摘要:

Magnetic field and plasma data, obtained by the Helios 1 and 2 spacecraft in the solar wind near 0.3 AU during the years 1975 to 1976, have been analyzed by calculating 12 kinds of spectra related to the Elsässer variables, δZ+= δV+ δVA, and δZ−= δV− δVA, where δVand δVAare the bulk velocity and Alfvén velocity fluctuations, respectively. For small amplitude Alfvén waves the fluctuation variable δZ+simply relates to outward propagation and δZ−to an inward sense of propagation, if the ambient magnetic fieldB0is directed inward. The frequency range analysed in this paper is 6×10−6Hz to 6×10−3Hz. It is found that (1) the autocorrelation length for δZ−is much larger than for δZ+in both the high‐speed and low‐speed wind. (2) The power spectra of δZ−, especially in high‐wind speed, are steeper in the low‐frequency range and flatten in the high‐frequency range. (3) In the low‐frequency range, the power spectra for the components of δZ+tend to be isotropic with respect to the three polarization directions, while the spectra of δZ−are dominated by the radial component. In the high‐frequency domain, the spectra of both δZ+and δZ−are dominated by the transverse component in high‐speed wind and are more isotropic in low‐speed wind. (4) The spectra related to the residual energy or the cross‐correlation in low‐speed flows have a power law with the slope near to −5/3. However, in high‐speed flows the corresponding data are widely distributed in a cloud of points with an upper envelope near to the spectrum of δZ−. The origin of all these spectra and their importance for the solar wind physics have also been discussed. Several generation mechanisms are suggested as candidates. In the flat part ofe−spectrum, the fluctuations may be generated by non‐local (in wave number space) interactions with the low‐frequency part of thee+spectrum, or just by parametric decay of the high‐frequency part of thee+spectrum. The steep part ofe−(f) may be related to small‐scale stream tubes, or be influenced by pressur

ISSN:0148-0227    

DOI:10.1029/JA094iA09p11739

年代:1989

数据来源: WILEY

摘要:

A distinctive magnetic structure in which azimuthal and latitudinal angles of field vectors are closely related to each other has been found in the interplanetary magnetic field data obtained by Sakigake at 0.8–1.0 AU. In this structure, termed a ”planar magnetic structure” (PMS), the magnetic field vectors are nearly parallel to a fixed plane. This plane includes the spiral direction but is inclined to the ecliptic plane from 30° to 85°. The field vectors take almost all directions parallel to this plane. The PMS consists of several segments in which field directions are almost constant, and the segments are separated by tangential discontinuities where directional changes of the field vector occur abruptly without showing any preferred polarization. The ion number density, the ion temperature, and the plasma β tend to be higher in the PMS than in the surrounding plasma. The PMS events are clearly distinct from ”magnetic clouds” both in the field configuration and in the plasma conditions. During the 25‐month period from July 1985 to July 1987, eight PMS events with durations of several hours have been identified. The PMS events may be associated with newly emerging magnetic structure in the photosphere from which magnetic tongues are extended into interp

ISSN:0148-0227    

DOI:10.1029/JA094iA09p11761

年代:1989

数据来源: WILEY

摘要:

In this work an empirical modeling technique is used to analyze certain Io‐caused emission (ICE) structures in the Jovian decametric radio spectrographic data. Taking into account that some of these ICE structures have the appearance of great arcs with internal arclike fine structure, a geometrical model is developed in which the observed emission is seen to result from conical emission beams emanating from source regions carried on corotating field lines which are excited only as they cross an excitation zone centered on the Io flux tube. The model is also applied to the analysis of a limited set of spectrograms of the traditional Io‐related sources Io A, Io B, and Io D. In some of these spectra it was possible to identify multiple‐ICE structures. The model results for these spectra are consistent with a physical model in which emission is occurring from multiple spaced flux tubes positioned ahead of and moving with Io, and lend credence to the multiple Alfvén wave reflection hypo

ISSN:0148-0227    

DOI:10.1029/JA094iA09p11777

年代:1989

数据来源: WILEY

摘要:

We evaluate several models of Jovian plasma sheet structure by determining how well they organize several aspects of the observed Voyager 2 magnetic field characteristics as a function of Jovicentric radial distance. The present study focuses exclusively on the data from the Voyager 2 spacecraft because the magnetosphere was more stable during that flyby than it was during the Pioneer 10 and Voyager 1 flybys. It is shown that in the local time sector of the Voyager 2 outbound pass (near 0300 LT) the published hinged‐magnetodisc models with wave (i.e., models corrected for finite wave velocity effects) are more successful than the published magnetic anomaly model in predicting locations of current sheet crossings. We also consider the boundary between the plasma sheet and the magnetotail lobe which is expected to vary slowly with radial distance. We use this boundary location as a further test of the models of the magnetotail. The plasma‐sheet‐lobe boundary is often identified from the observed field gradients, the gradients in the lobes being much smaller than those in the plasma sheet. We show that the compressional MHD waves have much smaller amplitude in the lobes than in the plasma sheet and use this criterion to refine the identification of the plasma‐sheet‐lobe boundary. When the locations of crossings into and out of the lobes are examined, it becomes evident that the magnetic‐anomaly model yields a flaring plasma sheet with a halfwidth of ∼3RJat a radial distance of 20RJand ∼12RJat a radial distance of 100RJ. The hinged‐magnetodisc models with wave, on the other hand, predict a halfwidth of ∼3.5RJindependent of distance beyond 20RJ. New optimized versions of the two models locate both the current sheet crossings and lobe encounters equally successfully. The optimized hinged‐magnetodisc model suggests that the wave velocity decreases with increasing radial distance. The optimized magnetic anomaly model yields lower velocity contrast than the model of Vasyliunas and Dessler (1981). The hinged‐magnetodisc models are shown to satisfy the constraints on the plasma sheet thickness imposed by MHD theory. The magnetic anomaly model can be made consistent with the expectations from MHD theory only by assuming plasma anisotropy so large that the plasma would be unstable to mirror mode instability. We recognize that the magnetic anomaly model is a comprehensive model designed to account for many features of the Jovian system as observed in multiple flyby missions. The plasma sheet structure for the Voyager 2 epoch is only one aspect of the model's predictions. Nonetheless, we believe that the model's failure to satisfy constraints on plasma sheet structure imposed by MHD equilibrium theory presents a challenge to the magnetic anomaly model as it

ISSN:0148-0227    

DOI:10.1029/JA094iA09p11791

年代:1989

数据来源: WILEY

摘要:

An assumption of quasi‐one‐dimensionality is used to derive a simple set of equations describing the comet Giacobini‐Zinner tail configuration. The MHD equations are expanded in terms of a parameter representing the ratio of the length scale in the direction perpendicular to the neutral sheet over the length scale in the direction parallel to the tail. It is shown that in this way it is possible to obtain much information on the structure of the tail and to fit reasonably well the observations made by the ICE space

ISSN:0148-0227    

DOI:10.1029/JA094iA09p11805

年代:1989

数据来源: WILEY

摘要:

The linear stability of the plasma tail of a comet is numerically investigated. The zeroth‐order model of the tail plasma configuration is based on the result of a companion paper (Malara et al., this issue), discussing the possible modeling of the data obtained during the ICE encounter with comet Giacobini‐Zinner. Effects both of finite resistivity and of velocity shear are taken into account. The tail appears to be unstable against Kelvin‐Helmholtz‐like modes in which a certain amount of reconnection occurs. Because of the velocity flow shear the perturbation extends far beyond the singular layer and affects in depth the magnetic lobes. The relative importance of the Kelvin‐Helmholtz turbulence in the lobes and the level of reconnection, the typical wavelengths, and the growth time of the mode depend on the particular cometary tail c

ISSN:0148-0227    

DOI:10.1029/JA094iA09p11813

年代:1989

数据来源: WILEY

摘要:

We give a systematic theoretical analysis of trapped nonadiabatic charged particle motion in two‐dimensional taillike magnetic field reversals. Particle dynamics is shown to be controlled by the curvature parameter κ, i.e., the ratio κ² =Rmin/ρmaxbetween the minimum radius of curvature of the magnetic field and the maximum Larmor radius in it for a particle of given energy. κ≫1 corresponds to the usual adiabatic case with the magnetic moment μ as a first‐order invariant of motion. As κ decreases toward unity, the particle motion becomes stochastic due to deterministic chaos, caused by the overlapping of nonlinear resonances between the bounce‐ and the gyro‐motion. We determine the threshold of deterministic chaos and derive the related pitch angle diffusion coefficient which describes statistically the particle behavior in the limit κ → 1. Such behavior, which for κ ≅ 1 becomes strongly chaotic, applies, e.g., to thermal electrons in Earth's magnetotail and makes its collisionless tearing mode instability possible. We also show that in sharply curved field reversals, i.e., for κ1. Both types of trapped particle motion in sharply curved magnetic field reversals κ<1 are closely connected with fast oscillations perpendicular to the reversal plane. However, the trajectories are adiabatic only in the case that they permanently remain crossing the reversal plane. The adiabatic are of a ring type, i.e., they resemble rings in phase space and also in real physical space. For ring‐type orbits the action integral over the fast oscillations is an adiabatic invariant in the usual sense. On the other hand, the most common particle trajectories in a sharply curved field reversal with κ<1 are essentially of a cucumberlike quasi‐adiabatic type. For quasi‐adiabatic cucumberlike orbits the action integral over the fast oscillations is an adiabatic invariant only in a piecemeal way between successive traversals in the phase space of the fast motion of a separatrix between orbits which do and those, which do not cross the reversal plane. Due to the effect of separatrix traversals the slow motion shifts between different cucumber orbits with a conservation of the action integral on average but with its chaotic phase space diffusion even for very small perturbation parameters κ. The case κ<1 is applicable, e.g., to thermal ions and high‐energy electrons in Earth's magnetotail. Our findings lead to a systematic interpretation of particle observations in Earth's magnetotail and of numerous numerical calculations, carried out in the past. They also explain rather well, e.g., the pitch angle diffusion of plasma sheet particles, the isotropization of the plasma sheet electron distribution immediately before a substorm and provide with the transition to chaos a mechanism for the onset of a large‐scale tail instability and the explosion of isolated substorms. Further implications for magnetotail physics, such as acceleration processes and the influence of the particle escape from the field reversal wil

ISSN:0148-0227    

DOI:10.1029/JA094iA09p11821

年代:1989

数据来源: WILEY

摘要:

Impulsive changes in the solar wind can deposit energy into magnetospheric cavity modes. We discuss the coupling of cavity modes to the field line continuum, and show that the time scales for dissipative phase mixing and mode conversion to kinetic Alfvén waves in the magnetosphere are long compared with lifetimes of MHD wave events with periods that vary continuously with radial distance. Therefore the ultimate dissipative sink for cavity mode energy should be the ionosphere. For model magnetospheres in which all wave components have a complex field aligned wave numberkz=kz0(1+iκ) to simulate a Poynting flux into the ionosphere, we use simple arguments to show that the coupled cavity mode damping decrement is given approximately by γ/ω0= (γ/ω0)c+ (kz0/k)² κ. Here (γ/ω0)cis the damping via coupling to the field line continuum alone, andkis the total wave number magnitude. Detailed numerical calculations in a cylindrical magnetospheric model support this, although significant departures from the approximate expression can occur for small κ. Following earlier work by other authors, we emphasize that complexkzmay not simulate the appropriate boundary condition for the coupled cavity mode outside resonan

ISSN:0148-0227    

DOI:10.1029/JA094iA09p11843

年代:1989

数据来源: WILEY

摘要:

The magnetospheric generation and propagation characteristics of ion cyclotron waves associated with a Pc 1 emission observed by a network of four middle and low‐latitude ground stations are determined using ground source location techniques and ISEE 1 plasma data. The source region at L = 4.7 ± 0.7 is determined from propagation in the F2region ionospheric duct using wave polarization characteristics at ground stations. This source is just inside the steep plasmapause seen by ISEE 1 at L = 4.9 ± 0.1. The ion cyclotron wave packet interhemispheric bounce period measured from the ground spectra increases with time from 140 to 155 s during the event but is in agreement with dispersion calculations undertaken using ISEE 1 electron density and cool to cold ion composition data in a H+plasma with 8–10% He+and<1% 0+ions. The frequency of the Pc 1 emission band seen on the ground (0.5–0.8 Hz) corresponds to the propagation region between the equatorial 0+cutoff frequency and the He+cyclotron frequency. Linear convective growth rate calculations inside the plasmapause show significant wave amplitudes in this band. With these results it is possible to completely describe the wave generation mechanism and the magnetosphere ionosphere propagation characteristics for the Pc

ISSN:0148-0227    

DOI:10.1029/JA094iA09p11855

年代:1989

数据来源: WILEY

摘要:

We present a comprehensive study of the composition, and the density and temperature distributions of the thermal (energy≤110 eV) ion population in the terrestrial plasmasphere under quiet geomagnetic conditions. The data were collected by the Ion Composition Experiment (ICE) on board the European Space Agency's GEOS 1 satellite and cover the period from June 1977 to May 1978. For the data reduction we employ a method based on the modulation of the detector count rates by the rotation of the spacecraft. We find typical quiet time proton densities to vary smoothly between ∼10² cm−3(L≈ 6) and 2×10³ cm−3(L≈ 3). In the local time sector 1700–2200 the equatorial proton concentration obeys an inverse fourth power dependence with dipoleL. He+is a major ionic component in allLranges investigated. Its concentration relative to H+is highly variable, ranging from ∼1% to, on occasion, over 100%. The most frequent values we obtain lie in the range ∼2–6%. The averaged variation of He+with dipoleLin the 1700–2200 local time range shows a somewhat more rapid decrease with increasingLthan does H+. The temperatures in the quiet plasmasphere are between 4×10³ K and 1.5×104K and generally exhibit a slow increase withLvalue. The average radial temperature gradient near the equator is ∼0.15 K/km. The main ionic constituents are usually in thermal equilibrium throughout the plasmasphere. There are indications that the ionic component is in a state of thermal equilibration with the electronic component in the outer plasmasphere. Using alternate passes in the same region, we discuss the poststorm recovery of the plasmasphere onLshells>4 for both H+and He+. Data are also presented on the minor ions O+, O++, D+, and He++. The O+densities aroundL≈ 3 are ≲1 cm−3, and the densities of O++are comparable. Cases are shown where the mass/charge = 2 ion is predominantly D+and others where it is mainly He++. Typical densities of this ion in theLrange 2.6–3.6 are a few tenths per cubic centimeter. With one exception we found these minor ions to share a common temperature with the main constituents of the plasmasphere. The results of our survey are compared with previous studies and with theoretical modeling. In particular, the GEOS 1 H+temperature structure is in substantial agreement with those from the Plasma Composition Experiment (PCE) on ISEE 1 and the retarding ion mass spectrometer (RIMS) on DE 1. We confirm the enhanced O++to O+density ratio in the equatorial plasmasphere with respect to values in the mid‐latitude ionosphere. Our observations on this ratio are compared with the

ISSN:0148-0227    

DOI:10.1029/JA094iA09p11865

年代:1989

数据来源: WILEY