摘要:

The AMPTE CCE spacecraft observed a transverse Pc 5 magnetic pulsation (period ∼200 s) at 2155–2310 UT on November 20 (day 324), 1985, at a radial distance of 5.7–7.0RE, at a magnetic latitude of 1.2°–1.9°, and near 1300 magnetic local time. The magnetic field perturbation was observed primarily in the radial component with an amplitude of 15 nT peak to peak. Ion fluxes (energy>50 keV) measured by the medium energy particle analyzer (MEPA) on board CCE were also observed to oscillate at the frequency of the magnetic pulsation. The wide range of energy and pitch angle of ions covered by the MEPA allowed us to study the ion flux oscillations in great detail. It is found that (1) regardless of energy the oscillation amplitude tends to maximize near the field‐aligned directions while it is essentially zero at 90° pitch angle, (2) for a given energy and the given location (east or west) of ion guiding centers, flux oscillations at pitch angle α and at its conjugate, 180°‐α, are 180° out of phase, (3) for a given look direction, the oscillation phase changes with energy, and (4) for a given pitch angle and energy, the eastside flux oscillation leads the westside flux oscillation. These observations can be explained by the adiabatic theory of ion flux pulsations with finite Larmor radius effects included (Southwood and Kivelson, 1981; Kivelson and Southwood, 1983), if we assume an antisymmetric standing wave on the field line, westward propagation of the wave, and a large azimuthal wave number |m| ∼110. These properties of the wave are consistent with a second‐harmonic standing Alfvén wave excited in the region where the ring current ions have an

ISSN:0148-0227    

DOI:10.1029/JA095iA04p03717

年代:1990

数据来源: WILEY

摘要:

On April 21, 1985, an intense Ps 6 pulsation event was observed with the EISCAT radar and the EISCAT magnetometer cross. These measurements serve as a reference for a new electrostatic model for the ionospheric conductances, electric fields, and currents of the auroral structures associated with the pulsations, whose auroral signatures are the Ω bands. All parameters are essentially derived from the input field‐aligned current distribution. By varying this distribution and a few free parameters in the relation between the conductances and the upward current, the model is adjusted to the data. We find that by a rearrangement of the upward current from a one‐dimensional sheet configuration to tonguelike poleward extensions the observed event is reproduced in a satisfactory way. Compared to previous works, the Hall current is modulated in a different, less symmetric way, and considerably lower field‐aligned current densities are re

ISSN:0148-0227    

DOI:10.1029/JA095iA04p03733

年代:1990

数据来源: WILEY

摘要:

The evolution of Io's Alfvén waves is modeled in a realistic magnetic field and torus density distribution. This is performed by calculating the normal modes of the field lines disturbed by Io and synthesising the waves near Io from a complex sum over the eigenmodes. The wave pattern produced downstream from the satellite exhibits periodic structure over a range of scales. In terms of the Jovian longitude (or CML) of a stationary observer, we expect large‐scale structure (>60°), small‐scale structure (<6°), and intermediate periods too. These are close to observed intervals in decametric (DAM) emissions, such as the length of a DAM storm, bunching of arcs within a storm, and individual arc sepa

ISSN:0148-0227    

DOI:10.1029/JA095iA04p03745

年代:1990

数据来源: WILEY

摘要:

Brief, impulsive, large‐amplitude (δp/p∼ 1) solar wind dynamic pressure pulses, recurring on time scales of 5 to 15 min, are common just upstream of the Earth's bow shock. When each pulse strikes the magnetopause, it launches a fast‐mode compressional wave in the magnetosphere that can propagate antisunward faster than the magnetosheath flow. Consequently, the magnetopause bulges outward ahead of each contraction associated with a pressure pulse. These ridges generally propagate antisunward, although sunward motion is common on the early post‐noon magnetopause. The greatest amplitude (∼1 to 2RE) magnetopause motion occurs on the prenoon magnetopause, at high‐latitudes, and during periods of southward interplanetary magnetic field. The signatures of the pressure‐pulse‐driven magnetopause motion include a bipolar magnetic field signature normal to the nominal magnetopause, a rotation of the magnetic field away from both magnetosheath and magnetospheric orientations, a mixture of magnetosheath and magnetospheric plasmas, and high‐speed magnetosheath plasma flows. The magnetopause boundary motion, in turn, drives transient compressions and shears in the dayside magnetospheric magnetic field. These compressions and shears map to the dayside auroral ionosphere, where the ground signatures produced by a single, brief, solar wind dynamic pressure pulse are an antisunward moving (sunward at early post‐noon local times) double‐convection vortex, associated with north–south magnetic field perturbations, increased ELF/VLF wave activity, precipitating particles, and cosmic noise absorption. The ionospheric and magnetospheric signatures driven by solar wind pressure pulses greatly resemble those previously associated w

ISSN:0148-0227    

DOI:10.1029/JA095iA04p03755

年代:1990

数据来源: WILEY

摘要:

Magnetic field enhancements and depressions on the time scales of minutes were frequently observed simultaneously by the AMPTE CCE, GOES 5, and GOES 6 spacecraft in the subsolar magnetosphere. The source of these perturbations has been detected in the high time resolution AMPTE IRM measurements of the kinetic pressure of the solar wind upstream of the bow shock. It is argued that these upstream pressure variations are not inherent in the solar wind but rather are associated with the bow shock. This conclusion follows from the facts that (1) the upstream field strength and the density associated with the perturbations are highly correlated with each other whereas these quantities tend to be anticorrelated in the undisturbed solar wind, and (2) the upstream perturbations occur within the foreshock or at its boundary. The results imply a mode of interaction between the solar wind and the magnetosphere whereby density changes produced in the foreshock subsequently convect through the bow shock and impinge on the magnetosphere. Also velocity decreases deep within the foreshock sometimes reach many tens of kilometers per second and may be associated with further pressure variations as a changing interplanetary field direction changes the foreshock geometry. Upstream pressure perturbations should create significant effects on the magnetopause and at the foot of nearby field lines that lead to the polar cusp ionosphere.

ISSN:0148-0227    

DOI:10.1029/JA095iA04p03773

年代:1990

数据来源: WILEY

摘要:

We describe a global magnetospheric model with a single subsolar merging line whose position is determined neither locally by the relative orientations and strengths of the merging fields nor globally by the orientation of a separator line—the governing parameters of most previous models—but by the condition of tangential contact between the external field and the magnetopause. As in previous models, the tilt of the merging line varies with IMF orientation, but here it also depends upon the ratio of Earth's magnetic flux that leaks out of the magnetopause to IMF flux that penetrates in. In the limiting case treated by Alekseyev and Belen'kaya, with no leakage of Earth's field and total IMF penetration, the merging line forms a great circle around a spherical magnetosphere where undeviated IMP lines lie tangent to its surface. This tangent merging line lies perpendicular to the IMF. We extend their work to the case of finite leakage and partial penetration, which distort the IMF into a draped pattern, thus changing the locus of tangency to the sphere. In the special case where the penetrating IMF flux is balanced by an equal amount of Earth flux leakage, the tangent merging line bisects the angle between the IMF and Earth's northward subsolar field. This result is identical to the local merging line model result for merging fields with equal magnitude. Here a global flux balance condition replaces the local equal magnitude condit

ISSN:0148-0227    

DOI:10.1029/JA095iA04p03787

年代:1990

数据来源: WILEY

摘要:

If a flux transfer event is modeled as a round patch of open field lines on an otherwise closed magnetopause, it maps to the ionosphere as an elongated shape pointing radially away from the cusp. We demonstrate this distortion by superposing a cylindrical flux rope field with the shielded Chapman‐Ferraro magnetospheric field. The axial component of the flux rope is normal to the planar magnetopause and decreases to zero at the edge of the rope. Field lines mapped from the edge nearest the cusp trace along the magnetopause surface to the cusp and then follow the singular cusp field line down to the ionosphere. However, field lines traced from the far edge must pass through the flux rope, where the axial component is strong. Consequently, they extend deeper into the magnetosphere, where the field is less cusp‐configured and more dipolar, and map down to the ionosphere away from the cusp, elongating the footprint of the round patch. Elongation increases with displacement of the patch from the subsolar region of the magnetopause. A series of patches mapped from distances successively further from the subsolar point forms a pattern of striations radiating from the cusp that resembles an observed pattern of auroral arcs in the midday oval, previously interpreted as signatures of magnetosheath plasma injection at the cusp by an unspecified mechanism. The mapping suggests that the mechanism could be patchy reconnection at distances which can be considerably removed from the c

ISSN:0148-0227    

DOI:10.1029/JA095iA04p03795

年代:1990

数据来源: WILEY

摘要:

Using 8 months of tail data obtained with the AMPTE/IRM satellite, more than 270,000 ion moments and magnetic field measurements were analyzed with respect to the occurrence rates and typical characteristics of high‐speed ion flows with velocities in excess of 400 km/s. The occurrence rates in the plasma sheet boundary layer, the outer central plasma sheet and the neutral sheet neighborhood have a 4∶1∶2 ratio for flows of 400–600 km/s. For flows in excess of 800 km/s, there is only a minimal chance to detect them in the outer central plasma sheet but equal chances in the two other regions. For highAEthe chances to detect high‐speed flows in the inner central plasma sheet are greater than to find them in the plasma sheet boundary layer. In the outer central plasma sheet the high‐speed flow occurrence rate is small and independent ofAE. In all three regions the largest occurrence rates are found near the midnight meridian at the largest radial distances accessible to IRM. High‐speed flow occurrence rates and ion densities are anticorrelated. The high‐speed flows are bursty with the majority of the flows lasting less than 10s. The occurrence of the high‐speed flows is strongly peaked in the sunward direction. Virtually no tailward high‐speed ion flow could be detected. About 60–70% of all high‐speed flows near the neutral sheet have a dominant component perpendicular to the magnetic field and are associated with comparatively large northward and duskward magnetic field directions. At times, also appreciable duskward flow components appear. Overall, our results indicate that both the plasma sheet boundary layer and the inner central plasma sheet are important regions for the dynamics of t

ISSN:0148-0227    

DOI:10.1029/JA095iA04p03801

年代:1990

数据来源: WILEY

摘要:

Using 4 months of tail data obtained by the ELF/MF spectrum analyzer and the plasma instrument on board the AMPTE/IRM satellite, more than 50,000 ten‐second‐averaged electric wave spectra were analyzed in order to establish typical spectra for periods of high and low ion density and high and low ion β. The general spectral slope of the spectra in the plasma sheet follows anf−2law. Ion β has a stronger influence on the spectral form than the ion density. Highest average spectral densities are obtained in the low‐β plasma sheet boundary layer, where the spectrum is that of broadband electrostatic noise extending to frequencies near and above the upper hybrid frequency. Lowest wave intensities are encountered in the high‐β inner central plasma sheet. The outer central plasma sheet has generally low wave intensities and is dominated by electron cyclotron odd half‐harmonics and electron plasma wave emissions. Lower electron cyclotron harmonics are found in the high‐β cases in all three regions of the plasma sheet while higher odd half‐harmonics dominate the low‐β and low‐density i

ISSN:0148-0227    

DOI:10.1029/JA095iA04p03811

年代:1990

数据来源: WILEY

摘要:

Magnetic field measurements made in the Earth's tail region during the growth phases of substorms on May 23, 1979, by GEOS 2 and ISEE 1, and on June 23, 1979, by GEOS 2 and IMP‐J, were used to model the magnetic configuration in the location conjugate to the Scandinavian riometer network. Toward the end of the growth phase the time variation of the magnetic configuration explains about 80% of the equatorward expansion of the central part of the auroral oval. Assuming the origin of the observed equatorward moving band of energetic electron precipitation into the ionosphere to be caused by pitch angle scattering in the equatorial current sheet, we inferred the half thickness of the current sheet atr≈ 9REas thin as 0.1REnear the end of the growth phase. Combined with the modeled evolution of the magnetic configuration, this mechanism consistently explains the main features of the electron precipitat

ISSN:0148-0227    

DOI:10.1029/JA095iA04p03819

年代:1990

数据来源: WILEY