Brian Charles Wolven

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

Lyman‐α imaging of the SO2 distribution on Io

Imaging spectroscopy of Io in the ultraviolet (1160–1720 A) was carried out with the Space Telescope Imaging Spectrograph on HST on three dates in October 1997 and August 1998. Among the initial results was the observation of concentrated regions of Hi Lyman-α flux near the poles of Io that exhibited a morphology and temporal variability different from those of the atomic oxygen and sulfur emission regions seen near the equatorial limbs. We examine the suggestion that the primary source of Lyman-α emission is surface reflected solar radiation that penetrates the thin polar atmosphere, but is strongly absorbed by the thicker SO2 atmosphere near Ios equator. Spectral and spatial analyses lead to derived SO2 column densities that are in good agreement with those derived from earlier HST observations of Ios albedo in the 2000–2300 A wavelength range. The Lyman-α images clearly illustrate features of Ios atmosphere that have been deduced from previous observations and theoretical modeling: a non-uniformity with respect to the sub-solar point dominated by a freezing out of the SO2 near the poles and variation with both longitude and time due to the variability of the sources of the atmospheric gas. Lyman-α imaging is demonstrated to be an extremely powerful and direct way to globally map the dynamic atmosphere of Io.

Io's equatorial spots: Morphology of neutral UV emissions

The first observations of Io with the Space Telescope Imaging Spectrograph (STIS) on the Hubble Space Telescope (HST) showed that the brightest ultraviolet emissions come from localized regions near Ios equator, designated “equatorial spots.” This paper presents a detailed study of the location, shape, and brightness of the equatorial spots in near-monochromatic images obtained using STIS in the first-order long-slit spectroscopy mode. This study provides evidence that the equatorial emissions are linked to the interaction between the Jovian magnetosphere and Ios atmosphere. The morphology of the equatorial spots reported here provides additional information on the nature of this complex electrodynamic interaction. We find the following principal results: the locations of the equatorial spots are correlated with the Jovian magnetic field orientation at Io, but with a relation that is not 1:1; the equatorial spots are centered 10°–30° longitude downstream from Ios sub-Jovian longitude; the brightness of the emissions in this data set is correlated with Ios distance from the plasma torus centrifugal equator; and the anti-Jovian equatorial spots are ∼20% brighter than the sub-Jovian equatorial spots.

Searching for Far-ultraviolet Auroral/Dayglow Emission from HD?209458b

We present recent observations from the HST -Cosmic Origins Spectrograph aimed at characterizing the auroral emission from the extrasolar planet HD209458b. We obtained medium-resolution (R ∼ 20,000) far-ultraviolet (1150 – 1700 Å) spectra at both the Phase 0.25 and Phase 0.75 quadrature positions as well as a stellar baseline measurement at secondary eclipse. This analysis includes a catalog of stellar emission lines and a star-subtracted spectrum of the planet. We present an emission model for planetary H2 emission, and compare this model to the planetary spectrum. No unambiguously identifiable atomic or molecular features are detected, and upper limits are presented for auroral/dayglow line strengths. An orbital velocity cross-correlation analysis finds a statistically significant (3.8 σ) feature at +15 (± 20) km s in the rest frame of the planet, at λ1582 Å. This feature is consistent with emission from H2 B – X (2 – 9) P(4) (λrest = 1581.11 Å), however the physical mechanism required to excite this transition is unclear. We compare limits on relative line strengths seen in the exoplanet spectrum with models of ultraviolet fluorescence to constrain the atmospheric column density of neutral hydrogen between the star and the planetary surface. These results support models of short period extrasolar giant planets with weak magnetic fields and extended atomic atmospheres. Subject headings: stars: planetary systems — stars: individual (HD209458) — stars: atmospheres — ultraviolet: stars

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.

Emission profiles of neutral oxygen and sulfur in Io's exospheric corona

Hubble Space Telescope (HST)/Space Telescope Imaging Spectrograph (STIS) observations of Io acquired in 1997 [Roesler et al., 1999] provided the first simultaneous spatially resolved measurements of emission from neutral sulfur and oxygen, the dominant atomic species in Ios exospheric corona. Previous measurements of Ios corona relied primarily on sunlight resonantly scattered from sodium, a trace element in Ios atmosphere, and required measurement during mutual satellite eclipses to obtain the necessary spatial resolution. We present here spatial profiles of Ios extended emissions derived from observations spanning the time period from October 1997 to February 2000. The STIS Far Ultraviolet Multi-Anode Microchannel Array (FUV-MAMA) detector permits measurement of the emissions with a spatial resolution of ∼0.05 Io radii out to distances of ∼20 Io radii. Useful measurements are limited to ∼10 Io radii owing to the low signal-to-noise ratio of the extended emission features. The coronal emission profiles vary considerably in slope and intensity and are generally brighter for Io west (duskside) of Jupiter. Emission profiles obtained near western elongation are relatively symmetric about Io; profiles obtained in other orbital positions display varying degrees of asymmetry, with enhanced emissions and generally steeper slopes in the downstream direction relative to the plasma flow. The downstream-upstream profile asymmetry is thought to be caused by higher electron densities in Ios plasma wake. While the coverage of the data is limited in both Jovian System III coordinates and geocentric phase, the intensities of emission from regions both near Io and in the extended corona vary with System III longitude in a near-simultaneous fashion, suggesting local torus electron density as the probable source of this modulation. The observed ratio of oxygen to sulfur emission is relatively constant in time, perhaps reflecting the stoichiometric ratio of the SO 2 source molecules. Eclipse and posteclipse observations on February 25, 2000, show a dramatic increase in profile emission brightness and slope, suggesting a dynamic response by a sublimation-supported component of Ios SO 2 atmosphere and associated atomic species.

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.

Excitation of the Ganymede Ultraviolet Aurora

We analyze the ultraviolet aurorae observed on Ganymede by means of the Hubble Space Telescope and compare them to similar phenomena on Earth. We find that the tenuous nature of Ganymedes atmosphere precludes excitation of the aurora by high-energy electrons and requires a local acceleration mechanism. We propose the following as plausible mechanisms for generating both the continuous background emission and the intense auroral bright spots:

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.

Self-absorption by vibrationally excited H2 in the Astro-2 Hopkins Ultraviolet Telescope spectrum of the Jovian aurora

Observed H2 band intensities in the spectrum of the northern Jovian aurora, obtained by the Hopkins Ultraviolet Telescope (HUT) during the Astro-2 mission in March 1995, are shown to be consistent with attenuation due to absorption by vibrationally excited H2 (ν≤2) in the overlying atmosphere. Strong Werner band transitions near 1050 and 1100 A, in a region of the spectrum not accessible to IUE or HST, have intensities significantly weaker than those predicted by the theoretical branching ratios for optically thin emission. Absorbed photons escape primarily by fluorescent scattering in transitions which connect to unpopulated vibrational levels at longer wavelengths. A spectrum of the Jovian day glow (produced by a combination of solar and electron-impact fluorescence) obtained a few days earlier displays considerably less attenuation in the 1050–1100 A region. We present synthetic H2 fluorescence spectra which include the effects of self-absorption and demonstrate the ability to reproduce the features observed in the HUT spectra.