共查询到20条相似文献，搜索用时 81 毫秒
1.
C. C. Harvey 《Solar physics》1976,46(2):509-509
An attempt is made to explain the observed frequency-time profiles of type III solar radiobursts in terms of a rapid plasma wave decay rate combined with the exciter model recently proposed by the author. The decay rate is assumed to be sufficiently rapid for the plasma wave energy density profile to be similar to the excitor power density time profile; this is consistent with the exciter model, the rapid decay being caused by Landau damping on the electrons of the modified high energy tail of the ambient plasma electron velocity distribution. The model is compared with radio observations by making simple assumptions about the dependence of the radio intensity upon the plasma wave energy. A comparison is made with simultaneous radio and electron observations by further assuming a simple power-law velocity distribution for the electrons at their point of ejection from the Sun. 相似文献
2.
L. Muschietti 《Solar physics》1990,130(1-2):201-228
The dynamics of fast electrons streaming from the Sun through the interplanetary plasma is reviewed from the kinetic viewpoint. How can a bump appear on the tail of the electron distribution function? How can the beam survive the beam-plasma instability and propagate up to 1 AU as observed? These two questions are discussed in light of the recent data acquired in situ on electron distributions, Langmuir waves, ion acoustic waves, and background density fluctuations. 相似文献
3.
Krzysztof Stasiewicz 《Planetary and Space Science》1985,33(6):591-596
The influence of low-frequency electrostatic turbulence on the flux of precipitating magnetospheric electrons is analyzed in the framework of the quasilinear kinetic equation. It is shown that an electron population in a turbulent region, with an electric field parallel to the ambient magnetic field, can be separated into two parts by introducing a pitch angle dependent runaway velocity vr(θ). Lower energy electrons with parallel velocity v∥ < vr are effectively scattered by plasma waves, so that they remain in the main population and are subjected to an anomalous transport equation. A distribution function f ∞ v?4 (or the particle flux vs energy J ∞ E?1) is established in this velocity range. Faster electrons with v∥ ? vr are freely accelerated by a parallel electric field, so that they contribute directly to hot electron fluxes which are observed at ionospheric altitudes. New expressions are derived for the magnetic-field aligned current and the electron energy flux implied by this model. These expressions agree well with empirical relations observed in auroral inverted-V structures. 相似文献
4.
T. Takakura 《Solar physics》1979,61(1):161-186
A simulation of normal type III radio bursts has been made in a whole frequency range of about 200 MHz to 30 kHz by the usage of the semi-analytical method as developed in previous papers for the plasma waves excited by a cloud of fast electrons. Three-dimensional plasma waves are computed, though the velocities of fast electrons are assumed to be one-dimensional. Many basic problems about type III radio bursts and associated solar electrons have been solved showing the following striking or unexpected results.Induced scattering of plasma waves, by thermal ions, into the plasma waves with opposite wave vectors is efficient even for a solar electron cloud of rather low number density. Therefore, the second harmonic radio emission as attributed to the coalescence of two plasma waves predominates in a whole range from meter waves to km waves. Fundamental radio emission as ascribed to the scattering of plasma waves by thermal ions is negligibly small almost in the whole range. On the other hand, third harmonic radio emission can be strong enough to be observed in a limited frequency range.If, however, the time integral of electron flux is, for example, 2 × 1013 cm–2 (>5 keV) or more at the height of 4.3 × 1010 cm (
p = 40 MHz) above the photosphere, the fundamental may be comparable with or greater than the second harmonic, but an effective area of cross-section of the electron beam is required to be very small, 1017 cm2 or less, and hence much larger sizes of the observed radio sources must be attributed to the scattering alone of radio waves.The radio flux density expected at the Earth for the second harmonic can increase with decreasing frequencies giving high flux densities at low frequencies as observed, if x-dependence of the cross-sectional area of the electron beam is x
1.5 or less instead of x
2, at least at x 2 × 1012 cm.The second harmonic radio waves are emitted predominantly into forward direction at first, but the direction of emission may reverse a few times in a course of a single burst showing a greater backward emission at the low frequencies.In a standard low frequency model, a total number of solar electrons above 18 keV arriving at the Earth orbit reduces to 12% of the initial value due mainly to the collisional decay of plasma waves before the waves are reabsorbed by the beam electrons arriving later. However, no deceleration of the apparent velocity of exciter appears. A change in the apparent velocity, if any, results from a change in growth rate of the plasma waves instead of the deceleration of individual electrons.Near the Earth, the peak of second harmonic radio flux as emitted from the local plasma appears well after the passage of a whole solar electron cloud through this layer. This is ascribed to the secondary and the third plasma waves as caused in non-resonant regions by the induced scattering of primary plasma waves in a resonant region. 相似文献
5.
Numerical analysis has been carried out on the one-dimensional quasi-linear relaxation of a group of fast electrons travelling through the plasma. It is demonstrated that the electron velocity distribution of fast electrons tends to be a plateau form exciting the electron plasma waves and that the plasma waves are almost completely reabsorbed later by electrons arriving later. Both the velocity range and time interval in which quasi-plateau distribution is formed increase with distance from the origin of the fast electrons. There is no net energy loss of the electron cloud during the travel through the plasma if we neglect both the collisional losses and the scattering of plasma waves. Although the present computation is preliminary and limited to rather low beam density, we can see that the characteristics of both the electron beam and the plasma waves tend, with distance, to those of the analytical solution given by Ryutov and Sagdeev; though a modification to set a low velocity cutoff on the plasma waves due to the thermal electrons is necessary. 相似文献
6.
Observations of low frequency solar type III radio bursts and the associated fast solar electrons show that the total path length traveled by the particles between the Sun and the Earth is significantly greater than the length of the smooth Archimedean spiral trajectory followed by the centroid of the type III exciter (Alvarez et al., 1975). Here we assume that the ratio of electron path length and the spiral length increases approximately as r
n, where r is heliocentric distance, and then compute the radio bursts arrival time at 1 AU for different values of n. A comparison with the radio observations indicates that the best fit occurs for n = 1.5 ± 1.0. We interpret these results in terms of the variation of electron scattering with heliocentric distance. 相似文献
7.
An extensive study of the IMP-6 and IMP-8 plasma and radio wave data has been performed to try to find electron plasma oscillations associated with type III radio noise bursts and low-energy solar electrons. This study shows that electron plasma oscillations are seldom observed in association with solar electron events and type III radio bursts at 1.0 AU. In nearly four years of observations only one event was found in which electron plasma oscillations are clearly associated with solar electrons. For this event the plasma oscillations appeared coincident with the development of a secondary maximum in the electron velocity distribution functions due to solar electrons streaming outwards from the Sun. Numerous cases were found in which no electron plasma oscillations with field strengths greater than 1 μV m?1 could be detected even though electrons from the solar flare were clearly detected at the spacecraft. For the one case in which electron plasma oscillations are definitely produced by the electrons ejected by the solar flare the electric field strength is relatively small, only about 100 μV m?1. This field strength is about a factor of ten smaller than the amplitude of electron plasma oscillations generated by electrons streaming into the solar wind from the bow shock. Electromagnetic radiation, believed to be similar to the type III radio emission, is also observed coming from the region of the more intense electron plasma oscillations upstream of the bow shock. Quantitative calculations of the rate of conversion of the plasma oscillation energy to electromagnetic radiation are presented for plasma oscillations excited by both solar electrons and electrons from the bow shock. These calculations show that neither the type III radio emissions nor the radiation from upstream of the bow shock can be adequately explained by a current theory for the coupling of electron plasma oscillations to electromagnetic radiation. Possible ways of resolving these difficulties are discussed. 相似文献
8.
Biswajit Sahu 《Astrophysics and Space Science》2013,345(2):415-420
A first theoretical work is presented to study the propagation of two-solitons in an electron acoustic waves (EAWs) within the theoretical framework of the Tsallis statistical mechanics. For this purpose, cylindrical and spherical Korteweg-de Vries (KdV) equations are derived for electron acoustic solitary waves (EASWs) in an unmagnetized three species plasma system comprised of cold electrons, immobile ions and hot electrons featuring Tsallis statistics by employing the standard reductive perturbation method. The effects of electron nonextensivity and the fractional number density of the hot electrons relative to that of the cold ones number density (α) on the profiles of two-soliton structures are investigated numerically. Results would be helpful for understanding the localized structures that may occur in space plasmas. 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.
S. K. El-Labany R. Sabry W. M. Moslem E. A. Elghmaz 《Astrophysics and Space Science》2014,349(2):773-780
Generation of quasielastic electron-acoustic (EA) waves head-on collision are investigated in non-planar (cylindrical/spherical) plasma composed of cold electrons fluid, hot electrons obeying nonthermal distribution, and stationary ions. The cylindrical/spherical Korteweg-de Vries (KdV) equations describing two bidirectional EA waves are derived and solved analytically. Numerical investigation have shown that only positive electron-acoustic (EA) structures can propagate and collide. The analytical phase shift |Δ A | due to the non-Maxwellian (nonthermal) electrons is different from the Maxwellian case. Both the hot-to-cold electron number density ratio α and nonthermal parameter β have opposite effect on the phase shift behavior. The phase shift of the spherical EA waves is smaller than the cylindrical case, which indicates that the former is more stable for collision. The relevance of the present study to EA waves propagating in the Earth’s auroral zone is highlighted. 相似文献
11.
We examine a mechanism for breaking down solar wind (SW) speed shears within 1 astronomical unit (a.u.), initiated by the development of the Kelvin-Helmholtz (K-H) instability for typical parameters of the plasma and magnetic field in the interplanetary medium. A semi-empirical SW model has been invoked to derive a distribution of the plasma parameters β = 8πP/B2 and MA2 = (ρν2/2)/(B2/8π) between the Sun and 1 a.u. It is shown that in the vicinity of the Sun, up to heliocentric distances r ≈ 0.1 a.u., the parameters β ? 1, and M2A ? 1 and therefore the magnetic field here may be considered a very strong one. Because of the stabilizing effect of the magnetic field the K-H instability in this region does not develop and a presence of great shears in SW speed with large velocity gradients is possible here.At distances r > 0.1 a.u. the parameters β ? 1, and M2A > 1. Examination of a variety of SW speed profiles showed that the presence of plasma flow velocity shears in this region leads to an excitation of the K-H instability. Numerical analysis results indicate that a principal role in the excitation of this instability is played by oblique waves that propagate at an angle α ≈ 45° to the stream velocity vector.The question of the evolution of the leading front of a high speed SW streams within 1 a.u. is discussed, with a proper account of the influence of competing effects of kinematic steepening and turbulent viscosity, the latter being due to the development of the K-H instability. It is shown that the turbulent viscosity effect in this region is substantial and is capable of ensuring an expansion of the leading front of the high speed SW stream as this moves from 0.3 to 1 a.u., in agreement with experimental evidence reported by Rosenbauer et al. (1977). 相似文献
12.
IMP-6 spacecraft observations of low frequency radio emission, fast electrons, and solar wind plasma are used to examine the dynamics of the fast electron streams which generate solar type-III radio bursts. Of twenty solar electron events observed between April, 1971 and August, 1972, four were found to be amenable to detailed analysis. Observations of the direction of arrival of the radio emission at different frequencies were combined with the solar wind density and velocity measurements at 1 AU to define an Archimedean spiral trajectory for the radio burst exciter. The propagation characteristics of the exciter and of the fast electrons observed at 1 AU were then conpared. We find that: (1) the fast electrons excite the radio emission at the second harmonic; (2) the total distance travelled by the electrons was between 30 and 70% longer than the length of the smooth spiral defined by the radio observations; (3) this additional distance travelled is the result of scattering of the electrons in the interplanetary medium; (4) the observations are consistent with negligible true energy loss by the fast electrons. 相似文献
13.
Numerical solutions of the general time-dependent gas-dynamical equations in linear adiabatic approximation are given for initial conditions imitating: (a) a central perturbation, (b) a boundary perturbation (in the convective envelope), and (c) a ‘shrinking’ of the Sun as a whole. For a variety of models of the Sun it is found that at the surface the radial component v r of velocity is much greater than the tangential component v t , and that the period T of stationary oscillations does not exceed 131m. The appearance at the surface of a g mode with period 160m is found to be improbable. With the initial conditions adopted, a propagating wave is produced which is reflected successively from the centre to the periphery and back, producing 5-min oscillations at the surface of the Sun. Expansion of this wave into separate modes leads to a power spectrum qualitatively similar to that observed. 相似文献
14.
Jianfei Tang Dejin Wu Guoqing Zhao Ling Chen Chengming Tan 《Astrophysics and Space Science》2014,353(1):131-135
Langmuir waves (LWs), which are believed to play a crucial role in the plasma emission of solar radio bursts, can be excited by streaming instability of energetic electron beams. However, solar hard X-ray observations imply that the energetic flare electrons usually have a power-law energy distribution with a lower energy cutoff. In this paper, we investigate LWs driven by the power-law electrons. The results show that power-law electrons with the steepness cutoff behavior can excite LWs effectively because of the population inversion distribution below the cutoff energy (E c ). The growth rate of LWs increases with the steepness index (δ) and decreases with the power-law index (α). The wave number of the fastest growing LWs (kλ D ), decreases with the characteristic velocity of the power-law electrons ( \(v_{c}=\sqrt{2E_{c}/m_{e}}\) ) and increases with the thermal velocity of ambient electrons (v T ). This can be helpful for us to understand better the physics of LWs and the dynamics of energetic electron beams in space and astrophysical plasmas. 相似文献
15.
Jan Kuijpers 《Solar physics》1975,44(1):173-193
The possible generation of intermediate drift bursts in type IV dm continua through coupling between whistler waves, traveling along the magnetic field, and Langmuir waves, excited by a loss-cone instability in the source region, is elaborated. We investigate the generation, propagation and coupling of whistlers. It is shown that the superposition of an isotropic background plasma of 106K and a loss-cone distribution of fast electrons is unstable for whistler waves if the loss-cone aperture 2α is sufficiently large (sec α?4); a typical value of the excited frequencies is 0.1 ω ce (ω ce is the angular electron cyclotron frequency). The whistlers can travel upwards through the source region of the continuum along the magnetic field direction with velocities of 21.5–28 v A (v A is the Alfvén velocity). Coupling of the whistlers with Langmuir waves into escaping electromagnetic waves can lead to the observed intermediate drift bursts, if the Langmuir waves have phase velocities around the velocity of light. In our model the instantaneous bandwith of the fibers corresponds to a frequency of 0.1–0.5 ω ce and leads to estimates of the magnetic field strength in the source region. These estimates are in good agreement with those derived from the observed drift rate, corresponding to 21.5–28 v A, if we use a simple hydrostatic density model. 相似文献
16.
Noble P. Abraham Samuel George G. Sreekala Sijo Sebastian Chandu Venugopal G. Renuka 《Earth, Moon, and Planets》2014,111(3-4):115-125
We have studied the stability of the electrostatic electron cyclotron wave in a plasma composed of hydrogen, oxygen and electrons. To conform to satellite observations in the low latitude boundary layer we model both the ionic components as drifting perpendicular to the magnetic field. Expressions for the frequency and the growth rate of the wave have been derived. We find that the plasma can support electron cyclotron waves with a frequency slightly greater than the electron cyclotron frequency ω ce ; these waves can be driven unstable when the drift velocities of both the ions are greater than the phase velocity of the wave. We thus introduce another source of instability for these waves namely multiple ion beams drifting perpendicular to the magnetic field. 相似文献
17.
18.
Solar flare accelerated electrons escaping into the interplanetary space and seen as type III solar radio bursts are often detected near the Earth. Using numerical simulations we consider the evolution of energetic electron spectrum in the inner heliosphere and near the Earth. The role of Langmuir wave generation, heliospheric plasma density fluctuations, and expansion of magnetic field lines on the electron peak flux and fluence spectra is studied to predict the electron properties as could be observed by Solar Orbiter and Solar Probe Plus. Considering various energy loss mechanisms we show that the substantial part of the initial energetic electron energy is lost via wave–plasma processes due to plasma inhomogeneity. For the parameters adopted, the results show that the electron spectrum changes mostly at the distances before ～?20 R ⊙. Further into the heliosphere, the electron flux spectrum of electrons forms a broken power law relatively similar to what is observed at 1 AU. 相似文献
19.
A dispersion relation which takes into account the nonuniformity of the magnetic field as well as the plasma density along the field lines is obtained for an electrostatic wave propagating parallel to the magnetic field. This dispersion relation is solved for a particular case in which a group of electrons with a monochromatic distribution in magnetic moment is mixed with a low energy plasma. Such electrons are shown to excite ion acoustic waves carried by the low-energy plasma component near multiples of the bounce frequency of these electrons. The theoretical results are applied to explain electrostatic oscillations with a period of approximately fifty seconds observed in the high energy electron fluxes at synchronous altitude. 相似文献
20.
T. Takakura 《Solar physics》1991,136(2):303-316
Numerical simulation is made of the transient heat conduction during local heating in a model coronal magnetic loop with an axial electric current. It is assumed that a segment near the top of the normal coronal loop is heated to above 107 K by a sufficiently small heat input as compared with the total flare energy. A hump appears in the velocity distribution of electrons moving down the temperature gradient with speeds slightly below the thermal one. Consequently, electron plasma waves are excited. The high intensity of the waves persists in the upper region of the loop for more than a second until the termination of the simulation. The energy density of the plasma waves normalized with respect to thermal density is 10–3.5 at maximum. A theoretical estimate gives an anomalous resistivity 5 orders of magnitude greater than an initial value. Based on the above result, we propose a model for impulsive loop flares. 相似文献