摘要:

A radial alignment study of three solar wind stream structures observed by IMP 7 and 8 (at 1.0 AU) and Voyager 1 and 2 (in the range 1.4–1.8 AU) in late 1977 is presented. It is demonstrated that several important aspects of the observed dynamical evolution can be both qualitatively and quantitatively described with a single‐fluid two‐dimensional MHD numerical model of quasi‐steady corotating flow, including accurate prediction of (1) the formation of a corotating shock pair at 1.75 AU in the case of a simple, quasi‐steady stream, (2) the coalescence of the thermodynamic and magnetic structures associated with the compression regions of a fast corotating stream interacting with a slower corotating stream, and (3) the dynamical “destruction” of a “small” (i.e., low velocity‐amplitude, short spatial scale) stream by its overtaking of a slower‐moving, high‐density region associated with a preceding shock and transient flow. The evolution of these flow systems is discussed in terms of the concepts of “filtering” and “entrainment,” which have appeared in the literature. It is concluded that this study provides support at a detailed, quantitative level for our general conceptual notions of stream evol

ISSN:0148-0227    

DOI:10.1029/JA090iA08p07377

年代:1985

数据来源: WILEY

摘要:

The ion composition experiment on ISEE 3 obtains the velocity distributions for4He++, completely separated from protons. Temperatures and velocities derived from these distributions have been compared with the Los Alamos National Laboratory proton temperatures and velocities obtained from the National Space Science Data Center. The results, which apply on the macroscale, confirm and extend previous observations and show the following: (1) The temperature ratioTα/Tpdrops with increasing transit timeR/VThis indicates that interactions among the ions take place, but it does not identify the collision mechanism. (2) At low τe/τc(the ratio solar wind expansion time/collision time) the temperatures of the individual species are proportional to their mass. As τe/τcincreases, the He temperature drops, and the H temperature increases to a common intermediate value determined by equipartition of energy. (3)Tα/Tpis approximately proportional to (mpTp+mαTα)/(mα+mp), as suggested for Coulomb collisions. The temperature ratio is more sensitive to the prevailing value ofTthan to that of the densityN. Thus the scattering depends more strongly on the thermal speed than on the interparticle separation. (4) The state of ion equilibrium is determined by the value of τe/τc, independent of the type of flow. (5) As τe/τcincreases from 0.01 to unity and above, the temperature ratio first remains constant near 4, then drops uniformly to unity, and remains constant for τe/τc≳ 1. The relative velocity difference |Vα‐Vp|/VAdecreases monotonically to zero as a function of τe/τs, where τsis the slowing down tune due to collisions. This evidence establishes Coulomb collisions as the important ion‐ion equilibrating process in the solar wind whe

ISSN:0148-0227    

DOI:10.1029/JA090iA08p07389

年代:1985

数据来源: WILEY

摘要:

Recent evidence indicates that magnetic reconnection at the earth's magnetopause may not be a steady process, but rather it is frequently impulsive and limited in spatial extent. These limited reconnection events are called flux transfer events (FTE's). We have searched the magnetic field observations at Jupiter from Pioneer 10 and 11 and Voyager 1 and 2 for evidence of FTE's and have found 14 possible events. The FTE's at Jupiter are associated with northward magnetosheath fields. The electric fields generated by Jovian FTE's are small in comparison with the corotationEfield throughout much of the magnetosphere. Thus FTE's are probably not an important source of flow within the Jovian magnetosphere.

ISSN:0148-0227    

DOI:10.1029/JA090iA08p07397

年代:1985

数据来源: WILEY

摘要:

We derive the potential distribution in a plasma containing dust grains where the Debye length can be larger or smaller than the average intergrain spacing. We treat three models for the grain‐plasma system, with the assumption that the system of dust and plasma is charge‐neutral: the permeable grain model of Goertz and Ip (1984), an impermeable grain model, and a capacitor model that does not require the nearest neighbor approximation of the other two models. We use a gauge‐invariant form of Poisson's equation which is linearized about the average potential in the system. The charging currents to a grain are functions of the difference between the grain potential and this average potential. We obtain expressions for the equilibrium potential of the grain and for the gauge‐invariant capacitance between the grain and the plasma. The charge on a grain is determined by the product of this capacitance and the grain‐plasma potential difference. The three models give similar but not identical results. The results depend primarily on the parameterZ= 4πλ²NC, where λ is the Debye length,Nis the grain concentration, andCis the grain to plasma capacitance. WhenZ≫ 1, the number of charges on a grain that is only charged by plasma currents is given by where µ is the square root of the ion to electron mass ratio, andand are the average ion and electron densities. We confirm the result of Goertz and Ip (1984) that the charge on a grain in regions such as Saturn's F ring and spokes can be severely decreased from its free space value. The charge reduction occurs because the plasma electrons are depleted so that the grain does not need to be as negatively charged to equalize the ion and electron fluxes to its surface, despite the increased grain to pl

ISSN:0148-0227    

DOI:10.1029/JA090iA08p07405

年代:1985

数据来源: WILEY

摘要:

Ionization troughs are frequently observed in the Venus nightside ionosphere by the orbiter ion mass spectrometer (OIMS) on the Pioneer Venus Orbiter (PVO). These events typically exhibit density depletions which may range from a factor of 2 to an order of magnitude or more, often feature sharp density gradients, and frequently are associated with superthermal ion fluxes. Although their full spatial extent cannot be determined, the ion trough dimensions as perceived along the PVO orbit track extend from 1° to more than 10° in latitude and longitude. Troughs have been encountered at altitudes ranging from more than 1000 to less than 250 kilometers. The nightside trough boundaries are similar to both the ionization gradients which mark the Venus dayside ionopause and the ionization gradients which define the high‐latitude ionization trough in the earth ionosphere. In each of these Venus and earth phenomena there is a close association between sharp gradients in thermal ion distributions and energetic and dynamic processes stimulated by the interaction between the solar wind/interplanetary magnetic field (IMF) and the thermal ionosphere. A close correlation is identified between ion trough events and prominent published examples of 100‐Hz plasma waves detected by the orbiter electric field detector (OEFD), which have been interpreted by Scarf and Russell (1983) and by Ksanfomaliti et al. (1983) as whistler mode propagation stimulated by lightning in the lower atmosphere and frequently occurring over mountains. While the occasional coincidence between the detection of whistler waves and the overflight of Venus mountain ranges is also observed for the ion troughs, neither phenomenon is seen to exhibit convincing evidence of an association with the mountains. Overall, these findings lead us to believe that many of the 100‐Hz noise bursts previously attributed to lightning are, like the ion troughs, actually the result of the interaction between the ionosphere and the combined effects of the solar wind and the draped IMF. If this interpretation is correct, there may be little evidence for lightning from the PVO measurements. Similarly, a supporting argument for active volcanism as a possible source for the deduced lightning events would also be dis

ISSN:0148-0227    

DOI:10.1029/JA090iA08p07415

年代:1985

数据来源: WILEY

摘要:

In the current “conceptual” model of magnetospheric substorms, the growth phase terminates and the expansion phase commences with the onset of rapid reconnection at a new, near‐earth X‐type neutral line. Physical concepts developed in the analysis of the collisionless tearing mode and the flow of collisionless plasma in weakly magnetized, thin current sheets are combined to construct a model of purely collisionless, time‐dependent, ion‐dominated reconnection. Formulated in the context of time‐dependent magnetospheric convection, the model describes the reconnection collapse of the initially thick plasma sheet. In the nonlinear phase the reconnection rate grows explosively in time and saturates into a steady collisionless reconnection flow when the initial magnetic flux in the current sheet has reconnected; at saturation the reconnection rate is comparable to the maximum Petschek rate. The time scale and dynamics of the explosive reconnection model are broadly consistent with observations of substorm growth phase and expansion phase onset. For typical plasma sheet parameters the explosive reconnection electromotive force across the tail approaches 1 MV

ISSN:0148-0227    

DOI:10.1029/JA090iA08p07427

年代:1985

数据来源: WILEY

摘要:

The analysis of 16 plasma vortex occurrences in the magnetotail plasma sheet of Hones et al. (1983) is extended. We used two‐ and three‐dimensional plasma measurements and three‐dimensional magnetic field measurements to study phase relations, energy propagation, and polarization properties. The results point toward an interpretation as a slow strongly damped MHD eigenmode which is generated by tailward traveling perturbations at the low‐latitude interface between plasma sheet and magnet

ISSN:0148-0227    

DOI:10.1029/JA090iA08p07449

年代:1985

数据来源: WILEY

摘要:

New evidence of phase perturbations of subionospheric VLF/LF signals, produced by lightning‐whistler‐induced precipitation of bursts of radiation belt electrons, is reported. Phase changes of 0.1–1.0 µs on a 12.9‐kHz signal arriving at Palmer, Antarctica (L∼ 2.4), over a ∼2400‐km path are regularly observed with a characteristic signature of a rapid (≤ 1 s) onset followed by a relatively slow (10–30 s) recovery. Event occurrence peaks during equinoctial periods, with typical rates of 1–40 events per hour lasting for several hours per night. The observed phase perturbations can be interpreted to correspond to a rapid reduction of up to ∼0.1–1.0 km in the effective nighttime ionospheric reflection height (∼85 km) for the subionospheric VLF signals. The incident energetic particle fluxes that are required to produce the extra ionization are estimated to be ∼10−3‐10−2ergs cm−2s−1. Such flux levels are generally consistent with those estimated using theoretical models of whistler‐particle resonant scattering as well as direct satellite‐based observations of whistler‐induced precipitation. This phenomenon provides a sensitive means for detecting wave‐induced burst particle precipitation. The ionospheric perturbations resulting from such precipitation may cause significant errors in certain global navigation aids, such

ISSN:0148-0227    

DOI:10.1029/JA090iA08p07457

年代:1985

数据来源: WILEY

摘要:

This paper discusses the constraints on auroral z mode waves implied by damping. Damping of the magnetoionic z mode in a thermal plasma with ωp≪ Ω is calculated and shown to be strong near the gyrofrequency. Analytic formulae for the optical depth of the corresponding absorption layer are presented. Ray tracing calculations are used to investigate the effect of refraction during the propagation of a z mode ray: refraction is found to be particularly important in the vicinity of the gyrofrequency layer and tends to cause the wave vector of the ray to change so that it is strongly damped. The results are applied to models for the generation of auroral z mode waves. A point of particular interest concerns the possibility of explaining observed emissions whose bandwidths straddle the gyrofrequency by an emission mechanism which amplifies waves either only above or only below the gyrofrequency. For a dipolar magnetic field and radial plasma density gradient it is found that a z mode ray in the auroral zone is unlikely to be able to pass through the gyrofrequency layer without significant damping occurr

ISSN:0148-0227    

DOI:10.1029/JA090iA08p07471

年代:1985

数据来源: WILEY

摘要:

Nearly simultaneous measurements of auroral zone electric fields are obtained by the Dynamics Explorer spacecraft at altitudes below 900 km and above 4500 km during magnetic conjunctions. The measured electric fields are usually nearly perpendicular to the magnetic field lines. The north‐south meridional electric fields are “projected” to a common altitude by a mapping function which accounts for the convergence of the magnetic field lines. When plotted as a function of invariant latitude, graphs of the projected electric fields measured by both DE 1 and DE 2 show that the large‐scale electric field is the same at both altitudes, as expected. Superimposed on the large‐scale fields, however, are small‐scale features with wavelengths of less than 100 km which are larger in magnitude at the higher altitude. Fourier transforms of the electric fields show that the magnitudes depend on wavelength. Outside of the auroral zone the electric field spectrums are nearly identical. But within the auroral zone the high‐ and low‐altitude electric fields have a ratio which increases with the reciprocal of the wavelength. The small‐scale electric field variations are associated with field‐aligned currents. These currents are measured with both a plasma instrument and magnetometer on DE 1. A Fourier transform of the east‐west magnetic field component measured on the high‐altitude satellite is found to be nearly identical to the Fourier transform of the north‐south electric field measured on the low‐altitude satellite, with a constant ratio. This ratio is proportional to the ionospheric conductivity. The experimental measurements are found to agree with a steady state theory which postulates that there are parallel potential drops associated with the variations in the perpendicular electric fields. It is assumed that there is a linear relationship between the field‐aligned current and the total parallel potential drop and that the field‐aligned currents close through Pedersen currents in the ionosphere. The theory predicts that the ratio between the low‐ and high‐ altitude electric fields varies with the wavelength. Below a “critical” wavelength the electric field is not effectively transmitted to low altitudes. Owing to the good agreement between the theory and observations, it is concluded that the linear relationship between the current density and

ISSN:0148-0227    

DOI:10.1029/JA090iA08p07479

年代:1985

数据来源: WILEY