differential emission measure vs electron temperature T from the transition region to the corona of an active region (105
T <5 × 106 K). The total emission measure and radiative losses are of order 3 × 1048 cm–3 and 4 × 1026 ergss–1 respectively. The emission measure at T > 106 K (i.e. that mainly responsible for the X-ray emission) is about 75% of the total. We also examine the use of Mg x 625 Å as an indicator of coronal electron density. A set of theoretical energy balance models of coronal loops in which the loop divergence is a variable parameter is presented and compared with the observations. Particular attention is given to the limitations inherent in any such comparison. 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.
Solar plasma that exists at around 105 K, which has traditionally been referred to as the solar transition region, is probably in a dynamic and fibril state with a small filling factor. Its origin is as yet unknown, but we suggest that it may be produced primarily by one of five different physical mechanisms, namely: the heating of cool spicular material; the containment of plasma in low-lying loops in the network; the thermal linking of cool and hot plasma at the feet of coronal loops; the heating and evaporating of chromospheric plasma in response to a coronal heating event; and the cooling and draining of hot coronal plasma when coronal heating is switched off. We suggest that, in each case, a blinker could be produced by the granular compression of a network junction, causing subtelescopic fibril flux tubes to spend more of their time at transition-region temperatures and so to increase the filling factor temporarily. 相似文献
8.
A model of filament formation based on the condensation of coronal arches is described. The condensation results from initiating the radiative instability within an arch by superimposing a transient energy supply upon the steady state heating mechanism. The transient energy supply increases the density within the arch so that when it is removed the radiative losses are sufficient to lead to cooling below the minimum in the power loss curve.Times from the initial formation of the condensation to its temperature stabilization as a cool filament have been calculated for various initial conditions. They lie in the range 104 to 105 s with the majority of the time spent above a temperature of 1 × 106 K.Under the assumption that the condensation of a single arch forms an element of the filament, a complete filament requires the condensation of an arcade of loops. Using experimentally derived parameters, filament densities of 1011 to 1012 cm–3 can be obtained. 相似文献
9.
White-light flares are defined as those flares that produce significant enhancement of emission in the visible light continuum. The source of energy for this emission has not yet been identified with several possibilities being suggested: heating of the lower chromosphere by some mechanical or magnetic means, or by soft X-ray or extreme ultraviolet radiation from coronal loops being absorbed in the lower chromosphere and re-emitted in the visible.Using non-LTE radiative transfer calculations for hydrogen and helium in a simple model atmosphere we show that EUV ( < 912 Å) radiation cannot be the main energy source for white-light flares. Estimates of the observed energy emitted in the visible and the EUV indicate that there may be enough energy in the EUV to account for the white light flare with this mechanism. Using enhancements in the wavelength region below 912 Å of up to 7 × 109 ergs cm–2 s–1 ster–1 (5 × 105 times the estimated q radiation field) to represent flare EUV emission from above we investigated the non-LTE level populations for hydrogen and helium and the lower atmospheric heating resulting from this radiation. The basic result is that the opacities in the Lyman continuum and the helium I and II continua are so much larger than even the enhanced opacity in the visible hydrogen continuum that the EUV radiation is absorbed before it can have a significant effect in the visible light continuum. However, the EUV radiation can cause a significant enhancement of H emission.Operated by the Association of Universities for Research in Astronomy Inc. for the National Aeronautics and Space Administration. 相似文献
10.
Van Driel-Gesztelyi L. Wiik J.E. Schmieder B. Tarbell T. Kitai R. Funakoshi Y. Anwar B. 《Solar physics》1997,174(1-2):151-162
Observations of the post-flare loops after the X3.9 flare which occurred on 25 June, 1992 at 20:11 UT by the Yohkoh/SXT in X-rays, as well as in H obtained at 5 different observatories, have provided a unique, longest ever, set of data for a study of the relationship between the hot and cool post-flare loops as they evolve. At any given time, the altitude difference between the hot X-ray loops of 6–7× 106 K and the cool H loops of 1.5× 104 K is related to the expansion rate of the loop systems and their cooling time. Therefore, measurements of the expansion rate and relative height of hot and cool loops can provide direct observational values for their cooling times. We measured the altitude of hot and cool loops for 15 and 19 hours, respectively, and found that the cooling time increased as the density of the loops decreased. We found a reasonably good agreement between the observed cooling times and those obtained from model calculations, although the observed values were always somewhat longer than the theoretical ones. Taking into account evolutionary effects, we also found similar shapes and configurations of hot and cool loops during the entire observing period and confirmed that, at any time, hot loops are at higher altitude than cool loops, suggesting that cool loops indeed evolve from hot loops. These results were used to check the validity of the reconnection model. 相似文献
11.
Equations of thermal equilibrium along coronal loops with footpoint temperatures of 2 × 104 K are solved. Three fundamentally different categories of solution are found, namely hot loops with summit temperatures above about 4 × 105 K, cool loops which are cooler than 8 × 104 K along their whole length and hot-cool loops which have summit temperatures around 2 × 104 K but much hotter parts at intermediate points between the summit and the footpoints. Hot loops correspond to the hot corona of the Sun. The cool loops are of relevance for fibrils, for the cool cores observed by Foukal and also for active-region prominences where the magnetic field is directed mainly along the prominence. Quiescent prominences consist of many cool threads inclined to the prominence axis, and each thread may be modelled as a hot-cool loop. In addition, it is possible for warm loops at intermediate summit temperatures (8 × 104K to 4 × 105 K) to exist, but the observed differential emission measure suggests that most of the plasma in the solar atmosphere is in either the hot phase or the cool phase. Thermal catastrophe may occur when the length or pressure of a loop is so small that the hot solution ceases to exist and there are only cool loop solutions. Many loops can be superimposed to form a coronal arcade which contains loops of several different types. 相似文献
12.
Yohkoh and the Coronal Diagnostic Spectrometer (CDS) on the Solar and Heliospheric Observatory (SOHO) jointly observed two
brightenings in active region NOAA 7981 on 6 August 1996. Combining the UV data from CDS with information from the high time
resolution coronal images obtained with the Soft X-ray Telescope (SXT) on Yohkoh, provides us with important information on
the relationship between the transition region and corona. Our observations show that cool plasma (Te = 2.2 x 10-5 K) can lie at the same altitude as the hot coronal plasma (Te = 1–4 x 106 K). The lower temperature structure is not formed from the cooling of the hotter coronal loop. We are also able to observe
a low temperature cut-off of Te = 1–4 x 106 K for a loop which repeatedly brightened over the period of approximately one day. 相似文献
13.
B. Schmieder P. Heinzel J. E. Wiik J. Lemen B. Anwar P. Kotrč E. Hiei 《Solar physics》1995,156(2):337-361
We have analyzed the physical conditions of the plasma in post-flare loops with special emphasis on dynamics and energy transport using SXT-data (hot plasma) and optical ground-based data from Pic du Midi, Wrocaw, and Ondejov (cool plasma). By combining the H observations with the SXT images we can understand the relationship between cool and hot plasmas, the process of cooling post-flare loops and the mechanism which maintains the long duration of these loops. Using recent results of NLTE modeling of prominence-like plasmas, we derive the emission measure of cool H loops and this gives us a realistic estimate of the electron density (2.2 × 1010 cm–3). Then, by comparing this emission measure with that of hot loops derived from SXT data, we are able to estimate the ratio between electron densities in hot and cool loops taking into account the effect of geometrical filling factors. This leads to the electron density in hot loops 7 × 109 cm–3. We also derive the temperature of hot X-ray loops ( 5.5 × 106 K), which, together with the electron density, provides the initial values for solving the time-dependent energy balance equation. We obtain the cooling times which are compared to a typical growth-time of the whole loop system ( 2000 s). In the legs of cool H loops, we observe an excess of the emission measure which we attribute to the effect of Doppler brightening (due to large downflow velocities). 相似文献
14.
Although back conduction from the corona has been shown to be inadequate for powering EUV emission below T 2 × 105 K, it is thought to be adequate in the temperature range 2 × 105 K < T < 106 K. No models to date, however, have included the large magnetic constriction which should occur in the legs of coronal loops where conductive transition regions, hitherto thought to contain the bulk of the plasma in this higher temperature range, are located. On the basis of fine scale magnetograms, Dowdy et al. (1986) have estimated that these magnetic flux tubes are constricted from end to end by an areal factor of approximately 100. Furthermore, on the basis of simple steady-state conductive models, Dowdy et al. (1985) have shown that the large constriction can inhibit the conductive flow of heat by an order of magnitude. We are thus led to re-examine static models of this region of the atmosphere which incorporate not only conduction and radiation but also the effects of large magnetic constrictions. We find that the structure of this plasma depends not only on the magnitude of the constriction but also on the tube's shape.Our results show that no model with a constriction of order 100 can simultaneously (a) produce the variation of differential emission measure with temperature derived from measured line intensities and (b) satisfy the observed constraint (Reeves, 1976) that EUV emission from below T 7 × 105 K be confined to the supergranular network, covering no more than 0.45 of the solar surface. The failure of the models suggests that the bulk of the 105–106 K plasma in the quiet solar atmosphere is not in transition region structures, but is instead magnetically isolated from the corona and heated internally. Even though the transition region component of 105–106 K plasma in the legs of coronal loops should exist, it comprises only a small fraction of the total 105–106 K plasma and, hence, produces only a small fraction of the observed EUV emission from this temperature range.We also find that for any permitted tube shape, constriction factors of order 100 reduce the coronal conductive energy losses to the transition region to a value which is less than a third of the value for an unconstricted field, i.e., to less than 2 × 105 erg cm –2 s –1. In particular, if the magnetic geometry of the upper transition region is extremely concave (i.e., horn-shaped geometry with most of the areal divergence near the hot end), then a constriction of order 100 results in a conductive loss of less than 1 × 104 erg cm–2 s–1 and, hence, much less than the coronal radiative energy loss. For such geometries, the constriction in the magnetic field hence provides an effective thermal insulation of the corona from the cooler parts of the solar atmosphere.Presidential Young Investigator. 相似文献
15.
We present a detailed study of coronal loop brightenings observed in an active region on the solar limb. These brightening loops show expanding and shrinking motions in EUV coronal line images and also show downflow along the loops in Lα and Hα images. By means of time-slice analysis of the images, we found that both the expanding and shrinking motions of the loops are not real motions of plasma but apparent motions like post-flare loops, where the loops at the different height are successively heated and cooled. From a temperature analysis, the time delay between the brightenings of hot 195 Å and cool Lα loops is found to be nearly equal to the time-scale of the conduction cooling. We conclude that these loop brightenings are sources of so called Hα coronal rains. 相似文献
16.
Thomas R. Ayres 《Solar physics》1980,68(1):125-133
I examine the controversial problem of H– radiative cooling in the solar chromosphere. I find, in agreement with Praderie and Thomas, that H– is a substantial source of radiative heating in the outer atmosphere, especially when departures from LTE are important. The role of H– as a chromospheric heating agent must be considered carefully before net radiative cooling rates can be assessed from empirical chromospheric models, or calculations of nonradiative heating, for example by acoustic waves, can be pursued meaningfully. 相似文献
17.
This paper reports results of an analysis of Skylab observations of coronal bright points made in EUV spectral lines formed in the chromosphere, chromospheric-coronal transition region and corona. The most important result is that the observed bright points exhibited large variations in EUV emission over time scales as short as 5.5 min, the temporal resolution of the data. In most cases strong enhancements in the coronal line were accompanied by strong enhancements in the chromospheric and transition region lines. The intensity variations appear to take place within substructures of the bright points, which most likely consist of miniature loops evolving on time scales of a few minutes. Coronal cooling times derived from the data are consistent with an intermittent, impulsive coronal heating mechanism for bright points. 相似文献
18.
The total radiative output in the EUV continuum (1400–1960 Å) from the 5 September 1973 flare has been obtained from the EUV spectra of the flare observed with the NRL slit spectrograph (SO82B) on Skylab. The radiative energy in the EUV continuum is of the order of 1029 ergs, which is more than a factor of 2 greater than those radiated in soft X-rays (8–20 Å) and in H for the flare. Thus, the EUV continuum emission is an important radiative energy loss, and should be included in the consideration of the energy balance of the flare.Ball Corporation.Now at the Institute of Theoretical Astrophysics, University of Oslo, Oslo, Norway. 相似文献
19.
Neupert W.M. Newmark J. Delaboudinière J.-P. Thompson B.J. Catura R.C. Moses J.D. Gurman J.B. Portier-Fozzani F. Maucherat A.J. Defise J.M. Jamar C. Rochus P. Dere K.P. Howard R.A. Michels D.J. Freeland S. Lemen J.R. Stern R.A. 《Solar physics》1998,183(2):305-321
Solar EUV images recorded by the EUV Imaging Telescope (EIT) on SOHO have been used to evaluate temperature and density as a function of position in two largescale features in the corona observed in the temperature range of 1.0–2.0 MK. Such observations permit estimates of longitudinal temperature gradients (if present) in the corona and, consequently, estimates of thermal conduction and radiative losses as a function of position in the features. We examine two relatively cool features as recorded in EIT's Feix/x (171 Å) and Fexii (195 Å) bands in a decaying active region. The first is a long-lived loop-like feature with one leg, ending in the active region, much more prominent than one or more distant footpoints assumed to be rooted in regions of weakly enhanced field. The other is a near-radial feature, observed at the West limb, which may be either the base of a very high loop or the base of a helmet streamer. We evaluate energy requirements to support a steady-state energy balance in these features and find in both instances that downward thermal conductive losses (at heights above the transition region) are inadequate to support local radiative losses, which are the predominant loss mechanism. The requirement that a coronal energy deposition rate proportional to the square of the ambient electron density (or pressure) is present in these cool coronal features provides an additional constraint on coronal heating mechanisms. 相似文献
20.
We present here the results of emission tomography studies, based on a new differential deconvolution method (DDM) of Laplace transform inversion, which we use for reconstruction of the coronal emission measure distributions in the quiet Sun, coronal holes and plage areas. Two methods are explored. The first method is based on the deconvolution of radioemission brightness spectra in a wide wavelength range (1 mm–100 cm) for temperature profile reconstructions from the corona to the deeper chromosphere. The second method uses radio brightness measurements in the cm–dm range to give a coronal column emission measure (EM).Our results are based on RATAN-600 observations in the range 2.0–32 cm supplemented by the data of other observatories during the period near minimum solar activity. This study gives results that agree with known estimates of the coronal EM values, but reveals the absence of any measurable quantities of EM in the transition temperature region 3 × 104 –105 K for all studied large-scale structures. The chromospheric temperature structure (T
e = 20,000–5800 K) is quite similar for all objects with extremely low-temperature gradients at deep layers.Some refraction effects were detected in the decimeter range for all Types of large-scale structures, which suggests the presence of dense and compact loops (up to N
e =(1–3)× 109 cm-3 number density) for the quiet-Sun coronal regions with temperature T
e > 5× 10-5 K. 相似文献