G. J. Romick

Energy distribution of energetic O+ precipitation into the atmosphere

Measurements of ring current ion densities by the Active Magnetospheric Particle Tracer Explorers CCE satellite suggest that a large flux of energetic (tens of keV) O+ precipitates in the mid-latitude atmosphere during major geomagnetic storms. The mass spectrometer (>30 keV) on board the NOAA 6 satellite measured an ion flux of 30 ergs cm−2 s−1 at 52° invariant latitude during a very large geomagnetic storm. To calculate atmospheric response to precipitation of the energetic O+ fluxes, we have revised an O+ transport model to include energies up to 200 keV. The models sensitivity to the estimated cross sections and model atmospheres for various monoenergetic O+ fluxes are presented in this paper. Calculation of the atmospheric response to O+ precipitation shows that (1) most of the incident energy is immediately transformed into atmospheric heating, (2) the total number of escape particles is smaller than the total number of incident particles, in contrast to the results of previous models, and (3) the peak heating and ionization altitudes vary from 104 to 300 km depending on the incident energy. The results are most sensitive to the estimated differential scattering cross sections at large scattering angles. The use of forward scattering cross sections at both extremes of the possible range results in very different energy allocations and altitudes of peak ionization and heating (a difference of as much as 80 km). The MSIS-86 model atmosphere with extreme parametric values caused little change in the energy allocation, but the large variation in the model atomic oxygen density (F10.7 index = 70 and 210) alters the peak heating and ionization altitude for low-energy incident O+ (a few keV) by 50 km.

Altitudes of polar mesospheric clouds observed by a middle ultraviolet imager

The first images of Earth limb in the middle ultraviolet (235–263 nm) have revealed in detail the altitude structures of polar mesospheric clouds (PMCs). The images were obtained from the Ultraviolet and Visible Imaging and Spectrographic Imaging (UVISI) instrument on the Midcourse Space Experiment (MSX) spacecraft during the austral summer of 1997–1998. The satellite made multiple passes over the Antarctic and obtained over 750 images of PMCs at latitudes poleward of 70°S. Even without correction for scene backgrounds, the imager easily observed PMCs distinct from the atmospheric backgrounds. The clouds appeared as discrete, filamentary structures having altitudes between 80 and 85 km, although most PMCs appeared at altitudes between 82.0 and 83.0 km with a mean of 82.3 ± 0.8 km. The clouds were randomly distributed on a trans-polar scale of ∼1000 km, although in some instances the clouds clustered for distances of 200–300 km across the polar mesosphere. In other instances, PMCs were wholly absent on the mesospheric horizon. The imager also noted enhanced radiances on the topsides of the PMC altitude profiles; this excess radiance may be caused by “subvisible” particles not apparent at visible wavelengths. The PMC altitudes do not appear correlated with latitude or local time on the scale of the observations discussed here.

Atmospheric remote sensing using a combined extinctive and refractive stellar occultation technique 1. Overview and proof-of-concept observations

[1]xa0A new approach to optical remote sensing of the Earths atmosphere using a combination of extinctive and refractive stellar occultation measurements is presented. In this combined method, spectrographic imagers are used to measure the wavelength-dependent atmospheric extinction of starlight while a co-aligned imager is used to measure the atmospheric refraction along the same line of sight. By simultaneously measuring both the refraction and extinction of the starlight, the composition-dependent extinction measurements can more accurately probe the Earths lower atmosphere, where refraction effects are significant. The refraction measurements provide the bulk atmospheric properties and the actual light path through the atmosphere, both of which are necessary to correctly infer the total extinction in the refractive regime. The technique is demonstrated on a proof-of-concept basis using data from the Ultraviolet and Visible Imagers and Spectrographic Imagers (UVISI) on the Midcourse Space Experiment (MSX) satellite. These preliminary results show that the combined approach has the potential to be a powerful, self-calibrating method for remotely sensing the Earths atmosphere in general and for the determination of ozone profiles in the stratosphere and upper troposphere in particular.

Transpolar structure of polar mesospheric clouds

Middle-ultraviolet (210- to 252-nm) images have revealed the transpolar structure of polar mesospheric clouds (PMCs) at a spatial resolution of 3 km. The ultraviolet and visible imaging and spectrographic imaging instrument on the Midcourse Space Experiment (MSX) satellite collected over 27,000 mid-UV images of PMCs during 26 passes over the north and south polar regions during the summer seasons of 1997, 1998, and 1999. A Lomb periodogram analysis of PMC radiance projected to an 83-km altitude reveals periodic structures with wavelengths ranging from ∼100 to ∼3000 km. In either hemisphere, more PMCs have features with wavelengths shorter than 1000 km than longer, and a crude spectrum of the PMC structures suggests a spectral peak between 500 and 1000 km. There is little evidence of structures having wavelengths short of ∼100 km, and the longer wavelengths generally have more spectral “power” than the shorter wavelengths. PMC structures do not remain stable over time periods of weeks, but may retain similar structural features for at least as long as 24 hours. The clouds may be considered markers of gravity waves, which carry energy from the lower atmosphere to the mesosphere and modulate the appearance of PMCs.

Midcourse Space Experiment/Ultraviolet and Visible Imaging and Spectrographic Imaging limb observations of combined proton/hydrogen/electron aurora

Simultaneous measurements of auroral limb H Lyman α, H Balmer α, H Balmer β, N2+ 1 NG 391.4-nm, and N2 2 PG 337.1-nm emissions excited by combined proton/hydrogen/electron precipitation are reported. The data were recorded by the Ultraviolet and Visible Imaging and Spectrographic Imaging (UVISI) spectrographic imagers on the Midcourse Space Experiment (MSX) satellite on November 10, 1996, while viewing a diffuse emission region during a limb-scanning data collection event. Energy fluxes associated with the electron and proton/hydrogen precipitation components are estimated with one-dimensional fits to selected limb profiles using a transport-theoretic model. Spectral radiances (110–900 nm) are also presented. The data presented here are but one example of the large number of MSX/UVISI data sets that have been collected offering opportunities for scientific investigations of auroral emission phenomena and for retrieval of composition and particle precipitation parameters from optical remote sensing data.

Variations, with peak emission altitude, in auroral O2 atmospheric (1, 1)/(0, 1) ratio and its relation to other auroral emissions

Spectral distributions of auroral optical emissions, peaked at distinctly different heights in the thermosphere, show significant variations, with altitude, in the O2 atmospheric (1, 1)/(0, 1) band ratio. The latter increases with height in auroras peaked between 110 and 150 km and then gradually decreases at higher altitudes. To minimize ambiguities associated with auroral height determination needed for investigating this effect, four independent height-assessment methods are employed. The first one is based on the incoherent scatter radar (ISR) soundings of the auroral ionization profile from which the height, where precipitating particles dissipate most of their energy, can be determined. Concurrent spectroscopic observations of the thermalized rotational distributions of auroral band emissions yield the ambient air temperature, and hence an independent assessment of the height, of the thermospheric region where these emissions peak. Changes in O/O2 and O/N2 ratios with height lead to changes in the ratios of auroral emissions, from these species, peaked at different heights. Finally, changes in collision frequency with height lead to changes in the brightness of the auroral emissions, resulting from radiatively allowed transitions relative to those produced from radiatively forbidden transitions. The four methods yield comparable values for the height of the thermospheric region where emissions, from each auroral event, peak. The observed variations in O2 atmospheric (1, 1)/(0, 1) with auroral height is compared with that expected from O (1D) + O2 excitation source and quenching by O2 and O. The effects of electron impact excitation of O2(b1∑g+, v′) and high rotational levels of the P branch of O2 atmospheric (0, 0) band on O2 atmospheric (1, 1)/(0, 1) ratio are discussed. Quantitative ratios of various auroral emissions, from O, N2, and N+2, peaked at different heights, that can provide an assessment of auroral heights where these emissions peak, are listed.

Evidence for bimodal particle distribution from the spectra of polar mesospheric clouds

[1]xa0The spectrographic imagers on the MSX satellite have made the first observations of the middle ultraviolet spectra (200–315 nm) of polar mesospheric clouds (PMCs). Dividing the PMC spectra by the solar spectrum yields a scattering spectrum expressible as a matrix-vector formalism of Mie scattering functions and the particle distribution. Using this formalism, PMC particle distributions are related to the observed scattering spectrum. The scattering spectrum always exhibits a peculiar “hump” at ∼260 nm that cannot be explained by any effect other than the particle distribution. A lognormal distribution of small particles (mode ∼ 50 nm) produces the overall shape of the spectrum but not the “hump.” Although not unique, a simple bimodal distribution of small particles (r ∼ 50 nm) and large particles (r ∼ 200 nm) describes the scattering spectrum and its hump very well. The clouds may therefore consist of two different populations, as suggested by some models of the clouds. Numerically, smaller particles dominate by about 10:1, but the larger particles strongly influence the scattering spectrum.

Night uv spectra (1100–2900Å) at mid and low latitude during a magnetic storm

Spectral analyses of night atmospheric emission observations from the S3-4 satellite revealed enhanced OI 1304-A, OI 1356- A, and OI 1640- A, NI 1493- A, NI 1744- A and NI 12143-A lines and Lyman-Birge-Hopfield (LBH) band emissions at low magnetic latitudes. The satellite, at about 260-km altitude, was over the region between −41° to −15° geomagnetic latitudes at7:55 UT on August 28, 1978 (Dst = −193) during the main phase of a major geomagnetic storm (Maximum Dst = −242 at 10 UT). n nThe observed ratios of the LBH bands to the NI 1493-A and NI 1744-A line emissions are an order of magnitude lower than those produced by electron impact and are similar to laboratory measurements of heavy particle impact (H and H+) on N2. Significant precipitation of neutral O atoms, formed by charge exchange of the ring current O+ with exospheric hydrogen and oxygen, can occur when |Dst| is large. Oxygen is observed to dominate the ring current at corresponding low L-values during a storm main phase. Consideration of these and other factors leads us to conclude that the most likely cause of the observed emissions is oxygen precipitation from the ring current.

Model calculation of atmospheric emission caused by energetic O+ precipitation

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−2 s−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), N2 second positive(2P), and N2 Lyman-Birge-Hopfield (LBH) bands; the N I lines at 1493A, 1744A, and 8680A; the N II line at 5005A, and the O I lines at 1304A, 1356A, and 6300A. Most atomic oxygen line emissions are from primary oxygen atoms traveling downward at a speed of 107 and 108 cm s−1. Because of the high speed at which the atoms travel, most ¹D is quenched before emission. Atomic oxygen line spectra at 1304A and 1356A display a 1A 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 caused by O+ precipitation.

Ultraviolet imaging and spectrographic imaging of polar mesospheric clouds

Abstract The Ultraviolet and Visible Imaging and Spectrographic Imaging (UVISI) instrument on the Midcourse Space Experiment (MSX) satellite obtained unique ultraviolet (200–350 nm) images and spectrographic images of polar mesospheric clouds over both the northern and southern poles during the summers of 1997–1999. Imager observations revealed that PMCs have essentially the same altitudes in the north and south (82.7 ± 1.3 km) and that cloud altitudes remain essentially constant with latitude. Imagery from below the horizon showed PMC structures with scales ranging from 100 to over 500 km. Integrated from 235 nm to 265 mn, PMC limb intensities were essentially the same in both hemispheres, averaging 12 mega-Rayleighs at peak altitude. Intensities increased with latitude toward the poles. Spectrographic observations reveal the clouds have a solar-like spectrum from 200 nm to 600 nm, but that short of 315 nm the spectrum is contaminated with ground albedo effects. Between 200 nm and 315 nm, the scattering ratio (PMC spectral intensity to solar irradiance) is less steep than that expected from pure Rayleigh scattering. When the scattering ratio is least-squares fit to a log-normal distribution of Mie scatterers, the distribution has a mode radius of 65 nm and a dispersion of 1.2 with no dependence on latitude or time from solstice. The slope of the scattering ratio changes as a function of altitude relative to the altitude peak of the PMC intensity, suggesting that particles 3 km above this peak are smaller than particles at the peak and 3 km below.