共查询到20条相似文献，搜索用时 15 毫秒
1.
William M. Jackson Victor Blunt Hua Lin Martina Green Gregory Olivera William H. Fink Yihan Bao Randall S. Urdahl Fida Mohammad Mansour Zahedi 《Astrophysics and Space Science》1996,236(1):29-47
A unified picture of the photodissociation of theC
2
H radical has been developed using the results from the latest experimental and theoretical work. This picture shows that a variety of electronic states ofC
2 are formed during the photodissociation of theC
2
H radical even if photoexcitation accesses only one excited state. This is because the excited states have many avoided corssings and near intersections where two electronic states come very close to one another. At these avoided crossings and near intersections, the excited radical can hop from one electronic state to another and access new final electronic states of theC
2 radical. The complexity of the excited state surfaces also explains the bimodal rotational distributions that are observed in all of the electronic states studied. The excited states that dissociate through a direct path are limited by dynamics to produceC
2 fragments with a modest amount of rotational energy, whereas those that dissociate by a more complex path have a greater chance to access all of phase space and produce fragments with higher rotational excitation. Finally, the theoretical transition moments and potential energy curves have been used to provide a better estimate of the photochemical lifetimes in comets of the different excited states of theC
2
H radical. The photochemically active states are the 22+, 22II, 32II, and 32+, with photodissociation rate constants of 1.0×10–6, 4.0×10–6, 0.7×10–6, and 1.3×10–6s–1, respectively. These rate constants lead to a total photochemical lifetime of 1.4×105 s. 相似文献
2.
The effects of the production on dust grain surfaces of molecular hydrogen in excited states have been investigated. On the
assumption that all of the H2 formed on the surface of grains has a sufficient level of excitation too vercome the energy barriers in the formation reactions
for the important OH and CH+ radicals, we consider the likely abundances of excited H2 (H2
*), OH and CH+ in various situations. Two different models are employed; the first links the H2
* abundance directly to that of H2 using a steady-state approximation, whilst the second considers the time-dependence of H2
*. The second model is applied to gas that has been subjected to a strong isothermal shock (specifically, the shock-induced
collapse of a diffuse cloud), which results in an extreme (high density, high atomic hydrogen abundance) environment. In general,
it is found that the presence of the excited H2 has only marginal effects on the chemistry of interstellar clouds. However, in the isothermal shock model, the abundances
of CH+ are significantly enhanced, but only on short timescales, whilst the effects on the OH abundances are smaller, but last longer.
We conclude that other than in such exceptional environments there are no obvious chemical signatures of the formation of
H2
*.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
3.
The effect of quasi-resonance energy transfer in collisions between H2 and H2O molecules in H2O maser sources is investigated. New data on the state-to-state rate coefficients for collisional transitions for H2O and H2 molecules are used in the calculations. The results of ortho-H2O level population inversion calculations for the 22.2-, 380-, 439-, and 621-GHz transitions are presented. The ortho-H2O level population inversion is shown to depend significantly on the population distribution of the para-H2 J = 0 and 2 rotational levels. The possibility of quasi-resonance energy transfer in collisions between H2 molecules at highly excited rotational-vibrational levels and H2O molecules is considered. The quasi-resonance energy transfer effect can play a significant role in pumping H2O masers in the central regions of active galactic nuclei and in star-forming regions. 相似文献
4.
A critical examination of the components of the recent impulsive loop flare model of Takakura is made. It is found that his analysis of the stability of the electron distribution resulting from anomalous heat conduction is in error and electron plasma waves would not be excited. Rather, in the regions where the electron/proton temperature ratioT
e/T
i 10, electrostatic ion-cyclotron waves would be excited and in the regions whereT
e 10, ion-acoustic waves would be excited. Ratios ofT
e/T
i 10 occur only in the late time development behind the conduction fronts. Since the anomalous resistivity due to electrostatic ion-cyclotron waves is fortuitously about 70% of the one used by Takakura, the general development will follow closely the one calculated by him. Because the anomalous resistivity due to ion-acoustic waves is about 95 times the one used by Takakura, the development in the parts of the loop whereT
e/T
i 10 for late times would be altered considerably.Also Guest Worker at NOAA Space Environment Laboratory, Boulder, Colorado, U.S.A. 相似文献
5.
This paper reports results from an experiment designed to measure the nascent rovibrational population of H2 molecules that have formed through the heterogeneous recombination of H atoms on the surface of cosmic dust analogues under
conditions approaching those of the interstellar medium (ISM). H2 that has formed on a highly oriented pyrolytic graphite (HOPG) surface has been detected, using laser induced resonance-enhanced
multi-photon ionization (REMPI), in the v = 1 (J= 0–3) rovibrational states at surface temperatures of 30 K and 50 K. These excited product molecules display rotational temperatures
significantly higher than the target surface temperature. These first results suggest that a considerable proportion of the
binding energy released on formation of the H2 is deposited in the surface, in addition to internal excitation of the product molecules.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
6.
Suresh Chandra 《Solar physics》1978,58(2):291-297
The populations of the excited state 2
P
3/2 relative to the ground state 2
P
1/2 have been investigated in C ii, N iii, O iv, Ne vi, Mg viii, Si x, and Si ii by considering all the radiative and collisional processes including the collisional transitions to the higher states which cascade to the upper level. The relative populations are used for the calculation of the line emissivities. The intensities of 76 320, 30 258 and 14 302 lines of Nevi, Mgviii, and Six ions respectively in the chromosphere-corona transition region are also calculated. 相似文献
7.
Suresh Chandra 《Solar physics》1982,75(1-2):133-137
The intensities of the lines for the transition 2
P
3
2/0
-2
P
1
2/0
in C ii, N iii, O iv, Ne vi, Mg viii, Si x, and Si ii in the chromosphere-corona transition region are investigated. The populations of the excited state 2
P
f3/2
p0
relative to the ground state 2
P
f1/2
0
are obtained by considering all the radiative and collisional transition processes including the collisional transitions to the higher states which cascade to the upper level. It is found that in the transition region the intensity (which may be expressed as a function of temperature only) increases with the charge on the ion for a sequence.On leave from D.N. College, Gulaothi 245 408, India. 相似文献
8.
G. S. Lakhina 《Astrophysics and Space Science》1990,165(1):153-161
The velocity shear of ion beams observed in the polar cusp region can drive the kinetic Alfvén modes unstable. A hot ion beam can excite both a resonant kinetic Alfvén wave instability and a nonresonant coupled Alfvén ion-acoustic wave instability. For the case of a cold ion beam only the latter instability is excited. For the altitude range of 5–7R
e
, velocity shearS0.04–1.0 is needed to excite the kinetic Alfvén wave instabilities. HereS=(dV
B
/
cB
dx), whereV
b
is the streaming velocity,and
cB
is the gyrofrequency of the bean ions. The excited modes have frequencies, in the satellite frame of reference, in the ULF frequency range. The noise generated by the velocity shear-driven Alfvén modes is electromagnetic in nature. These modes have a substantial component of parallel electric fields and, therefore, they can play an important role in the ionosphere-magnetosphere coupling process occurring in the polar cusp region. 相似文献
9.
The equations of motion of the 2+2 body problem (two interacting particles in the gravitational field of two much more massive primaries m1 and m2 in circular keplerian orbit) have an integral analogous to the Jacobi integral of the circular 2+1 body problem. We show here that with 2+2 bodies this integral does not give rise to Hill stability, i.e. to confinement for all time in a portion of the configuration space not allowing for some close approaches to occur. This is because all the level manifolds are connected and all exchanges of bodies between the regions surroundingm
1,m
2 and infinity do not contradict the conservation of the integral. However, it is worth stressing that some of these exchanges are physically meaningless, because they involve either unlimited extraction of potential energy from the binary formed by the small bodies (without taking into account their physical size) or significant mutual perturbations between the small masses without close approach, a process requiring, for the Sun-Jupiter-two asteroids system, timescales longer than the age of the Solar System. 相似文献
10.
We study a nonlinear mechanism for the excitation of kinetic Alfvén waves (KAWs) by fast magneto-acoustic waves (FWs) in the
solar atmosphere. Our focus is on the excitation of KAWs that have very small wavelengths in the direction perpendicular to
the background magnetic field. Because of their small perpendicular length scales, these waves are very efficient in the energy
exchange with plasmas and other waves. We show that the nonlinear coupling of the energy of the finite-amplitude FWs to the
small-scale KAWs can be much faster than other dissipation mechanisms for fast wave, such as electron viscous damping, Landau
damping, and modulational instability. The nonlinear damping of the FWs due to decay FW = KAW + KAW places a limit on the
amplitude of the magnetic field in the fast waves in the solar corona and solar-wind at the level B/B
0∼10−2. In turn, the nonlinearly excited small-scale KAWs undergo strong dissipation due to resistive or Landau damping and can
provide coronal and solar-wind heating. The transient coronal heating observed by Yohkoh and SOHO may be produced by the kinetic Alfvén waves that are excited by parametric decay of fast waves propagating from
the reconnection sites. 相似文献
11.
Under a fully electromagnetic treatment, the threshold for excitation of the lower hybrid instability driven by solar wind electron heat flux is found to be much higher than that predicted by electrostatic approximation. For average solar wind conditions at 1 AU, the fully electromagnetic lower hybrid instability is excited when the core electron drift speed is about 8V
A, whereV
A is the Alfvén speed. The region between the Sun and 1 AU is expected to be more favourable than 1 AU for this instability. 相似文献
12.
T. Takakura 《Solar physics》1987,107(2):283-297
Numerical simulation for the dynamics of a coronal filamentary magnetic loop has been made under the assumption that the field is initially force-free and an electric resistivity suddenly increases at a given moment due to an appearance of ion sound waves, which can be excited due to a high current density if a characteristic radius r
0 of the magnetic loop is about 3 km or less in a magnetic field B
0 of 1000 G. During the resistive decay of the magnetic field a strong field-aligned electric field is created and maintained for a sufficient time to acceleratie both electrons and protons to a high energy, which is proportional to B
0/r
0 and can be 100 MeV if r
0 = 10 km and B
0 = 1000 G. If the coronal magnetic tube is composed of many such filamentary loops, the total number of accelerated electrons is consistent with the observations. 相似文献
13.
M. H. Rodríguez 《Astrophysics and Space Science》1984,102(1):123-129
The shell spectrum of CH Cyg observed in 1981 has been analyzed by the curve of growth method and using the equivalent widths of Balmer lines. The following parameters for the region of the shell where the absorption spectrum is formed have been obtained: excitation temperatureT
ex=9000 K, microturbulent velocityv
t
=16 km s–1, electron densityn
e=6.3×1012, number of active hydrogen atoms in the second excited stateN
0.2=5.8×1015, and the effective height of the absorption spectrum formation regionH=105 km. 相似文献
14.
Dieter Biskamp 《Astrophysics and Space Science》1996,242(1-2):165-207
A review of the present status of the theory of magnetic reconnection is given. In strongly collisional plasmas reconnection proceeds via resistive current sheets, i.e. quasi-stationary macroscopic Sweet-Parker sheets at intermediate values of the magnetic Reynolds numberR
m
, or mirco-current sheets in MHD turbulence, which develops at highR
m
. In hot, dilute plasmas the reconnection dynamics is dominated by nondissipative effects, mainly the Hall term and electron inertia. Reconnection rates are found to depend only on the ion mass, being independent of the electron inertia and the residual dissipation coefficients. Small-scale whistler turbulence is readily excited giving rise to an anomalous electron viscosity. Hence reconnection may be much more rapid than predicted by conventional resistive theory. 相似文献
15.
T. N. Chakrabarty 《Journal of Astrophysics and Astronomy》2004,25(1-2):93-101
Radial dipole matrix elements having astrophysical importance have been computed for highly excited states of hydrogen atom.
Computation is based on Heisenberg’s form of correspondence principle for Coulomb potential. Particular attention has been
paid to the choice of classical analogue(n
c) of principal quantum number (n). The computed radial matrix elements are in good agreement with quantum mechanical results. Further, radial matrix elements
for few transitions involving highn neighboring states of hydrogen atom are presented. 相似文献
16.
It is proposed that energy transfer from excited O2 contributes to the production of O(1S) in aurora. An analysis is presented of the OI5577 Å emission in an IBC II+ aurora between 90 and 130 km. The volume emission rate of the emission at these altitudes is consistent with the production rate of O(1S) by energy transfer to O(3P) from N2 in the A3Σ2+ state and O2 in the A3Σu+, C3Δc1Σu? states, the N2A state being populated by direct electron impact excitation and B → A cascade and the excited O2 states by direct excitation. Above the peak emission altitude (~105 km), energy transfer from N2A is the predominant production mechanism for O(1S). Below it, the contribution from quenching of the O2 states becomes significant. 相似文献
17.
We consider a model for the cyclic activity of young binary stars that accrete matter from the remnants of a protostellar
cloud. If the orbit of such a binary system is inclined at a small angle to the line of sight, then the streams of matter
and the density waves excited in the circumbinary disk can screen the primary component of the binary from the observer. To
study these phenomena by the SPH (smoothed particle hydrodynamics) method, we have computed grids of hydrodynamic models for
binary systems based on which we have constructed the light curves as a function of the orbital phase. The main emphasis is
on investigating the properties of the brightness oscillations. Therefore, the model parameters were varied within the following
ranges: the component mass ratio q = M
2: M
1 = 0.2–0.5 and the eccentricity = 0–0.7. The parameter that defined the binary viscosity was also varied. We adopted optical
grain characteristics typical of circumstellar dust. Our computations have shown that bimodal oscillations are excited in
binaries with eccentric orbits, provided that the binary components do not differ too much in mass. In this case, the ratios
of the periods and amplitudes of the bimodal oscillations and their shape depend strongly on the inclination of the binary
plane and its orientation relative to the observer. Our analysis shows that the computed light curves can be used in interpreting
the cyclic activity of UX Ori stars. 相似文献
18.
The N2 vibrational distribution is calculated for a specific IBC Class II aurora using as input, data obtained from coordinated rocket and satellite observations and currently accepted excitation and quenching rates. The results of the calculations indicate no significant vibrational enhancements for this specific aurora nor for “upper limit” estimates for more intense aurorae. We conclude from this study that if significantly larger concentrations of vibrationally excited N2 molecules exist in the aurora, as recent rocket e.u.v. measurements suggest, current concepts of the sources and sinks of N2 vibrational excitation will require significant revision. 相似文献
19.
Some astrophysical observations of molecular hydrogen point to a broadening of the velocity distribution for molecules at excited rotational levels. This effect is observed in both Galactic and high-redshift clouds. Analysis of H2, HD, and CI absorption lines has revealed the broadening effect in the absorption system of QSO 1232+082 (z abs = 2.33771). We analyze line broadening mechanisms by considering in detail the transfer of ultraviolet radiation (in the resonance lines of the Lyman and Werner H2 molecular bands) for various velocity distributions at excited rotational levels. The mechanism we suggest includes the saturation of the lines that populate excited rotational levels (radiative pumping) and manifests itself most clearly in the case of directional radiation in the medium. Based on the calculated structure of a molecular hydrogen cloud in rotational level populations, we have considered an additional mechanism that takes into account the presence of a photodissociation region. Note that disregarding the broadening effects we investigated can lead to a significant systematic error when the data are processed. 相似文献
20.
N. I. Rovenskaya 《Astrophysics and Space Science》1992,188(1):89-107
Treating it as a boundary-value problem, the equations of highly excited state populations of atoms and ions are theoretically studied in case of recombining plasma. Scattering and spontaneous transitions as well as those induced by background radiation, are taken into account in the kinetic equations. The kinetic coefficients for inelastic scattering of incident charged particles on highly excited atoms and ions have been calculated in the asymptotically exact case: 1.6×105 (z/n)2)T
e
–1
1.The distribution functions over the Rydberg states, analytically found, allow to determine amplification factor and optical depth of radio-recombination lines as functions of cosmic plasma parameters. 相似文献