摘要:

The capture of newly ionized lithium ions in the solar wind by means of electromagnetic instabilities is investigated through linear analysis and computer simulation. Three instabilities, driven by a lithium velocity ring perpendicular to and drifting along the magnetic field, are considered. The capture time of the lithium by the solar wind is roughly 10 linear growth times, regardless of whether resonant or nonresonant modes dominate initially. Possible implications of the results for the Active Magnetosphere Particle Tracer Explorer (AMPTE) mission are discussed.

ISSN:0148-0227    

DOI:10.1029/JA089iA09p07327

年代:1984

数据来源: WILEY

摘要:

We present the results from a test‐particle simulation designed to study how lithium tracer ions injected upstream of the earth's bow shock interact with the bow shock when waves are present in the shock's upstream and downstream vicinity. The wave activity is assumed to consist of parallel and antiparallel propagating Alfvén waves characterized by a frequency power spectrumP(f)∼f−1within a frequency interval and range of amplitudes defined separately in the upstream and downstream regions. At a “single encounter” (defined in this paper as the time during which an ion remains within ∼2 gyroradii of the shock) the waves act mainly to perturb an ion's orbit, leading to an increase (or decrease) in the number of orbital shock crossings, and therefore increase (or decrease) the energy gain (via drift along theU × Belectric field) relative to the situation when waves are absent. Complete transmission of the injected ion distribution is predicted with and without wave activity present when the angle ψ1between the shock normal and the nominal upstream magnetic field satisfies 70° ≲ ψ1≤ 90°. A wave field of sufficient amplitude should increase ion transmission for 45° ≲ ψ1≲ 70°, where significant reflection (∼ 50%) is possible in the absence of waves. At a fixed ψ1, increasing wave activity yields larger average energy gains, reduced pitch angle anisotropies, and increased spatial dispersion on the bow shock for both the reflected ions and those t

ISSN:0148-0227    

DOI:10.1029/JA089iA09p07331

年代:1984

数据来源: WILEY

摘要:

The transport of test ions through the magnetosheath depends upon the macroscopic structure defined by the average magnetic and flow fields,B0andU, and the fluctuating fields, δEand δB, encountered by the particles. In this paper an approximate model is used to generate the macroscopic configuration: it satisfies the jump conditions at the bow shock, treats the magnetopause as a tangential discontinuity (B0andUtangent to the magnetospheric boundary), and uses intermediate field amplitudes and orientations that resemble the more precise results of gas dynamic simulations. Equations of motion for a fictitious ensemble average particle describe the ion propagation, with the effects of the inhomogeneous turbulence phenomenologically incorporated into a friction tensor whose components reflect the coupling of the ions to the bulk plasma along and across the magnetic fieldB0. Application to the solar wind releases of lithium ions by the Active Magnetospheric Particle Tracer Explorers mission illustrates the approach: we follow the guiding center trajectory of the ensemble average “particle” for several upstream solar wind and interplanetary magnetic field conditions, different locations of the release site, and quiet and disturbed magnetosheath turbulence regimes. Implications of the results for the release strategy are disc

ISSN:0148-0227    

DOI:10.1029/JA089iA09p07339

年代:1984

数据来源: WILEY

摘要:

In a magnetically closed magnetosphere, none of the geomagnetic field lines intersects the magnetopause. Solar wind particles incident upon such a field topology are normally thought to be specularly reflected and therefore none gains entry to the magnetosphere. Strictly speaking, however, specular reflection occurs only when the incident charged particle encounters a uniform magnetic field. Although the geomagnetic field at the magnetopause does appear quite uniform to incident particles (its scale size is much greater than the gyroradii of solar wind particles), it possesses a small gradient parallel to the magnetopause. When this gradient is taken into account, a fraction of the incident particles can gain access to the magnetosphere. This gradient drift entry (GDE) process is a feature of the basic structure of the geomagnetic field and should occur at all times. It is therefore expected that the Active Magnetospheric Particle Tracer Explorers (AMPTE) lithium ion releases will penetrate certain regions of the magnetopause. An excellent test of the GDE process could be made by initiating a lithium release during a northward interplanetary magnetic field (when the magnetosphere is thought to be magnetically closed).

ISSN:0148-0227    

DOI:10.1029/JA089iA09p07347

年代:1984

数据来源: WILEY

摘要:

Using a simple time dependent cold plasma density model and assuming a typical ambient radiation belt environment, we study the evolution of the whistler mode turbulence and particle precipitation in a cold plasma release experiment similar to one that may be conducted as part of the Active Magnetospheric Particle Tracer Explorers (AMPTE) program. It is known from earlier work that the release of cold lithium ions can significantly lower the critical energyEcabove which the resonant radiation belt electrons can pitch angle scatter. We study the time evolution of the one pass gain factor for a whistler wave packet and find that for parameters accessible to AMPTE type experiments the gain factor is large enough to ensure strong whistler turbulence and strong pitch angle diffusion of radiation belt particles with energies in the range between the ambient value ofEcand the reduced value ofEc. Estimates for the total power input to the ionospheric footprint of the release are of the order of an erg/cm²/s. This precipitated energy should produce a patch of visible aurora. This effect should also persist for many hours

ISSN:0148-0227    

DOI:10.1029/JA089iA09p07351

年代:1984

数据来源: WILEY

摘要:

One of the active experiments to be performed as part of the Active Magnetospheric Particle Tracer Explorers mission involves the release of lithium ions in the geomagnetic tail and their subsequent detection after earthward convection into the nightside outer ring current region. In this paper we have used the guiding center approximation to integrate ion trajectories in a simple two‐dimensional model of the quiet time nightside magnetosphere in order to estimate expected ion properties at the latter location. Our principal conclusion is that under typical quiet time conditions these ions will predominantly form a field‐aligned beam (∼5°–20° pitch angle) at energies from a few hundred eV

ISSN:0148-0227    

DOI:10.1029/JA089iA09p07357

年代:1984

数据来源: WILEY

摘要:

An unsteady one‐dimensional MHD model is developed to study (1) the essential physical processes involved in the development of the forward‐reverse shock pair at large heliocentric distances, (2) the interaction of the shock pair with the rarefaction regions of the stream structure, and (3) the merging of two forward or reverse shocks. We use a method of solution which is quite different from the finite difference methods: MHD shocks (or contact surfaces) are treated as boundary surfaces. The presence of moving shocks divides the domain of interest in ther‐tplane into several flow regions. The jump conditions of MHD shocks describe the flow conditions across the moving boundaries between flow regions. The method of characteristics describes the variation of flow conditions in each region. The solutions explain the formation of the shock pair in the leading edge region as resulting from the merging of fast waves. The strong MHD disturbances generated in the corotating interaction region (CIR) propagate at a fast speed relative to the moving material. The wave propagation speed is greater in CIR than in its surroundings. This causes the disturbances in CIR to pile up to form a shock pair. During the formation process the shocks continuously grow into a fully developed state. The newly formed shock pair will then propagate outward from the leading edge to interact with the ambient rarefaction regions. The double‐sawtooth configuration of the velocity profile is a result of this interaction. We also obtain solutions to demonstrate that the merging of two shocks produces a stronger shock and a contact surface on its b

ISSN:0148-0227    

DOI:10.1029/JA089iA09p07367

年代:1984

数据来源: WILEY

摘要:

When considering local solutions for the jump in plasma conditions across planetary bow shocks, a magnetohydrodynamic (MHD) formalism of the Rankine‐Hugoniot relations is used. However, when planetary shocks are modeled on a global scale, gas dynamic (GD) treatments are frequently employed which neglect the magnetic field effect on the shock structure and the flowing plasma. These treatments require the specification of a free‐stream Mach number and a ratio of specific heats. Herein observations of the Venus bow shock are used to illustrate how closely one can approximate the observed magnetic field jump with MHD and GD treatments if one chooses a particular Mach number and ratio of specific heats. As expected theoretically, it is found that the observed magnetosonic Mach number (and not the sonic Mach number) when used in the GD calculations gives the best approximate solutions to the MHD problem using reasonable value for the ratio of specific heats. At Venus we find that the best value for the ratio of specific heats (γ) is 1.85 (rather than 5/3 or 2), although this ratio depends on the Alfvenic Mach numberMAand the angle between the shock normal and the upstream field, θBN. We cannot determine how well the GD solution reproduces the shock shape at Venus, because of the possible influence of both MHD and mass‐loading effects. In fact, the Venus bow shock is much farther away from the planet at the terminator than the GD model would predict for either γ = 5/3 or 2. Thus we must depend on terrestrial studies to determine how best to approximate shock shape with

ISSN:0148-0227    

DOI:10.1029/JA089iA09p07381

年代:1984

数据来源: WILEY

摘要:

We propose a mechanism for the transport of energy from the rotation of Jupiter into the heavy ions whose precipitation into the atmosphere is responsible for the intense ultraviolet aurora. The process involves charge exchange of heavy ions in the Io torus, their flight to the outer magnetosphere as neutrals, and their reionization in a region where the pickup magnetic moment is large. The particles diffuse back inward adiabatically and are precipitated at 9RJwith an energy exceeding 10 keV/nucleon. A field and flow model based on Voyager observations is used that defines a source region extending from 32RJto 80RJon the nightside. This region is capable of delivering 4×1013W of power to the auroral region, well within the realm of the power required by observations. We also demonstrate that this mechanism provides a neutral planetary wind from Jupiter. The predicted wind will consist of two beams: one intense (5×1028s−1) at 25 eV/nucleon and the other tenuous (1×1025s−1) at 600 eV/nucleon and above. These neutral particles will appear as a suprathermal heavy ion component in the sola

ISSN:0148-0227    

DOI:10.1029/JA089iA09p07393

年代:1984

数据来源: WILEY

摘要:

It is proposed that the transport of material from Io to the plasma torus requires the presence of an atmosphere as an intermediate buffer. Direct escape of volcanic SO2is expected to be negligible. The sputtering of surface SO2frost by corotating ions (if they reach the surface) or by magnetospheric MeV ions would not provide sufficient escape flux. Instead, such sputtering process produces an ∼97% fraction with energy less than the escape energy; hence this process alone could produce an atmosphere thick enough to prevent the corotating ions from reaching the surface and thick enough to disallow even the energetic sputtered products created at the surface to escape. The escape from an atmosphere could take place by several mechanisms, of which secondary sputtering at the exobase is dominant. The escape rate from a thin atmosphere increases with increasing atmospheric SO2content. By preparing photochemical models of SO2atmospheres for varying total atmospheric pressure and calculating the escape rates, it is concluded that the globally averaged atmospheric SO2pressure on Io at Voyager 1 encounter must have been ≳ 1.4×10−10bar (surface densityn0(SO2) ≳ 1.2×1010cm−3, or column density of ≳ 1×1016cm−2), as the escape rates in this limiting case are just adequate to populate the torus if the Brown et al. (1983) estimates of the supply rate are applicable. However, if the higher estimates of Pontius and Hill (1983) are appropriate, then more SO2would be needed in t

ISSN:0148-0227    

DOI:10.1029/JA089iA09p07399

年代:1984

数据来源: WILEY