摘要:

An expression for the parallel current density is derived for a plasma characterized by negligible bulk flow (magnetostatic) velocity and a two‐component (anisotropic) pressure tensor by expanding the equilibrium Vlasov equation for each species in the adiabatic parameter until such point as a nonvanishing momentj∥= ∫d³ υυ∥ƒ is identified. The result is a nonlocal one: it relatesj∥at one pointsalong a field line toj∥at another (reference) points0plus an integral function of the pressure and magnetic field between them. It is a generalization and elaboration of results obtained by Bostrom (1975), Heinemann (1990), and Heinemann and Pontius (1991). The expression could have been obtained by integrating the current continuity equation with −▽ ·j⊥as a source term andj⊥given by perpendicular momentum balance. We explicitly show the equivalency. The widely used Vasyliunas (1970) equation follows whenP⊥is set equal toP∥andsands0are taken to be at the ionosphere and the equator. An extended discussion of the relationship of results derived here to others in the literature is carried out in an effort to bring unity and pers

ISSN:0148-0227    

DOI:10.1029/91JA03029

年代:1992

数据来源: WILEY

摘要:

The expression for the Birkeland current density in an anisotropicP⊥,P∥plasma derived in a companion paper (Birmingham, this issue) is simplified by linearization: for purposes of computingj∥the magnetic configuration is assumed to be axisymmetric, either a dipole or a dipole stretched by the field arising from an azimuthal current sheet. The anisotropy is that of a bi‐Maxwellian and characterized by equatorial valuesP⊥(s0) andr, the equatorial temperature (pressure) ratio. Parameterj∥is proportional to the azimuthal derivatives ofP⊥(s0) andr, with coefficients that are integral functions along the backgroundBof the pressure and field strength from the equators0, where there is (by assumption) no Birkeland current, to the point of reference. Values ofj∥at the ionosphere are compared between the two magnetic models for 0.1

ISSN:0148-0227    

DOI:10.1029/91JA03030

年代:1992

数据来源: WILEY

摘要:

A global simulation of the magnetosphere with a long tail (∼100RE) is performed. A magnetosphere with a neutral sheet is constructed from a dipole field by solar wind dynamic pressure (no interplanetary magnetic field (IMF)). Concentration of the plasma sheet current occurs preferentially at 14–18REon the tail side of the Earth, which is an indication that, magnetically, this is the most fragile region of the tail structure. It is the demarcation region between the dipolar and streaming field line structures. Therefore, in the presence of resistivity, magnetic reconnection can be preferentially driven here by a compressional disturbance of some sort. In the present initial value problem, reconnection occurs at 15–20REin the tail and develops into a large lump of plasma surrounded by reconnected field lines, a plasmoid, which is ejected tailward. The time scale of the plasmoid formation and ejection process is very slow, of the order of several hours, when no IMF exists. After ejecting one plasmoid, reconnection occurs again in the plasma sheet, and a second plasmoid is formed and ejected. This result shows that a magnetosphere that has a sufficiently long tail and a neutral sheet is fragile and subject to plasmoid formation. We also show that the hot plasma, when mapped down to the ionosphere along the field lines, encompasses the auroral oval in agreement with the DE satellite observa

ISSN:0148-0227    

DOI:10.1029/91JA02928

年代:1992

数据来源: WILEY

摘要:

Measurements of the polar cap potential drop and size have been obtained during magnetospheric substorms. Using double‐probe electric field measurements on the DE 2 satellite, 148 measurements have been obtained at random times preceding, during, and after 64 substorms. The polar cap potentials are graphed as a function of the difference between the time of the polar cap measurement and the time of the expansion onset of the corresponding substorm. The ratios of the auroral electrojet (AE) indices and the potential are also determined. The results show that on the average the polar cap potential starts to increase at 1.5 hours before onset, reaches a level of 70 kV in the 30‐min period before onset, and starts to decline at 1.5 hours after onset. However, on a case‐by‐case basis there are substantial variations from the average, as polar cap potentials over 120 kV were measured as early as 1 hour before substorm onset and values as low as 40 kV were observed during the expansion phase. The size of the polar cap ranged from 23° to 38° invariant latitude at the time of onset, and had an average value of 31°. TheAE/ΦPCratio is nearly constant before and after substorms, but decreases slightly during the substorm growth phase and increases greatly during the expansion phase. This increase is most likely due to a higher conductivity and westward electric field within the electrojet during expansion, which causesAEto increase without a corresponding change in the polar ca

ISSN:0148-0227    

DOI:10.1029/91JA03159

年代:1992

数据来源: WILEY

摘要:

The self‐consistent electron kinetics of Alfvén waves on the electron inertial scale is studied using a two‐dimensional hybrid‐kinetic description. The ions follow a fluid description for Alfvén waves at frequencies below the ion cyclotron frequency. The parallel electron dynamics are treated kinetically using particle‐in‐cell techniques. In this model the electron plasma mode is eliminated and only the physics of the Alfvén waves is retained. At sufficiently large amplitudes, it is found that oblique Alfvén waves break due to finite electron inertia in a cold plasma. The consequence of wave breaking is the formation of an electron beam which can be unstable to the beam‐plasma instability. The electrons supporting the parallel current thermalize into a non‐Maxwellian distribution with an energetic tail up to several keV, assuming a reasonable magnetospheric Alfvén speed. In hot plasma simulations, electron trapping is the principal mechanism of electron acceleration. It is proposed that wave breaking or electron trapping of oblique Alfvén waves at 1REcan result in electron acceleration and may explain some observe

ISSN:0148-0227    

DOI:10.1029/91JA03101

年代:1992

数据来源: WILEY

摘要:

The evolution of the three‐dimensional resistive tearing instability in a magnetotail configuration is analyzed by means of a resistive MHD code for various forms of the energy equation including ohmic and compressional heating. As an introduction we present a complete derivation of the set of MHD equations from a kinetic formalism noting all simplifying assumptions. The following study emphasizes the changes in magnetotail evolution brought about by a variation of the assumed value of the polytropic exponent γ, representing the ratio of specific heats in the energy equation, which closes the set of MHD equations. We find that within the range of 2/3 ≤ γ ≤ 4, a tearing mode develops in all cases. Certain properties, such as the formation and ejection of a plasmoid, associated with the formation of the reversal region of the north‐south component of the magnetic field, and strong earthward and tailward flow fields prevail as qualitative features independent of the value of γ chosen. Quantitatively, however, magnetotail evolution can be quite different for different values of γ. This is found primarily in the evolution of the temperature, where we find heating by a factor of order two with higher temperatures for higher values of γ. This is combined with a density reduction by a similar factor. Both features are consistent with recent observational results from a superposed epoch analysis of 39 substorm events in the near tail (Baumjohann et al., 1991). Ohmic dissipation is found to be the major source of the heating, while adiabatic or nonadiabatic compression may contribute primarily in the closed field region earthward of the reconnection site. Furthermore, quantitative values of typical flow speeds and evolution time scales show some dependence on the polytropic index. These results suggest that the qualitative features of the nonlinear evolution of the resistive tearing mode are not strongly influenced by the energy equation, whereas details can show stron

ISSN:0148-0227    

DOI:10.1029/91JA03003

年代:1992

数据来源: WILEY

摘要:

A case study of a transient geomagnetic field variation event associated with a traveling magnetospheric convection twin vortex is presented. The characteristics of this event are different from those of other presented cases, as the sense of rotation of the associated ionospheric current system is reversed, that is, it exhibits upward (downward) field‐aligned currents in the tailward (dayside) part of the vortex structure. Other features are as usually observed of such transients. In particular, a clear tailward motion at high latitudes can be deduced from local as well as global magnetic field observations. At low dayside latitudes, magnetic field variations similar to sudden impulse (SI) variations are recorded. Almost simultaneous onset and no westward propagation are observed, too. As possible source mechanisms of such transient events, localized magnetic field reconnection as well as pressure pulses at the dayside magnetopause are discussed. If the switch‐on of magnetic reconnection can be regarded as equivalent to an equivalent current flowing against the existing magnetopause current and if this countercurrent is spatially localized, an Alfvén wave is generated with downward (upward) field‐aligned current flow in the tailward (dayside) part of the wave, as observed for some of the reported transients. However, the SI‐like behavior of the event studied in this paper, its reversed sense of rotation, and the vortex associated field‐aligned current density excludes such localized magnetic reconnection as a possible source mechanism and argues in favour of the existence of pressure pulses at the magnetopause. It is shown that a spatially localized compression or dilatation of the dayside magnetopause is also associated with the onset of a current flowing parallel or antiparallel to the magnetopause current, respectively. The additional current associated with compression gives rise to a system of upward and downward field‐aligned currents in the tailward and dayside part of the system, much as observed for the event analysed. As the pressure pulse is moving along the magnetopause, the generated Alfvén wave pattern is that of an Alfvén wing structure, with the ground‐magnetic transient representing the passing by of the first ionospheric reflection site. Such an Alfvén wing structure may also explain recent observations of multiple tw

ISSN:0148-0227    

DOI:10.1029/91JA02464

年代:1992

数据来源: WILEY

摘要:

Dayside Pc 1 geomagnetic pulsation bursts have been studied using a three‐station array of induction magnetometers located at high latitudes (geomagnetic latitude (MLAT) of −70° to −81°). The data set reveals that when these emissions are continually recorded at Davis (MLAT = −74.5°;L= 14), the low‐latitude cleft, observed by the DMSP F7 satellite, is located at the Davis latitude. Associated magnetic variations in the form of solitary pulses often lead the Pc 1 bursts by 1 to 2 min. These pulses are typically associated with riometer absorption events and consequently the precipitation of fluxes of keV electrons. The Pc 1 bursts are interpreted as resulting from ion cyclotron waves which have propagated to the ionosphere from the equatorial boundary layer region. The associated boundary layer ions, identified by the low‐altitude DMSP F7 satellite, range between 1 and 5 keV in energy. These particles are considered to be the most likely free energy source for the ion cyclotron waves. It is considered that such resonant ions enter the magnetosphere via the cleft and cusp because this enables a prenoon time of occurrence of most of the observations to be explained. Measured time delays of 40 to 120 s between the associated riometer absorption and Pc 1 bursts are consistent with an ion cyclotron wave generation region located in the equatorial

ISSN:0148-0227    

DOI:10.1029/91JA01456

年代:1992

数据来源: WILEY

摘要:

We present simultaneous solar wind and magnetospheric observations for a series of flux transfer events (FTEs) observed by the UKS and IRM spacecraft in the outer prenoon magnetosphere on October 28, 1984 (day 302). The CCE satellite, located near local noon, observed an enhancement in the magnetospheric magnetic field strength 4 min prior to each IRM/UKS event, suggesting the antisunward propagation of a compression in the magnetopause surface. We suggest that these compressions resulted from wavy magnetopause motion driven by variations in the solar wind dynamic pressure applied to the magnetosphere, rather than the motion of FTEs formed by magnetic merging. In support of this hypothesis we note that the events previously identified as FTEs occurred during a sequence of quasi‐periodic (5–6 min) oscillations in the magnetospheric plasma velocity and magnetic field. The oscillations exhibited all the characteristics expected for antisunward moving magnetopause boundary motion and those cycles identified as FTEs simply exhibited larger amplitudes. The magnetospheric compressions observed by the CCE and IRM/UKS corresponded to enhancements in the interplanetary magnetic field strength (IMF) observed by the ISEE 2 satellite, located just upstream of the subsolar bow shock. Some of the magnetospheric compressions occurred for a strongly northward IMF. There was no evidence for the type of accelerated flows commonly associated with magnetic merging at the magnetopause on this day, and the orientation of the magnetic field inside the UKS/IRM events was not consistent with that expected for F

ISSN:0148-0227    

DOI:10.1029/91JA03017

年代:1992

数据来源: WILEY

摘要:

High‐speed flows in the inner central plasma sheet (first reported by Baumjohann et al. (1990)) are studied, together with the concurrent behavior of the plasma and magnetic field, by using AMPTE/IRM data from ≈ 9 to 19REin the Earth's magnetotail. The conclusions drawn from the detailed analysis of a representative event are reinforced by a superposed epoch analysis applied on 2 years of data. The high‐speed flows organize themselves in 10‐min time scale flow enhancements which we call bursty bulk flow (BBF) events. Both temporal and spatial effects are responsible for their bursty nature. The flow velocity exhibits peaks of very large amplitude with a characteristic time scale of the order of a minute, which are usually associated with magnetic field dipolarizations and ion temperature increases. The BBFs represent intervals of enhanced earthward convection and energy transport per unit area in they‐zGSM direction of the order of 5×10

ISSN:0148-0227    

DOI:10.1029/91JA02701

年代:1992

数据来源: WILEY