共查询到20条相似文献，搜索用时 31 毫秒
1.
We analyze multiple-wavelength observations of a two-ribbon flare exhibiting apparent expansion motion of the flare ribbons
in the lower atmosphere and rising motion of X-ray emission at the top of newly-formed flare loops. We evaluate magnetic reconnection
rate in terms of V
r
B
r by measuring the ribbon-expansion velocity (V
r) and the chromospheric magnetic field (B
r) swept by the ribbons. We also measure the velocity (V
t) of the apparent rising motion of the loop-top X-ray source, and estimate the mean magnetic field (B
t) at the top of newly-formed flare loops using the relation 〈V
t
B
t〉≈〈V
r
B
r〉, namely, conservation of reconnection flux along flare loops. For this flare, B
t is found to be 120 and 60 G, respectively, during two emission peaks five minutes apart in the impulsive phase. An estimate
of the magnetic field in flare loops is also achieved by analyzing the microwave and hard X-ray spectral observations, yielding
B=250 and 120 G at the two emission peaks, respectively. The measured B from the microwave spectrum is an appropriately-weighted value of magnetic field from the loop top to the loop leg. Therefore,
the two methods to evaluate coronal magnetic field in flaring loops produce fully-consistent results in this event. 相似文献
2.
Photospheric motion shears or twists solar magnetic fields to increase magnetic energy in the corona, because this process may change a current-free state of a coronal field to force-free states which carry electric current. This paper analyzes both linear and nonlinear two-dimensional force-free magnetic field models and derives relations of magnetic energy buildup with photospheric velocity field. When realistic data of solar magnetic field (B
0 103 G) and photospheric velocity field (v
max 1 km s–1) are used, it is found that 3–4 hours are needed to create an amount of free magnetic energy which is of the order of the current-free field energy. Furthermore, the paper studies situations in which finite magnetic diffusivities in photospheric plasma are introduced. The shearing motion increases coronal magnetic energy, while the photospheric diffusion reduces the energy. The variation of magnetic energy in the coronal region, then, depends on which process dominates. 相似文献
3.
Lennart Lindberg 《Astrophysics and Space Science》1978,55(1):203-225
A beam of collisionless plasma is injected along a longitudinal magnetic field into a region of curved magnetic field. Two unpredicted phenomena are observed: The beam becomes deflected in the directionopposite to that in which the field is curved, and itcontracts to a flat slab in the plane of curvature of the magnetic field.The plasma is produced by a conical theta-pinch gun and studied by means of high speed photography, electric and magnetic probes, ion analyser, and spectroscopy.The plasma beam is collisionless and its behaviour is, in principle, understood on the basis of gyro-centre drift theory. A fraction of the transverse electric fieldE=–v×B, which is induced when the beam enters the curved magnetic field, is propagated upstream and causes the reverse deflection byE×B drift. The upstream propagation of the transverse electric field is due to electron currents.The circuit aspect on the plasma is important. The transverse polarization current in the region with the curved field connects to a loop of depolarization currents upstream. The loop has limited ability to carry current because of the collisionless character of the plasma; curlE is almost zero and electric field components arise parallel to the magnetic field. These play an essential role, producing runaway electrons, which have been detected. An increased electron temperature is observed when the plasma is shot into the curved field. Runaway electrons alone might propagate the electric field upstream in case the electron thermal velocity is insufficient.The phenomenon is of a general character and can be expected to occur in a very wide range of ensities. The lower density limit is set by the condition for self-polarization,nm
i
/
0
B
2 1 or, which is equivalent,c
2/v
A
2
;1, wherec is the velocity of light, andv
A the Alfvén velocity. The upper limit is presumably set by the requirement
e
e
1.The phenomenon is likely to be of importance, for example, for the injection of plasma into magnetic bottles and in space and solar physics. The paper illustrates the complexity of plasma flow phenomena and the importance of close contact between experimental and theoretical work.Paper dedicated to Professor Hannes Alfvén on the occasion of his 70th birthday, 30 May, 1978 相似文献
4.
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
5.
The curvature -quanta emitted in the pulsar magnetospheres tangentially to the curved lines of force of the magnetic field are shown to be later canalized along the magnetic field—if it is strong enough,B0.1B
cr
=4×1012G—by gradually converting into a mutually bound electron-positron pair, i.e. a positronium atom. This happens before the photon reaches the threshold of free-pair creation. The positronium thus arising is stable against the ionizing action of the electric field near the pulsar unless it reaches a critical value about 4×107 CGSE forB1013G. This prevents the screening of the electric field up to the distances from the pulsar, where the magnetic field is already below the value of 0.1B
cr
and the free pair creation may become essential. This effect provides, at least within the Arons model, a higher theoretical estimate for the total luminosity of pulsars whose field at the surfaceB
s
exceeds 0.1B
cr
as compared with the conventional one. 相似文献
6.
Results of ourmeasurements of the longitudinal magnetic field B
z
for the young star RWAur A are presented. B
z
measured from the so-called narrow component of the He I 5876 line varies in the range from −1.47 ± 0.15 to +1.10 ± 0.15
kG. Our data are consistent with a stellar rotation period of }~5.6 days and the model of two hot spots with opposite magnetic
field polarities spaced about 180° apart in longitude. Relative to the Earth, the spot with B
z
< 0 lies in the hemisphere above the midplane of the accretion disk, while the spot with B
z
> 0 is below the midplane. The upper limit for B
z
(at the 3σ level) obtained by averaging all observations is 180 G for the photosphere and 220 and 230 G for the Hα and [OI] 6300 line formation regions, respectively. We have also failed to detect a field in the formation region of broad
emission line components: the upper limit for B
z
is 600 G. In two of 11 cases, we have detected a magnetic field in the formation region of the blue absorption wing of the
Na I D doublet lines, i.e., in the wind from RW Aur A: B
z
= −180 ± 50 and −810 ± 80 G. The radial velocity of the photospheric lines in RW Aur A averaged over all our observations
is }~+10.5 km s−1, i.e., a value lower than that obtained by Petrov et al. (2001) ten years earlier by 5.5 km s−1. Therefore, we discuss the possibility that RW Aur is not a binary but a triple system. 相似文献
7.
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
8.
T. Takakura 《Solar physics》1984,91(2):311-324
In some gradual hard X-ray bursts with high intensity, hard X-ray source (15–40 keV) is steadily located in the corona along with softer X-ray source (5–10 keV).Two stationary models, high density and high temperature models, are proposed to solve the difficult problem of confinement of hot (or nonthermal) plasma in the direction of the magnetic field along the loops in the corona. In both models, an essential point is that the effective X-ray source is composed of fine dense filamentary loops imbeded in a larger rarefied coronal loop, and the electron number density in the filaments is so high as 1011–1012 cm-3. If the density is so high heat conduction can be as reasonably small as of the order of 1027 erg s -1 for the given emission measures of observed X-rays, since the required cross-sectional area is small and also classical conduction is valid. Collisional confinement of thermal tail, and nonthermal electrons if any, up to 50–60 keV in the filaments is also possible, so that the hard X-ray images can be loop like structure instead of double source (foot points).High density model is applicable to the coronal filamentary loops with temperature T
m < 5 × 107 K at the loop summit. The heat flow from the summit downwards is lost almost completely by the radiation from the loop during the conduction to the foot points. A continuous energy release is assumed near the summit to maintain the stationary temperature T
m, and pressure balance is maintained along the loop. In this model, the number density at the summit is given by n
m - 106
T
m
2
/sm, where s
m is the length of the loop from the summit to the foot point, and the distribution of temperature and density along the loop are given by T = T
m(s/sm)1/3 and n = n
m(s/sm)-1/3, respectively.High temperature model is applicable to the filamentary loops with higher temperature up to about 108.5 K and comparatively lower number density as 1011 cm-3 for the requirement of magnetic confinement of the hot plasma in radial direction. The radiation from the loop is negligibly small in this model so that the heat flux is nearly conserved down to the foot points. In this case, temperature gradient is smaller than that of the high density model, depending on the tapering of the magnetic bottle.In both models, the differential emission measure is maximum at the highest temperature T
m and the brightness distribution along the loop shows a maximum around the summit of the loop if some magnetic tapering is taken into account. 相似文献
9.
An analysis, with a representative (canonical) example of solar-flare-generated equatorial disturbances, is presented for the temporal and spatial changes in the solar wind plasma and magnetic field environment between the Sun and one astronomical unit (AU). Our objective is to search for first order global consequences rather than to make a parametric study. The analysis - an extension of earlier planar studies - considers all three plasma velocity and magnetic field components (V r, Vφ, V0, and B r, B0, Bφ) in any convenient heliospheric plane of symmetry such as the ecliptic plane, the solar equatorial plane, or the heliospheric equatorial plane chosen for its ability (in a tilted coordinate system) to order northern and southern hemispheric magnetic topology and latitudinal solar wind flows. Latitudinal velocity and magnetic field gradients in and near the plane of symmetry are considered to provide higher-order corrections of a specialized nature and, accordingly, are neglected, as is dissipation, except at shock waves. The representative disturbance is examined for the canonical case in which one describes the temporal and spatial changes in a homogeneous solar wind caused by a solar-flare-generated shock wave. The ‘canonical’ solar flare is assumed to produce a shock wave that has a velocity of 1000 km s#X2212;1 at 0.08 AU. We have examined all plasma and field parameters at three radial locations: central meridian and 33° W and 90° W of the flare's central meridian. A higher shock velocity (3000 km s#X2212;1) was also used to demonstrate the model's ability to simulate a strongly-kinked interplanetary field. Among the global (first-order) results are the following: (i) incorporation of a small meridional magnetic field in the ambient magnetic spiral field has negligible effect on the results; (ii) the magnetic field demonstrates strong kinking within the interplanetary shocked flow, even reversed polarity that - coupled with low temperature and low density - suggests a viable explanation for observed ‘magnetic clouds’ with accompanying double-streaming of electrons at directions ～ 90° to the heliocentric radius. 相似文献
10.
Spectra, angular distributions, and polarization of two-photon annihilation radiation in a magnetic field are studied in detail in the case of small longitudinal velocities of annihilating electrons and positrons which occupy the ground Landau level. Magnetic field essentially affects the annihilation if its magnitudeB is not very low in comparison withB
cr=4.4×1013G, which may take place near the surface of a neutron star. The magnetic field broadens the spectra (the width depends on an angle betweenB and a wave vector) and leads to their asymmetry. The angular distribution may be highly anisotropic, being fan-like or pencillike for different photon energies . The total annihilation rate is suppressed by the magnetic field (B
–3 forBB
cr).The radiation is linearly polarized; the degree and orientation of the polarization depend onB, and . The polarization may reach several tens percent even for comparatively small fieldsB 0.1B
crtypical for neutron stars. This means that the polarization may be detected, e.g., in the annihilation radiation from the gamma-ray bursts. 相似文献
11.
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
12.
Thirteen synoptic maps of expansion rate of the coronal magnetic field (CMF; RBR) calculated by the so-called ‘potential model’ are constructed for 13 Carrington rotations from the maximum phase of solar activity cycle 22 through the maximum phase of cycle 23. Similar 13 synoptic maps of solar wind speed (SWS) estimated by interplanetary scintillation observations are constructed for the same 13 Carrington rotations as the ones for the RBR. The correlation diagrams between the RBR and the SWS are plotted with the data of these 13 synoptic maps. It is found that the correlation is negative and high in this time period. It is further found that the linear correlation is improved if the data are classified into two groups by the magnitude of radial component of photospheric magnetic field, |Bphor|; group 1, 0.0 G ≦ |Brpho| < 17.8 G and group 2, 17.8 G ≦ |Brpho|. There exists a strong negative correlation between the RBR and the SWS for the group 1 in contrast with a weak negative correlation for the group 2. Group 1 has a double peak in the density distribution of data points in the correlation diagram; a sharp peak for high-speed solar wind and a low peak for low-speed solar wind. These two peaks are located just on the axis of maximum variance of data points in the correlation diagram. This result suggests that the solar wind consists of two major components and both the high-speed and the low-speed winds emanating from weak photospheric magnetic regions are accelerated by the same mechanism in the course of solar activity cycle. It is also pointed out that the SWS can be estimated by the RBR of group 1 with an empirical formula obtained in this paper during the entire solar activity cycle. 相似文献
13.
A model of the equilibrium structure of the coronal magnetic field is developed, taking account of the fact that field lines are rooted in the photosphere, where field is concentrated into isolated flux tubes. The field is force-free, described by ×B =
B, with constant; this field has special physical significance, being the state of mininum energy after small-scale reconnections, and is also mathematically convenient in that the principle of superposition can be used to construct complex geometries. First a model of a single loop is presented, with a flux source and sink pair at the photosphere; both point flux tubes and finite radius flux tubes are considered. Then more complex topologies with multiple sources and sinks are investigated. It is shown that significant topology changes arise for different values of, indicating the possibility that there can be energy changes through magnetic reconnection if the field evolves ideally and then relaxes to a linear state. 相似文献
14.
T. Takakura 《Solar physics》1990,127(1):95-107
The Fokker-Planck equation is numerically solved to study the electron velocity distribution under steady heat conduction with an applied axial electric current in a model coronal loop.If the loop temperature is so high that the electron mean-free path is longer than the local temperature scale height along the loop, a velocity hump appears at about the local thermal electron velocity. The hump is attributed to cooler electrons moving up the temperature gradient to compensate for the runaway electrons moving down the gradient. If the ratio between the mean free path and temperature scale height is greater than about 2, negative absorption for the plasma waves can appear (waves grow). This effect is enhanced by the presence of axial electric current in the half of the coronal loop in which the electrons carrying the current are drifting up the temperature gradient. Thus, the plasma instability may occur in the coronal elementary magnetic flux tubes. Although the present paper is limited to show the critical condition and linear growth rate of the instability, the following scenarios may be inferred.If the flux tubes change from marginally stable to unstable against the plasma instability, due to an increase in the loop temperature, anomalous resistivity may suddenly appear because of the growth of plasma waves. Then a high axial electric field is induced that may accelerate particles. This could be the onset of impulsive loop flares.For a low electric current, if the loop temperature is sufficiently high to give the negative absorption for the plasma waves in a large part of the coronal loop, steady plasma turbulence may originate. This could be a source for the type I radio noise storm. 相似文献
15.
T. J. Bogdan 《Solar physics》1986,103(2):311-315
Previous efforts to construct solar coronal fields using surface magnetograph data have generally employed a least squares minimization technique in order to determine the spherical harmonic expansion coefficients of the magnetic scalar potential. Provided there is no source surface high up in the corona, we show that knowledge of the line-of-sight component of the surface magnetic field, B
i
= B
r
sin + B
cos , is sufficient to uniquely determine the potential coronal magnetic field by an explicit construction of the magnetic scalar potential for an arbitrary B
l
(, ).The National Center for Atmospheric Research is sponsored by the National Science Foundation. 相似文献
16.
From K-coronameter data we have obtained an electron density profile above the active region responsible for the Type IV burst observed on 14 September 1966. If the observed frequency cutoff in the burst's spectrum is caused by the Razin effect, then the coronal electron density may be derived from the intensity variation in the burst as it propagates outwards from the Sun. We show that the electron density profiles obtained from K-coronameter data (appropriate to 1.125 <r/R
< 2.0) and from the radio data (2.2< r/R
< 2.5) form a continuous distribution. We conclude that the cutoff is due to the Razin effect, and that radiation in the burst is due to relativistic electrons having a steep inverse power-law energy distribution. From the electron density profile derived from the radio data, we find that the coronal magnetic field was 0.26 G at r/R
= 2.2. 相似文献
17.
T. Takakura 《Solar physics》1988,115(1):149-160
Time-dependent Fokker-Planck equation was numerically solved to demonstrate the dynamics of electrons in a uniform coronal loop with an applied axial DC electric field in the presence of ion-sound waves. This electric field is attributed to an anomalous resistivity due to the ion-sound turbulence caused by an initially given critical current density.The electron momentum distribution becomes a steady state in the whole turbulent region in a short time for which some electrons can be accelerated to the maximum electric potential K
c. The steady energy distribution of electrons flowing out the end of the turbulent region has a very hard power-law-like spectrum with an index of about 0.75. The associated hard X-rays from a thick target also show a hard spectrum with a photon spectral index of 1.3. In order for to be much greater as observed in impulsive X-ray bursts, it is required that the source is a sum of many elementary loops with a power-law-like distribution in K
c with an index = – + 2.5. 相似文献
18.
Helios-1 and 2 spacecraft allowed a detailed investigation of the radial dependence of the interplanetary magnetic field components between 0.3 and 1 AU. The behaviour of the radial component B
ris in a very good agreement with Parker's model (B
r r
-2) and the azimuthal component B
also shows a radial dependence which is close to theoretical predictions (B
r
-1). Experimental results for the normal component B
and for the field magnitude B are consistent with those from previous investigations. The relative amplitude of the directional fluctuations with periods less than 12 hr is essentially independent of heliocentric distance, while their power decreases approximately as r
–3 without any appreciable difference between higher and lower velocity regimes.Also at Laboratorio Plasma nello Spazio, CNR, Frascati. 相似文献
19.
D. G. Yakovlev 《Astrophysics and Space Science》1984,98(1):37-59
The expressions are derived for thermal and electric conductivities as well as thermopower of a degenerate relativistic electron gas in the surface layers of neutron stars along the magnetic fieldB=4×1011–1014G for two scattering mechanisms of electrons, namely, for Coulomb scattering on ions in the ion-liquid regime and on high-temperature phonons in the solid regime. The results may be of use to study neutron star cooling rates, nuclear burning of the matter in the surface layers, diffusion of the magnetic field, etc. 相似文献
20.
Previous global models of coronal magnetic fields have used a geometrical construction based on a spherical source surface because of requirements for computational speed. As a result they have had difficulty accounting for (a) the tendency of full magnetohydrodynamic (MHD) models to predict non-radial plasma flow out to r 10r
and (b) the appreciable magnitude, 3, of B
r
, (the radial component of B) consistently observed at r 1 AU. We present a new modelling technique based on a non-spherical source surface, which is taken to be an isogauss of the underlying potential field generated by currents in or below the photosphere. This modification of the source surface significantly improves the agreement between the geometrical construction and the MHD solution while retaining most of the computational ease provided by a spherical source surface. A detailed comparison between the present source-surface model and the MHD solution is made for the internal dipole case. The resulting B field agrees well in magnitude and direction with the coronal B field derived from the full MHD equations. It shows evidence of the slightly equatorward meridional plasma flow that is characteristic of the MHD solution. Moreover, the B field obtained by using our non-spherical source surface agrees well with that observed by spacecraft in the vicinity of the Earth's orbit. Applied to a solar dipole field with a moment of 1 G-r
3
, the present model predicts that B
r
at r 1 AU lies in the range of 1–2 and is remarkably insensitive to heliomagnetic latitude. Our method should be applicable also to more general (i.e., more realistic) configurations of the solar magnetic field. Isogauss surfaces for two representative solar rotations, as calculated from expansions of observed photospheric magnetic-field data, are found to show large and significant deviations from sphericity. 相似文献