共查询到20条相似文献，搜索用时 31 毫秒
1.
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
2.
We present a careful investigation of the magnetofrictional relaxation and extrapolation technique applied to the reconstruction
of two test fields. These fields are taken from the family of nonlinear force-free magnetic equilibria constructed by Low
and Lou (Astrophys. J.
352, 343, 1990), which have emerged as standard tests for extrapolation techniques in recent years. For the practically relevant case that
only the field values in the bottom plane of the considered volume (vector magnetogram) are used as input information (i.e., not including the knowledge about the test field at the side and top boundaries), the test field is reconstructed to a higher
accuracy than obtained previously. Detailed diagnostics of the reconstruction accuracy show that the implementation of fourth-order
spatial discretization was essential to reach this accuracy for the given test fields and to achieve near machine precision
in satisfying the solenoidal condition. Different variants of boundary conditions are tested, which all yield comparable accuracy.
In its present implementation, the technique yields a scaling of computing time with total number of grid points only slightly
below N
5/3, which is too steep for applications to large (≥10242) magnetograms, except on supercomputers. Directions for improvement are outlined. 相似文献
3.
G. Rüdiger 《Astronomische Nachrichten》1977,298(5):245-252
This paper deals with the theory of the solar rotational law. We assume the turbulence to be of the largest influence compared with the momentum flux caused by molecular viscosity and meridional circulation. Firstly we use heuristical forms for the needed cross correlations Qrφ (turbulent radial momentum flux) and Qνφ (turbulent latitudinal momentum flux): Qrφ = −α0r ϑΩ/ϑr · sin ν + Q0 sin ν + Q2 sin3 ν, Qνφ = −δ0 ϑΩ/ϑν· sin ν + P2 sin2θ cos θ. It is shown that a radial dependence of the angular velocity Ω is given by Q0. Furthermore, the observed equatorial acceleration occurs in the case of non-negativity of Q2 and/or P2. Because of the spatial dependence of the solar angular velocity the coefficients of Q and P are unfortunately not to be measured. Secondly, we determine the coefficients with a theory founded upon the hypothesis that a rotating stochastical force field — independent from Ω — maintains an anisotropic turbulence. The global fast rotation produces, indeed, finite cross correlations Q2 and P2. It is suggested that horizontally directed turbulent motions with not too small radial correlations lengths and time scales of about 2 weeks could be responsible for the solar differential rotation. Finally, we show that also short-living turbulent horizontal modes provide the observed equatorial acceleration if they occur preferably at the equatorial region. 相似文献
4.
《Planetary and Space Science》1999,47(6-7):745-763
An improved magnetohydrodynamic (MHD) model with chemistry is presented. The analysis of the source and sink terms for H2O + shows that for small comets up to 11% of water molecules are finally ionized. For large comets (such as Halley) this fraction decreases to less than 3%. From the MHD scaling laws a similarity law for the individual ion densities is deduced which takes into account that the mother molecules are depleted by dissociation. This is applied to H2O + ions. Radial density profiles from model calculations, observations by Giotto near comet Halley, and ground based observations of three comets confirm this scaling law for H2O + ions. From the similarity law for the density a scaling law for the column density is derived which is more convenient to apply for ground based observations. From these scaling laws methods are derived which allow the determination of the water production rate from the ground based images of the H2O + ions. Finally, the two dimensional images of model column densities are compared with observations. 相似文献
5.
Cotemporal Fei 630.2 nm magnetograms from the Solar Optical Universal Polarimeter (SOUP) filter and the Advanced Stokes Polarimeter (ASP) are quantitatively compared using observations of active region AR 8218, a large negative polarity sunspot group observed at S20 W22 on 13 May 1998. The SOUP instrument produces Stokes V/I `filter magnetograms' with wide field of view and spatial resolution below 0.5 arc sec in good seeing, but low spectral resolution. In contrast, the ASP uses high spectral resolution to produce very high-precision vector magnetic field maps at spatial resolution values on the order of 1 arc sec in good seeing. We use ASP inversion results to create an ASP `longitudinal magnetic flux-density map' with which to calibrate the less precise SOUP magnetograms. The magnetograms from each instrument are co-aligned with an accuracy of about 1 arc sec. Regions of invalid data, poor field-of-view overlap, and sunspots are masked out in order to calibrate SOUP predominately on the relatively vertical `weak-field' plage magnetic elements. Pixel-to-pixel statistical comparisons are used to determine the SOUP magnetogram linear calibration constant relative to ASP flux-density values. We compare three distinct methods of scaling the ASP and SOUP data to a common reference frame in order to explore filling factor effects. The recommended SOUP calibration constant is 17000 ± 550 Mx cm–2 per polarization percent in plage regions. We find a distinct polarity asymmetry in SOUP response relative to the ASP, apparently due to a spatial resolution effect in the ASP data: the smaller, less numerous, minority polarity structures in the plage region are preferentially blended with the majority polarity structures. The blending occurs to a lesser degree in the high-resolution SOUP magnetogram thus leading to an apparent increase in SOUP sensitivity to the minority polarity structures relative to the ASP. One implication of this effect is that in mixed polarity regions on the Sun, lower spatial resolution magnetograms may significantly underestimate minority polarity flux levels, thus leading to apparent flux imbalances in the data.
*Visiting Astronomer, National Solar Observatory, operated by the Association of Universities for Research in Astronomy, Inc. (AURA), under cooperative agreement with the National Science Foundation.
The National Center for Atmospheric Research is sponsored by the National Science Foundation. 相似文献
6.
We present a new method to resolve the 180° ambiguity for solar vector magnetogram measurements. The basic assumption is that the magnetic shear angle (), which is defined as the difference between the azimuth components of observed and potential fields, approximately follows a normal distribution. The new method is composed of three steps. First, we apply the potential field method to determine the azimuthal components of the observed magnetic fields. Second, we resolve the ambiguity with a new criterion: –90°+mp
lele90°+mp, where mp is the most probable value of magnetic shear angle from its number distribution. Finally, to remove some localized field discontinuities, we use the criterion B
tB
mt
ge0, where B
t and B
mt are an observed transverse field and its mean value for a small surrounding region, respectively. For an illustration, we have applied the new ambiguity removal method (Uniform Shear Method) to a vector magnetogram which covers a highly sheared region near the polarity inversion line of NOAA AR 0039. As a result, we have found that the new ambiguity solution was successful and removed spatial discontinuities in the transverse vector fields produced in the magnetogram by the potential field method. It is also found that our solution to the ambiguity gives nearly the same results, for highly sheared vector magnetograms and vertical current density distributions, of NOAA AR 5747 and AR 6233 as those of other methods. The validity of the basic assumption for an approximate normal distribution is demonstrated by the number distributions of magnetic shear angle for the three active regions under consideration. 相似文献
7.
G. Allen Gary 《Solar physics》1996,163(1):43-64
The potential magnetic field from a finite planar boundary is extrapolated into the upper hemisphere using information from all three magnetic field components. The method determines, first, the transverse field associated with the observed normal magnetic intensity. Then by subtraction, the method determines the associated transverse magnetic field observed in the interior (i.e., in the field of view) of the magnetogram which is due to the normal flux exterior to the field of view of the magnetogram. Inverting this information gives an approximation to the exterior normal flux. The combination of the observed normal flux of the interior and the approximation of the exterior normal flux is employed to calculate the potential field. The formulation of the problem results in an ill-posed integral inversion problem in which a regularized solution is obtained using the singular value decomposition (SVD) technique in conjunction with an appropriate Tikhonov-Phillips filter. The technique can be applied to correcting potential field calculations which are influenced by out-of-view fluxes, e.g., for a high spatial resolution vector magnetogram with a small field of view in which there is no supporting exterior data. The problem studied is also important in providing a regularized solution of the Cauchy potential problem. The method provides a much larger range of convergence than the method of Gary and Musielak (1992), and, in fact, is stable in the total upper hemisphere.The U.S. Government right to retain a non-exclusive, royalty-free licence in and to any copyright is acknowledged. 相似文献
8.
The NOAA active region (AR) 11029 was a small but highly active sunspot region which produced 73 GOES soft X-ray flares during its transit of the disk in late October 2009. The flares appear to show a departure from the well-known power law frequency-size distribution. Specifically, too few GOES C-class and no M-class flares were observed by comparison with a power law distribution (Wheatland, Astrophys. J. 710, 1324, 2010). This was conjectured to be due to the region having insufficient magnetic energy to power the missing large events. We construct nonlinear force-free extrapolations of the coronal magnetic field of AR 11029 using data taken on 24 October by the SOLIS Vector SpectroMagnetograph (SOLIS/VSM) and data taken on 27 October by the Hinode Solar Optical Telescope SpectroPolarimeter (Hinode/SP). Force-free modeling with photospheric magnetogram data encounters problems, because the magnetogram data are inconsistent with a force-free model. We employ a recently developed “self-consistency” procedure which addresses this problem and accommodates uncertainties in the boundary data (Wheatland and Régnier, Astrophys. J. 700, L88, 2009). We calculate the total energy and free energy of the self-consistent solution, which provides a model for the coronal magnetic field of the active region. The free energy of the region was found to be ≈?4×1029?erg on 24 October and ≈?7×1031?erg on 27 October. An order of magnitude scaling between RHESSI non-thermal energy and GOES peak X-ray flux is established from a sample of flares from the literature and is used to estimate flare energies from the observed GOES peak X-ray flux. Based on the scaling, we conclude that the estimated free energy of AR 11029 on 27 October when the flaring rate peaked was sufficient to power M-class or X-class flares; hence, the modeling does not appear to support the hypothesis that the absence of large flares is due to the region having limited energy. 相似文献
9.
Knowledge regarding the coronal magnetic field is important for the understanding of many phenomena, like flares and coronal
mass ejections. Because of the low plasma beta in the solar corona, the coronal magnetic field is often assumed to be force-free
and we use photospheric vector magnetograph data to extrapolate the magnetic field into the corona with the help of a nonlinear
force-free optimization code. Unfortunately, the measurements of the photospheric magnetic field contain inconsistencies and
noise. In particular, the transversal components (say B x and B y) of current vector magnetographs have their uncertainties. Furthermore, the magnetic field in the photosphere is not necessarily
force free and often not consistent with the assumption of a force-free field above the magnetogram. We develop a preprocessing
procedure to drive the observed non–force-free data towards suitable boundary conditions for a force-free extrapolation. As
a result, we get a data set which is as close as possible to the measured data and consistent with the force-free assumption. 相似文献
10.
Ramon J. Quiroga 《Astrophysics and Space Science》1983,93(1):37-54
Statistics in absorption 21-cm data show two main types of clouds at low galactic latitudes: dense small clouds, many of them
with molecular cores, with dispersions σ≈1.5 km s−1 and large clouds forming the fine features of the spiral arms (the shingle like features) with a dispersion range α≈3–4 km
s−1. Sizes and dispersions of both types of clouds are compatible with the Kolmogorov law of turbulence: σ∞d
1/3. The large clouds forming the shingle-like features can be considered as the largest clouds of a Kolmogorov spectrum (the
initial vortices), or as the hydrodynamic features with minimum sizes in the Galaxy. In order to define hydrodynamic motions
in the same sense as given by Ogrodnikov (1965) we use here the tensorial form of the Helmholtz theorem to obtain an approximation
for the hydrodynamic motions depending on distances and seen from the local standard of rest:V
r
∞r. The intermediate range of sizes between turbulent motions and hydrodynamic motions is 100<d<300 pc which is also the range of sizes of the large clouds forming the fine features of the spiral arms.
A classification on of motions in the Galaxy is postulated: (a) a basic rotation motion given by an smooth unperturbed curveΘ
b
(R) associated to the old disk population. (b) Systematic motions of the spiral arms. (c) Systematic motions in the fine structure
of the arms. For scale sizes smaller than these fine features one has turbulent motions according to the Kolmogorov law.
The densities and sizes of the turbulent clouds behave asn
H
∝d
−2 in a range of sizes 7 pc<d<300 pc. The obtained gas densities of the clouds are confirmed with the dust densities from photometric studies. The conditions
for gravitational binding of the clouds are analyzed. Factors as the geometry and the magnetic field within the clouds increases
the critic densities for gravitational binding. When we consider these factors we find that the wide component clouds have
densities below such a critical value.
The narrow component clouds have densities similar or above the critical value; but the real fraction of collapsing clouds
remains unknown as far as the factor of geometry and the inner magnetic field of each cloud are not determinated. 相似文献
11.
D. Gerbal H.V. Capelato F. Durret G.B. Lima Neto I. Márquez 《Astrophysics and Space Science》2001,276(2-4):861-868
We suggest that elliptical galaxies, as stellar systems in a stage of quasi-equilibrium, may have a specific entropy. We use
the Sérsic law to describe the light profile. The specific entropy (the Boltzmann–Gibbs definition) is then calculated assuming
that the galaxy behaves as a spherical, isotropic, one-component system. We predict a relation between the three parameters
of the Sérsic law linked to the specific entropy, defining a surface in the parameter space, an ‘entropic plane’. We have
analysed a sample of simulated merging elliptical galaxies (virtual) and a sample of galaxies belonging to the Coma Cluster (real). Both virtual and realgalaxies are: 1) located in their own ‘entropic plane‘ and 2) in this plane, they are located on a straight line, indicating
constant entropy: another physical property A careful examination of the value of the specific entropy indicates a very small
increase in the specific entropy with the generation after merging (virtual sample). Although one cannot distinguish between various generations for real galaxies, the distribution of specific entropy in this sample is very similar to that in the virtual sample.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
12.
We investigate the limits of applicability of the Spitzer-Harm thermal conductivity in solar coronal loops and show that the ratio 0
/L
T
of electron mean-free path to temperature scale height in large-scale structures can approach the limits of the Spitzer-Harm theory. We use a non-local formulation of heat transport to compute a grid of loop models: the effects of non-local transport on the distribution of differential emission measure are particularly important in the coronal part of loops longer than the pressure scale height s
p
.We derive a scaling law for 0/L
T
in the corona, showing that it grows exponentially with L/s
p
, and discuss effects of non-local heat transport in the transition region.Now at Osservatorio Astrofisico di Catania, Italy. 相似文献
13.
A type of saturation is sometimes seen in sunspot umbrae in MDI/SOHO magnetograms. In this paper, we present the underlying
cause of such saturation. By using a set of MDI circular polarization filtergrams taken during an MDI line profile campaign
observation, we derive the MDI magnetograms using two different approaches: the on-board data processing and the ground data processing, respectively. The algorithms for processing the data are the same, but the former is limited by a 15-bit lookup table. Saturation
is clearly seen in the magnetogram from the on-board processing simulation, which is comparable to an observed MDI magnetogram taken one and a half hours before the campaign data. We analyze
the saturated pixels and examine the on-board numerical calculation method. We conclude that very low intensity in sunspot
umbrae leads to a very low depth of the spectral line that becomes problematic when limited to the 15-bit on-board numerical
treatment. This 15-bit on-board treatment of the values is the reason for the saturation seen in sunspot umbrae in the MDI
magnetogram. Although it is possible for a different type of saturation to occur when the combination of a strong magnetic
field and high velocity moves the spectral line out of the effective sampling range, this saturation is not observed. 相似文献
14.
G. A. Gurzadyan 《Astrophysics and Space Science》1991,179(2):293-302
The problem of the physical interpretation of the Wilson-Bappu effect is examined. A postulate is forwarded according to which this effect expresses the dependence of the complete kinetic energyE
t
of turbulent motions in the medium generating magnesium emission in the doublet 2800 Mgii on the velocityW
0 of turbulent motion. One of the consequences of the application of this postulate is the appearance of the relationships (8) and (11) which give the magnitude of the kinetic energyE
t
of turbulent motions in the first case (Figure 2) and the magnitude of the mass of emitting mediumM
0 in the second (Figure 3) depending on the turbulent velocityW
0
(k). This postulate unit in a common conception all variety of the magnesium emission escaped from a single star as well as from binary systems on the one hand and the magnesium emission generated in chromospheres as well as in intercomponent clouds or streams on the other. 相似文献
15.
The diffusion of scalar fields (temperature, density number of some admixture) in a compressible medium showing an isotropic, homogeneous and stationary turbulence is considered. The derived formulae for turbulent diffusivity χT(ξ) hold up to ξ ≈ 1, where ξ = u0 τ0/R0 (u0, τ0, and R0 are characteristic velocity, life-time, and correlation length of turbulent pulsations, respectively. The velocity field of turbulent motions u(r, t) is assumed to be known and the influence of the scalar field onto u(r, t) is neglected. It is shown that the velocity correlators, which change their signs in dependence on the space corrdinates, may give negative values for ξT(ξ) when ξ ≠ 0. 相似文献
16.
J. J. Rawal 《Astrophysics and Space Science》1986,119(1):159-166
Prentice (1978a, b), in his modern Laplacian theory of the origin of the solar system, has established a scenario in which he finds the ratio of the orbital radii of successively disposed gaseous rings to be a constant 1.69. In an attempt to understand this law in an alternative way, Rawal (1984a) assumes that during the collapse of the solar nebula the halts at various radii are brought about by the supersonic turbulent convection and arrives at the relation of the formR
p=Rap, whereR
is the radius of the present Sun anda=1.422, is referred to, here, as the Roche constant. Kepler's third law assumes the form:T
p=T0(a
3/2)
p
,T
0 being the rotational period of the Sun at the time it attained its present radius.R
p
satisfy Laplace's resonance relation without any exception. The present paper investigates inter-relations among the concepts of supersonic turbulent convection, rotational instability, and Roche limit.Paper presented at the IAU Third Asian-Pacific Regional Meeting, held in Kyoto, Japan, between 30 September–6 October, 1984. 相似文献
17.
A. Feoli 《Astronomische Nachrichten》2014,335(2):193-197
We propose a classification of supermassive black holes (SMBHs) based on their efficiency in the conversion of infalling mass in emitted radiation. We use a theoretical model that assumes a conservation of angular momentum between the gas falling inside the hole and the photons emitted outwards, and suggests the existence of the scaling relation M•‐Reσ3, where M• is the mass of the central SMBH, whereas Re and σ are the effective radius and velocity dispersion of the host galaxies (bulges), respectively. We apply our model on a data set of 57 galaxies of different morphological types and with M• measurements, obtained through the analysis of Spitzer /IRAC 3.6‐µ m images. In order to find the best fit of the corresponding scaling law, we use the FITEXY routine to perform a least‐squares regression of M• on Reσ3 for the considered sample of galaxies. Our analysis shows that the relation is tight and our theoretical model allows to easily estimate the efficiency of mass conversion into radiation of the central SMBHs. Finally we propose a new appealing way to classify the SMBHs in terms of this parameter. (© 2014 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim) 相似文献
18.
As was demonstrated in earlier studies, turbulence can result in a negative contribution to the effective mean magnetic pressure, which, in turn, can cause a large‐scale instability. In this study, hydromagnetic mean‐field modelling is performed for an isothermally stratified layer in the presence of a horizontal magnetic field. The negative effective magnetic pressure instability (NEMPI) is comprehensively investigated. It is shown that, if the effect of turbulence on the mean magnetic tension force vanishes, which is consistent with results from direct numerical simulations of forced turbulence, the fastest growing eigenmodes of NEMPI are two‐dimensional. The growth rate is found to depend on a parameter β* characterizing the turbulent contribution of the effective mean magnetic pressure for moderately strong mean magnetic fields. A fit formula is proposed that gives the growth rate as a function of turbulent kinematic viscosity, turbulent magnetic diffusivity, the density scale height, and the parameter β*. The strength of the imposed magnetic field does not explicitly enter provided the location of the vertical boundaries are chosen such that the maximum of the eigenmode of NEMPI fits into the domain. The formation of sunspots and solar active regions is discussed as possible applications of NEMPI (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim) 相似文献
19.
We describe the diffusion and random velocities of solid particles due to stochastic forcing by turbulent gas. We include the orbital dynamics of Keplerian disks, both in-plane epicycles and vertical oscillations. We obtain a new result for the diffusion of solids. The Schmidt number (ratio of gas to particle diffusivity) is Sc≈1+(Ωtstop)2, in terms of the particle stopping time tstop and the orbital frequency Ω. The standard result, Sc=1+tstop/teddy, in terms of the eddy turnover time, teddy, is shown to be incorrect. The main difference is that Sc rises quadratically, not linearly, with stopping time. Consequently, particles larger than 10 cm in protoplanetary disks will suffer less radial diffusion and will settle closer to the midplane. Such a layer of boulders would be more prone to gravitational collapse. Our predictions of RMS speeds, vertical scale height and diffusion coefficients will help interpret numerical simulations. We confirm previous results for the vertical stirring of particles (scale heights and random velocities), and add a correction for arbitrary ratios of eddy to orbital times. The particle layer becomes thinner for teddy>1/Ω with the strength of turbulent diffusion held fixed. We use two analytic techniques—the Hinze–Tchen formalism and the Fokker–Planck equation with velocity diffusion—with identical results when the regimes of validity overlap. We include simple physical arguments for the scaling of our results. 相似文献
20.
Samples of spiral galaxies from two catalogues of 21 cm line observations and a catalogue of near-infrared observations of
nearby galaxies have been used in conjunction with Infrared Astronomical Satellite data to study correlations involving MG, the dynamic mass of the galaxies, the luminosities in theH band (1.6Μm), the blue band and the far infrared bands and the mass of atomic hydrogen, it is found that both the blue and the far-IR
luminosities which are indicators of star formation averaged over ∼3 × l09 and ∼107 years respectively, have a linear dependence onM
G On the other hand, theH luminosity which is a measure of star formation averaged over the lifetime of galaxies, has a steeper power law dependence
onM
G. The correlations observed do not have significant dependence on the morophological type of the galaxies There is a poor correlation
between the far-infrared luminosity and the mass of atomic hydrogen. The mass of atomic hydrogen has a dependence of the formM
G. Because of the decrease in the mean mass for later morphological types and due to differences in power law dependences of
luminosities in different bands onM
G, the mean value of luminosity-to-mass ratio is a constant for blue and far-IR bands, decreases for theH band and the gas-to-mass ratio increases as morphological type increases. 相似文献