摘要:

In the study of UV airglow from the Earth's atmosphere, the N2Carroll‐Yoshino (CY)c4′1Σu+‐X1Σg+(0,0) and (0,1) Rydberg band emissions near 958 Å and 980 Å, respectively, are found to be weak relative to thec4′ (0) excitation rate. This result is surprising because laboratory measurements show that CY(0,0) and CY(0,1) are the brightest N2emission features between 910‐1010 Å even under optically thick conditions [Zipf and McLaughlin, 1978]. In order to investigate the cause of this weak emission quantitatively, we have developed a resonant fluorescent scattering model for CY(0,0) and CY(0,1). The model is intended to be comprehensive, including multiple scattering, extinction, branching, escape to space, predissociation, and temperature effects. Results show CY(0,0) photons are radiatively trapped and undergo resonant fluorescent scattering accompanied by substantial loss in the atmosphere. Indeed, the model predicts weak CY(0,0) intensities, consistent with observations. We find that the most important loss processes for the CY(0,ν″) system in the Earth's dayglow are predissociation and branching to CY(0,1) followed by absorption by the overlapping, 100% predissociated Bridge‐Hopfield I (BH I)b1Πu(2) ‐X1Σg+(0) band. Near solar minimum, model CY (0,1) and (0,2) dayglow zenith intensities between 160‐170 km range between 4‐9 R and 0.5‐1.5 R, respectively, where the lower number assumes 16.5% predissociation of thec4′(0) state and the higher number assumes 1% predissociation. These intensities are all consistent with observations reported by Morrison et al. [1990]. For the Earth's aurora, model CY(0,1) and (0,2) intensities averaged between 88°‐96° from the zenith at 118.5 km range between 60‐180 R and 150‐390 R, respectively, and CY(0,2) intensities at 170 km range between 200‐360 R. These results are consistent with upper limits from Feldman and Gentieu (1982) if the probability ofc4′ (0) predissociation is at least 9%. We also present qualitative arguments to explain the relatively b

ISSN:0148-0227    

DOI:10.1029/93JA01996

年代:1994

数据来源: WILEY

摘要:

Some anomalous auroral emissions, typically observed below 60° geomagnetic latitude during large geomagnetic storms, have distinctive spectral characteristics, that have been attributed to energetic ion/neutral particle precipitation. Mass spectrometers on satellites have observed energetic (keV) ion and neutral precipitation (up to 30 erg cm−2s−1) below 60° geomagnetic latitude. In this study we used a model to calculate emissions with the characteristics of the incident O+energy spectra observed from satellites and compared the emission intensities and intensity ratios of the model calculation to those from the anomalous auroral emission spectra. Using an oxygen transport model with estimated emission cross sections, we calculated the emission altitude distributions, vertically integrated column emission intensities and the spectral profile of atomic oxygen line emissions. The calculated emissions were for the N2+and O2+first negative(1N), N2second positive(2P), and N2Lyman‐Birge‐Hopfield (LBH) bands; the N I lines at 1493Å, 1744Å, and 8680Å; the N II line at 5005Å, and the O I lines at 1304Å, 1356Å, and 6300Å. Most atomic oxygen line emissions are from primary oxygen atoms traveling downward at a speed of 107and 108cm s−1. Because of the high speed at which the atoms travel, most ¹D is quenched before emission. Atomic oxygen line spectra at 1304Å and 1356Å display a 1Å Doppler shift and some broadening if viewed from above or below. The emission altitudes and intensities are most sensitive to the choice of elastic scattering cross sections and emission cross sections, respectively. The emission intensity ratios of both the N2+1N to N22P band and the N2LBH bands to the atomic lines, and the Doppler shift of atomic O lines strongly depend on the energy spectrum of incident O+. Seasonal and latitudinal model atmospheric differences have least impact. With an energy spectrum of incident O+, the general model results agree with the observed anomalous auroral emission spectra that were interpreted as being cau

ISSN:0148-0227    

DOI:10.1029/93JA01148

年代:1994

数据来源: WILEY

摘要:

In magnetospheric plasmas it is possible for plasma gradients to cause error in electric field measurements made by satellite double probes. The space charge enhanced plasma gradient induced error is discussed in general terms, the results of a laboratory experiment designed to illustrate this error are presented, and a simple expression that quantifies this error in a form that is readily applicable to satellite data is derived. The simple expression indicates that for a given probe bias current there is less error for cylindrical probes than for spherical probes. The expression also suggests that for Viking data the error is negligible.

ISSN:0148-0227    

DOI:10.1029/93JA00432

年代:1994

数据来源: WILEY

摘要:

The Langmuir plasma probe model is an important tool in spacecraft current collection and charging calculations. In ideal geometries, such as a sphere or an infinitely long cylinder, the model is well understood. However, the realistic geometries of current collectors, or spacecraft, are nonideal. An empirical formula for a Langmuir probe with a given nonideal geometry would be useful. We derive such a formula for the SCATHA satellite by using the SC10 potential data obtained during electron beam emissions. The satellite rotated perpendicular to sunlight with the SC10 booms in the equatorial plane. We choose one special mode of operation during a quiet space environment. In this mode the beam current increased continuously, while the energy remained constant. We analyzed the variations of the vehicle potential responding to the unique driving factor, the beam current. To provide physical explanation to the behavior of the SC10 potential data, we model the interactions between the beam, photoelectron, and ambient currents. We present an algorithm which successfully yields an empirical Langmuir probe formula for SCATHA, from which we obtain improved estimate of ambient electron temperatures and densities. The results predicted by the improved Langmuir probe model compare favorably with the very few published measurements from the region.

ISSN:0148-0227    

DOI:10.1029/93JA02728

年代:1994

数据来源: WILEY

摘要:

Collection of electrons by a satellite at a relatively high potential in low Earth orbit, including the effects of the satellite's orbital motion, remains a poorly understood phenomenon. Using a test particle simulation in which charged particle motion is tracked in prescribed electric fields, we calculate here the current collection and the nature of the energy distribution function of the electrons collected by the satellite, including the effects of the satellite orbital motion. Calculations of the collected current without the orbital motion show an excellent agreement with the current from the Parker‐Murphy model, but with the orbital motion of the satellite the current collection is enhanced, the degree of enhancement depending on the size of the satellite sheath extending along the magnetic field line. In the latter case, the flow of electrons around the satellite shows some interesting behaviors including azimuthalE×Bdrift around the satellite, axial trapping along the magnetic field, and formation of field‐aligned flow of electrons in the wake region. The total energy of the collected electrons is ≃eϕo, where ϕois the satellite potential, but the partition of the energy into components parallel (W∥) and perpendicular (W⊥) to the magnetic field shows interesting features. For the magnetic field along the polar axisZ, the energy distributions near the poles are perfectly field aligned, that is, the parallel energyW∥=eϕo. The perpendicular component (W⊥) progressively increases toward the “equator” of the spherical satellite. For a sufficiently large bias potential ϕo, the equatorial energy distributions f(W∥) and f(W⊥) show fine structures having multiple peaks. These features of the distribution functions result from the chaotic motion of electrons and the associated coupling between the electron motions parallel and perpendicular to the magnetic field in strongly nonuniform electric

ISSN:0148-0227    

DOI:10.1029/93JA01642

年代:1994

数据来源: WILEY