摘要:

During the outbound pass of the Jovian magnetosheath in February 1992, a large number of magnetic depressions, without a significant change in the direction of the magnetic field vector, were observed by the Ulysses magnetometer. These low‐frequency fluctuations, identified earlier as mirror mode waves, lasted about 1 day. The largest amplitude fluctuations (with a decrease in the field magnitude by a factor 3, on average) were observed just after the spacecraft crossed the magnetopause. The amplitude of the waves decreased as Ulysses approached the bow shock. We report here the results of a systematic study on the statistical properties of these field depressions. We have found that there is a saturation in the level of low field at about 1.3 nT. The average duration of the depressions is about 40 s, corresponding to a spatial size of about 20 gyroradii of the plasma protons. The angle between the magnetic field vector and the minimum variance direction is almost perpendicular, the average of that angle for the 350 individual field depressions we have analyzed is 83°. Constraints on the theoretical models which these observations impose are discussed. Many characteristics of the field depressions suggest the importance of nonlinear effects. The longitudinal size of the weak field seems rather large, raising the question whether the particles have enough time to fill in that space during the development of the instability. This difficulty might be resolved if energetic particles are involv

ISSN:0148-0227    

DOI:10.1029/95JA02207

年代:1996

数据来源: WILEY

摘要:

We report the discovery with the Ulysses unified radio and plasma wave (URAP) instrument of features in the Jovian hectometer (HOM) wavelength radio emission spectrum which recur with a period about 2–4% longer than the Jovian System III rotation period. We conclude that the auroral HOM emissions are periodically blocked from “view” by regions in the torus of higher than average density and that these regions rotate more slowly than System III and persist for considerable intervals of time. We have reexamined the Voyager planetary radio astronomy (PRA) data taken during the flybys in 1979 and have found similar features in the HOM spectrum. Contemporaneous observations by Brown (1994) show an [SII] emission line enhancement in the Io plasma torus that rotates more slowly than System III by the same amount as the HOM fe

ISSN:0148-0227    

DOI:10.1029/95JA02389

年代:1996

数据来源: WILEY

摘要:

The Voyager 2 encounter with Neptune and Triton in August 1989 showed a large ionosphere at Triton. Subsequent studies have tried to explain the production of such high levels of ionization but have ignored the possible plasma dynamics originating from the interaction between Neptune's magnetosphere and Triton. This study applies knowledge gained from studying the solar wind‐Venus interaction to this case. We find that observations made by Voyager 2 can be explained by downward convection of magnetospheric plasma into Triton's atmosphere, with the flow momentum transferred to the neutral atmosphere near an altitude of 650 km. We show that momentum transfer is accomplished as both the convective velocity and the magnetic field go to zero. The mechanism by which the ionosphere is produced was identified previously as impact ionization from hot electrons originating in Neptune's magnetosphere. These precipitating hot electrons are shown in this study to be unaffected by a magnetic field below roughly 650 km. This is a result not previously anticipated, and one which implies that the plasma interaction between Neptune's magnetosphere and Triton cannot be ignore

ISSN:0148-0227    

DOI:10.1029/95JA02761

年代:1996

数据来源: WILEY

摘要:

The interplanetary magnetic field (IMF) orientation controls foreshock densities and modulates the fraction of the solar wind dynamic pressure applied to the magnetosphere. Such pressure variations produce bow shock and magnetopause motion and cause the radial profiles for various magnetosheath parameters to sweep inward and outward past nearly stationary satellites. We report ISEE 2 observations of correlated density and speed fluctuations, and anticorrelated density and temperature fluctuations, on an outbound pass through the northern dawnside magnetosheath. Densities decreased when the magnetic field rotated southward and draped about the magnetopause. In the absence of any significant solar wind density or dynamic pressure variations, we interpret the magnetosheath fluctuations as evidence for radial magnetosheath motion induced by variations in the IMF orientation.

ISSN:0148-0227    

DOI:10.1029/95JA03141

年代:1996

数据来源: WILEY

摘要:

This paper is a study of a possible mechanism of magnetic field amplification in the thin transition layer just outside the magnetopause of a quickly spinning planet. Here we present analysis of a new magnetospheric magnetic field source arising due to the rapid rotation. When the ionosphere and magnetopause conductivities of the rapidly roating planet are sufficiently high (but less than the field‐aligned conductivity) the circulation exists at the magnetopause in the open field line bundles. The toroidal magnetic field is generated there from the poloidal ambient magnetosheath magnetic field by differential rotation. The requirement of the α effect existence is satisfied due to the deviation from axial symmetry about the planet's rotation axis of the initial magnetosheath velocity and magnetic fields. The increasing poloidal magnetic field can be regenerated from the toroidal one by the α effect. This mechanism may act as a hydromagnetic dynamo. The generated magnetic field penetrates into the magnetosphere and adds to the magnetospheric magnetic fi

ISSN:0148-0227    

DOI:10.1029/95JA02445

年代:1996

数据来源: WILEY

摘要:

The microstructure of pristine magnetopause crossings has been analyzed by using high‐resolution particle and field data obtained by the Active Magnetospheric Particle Tracer Explorers (AMPTE) Charge Composition Explorer (CCE) and ISEE 2 spacecraft. These crossings are pristine in the sense that they exhibit no adjoining magnetospheric boundary layer, or, at most, a low‐density plateau. The CCE crossings include the low‐latitude near‐noon region not typically sampled by ISEE 2, which covers all other local time sectors in a complementary way. Magnetopause crossings without a boundary layer are found to occur to all local times, and such crossings constitute about 10% of all magnetopause crossings. Total pressure balance across the magnetopause is observed to within experimental errors; however, electron data, full‐energy composition measurements, and occasionally field stress are needed to fully evaluate pressure balance. The microstructure of the magnetopause current layer is also found to depend on local time. Crossings within about 1 hour local time of the noon meridian often exhibit very sharp density gradients on scale lengths down to a few plasma skin depths. These gradients are reduced for crossings farther from local noon such that, for cases near the dawn‐dusk meridian, the scale length for density gradients and the magnetopause current are roughly comparable. Magnetopause crossings without a boundary layer impose severe constraints on various theories of boundary layer formation. Pristine magnetopause crossings may be direct cuts through the diffusion region for reconnection. With this interpretation our results are in qualitative agreement with recent simulations of the diffusion region and associated turbulence byDrake et al.[1994], who propose the current convective instability as the dominant process for current transport at the m

ISSN:0148-0227    

DOI:10.1029/95JA02757

年代:1996

数据来源: WILEY

摘要:

Theoretical and experimental studies suggest four main classes of magnetopause/boundary layer processes to explain how particles, momentum and energy transport from the solar wind into the magnetosphere‐ionosphere system. Assuming that the IMFBy‐related prenoon‐postnoon asymmetry of plasma convection is specific to the magnetic merging coupling mode, we determine statistically the local time distribution of transient auroral events as a function of the IMFBypolarity. This is based on concurrent observations of the interplanetary magnetic field componentsBzandByand optical auroral observations of cusp/cleft activities within approximately 1000–1400 magnetic local time (MLT) and 71°–81° magnetic latitudes. Intervals of negative IMFBzand nonzeroByare selected for this study. Earlier case studies have shown that transient auroral events moving eastward or westward in the midday sector, depending on the IMFBypolarity, are candidate ionospheric signature of magnetopause flux transfer events (FTEs). An example of this auroral event sequence, illustrating some main characteristics of the expected ionospheric footprints of FTEs, is presented. The statistical study reported here shows an asymmetric prenoon‐postnoon auroral occurrence distribution which depends on IMFBy, consistent with the predictions of the reconnection model within a 95% level of confidence. This result confirms the previously existing evidence in favor of an FTE‐related interpretation of the actual aur

ISSN:0148-0227    

DOI:10.1029/95JA02590

年代:1996

数据来源: WILEY

摘要:

The characteristics of particle precipitation in the polar cap during intervals of northward interplanetary magnetic field (IMF) are investigated. When the IMF is directed northward, localized and structured electron fluxes, called polar showers, appear in the polar cap and interrupt uniform and soft electron precipitation. We classify polar showers into two types, taking into account the association of ion precipitation: (1) polar showers accompanied by ion fluxes (type A) and (2) polar showers without ion fluxes (type B). In order to investigate the characteristics of each type, we examine the DMSP F8 plasma data obtained for one‐year period. We found that type A and B polar showers respond to the direction of the IMF in quite different ways. Type A polar showers do not significantly depend on either the IMFBxorBycomponent. Most type A ion fluxes are more intense and energetic than those of the solar wind, and their intensity is similar to that in the boundary plasma sheet. This intensity implies that type A polar showers occur on closed field lines. On the other hand, the statistical properties of the type B polar shower are similar to those of polar rain, which is believed to originate in the solar wind. We therefore suggest that the plasma in type B polar showers still originates in the solar wind and that some magnetic field lines in the polar cap are open even when the IMF is directed northwar

ISSN:0148-0227    

DOI:10.1029/95JA01848

年代:1996

数据来源: WILEY

摘要:

Magnetic pulsations and quasi‐periodic (QP) amplitude modulations of ELF‐VLF waves at Pc 3–4 frequencies (15–50 mHz) are commonly observed simultaneously in cusp‐latitude data. The naturally occurring ELF‐VLF emissions are believed to be modulated within the magnetosphere by the compressional component of geomagnetic pulsations formed external to the magnetosphere. We have examined data from South Pole Station (L ∼ 14) to determine the occurrence and characteristics of QP emissions. On the basis of 14 months of data during 1987 and 1988, we found that QP emissions typically appeared in both the 0.5–1 kHz and 1–2 kHz receiver channels at South Pole Station and occasionally in the 2–4 kHz channel. The QP emission frequency appeared to depend on solar wind parameters and interplanetary magnetic field (IMF) direction, and the months near fall equinox in both 1987 and 1988 showed a significant increase in the percentage of QP emissions only in the lowest‐frequency channel. We present a model consistent with these variations in which high‐latitude (nonequatorial) magnetic field minima near the magnetopause play a major role, because the field magnitude governs both the frequency of ELF‐VLF emissions and the whistler mode propagation cutoffs. Because the field in these regions will be strongly influenced by solar wind and IMF parameters, variations in the frequency of such emissions may be useful in providing ground‐based diagnostics of the outer

ISSN:0148-0227    

DOI:10.1029/95JA02223

年代:1996

数据来源: WILEY

摘要:

The average patterns of the electrostatic potential, current vectors, and Joule heating in the polar ionosphere, as well as the associated field‐aligned currents, are determined for a quiet time, the growth phase, the expansion phase, the peak epoch, and the recovery phase of substorms. For this purpose, the Kamide‐Richmond‐Matsushita magnetogram‐inversion algorithm is applied to a data set (for March 17, 18, and 19, 1978) from the six meridian magnetometer chains (the total number of magnetometer stations being 71) which were operated during the period of the International Magnetospheric Study (IMS). This is the first attempt at obtaining, on the basis of individual substorms, the average pattern of substorm quantities in the polar ionosphere for the different epochs. The main results are as follows: (1) The substorm‐time current patterns over the entire polar region consist of two components. The first one is related to the two‐cell convection pattern, and the second one is the westward electrojet in the dark sector which is related to the wedge current. (2) Time variations of the two components for the four substorm epochs are shown to be considerably different. (3) The dependence of these differences on the ionospheric electric field and the conductivities (Hall and Pedersen) is identified. (4) It is shown that the large‐scale two‐cell pattern in the electric potential is dominant during the growth phase of substorms. (5) The expansion phase is characterized by the appearance of a strong westward electrojet, which is added to the two‐cell pattern. (6) The large‐scale potential pattern becomes complicated during the recovery phase of substorms, but the two‐cell pattern appears to be relatively dominant again during their late recovery as the wedge current subsides. These and many other earlier results are consistent with the present ones, which are more quantitatively and comprehensively demonstrated in our global study. Thus the two components are tentatively identified as the directly driven and the unloading components, respectively, although there is some indication that both components are actually coupled in the ionosphere. In the present paper we show that the directly driven component is present throughout the lifetime of substorms, becoming a dominant feature during the recovery phase of substorms as the unloading component wanes. The fact that the two components exist and that their time variations are different indicates that different processes are in progress even for the same value, say, −500 nT, ofALand that we must be cautious in using a single electrojet index, such asAL, in

ISSN:0148-0227    

DOI:10.1029/95JA02990

年代:1996

数据来源: WILEY