摘要:

We present a detailed study of the highest‐frequency component of smooth radio emission observed during the Voyager 2 encounter with Neptune in August 1989. This emission occurs during three distinct periods on August 24 and 25, 1989, in the frequency range of 550 to 900 kHz. By assuming straight‐line propagation from sources of both fundamental and second harmonic gyroemission, we perform a detailed analysis of the observed polarization of the emission. The data are most consistent with an L‐O mode source in the north magnetic polar region, around 50°W, 50°N. A second possible source is in the north magnetic polar region, around 270°W, 50°N. This source must emit in the

ISSN:0148-0227    

DOI:10.1029/94JA02856

年代:1995

数据来源: WILEY

摘要:

We present a complete solution for a set of magnetohydrodynamic (MHD) Riemann problems in which the upstream and downstream states have the same total pressure, and in which the normal component of the magnetic field is very small. These solutions are pertinent to subfast flows in the earth's magnetic tail and near the magnetopause. In a coplanar situation a family of solutions exists that depend on two parameters as well as on dissipation mechanisms. In the parallel case the transverse magnetic field either does not change direction or changes the direction twice by involving two intermediate shocks. In the antiparallel case an intermediate shock is always required, except when the solution consists of two switch‐off shocks. In a noncoplanar case the solution is not self‐similar as a function ofx/t, but continues to evolve. At early times the evolution is similar to the coplanar case. In general two time‐dependent intermediate shocks are required to rotate the magnetic fields. The velocity shear has strong effect on the Riemann solution. In some cases no Riemann solution can exist because of the cavitation caused by the slow rarefaction waves. The calculated magnetopause structure resembles the observed structure for northward interplanetary magnetic field (IMF). However, for southward IMF, the MHD result shows the existence of a depletion layer, which is not supported by observations. We also show that on the magnetosheath side, the Walén relation, which is exact for a rotational discontinuity, can also be well satisfied by a slow shock, an intermediate shock, or the head of a slow rarefactio

ISSN:0148-0227    

DOI:10.1029/94JA03095

年代:1995

数据来源: WILEY

摘要:

Empirical data‐based models of the magnetospheric magnetic field have been widely used during recent years. However, the existing models (Tsyganenko, 1987, 1989a) have three serious deficiencies: (1) an unstable “de facto” magnetopause, (2) a crude parametrization by theKpindex, and (3) inaccuracies in the equatorial magnetotailBzvalues. This paper describes a new approach to the problem; the essential new features are (1) a realistic shape and size of the magnetopause, based on fits to a large number of observed crossings (allowing a parametrization by the solar wind pressure), (2) fully controlled shielding of the magnetic field produced by all magnetospheric current systems, (3) new flexible representations for the tail and ring currents, and (4) a new “directional” criterion for fitting the model field to spacecraft data, providing improved accuracy for field line mapping. Results are presented from initial efforts to create models assembled from these modules and calibrated against spacecraft

ISSN:0148-0227    

DOI:10.1029/94JA03193

年代:1995

数据来源: WILEY

摘要:

A tilt‐dependent magnetic field model of the Earth's magnetosphere with variable magnetopause standoff distance is presented. Flexible analytic representations for the ring and cross‐tail currents, each composed of elements derived from the Tsyganenko and Usmanov (1982) model, are combined with the fully shielded vacuum dipole configurations of Voigt (1981). The ring current, consisting of axially symmetric eastward and westward currents fixed about the dipole axis, resembles that inferred from magnetic field observations yet permits easy control of inner magnetospheric inflation. The cross‐tail current contains a series of linked current sheet segments which allow for the tilt‐dependent flexing of the current sheet in thex‐zplane and arbitrary variations in current sheet position and intensity along the length of the magnetotail. Although the current sheet does not warp in they‐zplane, changes in the shape and position of the neutral sheet with dipole tilt are consistent with both MHD equilibrium theory and observations. In addition, there is good agreement with observed ΔBprofiles and the average equatorial contours of magnetic field magnitude. While the dipole field is rigorously shielded within the defined magnetopause, the ring and cross‐tail currents are not similarly confined, consequently, the model's region of validity is limited to the inner magnetosphere. The model depends on four independent external parameters, namely, (1) the dipole tilt angle, (2) the magnetopause standoff distance, (3) the midnight equatorward boundary of the diffuse aurora, and (4) the geomagnetic indexDst. In addition, we present a simple but limited method of simulating several substorm related magnetic field changes associated with the disruption of the near‐Earth cross‐tail current sheet and collapse of the midnight magnetotail field region. These include the classic dipolarization of the near‐Earth field and the reduction of the far‐tail equatorial field accompanying current sheet thinning. This feature further facilitates the generation of magnetic field configuration time sequences useful in plasma convection simulations of real

ISSN:0148-0227    

DOI:10.1029/94JA03139

年代:1995

数据来源: WILEY

摘要:

In order to facilitate bounce‐averaged guiding center simulations of geomagnetically trapped particles, we express the kinetic energy of a particle with magnetic coordinates (L,ϕ) as an analytic function of the first two adiabatic invariants (M, J) and theLvalue of the field line. The magnetic field model is axisymmetric, consisting of a dipolarBfield plus a uniform southward magnetic field parallel to the dipole momentμE. This model magnetosphere is surrounded by a circular equatorial neutral line whose radiusbis an adjustable parameter. TheLvalue of a field line is (by definition) inversely proportional to the flux enclosed by the corresponding magnetic shell of equatorial radiusr0, and theLvalue at the neutral line (r0=b) is denotedL*. The azimuthal coordinate ϕ measures magnetic local time. The best functional representation found for the normalized difference (L³a³/μE)(Bm‐B0) between mirror‐point fieldBmand equatorial fieldB0along any field line is a 5‐term expansion in powers (2/3 through 6/3) of the quantityX≡ (La/μE)1/2K, whereK≡ (J²/8m0M)1/2is an adiabatically conserved quantity independent of particle energy,m0is the rest mass of the particle, andais the radius of the Earth. This functional form is motivated by results for limiting cases in which particles mirror very near and very far from the magnetic equator. Expansion coefficients corresponding to various powers ofXare obtained from least squares fits to numerically computed results forXas a function ofLandBm. These are accurately expressible as fourth‐order polynomials in (r0/b)³, hence indirectly as functions ofL/L* = 3La/2b. This representation, which leads (except for a manageably small region of parameter space) to better than 1% accuracy in the specification ofBmas a function ofKandL, allows bounce‐averaged guiding center simulations to be performed without actually tracing the bounce motions of individual particles. Bounce‐averaged driftsL(meridional) and ϕ (azimuthal) are proportional to derivatives of the HamiltonianH(sum of kinetic and potential energies) with respect to ϕ andL, respectively. Our formulation thus provides a computationally efficient method for tracing the bounce‐averaged adiabatic motion (conserving all three invariants) and nonadiabatic transport (violating the third invariant while conserving the first two invariants) of geomagnetically trapped partic

ISSN:0148-0227    

DOI:10.1029/94JA02855

年代:1995

数据来源: WILEY

摘要:

The relation between the solar wind input to the magnetosphere, υBSouth, and the auroral geomagnetic indexALis modeled with two linear moving‐average filtering methods: linear prediction filters and a driven harmonic oscillator in the form of an electric circuit. Although the response of the three‐parameter oscillator is “simpler” than the filter's, the methods yield similar linear timescales and values of the prediction‐observation correlation and the prediction χ². Further the filter responses obtained by the two methods are similar in their long‐term features. In these aspects the circuit model is equivalent to linear prediction filtering. This poses the question of uniqueness and proper interpretation of detailed features of the filters such as response peaks. Finally, the variation of timescales and filter responses with theALactivity leve

ISSN:0148-0227    

DOI:10.1029/94JA03303

年代:1995

数据来源: WILEY

摘要:

We present a statistical survey of Prognoz 10 solar wind observations at the times of transient (step function and impulsive) variations in the dayside magnetospheric magnetic field strength measured by the GOES 5 and 6 geosynchronous satellites. The results indicate that 51% of the transient magnetospheric events can be associated with corresponding variations in the solar wind dynamic pressure. A further 17% of the events can be associated with fluctuations in the interplanetary magnetic field orientation in the sense previously associated with foreshock pressure pulses. We find no tendency for impulsive events at dayside geosynchronous orbit to be associated with north/south fluctuations in the interplanetary magnetic field (IMF) orientation, nor for the events to occur primarily during intervals of southward IMF. The success rate for associating transient events at dayside geosynchronous orbit with solar wind features decreases as Prognoz 10 moves farther from the Earth‐Sun line. The observations indicate that variations in the solar wind dynamic pressure and foreshock pressure pulses associated with variations in the IMF cone angle are the predominant causes of large‐amplitude transient events observed at dayside geosynchronous or

ISSN:0148-0227    

DOI:10.1029/94JA03144

年代:1995

数据来源: WILEY

摘要:

Vortex‐induced magnetic reconnection (VIMR) and bursty single X line reconnection (BSXR) at the magnetopause are studied using two‐dimensional MHD simulation. The VIMR and BSXR occur when the flow‐aligned Alfvén Mach number is greater or less, respectively, than a critical value which typically ranges from 0.3 to 0.6 for different anomalous resistance conditions. In the former case, the streamlines and magnetic field lines are in alignment and construct a concentric vortical pattern, whereas in the latter case, magnetic field lines do not form closed loops, and vortices occur around the X line. The topological structure of the asymptotic quasi‐steady state of 2‐D reconnection depends strongly upon the topological invariants such as the cross helicity. The different sites where VIMR and BSXR work at the magnetopause to form flux transfer events are

ISSN:0148-0227    

DOI:10.1029/94JA02898

年代:1995

数据来源: WILEY

摘要:

There are four low‐frequency modes which may propagate in a high‐beta nearly bi‐Maxwellian plasma. These are the magnetosonic, Alfvén, ion acoustic, and mirror modes. This manuscript defines a procedure based on linear Vlasov theory for the unique identification of these modes by use of transport ratios, dimensionless ratios of the fluctuating field and plasma quantities. A single parameter, the mode deviation, is defined which characterizes the difference between the theoretical transport ratios of a particular mode and the observed ratios. The mode deviation is calculated using the plasma and magnetic field data gathered by the Active Magnetospheric Particle Tracer Explorers/Ion Release Module spacecraft to identify the modes observed in the terrestrial magnetosheath near the magnetopause. As well as determining the mode which best describes the observed fluctuations, it gives us a measure of whether or not the resulting identification is unique. Using 17 time periods temporally close to a magnetopause crossing, and confining our study to the frequency range from 0.01 to 0.04 Hz, we find that the only clearly identified mode in this frequency range is the mirror mode. Most commonly, the quasi‐perpendicular mirror mode (with wave vectorkroughly perpendicular to the background magnetic fieldB0) is observed. In two events the quasi‐parallel mirror mode (k∥B0) wa

ISSN:0148-0227    

DOI:10.1029/94JA03024

年代:1995

数据来源: WILEY

摘要:

VLF line radiation received by the ISIS 1 and 2 satellites over New Zealand is found to fall into two distinct classes. The first of these consists of magnetospheric lines (MLs) which are characterized by a broadband appearance and by frequency drifts of a few tens of hertz per minute, similar to those reported elsewhere. Both their initial frequencies and their frequency spacings were, however, found to be essentially random rather than multiples of 50 or 60 Hz. The hourly variation in ML occurrence showed no correlation with electrical load in possible electrical mains systems sources. No clear decrease in occurrence on weekends was evident. This first satellite survey with significant numbers of MLs found no evidence of a relationship with power line harmonics. The second class of satellite‐received VLF lines consists of “tram lines” (TLs) which are characterized by their very narrow bandwidth and zero frequency drift. TLs appear to lie close to harmonics of 50 or 60 Hz. An example of this class is pres

ISSN:0148-0227    

DOI:10.1029/94JA02865

年代:1995

数据来源: WILEY