摘要:

We have measured the vacuum ultraviolet (VUV) emission cross sections of SO2from 40 to 200 nm in the laboratory in a crossed beam experiment. The wavelength range comprised the extreme ultraviolet (EUV) spectrum, from 40 to 120 nm, and the far ultraviolet (FUV) spectrum, from 120 to 200 nm. The EUV and FUV emission spectra induced by electron impact consist of atomic multiplets of S I, II, and III and O I, II, and III. The studies included the measurement of excitation functions of the strongest multiplets from 0 to 1 keV impact energy and the identification of dissociation processes from the analysis of the threshold excitation function. The O I (98.9 nm) resonance multiplet (g3P‐3D) is the strongest EUV feature from electron impact induced fluorescence of SO2with a peak cross section of 7.82 ± 1.59 × 10‐19cm2at 125 eV electron impact energy. The O I (130.4 nm) multiplet (g3P‐3S) resonance transition is the strongest feature in the FUV, with a peak cross section of 1.98 ± 0.44 × 10‐18cm2at 90 eV. The strongest feature of S I in the VUV is the S I (147.9 nm) multiplet (g3P‐3D) resonance transition, with a peak cross section of 1.52 ± 0.33 × 10‐18cm2at 90 eV. Dissociation‐excitation cross sections of SO2are important contributors to the total inelastic cross section of SO2gas interacting with electrons. These cross sections are used in neutral cloud theory models of the Io atmosphere. The laboratory spectrum at low energy (~22 eV) matches a recently obtained IUE FUV spectrum of Io. The comparison represents evidence of a thin or intermediate atmosphere of SO2that is directly impacted by the plasma torus. The existence of an atmosphere on Io embedded in the corotating torus plasma is an important remote sensing target for the Hubble Space Telescope, Goddard High Resolution Spectrograph and Faint Object Spectrograph, and Galileo EUV and FUV spectrometers, among other present and planned VUV studies o

ISSN:0148-0227    

DOI:10.1029/92JA00296

年代:1992

数据来源: WILEY

摘要:

We have measured the middle ultraviolet (MUV) electron impact induced fluorescence spectrum of SO2from 200 to 430 nm in a crossed beam experiment. The spectrum is dominated by two features at the experimental resolution of 0.5 nm. These two features are referred to as MUV1 and MUV2. MUV1 is the SO(A3∏ →X3∑‐) band system extending from 240 to 265 nm produced by dissociative excitation. MUV2 is a blend of theSO2(ā(1A2),B~(1B1)→X~(1A1))andSO2+(C~(2B2)→X~(2A1))molecular band systems in the range 264‐430 nm. In addition, the excitation function measurements of MUV2 indicate that low‐energy electrons effectively prepare SO2in one or more electronically excited triplet states that involve the SO2(ā(3B1)) state by direct excitation and/or cascading. A candidate triplet band system for this emission process is theE~‐āelectronic transition. This emission process is the largest contributor to the MUV2 emission cross section at low electron impact energy. The peak cross section for MUV1 occurs at 20 eV with a value of 25.0 ± 5.5 × 10‐19cm2. The peak cross section for MUV2 arises at 9 eV with a value of 368 ± 81 × 10‐19cm2. The laboratory measurements of the excitation functions of both MUV features were made from 0 to 1 keV. The emission cross sections are an important part of the total inelastic cross section of SO2needed in modeling the Io atmosphere. The laboratory results give a plausible explanation of the Io auroral hot

ISSN:0148-0227    

DOI:10.1029/92JA00295

年代:1992

数据来源: WILEY

摘要:

The response of theE‐F1region ionosphere to different solar EUV flux models is investigated theoretically using two different photochemical schemes, and the results are compared with incoherent scatter radar electron density measurements taken at Millstone Hill. The latest EUV flux model (Tobiska, 1991), which incorporates more recent measurements, has generally more flux at short wavelengths compared to the Hinteregger et al. (1981) flux model based on AE‐E satellite data. This results in better agreement with the measurements in theE‐F1region and above. The Tobiska flux model, however, gives a smallerEregion peak density, due to the influence of low Lyman ß flux in the November 10, 1988, rocket measurements of Woods and Rottman (1990). The photochemical scheme of Buonsanto (1990) has been improved and now gives results similar to the more comprehensive scheme of (Solomon et al., 1988; Solomon and Abreu, 1989; S. C. Solomon and R. G. Roble, Simulation of the global thermospheric airglow, 1, Methodology, submitted to Journal of Geophysical Research, 1992), provided that the ratios of photoelectron impact ionization to photoionization (pe/pi) given by this latter model are included. The pe/pi ratios calculated by this model and by the models of Lilensten et al. (1989) and Richards and Torr (1988) differ significantly, and work is needed to resolve these differences. In general, the photochemical model results underestimate the data, especially in winter. This result agrees with that of the earlier paper by Buonsanto and could be resolved by decreasing MSIS‐86 N2and O2densities in winter if additional ions were produced in theEregion either by photoionization or by photoelectron impact ionization. The photoionization and photoabsorption cross sections of Conway (1988) give results in somewhat better agreement with observations than the cross sections of Torr et al. (1979). For the zenith angles considered (daytime conditions), the Chapman function method for calculating photoabsorption gives results in satisfactory agreement with a more rigorous calculation method using a formula from Ree

ISSN:0148-0227    

DOI:10.1029/92JA00792

年代:1992

数据来源: WILEY

摘要:

We present a solar EUV index for aeronomical studies at Earth, obtained from the Langmuir probe measurement of photoelectron current on the Pioneer Venus orbiter (PVO). When the probe is in the solar wind and is negatively biased, the current,Ipe, is mainly due to the impact of solar EUV photons with energies above the work function of the probe.Ipethus measures the total EUV flux over the wavelength interval from about 30 nm to Lyman alpha; on Venus this solar flux,VEUV, was defined by Brace et al. (1988). When translated to the solar longitude of Earth, for Earth use, it is calledEEUV. To examine the potential ofEEUVas a solar EUV flux index, we study the behavior of ionospheric parameters ƒ0E, ƒ0F1, and ƒ0F2at mid‐latitude stations and compare their relationship withEEUVand with the 10.7‐cm solar radio flux. We find ƒ0F1and ƒ0F2to be better correlated withEEUVthan with the 10.7‐cm flux. However, ƒ0Eis better correlated with the 10.7‐cm flux, because the 10.7‐cm flux is also a proxy for soft X rays, which are an important ionizing source in theEregion. We also provide a table of the EUV index,EEUV, for the period February 12, 1979, through most of 1991 which extends in time the table (of Venus values) published earlier by Brace et al. (1988). The EUV index will continue to grow until the end of the PVO missi

ISSN:0148-0227    

DOI:10.1029/92JA00384

年代:1992

数据来源: WILEY

摘要:

Observations of thermospheric parameters, made from the Dynamics Explorer 2 (DE 2) spacecraft during three successive orbital crossings of a quiescent dusk sector auroral arc, have been compared with the predictions of three fine‐grid auroral arc models. DE 2 measured the ion and neutral winds, electron and neutral temperatures, neutral composition, and energetic auroral electron spectra (5 eV to 32 keV) at ∼320 km altitude. These observations were at high spatial and temporal resolution, suitable for comparisons with the models. The observed zonal and meridional neutral winds near the arc were greater than the model predictions, probably because of the presence of stronger electric fields and higher ion and neutral densities during the observations than were used in the models. The measured vertical winds were also larger than the corresponding model values. The DE 2 composition measurements showed a local increase of the N2/O ratio in the arc, which is interpreted to be a result of the upward motion of N2‐rich air. A reduction in the measured neutral temperature of ∼100 K in the arc, relative to temperatures on either side, was found for all three arc crossings. Of the three theoretical models examined, only one shows any tendency for the neutral thermospheric temperature to drop in the auroral arc, and the decrease calculated is significantly less than the observed temperature change. A simple calculation of the adiabatic cooling effect of the observed upward motion yields a temperature drop of ∼160 K, comparable with the observed temperature reduction

ISSN:0148-0227    

DOI:10.1029/92JA00168

年代:1992

数据来源: WILEY

摘要:

We apply a collisional semikinetic model to the study of the topside ionosphere transition region. In this study we consider the transition from (1) O+to H+dominance, (2) subsonic to supersonic H+flow, and (3) collisional to collisionless plasma. We focus primarily on the H+ions as they flow upward from a relatively low altitude of 500‐1000 km to a high altitude of 7000 km. These ions are subjected to collisions with O+ions and self collisions, as well as the effects of macroscopic forces such as the magnetic mirror force, gravity, and the ambipolar electric force. We find that when supersonic outflow occurs, the H+velocity distribution function shows a large departure from Maxwellian with a large positive and then negative heat flow, particularly in the region where the flow Mach number passes through one. In cases where subsonic outflow occurs and the H+density is relatively low, distribution functions with two peaks can occur. In these situations the H+ions have large parallel temperatures and large heat flows when the two peaks are of unequal heigh

ISSN:0148-0227    

DOI:10.1029/92JA00643

年代:1992

数据来源: WILEY

摘要:

Observations from the Swedish satellite Viking allow us to study the relationship between auroral kilometric radiation (AKR) and discrete auroral features. Previous work has shown that AKR generation is most often associated with nightside aurora. We present wave data which show that under certain circumstances the source regions may also occur on discrete features, identified as transpolar arcs. The wave spectrograms detected during crossings or closest approaches to such sources exhibit structures similar to those observed during nightside AKR source crossings. Also, the associated ion beams and trapped conical electron populations with enhanced upward directed loss cones peak at comparable energies (∼1 keV

ISSN:0148-0227    

DOI:10.1029/92JA00410

年代:1992

数据来源: WILEY

摘要:

Recent results in the search for strong Langmuir turbulence effects during ionospheric modification experiments performed at the Arecibo Observatory are presented. Indirect evidence of Langmuir wave collapse is obtained through the observation of theoretically predicted “caviton‐type” enhanced plasma waves spectra using the 430 MHz incoherent radar at Arecibo. A typical spectrum consists of a “free‐mode” peak with a frequency that is significantly higher than the heater frequency, and a broad “caviton continuum” with frequencies below the heater frequency. Free modes are freely propagating Langmuir waves radiated by collapsing cavitons during collapse. The generation and dynamics of these “free modes” will be discussed. Asymmetries between the frequency shifts and strengths of the upshifted and downshifted free‐mode lines and their dependence on the time delay following the onset of heating are explained in terms of the radiation of free Langmuir modes by cavitons and the subsequent propagation of free modes down or up the density gradient. Experimental results are compared with theoretical predictions. Results on the transition of “caviton‐type” plasma line spectra to the commonly observed “decay‐type

ISSN:0148-0227    

DOI:10.1029/92JA00645

年代:1992

数据来源: WILEY

摘要:

A comprehensive set of wave analysis techniques is applied to dayside ELF hiss observed over three typical orbits of the low‐altitude AUREOL 3 satellite. Validity domains are established for each technique, whose results indicate the following propagation characteristics. Within the plasmasphere the waves are narrow‐band, have a lower cutoff frequency close to the local proton gyrofrequency ƒH+, and propagate downward with very obliquekvectors directed toward lowerLvalues. Left‐hand mode waves are detected just below ƒH+. Within the plasmapause gradient the waves are broad‐band, have a lower cutoff frequency at the vicinity of ƒH+, and propagate mainly upward with obliquekvectors directed toward lowerLvalues. Within the light‐ion trough region and the auroral zones the waves are broad‐band, have lower cutoff frequencies that can be below the local ƒH+, and propagate downward withkvectors alongB0. Narrowband emissions detected in the vicinity of the cusp propagate upward. Exceptions are found at frequencies just above ƒH+where, at nearly all invariant latitudes, waves are commonly upgoing. We conclude that (1) the reflection at the two‐ion cutoff frequency is a very efficient process for returning waves to higher altitudes and (2) the large density gradients that characterize the plasmapause and the cusp seem to act as traps for waves reflected b

ISSN:0148-0227    

DOI:10.1029/91JA02929

年代:1992

数据来源: WILEY

摘要:

This study extends the work of Sutcliffe and Nielsen (1990) in which a classical Pi 2 pulsation was first isolated in Scandanavian Twin Auroral Radar Experiment (STARE) data. A high‐pass‐filtering technique is used to remove the background electric field in the STARE data and so reveal the spatial and temporal ionospheric signatures of the Pi 2 pulsation electric fields. A number of events are identified and examples presented in which pulsation electric fields up to 50 mV/m are observed. Magnetic field oscillations computed from the filtered STARE data using the Biot‐Savart law correlate well with pulsation magnetometer data. A 180° phase difference is observed between high‐ and low‐latitudeXcomponent pulsations. The ionospheric signature of a Pi 2 is located slightly poleward of the core of the auroral breakup region where the southward, westward, and northward directed background electric fields converge; the strongest pulsation fields occur in the region of equatorward directed electric fields. The ionospheric electric field patterns of the Pi 2 pulsations determined from the STARE data correlate well with those modeled for a transverse Alfvén wave incident on an east‐west aligned high‐conductivity strip in

ISSN:0148-0227    

DOI:10.1029/92JA00440

年代:1992

数据来源: WILEY