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1.
Zmuda and Armstrong (1974) showed that the field-aligned currents consist of two pairs; one is located in the morning sector and the other in the evening sector. Our analysis of magnetic records from the TRIAD satellite suggests that in each pair the poleward field-aligned current is more intense than the equatorward current, a typical ratio being 2:1. This difference has a fundamental importance in understanding the coupling between the magnetosphere and the ionosphere. We demonstrate this importance by computing the ionospheric current distribution by solving the continuity equation using the “observed” distribution of j∥ for several models of the ionosphere with a high conductive annular ring (simulating the auroral oval).It is shown that the actual field-aligned and ionospheric current system is neither a simple Birkeland type, Boström type nor Zmuda-Armstrong type, but is a complicated combination of them. The relative importance among them varies considerably, depending on the conductivity distribution, the location of the peak of the field-aligned currents, etc. Further, it is found that the north-south segment of ionospheric current which connects the pair of the field-aligned currents in the morning sector does not close in the same meridian and has a large westward deflection. Thus, it has an appreciable contribution to the westward electrojet. One of the model calculations shows that the entire north-south closure current contributes to the westward electrojet. 相似文献
2.
We present results from a theoretical model which has been used to investigate the modulation of the magnetosphere-ionosphere coupling currents in the Jovian middle magnetosphere by solar wind-induced compressions and expansions of the magnetosphere. We consider an initial system in which the current sheet field lines extend to 50RJ in the equatorial plane, and where the iogenic plasma in the current sheet undergoes steady outward radial diffusion under the influence of the ionospheric torque which tends to maintain corotation with the planet. We show using typical Jovian parameters that the upward-directed field-aligned currents flowing throughout the middle magnetosphere region in this system peak at values requiring the existence of significant field-aligned voltages to drive them, resulting in large precipitating energy fluxes of accelerated electrons and bright ‘main oval’ UV auroras. We then consider the changes in these parameters which take place due to sudden expansions or compressions of the magnetosphere, resulting from changes in the solar wind dynamic pressure. Two cases are considered and compared, these being first the initial response of the system to the change, determined approximately from conservation of angular momentum of the radially displaced plasma and frozen-in field lines, and second the subsequent steady state of steady outward radial diffusion applied to the compressed or expanded system. We show that moderate inward compressions of the outer boundary of the current sheet field lines, e.g. from 50 to 40RJ, are effective in significantly reducing the coupling currents and precipitation in the initial state, the latter then recovering, but only partly so, during the evolution to the steady state. Strong inward compressions, e.g. to 30RJ cause significant super-corotation of the plasma and a reversal in sense of the current system in the initial state, such that bright auroras may then be formed poleward of the usual ‘main auroral oval’ due to the ‘return’ currents. The sense of the currents subsequently reverts back to the usual direction as steady-state conditions are restored, but they are weak, and so is the consequent electron precipitation. For outward expansions of the current sheet, however, the field-aligned currents and electron precipitation are strongly enhanced, particularly at the poleward border mapping to the outer weak field region of the current sheet. In this case there is little evolution of the parameters between the initial expansion and the subsequent steady state. Overall, the results suggest that the Jovian middle magnetosphere coupling currents and resulting ‘main oval’ auroral acceleration and precipitation will be strongly modulated by the solar wind dynamic pressure in the sense of anti-correlation, through the resulting compressions and expansions in the size of the magnetosphere. 相似文献
3.
Magnetospheric Alfvén waves are reflected by the ionosphere. We investigate the effect of horizontally varying ionospheric conductivity on the process of Alfvén wave reflection. Four idealised ionospheric models are considered in detail. We find that the reflection process is strongly dependent on the orientation of the wave electric field vector with respect to the boundary between high and low conductivities, and under certain conditions subsidiary Alfvén waves are generated. The field-aligned currents in the subsidiary Alfvén waves serve to close divergent horizontal currents resulting from the non-uniform ionospheric conductivity. The implications for ground-based pulsation studies are discussed. 相似文献
4.
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
5.
M. Lockwood 《Planetary and Space Science》1982,30(6):595-609
Topside ionospheric profiles are used to study the upward field-aligned flow of thermal O+ at high latitudes. On the majority of the field lines outside the plasmasphere, the mean flux is approximately equal to the mean polar wind measured by spacecraft at greater altitudes. This is consistent with the theory of thermal light ion escape supported, via charge exchange, by upward O+ flow at lower heights. Events of larger O+ flow are detected at auroral latitudes and their occurrence is found to agree with that of transversely accelerated ions within the topside ionosphere and the magnetosphere. The effects of low altitude heating of O+ by oxygen cyclotron waves, driven by downward field-aligned currents, are considered as a possible common cause of these two types of event. 相似文献
6.
Analysis of global hybrid simulations of Mercury’s magnetosphere-solar wind interaction is presented for northward and southward interplanetary magnetic field (IMF) orientations in the context of MESSENGER’s first two encounters with Mercury. The global kinetic simulations reveal the basic structure of this interaction, including a bow shock, ion foreshock, magnetosheath, cusp regions, magnetopause, and a closed ion ring belt formed around the planet within the magnetosphere. The two different IMF orientations induce different locations of ion foreshock and different magnetospheric properties: the dayside magnetosphere is smaller and cusps are at lower latitudes for southward IMF compared to northward IMF whereas for southward IMF the nightside magnetosphere is larger and exhibits a thin current sheet with signatures of magnetic reconnection and plasmoid formation. For the two IMF orientations the ion foreshock and quasi-parallel magnetosheath manifest ion-beam-driven large-amplitude oscillations, whereas the quasi-perpendicular magnetosheath shows ion-temperature-anisotropy-driven wave activity. The ions in Mercury’s belt remain quasi-trapped for a limited time before they are either absorbed by Mercury’s surface or escape from the magnetosphere. The simulation results are compared with MESSENGER’s observations. 相似文献
7.
Kinetic Alfven waves are examined in the presence of electron and ion beam and an inhomogeneous magnetic field with bi-Maxwellian distribution function. The theory of particle aspect analysis is used to evaluate the trajectories of the charged particles. The expressions for the field-aligned currents, perpendicular currents (with respect to B 0), dispersion relation and growth/damping rate with marginal instability criteria are derived. The effect of electron and ion beam and inhomogeneity of magnetic field are discussed. The results are interpreted for the space plasma parameter appropriate to the auroral acceleration region of the earth’s magnetoplasma. 相似文献
8.
Kinetic Alfven waves (KAWs) driven by the diamagnetic drift instability that is excited by the density inhomogeneity in low-β plasmas, such as plasmas in the auroral region, are investigated by adopting the particle aspect analysis and loss-cone distribution function. The results obtained in this paper indicate that the propagation and evolution of kinetic Alfven waves decrease and the kinetic Alfven wave excitation becomes not easier with increasing loss-cone index J. But the spatial scales of the perpendicular perturbation driving kinetic Alfven waves have a decreasing tendency with the larger values of J, which perhaps is in relation with the decreasing width of loss-cone. A single hump appears in the plots of the growth rate of the instability when J=2. But the hump cannot emerge when J=0 or J=1. The density inhomogeneity of ions plays an important role in driving KAWs and it cannot be ignored. KAWs can be easier driven and KAWs can propagate and evolve faster with the increasing level of density inhomogeneity. However, the range of the perpendicular wave number of the wave instability decreases, namely, the longer the scale of perpendicular disturbance the easier the excitation of KAW. As the density inhomogeneity increases, the tendency of numerical solutions of the dispersion relation is similar to that obtained by the kinetic theory and Maxwellian distribution function (Duan and Li, 2004). But the profiles of the plots of numerical solutions are different. This means that the velocity distribution function of particles is important for KAW driven in magnetoplasmas, especially in the active regions of the magnetosphere, such as auroral region, and plasma sheet boundary. 相似文献
9.
P. M. Banks T. Araki C. R. Clauer J. P. ST. Maurice J. C. Foster 《Planetary and Space Science》1984,32(12):1551-1557
This report investigates the suggestion that the pattern of plasma convection in the polar cleft region is directly determined by the interplanetary electric field (IEF). Owing to the geometrical properties of the magnetosphere, the East-West component of the IEF will drive field-aligned currents which connect to the ionosphere at points lying on either side of noon, while currents associated with the North-South component of the IEF will connect the two polar caps as sheet currents centered at noon. The effects of the hypothesized IEF driven cleft current systems on polar cap ionospheric plasma convection are investigated through a series of numerical simulations. The simulations demonstrate that this simple electrodynamic model can account for the narrow “throats” of strong dayside antisunward convection observed during periods of southward interplanetary magnetic field (IMF) as well as the sunward convection observed during periods of strongly northward IMF. Thedawn-dusk shift of polar cap convection which is related to the By component of the IMF is also accounted for by the model. 相似文献
10.
A detailed study of the mechanism of electromagnetic stratification of the large-scale stationary magnetospheric convection due to a friction of the convective flow in the ionosphere layer was performed. Magnetosphere-ionosphere interaction was taken into account by means of the effective boundary conditions on the ionosphere top and bottom boundaries including the actual height profile of charge particles velocity in the ionosphere. It has been shown that the magnetospheric convection is stratified into small-scale current sheets which are respective in the linear approximation to an oblique Alfvén wave. The dispersion equation was deduced for the Alfvén mode and its solution obtained determining the space-time scales and the increment of instability. The maximum increment is realized for the disturbances stretched along the convection velocity that is correspondent to the actual orientation of the auroral arcs. In the conditions of rapid growth of Alfvén velocity above the maximum of the ionosphere F layer, it was shown that small-scale disturbances with the transverse scales l⊥ ? 1 km are localized at the altitudes up to several thousand kilometers whereas the large-scale stratification penetrate into the equatorial plane of the magnetosphere. A mechanism is proposed to intensify the parallel electric field acting at that stratification stage when the field-aligned currents in the Alfvén wave are sufficient to form abnormal resistance along geomagnetic lines of force. 相似文献
11.
In the companion paper (Lam and Rostoker, 1978) we have shown that Pc 5 micropulsations are intimately related to the behaviour and character of the westward auroral electrojet in the morning sector. In this paper we show that Pc 5 micropulsations can be regarded as LC-oscillations of a three-dimensional current loop involving downward field-aligned current flow near noon, which diverges in part to form the ionospheric westward electrojet and returns back along magnetic field lines into the magnetosphere in the vicinity of the ionosphere conductivity discontinuity at the dawn meridian. The current system is driven through the extraction of energy from the magnetospheric plasma drifting sunwards past the flanks of the magnetosphere in a manner discussed by Rostoker and Boström (1976). The polarization characteristics of the pulsations on the ground can be understood in terms of the effects of displacement currents of significant intensity which flow near the F-region peak in the ionosphere and induced currents which flow in the earth. These currents significantly influence the magnetic perturbation pattern at the Earth's surface. Model current system calculations show that the relative phase of the pulsations along a constant meridian can be explained by the composite effect of oscillations of the borders of the electrojet and variations in the intensity of current flow in the electrojet. 相似文献
12.
It is assumed that the three-dimensional current system of a substorm passes three successive stages. (1) When a dawn-to-dusk magnetospheric electric field appears, a current system with field-aligned currents at the poleward boundary of the auroral zone arises. An equivalent ionospheric current system calculated, taking into account a day-night asymmetry of ionospheric conductivity, looks like the well-known DP-2 system including an eastward low-latitude current and a greater magnitude of the dusk vortex in comparison with the dawn one. (2) An electric drift of plasma towards the Earth leads to the appearance of a westward partial ring current increasing in time. This current is closed by field-aligned currents at the equatorward boundary of the auroral zone. The calculated equivalent current system is similar to the well-known one of the precursory phase. (3) An increase of the auroral ionospheric conductivity during the expansive phase produces an increase of all currents and a turning of field-aligned currents at the equatorward boundary of the auroral zone relative to those at the poleward one. The calculated equivalent current system is similar to the DP-1 system. 相似文献
13.
The role of the ionospheric conductance in the solar wind-magnetosphere coupling is studied using global MHD simulations. The simulations with varying conductance and a constant solar wind input show that the field-aligned currents, whose magnitude depends on the ionospheric conductance, affect the size of the magnetopause at the flanks by increasing the local magnetic pressure and thus altering the surface equilibrium at the magnetopause. A current system that generates the magnetic stresses required to account for the location and geometrical structure of the magnetosphere observed in the simulations is proposed. 相似文献
14.
We look at time-dependent normal mode solutions to the Alfven wave equation in a uniform magnetic field, between planar ionospheres. In particular, the effect of sharp gradients in ionospheric conductivity on the spatial and temporal structure of the waves is considered. We show that the electric field of the wave must always be perpendicular to any conductivity discontinuities present, and that this is achieved by the generation of circularly polarized Alfven waves at the discontinuity. The results are applied to an ionospheric strip of high conductivity; this being relevant to Pi2s. 相似文献
15.
Kiyohumi Yumoto 《Planetary and Space Science》1986,34(12):1329-1334
Polarization properties of Pc3 magnetic pulsations at very low latitudes cannot be explained by existing theories which are based on the field line resonance model, because magnetic field lines at ¦Φ¦ < 22° are almost entirely in the ionosphere. In order to interpret Pc3 polarization characteristics observed at very low latitudes (¦Φ¦ < 20°), I would like to propose a possible, new qualitative model in which two superimposed ionospheric eddy currents, oscillating with slight differences in frequency in the Pc3 range and in azimuthal wave number, move azimuthally at very low latitudes. The equatorial ionospheric Pedersen eddy currents are believed to be predominantly caused by inductive electric fields of compressional Pc3 source waves which may possibly arrive in the equatorial ionosphere from the outer magnetosphere. 相似文献
16.
Sharp density gradients coupled with field-aligned currents can give rise to short wavelength (?15 m) drift waves due to collisional effects in the F-region of the auroral ionosphere. In this wavelength range, ion-ion collisions at altitudes of 300–450 km render the ions unmagnetized and a field-aligned current can drive a drift wave, propagating almost transverse to the magnetic field, unstable due to the resistance in electron parallel motion arising from electron collisions. 相似文献
17.
The equation of small oscillations of ULF waves in the Earth’s magnetosphere is derived accounting for a fast magnetosonic wave. The spectrum of discrete Alfven modes near the Alfven frequency minimum is studied on the basis of this equation. 相似文献
18.
After reviewing the basic characteristics of the polar cap arcs, it is suggested that their appearance can be explained if the open region splits into two, one located in the dawn sector and the other in the dusk sector. It is suggested that a distinct splitting occurs temporarily when an IMF tangential discontinuity passes by the magnetosphere and the sign of the IMF By component changes at the discontinuity, provided that the IMF Bz component is positive on both sides. As a result, the dawn or the dusk side of the polar region will be connected to either the front side or the hind side of the discontinuity, depending on the sign of the By component across the discontinuity. As the dynamo process is expected to operate in each of the two open regions (as is the case in the single open region), it is reasonable to infer that a sheet of plasma and of field-aligned currents forms in the region between the two open regions, resulting in the polar cap arcs across the polar region. The four-cell convection pattern may also appear. A model of the magnetosphere is constructed to demonstrate such a possibility. 相似文献
19.
The particle aspect approach is adopted to investigate the trajectories of charged particles in the electromagnetic field of kinetic Alfven wave. Expressions are found for the dispersion relation, damping rate and associated currents in homogenous plasma. Kinetic effects of electrons and ions are included to study kinetic Alfven wave because both are important in the transition region. It is found that the ratio β of electron thermal energy density to magnetic field energy density and the ratio of ion to electron thermal temperature (Ti/Te) affect the dispersion relation, damping-rate and associated currents in both cases (warm and cold electron limits). The treatment of kinetic Alfven wave instability is based on the assumption that the plasma consists of resonant and non-resonant particles. The resonant particles participate in an energy exchange process, whereas the non-resonant particles support the oscillatory motion of the wave. 相似文献
20.
S.-I. Akasofu 《Planetary and Space Science》1982,30(4):389-393
It is suggested that the ionosphere plays an active role in many substorm processes by generating field-aligned currents associated with the Hall current. Various substorm phenomena, such as the cross-tail current interruption, the asymmetric main phase field and the poleward expanding auroral bulge, may be closely related to the closure of the Hall current circuits in the magnetosphere. 相似文献