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1.
The effect of radiation losses on the dispersion and damping of magnetohydrodynamic waves in the solar corona is studied. The conditions are determined under which radiation losses are most appreciable. A damping of kink modes of coronal loops with plasma temperatures within 106–106.3 K and 106.3–107 K are compared. It is concluded that the radiation damping dominates in the temperature range 106–106.3 K, which can cause the observed fast damping of kink oscillations of coronal loops. Radiation losses should be taken into account in full magnetohydrodynamic equations with radiative transfer. 相似文献
2.
The rates of photodissociation of the OH and OD molecules from absorption of solar radiation in he X2Π-A2Σ+ electronic transition are calculated to lie between 3.5 and 6.7 × 10?6 sec?1 for OH for heliocentric velocities between -60 and +60 km sec?1 and to be about 4.7 × 10?7 sec?1 for OD at 1 AU from the Sun. The corresponding lifetimes, which are upper bounds to the actual lifetimes, are generally consistent with the observational cometary data. 相似文献
3.
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
4.
Samuel C. Vila 《Astrophysics and Space Science》1982,88(2):293-297
This paper presents disk models for cataclysmic variables in which convection in the central layers has been included. The calculation of the vertical structure at different points is presented. The models have a central mass of 1M ⊙ and matter fluxes of 10?9, 10?8, and 10?7 M ⊙ yr?1. The corresponding luminosities are 1.86, 1.86×10 and 1.86×102 L ⊙. 相似文献
5.
It is generally assumed that the magnetic fields of millisecond pulsars (MSPs) are ～108 G. We argue that this may not be true and the fields may be appreciably greater. We present six evidences for this: (1) The ～108G field estimate is based on magnetic dipole emission losses which is shown to be questionable; (2) The MSPs in low mass X-ray binaries (LMXBs) are claimed to have <1011 G on the basis of a Rayleygh-Taylor instability accretion argument. We show that the accretion argument is questionable and the upper limit 1011 G may be much higher; (3) Low magnetic field neutron stars have difficulty being produced in LMXBs; (4) MSPs may still be accreting indicating a much higher magnetic field; (5) The data that predict ～108 G for MSPs also predict ages on the order of, and greater than, ten billion years, which is much greater than normal pulsars. If the predicted ages are wrong, most likely the predicted ～108 G fields of MSPs are wrong; (6) When magnetic fields are measured directly with cyclotron lines in X-ray binaries, fields ?108 G are indicated. Other scenarios should be investigated. One such scenario is the following. Over 85% of MSPs are confirmed members of a binary. It is possible that all MSPs are in large separation binaries having magnetic fields >108 G with their magnetic dipole emission being balanced by low level accretion from their companions. 相似文献
6.
The u.v. spectrometer polarimeter on the Solar Maximum Mission has been utilized to measure mesospheric ozone vs altitude profiles by the technique of solar occultation. Sunset data are presented for 1980, during the fall equinoctal period within ± 20° of the geographic equator. Mean O3, concentrations are 4.0 × 1010 cm?3at 50 km, 1.6 × 1010 cm?3 at 55 km. 5.5 × 109 cm?3 at 60 km and 1.5 × 109 cm?3 at 65 km. Som profiles exhibit altitude structure which is wavelike. The mean ozone profile is fit best with the results of a time-dependent model if the assumed water vapor mixing ratio employed varies from 6 ppm at 50 km to 2–4 ppm at 65 km. 相似文献
7.
G. E. Brueckner 《Solar physics》1983,86(1-2):259-265
Observations of high-speed coronal clouds (OSO-7), flare ejecta (Skylab) and high-energy jets (HRTS) are compared. It is possible that the same physical mechanism - an expanding loop - which is responsible for the high speed jets (400 km sec?1, 2.5 × 1026 ergs) can also account for the high-speed coronal clouds (1300 km sec?1, 4 × 1030 ergs), which were correlated with a flare-connected spray. Field strength of 15 gauss and 2500 gauss are required for the jets and the sprays, respectively. 相似文献
8.
D. B. Melrose 《Solar physics》1975,43(1):211-236
The theory of plasma emission is developed under the assumption that the Langmuir waves are generated by an isotropic distribution of fast electrons. Emission from inverse power-law distributions tend to favor emission at the second harmonic with brightness temperatures up to about 108 K at 100 MHz. The concept of a gap (in velocity space) distribution is developed. Very bright plasma emission can result from a gap distribution. For brightness temperatures between 109 K and 1011 K for the second harmonic the fundamental has a brightness temperature between 108 K and 109 K. For higher brightness temperatures the fundamental is amplified and can be very much brighter than the second harmonic. The maximum brightness temperatures for the fundamental and second harmonic at 100 MHz are about 1016 K and 1013 K respectively. Mechanisms by which a gap distribution might be formed are discussed and two effective mechanisms are identified. The theory is applied to the interpretation of radio bursts of types I, II, stationary IV and V. In each case the suggested mechanism appears to be favorable. 相似文献
9.
If the dwarf spheroidals are embedded in an extended cloud of dark matter then their density profiles can be reproduced by
assuming a Maxwellian distribution of velocities for the constituent stars. The observed luminosity profiles of dwarf spheroidals
imply densities for the dark matter in the range 10-26 to 10-25 g cm-3, and mass-to-luminosity ratios which are typically an order of magnitude greater than those of globular clusters. Neutrinos
of mass ∼ 10 eV and (v) ∼ 1000 km s-1 can provide this requisite density for the background. 相似文献
10.
New theoretical emission line ratios for the Be-sequence ions Mgix and Sixi are presented. A comparison with observational data for two solar flares and an active region loop obtained with the Harvard EUV spectrometer and NRL XUV spectroheliograph aboard Skylab reveals that these plasmas are in ionization equilibrium at coronal temperatures. Unfortunately most of the density diagnostics are not particularly useful under solar plasma conditions, as they vary only slightly over the electron density range 108–1013cm–3. However the Sixi ratioI(3
P
e
2 -3
P
o
2)/I(3
P
o
1 –1
S
e
0) is density sensitive in the range 108 to 1010cm–3, which is representative of electron densities found in solar active regions or small flares. 相似文献
11.
Electrical currents should flow in Europa because of its presence in Jupiter's corotating magnetosphere. The possible magnitudes of these currents are calculated assuming that Europa is a differentiated body consisting of an outer H2O layer and a silicate core. Two types of models are considered here: one in which the water is completely frozen and a second in which there is an intermediate liquid layer. For the transverse electric mode (eddy currents), the calculated current density in a liquid layer is approximately 10?5 A m?2. For the transverse magnetic mode (unipolar generator), the calculated current density in the liquid is severely constrained by the ice layer to only 10?10 to 10?11 A m?2, for a total H2O thickness of 100 km, provided that neither layer is less than 4 km thick. The current density is less for a completely frozen H2O layer. If transient cracks were to appear in the ice layer, exposing liquid, the calculated current density could rise to a range of 10?6 to 10?5 A m?2, depending on layer thicknesses, requiring an exposed area of 10?9 to 10?8 of the Europa surface. Electrical heating would be significant only if the ice layer thickness were on the order of 1 m, such as might occur if an exposed liquid surface were to freeze over; the heating under this condition could hinder the thickening of the ice layer. 相似文献
12.
Taking into account meteoroid measurements by in situ experiments, zodiacal light observations, and oblique angle hypervelocity impact studies, it is found that the observed size distributions of lunar microcraters usually do not represent the interplanetary meteoroid flux for particles with masses ?10?10g. From the steepest observed lunar crater size distribution a “lunar flux” is derived which is up to 2 orders of magnitude higher than the interplanetary flux at the smallest particle masses. New models of the “lunar” and “interplanetary” meteoroid fluxes are presented. The spatial mass density of interplanetary meteoritic material at 1 AU is ～10?16g/m3. A large fraction of this mass is in particles of 10?6 to 10?4 g. A detailed analysis of the effects of mutual collisions (i.e., destruction of meteoroids and production of fragment particles) and of radiation pressure has been performed which yielded a new picture of the balance of the meteoritic complex. It has been found that the collisional lifetime at 1 AU is shortest (～104years) for meteoroids of 10?4 to 1 g mass. For particles with masses m > 10?5g, Poynting-Robertson lifetimes are considerably larger than collisional lifetimes. The collisional destruction rate of meteoroids with masses is about 10 times larger than the rate of collisional production of fragment particles in the same mass range. About 9 tons/sec of these “meteor-sized” (m > 10?5g) particles are lost inside 1 AU due to collisions and have to be replenished by other sources, e.g., comets. Under steady-state conditions, most of these large particles are “young”; i.e., they have not been fragmented by collisions and their initial orbits are not altered much by radiation pressure drag. Many more micrometeoroids of masses m ? 10?5g are generated by collisions from more massive particles than are destroyed by collisions. The net collisional production rate of intermediate-sized particles 10?10g ? m ? 10?5g is found to be about 16 times larger at 1 AU than the Poynting-Robertson loss rate. The total Poynting-Robertson loss rate inside 1 AU is only about 0.26 tons/sec. The smallest fragment particles (m ? 10?10g) will be largely injected into hyperbolic trajectories under the influence of radiation pressure (β meteoroids). These particles provide the most effecient loss mechanism from the meteoritic complex. When it is assumed that meteoroids fragment similarly to experimental impact studies with basalt, then it is found that interplanetary meteoroids in the mass range 10?10g ? m ? 10?5g cannot be in temporal balance under collisions and Poynting-Robertson drag but their spatial density is presently increasing with time. 相似文献
13.
Alexander Kuznetsov 《Astrophysics and Space Science》2008,314(4):311-318
This paper establishes united classification of gamma-ray bursts and their counterparts on the basis of measured characteristics:
photon energy E and emission duration T. We find that the interrelation between these characteristics is such that as the energy increases, the duration decreases
(and vice versa). The given interrelation reflects the nature of the phenomenon and forms the E–T diagram, which represents a natural classification of all observed events in the energy range from about 109 to 10−6 eV and in the corresponding interval of durations from about 10−2 up to 108 s. The proposed classification results from our findings, which are principal for the theory and practical study of the phenomenon. 相似文献
14.
We have carried out 1.25 pc resolution MHD simulations of the ISM, on a Cartesian grid of 0 ≤ (x, y) ≤ 1 kpc size in the galactic plane and ?10 ≤ z ≤ 10 kpc into the halo, thus being able to fully trace the time-dependent evolution of the galactic fountain. The simulations show that large scale gas streams emerge, driven by SN explosions, which are responsible for the formation and destruction of shocked compressed layers. The shocked gas can have densities as high as 800 cm?3 and lifetimes up to 15 Myr. The cold gas is distributed into filaments which tend to show a preferred orientation due to the anisotropy of the flow induced by the galactic magnetic field. Ram pressure dominates the flow in the unstable branch 102 < T ≤ 103.9 K, whereas for T ≤ 100 K (stable branch) magnetic pressure takes over. Near supernovae thermal and ram pressures determine the dynamics of the flow. Up to 80% of the mass in the disk is concentrated in the thermally unstable regime 102 < T ≤ 103.9 K with ～30% of the disk mass enclosed in the T ≤ 103 K gas. The hot gas in contrast is controlled by the thermal pressure, since magnetic field lines are swept towards the dense compressed walls. 相似文献
15.
D. J. Stevenson 《Astronomische Nachrichten》1984,305(5):257-264
It is proposed that the existence and nature of a planetary dynamo can be characterized by a dimensionless number Φ ≡ FeR/ϱλ2ω, called the energy flux number, where Fe is the energy flux available for dynamo generation, R is the core radius (or thickness of the dynamo generating region), ϱ is the fluid density, λ is the magnetic diffusivity and ω is the angular velocity. For Φ ≲ 1, there is no dynamo. For 1 ≲ Φ ≲ 102.5 there is an “energy-limited dynamo”, in which Fe is insufficient to enable the dynamo to reach the dynamically desirable state A ≡ B2/8πϱλω ∼ 1, where B is a typical field amplitude (in Gauss). For 102.5 ≲ Φ ≲ 105, there is a dynamically determined dynamo (Λ ∼ 1) in which the magnetic Reynolds number of turbulent eddies is small. For Φ ≳ 105, there is a turbulent dynamo. Probable planetary examples of these three dynamo states are Mercury (Φ ∼ 102-103), Earth (Φ ∼ 104) and Jupiter (Φ ∼ 1011), respectively. 相似文献
16.
The relative abundances of seven constitutent nuclei, He4, C12, O16, Ne20, Mg24, Si28 and Fe56, are calculated as a function of time for neutron star atmospheres within which exist magnetic fields of the order of 1013G. The opacity, equation of state of the electrons, and cooling rate of the magnetic star are discussed, and it is shown to be a reasonable approximation to assume an atmosphere to be isothermal. The effects of particle diffusion are included in the nuclear reaction network. Computations are performed both for a constant mass atmosphere and for an atmosphere in which mass is being ejected. It is found that the final abundances are model independent as long as the initial model contains predominantly He4. The relative abundances are compared to the cosmic ray spectrum. For both the constant mass and mass loss atmospheres, nucleosynthesis proceeds virtually completely to Fe56. However the outermost layers of the envelope, in which no mass is being ejected, are composed almost entirely of He4 with trace amounts of Fe56. After the loss of about 1021 g, only Fe56 is ejected from atmospheres expelling mass.A portion of the research on which this paper was based was performed while L. C. Rosen was present at the Lawrence Radiation Laboratory, Livermore, California. 相似文献
17.
Oxidation of CH4 provides the major source for atmospheric H2 which is removed mainly by reaction with OH. Biological activity at the Earth's surface appears to represent at most a minor sink for H2. Anthropogenic activity is a significant source for both H2 and CO in the present atmosphere and may be expected to exert a growing influence in the future. Models are presented which suggest a rise in the mixing ratio of H2 from its present value of 5.6 × 10?7 to about 1.8 × 10?6 by the year 2100. The mixing ratio of CO should grow from 9.7 × 10?8 to 2.3 × 10?7 over the same time period and there should be a rise in CH4 by about a factor of 1.5 associated with anthropogenically induced reductions in tropospheric OH. 相似文献
18.
A compact structure of a low-mass Type I presupernovae is assumed to be an essential feature of the hydrodynamical problem
dealing with the supernova Type I (SNI) envelope outbursts. This structure is characterized by a degenerate carbon-oxygen
core, which suffers a thermonuclear explosion of carbon fuel (M
0≃1.40M
⊙), and by a compact lowmass envelope (M
e
≲0.1M
⊙) with external radiusR
e≃109 cm. The parameters, of this hydrostatic envelope are specified and then, for a relatively small explosion energy, ofW
0≃(2–10)×1049 erg, hydrodynamic problem of the envelope ejection is solved numerically. This energy comes from neutrino-induced detonative
carbon burning. The resulting structure of the SNI atmosphere expanding with the velocity gradient can be employed for an
interpretation of the observed SNI spectra. In accordance with our previous papers, the SNI light curves are considered to
occur due to an additional slow (with time-scale 106–107 s) release of the bulk of the SNI energy,W≃1051, erg. The slow energy release does not, however, affect the structure of the outermost expanding layers of the envelope which
are responsible for the SNI spectra.
A short (Δt≃10−2 s) burst of soft (2–10 keV) X-rays with total radiated energy of about 1040 erg is found to appear 10–20 days before the SNI optical maximum. 相似文献
19.
Shock wave and thermodynamic data for rock-forming and volatile-bearing minerals are used to determine minimum impact velocities (vcr) and minimum impact pressures (pcr) required to form a primary H2O atmosphere during planetary accretion from chondritelike planetesimals. The escape of initially released water from an accreting planet is controlled by the dehydration efficiency. Since different planetary surface porosities will result from formation of a regolith, vcr and pcr can vary from 1.5 to 5.8 km/sec and from 90 to 600 kbar, respectively, for target porosities between 0 and ～45%. On the basis of experimental data, hydration rates for forsterite and enstatite are derived. For a global regolith layer on the Earth's surface, the maximum hydration rate equals 6 × 1010 g H2O sec?1 during accretion of the Earth. Attenuation of impact-induced shock pressure is modeled to the extent that the amount of released water as a function of projectile radius, impact velocity, weight fraction of water in the target, target porosity, and dehydration efficiency can be estimated. The two primary processes considered are the impact release of water bound in hydrous minerals (e.g., serpentine) and the subsequent reincorporation of free water by hydration of forsterite and enstatite. These processes are described in terms of model calculations for the accretion of the Earth. Parameters which lead to a primary atmosphere/hydrosphere are: an accretion time of ? 1.6 × 108years, the use of an accretion model defined by Weidenschilling (1974, 1976), a mean planetesimal radius of 0.5 km, a hydration rate of 6 × 1010 g H2O sec?1 inferred from a mean porosity of ～ 10% for the upper 1 km of the accreting Earth, and values for the dehydration efficiency, DE, of 0.55 and 0.07 for the maximum and minimum pressure decay model, respectively. Conditions which prohibit the formation of a primary atmosphere include an accretion time much longer than 1.6 × 108 years, a hydration rate for forsterite and enstatite well in excess of 6 × 1010 g H2O sec?1, and a dehydration efficiency DE < 0.07. We conclude that the concept of dehydration efficiency is of dominant importance in determining the degree to which an accreting planet acquires an atmosphere during its formation. 相似文献
20.
The structure of a hydrogen atom situated in an intense magnetic field is investigated. Three approaches are employed. An elementary Bohr picture establishes a crucial magnetic field strength,H a ?5×109G. Fields in excess ofH a are intense in that they are able to modify the characteristic atomic scales of length and binding energy. A second approach solves the Schrödinger equation by a combination of variational methods and perturbation theory. It yields analytic expressions for the wave functions and energy eigenvalues. A third approach determines the energy eigenvalues by reducing the Schrödinger equation to a one-dimensional wave equation, which is then solved numerically. Energy eigenvalues are tabulated for field strengths of 2×1010G and 2×1012 G. It is found that at 2×1012 G the lowest energy eigenvalue is changed from ?13.6 eV to about ?180 eV in agreement with previous variational computations. 相似文献