Andrew C. Nicholas

Variations in the FUV dayglow after intense auroral activity

A localized {approximately}55% decrease is observed in the brightness of Earth`s FUV dayglow in the morning sector at 130.4 nm after an interval of intense geomagnetic activity. This large decrease is interpreted as being the consequence of auroral-associated heating which reduces the thermospheric column density of O relative to that of N{sub 2}. Spatial extent of the observed decrease exceeds 10{sup 7} km{sup 2} at the {minus}25% level, and the depth of the decrease lessens during more than two hours of observations. These remote observations provide the first instantaneous, two-dimensional measurement of the large-scale spatial extent of such a change in thermospheric composition. 16 refs., 4 figs., 1 tab.

A Survey of Large-Scale Variations in Thermospheric Oxygen Column Density with Magnetic Activity as Inferred from Observations of the FUV Dayglow

Brightness of the terrestrial far-ultraviolet (FUV) dayglow is dominated by the 130.4-nm emission of neutral atomic oxygen, O I, and variations in the brightness observed from altitudes high above the emitting region reflect variations in thermospheric oxygen density. This paper summarizes the results of an initial survey of the Dynamics Explorer 1 observations of the FUV dayglow through a presentation of 13 representative events selected to demonstrate the spatial extent and short-term temporal stability of the brightness perturbations. The emphasis here is on the morning sector of local time and the polar cap for observations obtained in the time interval from September 23, 1981, through January 19, 1982. An analytic expression is derived for the average response of the FUV photometer to the dayglow during periods of high-latitude magnetic quiescence. The remaining observations in this time interval are then analyzed for their deviations from the established quiet time values. Deviations of −40% to +30% are found following intervals of increased magnetic activity. The most significant decreases (−30% to −40%) are observed equatorward of the instantaneous auroral oval only after sustained periods (∼6 hours) of intense magnetic activity (average AE greater than ∼700 nT). Decreases extend equatorward from the aurora to geographic latitudes as low as ∼30° N. Decreases of lesser magnitude that do not extend as far equatorward are associated with sustained periods of more moderate activity in which the average value of AE is smaller (∼300–400 nT). Also, the spatial extent and magnitude of the decreases in the morning sector appear greater when the IMF By component is positive. In both cases, decreases are readily observed within the polar cap. Localized enhancements of +20% to +30% occur much less frequently and are detected at the middle latitudes, well equatorward of the auroral oval.

An empirical model of the OI FUV dayglow from DE-1 images

Abstract A refined empirical model of the Dynamics Explorer-1 far-ultraviolet (FUV) imaging photometer’s response to Earth’s quiet time FUV dayglow has been developed for thermospheric studies. The mean photometer response is based upon FUV observations in 156 images obtained during the first five months of imager operations (September 1981–January 1982) and is determined as a function of solar and satellite zenith angles, observational azimuth and solar clock angles, and solar radio flux. Variations with each parameter are characterized and, where possible, fitted with an appropriate function. The fitted response, based on the n-th power of the cosine of the solar zenith angle, is within 10% of actual mean values at all observed solar and satellite zenith angles and is consistent with the results of a first-principles calculation. Subtraction of the model background from other DE-1 images indirectly reveals the enhancement or diminution of thermospheric O/N2 column density ratios due to transport and Joule heating effects. An analysis of summer storm-time images from the Southern Hemisphere demonstrates the use of the model in revealing these effects. The technique developed here is readily applicable to other FUV data sets.

Atomic oxygen in the thermosphere during the July 13, 1982, solar proton event deduced from far ultraviolet images

The far ultraviolet (FUV) analysis technique of Strickland et al. [this issue] is used to infer height-integrated O/N2 column densities from O I-130.4-nm FUV dayglow images taken by the Dynamics Explorer 1 (DE 1) spin scan auroral imager (SAI) [Frank et al., 1981] during a large solar proton event that occurred on July 13, 1982. The FUV results show that O/N2 decreases in the northern hemisphere as a result of seasonal and storm time effects. Furthermore, these decreases differ significantly from corresponding predictions made by the MSISE-90 model of Hedin et al. [1991]. The FUV-derived O/N2 column densities are compared to in situ volume density and neutral temperature ([O], [N2], Tn) measurements made from the Dynamics Explorer 2 (DE 2) satellite [Carignan et al., 1981; Spencer et al., 1981]. The comparisons are made using two techniques, In the first approach, ground truth estimates of O/N2 column densities are made from the in situ measurements. In the second approach, [O] volume densities are inferred from the FUV measurements of O/N2 column density and directly compared to direct DE 2 neutral atmosphere composition spectrometer (NACS) measurements of [O]. While some discrepancies exist, the results show that reasonable estimates of the atomic oxygen volume density [O] during the July 13, 1982, solar proton event could be made using the FUV images.

Electron densities determined by inversion of ultraviolet limb profiles

We present electron density profiles derived by inversion of ultraviolet limb radiances observed on November 24, 1999, by the Low-Resolution Airglow and Aurora Spectrograph instrument on the Advanced Research and Global Observing Satellite. The solar 10.7-cm radio flux was 181 solar flux units, and the daily ap was 21, indicating moderate geomagnetic activity. The O+ density profile, which is approximately equal to the electron density profile in the F region ionosphere, was determined by inverting the limb radiance profile of O I 911-A emission of atomic oxygen. The 911-A emission is produced by radiative recombination of O+ ions and electrons. Sounding rocket and satellite measurements of the Earths extreme ultraviolet dayglow indicated significant contamination in the 900- to 920-A passband by emissions from atomic, molecular, and ionized nitrogen [Gentieu et al., 1979, 1981; Chakrabarti et al., 1983]. As a result of these observations, the radiative recombination emission was thought to be of little use for ionospheric sensing during the daytime. Feldman et al. [2001] have recently measured spectra in the 905- to 1184-A passband and show that the contamination at F region altitudes is less than that present in the earlier observations [Gentieu et al., 1979, 1981; Chakrabarti et al., 1983]. We found the contamination of the O I 911-A emission to be negligible at F region altitudes and have been able to use the 911-A emission to accurately characterize the ionospheric state. We have compared the peak electron density and peak height determined by inversion of the 911-A altitude profiles with nearly coincident ionosonde measurements, and we find the measurements from the two techniques to be in good agreement, demonstrating the accuracy of this technique for sensing the ionospheric state.

Enhanced empirical models of the thermosphere

Abstract The Naval Research Laboratory (NRL) has embarked on a development program to upgrade empirical models of the neutral upper atmosphere (thermosphere and upper mesosphere) and to apply these models to scientific and engineering problems. The program focus has been the Mass Spectrometer — Incoherent Scatter Radar (MSIS) model of composition and temperature. The new NRLMSIS model, due for release in 2000, has ingested additional data sets, including, for the first time, the drag and accelerometer data of Jacchia and others. The formulation in the lower thermosphere now has improved flexibility, and a new species, “anomalous oxygen,” allows for appreciable O + and hot atomic oxygen contributions to the total mass density at high altitudes. A new full disk proxy for the solar chromospheric EUV driver of thermospheric variability is available for studies in combination with the NRLMSIS model and database. This will determine the value of augmenting F 10.7 in the model formulation — a longstanding issue. Whereas F 10.7 correlates more closely with coronal EUV flux, chromospheric fluxes provide the primary thermospheric heating.

Update on the calibration and performance of the special sensor ultraviolet limb imagers (SSULI)

The Naval Research Laboratory has built give Special Sensor Ultraviolet Limb Imagers (SSULIs) for the Defense Meteorological Satellite Program. These sensors are designed to measure vertical intensity profiles of the Earths airglow in the extreme and far ultraviolet (800 to 1700 angstroms). The data from these sensors will be used to infer altitude profiles of ion, electron and neutral density. The first SSULI is scheduled to launch in 2000. An identical copy of the SSULI sensor called LORAAS was launched aboard the ARGOS spacecraft on February 23, 1999. Data from LORAAS will be used to verify the performance of the SSULI sensors, ground analysis software and validate the UV remote sensing technique. Together with the LORAAS instrument the SSULI program will collect data on the composition of the upper atmosphere for a complete solar cycle.

On-orbit characterization and performance of the HIRAAS instruments aboard ARGOS: LORAAS sensor performance

The Advanced Research and Global Observation Satellite (ARGOS) has been operating since February 1999 and includes three spectrographs comprising the High Resolution Airglow and Auroral Spectroscopy (HIRAAS) experiment. The HIRAAS instruments remotely sense the Earths mid-, far- and extreme-ultraviolet airglow to study the density, composition, and temperature of the thermosphere and ionosphere. The Low Resolution Airglow and Aurora Spectrograph (LORAAS) is a limb scanner covering the 80-170 passband nm with 1.8 nm spectral resolution. Repeated serendipitous observations of hot O- and B-type stars have been used to improve the aspect solution, characterize the instrument field-of-view, and monitor relative sensitivity degradation of the instrument during the mission. We present the methodology of performance characterization and report the observed performance degradation of the LORAAS wedge-and-strip microchannel plate detector. The methods and results herein can be utilized directly in on-orbit characterization of the SSULI operational sensors to fly aboard the DMSP Block 5D3 satellites.

Ionospheric-thermospheric UV tomography: 2. Comparison with incoherent scatter radar measurements

The Special Sensor Ultraviolet Limb Imager (SSULI) instruments are ultraviolet limb scanning sensors that fly on the Defense Meteorological Satellite Program F16-F19 satellites. The SSULIs cover the 80–170 nm wavelength range which contains emissions at 91 and 136 nm, which are produced by radiative recombination of the ionosphere. We invert the 91.1 nm emission tomographically using a newly developed algorithm that includes optical depth effects due to pure absorption and resonant scattering. We present the details of our approach including how the optimal altitude and along-track sampling were determined and the newly developed approach we are using for regularizing the SSULI tomographic inversions. Finally, we conclude with validations of the SSULI inversions against Advanced Research Project Agency Long-range Tracking and Identification Radar (ALTAIR) incoherent scatter radar measurements and demonstrate excellent agreement between the measurements. As part of this study, we include the effects of pure absorption by O2, N2, and O in the inversions and find that best agreement between the ALTAIR and SSULI measurements is obtained when only O2 and O are included, but the agreement degrades when N2 absorption is included. This suggests that the absorption cross section of N2 needs to be reinvestigated near 91.1 nm wavelengths.

The atmospheric neutral density experiment (ANDE) and modulating retroreflector in space (MODRAS): combined flight experiments for the space test program

The Atmospheric Neutral Density Experiment (ANDE) is a low cost mission proposed by the Naval Research Laboratory to demonstrate a method to monitor the thermospheric neutral density at an altitude of 400 km. The primary mission objective is to provide total neutral density along the orbit for improved orbit determination of resident space objects. The ANDE mission also serves as a test platform for a new space-to-ground optical communications technique, the Modulating Retro-reflector Array in Space (MODRAS) experiment. Both are sponsored in part by the Department of Defense Space Test Program. The mission consists of two spherical spacecraft fitted with retro-reflectors for satellite laser ranging (SLR). One spacecraft is completely passive; the other carries three active instruments; a miniature Wind And Temperature Spectrometer (WATS) to measure atmospheric composition, cross-track winds and neutral temperature; a Global Positioning Sensor (GPS); and a Thermal Monitoring System (TMS) to monitor the temperature of the sphere. A design requirement of the active satellite is to telemeter the data to the ground without external protrusions from the spherical spacecraft (i.e. an antenna). The active satellite will be fitted with the MODRAS system, which is an enabling technology for the ANDE mission. The MODRAS system consists of a set of multiple quantum well (MQW) modulating retro-reflectors coupled with an electronics package, which will telemeter data to the ground by modulating the reflected light from laser interrogation beam. This paper presents a mission overview and emphasis will be placed on the design, optical layout, performance, ground station, and science capabilities of the combined missions.