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1.
The geometrical and scaling properties of the energy flux of the turbulent kinetic energy in the solar wind have been studied. Using present experimental technology in solar wind measurements we cannot directly measure the real volumetric dissipation rate, <varepsilon>(t), but are constrained to represent it by its surrogate the energy flux near the dissipation range at the proton gyro scale. There is evidence for the multifractal nature of the so defined dissipation field <varepsilon>(t), a result derived from the scaling exponents of its statistical moments. The generalized dimension D q has been determined and reveals that the dissipation field has a multifractal structure, which is not compatible with a scale-invariant cascade. The related multifractal spectrum f(<alpha>) has been estimated for the first time for MHD turbulence in the solar wind. Its features resemble those obtained for turbulent fluids and other nonlinear multifractal systems. The generalized dimension D q can for turbulence in high-speed streams be fitted well by the functional dependence of the p-model with a comparatively large parameter p 1=0.87, indicating a strongly intermittent multifractal energy cascade. The experimental value for D p/3 used in the scaling exponent s(p) of the velocity structure function gives an exponent that can describe some of the observations. The scaling exponent <mu> of the autocorrelation function of <varepsilon>(t) has also been directly evaluated, being 0.37. Finally, the mean dissipation rate was determined, which could be used in solar wind heating models. 相似文献
2.
An extended structure-function model is developed by including the new effect in the p-model of Meneveau and Sreenivasan which shows that the averaged energy cascade rate changes with scale, a situation which has been found to prevail in nonfullydeveloped turbulence in the inner solar wind. This model is useful for the small-scale fluctuations in the inner heliosphere, where the turbulence is not fully developed and cannot be explained quantitatively by any of the previous intermittency turbulence models. With two model parameters, the intrinsic index of the energy spectrum <alpha>, and the fragmentation fraction P 1, the model can fit, for the first time, all the observed scaling exponents of the structure functions, which are calculated for time lags ranging from 81 s to 0.7 h from the Helios solar wind data. From the cases we studied we cannot establish for P 1 either a clear radial evolution trend, or a solar-wind-speed or stream-structure dependence or a systematic anisotropy for both the flow velocity and magnetic field component fluctuations. Generally, P 1 has values between 0.7 and 0.8. However, in some cases in low-speed wind P 1 has somewhat higher values for the magnetic components, especially for the radial component. In high-speed wind, the inferred intrinsic spectral indices (<alpha>) of the velocity and magnetic field components are about equal, while the experimental spectral indices derived from the observed power spectra differ. The magnetic index is somewhat larger than the index of the velocity spectrum. For magnetic fluctuations in both high- and low-speed winds, the intrinsic exponent <alpha> has values which are near 1.5, while the observed spectral exponent has much higher values. In the solar wind with considerable density fluctuations near the interplanetary current sheet near 1 AU, it is found that P 1 has a comparatively high value of 0.89 for V x . The impact of these results on the understanding of the nature of solar wind fluctuations is discussed, and the limitations in using structure functions to study intermittency are also described. 相似文献
3.
Eckart Marsch 《Astrophysics and Space Science》1998,264(1-4):63-76
A physical model of the solar transition region and corona is presented, in which plasma flows in rapidly-diverging coronal
funnels and holes are described within the framework of a two-fluid model including wave-particle interactions. The ions are
heated by wave dissipation and accelerated by the pressure gradient of high-frequency Alfvén waves, which are assumed to originate
at the bottom of the magnetic network by small-scale reconnection. The heating is assumed to be due to cyclotron-resonant
damping of the waves near the local ion gyrofrequency. The EUV emission lines observed by the SUMER spectrometer on SOHO show
very strong broadenings, which seem to be ordered according to the ion charge-per-mass ratio and thus to indicate cyclotron-resonant
heating by waves. Based on quasilinear theory, a closure scheme for anisotropic multi-component fluid equations is developed
for the wave-particle interactions of the ions with Alfvén waves. The acceleration and heating rates are calculated.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
4.
The origin of the solar wind is a long-standing issue in both observational and theoretical studies. To understand how and
where in the solar atmosphere the mass and energy of the solar wind are supplied is very important. Previous observation suggests
a scenario in which the fast solar wind originates at heights above 5 Mm in the magnetically open funnel, a process that is
accompanied by downward flow below 5 Mm, whereby the mass and energy are supplied through reconnection between the open funnel
and adjacent closed loops. Based on this scenario, we develop a fluid model to study the solar wind generation under the assumption
that mass and energy are deposited in the open funnel at 5 Mm. The mass supply rate is estimated from the mass loss rate as
given by the emptying of the side loops as a result of their assumed reconnection with the open funnel. Similarly, the energy
input rate is consistent with the energy release rate as estimated from the energy flux associated with the reconnection between
the open magnetic funnel and the closed magnetic loops. Following the observations, we not only simulate the plasma flowing
upward to form the solar wind but also calculate the downward flow back to the lower atmosphere. This model is a first attempt
to study physically the proposed scenario of solar wind origin and gives a new physical illustration of the possible initial
deposition and consequent transportation of mass and energy in the coronal funnel. 相似文献
5.
A physical model of the transition region, including upflow of the plasma in magnetic field funnels that are open to the overlying corona, is presented. A numerical study of the effects of Alfvén waves on the heating and acceleration of the nascent solar wind originating in the chromospheric network is carried out within the framework of a two-fluid model for the plasma. It is shown that waves with reasonable amplitudes can, through their pressure gradient together with the thermal pressure gradient, cause a substantial initial acceleration of the wind (on scales of a few Mm) to locally supersonic flows in the rapidly expanding magnetic field trunks of the transition region network. The concurrent proton heating is due to the energy supplied by cyclotron damping of the high-frequency Alfvén waves, which are assumed to be created through small-scale magnetic activity. The wave energy flux of the model is given as a condition at the upper chromosphere boundary, located above the thin layer where the first ionization of hydrogen takes place.Among the new numerical results are the following: Alfvén waves with an assumed f
-1 power spectrum in the frequency range from 1 to 4 Hz, and with an integrated mean amplitude ranging between 25 and 75 km s4, can produce very fast acceleration and also heating through wave dissipation. This can heat the lower corona to a temperature of 5× 105 K at a height of h=12,000 km, starting from 5× 104 K at h=3000 km. The resulting thermal and wave pressure gradients can accelerate the wind to speeds of up to 150 km s-1 at h=12,000 km, starting from 20 km s-1 at h=3000 km in a rapidly diverging flux tube. Thus the nascent solar wind becomes supersonic at heights well below the classical Parker-Type sonic point. This is a consequence of the fact that any large wave-energy flux, if it is to be conducted through the expanding funnel to the corona, implies the building-up of an associated wave-pressure gradient. Because of the diverging field geometry, this might lead to a strong initial acceleration of the flow. There is a multiplicity of solutions, depending mainly on the coronal pressure. Here we discuss two new (as compared with a static transition region model) possibilities, namely that either the flow remains supersonic or slows down abruptly by shock formation, which then yields substantial coronal heating up to the canonical 106 K for the proton temperature. 相似文献
6.
A model of the solar corona and wind is developed which includes for the first time the heating and acceleration effects of high-frequency Alfvén waves in the frequency range between 1 Hz and 1 kHz. The waves are assumed to be generated by the small-scale magnetic activity in the chromospheric network. The wave dissipation near the gyro-frequency, which decreases with increasing solar distance, leads to strong coronal heating. The resulting heating function is different from other artificial heating functions used in previous model calculations. The associated thermal pressure-gradient force and wave pressure-gradient force together can accelerate the wind to high velocities, such as those observed by Helios and Ulysses. Classical Coulomb heat conduction is also considered and turns out to play a role in shaping the temperature profiles of the heated protons. The time-dependent two-fluid (electrons and protons) model equations and the time-dependent wave-spectrum equation are numerically integrated versus solar distance out to about 0.3 AU. The solutions finally converge and settle on time-stationary profiles which are discussed in detail. The model computations can be made to fit the observed density profiles of a polar coronal hole and polar plume with the sonic point occurring at 2.4 R
and 3.2 R
, respectively. The solar wind speeds obtained at 63 R
are 740 km s-1 and 540 km s-1; the mass flux is 2.1 and 2.2 × 108 cm-2 s-1 (normalized to 1 AU), respectively. The proton temperature increases from a value of 4 × 105 K at the lower boundary to 2 × 106 K in the corona near 2 R
. 相似文献
7.
This paper extends some previous work on the acceleration of minor ions in the solar wind to include the effects of wave acceleration and heating arising from minor ions interacting via the gyroresonance with ion cyclotron waves. Resonant wave acceleration is made up of two contributions, the first, and generally the more important, is a local acceleration which is proportional to the wave power and the number of resonant particles and is also sensitive to the details of the distribution function; while the other contribution is basically fluid dynamic in character, arises from the inhomogeneity of the medium and is proportional to the radial gradient of the resonant wave power. Under suitable cir-cumstances both contributions exhibit the feature that heavier ions receive greater acceleration than lighter ones. Also the kinematics of the resonance shows that the resonance wave acceleration switches off above a maximum differential speed, between ions and protons, which increases with increasing ratio of mass to charge. We also examine briefly possible beam instabilities driven by the streaming of minor ions relative to protons. 相似文献
8.
A sizable total-pressure (magnetic pressure plus kinetic pressure) enhancement was found within the high-speed wind stream observed by Helios 2 in 1976 near 0.3 AU. The proton density and temperature and the magnetic magnitude simultaneously increased for about 6 h. This pressure rise was associated with a comparatively large southward now velocity component (with Vz – 100 km · s–1) and magnetic-field rotation. The pressure enhancement was associated with unusual features in the electron distribution function. It shows a wide angular distribution of electron counting rates in the low-energy (57.8 eV) channel, while previous to the enhancement it exhibits a wide angular distribution of electron count rate in the high-energy (112, 221 and 309 eV) channels, perhaps indicating the mirroring of electrons in the converging field lines of the background magnetic field. These fluid and kinetic phenomena may be explained as resulting from an interplanetary magnetic flux rope which is not fully convected by the flow but moves against the background wind towards the Sun. 相似文献
9.
Jörg Büchner Ian Axford Eckart Marsch Vytenis Vasyliūnas 《Astrophysics and Space Science》1998,261(1-4):9-10
10.
Wilhelm K. Lemaire P. Curdt W. Schühle U. Marsch E. Poland A. I. Jordan S. D. Thomas R. J. Hassler D. M. Huber M. C. E. Vial J.-C. Kühne M. Siegmund O. H. W. Gabriel A. Timothy J. G. Grewing M. Feldman U. Hollandt J. Brekke P. 《Solar physics》1997,170(1):75-104
SUMER – the Solar Ultraviolet Measurements of the Emitted Radiation instrument on the Solar and Heliospheric Observatory (SOHO) – observed its first light on January 24, 1996, and subsequently obtained a detailed spectrum with detector B in the wavelength range from 660 to 1490 Å (in first order) inside and above the limb in the north polar coronal hole. Using detector A of the instrument, this range was later extended to 1610 Å. The second-order spectra of detectors A and B cover 330 to 805 Å and are superimposed on the first-order spectra. Many more features and areas of the Sun and their spectra have been observed since, including coronal holes, polar plumes and active regions. The atoms and ions emitting this radiation exist at temperatures below 2 × 106 K and are thus ideally suited to investigate the solar transition region where the temperature increases from chromospheric to coronal values. SUMER can also be operated in a manner such that it makes images or spectroheliograms of different sizes in selected spectral lines. A detailed line profile with spectral resolution elements between 22 and 45 mÅ is produced for each line at each spatial location along the slit. From the line width, intensity and wavelength position we are able to deduce temperature, density, and velocity of the emitting atoms and ions for each emission line and spatial element in the spectroheliogram. Because of the high spectral resolution and low noise of SUMER, we have been able to detect faint lines not previously observed and, in addition, to determine their spectral profiles. SUMER has already recorded over 2000 extreme ultraviolet emission lines and many identifications have been made on the disk and in the corona. 相似文献