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1.
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
2.
By the test particle method, we have investigated the kinematic characteristics of the electrons in the reconnecting current sheet with a guiding magnetic field Bz after they are accelerated by the supper-Dreicer electric field Ez. Firstly, the influence of the guiding magnetic field Bz on the particle acceleration is discussed under the assumption that Bz is constant in magnitude but different in orientation with respect to the electric field. In this case, the variation of the Bz direction directly leads to the variation of electron trajectories and makes electrons leave the current sheet along different paths. If Bz is parallel to Ez, the pitch angles of the accelerated electrons are close to 180°. If Bz is anti-parallel to Ez, the pitch angles of the accelerated electrons are close to 0°. The orientation of the guiding magnetic field just makes the electric field accelerate selectively the electrons in different regions, but does not change the energy distribution of electrons, and the finally derived energy spectrum is the common power-law spectrum E?γ. In typical coronal conditions, γ is about 2.9. The further study indicates that the magnitude of γ depends on the strengths of the guiding magnetic field and reconnecting electric field, as well as the scale of the current sheet. Then, the kinematic characteristics of the accelerated electrons in the current sheet with multiple X-points and O-points are also studied. The result indicates that the existences of the X-points and O-points have the particles constrained in the accelerating region to obtain the maximum acceleration, and the final energy spectrum has the characteristics of multi-power law spectra. 相似文献
3.
It is well known that the parallel cuts of the parallel and perpendicular electric field in electron phase-space holes (electron
holes) have bipolar and unipolar structures, respectively. Recently, electron holes in the Earth’s plasma sheet have been
observed by THEMIS satellites to have detectable fluctuating magnetic field with regular structures. Du et al. (2011) investigated the evolution of a one-dimensional (1D) electron hole with two-dimensional (2D) electromagnetic particle-in-cell
(PIC) simulations in weakly magnetized plasma (Ω
e
<ω
pe
, where Ω
e
and ω
pe
are the electron gyrofrequency and electron plasma frequency, respectively), which initially exists in the simulation domain.
The electron hole is unstable to the transverse instability and broken into several 2D electron holes. They successfully explained
the observations by THEMIS satellites based on the generated magnetic structures associated with these 2D electron holes.
In this paper, 2D electromagnetic particle-in-cell (PIC) simulations are performed in the x–y plane to investigate the nonlinear evolution of the electron two-stream instability in weakly magnetized plasma, where the
background magnetic field (B0 = B0[(e)\vec] x)(\mathbf{B}_{0} =B_{0}\vec{\mathbf{e}} _{x}) is along the x direction. Several 2D electron holes are formed during the nonlinear evolution, where the parallel cuts of E
x
and E
y
have bipolar and unipolar structures, respectively. Consistent with the results of Du et al. (2011), we found that the current along the z direction is generated by the electric field drift motion of the trapped electrons in the electron holes due to the existence
of E
y
, which produces the fluctuating magnetic field δB
x
and δB
y
in the electron holes. The parallel cuts of δB
x
and δB
y
in the electron holes have unipolar and bipolar structures, respectively. 相似文献
4.
We studied the behavior of magnetic field, horizontal motion and helicity in a fast emerging flux region NOAA 10488 which
eventually forms a δ spot. It is found that the rotation of photospheric footpoints forms in the earlier stage of magnetic
flux emergence and the relative shear motion of different magnetic flux systems appears later in this active region (AR).
Therefore the emerging process of the AR can be separated into two phases: rotation and shear. We have computed the magnetic
helicity injected into the corona using the local correlation tracking (LCT) technique. Furthermore we determined the vertical
component of current helicity density and the vertical component of induction electric fields Ez = (V× B)z in the photosphere. Particularly we have presented the comparison of the injection rate of magnetic helicity and the variation
of the current helicity density. The main results are as follows: (1) The strong shear motion (SSM) between the new emerging
flux system and the old one brings more magnetic helicity into the corona than the twisting motions. (2) After the maturity
of the main bipolar spots, their twist decreases and the SSM becomes dominant and the major contributor of magnetic non-potentiality
in the solar atmosphere in this AR. (3) The positions of the maxima of Ez (about 0.1 ∼ 0.2 V cm−1) shift from the twisting areas to the areas showing SSMs as the AR evolved from the rotation phase to the shear one, but
no obvious correlation is found between the kernels of Hα flare and Ez for the M1.6 flare in this AR. (4) The coronal helicity inferred from the horizontal motion of this AR amounts to −6 × 1043 Mx2. It is comparable with the coronal helicity of ARs producing flares with coronal mass ejections (CMEs) or helicity carried
away by magnetic clouds (MCs) reported in previous studies (Nindos, Zhang, and Zhang, 2003; Nindos and Andrews, 2004). In
addition, the formation of the δ configuration in this AR belongs to the third formation type indicated by Zirin and Liggett
(1987), i.e., collision of opposite polarities from different dipoles, and can be naturally explained by the SSM. 相似文献
5.
B. Buti 《Astrophysics and Space Science》1982,85(1-2):35-41
The longitudinal electric field associated with the observed electrostatic turbulence in the solar wind is shown to modify the dispersive characteristics of the hydromagnetic waves propagating along the interface between the solar wind and the cometary plasma. Extremely weak turbulence has a tendency to stabilize these surface waves, whereas turbulence of moderate level can be stabilizing or destabilizing depending on the strength of the cometary magnetic fieldB
oc
relative to the interplanetary magnetic fieldB
os
. ForB
oc
B
os
, destabilization is not possible. 相似文献
6.
Coordinated observations involving ion composition, thermal plasma, energetic particle, and ULF magnetic field data from GEOS 1 and 2 often reveal the presence of electromagnetic ion cyclotron and magnetosonic waves, which are distinguished by their respective polarization characteristics and frequency spectra. The ion cyclotron waves are identified by a magnetic field perturbation that lies in a plane perpendicular to the Earth's magnetic field B0 and propagate along B0. They are associated with the abundance of cold He+ in the presence of anisotropic pitch angle distributions of ions having energies E > 20 keV, and were observed at frequencies near the He+ gyrofrequency. The magnetosonic waves are characterized by a magnetic field perturbation parallel to B0 and thus seem to be propagating perpendicular to the Earth's magnetic field. They often occur at harmonics (not always including the fundamental) at the proton gyrofrequency and are associated with phase-space-density distributions that peak at energies E ～ 5–30 keV and at a pitch angle of 90°. Such a ring-like distribution is shown to excite instability in the magnetosonic mode near harmonics of the proton gyrofrequency. Magnetosonic waves are associated in other cases with sharp spatial gradients in energetic ion intensity. Such gradients are encountered in the early afternoon sector (as a consequence of the drift shell distortion caused by the convection electric field) and could likewise constitute a source of free energy for plasma instabilities. 相似文献
7.
Powerful flares are closely related to the evolution of the complex magnetic field configuration at the solar surface. The
strength of the magnetic field and speed of its evolution are two vital parameters in the study of the change of magnetic
field in the solar atmosphere. We propose a dynamic and quantitative depiction of the changes in complexity of the active
region: E=u×B, where u is the velocity of the footpoint motion of the magnetic field lines and B is the magnetic field. E represents the dynamic evolution of the velocity field and the magnetic field, shows the sweeping motions of magnetic footpoints,
exhibits the buildup process of current, and relates to the changes in nonpotentiality of the active region in the photosphere.
It is actually the induced electric field in the photosphere. It can be deduced observationally from velocities computed by
the local correlation tracking (LCT) technique and vector magnetic fields derived from vector magnetograms. The relationship
between E and ten X-class flares of four active regions (NOAA 10720, 10486, 9077, and 8100) has been studied. It is found that (1)
the initial brightenings of flare kernels are roughly located near the inversion lines where the intensities of E are very high, (2) the daily averages of the mean densities of E and its normal component (E
n) decrease after flares for most cases we studied, whereas those of the tangential component of E (E
t) show no obvious regularities before and after flares, and (3) the daily averages of the mean densities of E
t are always higher than those of E
n, which cannot be naturally deduced by the daily averages of the mean densities of B
n and B
t. 相似文献
8.
A series of solar cm-radio bursts are analyzed by a new inverse method estimating spatial changes of the superthermal electron
distribution in solar cm-radio burst sources. It is found that the measure of the spatial change of superthermal electrons
in the radio source ν
n
is always greater than that for the magnetic field ν
B
and it is linearly dependent on the spectral index of the electrons δ as ν
n
≈0.5δ. This relation is explained in the simplified flare-loop model integrating the analytical solutions of the Fokker – Planck
equation. The mean value of ν
B
is found to be 0.36±0.04, which is very close to the value of ν
B
=0.38±0.02 derived from the dependence of the magnetic field strength on the height in the active region measured by RATAN-600. 相似文献
9.
Wen-Rui Hu 《Astrophysics and Space Science》1983,97(2):353-367
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.
We extend Jokipii and Lerche's analysis of the turbulent structure of our Galaxy by means of a study of the rotation measure of extragalactic sources. Like them we use a simple, statistically homogeneous and isotropic disc model of the Galaxy and assume that the magnetic field has both an average component and a fluctuating one. We assume that the electron density is proportional to some power of the magnetic field (N
eB
n with 1n2). Using the rotation measure data on 242 extragalactic sources given by Vallée and Kronberg we consider both an exponential and a Gaussian two-point correlation function for the (Gaussian) fluctuating component of the magnetic field with a correlation lengthL. We find reasonable agreement between theory and observations for an average magnetic field of about 3 G, a fluctuating magnetic field component with an amplitude of about 2.6G, an average electron density of about 0.03 cm–3, a fluctuating density component of about 0.05 cm–3, and a correlation length of about 300 pc. 相似文献
11.
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
12.
Macroscopic electric fields in the solar atmosphere have received much less attention than magnetic fields, although they must play a role of comparable importance in plasma heating, and in charged particle acceleration and transport. We review various remote sensing techniques that have been developed, whose sensitivity is now 5–10 V cm –1 for measurement of the electric field component transverse to the line-of-sight. Our review of the processes most likely to produce observable fields in the solar atmosphere indicates that quasi-static, macroscopic values of E
(the electric field component parallel to the magnetic vector) well above this detection threshold are predicted by the discharge model of flares, by models of return currents associated with flare particle beams, and by models of neutral sheets associated with two-ribbon flares and post-flare loops. In addition, both E
and E
components may be detectable in time dependent electric fields associated with MHD and plasma waves, and with plasma turbulence. The emission measures and time-scales associated with these electrified plasma volumes are as highly uncertain as our present understanding of the volumes, plasma conditions and processes involved in the liberation of flare energy. Observations of electric field vector intensities, orientations, time-behaviour and spatial distribution at the presently attained electric field sensitivity levels could provide new, direct information of great interest in the electrodynamics of solar magnetic structures. 相似文献
13.
The pulsed plasma probe technique has been expanded to include simultaneous determinations of absolute electron density, density fluctuations, electron temperature, and mean-ion-mass with resolution limited only by probe geometry, sheath size, and telemetry. The technique has been designed to test for coupling of electron density variations and ion composition irregularities in multi-component plasmas by the comparison of electron density fluctuation power spectraP
N(k) and a newly-developed diagnostic parameter, the mean-ion-mass fluctuation spectraM
i/M
iP
M(k). In addition, the experiment extends satellite-borne irregularity spectral analyses down to the 5–20 m range while attempting to identify F-region plasma instability processes on the basis of characteristics inN
e,T
e, N
e,P
N,M
i, andP
M. Initial results demonstrate the expanded diagnostic capability for high spatial resolution measurements of mean-ion-mass and provide experimental evidence for the role of ion composition in multi-stepped plasma instability processes. Specific results include a spectral indexX
n inP
N=A
nf–X
n of 1.6–2.9 over the wavelength range from 1 km to 6 m under conditions identified with an unstable equatorial nighttime ionosphere. Simultaneous measurements ofM
i/M
i(P
M=A
M
f
–X
m) andN
e/N
e(P
N=A
n
f
–X
n) have shown a general behavior tending to lower power (A
m<A
n) and softer spectra (X
m<X
n) in ion mass fluctuations when compared with fluctuations in total plasma density. Limited analyses of the two power spectral elements raise hopes for the differentiation between plasma mechanisms that can lead to similar indices inP
N.Paper originally submitted to the journalSpace Science Instrumentation. 相似文献
14.
The 1968–2000 data on the mean magnetic field (MMF, longitudinal component) of the Sun are analysed to study long-time trends of the Sun's magnetic field and to check MMF calibration. It is found that, within the error limits, the mean intensity of photospheric magnetic field (the MMF strength, |H|), did not change over the last 33 years. It clearly shows, however, the presence of an 11-year periodicity caused by the solar activity cycle. Time variations of |H| correlate well with those of the radial component, |B
r|, of the interplanetary magnetic field (IMF). This correlation (r=0.69) appears to be significantly higher than that between |B
r| and the results of a potential source-surface extrapolation, to the Earth's orbit, of synoptic magnetic charts of the photosphere (using the so-called `saturation' factor –1 for magnetograph measurements performed in the line Fei 525.0 nm; Wang and Sheeley, 1995). It seems therefore that the true source surface of IMF is the `quiet' photosphere – background fields and coronal holes, like those for MMF. The average `effective' magnetic strength of the photospheric field is determined to be about 1.9 G. It is also shown that there is an approximate linear relation between |B
r| and MMF intensity |H| (in gauss)|B
r|(H
0)min×(1+C|H|)where =1.5×10–5 normalizes the photospheric field strength to 1 AU distance from the Sun, (H
0)min=1.2 G is some minimal `effective' intensity of photospheric background fields and C=1.3 G–1 an empirical constant. It is noted that good correlation between time variations of |H| and |B
r| makes suspicious a correction of the photospheric magnetic fields with the use of saturation factor –1. 相似文献
15.
We study the evolution of growth and decay laws for the magnetic field coherence length ξ, energy EM and magnetic helicity H in freely decaying 3D MHD turbulence. We show that with certain assumptions, self‐similarity of the magnetic power spectrum alone implies that ξ ∼ t1/2. This in turn implies that magnetic helicity decays as H ∼ t–2s, where s = (ξdiff/ξH)2, in terms of ξdiff, the diffusion length scale, and ξH, a length scale defined from the helicity power spectrum. The relative magnetic helicity remains constant, implying that the magnetic energy decays as EM ∼ t–1/2–2s. The parameter s is inversely proportional to the magnetic Reynolds number ReM, which is constant in the self‐similar regime. (© 2005 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim) 相似文献
16.
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
17.
S. Chatterjee 《Astrophysics and Space Science》1989,162(1):167-170
It is shown that, in the non-interacting limit, a one-dimensional metallic system has a transition to an insulating phase, in presence of external magnetic fieldH, if B
H>E
B where B is the Bohr magneton andE
Bis the energy band width of the electronic states. Possible realization of this effect in the atmosphere of neutron stars is pointed out. 相似文献
18.
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
19.
We study the dispersion characteristics of fast MHD surface waves travelling on a plasma slab immersed in a complex magnetic field consisting of a large longitudinal B
0z
component and a small sheared B
0y
component. The analysis shows that for typical coronal conditions both the sausage and kink waves are generally pseudo-surface waves. The tangential magnetic field, B
0y
, modifies the dispersion curves, and for sufficiently large sheared fields there is a transition from pseudo-surface to pure-surface fast kink waves.On leave from Faculty of Physics, Sofia University, BG-1126 Sofia, Bulgaria. 相似文献
20.
Mohamed Adel Sharaf 《Astrophysics and Space Science》1979,63(2):295-317
General integral transform of the exponential integralsE
n
is considered and will be denoted asB
(k)
n
(). Different expressions and the equations satisfied byB
(k)
n
are developed. Two-term recurrence formula forB
(k)
n
(0) and three-term recurrence formula forB
(k)
n
(); 0 will be established for a givenk1 andn=2,3, ...,N. The computational algorithms based on these formulae are also constructed for the casesk=1,2,3, andn2. Finally the numerical results fork=2,3 andn=2(1)25 are presented to 15-digit accuracy 相似文献