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1.
The negative effective magnetic-pressure instability operates on scales encompassing many turbulent eddies, which correspond to convection cells in the Sun. This instability is discussed here in connection with the formation of active regions near the surface layers of the Sun. This instability is related to the negative contribution of turbulence to the mean magnetic pressure that causes the formation of large-scale magnetic structures. For an isothermal layer, direct numerical simulations and mean-field simulations of this phenomenon are shown to agree in many details, for example the onset of the instability occurs at the same depth. This depth increases with increasing field strength, such that the growth rate of this instability is independent of the field strength, provided the magnetic structures are fully contained within the domain. A linear stability analysis is shown to support this finding. The instability also leads to a redistribution of turbulent intensity and gas pressure that could provide direct observational signatures.  相似文献   

2.
As was demonstrated in earlier studies, turbulence can result in a negative contribution to the effective mean magnetic pressure, which, in turn, can cause a large‐scale instability. In this study, hydromagnetic mean‐field modelling is performed for an isothermally stratified layer in the presence of a horizontal magnetic field. The negative effective magnetic pressure instability (NEMPI) is comprehensively investigated. It is shown that, if the effect of turbulence on the mean magnetic tension force vanishes, which is consistent with results from direct numerical simulations of forced turbulence, the fastest growing eigenmodes of NEMPI are two‐dimensional. The growth rate is found to depend on a parameter β* characterizing the turbulent contribution of the effective mean magnetic pressure for moderately strong mean magnetic fields. A fit formula is proposed that gives the growth rate as a function of turbulent kinematic viscosity, turbulent magnetic diffusivity, the density scale height, and the parameter β*. The strength of the imposed magnetic field does not explicitly enter provided the location of the vertical boundaries are chosen such that the maximum of the eigenmode of NEMPI fits into the domain. The formation of sunspots and solar active regions is discussed as possible applications of NEMPI (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)  相似文献   

3.
In this study we provide the first numerical demonstration of the effects of turbulence on the mean Lorentz force and the resulting formation of large‐scale magnetic structures. Using three‐dimensional direct numerical simulations (DNS) of forced turbulence we show that an imposed mean magnetic field leads to a decrease of the turbulent hydromagnetic pressure and tension. This phenomenon is quantified by determining the relevant functions that relate the sum of the turbulent Reynolds and Maxwell stresses with the Maxwell stress of the mean magnetic field. Using such a parameterization, we show by means of two‐dimensional and three‐dimensional mean‐field numerical modelling that an isentropic density stratified layer becomes unstable in the presence of a uniform imposed magnetic field. This large‐scale instability results in the formation of loop‐like magnetic structures which are concentrated at the top of the stratified layer. In three dimensions these structures resemble the appearance of bipolar magnetic regions in the Sun. The results of DNS and mean‐field numerical modelling are in good agreement with theoretical predictions. We discuss our model in the context of a distributed solar dynamo where active regions and sunspots might be rather shallow phenomena (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)  相似文献   

4.
The property of inhomogeneous turbulence in conducting fluids to expel large‐scale magnetic fields in the direction of decreasing turbulence intensity is shown as important for the magnetic field dynamics near the base of a stellar convection zone. The downward diamagnetic pumping confines a fossil internal magnetic field in the radiative core so that the field geometry is appropriate for formation of the solar tachocline. For the stars of solar age, the diamagnetic confinement is efficient only if the ratio of turbulent magnetic diffusivity ηT of the convection zone to the (microscopic or turbulent) diffusivity ηin of the radiative interior is ηT/ηin ≥ 105. Confinement in younger stars requires larger ηT/ηin. The observation of persistent magnetic structures on young solar‐type stars can thus provide evidence for the nonexistence of tachoclines in stellar interiors and on the level of turbulence in radiative cores. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)  相似文献   

5.
The MARSIS (Mars Advanced Radar for Subsurface and Ionosphere Sounding) instrument on the Mars Express spacecraft provides both local and remote measurements of electron densities and measurements of magnetic fields in the martian ionosphere. The density measurements show a persistent level of large fluctuations, sometimes as much as a factor of three or more at high altitudes. Large magnetic field fluctuations are also observed in the same region. The power spectrums of both the density and magnetic field fluctuations have slopes on a log-log plot that are consistent with the Kolmogorov spectrum for isotropic fluid turbulence. The fractional density fluctuation, Δne/ne, of the turbulence increases with altitude, and reaches saturation, Δne/ne ∼ 1, at an altitude of about 400 km, near the nominal boundary between the ionosphere and the magnetosheath. The fluctuations are usually so large that a well-defined ionopause-like boundary between the ionosphere and the solar wind is seldom observed. Of mechanisms that could be generating this turbulence, we believe that the most likely are (1) solar wind pressure perturbations, (2) an instability in the magnetosheath plasma, such as the mirror-mode instability, or (3) the Kelvin-Helmholtz instability driven by velocity shear between the rapidly flowing magnetosheath and the ionosphere.  相似文献   

6.
We provide a theory of magnetic diffusion, momentum transport, and mixing in the solar tachocline by considering magnetohydrodynamics (MHD) turbulence on a β plane subject to a large scale shear (provided by the latitudinal differential rotation). In the strong magnetic field regime, we find that the turbulent viscosity and diffusivity are reduced by magnetic fields only, similarly to the two-dimensional MHD case (without Rossby waves). In the weak magnetic field regime, we find a crossover scale (LR) from a Alfvén dominated regime (on small scales) to a Rossby dominated regime (on large scales). For parameter values typical of the tachocline, LR is larger than the solar radius so that Rossby waves are unlikely to play an important role in the transport of magnetic field and angular momentum. This is mainly due to the enhancement of magnetic back-reaction by shearing which efficiently generates small scales, thus strong currents. (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)  相似文献   

7.
We model solar coronal mass ejections (CMEs) as expanding force-free magnetic structures and find the self-similar dynamics of configurations with spatially constant ??, where J=?? B, in spherical and cylindrical geometries, expanding spheromaks and Lundquist fields, respectively. The field structures remain force-free, under the conventional non-relativistic assumption that the dynamical effects of the inductive electric fields can be neglected. While keeping the internal magnetic field structure of the stationary solutions, expansion leads to complicated internal velocities and rotation, caused by inductive electric fields. The structure depends only on overall radius R(t) and rate of expansion $\dot{R}(t)$ measured at a given moment, and thus is applicable to arbitrary expansion laws. In case of cylindrical Lundquist fields, magnetic flux conservation requires that both axial and radial expansion proceed with equal rates. In accordance with observations, the model predicts that the maximum magnetic field is reached before the spacecraft reaches the geometric center of a CME.  相似文献   

8.
C. L. Tang  S. Q. Song 《Solar physics》2012,279(2):537-550
Mirror mode waves in the solar wind are typically observed not as quasi-periodic sinusoidal signatures, but as trains of nonperiodic structures of two types: magnetic ??peaks?? and magnetic ??dips.?? Some trains of long durations have been called mirror mode storms. In this work we report mirror mode waves downstream of a stream interaction region (SIR) forward shock observed near 1?AU on 7?May 2007 with Solar Terrestrial Relations Observatory (STEREO) and Time History of Events and Macroscale Interactions during Substorms (THEMIS) data. The high-resolution magnetic-field data (0.125-second resolution) from STEREO are scanned to search for magnetic dips and peaks (or upgoing magnetic ??mesas??) in the solar wind. STEREO-A observes a mirror mode storm: the appearance of mirror mode waves (mainly magnetic peaks and upgoing mesas) is simultaneous with the entry into a high-density, high-temperature, and high plasma ?? accompanied by a depressed field region; the magnetic dips survive in the lower plasma-?? region. STEREO-B observes mirror mode waves (mainly magnetic peaks) with different amplitudes and asymmetric forms, which can survive in a low plasma ?? region. THEMIS-D, which was located in the solar wind, also observes mirror mode waves (mainly magnetic peaks and upgoing mesas) as well as an enhanced ion temperature anisotropy (T ????3T ??). The enhanced ion temperature anisotropy and high plasma ?? satisfy the mirror-instability criterion. These observations of STEREO and THEMIS-D show that mirror mode waves can be excited downstream of a SIR forward shock near 1?AU.  相似文献   

9.
In Paper I (Hu, 1982), we discussed the the influence of fluctuation fields on the force-free field for the case of conventional turbulence and demonstrated the general relationships. In the present paper, by using the approach of local expansion, the equation of average force-free field is obtained as (1+b)?×B 0=(α#x002B;a)B 0#x002B;a (1)×B 0#x002B;K. The average coefficientsa,a (1),b, andK show the influence of the fluctuation fields in small scale on the configurations of magnetic field in large scale. As the average magnetic field is no longer parallel to the average electric current, the average configurations of force-free fields are more general and complex than the usual ones. From the view point of physics, the energy and momentum of the turbulent structures should have influence on the equilibrium of the average fields. Several examples are discussed, and they show the basic features of the fluctuation fields and the influence of fluctuation fields on the average configurations of magnetic fields. The astrophysical environments are often in the turbulent state, the results of the present paper may be applied to the turbulent plasma where the magnetic field is strong.  相似文献   

10.
Magnetically mediated disk outflows are a leading paradigm to explain winds and jets in a variety of astrophysical sources, but where do the fields come from? Since accretion of mean magnetic flux may be disfavored in a thin turbulent disk, and only fields generated with sufficiently large scale can escape before being shredded by turbulence, in situ field production is desirable. Nonlinear helical inverse dynamo theory can provide the desired fields for coronae and outflows. We discuss the implications for contemporary protostellar disks, where the (magneto-rotational instability (MRI)) can drive turbulence in the inner regions, and primordial protostellar disks, where gravitational instability drives the turbulence. We emphasize that helical dynamos are compatible with the magneto-rotational instability, and clarify the relationship between the two.  相似文献   

11.
The Helioseismic and Magnetic Imager (HMI) onboard the Solar Dynamics Observatory (SDO) is designed to study oscillations and the magnetic field in the solar photosphere. It observes the full solar disk in the Fe?i absorption line at 6173 Å. We use the output of a high-resolution, 3D, time-dependent, radiation-hydrodynamic simulation based on the CO 5 BOLD code to calculate profiles F(??,x,y,t) for the Fe?i 6173 Å line. The emerging profiles F(??,x,y,t) are multiplied by a representative set of HMI filter-transmission profiles R i (??, 1??i??6) and filtergrams I i (x,y,t; 1??i??6) are constructed for six wavelengths. Doppler velocities V HMI(x,y,t) are determined from these filtergrams using a simplified version of the HMI pipeline. The Doppler velocities are correlated with the original velocities in the simulated atmosphere. The cross-correlation peaks near 100 km, suggesting that the HMI Doppler velocity signal is formed rather low in the solar atmosphere. The same analysis is performed for the SOHO/MDI Ni?i line at 6768 Å. The MDI Doppler signal is formed slightly higher at around 125 km. Taking into account the limited spatial resolution of the instruments, the apparent formation height of both the HMI and MDI Doppler signal increases by 40 to 50 km. We also study how uncertainties in the HMI filter-transmission profiles affect the calculated velocities.  相似文献   

12.
Current theoretical models for what drives star formation (especially low-mass star formation) are: (1) magnetic support of self-gravitating clouds with ambipolar diffusion removing support in cores and triggering collapse and (2) compressible turbulence forming self-gravitating clumps that collapse as soon as the turbulent cascade produces insufficient turbulent support. Observations of magnetic fields can distinguish between these two models because of different predictions in three areas: (1) magnetic field morphology, (2) the scaling of field strength with density and non-thermal velocities, and (3) the mass to magnetic flux ratio, M/Φ. We first discuss the techniques and limitations of methods for observing magnetic fields in star formation regions, then describe results for the L1544 prestellar core as an exemplar of the observational results. Application of the three tests leads to the following conclusions. The observational data show that both magnetic fields and turbulence are important in molecular cloud physics. Field lines are generally regular rather than chaotic, implying strong field strengths. But fields are not aligned with the minor axes of oblate spheroidal clouds, suggesting that turbulence is important. Field strengths appear to scale with non-thermal velocity widths, suggesting a significant turbulent support of clouds. Giant Molecular Clouds (GMCs) require mass accumulation over sufficiently large volumes that they would likely have an approximately critical M/Φ. Yet H I clouds are observed to be highly subcritical. If self-gravitating (molecular) clouds form with the subcritical M/Φ of H I clouds, the molecular clouds will be subcritical. However, the observations of molecular cloud cores suggest that they are approximately critical, with no direct evidence for subcritical molecular clouds or cloud envelopes. Hence, the observations remain inconclusive in deciding between the two extreme-case models of what drives star formation. What is needed to further advance our understanding of the role of magnetic fields in the star formation process are additional high sensitivity surveys of magnetic field strengths and other cloud properties in order to further refine the assessment of the importance of magnetic fields in molecular cores and envelopes.  相似文献   

13.
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14.
The role of shear in alleviating catastrophic quenching by shedding small‐scale magnetic helicity through fluxes along contours of constant shear is discussed. The level of quenching of the dynamo effect depends on the quenched value of the turbulent magnetic diffusivity. Earlier estimates that might have suffered from the force‐free degeneracy of Beltrami fields are now confirmed for shear flows where this degeneracy is lifted. For a dynamo that is saturated near equipartition field strength those estimates result in a 5‐fold decrease of the magnetic diffusivity as the magnetic Reynolds number based on the wavenumber of the energy‐carrying eddies is increased from 2 to 600. Finally, the role of shear in driving turbulence and large‐scale fields by the magneto‐rotational instability is emphasized. New simulations are presented and the 3π /4 phase shift between poloidal and toroidal fields is confirmed. It is suggested that this phase shift might be a useful diagnostic tool in identifying mean‐field dynamo action in simulations and to distinguish this from other scenarios invoking magnetic buoyancy as a means to explain migration away from the midplane. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)  相似文献   

15.
On the basis of quasi-linear theory of ion-acoustic turbulence it is shown that the angular and energy distribution of the electron spectra observed in quasi-static inverted-V structures are natural products of electron heating and runaway processes occuring in a region of current-driven turbulence located at h ≈ 1 Re. The power law population J ∞ E?γ, with γ ≈ 1 observed in the energy range ~ 25–1000 eV, is interpreted as a quasi-stationary distribution of suprathermal electrons interacting resonantly with the ion sound waves. This spectrum is generated in the turbulent region and convectively transported earthward along the magnetic field lines. Field-aligned intense electron fluxes with collimation angle < 10° are explained as due to particles escaping from the turbulent region through the runaway cone—a characteristic feature of velocity-space in ion-acoustic turbulence. A complete, new interpretation of the observed electron spectra is given on the basis of the proposed physical acceleration mechanism along with many other implications of this theory.  相似文献   

16.
Magnetic reconnection, or the ability of the magnetic field lines that are frozen in plasma to change their topology, is a fundamental problem of magnetohydrodynamics (MHD). Webriefly examine the problem starting with the well-known Sweet-Parker scheme, discuss effectsof tearing modes, anomalous resistivity and the concept of hyperresistivity. We show that the field stochasticity by itself provides a way toenable fast reconnection even if, at the scale of individual turbulent wiggles,the reconnection happens at the slow Sweet-Parker rate. We show that fast reconnectionallows efficient mixing of magnetic field in the direction perpendicular tothe local direction of magnetic field. While the idea of stochastic reconnection still requiresnumerical confirmation,our numerical simulations testify that mixing motions perpendicular to the local magnetic field are upto high degree hydrodynamical. This suggests that the turbulent heattransport should be similar to that in non-magnetized turbulent fluid, namely,should have a diffusion coefficient ～V L L, whereV L is the amplitude of the turbulent velocity and L is the scale of the turbulent motions. We present numericalsimulations which support this conclusion. The applicationof this idea to thermal conductivity in clusters of galaxies shows that thismechanism may dominate the diffusion of heat and may be efficient enoughto prevent cooling flow formation.  相似文献   

17.
We present the results of magnetic field measurements in three active prominences, July 24, 1981, July 24, 1999, and July 12, 2004, obtained from observations with the echelle spectrograph of the horizontal solar telescope at the Astronomical Observatory of the Taras Shevchenko Kiev National University. The magnetic fields were measured from the Zeeman splitting of the I ± V profiles in the He I D3 and H?? lines in the atmosphere at heights from 3 to 14 Mm. Our measurements of the effective magnetic fields B eff from the shift of the profile centroids have shown that the magnetic fields averaged over the entrance slit area were within the range from ?600 to +1500 G. The amplitude values of the local fields have been estimated from the splitting of the bisectors of the central parts of the line profiles at 0.9 of the peak intensity. The corresponding fields B 0.9 have turned out to be approximately twice B eff and reached 4000 G in absolute value. Narrow (1?C2 Mm) height peaks at heights of 6?C11 Mm have been found in the height distributions of the magnetic field. We have found an interesting effect in two prominences-an anticorrelation between the magnetic field strengths measured from the D3 and H?? lines.  相似文献   

18.
The waves, propagating nearly transverse to the ambient magnetic field, with frequencies near the harmonics of the proton-cyclotron frequency are studied in an inhomogeneous plasma with protons having loss-cone distributions. Three types of drift cyclotron instabilities have been studied: (i) non-flute instability; (ii) B-resonant instability; and (iii) non-resonant instability. Increases of loss-cone and density gradient increase the growth rates of all three instabilities. Increases in the positive temperature gradient and t (ratio of thermal pressure of trapped protons to magnetic field pressure) have a stabilizing effect on the non-flute and non-resonant instabilities and a destabilizing effect on the B-resonant instability. The non-resonant instability has an interesting feature: a particular harmonic can be excited in two separate bands of unstable wave numbers. These instabilities can play an important role in the dynamics of the ring current and the inner edge of the plasma sheet region of the magnetosphere. The discrete turbulence generated by them would give rise to precipitation of protons on the auroral field lines, which may contribute to the excitation of diffuse aurora. These instabilities may be relevant to the observation of harmonic waves at 6R E by Perrautet al. (1978).  相似文献   

19.
The local magnetic structure of the heliospheric current sheet (HCS) is observed as a boundary through which the magnetic field inverts its direction toward or away from Sun. The local variability of the HCS has been studied by means of a comparison of its local orientation estimated from data of different spacecraft. With the aim of determining possible variations in the local orientation, the selected events have been grouped according to their magnetic connection. A rough estimate of the magnetic connection (C) between two observation points has been found by considering the absolute value of the difference between the elapsed and expected times (C=|??t el??C???t ex|/??t ex). Lower values of C imply better connections, and smaller variation in the HCS orientation is expected if variations, temporal or spatial, in the HCS shape are negligible. Two periods have been analyzed: the ascending phase of Solar Cycle 23 and the minimum of the cycle in 2007??C?2008. It has been observed that, during the ascending phase, changes in the local HCS shape are mainly due to spatial variations. During minimum, the results show an increasing trend of the variation of the HCS local inclination with distance between spacecraft up to 5000 Earth radii (R E). For larger distances the results show a downward tendency. This inversion could be related to a continuous interaction of the HCS with the solar wind and with a poor magnetic connection, which could lead to changes in the local HCS shape making it unrecognizable to analyze the evolution of the structure from one observation point to another.  相似文献   

20.
Non-linear, three-dimensional, time-dependent fluid simulations of whistler wave turbulence are performed to investigate role of whistler waves in solar wind plasma turbulence in which characteristic turbulent fluctuations are characterized typically by the frequency and length-scales that are, respectively, bigger than ion gyrofrequency and smaller than ion gyroradius. The electron inertial length is an intrinsic length-scale in whistler wave turbulence that distinguishably divides the high-frequency solar wind turbulent spectra into scales smaller and bigger than the electron inertial length. Our simulations find that the dispersive whistler modes evolve entirely differently in the two regimes. While the dispersive whistler wave effects are stronger in the large-scale regime, they do not influence the spectral cascades which are describable by a Kolmogorov-like   k −7/3  spectrum. By contrast, the small-scale turbulent fluctuations exhibit a Navier–Stokes-like evolution where characteristic turbulent eddies exhibit a typical   k −5/3  hydrodynamic turbulent spectrum. By virtue of equipartition between the wave velocity and magnetic fields, we quantify the role of whistler waves in the solar wind plasma fluctuations.  相似文献   

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