D. J. Morrison

Initial observations with the Global Ultraviolet Imager (GUVI) in the NASA TIMED satellite mission

[1] The Global Ultraviolet Imager (GUVI) instrument carried aboard the NASA TIMED satellite measures the spectral radiance of the Earth’s far ultraviolet airglow in the spectral region from 120 to 180 nm using a cross-track scanning spectrometer design. Continuous operation of the instrument provides images of the Earth’s disk and limb in five selectable spectral bands. Also, spectra at fixed scanning mirror position can be obtained. Initial results demonstrate the quantitative functionality of the instrument for studies of the Earth’s dayglow, aurora, and ionosphere. Moreover, through forward modeling, the abundance of the major constituents of the thermosphere, O, N2, and O2 and thermospheric temperatures can be retrieved from observations of the limb radiance. Variations of the column O/N2 ratio can be deduced from sunlit disk observations. In regions of auroral precipitation not only can the aurora regions be geographically located and the auroral boundaries identified, but also the energy flux Q, the characteristic energy Eo, and a parameter fo that scales the abundance of neutral atomic oxygen can be derived. Radiance due to radiative recombination in the ionospheric F region is evident from both dayside and nightside observations of the Earth’s limb and disk, respectively. Regions of depleted F-region electron density are evident in the tropical Appleton anomaly regions, associated with so-called ionospheric ‘‘bubbles.’’ Access to the GUVI data is provided through the GUVI website www.timed.jhuapl.edu\guvi. INDEX TERMS: 0310 Atmospheric Composition and Structure: Airglow and aurora; 0355 Atmospheric Composition and Structure: Thermosphere—composition and chemistry; 0358 Atmospheric Composition and Structure: Thermosphere—energy deposition; 2407 Ionosphere: Auroral ionosphere (2704); KEYWORDS: airglow, aurora, ultraviolet, imaging, satellite, atmosphere

The Apollo 17 Ultraviolet Spectrometer: Lunar atmosphere measurements revisited

Ultraviolet resonance fluorescence of solar radiation provides the most sensitive means of detecting the expected major constituents of the tenuous daytime lunar atmosphere. Such an experiment was carried out with the Apollo 17 Ultraviolet Spectrometer in December 1972 [Fastie et al, 1973b] and produced only upper limits to the number density of H, H2, O, C, N, CO and two noble gases near the surface of the moon. The complete data set of 47 terminator crossing observations, which were not utilized in the earlier analysis, has been re-examined and more stringent upper limits to the column emission rates for several species have been derived. These results, together with most recent values for the atomic and molecular fluorescence efficiencies, lead to more definitive limits on the content of the lunar atmosphere. The revised upper limit on atomic oxygen density allows for the possibility of comparable O/Na ratios in the atmosphere of both the Moon and Mercury.

Validation of remote sensing products produced by the Special Sensor Ultraviolet Scanning Imager (SSUSI): a far UV-imaging spectrograph on DMSP F-16

Operational sensors are designed and intended to reliably produce the measurements needed to develop high-value key environmental parameters. The Special Sensor Ultraviolet Spectrographic Imager (SSUSI) is slated to fly on the next five Defense Meteorological Satellite Program launches (beginning with the launch of F16 in Fall 2001). SSUSI will routinely produce maps of ionospheric and upper atmospheric composition and image the aurora. In this paper we describe these products and our validation plans and the process through which we can assure our sponsors and data products users of the reliability and accuracy of these products.

Remote Sensing of Earth's Limb by TIMED/GUVI: Retrieval of thermospheric composition and temperature

The Global Ultraviolet Imager (GUVI) onboard the Thermosphere-Ionosphere-Mesosphere Energetics and Dynamics (TIMED) satellite senses far ultraviolet emissions from O and N2 in the thermosphere. Transformation of far ultraviolet radiances measured on the Earth limb into O, N2, and O2 number densities and temperature quantifies these responses and demonstrates the value of simultaneous altitude and geographic information. Composition and temperature variations are available from 2002 to 2007. This paper documents the extraction of these data products from the limb emission rates. We present the characteristics of the GUVI limb observations, retrievals of thermospheric neutral composition and temperature from the forward model, and the dramatic changes of the thermosphere with the solar cycle and geomagnetic activity. We examine the solar extreme ultraviolet (EUV) irradiance magnitude and trends through comparison with simultaneous Solar Extreme EUV (SEE) measurements on TIMED and find the EUV irradiance inferred from GUVI averaged (2002–2007) 30% lower magnitude than SEE version 11 and varied less with solar activity. The smaller GUVI variability is not consistent with the view that lower solar EUV radiation during the past solar minimum is the cause of historically low thermospheric mass densities. Thermospheric O and N2 densities are lower than the NRLMSISE-00 model, but O2 is consistent. We list some lessons learned from the GUVI program along with several unresolved issues.

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.

Height‐integrated Joule and auroral particle heating in the night side high latitude thermosphere

[1] Energy deposited at high latitudes by Joule and auroral particle heating perturbs the chemistry and dynamics of the neutral thermosphere. Height-integrated heating rates can be calculated by combining Super Dual Auroral Radar Network (SuperDARN) measurements with TIMED spacecraft Global Ultraviolet Imager (GUVI) auroral images. Spatial maps of heating rates are examined during two TIMED orbits and statistical averages are presented for quiet to moderate levels of geomagnetic activity. Joule heating dominates in the evening sector auroral oval, whereas particle heating dominates after midnight. There is also a tendency for Joule heating (particle heating) to dominate in the vicinity of the region-1 (region-2) currents. This complementary behavior is associated with suppression of the electric field in regions of bright aurora.

The use of far ultraviolet remote sensing to monitor space weather

Abstract This paper discusses the connection between changes in Earths thermosphere and ionosphere induced by changes in the Earths local space environment (or “space weather”) and the phenomena observed in far ultraviolet images of the Earth. Two new experiments, the Global Ultraviolet Imager (GUVI) and the Special Sensor Ultraviolet Imager (SSUSI), will provide a new capability for monitoring changes in thermospheric composition and ionospheric density as they change in response to space weather. These sensors provide a ten-fold improvement in spatial and temporal resolution and a greater than ten-fold improvement in sensitivity over that provided by sensors on the POLAR and IMAGE satellites. These sensors are expected to provide new insights into the mesoscale coupling between the ionosphere and thermosphere, as well as allowing us to develop a better specification of the high latitude convection electric field pattern.

Hemispheric comparison of PMC altitudes

A middle ultraviolet imager observed the vertical structure of polar mesospheric clouds (PMCs) in the northern and southern polar regions during the austral summer of 1997–1998 and the arctic summer of 1999. During 23 transpolar passes, the imager obtained over 15,000 images at latitudes poleward of 55°. The accuracy and stability of the satellite platform and the large database permit the first statistical investigation of the small scale (∼1 km) altitude structure of PMCs on a transpolar scale. During a satellite pass over either polar region, PMCs clustered at different altitudes between 80 and 85 km. Southern clouds had a mean altitude of 83.2±1.4 km above the Earth ellipsoid, while northern clouds had a mean altitude of 82.6d±1.3 km, so southern and northern clouds had the same altitudes to within statistical variations. The mean PMC altitude was 82.7±1.3 km for both hemispheres. In neither hemisphere did cloud altitudes exhibit systematic variation with latitude.

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.