摘要:

The interaction between water outgassed from the space shuttle and the ionospheric plasma leads to production of water ions by charge exchange and an active and complex plasma wave environment for the space shuttle. We show that the amplitude and spectral character of some of these waves are controlled by the angle between the magnetic field and the shuttle's velocity vectorVTrelative to the ionospheric plasma. When the flow is approximately perpendicular to the magnetic field (V∥/VT∼ 0), large wave amplitudes and characteristic “mushroom” wave structures are observed, whereas more nearly parallel flows |V∥| ∼V⊥are characterized by low wave levels. We show that linear instability theory predicts the growth of Doppler‐shifted lower hybrid waves in the observed frequency range when driven by the ring and/or beam distributions of water ions produced by charge exchange in the vicinity of the space shuttle. Two mutually compatible interpretations for theV∥/VTeffect exist. The first interpretation involves the path lengths available for growth of waves driven by pickup ions varying with the quantityV∥/VTand being limited by spatial variations in the water ion distribution. The second interpretation follows directly from the linear theory: decreasing the ring/beam speedV⊥of the pickup ions driving the waves (increasingV∥/VT) results in smaller growth rates, with zero growth rate below some threshold value ofV⊥. The linear theory shows that decreased growth lengths or growth rates should naturally produce the observed amplitude and bandwidth changes constituting theV∥/VTeffect. These results have immediate implications for future shuttle missions and orbiting platforms subject to outgassing of water. If these facilities are used for ionospheric plasma studies or active experiments involving plasma waves, the plasma wave background due to pickup ions associated with the orbiter should be minimized. This requires orbits with large |V∥| ≥V⊥; that is, orbital velocities within about 45° of the magnetic field over as much of the orbit as possible. These constraints favor more nearly polar orbits and argue strongly against equatorial orbits. Alternatively, free‐flying spacecraft situated upstream from the

ISSN:0148-0227    

DOI:10.1029/90JA02564

年代:1991

数据来源: WILEY

摘要:

Laboratory experiments designed to uncover mechanistic information about the spectral and spatial characteristics of shuttle glow were conducted. Pulsed oxygen atoms traveling at orbital velocities were directed toward a substrate which was previously dosed with NO molecules. Heterogeneous recombination of the O and NO species resulted in NO2*exiting the surface of the sample, and an associated emission was found to extend from the sample plane. In the experiments the materials investigated were Z306 Chemglaze®(a common baffle black paint), aluminum, and nickel. The sample temperatures were varied from 300 to 77 K, and the oxygen atom velocity was varied from 5 to 10 km s−1. The experimental results include the measure of (1) an effective NO2*lifetime of 185 μs, (2) complete surface thermal accommodation of the formed NO2*, (3) a large NO2*emission brightness which was inversely related to surface temperature, and (4) a spectral shape which indicates a red shifting to distance from the sample as well as (5) a slight spectral shift which appears to be material related. The preliminary experimental data from this experiment were presented by Caledonia et al. (19

ISSN:0148-0227    

DOI:10.1029/90JA02747

年代:1991

数据来源: WILEY

摘要:

We have made a numerical investigation of the wake potential and density perturbations caused by the presence of a large negative voltage on a sector situated on the wake side of a spacecraft moving at orbital speed through the ionosphere. The spacecraft is represented as an infinite cylinder moving in an unmagnetized plasma. The rest of the cylinder's surface is biased to a moderate negative voltage representing floating potential. It is shown that in the case of large Debye ratio (spacecraft size much larger than Debye length) the geometrical wake is almost ion‐empty for a high differential bias voltage on the sector, and that in the case of low Debye ratio and low Mach number, ion focusing effects can lead to the formation of a high‐ion‐density strip parallel to the surface when the differential bias is made very large. In the latter case, the results show evidence of an ionic emptying effect in the near wake and in the midwake. We propose a mechanism to explain this result. Ion collection on the biased sector is also addr

ISSN:0148-0227    

DOI:10.1029/90JA02754

年代:1991

数据来源: WILEY

摘要:

A fully three‐dimensional model for an artificial plasma cloud in the ionosphere is developed. Such a cloud could be a result of a deliberate release or contamination about a spacecraft. Finite perpendicular and parallel lengths, all elastic collisions between the considered species, finite temperature effects, variable ionospheric densities, variable neutral winds, ambient electric fields and gravity are taken into account. The velocities of the charged species are written with the use of transport coefficient tensors. The current balance equation includes conductive currents, diffusion currents, currents due to the ambient and contaminant neutral winds, and gravitational drift currents. The numerical solution for a plasma sphere immersed in a plasma with an external electric field is obtained and compared with known analytic solutions. The electrodynamical behavior of Gaussian‐like clouds is examined. The neutral cloud simulates an orbital release (U=8 km/s) with various masses. The effects of the central density of the contaminant neutrals and ions are investigated. The perturbation potential increases with increasing ion and neutral density. The perturbation electric field consists of a dipole field due to the action of the neutral wind and a diffusion field due to the density gradients. The dipole field is mapped along the magnetic field lines while the diffusion field is bounded within the density perturbation. With cloud densities of 1015m−3the ion cloud drifts almost with the neutral cloud velocity. The effects of the coupling of the lowerEand upperFregion are also investigated. The ambient plasma is expected to develop image clouds with differential rotation which will stretch the flux tube. The effect of the altitude of the release is examined. It is found that releases at higher altitudes result in faster moving ion clouds. As a result of the nonuniformity of the perturbation electric field, the ion cloud drifts with differential speed which will result in fingerlike sh

ISSN:0148-0227    

DOI:10.1029/90JA02249

年代:1991

数据来源: WILEY

摘要:

The one‐ and two‐dimensional behavior of obliquely propagating hydromagnetic waves is analyzed by means of analytical theory and numerical simulations. It is shown that the nonlinear evolution of a one‐dimensional MHD wave leads to the formation of a rotational discontinuity and a compressive steepened quasi‐linearly polarized pulse whose structure is similar to that of a finite amplitude magnetosonic simple wave. For small propagation angles, the pulse mode (fast or slow) depends on the value of β with respect to unity while for large propagation angles the wave mode is fixed by the sign of the initial density‐field correlation. The two‐dimensional evolution shows that an MHD wave is unstable against a small‐amplitude long‐wavelength modulation in the direction transverse to the wave propagation direction. A “two‐dimensional magnetosonic wave” solution is found, in which the density fluctuation is driven by the corresponding total pressure fluctuation, exactly as in the one‐dimensional simple wave. Along with the steepening effect, the wave experiences both wave front deformation and a self‐focusing effect which may eventually lead to the “collapse” of the wave. The results compare well with observations of MHD waves in the E

ISSN:0148-0227    

DOI:10.1029/90JA01588

年代:1991

数据来源: WILEY

摘要:

Thermospheric wind data obtained from the Atmosphere Explorer E and Dynamics Explorer 2 satellites have been combined with wind data for the lower and upper thermosphere from ground‐based incoherent scatter radar and Fabry‐Perot optical interferometers to generate a revision (HWM90) of the HWM87 empirical model and extend its applicability to 100 km. Comparison of the various data sets with the aid of the model shows in general remarkable agreement, particularly at mid and low latitudes. The ground‐based data allow modeling of seasonal/diurnal variations, which are most distinct at mid latitudes. While solar activity variations are now included, they are found to be small and not always very clearly delineated by the current data. They are most obvious at the higher latitudes. The model describes the transition from predominately diurnal variations in the upper thermosphere to semidiurnal variations in the lower thermosphere and a transition from summer to winter flow above 140 km to winter to summer flow below. Significant altitude gradients in the wind are found to extend to 300 km at some local times and pose complications for interpretation of Fabry‐Perot obser

ISSN:0148-0227    

DOI:10.1029/91JA00251

年代:1991

数据来源: WILEY

摘要:

In situ measurement of electron density height profile of the equatorial nighttime ionosphere, under a developing spread F event, was carried out using a dual mode high frequency capacitance (HFC) probe that was flown on board a SONDA‐III rocket launched from Natal on December 11, 1985. This represents the first measurement of plasma bubble characteristics using an HFC probe. A series of plasma bubbles in varying degrees of their growth phase was detected mostly during the upleg of the flight. A somewhat detailed discussion, and comparative studies, of the measurements at the two probe frequencies (6.17 MHz and 11.75 MHz) are presented in this paper. Among the important findings from the present study is an evidence of electron temperature enhancement in the plasma bubble possibly caused by electron heating from energetic particle precipitation in the South Atlantic magnetic anomal

ISSN:0148-0227    

DOI:10.1029/90JA02384

年代:1991

数据来源: WILEY

摘要:

We examine the implications of a traditional assumption of exospheric physics, that collisions of hydrogen atoms and protons preferentially result in charge exchange with negligible momentum transfer. Initially adopted as a necessary convenience to accommodate limited computer resources in exosphere model calculations, this approximation results in a direct transformation of the proton velocity distribution into a hot component of neutral hydrogen. It leads to significant enhancements in both escape and in the formation of extended planetary coronae. With expanding computational facilities, the need for the approximation has passed. As the first step toward its replacement with a realistic, quantum mechanical model of the H‐H+collision process, we calculate differential and cumulative cross sections for quantum elastic scattering of indistinguishable nuclei for a fine grid of encounter energies and scattering angles. These data are used to study the nature of ionosphere‐exosphere coupling through H‐H+collisions, and to demonstrate that the distribution of velocities of scattered H produced in the traditional exospheric charge exchange approximation, as well as that arising from an alternative, fluid dynamics approach, leads to unacceptable abundances of coronal atoms in long‐term, highly elliptic traje

ISSN:0148-0227    

DOI:10.1029/90JA02744

年代:1991

数据来源: WILEY

摘要:

Observations of the aurora from South Pole station (magnetic latitude = −74.2°) have been used to study the intensification and fading of polar arcs observed near the dusk meridian. Most of the cases examined have the following features in common: (1) a preexisting auroral form intensifies for about 10 min; (2) this activation is followed by a pronounced decrease of luminosity; (3) the auroral fade terminates after 30–60 min with the onset of intense aurora which sweeps rapidly overhead. The brief intensifications involve O (630.0 nm) enhancements of 0.1–0.3 kR and N2+(427.8 nm) enhancements of 0.3–1.5 kR giving an emission ratio which in each case is consistent with the precipitation of electrons with an average energy of 3–12 keV, assuming a Maxwellian distribution. The auroral brightening is not accompanied by riometer absorption. Ground magnetic perturbations of the order 20–40 nT occur, and in about half the instances, weak broadband impulsive hiss is present. The availability of all‐sky camera, auroral electrojet (AE) index and interplanetary magnetic field (IMF) data for some of the cases enables the following additional characterizations of these events. The preexisting form is a Sun‐ or oval‐aligned arc (or part of a multiple arc system) which disappears following the activation; equatorward drift of the arc (or system) accompanies the luminosity change. There is some evidence to suggest that the arc is poleward of the auroral oval. The brief intensification and/or the onset of fading occurs during the growth phase or near the start of the expansive phase of a substorm; termination of the fade is near the maximum in AE and is probably indicative of the beginning of the recovery phase of the substorm. For all three cases for which IMF data were available the onset of fading occurred 20–30 min after Bzturned southward. Two of these cases were preceded by auroral arc enhancement when Bxwas sunward; in the third case there was no prior enhancement when Bxwas zero or slightly antisunward. Bywas negative (dawnward) in each case. Sun‐aligned arcs are a common feature of the polar cap during northward Bzbut disappear during the increasingly disturbed conditions that accompany southward Bz. The present results suggest that brief intensifications of southern hemisphere polar cap arcs near dusk may be linked in part to the sunward orientation of the IMF which favors enhanced electron fluxes in the southern l

ISSN:0148-0227    

DOI:10.1029/91JA00256

年代:1991

数据来源: WILEY

摘要:

ISEE 2 magnetometer data and ISEE 1/2 particle data from the first half of 1978 were examined to search for signatures of field‐aligned currents in the Earth's inner magnetosphere (2.6–9.5RE, L= 4.8–13.1). Eighteen field‐aligned current sheet “events” were found. Most of them occur during magnetically disturbed periods (AE= 226–724 nT). The surface current densities range from 12 to 113 mA/m and are compatible with magnitudes measured at low altitudes by TRIAD. In nearly every “event” gradients in the proton flux (32–50 keV and 80–126 keV are found in the vicinity of the field‐aligned currents indicating that the currents are located on or adjacent to the outer boundary of the plasma sheet. Using the Tsyganenko‐Usmanov magnetospheric field model to trace the field line from where the field‐aligned current signature was observed down to the ground, we found that the pattern is consistent with Iijima and Potemra's (1978) observation of large‐scale Birkeland currents in the polar region. Geomagnetic field data on the nightside, measured by about 10 observatories for each “event”, were used to study the magnetic field behavior on the ground. Even though we found both region 1 and region 2 currents from the ISEE data in the inner magnetosphere during magnetically disturbed periods, the ground magnetic signatures favor only the region 1 currents and also indicate the tran

ISSN:0148-0227    

DOI:10.1029/90JA02745

年代:1991

数据来源: WILEY