共查询到20条相似文献，搜索用时 46 毫秒
1.
Doyle J. G. O'Shea E. Erdélyi R. Dere K. P. Socker D. G. Keenan F. P. 《Solar physics》1997,173(2):243-258
Nonthermal velocities are presented for spectral lines covering the temperature range 10 4–10 6 K, measured from high-spectral-resolution data for several solar features observed at the limb by the high resolution telescope and spectrograph (HRTS), including a coronal hole, quiescent regions and several small-scale active regions. These results are compared with predictions based on acoustic waves and heating via Alfvén waves. It is likely that more than one mechanism is operating simultaneously, in particular, resonant Alfvén wave heating, which is very sensitive to background plasma motions. 相似文献
2.
The excitation and dissipation of global and surface Alfvén waves and their conversion into kinetic Alfvén waves have been analyzed for solar coronal loops using a cylindrical model of a magnetized plasma. Also the optimal conditions for coronal loop heating regimes with density of dissipated power 103 erg cm–3 s–1 by the new scheme named combined Alfvén wave resonance are found. Combined Alfvén wave heating regime appears when the global Alfvén wave is immersed into the Alfvén continuum with the condition of not-so-sharp distribution of axial current.Instituto de Matemática, Universidade Federal Fluminense, Niterói, RJ, Brazil 相似文献
3.
We have modeled the solar coronal active loop heating by discrete Alfvén waves. Discrete Alfvén waves (DAW) are a new class of Alfvén waves which can be described by the two-fluid model with finite ion-cyclotron frequency, or the MHD model with plasma current along the magnetic field line as shown by Appert, Vaclavik, and Villar (1984). We have modeled the coronal loop as a semi-toroidal plasma with the major toroidal radius much larger than the plasma radius. We have shown that the absorption of discrete Alfvén waves by the plasma through viscosity can account for at least 30% of the coronal heating rate density of 10–4 J m–3 s–1. 相似文献
4.
Andreev V. E. Efimov A. I. Samoznaev L. N. Chashei I. V. Bird M. K. 《Solar physics》1997,176(2):387-402
A statistical study of Faraday rotation fluctuations (FRF) has been performed using polarization angle data of S-band (f = 2.3 GHz) radio spacecraft signals. The measurements were recorded during the recurring superior conjunctions of the Helios probes, during which the solar proximate point of the radio ray path reached heliocentric distances between 3 and 34 R. The most commonly found temporal FRF spectra are power laws with an average spectral index 1.5 over the frequency range from 1 mHz < v < 10 mHz. The FRF variance decreases with heliocentric distance, the falloff exponent being 8 for R < 6 R and 3 for distances 8 < R < 6 R < 16 R. The results are interpreted under the assumption that the FRF are produced by Alfvén waves propagating in the coronal plasma. For the applicable range of heliocentric distances it is shown that Alfvén waves are in a regime of free propagation and probably transfer much of their energy to the kinetic energy of the solar wind. The spatial power spectrum of magnetic field fluctuations is inferred to be strongly anisotropic, the irregularities extending along the background magnetic field with axial ratios of the order of 10. 相似文献
5.
The effect of equilibrium flow on linear Alfvén resonances in coronal loops is studied in the compressible viscous MHD model. By means of a finite element code, the full set of linearised driven MHD equations are solved for a one-dimensional equilibrium model in which the equilibrium quantities depend only on the radial coordinate. Computations of resonant absorption of Alfvén waves for two classes of coronal loop models show that the efficiency of the process of resonant absorption strongly depends on both the equilibrium parameters and the characteristics of the resonant wave. We find that a steady equilibrium shear flow can also significantly influence the resonant absorption of Alfvén waves in coronal magnetic flux tubes. The presence of an equilibrium flow may therefore be important for resonant Alfvén waves and coronal heating. A parametric analysis also shows that the resonant absorption can be strongly enhanced by the equilibrium flow, even up to total dissipation of the incoming wave. 相似文献
6.
The nonlinear excitation of fast magnetosonic waves by phase mixing Alfvén waves in a cold plasma with a smooth inhomogeneity of density across a uniform magnetic field is considered. If initially fast waves are absent from the system, then nonlinearity leads to their excitation by transversal gradients in the Alfvén wave. The efficiency of the nonlinear Alfvén–fast magnetosonic wave coupling is strongly increased by the inhomogeneity of the medium. The fast waves, permanently generated by Alfvén wave phase mixing, are refracted from the region with transversal gradients of the Alfvén speed. This nonlinear process suggests a mechanism of indirect plasma heating by phase mixing through the excitation of obliquely propagating fast waves. 相似文献
7.
The propagation and interference of Alfvén waves in magnetic regions is studied. A multilayer approximation of the standard models of the solar atmosphere is used. In each layer, there is a linear law of temperature variation and a power law of Alfvén velocity variation. The analytical solutions of a wave equation are stitched at the layer boundaries. The low-frequency Alfvén waves (P > 1 s) are able to transfer the energy from sunspots into the corona by tunneling only. The chromosphere is not a resonance filter for the Alfvén waves. The interference and resonance of Alfvén waves are found to be important to wave propagation through the magnetic coronal arches. The transmission coefficient of Alfvén waves into the corona increases sharply on the resonance frequences. To take into account the wave absorption in the corona, a method of equivalent schemes is developed. The heating of a coronal arch by Alfvén waves is discussed. 相似文献
8.
It has been shown that Alfvén waves can drive non-inductive current in solar coronal loops via collisional or collisionless damping. Assuming that all the coronal-loop density of dissipated wave power (W= 10–3 erg cm–3 s–1), which is necessary to keep the plasma hot, is due to Alfvén wave electron heating, we have estimated the axial current density driven by Alfvén waves to be jz 103–105 statA cm–2. This current can indeed support the quasi-stationary equilibrium and stability of coronal loops and create the poloidal magnetic field up to B
1–5 G. 相似文献
9.
In 1988, Uchida and Shibata proposed a model for compact loop flares as due to the collision of two large amplitude torsional Alfvén wave packets coming up along a coronal magnetic loop, leaking out from the subphotospheric convective layers of the solar atmosphere. We investigate the possibility that active region transient brightenings occur when a single torsional Alfvén wave packet transits a coronal loop. Assuming this related origin for flares and transient brightenings, the statistics of the two phenomena must also be closely related. It is shown that the observed power-law frequency-energy distributions of flares and transient brightenings may be accounted for in a natural way if the energy distribution of the underlying torsional Alfvén wave packets is itself a power law. 相似文献
10.
G. S. Lakhina 《Astrophysics and Space Science》1987,133(2):203-218
The pick up cometary ion distributions are shown to excite Alfvénic mode instabilities, slow ion-acoustic mode instability and a lower hybrid instability during solar wind-comet interaction. The growth rates of all these instabilities become larger as the comet is approached. The lower hybrid instability is shown to account for the low-frequency 0–300 Hz electrostatic turbulence observed near comet Halley. The Alfvén modes can grow to large amplitudes and become modulationally unstable, in the presence of low-frequency density fluctuations, going over to envelope Alfvén solitons. A model consisting of a gas of Alfvén solitons is suggested to explain the hydromagnetic turbulence observed near comet Halley and comet Giacobini-Zinner. 相似文献
11.
The importance of Alfvén wave generation in interacting plasmas is discussed in general and illustrated by the example of solar wind interaction with cometary plasma. The quasi-linear theory of Alfvén wave generation by cometary ions at distances far from the cometary nucleus is reviewed. The incorporation of a diabatic plasma compression effects into this theory modifies the spectrum of Alfvén waves and the integral intensity of magnetic field fluctuations previously published. These results are in quantitative agreement with thein situ observations near the comets Giacobini-Zinner and Halley. However, the polarization of quasi-linearly excited waves needs further detailed comparison with observations.Paper dedicated to Professor Hannes Alfvén on the occasion of his 80th birthday, 30 May 1988. 相似文献
12.
Yurii M. Voitenko 《Solar physics》1996,168(1):219-222
The fast heating of coronal loops at the very beginning of solar flares is discussed as a first observable signature of downstreaming 0.1–1 MeV proton beams. We place emphasis on the produced by proton beams large-scale upward electric field, supporting beam-driven instability of the kinetic Alfvén waves (KAW). The considerable part of a beam kinetic energy may be converted into heat via intermediate KAW with heated region initially spreading from the top downward to footpoints of the loop-like coronal structures with velocity of about a few thousands km/s. 相似文献
13.
The analysis of observational data has shown that the duration of a pulse train in type IV radio bursts decreases with increasing hardness of the spectrum of high-energy protons and increases with decreasing proton fluxes from the Sun. It is shown that such a correlation corresponds to a magnetohydrodynamic (MHD) model of pulsations and is inexplicacable within the framework of a nonlinear periodical regime of plasma instabilities. The pulse train duration is determined by proton pitch-angle diffusion caused by Alfvén waves in coronal magnetic loops. A method of predicting solar proton hardness and proton fluxes using type IV radio burst pulsations is proposed. 相似文献
14.
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
15.
Chuan-Yi Tu 《Solar physics》1971,109(1):149-186
A new solar wind model has been developed by including in the model the Alfvénic fluctuation power spectrum equation proposed by Tu et al. (1984). The basic assumptions of the model are as follows: (1) for heliocentric distances r > 10 R
, the radial variation of the power spectrum of Alfvénic fluctuations is controlled by the spectrum equation (1), (2) for heliocentric distances r < 10 R
, the radial variation of the fluctuation amplitude is determined by the Alfvén wave WKB solution, (3) no energy cascades from the low-frequency boundary of the Alfvénic fluctuation power spectrum into the fluctuation frequency range, and the energy which cascades from the high-energy boundary of the spectrum into the higher frequency range is transported to heat of the solar wind flow. Some solutions of this model which, on one hand, describe the major properties of the Alfvénic fluctuations and the high-speed flow observed by Helios in the space range between 0.3–1 AU and, on the other hand, are consistent with the observational constraints at the coronal base have been obtained under the following conditions: (1) the spectrum index of the fluctuations is near to -1 for almost the whole frequency range at 10 R
, (2) the particle flux density at 1 AU is not greater than 3 × 108 cm–2 s–1, (3) the solution is for spherically-symmetric flow geometry or the solution passes through the outermost of the three critical points of the rapidly diverging flow geometry with f
max = 7. Some solutions passing through the innermost critical point of the rapidly diverging flow geometry with f
max = 7 have been found, however, with too low pressure at the coronal base to compare with the observational constraints. Heat addition or other kind of momentum addition for r < 10 R
is required to modify this model to yield better agreement with observations. A cascade energy flux function which leads to Kolmogorov power law in the high-frequency range of Alfvénic fluctuations is presented in Appendix A. More detailed discussions about the characteristics, the boundary conditions and the solution of the spectrum equation (1) are given in Appendix B.Projects supported by the Science Fund of the Chinese Academy of Sciences. 相似文献
16.
A time sequence over 80 min of coronal green-line spectra was obtained with a corona- graph at the Norikura Solar Observatory. Doppler velocities, line intensities, and line widths were derived through fitting a single Gaussian to the observed line profiles. Coronal waves have been clearly detected in the Doppler velocity data. The Fourier analysis shows powers in a 1–3 mHz range, and in higher frequencies (5–7 mHz) at localized regions. The propagation speed of the waves was estimated by correlation analysis. The line intensity and line width did not show clear oscillations, but their phase relationship with the Doppler velocity indicates propagating waves rather than standing waves. The existence of Alfvén waves whose speed is 500 km s–1 or faster is possible but inconclusive, while the existence of slower waves (of the order of 100 km s–1, possibly sound waves) is evident. The energy carried by the detected sound waves is far smaller than the required heat input rate to the quiet corona. 相似文献
17.
A. I. Efimov L. N. Samoznaev V. E. Andreev I. V. Chashei M. K. Bird 《Astronomy Letters》2000,26(8):544-552
A statistical analysis of the Faraday-rotation fluctuations (FRFs) of linearly polarized radio signals from the Helios 1 and Helios 2 spacecraft shows that the FRF time power spectra can be of three types. Spectra of the first type are well fitted by a single power law in the range of fluctuation frequencies 1–10 mHz. Spectra of the second type are a superposition of a power law and two quasi-harmonic components with fluctuation frequencies of about v1=4 mHz (fundamental frequency) and v2=8 mHz (second harmonic). Spectra of the third type exhibit only one of the two quasi-harmonic components against the background of a power law. The spectral density of the quasi-harmonic components can be represented by a resonance curve with a fairly broad [Δυ ≈ (0.5–1.3)υ1,2] distribution relative to the v=v1, 2 peak. The intensity of the quasi-harmonic FRF has a radial dependence that roughly matches the radial dependence for the background FRF, while their period at the fundamental frequency is approximately equal to the period of the wellknown 5-min oscillations observed in the lower solar atmosphere. The fluctuations with 5-min periods in FRF records can be explained by the presence in the outer corona of isolated trains of Alfvén waves generated at the base of the chromosphere-corona transition layer and by acoustic waves coming from deeper layers. 相似文献
18.
The propagation of Alfvén waves in a simple model of a sunspot is considered. The vertical structure near the center of the umbra is modelled realistically, but the horizontal structure is not considered. The full wave equation is solved, without recourse to the WKB approximation. Only wave propagation in the vicinity of the central field line in an axially symmetric spot is examined, and it is assumed that this field line is open. By taking wave reflections into account, we find that the observations of non-thermal motions near the temperature minimum (Beckers, 1976) and in the corona (Beckers and Schneeberger, 1977) are both consistent with an upward-propagating Alfvénic energy flux density of a few times 107 erg cm–2 s–1. This flux density is too small to cool the sunspot, but it is large enough to supply the energy requirements of the transition region and corona above a sunspot. This conclusion depends on the assumptions that the observed motions are indeed Alfvénic with periods near 180 s.The National Center for Atmospheric Research is sponsored by the National Science Foundation. 相似文献
19.
In a binary system of a background fluid-wave field, the wave effect may be important in some cases. From general properties of thermodynamics of the medium, we derive the coupling equations for the mean flow-wave field. For six wave modes (Langmuir wave, ion-acoustic oscillations, whistlers, Alfvén waves, magneto-acoustic oscillations, and transverse plasma wave) the corresponding representation of the wave stress tensor is found. Finally, the representation for the Alfvén waves is applied to the faculae heating and a result consistent with observations is obtained. 相似文献
20.
A source mechanism for the generation of shear Alfvén waves in the low solar corona is suggested. It is attributed to newly created ions associated with the ionization of neutral atoms in the chromosphere – corona transition region. In the present discussion attention is mainly paid to the excitation of shear Alfvén waves rather than compressional Alfvén waves, i.e., fast and slow magnetosonic waves. A kinetic instability, which amplifies Alfvén waves propagating in an arbitrary direction, due to the newborn ions, is studied. In the present analysis heavy ion species are emphasized. 相似文献