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1.
An astrophysical electron acceleration process is described which involves turbulent plasma effects: the acceleration mechanism will operate in ‘collision free’ magnetoactive astrophysical plasmas when ion-acoustic turbulence is generated by an electric field which acts parallel to the ambient magnetic lines of force. The role of ‘anomalous’ (ion-sound) resistivity is crucial in maintaining the parallel electric field. It is shown that, in spite of the turbulence, a small fraction of the electron population can accelerate freely, i.e. runaway, in the high parallel electric potential. The number density n(B) of the runaway electron component is of order n(B)?n2(csU∥?)2, where n = background electron number density, cs = ion-sound speed and U∥? = relative drift velocity between the electron and ion populations. The runaway mechanism and the number density n(B) do not depend critically on the details of the non-linear saturation of the ion-sound instability.  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.
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   

4.
Assuming a certain horizontal distribution of the convection field at a certain altitude above the ionosphere, the associated electric field and current distributions in a vertical plane are calculated using a model with finite current-dependent conductivity along the magnetic field lines. It is seen that given the kind of horizontal distribution of E6 commonly observed by polar-orbiting satellites at inverted-V electron precipitation events, the calculated distribution of E∥ is able to reproduce the basic spatial structure of the precipitation. It is also seen that the combined effect of a locally increased ionization within auroral forms and a large potential difference (ΔV)∥ along the magnetic field lines at higher altitudes is a strong reduction of E6 within the auroral forms. From the basic features of the electric field, it is concluded that an interpretation of auroral precipitation in terms of a static E∥ may require a mechanism that can support a large (ΔV)∥ even at relatively weak current densities and at the same time allow local enhancements of the parallel conductivity within the region of non-zero E∥. It is suggested that the magnetic mirroring combined with gyro-resonant wave-particle interactions may be a suitable mechanism.  相似文献   

5.
Hot spots similar to those in the radio galaxy Cygnus A can be explained by the strong shock produced by a supersonic but classical jet \(\left( {u_{jet}< c/\sqrt 3 } \right)\) . The high integrated radio luminosity (L?2×1044 erg s?1) and the strength of mean magnetic field (B?2×10?4 G) suggest the hot spots are the downstream flow of a very strong shock which generates the ultrarelativistic electrons of energy ?≥20 MeV. The fully-developed subsonic turbulence amplifies the magnetic field of the jet up to 1.6×10?4 G by the dynamo effect. If we assume that the post-shock pressure is dominated by relativistic particles, the ratio between the magnetic energy density to the energy density in relativistic particles is found to be ?2×10?2, showing that the generally accepted hypothesis of equipartition is not valid for hot spots. The current analysis allows the determination of physical parameters inside hot spots. It is found that:
  1. The velocity of the upstream flow in the frame of reference of the shock isu 1?0.2c. Radio observations indicate that the velocity of separation of hot spots isu sep?0.05c, so that the velocity of the jet isu jet=u 1+u sep?0.25c.
  2. The density of the thermal electrons inside the hot spot isn 2?5×10?3 e ? cm?3 and the mass ejected per year to power the hot spot is ?4M 0yr?1.
  3. The relativistic electron density is less than 20% of the thermal electron density inside the hot spot and the spectrum is a power law which continues to energies as low as 30 MeV.
  4. The energy density of relativistic protons is lower than the energy density of relativistic electrons unlike the situation for cosmic rays in the Galaxy.
  相似文献   

6.
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   

7.
High resolution electric field and particle data, obtained by the S23L1 rocket crossing over a discrete prebreakup arc in January 1979, are studied in coordination with ground observations (Scandinavian Magnetometer Array—SMA, TV and all-sky cameras) in order to clarify the electrodynamics of the arc and its surroundings. Height-integrated conductivities have been calculated from the particle data, including the ionization effects of precipitating protons and assuming a steady state balance between ion production and recombination losses. High resolution optical information of arc location relative to the rocket permitted a check of the validity of this assumption for each flux tube passed by the rocket. Another check was provided by a comparison between calculated (equilibrium values) and observed electron densities along the rocket trajectory. A way to compensate for the finite precipitation time when calculating the electron densities is outlined. The height-integrated HalI-Pedersen conductivity ratio is typically 1.4 within the arc and about 1 at the arc edges, indicative of a relatively softer energy spectrum there. The height-integrated conductivities combined with the DC electric field measurements permitted calculation of the horizontal ionospheric current vectors (J⊥), Birkeland currents (from div J⊥) and energy dissipation through Joule heating (ΣpE2). An eastward current of typically 1 A m?1 was found to be concentrated mainly to the arc region and equatorward of it. A comparison has been made with the equivalent current system deduced from ground based magnetometer data (SMA) showing a generally good agreement with the rocket results. An intense Pedersen current peak (1.2 A m?1) was found at the southern arc edge. This edge constituted a division line between a very intense (> 10 μA m?1) and localized (~ 6 km) downward current sheet to the south, probably carried by upward flowing cold ionospheric electrons and a more extended upward current sheet (> 10 μA m?2) over the arc carried by measured precipitating electrons. Joule and particle heating across the arc were anticorrelated, consistent with the findings of Evans et al. (1977) with a total value of about 100mW m?2.  相似文献   

8.
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   

9.
We calculate the kinetic coefficients and the transport mean free paths of high-energy particles parallel to the regular magnetic field in the approximation of a large-scale anisotropic random magnetic field by using a nonlinear collision integral, i.e., by taking into account the processes of strong random scattering. We consider the diffusion of solar and Galactic cosmic rays by two-dimensional turbulence. Strong random scattering by two-dimensional turbulence is shown to reduce the parallel transport mean free path several fold. The momentum dependence of the parallel mean free path does not change, Λ∥ ∝ p2?v. In the case of strong random scattering by turbulence formed by several modes, the parallel transport mean free path is Λ∥ ∝ p. We show that two-dimensional turbulence can make a major contribution to the parallel transport mean free paths of cosmic rays in the heliosphere and the interstellar medium.  相似文献   

10.
The propagation and modulation of electrons in the heliosphere play an important part in improving our understanding and assessment of the modulation processes. A full three-dimensional numerical model is used to study the modulation of galactic electrons, from Earth into the inner heliosheath, over an energy range from 10 MeV to 30 GeV. The modeling is compared with observations of 6–14 MeV electrons from Voyager 1 and observations at Earth from the PAMELA mission. Computed spectra are shown at different spatial positions. Based on comparison with Voyager 1 observations, a new local interstellar electron spectrum is calculated. We find that it consists of two power-laws: In terms of kinetic energy E, the results give E ?1.5 below ～500 MeV and E ?3.15 at higher energies. Radial intensity profiles are computed also for 12 MeV electrons, including a Jovian source, and compared to the 6–14 MeV observations from Voyager 1. Since the Jovian and galactic electrons can be separated in the model, we calculate the intensity of galactic electrons below 100 MeV at Earth. The highest possible differential flux of galactic electrons at Earth with E=12 MeV is found to have a value of 2.5×10?1 electrons m?2?s?1?sr?1?MeV?1 which is significantly lower (a factor of 3) than the Jovian electron flux at Earth. The model can also reproduce the extraordinary increase of electrons by a factor of 60 at 12 MeV in the inner heliosheath. A lower limit for the local interstellar spectrum at 12 MeV is estimated to have a value of (90±10) electrons m?2?s?1?sr?1?MeV?1.  相似文献   

11.
A simple model of the motion of charged particles in the closed field line magnetic field for L ? 4·5 is used together with Injun 3 measurements of 40 keV precipitated electrons made in the northern hemisphere to estimate theoretically the extent of electron precipitation, the energy input and the 3914 Å airglow in the South Atlantic geomagnetic anomaly. Using average values of the northern hemisphere precipitated electron flux, two regions of significantly enhanced electron precipitation are found in the southern hemisphere. One occurs in the region 10–20°E and 40–50°S, with L ≈ 2, and the second near 30°E and 65°S, with L ≈ 4.5. Approximately 0.04 erg cm?2 sec?1 are deposited by 40 keV electrons for 50 per cent of the time in the first region and half that amount in the second. This increases to ～0·1 and 0·02 erg cm?2 sec?1 respectively for 15 per cent of the time for near sunspot minimum conditions. The results show a gradual increase in precipitation on the western side of the anomaly followed by a rapid increase and sudden cut-off in precipitation within a few degrees west of minimum B. The flux on L = 2 reaches a “spike” in the southern hemisphere ～f35 times greater than the average flux precipitated on L = 2 in the northern hemisphere. This increase in precipitation arises from the loss of “trapped” particles to the atmosphere where the mirror heights are lowest.  相似文献   

12.
Starting from the Vlasov equation the steady state and stability properties of the electron sheet in the Cowley neutral sheet model of the geomagnetic tail are considered. Electrostatic ion plasma oscillations propagating from dusk to dawn are found to be unstable provided the thermal spread normal to the current is sufficiently large. Assuming the population of the neutral sheet to be supplied by the polar wind it is shown how a localisation of the cross tail electric field could lead to the instability first appearing around midnight. It is conjectured that the localisation of the cross tail electric field could continually feed the instability, so leading to enough turbulence to give enhanced reconnection of the magnetic field.List of symbols f distribution function - B magnetic field strength far from the neutral sheet - a sheet half thickness - total potential drop across the tail which is localised to the dusk end of the tail in Cowley's model - potential for the steady state electric field normal to the electron current sheet. This potential exists in that region of the tail that excludes the localised region of cross tail electric field - average velocity across the tail of electrons in the current sheet - v average velocity of the electrons normal to the current sheet - p canonical momentum of a particle - energy of a particle - KT electron energy normal to the sheet (1/2m e v 2 ) - KT i ion energy (1/2m i V 2 ) - electron gyrofrequency far from the neutral sheet - i ion gyrofrequency far from the neutral sheet - Ay steady state vector potential for the magnetic field - A –Ay/aB 0 (normalised vector potential) When perturbing the steady state, dashes have been used to denote the time dependent first order quantities. Where no confusion could arise the dashes are dropped, e.g.Ey=Ey since there is no zero orderEy in the region considered in the stability analysis.  相似文献   

13.
Simultaneous optical and particle data from the ISIS-2 satellite are used to characterize polar cap arcs. Polar cap arcs are identified from two-dimensional geomagnetic transforms of the optical data along with precipitating electron data for the time at which the satellite is on the field line intersecting the arc. No precipitating protons were detected for any of the arc crossings. The pitch angle. distribution of the precipitating electrons is generally isotropic and the differential electron spectra show enhancements in the flux in the 300–750 eV energy range. The average energy of the precipitating electrons for the different arcs ranges from about 300 to 600 eV. A possible explanation of the observed precipitating particle characteristics is that parallel electric fields are accelerating polar rain type spectra at an altitude of several thousand km. For the arc crossings reported here the equivalent 4278 Å emission rate per unit energy deposition rate has a mean value of 162 R/(erg cm?2 s?1). Average 3914 Å intensities are about 0.8 kR while 6300 Å intensities range from 0.5 to 3 kR. Model calculations indicate that direct impact excitation is a minor source for the 5577 Å emission rate, but supplies approx. 40% of the 6300 Å emission.  相似文献   

14.
We propose an accurate analytical model for the source of hard X-ray emission from a flare in the form of a “thick target” with a reverse current to explain the results of present-day observations of solar flares onboard the GOES, Hinode, RHESSI, and TRACE satellites. The model, one-dimensional in coordinate space and two-dimensional in velocity space, self-consistently takes into account the fact that the beam electrons lose the kinetic energy of their motion along the magnetic field almost without any collisions under the action of the reverse-current electric field. Some of the electrons return from the emission source to the acceleration region without losing the kinetic energy of their transverse motion. Based on the observed hard X-ray bremsstrahlung spectrum, the model allows the injection spectrum of accelerated electrons to be reconstructed with a high accuracy. As an example, we consider the white-light flare of December 6, 2006, which was observed with a high spatial resolution in the optical wavelength range at the main maximum of hard X-ray emission. Within the framework of our model, we show that to explain the hard X-ray spectrum, the flux density of the energy transferred by electrons with energies above 18 keV was ～3 × 1013 erg cm?2 s?1. This exceeds the habitual values typical of the classical model of a thick target without a reverse current by two orders of magnitude. The electron density in the beam is also very high: ～1011 cm?3. A more careful consideration of plasma processes in such dense electron beams is needed when the physical parameters of a flare are calculated.  相似文献   

15.
The structure of a hydrogen atom situated in an intense magnetic field is investigated. Three approaches are employed. An elementary Bohr picture establishes a crucial magnetic field strength,H a ?5×109G. Fields in excess ofH a are intense in that they are able to modify the characteristic atomic scales of length and binding energy. A second approach solves the Schrödinger equation by a combination of variational methods and perturbation theory. It yields analytic expressions for the wave functions and energy eigenvalues. A third approach determines the energy eigenvalues by reducing the Schrödinger equation to a one-dimensional wave equation, which is then solved numerically. Energy eigenvalues are tabulated for field strengths of 2×1010G and 2×1012 G. It is found that at 2×1012 G the lowest energy eigenvalue is changed from ?13.6 eV to about ?180 eV in agreement with previous variational computations.  相似文献   

16.
Experimental data describing the effect of the South Atlantic anomaly on E? 280 keV electron flux at L = 2 and high B values, are compared to the numerical solution of a pitch-angle diffusion equation with a varying loss cone. The diffusion coefficient needed to explain replenishment of the electrons lost over the anomaly is found to be 3.2 × 10?2 sec?1 Calculation of the diffusion coefficient due to cyclotron resonant interaction with VLF electro-magnetic waves leads to the conclusion that the observed wave spectral density can yield the needed diffusion coefficient.  相似文献   

17.
The observations of electron inverted ‘V’ structures by the MGS and MEX spacecraft, their resemblance to similar events in the auroral regions of the Earth, and the discovery of strong localized magnetic field sources of the crustal origin on Mars, raised hypotheses on the existence of Martian aurora produced by electron acceleration in parallel electric fields. Following the theory of this type of structures on Earth we perform a scaling analysis to the Martian conditions. Similar to the Earth, upward field-aligned currents necessary for the generation of parallel potential drops and peaked electron distributions can arise, for example, on the boundary between ‘closed’ and ‘open’ crustal field lines due to shears of the flow velocity of the magnetosheath or magnetospheric plasmas. A steady-state configuration assumes a closure of these currents in the Martian ionosphere. Due to much smaller magnetic fields as compared to the Earth case, the ionospheric Pedersen conductivity is much higher on Mars and auroral field tubes with parallel potential drops and relatively small cross scales to be adjusted to the scales of the localized crustal patches may appear only if the magnetosphere and ionosphere are decoupled by a zone with a strong E∥. Another scenario suggests a periodic short-circuit of the magnetospheric electric fields by a coupling with the conducting ionosphere.  相似文献   

18.
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   

19.
The response of an isothermal atmosphere of a thin vertical magnetic flux tube to the presence of an acoustic-gravity wave field in the external medium is considered. The Laplace transform method is used to solve a problem with initial conditions. The structure of the solution for disturbances in the tube is a superposition of forced oscillations at the source frequency and oscillations decaying as ～ t ?3/2 (the so-called wave wake). Both components are analogues of the corresponding disturbances in an external medium with a modified amplitude. The excitation under consideration is shown to be effective in the ranges of external oscillation frequencies 0 mHz ≤ v ≤ 3.3 mHz and v ≥ 6.5 mHz. The time-averaged energy flux density for high-frequency magnetoacoustic-gravity waves in the tube is estimated to be ∝ 3.0 × 107 erg cm?2 s?1, a value of the same order of magnitude as that required for heating local regions in the solar chromosphere, ∝ 107 erg cm?2 s?1.  相似文献   

20.
The continuity, momentum and energy hydrodynamic equations for an O+-H+ ionosphere have been solved self-consistently for steady state conditions when a perpendicular (convection) electric field is present. Comparison of the H+ temperature profiles obtained with and without the electric field show that the effect of the electric field is to enhance the H+ temperature at high altitudes from about 3600 to 6400 K. Due to ion heating by the electric field, there is a net reduction of O+ in the F2-region as compared with the case of a non-convecting ionosphere. When the reduction of O+ is neglected, the electric field acts to increase the H+ outward flux from 8.3 × 107 to 2.7 × 108 cm?2 sec?1 for average ionospheric conditions. However, when the reduction of O+ is included, there is a net reduction in the outward H+ flux. Nevertheless, the convection electric field still results in an increase in the rate of depletion of the F-re m?1 electric field.  相似文献   

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