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1.
A hydrodynamic model of high resolution is used to examine the stability of coronal loops to finite amplitude perturbations. The loop is heated by means of a low-amplitude energy input and its subsequent dynamic relaxation is followed.Firstly, the initial atmosphere is generated by solving the time independent form of the hydrodynamic equations. It is shown that the loop structure depends critically on the balance between the radiative losses and the quiescent heating at the base of the transition zone, i.e. on the concavity of the temperature profile in this region. This result already anticipates the need for high spatial resolution across the model transition zone.The dynamic evolution of the loop is then investigated for two classes of lower boundary conditions. In one case the chromospheric temperature is fixed throughout the simulation; in the other the low chromosphere is represented by a rigid insulating barrier. In both cases the loop is found to be stable: The loop is also unique to the extent that it relaxes to a state which is physically indistinguishable from its initial configuration. It is pointed out however, that a loop whose chromosphere is only marginally stable can evolve dynamically away from the initial static configuration.Finally, the observational consequences of the analysis are discussed. The differential emission measure profile is found to change its form as the loop cools, firstly, through an evaporative phase in which the coronal density increases; secondly, through a quasi-steady relaxation in which the enhanced coronal density gradually drains away to the chromosphere. This behaviour represents a possible observational test of the model. 相似文献
2.
The detailed thermal structure of the magnetized solar transition region, as measured by itsdifferential emission measure [DEM(T)], is unknown. Proposals have been made that envision a significant lower-temperature contribution to the energy balance from cross-field (ion) heat flux. In this paper, we describe a self-consistent, 2-D, MHD simulation (including the full effects of anisotropic thermal conduction) of a conceptual model due to Athay (1990). We display the detailed, irregular, thermal and magnetic structure of the transition region, and demonstrate that the predicted DEM agrees with observations, particularly in theT < 105 K regime where previous theories had difficulty. 相似文献
3.
4.
A. G. Hearn 《Solar physics》1977,51(1):159-168
The main differences between a coronal hole and quiet coronal regions are explained by a reduction of the thermal conduction coefficient by transverse components of the magnetic field in the transition region of quiet coronal regions.Calculations of minimum flux coronae show that if the flux of energy heating the corona is maintained constant while the thermal conductivity in the transition region is reduced, the coronal temperature, the pressure in the transition region and the corona, and the temperature gradient in the transition region all increase. At the same time the intensities of lines emitted from the transition region are almost unchanged. Thus all the main spectroscopically observed differences between coronal holes and quiet coronal regions are explained.The flux of energy heating the corona in both coronal holes and quiet coronal regions is 3.0 × 105 erg cm-2 s-1.The energy lost from coronal holes by the high speed streams in the solar wind is not sufficient to explain the difference in the coronal temperature in coronal holes and quiet coronal regions. The most likely explanation of the high velocity streams in the solar wind associated with coronal holes is that of Durney and Hundhausen. 相似文献
5.
C. Jordan 《Astrophysics and Space Science》1996,237(1-2):13-32
Observations of the Sun show that the chromosphere-corona transition region has a complex geometry and dynamic nature. In
spite of this, observations of stellar transition regions show common behaviour as well as systematic trends. The basic methods
used in making models of the transition region are set out. Observations relating to inhomogeneities in the solar transition
region are summarized. The structure and energy balance of stellar transition regions and the trends emerging are discussed. 相似文献
6.
The energy balance equation for a general solar atmosphere without assuming the plane-parallel approximation is solved analytically. This leads to models for both the transition region and inner corona as well as for the outer corona. The form of the latter is very similar to that of the hydrostatic conduction model of Chapman (1957). However, in this paper we confine ourselves only to the former.Model I is electron-pressure dependent but model II depends particularly on the maximum coronal temperature T
m and its corresponding altitude h
m. Both the models are compared with recently constructed temperature models of Chiuderi and Riani (1974), McWhirter et al. (1975), and Gabriel (1976a). It is concluded that our model II reproduces these models within a factor of not more than 2. 相似文献
7.
Two-dimensional numerical models of the solar transition region are calculated using an inverse coordinates method which attains pressure equilibrium between the network magnetic field and the external comparatively field-free gas. If A(y, z) is the magnetic potential (a scalar in 2D), which is constant on field lines, the method involves interchanging dependent and independent variables to obtain a quasi-linear PDE for y(A, z), which is solved iteratively. The advantage of this approach is that magnetic field lines, including any magnetic interface, become coordinate lines, thereby simplifying the energy equation and free boundary problem. In order to examine the effects of self-consistent geometry on the thermal structure of the transition region network, we calculate four models. The energy balance includes the effects of radiation, conduction, and enthalpy flux. It is confirmed that the lower branch of the emission measure curve cannot be explained within the single fluxtube model if the classical Spitzer thermal conductivity is used. However, by including a turbulent thermal conductivity as proposed by Cally (1990a), transition region models are obtained for which the resulting emission measure curves exhibit the correct behaviour, including the observed turn-up below about 200 000 K. In summary, the broad conclusions of previous non-turbulent 2D models are confirmed, but most importantly, the turbulent conductivity hypothesis tested in 1D by Cally is shown to produce excellent agreement with observations in the more realistic geometry. 相似文献
8.
Ion emission line intensities between 1170 and 1700 Å allow one to determine the differential emission measure (DEM) and electron pressure of the plasma in the solar transition region (TR). These line intensities together with their Doppler shifts and line widths are measured simultaneously for the first time above a sunsport from data obtained with the NRL High Resolution Telescope and Spectrograph with 0.06 Å spectral and 1 spatial resolution.The Doppler shifts show both subsonic and supersonic flow in the same line of sight over the umbra. The temperature structure for 40 resolution elements in the sunspot umbra and penumbra is derived from the DEM and the observed electron pressures.Extrapolation of the emission measure curves supports the previous EUV and X-ray observations that coronal plasma above sunspots with T
e>106 K is reduced while emission from TR plasma between 2×105 and 106 K is greatly enhanced relative to quiet or active regions. This enhancement shifts the minimum of the DEM to lower temperatures and increases the slope at 2×105 K by a factor of two.New pressure diagnostics using the emission line intensity ratios of C iv to N iv are presented, and applied to the data.The energy balance in the TR for the sunspot umbra is dominated by radiative losses from the large amount of TR plasma.An estimate of the energy budget shows that an energy input is required to balance the radiative energy losses above the umbra. The observed divergence of the enthalpy flux for the umbral downflows can balance these radiative losses for T
e between 30000 and 200 000 K.A typical umbral model of T
e versus reduced mass column density is compared with one for chromospheric temperatures determined from the Ca H and K lines.Institute of Theoretical Astrophysics, University of Oslo, Norway. 相似文献
9.
The temperature structure of the transition region between the chromosphere and corona is discussed in the context of current ideas about magnetic fields in these layers. Magnetic channeling of the downward conductive heat flow from the corona into the regions of enhanced field at the supergranulation boundaries is proposed as a mechanism for explaining the measured intensities of solar ultraviolet emission lines which originate in layers with temperatures below 105 °K. It is shown that nearly all of the observed ultraviolet line emission originates in interspicule regions, and that this emission plays an important part in the energy balance of the cooler layers of the transition region. It is suggested that certain motions observed in the upper chromosphere may represent the earliest visual evidence for conversion of inflowing conduction energy into kinetic motions.On leave from the Observatory Sonnenborgh at Utrecht, The Netherlands. 相似文献
10.
Roger A. Kopp 《Solar physics》1972,27(2):373-393
The classical picture of the transition region is that of a thin spherically symmetric shell maintained in a steady average thermodynamical state by a balance between conductive heating from the hot overlying corona and radiative losses. The further analysis of existing extreme ultraviolet flux data casts doubt on the correctness of this simple model. It is shown that the downward heat flux between the chromosphere and corona cannot be nearly as large as the value 6 × 105 erg cm–)= 13.46 - 2.99 sin2 s–1 derived in previous studies by assuming a planar atmosphere, and in fact is insufficient to balance transition-region radiative losses. An alternative picture is developed, consisting of a transition region network covering only a small fraction of the solar disk. The dissipation of mechanical energy, previously neglected in many calculations of theoretical transition region models, appears to play a dominant role in the local energy balance of the network.The National Center for Atmospheric Research is sponsored by the National Science Foundation. 相似文献
11.
The direct propagation of acoustic waves, driven harmonically at the solar photosphere, into the three-dimensional solar atmosphere
is examined numerically in the framework of ideal magnetohydrodynamics. It is of particular interest to study the leakage
of 5-minute global solar acoustic oscillations into the upper, gravitationally stratified and magnetised atmosphere, where
the modelled solar atmosphere possesses realistic temperature and density stratification. This work aims to complement and
bring further into the 3D domain our previous efforts (by Erdélyi et al., 2007, Astron. Astrophys. 467, 1299) on the leakage of photospheric motions and running magnetic-field-aligned waves excited by these global oscillations.
The constructed model atmosphere, most suitable perhaps for quiet Sun regions, is a VAL IIIC derivative in which a uniform
magnetic field is embedded. The response of the atmosphere to a range of periodic velocity drivers is numerically investigated
in the hydrodynamic and magnetohydrodynamic approximations. Among others the following results are discussed in detail: i) High-frequency waves are shown to propagate from the lower atmosphere across the transition region, experiencing relatively
low reflection, and transmitting most of their energy into the corona; ii) the thin transition region becomes a wave guide for horizontally propagating surface waves for a wide range of driver periods,
and particularly at those periods that support chromospheric standing waves; iii) the magnetic field acts as a waveguide for both high- and low-frequency waves originating from the photosphere and propagating
through the transition region into the solar corona.
Electronic Supplementary Material The online version of this article () contains supplementary material, which is available to authorized users. 相似文献
12.
The magnetic field that fills the corona is rooted in a small fraction of the solar surface. The consequent constriction of the field lines inhibits the conduction of heat down from the corona, thereby strongly affecting the energy balance in the corona and transition region. In this paper, we clarify how the shape of the constriction acts together with the amount of constriction to inhibit the heat flow. We analyze the heat flow in model tapered flux tubes in which the plasma properties are constant on cross sections, the plasma is static, and the only energy transfer is by thermal conduction. We find:
- From the general solution to the model, only two particular solutions are readily applied to the solar atmosphere. One is the steady-state case, appropriate for quiet regions and active regions which are not flaring; the other is the time-dependent case in which no heat enters the hot end, appropriate for conductive cooling of flare loops.
- In the steady-state case, the inhibition factor Φ/Φ P (the ratio of the heat flow through a constricted flux tube to the heat flow through the otherwise equivalent unconstricted tube) is simply the ratio of the harmonic mean area of the constricted tube to the area of the unconstricted tube. The inhibition of heat flow thus results from the amount and shape of the constriction.
- For any given shape and amount of monotonic tube taper, the inhibition factor in the steady-state case is a good estimate of that in the time-dependent case (within a factor of 2).
- The amount of constriction Γ (the ratio of the flux tube's hot end area to its cold end area) limits the range of possible values of the inhibition factor; the value of the inhibition factor within this range is set by the shape of the constriction. For the steady-state case, Γ ?1 ≤ Φ/Φ P ≤ 1. Therefore, the shape of the constriction is as important as the amount of constriction in determining the amount of inhibition.
- For the linear taper of a cone-shaped tube, the inhibition factor is an especially simple function of the amount of constriction: Φ/Φ P = Γ ?1/2. This inhibition-constriction relation for the cone correctly estimates the inhibition to order of magnitude for any tube in which the constriction occurs gradually all along the tube. If the constriction is more concentrated to the hot (cold) end of the tube than in a cone, then the inhibition is greater (less) than in a cone.
- Because most flux tubes that pass from the corona down through the transition region should not differ greatly from a cone in shape of constriction, the simple inhibition-constriction relation for the cone should correctly estimate the magnitude of the inhibition in quiet regions, active regions, and flare loops. The only observational input needed for this estimate is the amount of magnetic constriction through the transition region.
13.
A model for the energy balance and chemical equilibrium of the gas in photodissociation regions at the edge of molecular clouds, which are illuminated by strong FUV fields (6 eV ≦ hv ≦ 13.6 eV), has been developed. This model is used to calculate the emergent intensities in the fine structure lines of OI (63 μm, 145 μm), CI (609 μm, 370 μm), and CII (158 μm) and in the low-lying rotational transitions of CO. The numerical results show that column densities in the range 2 × 1017 to 2 × 1018 cm2 can be expected from the C+/C/CO transition region at the edge of molecular clouds. This difference with previous chemical calculations is partly due to a higher assumed carbon abundance, partly due to the charge exchange reactions of C+ with S and SiO, and partly due to carbon self-shielding which is taken into account. A detailed model is constructed for the Orion photodissociation region, which explains the observed OI (63 μm, 145 μm), CII (158 μm), CI (609 μm), and CO emission. In this model the CI (609 μm) emission originates in the warm (50°K) molecular gas behind Θ1C Ori but near the surface of OMCI. 相似文献
14.
Three independent observations by rocket, Skylab, and OSO-8 have all indicated the presence of steady downflows of the order of a few kilometers per second in the solar transition region overlying the chromospheric network. Using density estimates at these heights from traditional transition region models, we find that the downward mass fluxes associated with these velocities are comparable with the estimated upward mass flux in spicules, originating in the same regions. Since both observations and theoretical calculations show that the solar wind can accept only a small fraction of the upward spicule flux, we suggest that the downflow represents spicular material returning to the chromosphere after being heated to coronal temperatures. In this context, the differential velocity measurement of Cushman and Rense is interpreted as indicating a difference in downflow speeds rather than a difference in expansion speeds.Moreover, the enthalpy flux associated with the downflow of coronal material into these regions is shown by various estimates to exceed the inward heat flow expected by thermal conduction and it may constitute the dominant energy source for the transition region. Simplified analytical models are used to explore the nature of the transition region overlying the supergranulation boundaries, under the assumption that the thermal structure results from a balance of the downward convection of enthalpy and radiative losses. Models based upon these considerations are shown to be consistent with the observed emission measures.The National Center for Atmospheric Research is sponsored by the National Science Foundation. 相似文献
15.
James A. Klimchuk 《Solar physics》1989,119(1):19-34
The relationship between Doppler shift patterns observed in the transition region and magnetic field patterns observed in the photosphere is studied using coaligned pairs of Civ Dopplergrams and Fei magnetograms. Categories of magnetic features are defined - including neutral lines, unipolar regions, strong field regions, weak field regions, and magnetic boundaries - and from these, magnetic associations are determined for 159 V
0 lines separating areas of relative blueshift and redshift observed in and around active regions. The cases are subdivided on the basis of whether blueshifts or redshifts are observed on the side of the V
0 line nearest the solar limb.Two main results are that V
0 lines associated with neutral lines tend to have limbward blueshifts, while V
0 lines associated with unipolar regions tend to have limbward redshifts. These and other results provide supportive evidence for the active region model proposed recently by Klimchuk, in which relative redshifts occur where strong vertical fields penetrate the surface, and relative blueshifts occur where these same fields have spread out to become horizontal. It is likely that the relative blueshifts correspond to absolute Doppler shifts of very small amplitude, possibly even absolute redshifts.The National Center for Atmospheric Research is sponsored by the National Science Foundation. 相似文献
16.
Brandt J. C. Snow M. Yi Y. Larson S. M. Mikuz H. Petersen C. C. Liller W. 《Earth, Moon, and Planets》2002,90(1-4):15-33
The plasma tails of comets clearly show the demarcation of the solar wind into distinct equatorial and polar regions (Brandt
and Snow (2000), Icarus
148, 52–64).The boundary is determined by the maximum extent in latitude of the heliospheric current sheet (HCS). The observational
record contains many well-observed equatorial comets, but observations of comets in the polar region are relatively rare.
In addition to its size and brightness, comet Hale–Bopp had an orbital inclination of 89.4° and was well observed for months
in the polar region. We document the comet's large-scale appearance throughout the apparition, including the polar region
and its transition into the equatorial region. The bright dust tail hampered observations of the plasma tail, particularly
near the head, but images taken with a CO+ filter show a very large disconnection event (DE) on May 7 and May 8, 1997. The time of disconnection is estimated at approximately
May 4.0. This DE is associated with a crossing of the HCS. The model calculations of the HCS indicate that other crossings
might have occurred in late April, but given the uncertainty in the calculation, the comet might have missed the HCS. Sparse
observational coverage and the bright dust tail prevent further investigation of the potential earlier HCS crossings. The
plasma tail shows anomalous orientations at the highest latitudes and possible explanations are discussed. 相似文献
17.
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
18.
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
19.
D.J. Southwood M.K. Dougherty P. Canu A. Balogh P.J. Kellogg 《Planetary and Space Science》1993,41(11-12)
The spacecraft Ulysses flew through the Jovian magnetosphere during February 1992. This paper compares the magnetic field observations recorded during the inbound pass of the flyby with the electron density as derived from the URAP instrument. In general, it is expected that the density variations will anti-correlate with the magnetic field strength in order to maintain pressure balance, although there may be instances when a temperature or energy rise alone could balance the static stress. Furthermore, there is the possibility that a dynamic process could occur which would cause both the density and field magnitude to rise in unison. In the middle magnetosphere, anti-correlation is found to exist between the two data sets; however, in the outer magnetosphere (which was characterized by very disturbed fields) and in the transition region between the outer and middle magnetospheres, there is no simple relationship between the density and field. Examples of anti-correlation, temperature or energy increases and dynamic processes are found. 相似文献
20.
M. Sobotka M. V&#xE;zquez M. S&#xE;nchez Cuberes J. A. Bonet A. Hanslmeier 《Journal of Astrophysics and Astronomy》2000,21(3-4):289-292
Simultaneous time series of broad-band images of two active regions close to the disk center were acquired at the maximum
(0.80&#x039C;m) and minimum (1.55&#x039C;m) continuum opacities. Dark faculae are detected in images obtained as weighted intensity differences between both wave-length
bands. The elements of quiet regions can be clearly distinguished from those of faculae and pores in scatter plots of brightness
temperatures. There is a smooth transition between faculae and pores in the scatter plots. These facts are interpreted in
terms of the balance between the inhibition of convective energy transport and the lateral radiative heating. 相似文献