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1.
X-ray and EUV observations of the solar corona reveal a very complex and dynamic environment where there are many examples of structures that are believed to outline the Sun's magnetic field. In this present study, the authors investigate the temporal response of the temperature, density and pressure of a solar coronal plasma contained within a magnetic loop to an intermittent heating source generated by Ohmic dissipation. The energy input is produced by a one-dimensional MHD flare model. This model is able to reproduce some of the statistical properties derived from X-ray flare observations. In particular the heat deposition consists of both a sub-flaring background and much larger, singular dissipative events. Two different heating profiles are investigated: (a) the spatial average of the square of the current along the loop and (b) the maximum of the square of the current along the loop. For case (a), the plasma parameters appear to respond more to the global variations in the heat deposition about its average value rather than to each specific event. For case (b), the plasma quantities are more intermittent in their evolution. In both cases the density response is the least bursty signal. It is found that the time-dependent energy input can maintain the plasma at typical coronal temperatures. Implications of these results upon the latest coronal observations are discussed. 相似文献
2.
T. Takakura 《Solar physics》1984,91(2):311-324
In some gradual hard X-ray bursts with high intensity, hard X-ray source (15–40 keV) is steadily located in the corona along with softer X-ray source (5–10 keV).Two stationary models, high density and high temperature models, are proposed to solve the difficult problem of confinement of hot (or nonthermal) plasma in the direction of the magnetic field along the loops in the corona. In both models, an essential point is that the effective X-ray source is composed of fine dense filamentary loops imbeded in a larger rarefied coronal loop, and the electron number density in the filaments is so high as 1011–1012 cm-3. If the density is so high heat conduction can be as reasonably small as of the order of 1027 erg s -1 for the given emission measures of observed X-rays, since the required cross-sectional area is small and also classical conduction is valid. Collisional confinement of thermal tail, and nonthermal electrons if any, up to 50–60 keV in the filaments is also possible, so that the hard X-ray images can be loop like structure instead of double source (foot points).High density model is applicable to the coronal filamentary loops with temperature T
m < 5 × 107 K at the loop summit. The heat flow from the summit downwards is lost almost completely by the radiation from the loop during the conduction to the foot points. A continuous energy release is assumed near the summit to maintain the stationary temperature T
m, and pressure balance is maintained along the loop. In this model, the number density at the summit is given by n
m - 106
T
m
2
/sm, where s
m is the length of the loop from the summit to the foot point, and the distribution of temperature and density along the loop are given by T = T
m(s/sm)1/3 and n = n
m(s/sm)-1/3, respectively.High temperature model is applicable to the filamentary loops with higher temperature up to about 108.5 K and comparatively lower number density as 1011 cm-3 for the requirement of magnetic confinement of the hot plasma in radial direction. The radiation from the loop is negligibly small in this model so that the heat flux is nearly conserved down to the foot points. In this case, temperature gradient is smaller than that of the high density model, depending on the tapering of the magnetic bottle.In both models, the differential emission measure is maximum at the highest temperature T
m and the brightness distribution along the loop shows a maximum around the summit of the loop if some magnetic tapering is taken into account. 相似文献
3.
In recent papers by Priest et al., the nature of the coronal heating mechanism in the large-scale solar corona was considered. The authors compared observations of the temperature profile along large coronal loops with simple theoretical models and found that uniform heating along the loop gave the best fit to the observed data. This then led them to speculate that turbulent reconnection is a likely method to heat the large-scale solar corona. Here we reconsider their data and their suggestion about the nature of the coronal heating mechanism. Two distinct models are compared with the observations of temperature profiles. This is done to determine the most likely form of heating under different theoretical constraints. From this, more accurate judgments on the nature of the coronal heating mechanism are made. It is found that, due to the size of the error estimates in the observed temperatures, it is extremely difficult to distinguish between some of the different heat forms. In the initial comparison the limited range of observed temperatures (T>1.5 MK) in the data sets suggests that heat deposited in the upper portions of the loop, fits the data more accurately than heat deposited in the lower portions. However if a fuller model temperature range (T<1.0 MK) is used results in contridiction to this are found. In light of this several improvements are required from the observations in order to produce theoretically meaningful results. This gives serious bounds on the accuracy of the observations of the large-scale solar corona in future satellite missions such a Solar-B or Stereo. 相似文献
4.
It has been proposed that the million-degree temperature of the corona is due to the combined effect of barely detectable
energy releases, called nanoflares, that occur throughout the solar atmosphere. Unfortunately, the nanoflare density and brightness
implied by this hypothesis means that conclusive verification is beyond present observational abilities. Nevertheless, we
investigate the plausibility of the nanoflare hypothesis by constructing a magnetohydrodynamic (MHD) model that can derive
the energy of a nanoflare from the nature of an ideal kink instability. The set of energy-releasing instabilities is captured
by an instability threshold for linear kink modes. Each point on the threshold is associated with a unique energy release;
thus we can predict a distribution of nanoflare energies. When the linear instability threshold is crossed, the instability
enters a nonlinear phase as it is driven by current sheet reconnection. As the ensuing flare erupts and declines, the field
transitions to a lower energy state, which is modelled by relaxation theory; i.e., helicity is conserved and the ratio of current to field becomes invariant within the loop. We apply the model so that all
the loops within an ensemble achieve instability followed by energy-releasing relaxation. The result is a nanoflare energy
distribution. Furthermore, we produce different distributions by varying the loop aspect ratio, the nature of the path to
instability taken by each loop and also the level of radial expansion that may accompany loop relaxation. The heating rate
obtained is just sufficient for coronal heating. In addition, we also show that kink instability cannot be associated with
a critical magnetic twist value for every point along the instability threshold. 相似文献
5.
Radiative-convective equilibrium models for Jupiter and Saturn have been produced in a study centered primarily on the stratospheric energy balance and the possible role of aerosol heating. These models are compared directly to the thermal structure profiles obtained from Voyager radio occultation measurements. The method is based on a straightforward flux divergence formulation derived from earlier work (J. S. Hogan, S. I. Rasool, and T. Encrenaz 1969, J. Atmos. Sci.26, 898–905). The balance between absorbed and emitted energies is computed iteratively at each level in the atmosphere, assuming local thermodynamic equilibrium and employing a standard treatment of opacities. Results for Jupiter indicate that a dust-free model (no aerosol heating) furnishes a good mean thermal profile for the stratosphere when compared with the Voyager 1 radio occultation (RSS) measurements. These observations of the equatorial region (0° and 12°S, respectively) exhibit periodic vertical structure. Of course, among many possible complications, the Voyager profiles may not represent typical excursions from the mean. The aerosol heat depositions required to match these profiles exactly, relative to the nominal dust-free model, are reasonably consistent with independent estimates for “continuum” absorbers. Other interpretations are discussed, along with a survey of problems encountered in intercomparing the lower portions (P ? 300 mb) of the models, the RSS profiles, and a recent IRIS equatorial profile. Although aerosol heating cannot be ruled out at low latitudes on Jupiter, our results indicate that it may not be required to reproduce the Voyager 1 RSS profiles. On the other hand, heating by aerosols or some other absorber seems necessary in order to match the high-latitude Voyager 2 RSS temperature profile. The Saturn models are relatively simple and in good-to-excellent agreement with the Voyager 2 RSS profiles at all levels. Our comparisons indicate that aerosol heating played a minor role in Saturn's midlatitude stratospheric energy balance at the time of the Voyager 2 encounter. These models, however, may need to be reassessed once the hydrocarbon concentrations have been more precisely determined. 相似文献
6.
Temperature distribution in the cylindrically symmetric coronal magnetic loop, (i) with constant pressure and (ii) with the pressure varying along the radial distance, of the (a) hotter apex and (b) cooler apex than base is investigated analytically by considering the equilibrium between the heat conduction and radiation loss. If the temperature of the loop does not lie within one of the specified temperature ranges, then the distribution is calculated numerically.The effect of the inclusion of heating due to an external source is studied and found that it increases the length of the loop. On the basis of the observed phenomenon, that the magnetic field varies along the loop, the temperature distribution in the loop is investigated for the loop-geometries proposed by Antiochos and Sturrock (1976). It is concluded that for the larger compression in the area of cross section, the height of the loop decreases.Present investigation shows that no loop with equal apex and base temperatures can exist, but a small variation between the two temperatures supports the existence of the loop, which can be observed in nature. 相似文献
7.
The interpretation of red- and blueshifts in EUV spectral observations remains a challenge that could provide important clues to the heating processes in the solar atmosphere. Hinode/EUV Imaging Spectrometer (EIS) observations near the footpoints of coronal loops show blueshifts for emission lines with temperatures above 1 MK and redshifts for lines below 1 MK. The implications are addressed through numerical modeling of loop dynamics. The simulation results are converted into synthetic EIS observations. A single one-dimensional loop cannot reproduce the observed behavior. However, persistent red- and blueshifts can be understood as a collective spectral signature of a bundle of 10 or more loops that have an average temperature of around 1 MK and evolve in a similar way: small-scale heating events occur randomly along each loop on a timescale of several minutes. Strong blueshifts are accompanied by low intensities. The power-law index of the energy distribution has a minor role in determining the average Doppler shifts. 相似文献
8.
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
9.
In investigating the effects of collision Alfvén waves on the heating of a cool-type solar loop, like the post-flare loop, models are proposed, and the distributions of ion or electron density, temperature, pressure, and wave energy density are simulated. We assumed the magnetic field strength in the loop is about 100 G and found that Alfvén waves can propagate through the whole loop, that is to say, the decay length of collision Alfvén waves which we consider can reach to the height or length of the loop. Thus, the Alfvén wave heating is a considerable heating mechanism in cool loops. And we also found that the variations of density, pressure, and wave energy density are more significant than those of the temperature. In the whole loop, the temperature is of the order of 104 K. In comparison with other parameters, the temperature can be considered as homogeneous; hence, the heat conductive flux in the simulations is omitted. 相似文献
10.
《Icarus》1987,70(1):78-98
The discovery of large volcanic eruptions on Io suggests that Io is one of the most geologically active planetary bodies. The energy source of this geologic activity is believed to be tidal heating induced by Jupiter. A number of thermal history calculations were done to investigate the effect of tidal heating on the thermal history of Io taking into account solid state convection and advective heat transfer. These simulations show that the total tidal heating energy in Io is almost equal to the advectively transferred heat, indicating that the observed heat flow from Io is nearly equal to the total tidal heating energy. Since total tidal heating energy is dependent on the radius of the liquid mantle and the internal dissipation factor (Q), the radius of the liquid mantle can be estimated for a given value of Q. Some reasonable thermal history models of Io were obtained using a model with Q ≈ 25–50 in which the magma source of Ionian volcanism is at a depth of 100–300 km. The models satisfy the heat flow data and the existence of a thick lithosphere. Using a model with Q = 25 and L = 300 km (thickness of the advective region) as the standard model (model II), we then studied the effect of convective heat transfer and the initial temperature distribution on the Ionian thermal history. In these calculations, the other parameters are the same as in the standard model (model II). These calculations show that although the temperature distribution in the central region reflects the difference in the efficiency of convective heat transfer and initial temperature distribution, the temperature distribution in the outer region does not changes appreciably. 相似文献
11.
Numerical solutions are presented for the summit temperature and heating in a thermally isolated coronal loop that is in hydrostatic equilibrium. The extent to which gravity modifies the usual uniform-pressure scaling law is shown, and plots of the differential emission measure are also given. 相似文献
12.
In coronal holes the electron (proton) density is low, and heating of the proton gas produces a rapidly increasing proton temperature in the inner corona. In models with a reasonable electron density in the upper transition region the proton gas becomes collisionless some 0.2 to 0.3 solar radii into the corona. In the collisionless region the proton heat flux is outwards, along the temperature gradient. The thermal coupling to electrons is weak in coronal holes, so the heat flux into the transition region is too small to supply the energy needed to heat the solar wind plasma to coronal temperatures. Our model studies indicate that in models with proton heating the inward heat conduction may be so inefficient that some of the energy flux must be deposited in the transition region to produce the proton fluxes that are observed in the solar wind. If we allow for coronal electron heating, the energy that is needed in the transition region to heat the solar wind to coronal temperatures, may be supplied by heat conduction from the corona. 相似文献
13.
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
14.
Equations of thermal equilibrium along coronal loops are solved in the absence of gravity but where the cross-sectional area changes along the loop. The footpoint temperature is assumed to be 2 × 104 K. Several fundamental types of solution are found, namely hot loops, cool loops, hot-cool loops (where the footpoints and summits are cool but the intermediate parts are hotter) and warm loops (cool along most of their lengths except the summits). On increasing the cross-sectional area the summit temperature generally increases slightly except for warm loops where no increase in temperature is recorded and hot-cool loops where a dramatic increase in summit temperature may occur. The cool and hot-cool loops may model elementary fibril structures within prominences. 相似文献
15.
Donat G. Wentzel 《Solar physics》1976,50(2):343-360
I extend a previous paper which argued that Alfvén waves traveling up a large coronal loop may heat this loop at the top and increase its visibility. This heating is now evaluated more completely, taking into account the changes along the loop in field strength, gas density and flux of waves. The location and efficiency of the heating depend very non-linearly on the intensity of the waves, which allows rapid changes in the visibility of a loop. Observational and theoretical conditions for the applicability of the theory are summarized. Alfvén waves preferentially heat the upper portions of coronal helmets, but a measurable excess temperature on a loop requires somewhat implausibly high wave fluxes. Radiation losses from low-lying loops with strong magnetic fields cannot be explained without modifying the theory.The National Center for Atmospheric Research is sponsored by the National Science Foundation. 相似文献
16.
Many coronal heating mechanisms have been suggested to balance the losses from this tenuous medium by radiation, conduction, and plasma mass flows. A previous paper (Walsh, Bell, and Hood, 1995) considered a time-dependent heating supply where the plasma evolved isobarically along the loop length. The validity of this assumption is investigated by including the inertial terms in the fluid equations making it necessary to track the sound waves propagating in a coronal loop structure due to changes in the heating rate with time. It is found that the temperature changes along the loop are mainly governed by the variations in the heating so that the thermal evolution can be approximated to a high degree by the simple isobaric case. A typical isobaric evolution of the plasma properties is reproduced when the acoustic time scale is short enough. However, the cooling of a hot temperature equilibrium to a cool one creates supersonic flows which are not allowed for in this model. 相似文献
17.
A slowly evolving, flaring loop was observed by the UVSP, XRP, and HXIS instruments onboard SMM on June 10, 1980. Simultaneous radio observations from Toyokawa (Japan) are also available. The SMM instruments have an angular resolution ranging from 3 to 30 arc sec by which the loop structure may be determined. It appears that these observations cannot be accounted for by a single loop model even assuming a variable temperature and pressure. The additional presence of a hot and tenuous isothermal plasma is necessary to explain the harder emission (HXIS). X-ray and UV data are used to fit the differential emission measure as a function of temperature and a model of the flare is deduced, which is then checked against radio data. An estimate of the heating function along the loop and of the total energy content of the loop is also given. 相似文献
18.
Thermal models of planetary atmospheres can be calculated from assumptions of the energy budget of the atmosphere and from the knowledge of the effective temperature of the studied planet. On the other hand, the retrieval of the thermal atmospheric profiles from infrared measurements by means of the numerical inversion of the radiative transfer equation presents the advantages of not requiring such assumptions. The extent of the atmospheric range which can then be sounded is examined and the vertical resolution of the inferred profiles is discussed. Comparisons of thermal models and retrieved thermal profiles are made for the four giant planets. The retrieved profiles lead to brightness temperature spectra which fit all the available infrared measurements fairly well for Jupiter and Saturn but only part of them for Uranus and Neptune. The values of the planetary effective temperatures calculated from the retrieved profiles show that Jupiter, Saturn, and Neptune have strong internal heating sources while Uranus probably has a very small or null one. 相似文献
19.
We have shown that a current-carrying plasma loop can be heated by magnetic pinch driven by the pressure imbalance between inside and outside the loop, using a 3-dimensional electromagnetic (EM) particle code. Both electrons and ions in the loop can be heated in the direction perpendicular to the ambient magnetic field, therefore the perpendicular temperature can be increased about 10 times compared with the parallel temperature. This temperature anisotropy produced by the magnetic pinch heating can induce a plasma instability, by which high-frequency electromagnetic waves can be excited. The plasma current which is enhanced by the magnetic pinch can also excite a kinetic kink instability, which can heat ions perpendicular to the magnetic field. The heating mechanism of ions as well as the electromagnetic emission could be important for an understanding of the coronal loop heating and the electromagnetic wave emissions from active coronal regions. 相似文献
20.
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