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1.
本文根据波与介质相互作用的一套全MHD方程组,计算了无碰撞阿尔文波波能密度W和波能耗散项E_m,在太阳过渡区和内冕大气中随高度的分布。 计算结果表明:对于温度、密度偏低的大气,在过渡区底部几十甚至几百公里范围内,无碰撞阿尔文波的耗散引起的对大气的加热可超过热传导的贡献。从而说明这种阿尔文波的加热似乎是引起温度、密度偏低的大气(例如冕洞大气)在过渡区中温度陡升的重要原因。 相似文献
2.
It has been established that small scale heating events, known as nanoflares, are important for solar coronal heating if the
power-law distribution of their energies has a slope α steeper than −2 (α<−2). Forward modeling of impulsively heated coronal loops with a set of prescribed power-law indices α is performed. The power-law distribution is incorporated into the governing equations of motion through an impulsive heating
term. The results are converted into synthetic Hinode/EIS observations in the 40″ imaging mode, using a selection of spectral lines formed at various temperatures. It is shown
that the intensities of the emission lines and their standard deviations are sensitive to changes in α. A method based on a combination of observations and forward modeling is proposed for determining whether the heating in
a particular case is due to small or large scale events. The method is extended and applied to a loop structure that consists
of multiple strands. 相似文献
3.
We consider the heating of the solar plages to be due to the Alfven waves having ponderomotive force. Assuming a certain field configuration, we derived the formula for the turbulence heating of plages by Alfven waves. Our calculated plage temperature distribution is in agreement with the observed distribution. 相似文献
4.
Stability of magnetic configurations in the solar atmosphere under temperature anisotropy conditions
P. P. Malovichko 《Kinematics and Physics of Celestial Bodies》2008,24(5):236-241
The anisotropic instability of Alfven waves in the solar atmosphere is considered. This mechanism is shown to lead to the generation of not only Alfven but also kinetic Alfven waves, which is very important in investigating the heating and acceleration of particles in the chromospheric and coronal plasma. A criterion for the development of instability has been found. The conditions under which this instability can arise and the atmospheric regions in which its development is most probable are analyzed. It is shown that this generation mechanism of kinetic Alfven waves is fairly efficient and can play a significant role in some processes in the solar atmosphere. 相似文献
5.
Two competing fundamental hypotheses are usually postulated in the solar coronal heating problem: heating by nanoflares and heating by waves. In the latter it is assumed that acoustic and magnetohydrodynamic disturbances whose amplitude grows as they propagate in a medium with a decreasing density come from the convection zone. The shock waves forming in the process heat up the corona. In this paper we draw attention to yet another very efficient shock wave generation process that can be realized under certain conditions typical for quiet regions on the Sun. In the approximation of stationary dissipative hydrodynamics we show that a shock wave can be generated in the quiet solar chromosphere–corona transition region by the fall of plasma from the corona into the chromosphere. This shock wave is directed upward, and its dissipation in the corona returns part of the kinetic energy of the falling plasma to the thermal energy of the corona. We discuss the prospects for developing a quantitative nonstationary model of the phenomenon. 相似文献
6.
V. Krishan 《Journal of Astrophysics and Astronomy》1981,2(4):379-385
The mechanism of spatial resonance of Alfven waves for heating a collisionless plasma is studied in the presence of a twisted
magnetic field. In addition to modifying the equilibrium condition for a cylindrical plasma, the azimuthal component of the
magnetic field gives extra contribution to the energy deposition rate of the Alfven waves. This new term clearly brings out
the effects associated with the finite lifetime of the Alfven waves. The theoretical system considered here conforms to the
solar coronal regions. 相似文献
7.
Given the complexity involved in a flux-transport-type dynamo driven by both Babcock – Leighton and tachocline α effects, we present here a step-by-step procedure for building a flux-transport dynamo model calibrated to the Sun as a guide
for anyone who wishes to build this kind of model. We show that a plausible sequence of steps to reach a converged solution
in such a dynamo consists of (i) numerical integration of a classical α – ω dynamo driven by a tachocline α effect, (ii) continued integration with inclusion of meridional circulation to convert the model into a flux-transport dynamo
driven by only a tachocline α effect, (iii) final integration with inclusion of a Babcock – Leighton surface α effect, resulting in a flux-transport dynamo that can be calibrated to obtain a close fit of model output with solar observations. 相似文献
8.
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
9.
Kinetic Alfven waves (KAWs) driven by the diamagnetic drift instability that is excited by the density inhomogeneity in low-β plasmas, such as plasmas in the auroral region, are investigated by adopting the particle aspect analysis and loss-cone distribution function. The results obtained in this paper indicate that the propagation and evolution of kinetic Alfven waves decrease and the kinetic Alfven wave excitation becomes not easier with increasing loss-cone index J. But the spatial scales of the perpendicular perturbation driving kinetic Alfven waves have a decreasing tendency with the larger values of J, which perhaps is in relation with the decreasing width of loss-cone. A single hump appears in the plots of the growth rate of the instability when J=2. But the hump cannot emerge when J=0 or J=1. The density inhomogeneity of ions plays an important role in driving KAWs and it cannot be ignored. KAWs can be easier driven and KAWs can propagate and evolve faster with the increasing level of density inhomogeneity. However, the range of the perpendicular wave number of the wave instability decreases, namely, the longer the scale of perpendicular disturbance the easier the excitation of KAW. As the density inhomogeneity increases, the tendency of numerical solutions of the dispersion relation is similar to that obtained by the kinetic theory and Maxwellian distribution function (Duan and Li, 2004). But the profiles of the plots of numerical solutions are different. This means that the velocity distribution function of particles is important for KAW driven in magnetoplasmas, especially in the active regions of the magnetosphere, such as auroral region, and plasma sheet boundary. 相似文献
10.
《Chinese Astronomy and Astrophysics》2005,29(2):136-148
The method of Orthogonal Function Series Expansion (OFSE) is generalized and applied to the study of the evolution of the coupling of nondissipative torsional Alfven wave and fast wave in coronal loops. Using this method, the intrinsic angular frequency of the overall wave mode can be described mathematically and that of the Alfven waves along the magnetic lines in the coronal loop during the coupling of the Alfven and fast waves can be analyzed both theoretically and numerically. Also with this method, the relation between the coupling driven term and the Alfven wave resonance may be analyzed. Results of computation reveal the place of appearance of coupling resonance as well as the characteristics of the amplitudes of the Alfven and fast waves. As found by the calculations, if the footpoint driven angular frequency is not equal to the intrinsic angular frequency of the overall wave mode of the coronal loop and when a δ section appears at the place of coupled resonance, the radial gradient of the fast wave's amplitude is quite large. Sometimes it approximates to a discontinuity, and this is extremely favorable for the dissipation of the fast wave. If the footpoint driven angular frequency is equal to the intrinsic angular frequency of the overall wave mode and when a δ section occurs in the Alfven wave amplitude, abundant small-scale structures appear in the radial direction. Then the location of resonance approximately becomes a discontinuity, very favorable to the dissipation of the Alfven wave. 相似文献
11.
This paper is a pedagogical introduction to avalanche models of solar flares, including a comprehensive review of recent modeling
efforts and directions. This class of flare model is built on a recent paradigm in statistical physics, known as self-organized
criticality. The basic idea is that flares are the result of an ‘avalanche’ of small-scale magnetic reconnection events cascading
through a highly stressed coronal magnetic structure, driven to a critical state by random photospheric motions of its magnetic
footpoints. Such models thus provide a natural and convenient computational framework to examine Parker's hypothesis of coronal
heating by nanoflares.
Supplementary material to this paper is available in electronic form at http://dx.doi.org/10.1023/A:1013301521745 相似文献
12.
Peter J. Cargill 《Solar physics》1993,147(2):263-268
Implications of the conjecture that the solar corona is heated by small-scale events (nanoflares) are examined. It is shown that even if these nanoflares are small-scale (heating areas much smaller than 1 arc sec2) and impulsive, the corona may still be rather homogeneous. In particular, the filling factor (defined as the ratio of the coronal volume radiating in X-rays and EUV to the total volume) can easily be of order unity. Future experimental determination of the filling factor could prove useful in estimating the volume of coronal material that is heated during nanoflares.Work performed at Beam Physics Branch, Plasma Physics Division, Naval Research Laboratory, Washington, D.C., U.S.A.Mailing Address: Code 6790, NRL, Washington, D.C. 20375, U.S.A. 相似文献
13.
Mao Xin-jie 《Astrophysics and Space Science》1998,260(3):397-404
The configuration of the magnetic field associated with a protostar surrounded by a circumstellar disk is assumed to be a
kind of magnetic mirror, which reflects the particles at its throat located nearby the disk midplane, and then extracts them
out of the star and the disk. Turbulent Alfven waves are excited due to anisotropic temperature distribution caused by the
existing magnetic field in the environment. Accelerated by turbulent Alfven waves, the particles coming out of the young stellar
object and the circumstellar disk can reach the expected velocities around 300 km s-1 at a typical distance 0.1 pc from the central star. The wave energy is converted from the thermal energy stored in the system
consisting of the early stage star associated with the disk and their environment, and a small fraction of which is enough.
The coefficient η, indicating the efficiency of converting thermal energy to wave energy, is equal to 10-11.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
14.
The influences of the shock thickness and Alfven waves on the particle acceleration by diffusive shock waves are numerically studied through solving one-dimensional diffusive equation including the second-order Fermi effect. It is shown that the spectral index of the energetic particles strongly depends on the shock thickness. For example, the spectral index increases from 2.1 to 3.7 in the low energy range of 3—10 MeV and from 2.5 to 5.0 in the high energy range of 20—60 MeV as the thickness increases. The spectral index decreases from 4.3 to 3.1 as the particle injection energy increases. The spectral index decreases from 4.0 to 1.8 at the quasi-steady stage with the enhancement of the compression ratio from 2 to 4. The results indicate that under the influence of Alfven waves, the energetic particle spectrum at lower energy becomes flat and the spectral index decreases from 2.5 to 0.6 in the low energy range of 3—10 MeV and from 11.6 to 5.0 in the high energy range of 20—60 MeV. At the same time, the turning point energy reaches 19.6 MeV. The spectral index decreases from 5.8 to 2.9 as the energy density of Alfven waves increases. All these results are basically consistent with the theoretical models, as well as the observations of typical energetic particle events. 相似文献
15.
The effect of alpha particles on the dispersion relation of ion cyclotron waves and its influence on the heating of the solar
wind plasma are investigated. The presence of alpha particles can dramatically change the dispersion relation of ion cyclotron
waves, and significantly influence the way that ion cyclotron waves heat the solar wind plasma. We find that a spectrum of
ion cyclotron waves affects the thermal anisotropy of the solar wind protons and other ions differently in interplanetary
space: When alpha particles have a speed u
α>0.5v
A, and both protons and alpha particles have a thermal anisotropy T
⊥/T
∥>1, ion cyclotron waves heat protons in the direction perpendicular to the magnetic field, cool them in the parallel direction,
and exert the opposite effect on alpha particles. 相似文献
16.
Interaction of weak shock waves with a current sheet is investigated by a two-dimensional numerical magnetohydrodynamic model. In accordance with solar coronal conditions, a ratio of thermal to magnetic pressures of 0.1 and a shock Alfvén Mach number slightly above 1 are considered. It is found that even weak shock waves trigger magnetic field reconnection in current sheets. Based on this result, it is suggested that drifting chains of type I radio bursts are radio manifestations of the interactions of weakly super-Alfvénic shock waves with pre-existing current sheets distributed in an active region. This model of type I noise storms is then discussed in connection with the concept of nanoflares (localized reconnections) and the heating of the solar corona. 相似文献
17.
James A. Klimchuk 《Solar physics》2006,234(1):41-77
The question of what heats the solar corona remains one of the most important problems in astrophysics. Finding a definitive
solution involves a number of challenging steps, beginning with an identification of the energy source and ending with a prediction
of observable quantities that can be compared directly with actual observations. Critical intermediate steps include realistic
modeling of both the energy release process (the conversion of magnetic stress energy or wave energy into heat) and the response
of the plasma to the heating. A variety of difficult issues must be addressed: highly disparate spatial scales, physical connections
between the corona and lower atmosphere, complex microphysics, and variability and dynamics. Nearly all of the coronal heating
mechanisms that have been proposed produce heating that is impulsive from the perspective of elemental magnetic flux strands.
It is this perspective that must be adopted to understand how the plasma responds and radiates. In our opinion, the most promising
explanation offered so far is Parker's idea of nanoflares occurring in magnetic fields that become tangled by turbulent convection.
Exciting new developments include the identification of the “secondary instability” as the likely mechanism of energy release
and the demonstration that impulsive heating in sub-resolution strands can explain certain observed properties of coronal
loops that are otherwise very difficult to understand. Whatever the detailed mechanism of energy release, it is clear that
some form of magnetic reconnection must be occurring at significant altitudes in the corona (above the magnetic carpet), so
that the tangling does not increase indefinitely. This article outlines the key elements of a comprehensive strategy for solving
the coronal heating problem and warns of obstacles that must be overcome along the way. 相似文献
18.
Observations indicate that in plage areas (i.e. in active regions outside sunspots) acoustic waves travel faster than in the quiet Sun, leading to shortened travel times
and higher p-mode frequencies. Coupled with the 11-year variation of solar activity, this may also explain the solar cycle variation of
oscillation frequencies. While it is clear that the ultimate cause of any difference between the quiet Sun and plage is the
presence of magnetic fields of order 100 G in the latter, the mechanism by which the magnetic field exerts its influence has
not yet been conclusively identified. One possible such mechanism is suggested by the observation that granular motions in
plage areas tend to be slightly “abnormal”, dampened compared to the quiet Sun.
In this paper we consider the effect that abnormal granulation observed in active regions should have on the propagation of
acoustic waves. Any such effect is found to be limited to a shallow surface layer where sound waves propagate nearly vertically.
The magnetically suppressed turbulence implies higher sound speeds, leading to shorter travel times. This time shift Δ
τ is independent of the travel distance, while it shows a characteristic dependence on the assumed plage field strength. As
a consequence of the variation of the acoustic cutoff with height, Δ
τ is expected to be significantly higher for higher frequency waves within the observed regime of 3 – 5 mHz. The lower group
velocity near the upper reflection point further leads to an increased envelope time shift, as compared to the phase shift.
p-mode frequencies in plage areas are increased by a corresponding amount, Δ
ν/ν=ν
Δ
τ. These characteristics of the time and frequency shifts are in accordance with observations. The calculated overall amplitudes
of the time and frequency shifts are comparable to, but still significantly less than (by a factor of 2 to 5), those suggested
by measurements. 相似文献
19.
《Chinese Astronomy and Astrophysics》1984,8(1):21-25
We investigate the effect of Alfven waves on the profile of the g = 0 lines FeI 5691.505 and FeI 5434.534. We find nonnegligible oscillation and distortion of the profile and vastly different lines. We calculate the theoretical profiles of these two lines under the action of Alfven waves, compare them with observations and hence verify preliminarily that sunspots are cooled by the propagation of Alfven waves. 相似文献
20.
扼要地介绍了色球和日冕加热问题的研究历史。随着空间太阳观测技术的进步，人们认识到色球和日冕加热机制主要与MHD过程有关。因此，在本文中着重介绍四种MHD色球和日冕加热机制：（1）阿尔芬波；（2）MHD湍动；（3）场向电流；（4）磁重联。由于这四种加热机制的有效性都需要通过高分辨率观测来判定，所以空间太阳观测对于研究色球和日冕加热问题具有重大意义。 相似文献