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1.
The recovery phase of the magnetospheric substorm is studied numerically by means of a two-dimensional time-dependent nonlinear resistive MHD code. The initial configuration was chosen from the earlier numerical model in which the magnetospheric substorm was driven by the solar wind plasmas. In order to study the recovery phase, the entering solar wind energy flux was reduced when the magnetospheric substorm was in its expansive phase. The system was found to respond instantly to this change and the result showed many characteristic features related to the recovery phase including the tailward motion of thex-point of the reconnected magnetic field lines and the restoration of a tail-like configuration of the magnetic field. Thex-point moved at almost the same speed of the plasma flow in the upstream region, which was considerably smaller than the speed of the plasma jetting or the speed of the plasmoid. As the recovery phase progressed, the plasma jetting across thex-point was reduced very much in the Earthside region. Although the plasma flow was generally in the Earthward direction in the Earthside region of thex-point, the tailward flow was also found near thex-point. The current density was reduced near thex-point and the neutral sheet was broadened in the recovered region. The plasma sheet also became thick in this region. During the recovery of the substorm, the energy conversion rate, both in the form of plasma acceleration and the Joule heating, was reduced. These results on the recovery phase together with the earlier simulation result on the expansive phase indicate that driven reconnection can be a viable mechanism for the magnetospheric substorm including the recovery phase.  相似文献   

2.
The kinetic Alfven waves are investigated using Maxwell-Boltzmann-Vlasov equation to evaluate the kinetic dispersion relation and growth/damping rate with magnetic field gradient, density gradient, temperature gradient and velocity gradient with inhomogeneous plasma. The effect of gradient terms is included in the analysis for both the regions k ⊥ ρ i <1 and k ⊥ ρ i >1, where k ⊥ is the perpendicular wave number and ρ i is the ion gyroradius. This study elucidates a possible scenario to account for the particle acceleration and the wave dissipation in inhomogeneous plasmas. This model is able to explain many features observed in plasma sheet boundary layer as well as to evaluate the dispersion relation, growth rate, growth length and damping rate of kinetic Alfven wave. The applicability of this model is assumed for auroral acceleration region, plasma sheet boundary layer and cusp region.  相似文献   

3.
A comparison of the variations in the count of electrons E > 36 keV on the satellite Vela 4A, and in the Macquarie Island magnetometer H trace, shows for a time lag of 22-8 min a correlation, r = 0.95, over a 90 min period of the recovery phase of a magnetospheric substorm on 17 August 1968. All-sky camera data suggest that during the correlation period the auroral electrojet showed very little latitudinal movement. Each peak in electron count relates to a current surge in the electrojet as shown by a deepening of the negative bay at Macquarie Island.Using the Fairfield (1968) model of the location of auroral shells in the solar magnetic equatorial plane, and the known location of the satellite, an estimate of the velocity of tail to Earth plasma convection in the plasma sheet of about 0·33 Re/min is obtained for the recovery phase.The relationship is discussed between plasma sheet thinning and subsequent broadening, and the extension of the magnetic field lines into the tail region and their subsequent return. This discussion makes use of the estimated time lags between electron count at the satellite and the time of arrival of auroral particles at the antisolar meridian.From a somewhat speculative explanation, but one largely supported from the literature, of the magnetospheric processes involved in this auroral substorm, a plasma velocity estimate of 0·42 Re/min for the initial phase of the substorm is obtained. These velocities are of the same order as the 0·5 Re/min obtained by Lezniak and Winkler (1970) at 6·6 Re.  相似文献   

4.
The configuration of the magnetotail magnetic field has been calculated for a situation where a disruption of a portion of the tail current system develops. The decrease of the current in a localized region of the magnetotail leads to a collapse of the magnetic field in that vicinity. The calculated configuration of the field resembles what is predicted by reconnection models with the field lines moving toward the neutral sheet and then connecting and either moving toward or away from the earth. Associated with this changing magnetic field there is an induced electric field which will then influence the motion of the plasma in the magnetotail via E × B drifts.When the current from Xsm = ?20 to ?40 RE in the tail is decreasing with a tune-constant of 0.5 h the electric field produced, which is primarily westward, has a maximum value of 0.83 mV m?1 and produces plasma sheet thinning velocities of 0.3 km s?1. Higher velocities result for more rapid rates of current decrease, and they agree well with experimental observations. The plasma flows in the sunward direction are, however, much smaller than what has been observed. This is due in part to the inability of the magnetic field model to adequately represent the magnetic field in the immediate vicinity of the neutral sheet. Use of an improved model would give better agreement with the observations.The calculations show that the induced electric field of a time-dependent magnetic field is able to explain certain observed features of the plasma sheet motions. Also, this agreement suggests that the assumption that there is no charge separation contribution to the electric field may be reasonable during situations of large scale and rapid current disruptions in the magnetotail.  相似文献   

5.
Hydrodynamic and electrodynamic problems of solar wind interaction with the Earth's magnetosphere on the day-side are investigated.The initial fact, well established, is that the density of the magnetic field energy in the solar wind is rather small. Magnetic field intensity and orientation are shown to determine the character of the solar wind flow around the magnetosphere. For mean parameters of the wind, if the tangential component of the magnetic field is more or equal 5γ, the flow in the magneto-sheath will be laminar. For other cases the flow is of a turbulent type.For turbulent flow, typical plasma parameters are estimated: mean free path, internal scale of inhomogeneities and dissipated energy. The results obtained are compared with experimental data.For the case of laminar flow, special attention is paid to the situation when magnetic fields of the solar wind and Earth are antiparallel. It is suggested, on the basis of solid arguments, that the southward interplanetary field diffuses from the magnetosheath into the Earth's magnetosphere. These ideas are used for the estimation of the distance to the magnetopause subsolar point. A detailed comparison with results of observation is made. The coincidence is satisfactory. Theoretical investigation has been made to a great extent for thin magnetopause with thickness δ ～ RHe-gyroradius of an electron.It is shown that during magnetospheric substorms relaxation oscillations with the period τ = 100–300 sec must appear. A theorem is proved about the appearance of a westward electrical field during the substorm development, when the magnetosphere's day-side boundary moves Earthward and about the recovery phase, when the magnetopause motion is away from the Earth, when there is an eastward electrical field.In the Appendix, plasma wave exitation in the magnetopause is considered and conductivity magnitudes are calculated, including the reduction due to the scattering by plasma turbulence.  相似文献   

6.
The propagation of magnetoacoustic waves in the neighbourhood of a 2D null point is investigated for both β=0 and β ≠ 0 plasmas. Previous work has shown that the Alfvén speed, here v A ∝r, plays a vital role in such systems and so a natural choice is to switch to polar coordinates. For β=0 plasma, we derive an analytical solution for the behaviour of the fast magnetoacoustic wave in terms of the Klein–Gordon equation. We also solve the system with a semi-analytical WKB approximation which shows that the β=0 wave focuses on the null and contracts around it but, due to exponential decay, never reaches the null in a finite time. For the β ≠ 0 plasma, we solve the system numerically and find the behaviour to be similar to that of the β=0 system at large radii, but completely different close to the null. We show that for an initially cylindrically-symmetric fast magnetoacoustic wave perturbation, there is a decrease in wave speed along the separatrices and so the perturbation starts to take on a quasi-diamond shape; with the corners located along the separatrices. This is due to the growth in pressure gradients that reach a maximum along the separatrices, which in turn reduces the acceleration of the fast wave along the separatrices leading to a deformation of the wave morphology.  相似文献   

7.
Soft X-ray data from the XRP experiment on SMM are used to generate the temperature and density in the flaring region of the 1980, June 29 (18∶21 UT) solar flare. The temporal data (T max ～- 20 × 106 K and n max ～- 4 × 1011 cm?3), together with an assumed velocity, are used to simulate mass injection as the input pulse for the MHD model of Wu et al. (1982a, 1983a). The spatial and temporal coronal response is compared with the ground-based, Mark III K-coronameter observations of the subsequent coronal transient. The simulation produces a spatially-wide, large amplitude, temporarily-steepened MHD wave for either of the two ‘canonical’ magnetic topologies (closed and open), but no shock wave. This result appears to be confirmed by the fact that a type II radio event was observed late in the event for only a few minutes, thereby indicating that a steepening wave with temporary, marginal shock formation, was indeed present. The density enhancements produced by the simulation move away from the Sun at the same velocity observed by the K-coronameter. However, the observation of the coronal transient included a rarefaction that does not appear in the simulation. A probable explanation for this discrepancy is the likelihood that the magnitude and temporal profile of the density of the soft X-ray emitting plasma should not have been used as part of the mass injection pulse. We believe that the temperature profile alone, as suggested by earlier simulations, might have been a necessary and sufficient condition to produce both the compression and rarefaction of the ambient corona as indicated by the K-coronameter data. Hence, the dense plasma observed by XRP was probably confined, for the most part, close to the Sun during the ～ 17 min duration of the observations.  相似文献   

8.
We present a conceptual model of the formation of the plasma sheet and of its dynamical behavior in association with magnetospheric substorms. We assume that plasma mantle particles E～×B～ drift toward the current sheet in the center of the tail where they are accelerated by magnetic-field annihilation to form the plasma sheet. Because of the velocity-dependent access of mantle particles to the current sheet, we argue that the convection electric field and the corresponding rate of field annihilation decrease with increasing radial distance. As a consequence, there exists no steady-state configuration for the plasma sheet, which must instead shrink continuously in thickness until the near-earth portion of the current sheet is disrupted by the formation of a magnetic neutral line. The current-sheet disruption launches a large-amplitude hydromagnetic wave which is largely reflected from the ionosphere. The reflected wave sets the neutral line in motion away from the earth; the neutral line comes to rest at a distance (which we estimate to be a few hundred earth radii) where the incoming mantle particles enter the current sheet at the local Alfvén velocity. At this “Alfvén point” reconnection ceases and the thinning of the plasma sheet begins again. Within this model, the magnetospheric substorm (which is associated with the current-sheet disruption) is a cyclical phenomenon whose frequency is proportional to the rate of convection in the magnetospheric tail.  相似文献   

9.
Data taken by the NASA-GSFC magentometer aboard the Explorer 34 satellite are analysed in an effort ascertain the average motion and magnetic field structure of the plasma sheet near 30RE. We find that the flapping motion of the plasma sheet in the solar ecliptic Z-coordinate is characterized by a typical speed of 90 km/sec and an amplitude of ±2RE. Results suggest that there exists a layer of nearly uniform cross-tail current density in the central region of the plasma sheet approx 2· 3-2· 6RE thick within which the solar-magnetospheric X-component of the magnetic field changes from 10γ to ? 10γ. Near the centre of the plasma sheet, the average value of the Z-component of the field is 2γ. The analysis does not suggest a strong dependence of the magnetic-field configuration on Kp.  相似文献   

10.
Most of the MHD instabilities originating from the nonuniformity of a plasma excite MHD surface wave. When the excited wave has a frequency s which corresponds to the local shear Alfvén wave resonance (s = k v a (x), where v a is the Alfvén speed and k is the wave number in the direction of the magnetic field), the surface wave resonantly mode converts to the kinetic Alfvén wave, the Alfvén wave having a perpendicular wavelength comparable to the ion gyroradius and being able to propagate across the magnetic field. We discuss various linear and nonlinear effects of this kinetic Alfvén wave on the plasma including particle acceleration and heating. A specific example for the case of a MHD Kelvin-Helmholtz instability is given.  相似文献   

11.
The current sheet in Earth’s magnetotail often flaps, and the flapping waves could be induced propagating towards the dawn and dusk flanks, which could make the current sheet dynamic. To explore the dynamic characteristics of current sheet associated with the flapping motion holistically and provide reasonable physical interpretations, detailed direct calculation and analysis have been applied to one approximate analytic model of magnetic field in the flapping current sheet. The main results from the model demonstrate: (1) the magnetic fluctuation amplitude is attenuated from the center of current sheet to the lobe regions; The larger wave amplitude would induce the larger magnetic amplitude; (2) the curvature of magnetic field lines (MFLs), with maximum at the center of current sheet, is only dependent on the displacement Z along the south-north direction from the center of current sheet, regardless of the tilt of current sheet; (3) the half-thickness of neutral sheet, h, the minimum curvature radius of MFLs, Rcmin, and the tilt angle of current sheet, δ, satisfies h=Rcmin cos δ; (4) the gradient of magnetic strength forms a double-peak profile, and the peak value would be more intense if the local current sheet is more tilted; (5) current density j and its jy, jz components reach the extremum at the center of CS. j and jz would be more intense if the local current sheet is more tilted, but it is not the case for jy; and (6) the field-aligned component of current density mainly appears in the neutral sheet, and the sign of it would change alternatively as the flapping waves passing by. To check the validity of the model, one simulation on the virtual measurements has been made, and the results are in well consistence with actual observations of Cluster.  相似文献   

12.
It is suggested that localised electrostatic potential wells could be generated in the plasma sheet by large amplitude electrostatic ion cyclotron waves. It is shown from a consideration of a simple one dimensional model that such wells could possess a double structure of oppositely directed fields elongated in longitude. The possibility that the waves could evolve from a turbulent ion wave cascade driven by Earthward streaming protons is discussed and the magnitude of the potentials that could be established in this way is estimated using results for condensed state turbulent equilibria.The projections of these wells along the highly conducting geomagnetic field lines form potential valleys across the field lines in the high latitude auroral plasma. It is shown that these valleys would be of the scale and depth needed to establish electrostatic shocks which would be of sufficient intensity to accelerate electrons to energies comparable to those observed in “inverted-V” events. Potential wells are formed predominantly in the midnight sector of the plasma sheet and propagate Earthwards. This implies a corresponding equatorwards motion of the valley which, typically, would have a velocity of a few hundred m s?1.  相似文献   

13.
We present results from an investigation of the plasma sheet encounter signatures observed in the Jovian magnetosphere by the Energetic Particles Detector (EPD) and Magnetometer (MAG) onboard the Galileo spacecraft. Maxima in ion flux were used to identify over 500 spacecraft encounters with the plasma sheet between radial distances from Jupiter from 20 to 140RJ during the first 25 orbits (4 years of data). Typical signatures of plasma sheet encounters show a characteristic periodicity of either 5 or 10 hours that is attributed to an oscillation in the relative distance between the spacecraft and the plasma sheet that arises from the combination of planetary rotation and offset magnetic and rotational axes. However, the energetic particle and field data also display much variability, including instances of intense fluxes having little to no periodicity that persist for several Jovian rotation periods. Abrupt changes in the mean distance between the plasma sheet and the spacecraft are suggested to account for some of the transitions between typical flux periodicities associated with plasma sheet encounters. Additional changes in the plasma sheet thickness and/or amplitude of the plasma sheet displacement from the location of the spacecraft are required to explain the cases where the periodicity breaks down but fluxes remain high. These changes in plasma sheet characteristics do not display an obvious periodicity; however, the observations suggest that dawn/dusk asymmetries in both the structure of the plasma sheet and the frequency of anomalous plasma sheet encounters are present. Evidence of a thin, well-ordered plasma sheet is found out to 110RJ in the dawn and midnight local time sectors, while the dusk magnetosphere is characterized by a thicker, more disordered plasma sheet and has a potentially more pronounced response to an impulsive trigger. Temporal variations associated with changing solar wind conditions are suggested to account for the anomalous plasma sheet encounters there.  相似文献   

14.
Dipolarization fronts in the magnetotail plasma sheet   总被引：1，自引：0，他引：1  
We present a THEMIS study of a dipolarization front associated with a bursty bulk flow (BBF) that was observed in the central plasma sheet sequentially at X=−20.1, −16.7, and −11.0RE. Simultaneously, the THEMIS ground network observed the formation of a north-south auroral form and intensification of westward auroral zone currents. Timing of the signatures in space suggests earthward propagation of the front at a velocity of 300 km/s. Spatial profiles of current and electron density on the front reveal a spatial scale of 500 km, comparable to an ion inertial length and an ion thermal gyroradius. This kinetic-scale structure traveled a macroscale distance of 10RE in about 4 min without loss of coherence. The dipolarization front, therefore, is an example of space plasma cross-scale coupling. THEMIS observations at different geocentric distances are similar to recent particle-in-cell simulations demonstrating the appearance of dipolarization fronts on the leading edge of plasma fast flows in the vicinity of a reconnection site. Dipolarization fronts, therefore, may be interpreted as remote signatures of transient reconnection.  相似文献   

15.
Four models for geomagnetic substorms, a quiet tail model, and models incorporating structural effects of the tail are examined for consistency with magnetic-field data during satellite crossings of the tail neutral sheet/plasma sheet. For this data the tearing mode instability model is always consistent, and inward moving distant neutral line is sometimes consistent, quasi-steady reconnection with slow shock and intermediate wave structure and locally quiet tail rarely consistent, and an outward propagating rarefaction wave is never consistent with the magnetic observations. In several cases structural effects of the tail are consistent with key features of the magnetic signatures.  相似文献   

16.
The equilibrium structure of two-dimensional magnetic current sheets is investigated for systems in which the plasma pressure dominates the bulk flow energy, as appears appropriate for the quiet time plasmasheet in the geomagnetic tail. A simple model is studied in which the field is contained between plane parallel boundaries and varies exponentially along the system, while the plasma pressure is anisotropic, the anisotropy being arbitrary but constant along the centre plane. When the field is highly inflated by the plasma current it is found that adiabatic solutions exist only when the plasma pressure is close to isotropic. For the case P∥ > P⊥ it is argued that a thin, non-adiabatic current layer will in general form at the sheet centre, usually embedded within a much broader adiabatic current distribution. When P⊥ > P∥, a broad region of very depressed fields develops about the centre of the current sheet, terminated at its outer boundary by a spike in the current density. This central region becomes unstable to the mirror mode well before the limiting adiabatic solution is reached.  相似文献   

17.
K. Jockers  S. Szutowicz  T. Bonev 《Icarus》2011,215(1):153-185
Axisymmetric models of the outgassing of a cometary nucleus have been constructed. Such models can be used to describe a nucleus with a single active region. The models may include a solar zenith angle dependence of the outgassing. They retrieve the outgassing flux at distances from the nucleus where collisions between molecules are unimportant, as function of the angle with respect to the outgassing axis. The observed emissions must be optically thin. Furthermore the models assume that the outflow speed at large distance from the nucleus does not depend on direction. The value of the outflow speed is retrieved. The models are applied to CN images and HCN spectra of Comet 2P/Encke, obtained nearly simultaneously in November 2003 with the 2 m optical telescope on Mount Rozhen, Bulgaria, and with the 10 m Heinrich Hertz Submillimeter Telescope on Mount Graham, Arizona, USA. According to Sekanina (1988), Astron. J. 95, 911-924, at that time a single outgassing source was active. Input parameters to the models like the rotation period of the nucleus and a small correction to Sekanina’s rotation axis are determined from a simpler jet position angle model. The rotation is prograde with a sideric period of 11.056 ± 0.024 h, in agreement with literature values. The best fit model has an outflow speed of 0.95 ± 0.04 km s−1. The same value has been derived from the corkscrew appearing in the CN images. The location of the outgassing axis is at colatitude δa = 7.4° ± 2.9° and longitude λa = 235° ± 17° (a definition of zero longitude is provided). Comet Encke’s outgassing corresponds approximately to the longitudinally averaged solar input on a spherical nucleus (i.e. very likely comes from deeper layers) but with some deficiency of outgassing at mid-latitudes and non-zero outgassing from the dark polar cap. The presence of gas flow from the dark polar cap is explained as evidence of gas flow across the terminator. The models rely mostly on the CN images. The HCN spectra are more noisy. They provide information how to determine the best fit outflow velocity and the sense of rotation. The model HCN spectra are distinctly non-Gaussian. Within error limits they are consistent with the observations. Models based solely on the HCN spectra are also presented but, because of the lower quality of the data and the unfavorable observing geometry, yield inferior results. As a by-product we determine the CN parent life time from our CN observations. The solar EUV and Lyα radiation field at the time of our observations is taken into account.  相似文献   

18.
A number of fundamental questions as regards the physical nature of sunspots are formulated. In order to answer these questions, we apply the model of a round-shaped unipolar sunspot with a lower boundary consisting of cool plasma and with strong magnetic field at the depth of about 4 Mm beneath the photosphere, in accordance with the data of local helioseismology and with certain physically sound arguments (the shallow sunspot model). The magnetic configuration of a sunspot is assumed to be close to the observed one and similar to the magnetic field of a round solenoid of the appropriate size. The transverse (horizontal) and longitudinal (vertical) equilibria of a sunspot were calculated based on the thermodynamic approach and taking into account the magnetic, gravitational, and thermal energy of the spot and the pressure of the environment. The dependence of the magnetic field strength in the sunspot center, B 0, on the radius of the sunspot umbra a is derived theoretically for the first time in the history of sunspot studies. It shows that the magnetic field strength in small spots is about 700 Gauss (G) and then increases monotonically with a, tending asymptotically to a limit value of about 4000 G. This dependence, B 0(a) includes, as parameters, the gravity acceleration on the solar surface, the density of gas in the photosphere, and the ratio of the radius of the spot (including penumbra), a p, to the radius of its umbra a. It is shown that large-scale subsurface flows of gas in the sunspot vicinity, being the consequence but not the cause of sunspot formation, are too weak to contribute significantly to the pressure balance of the sunspot. Stability of the sunspot is provided by cooling of the sunspot plasma and decreasing of its gravitational energy due to the vertical redistribution of the gas density when the geometric Wilson depression of the sunspot is formed. The depth of a depression grows linearly with B 0, in contrast to the quadratic law for the magnetic energy. Therefore, the range of stable equilibria turns out to be limited: large spots, with radius a larger than some limit value (about 12–18 Mm, depending on the magnetic field configuration), are unstable. It explains the absence of very large spots on the Sun and the appearance of light bridges in big spots that divide the spot into a few parts. The sunspots with B 0≈2.6÷2.7 kilogauss (kG) and a≈5 Mm are most stable. For these spots, taken as a single magnetic structure, the period of their vertical eigen oscillations is minimal and amounts, according to the model, to 10–12 hours. It corresponds well to the period derived from the study of long-term oscillations of sunspots using SOHO/MDI data.  相似文献   

19.
The temperature and density of the plasma in the Earth's distant plasma sheet at the downstream distances of about 20–25 Re are examined during a high geomagnetic disturbance period. It is shown that the plasma sheet cools when magnetospheric substorm expansion is indicated by the AE index. During cooling, the plasma sheet temperature, T, and the number density, N, are related by T ∝ N23 (adiabatic process) in some instances, while by T ∝ N?1 (isobaric process) in other cases. The total plasma and magnetic pressure decreases when T ∝ N23 and increases when T ∝ N?1. Observation also indicates that the dawn-dusk component of plasma flow is frequently large and comparable to the sunward-tailward flow component near the central plasma sheet during substorms.  相似文献   

20.
Jun-Ichi Sakai 《Solar physics》1983,84(1-2):109-118
Transverse amplitude modulations of fast magnetosonic waves propagating perpendicular to the background magnetic field are shown to be unstable on a time scale τ ～- λ∥/V aφ, if the wave amplitude φ exceeds a critical value, φ c = C s/V a. The slow modes generated by the modulational instability under gravity can propagate along the magnetic field with the characteristic velocity, V ph = g/2k ∥ V aφ. The applications of this modulational instability and slow-mode generation mechanism to a solar plasma are discussed.  相似文献   

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