Venus Night Airglow Distibutions and Variability: NCAR VTGCM Simulations

in Bulletin of the American Astronomical Society (2008, September 01)

The National Center for Atmospheric Research (NCAR) thermospheric general circulation model for Venus (VTGCM) is producing results that are comparative to Pioneer Venus and Venus Express data. The model ... [more ▼]

The National Center for Atmospheric Research (NCAR) thermospheric general circulation model for Venus (VTGCM) is producing results that are comparative to Pioneer Venus and Venus Express data. The model is a three dimensional model that can calculate temperatures, zonal winds, meridional winds, vertical winds, and concentration of specific species. The VTGCM can also compute the O[SUB]2[/SUB]-IR and NO-UV night airglow intensity distributions. With a lower boundary set at 70 Km and a range of sensitivity tests, the VTGCM is able to show consistent set of results with the nightside temperature and the night airglows. These results can show possible controlling parameters of the O[SUB]2[/SUB]-IR, NO-UV night airglow layers, and the nightside hot spot. Being able to understand the night airglow distribution and variability provides valuable insight into the changing circulation of Venusâ upper atmosphere and leads to an overall planetary perception of the atmospheric dynamics. [less ▲]

Limb observations of the ultraviolet nitric oxide nightglow with SPICAV on board Venus Express

in Journal of Geophysical Research. Planets (2008), 113

Limb observations of the spectrum of nightglow emission in the delta (190-240 nm) and gamma (225-270 nm) bands of nitric oxide have been made with the Spectroscopy for Investigation of Characteristics of ... [more ▼]

Limb observations of the spectrum of nightglow emission in the delta (190-240 nm) and gamma (225-270 nm) bands of nitric oxide have been made with the Spectroscopy for Investigation of Characteristics of the Atmosphere of Venus (SPICAV) ultraviolet spectrometer on board Venus Express. These emissions arise from radiative recombination between O([SUP]3[/SUP]P) and N([SUP]4[/SUP]S) atoms that are produced on the dayside and recombine to form excited NO molecules on the nightside. No other emission feature has been identified. The mean altitude of the emission layer is located at 113 km, but it varies between 95 and 132 km. The mean brightness of the total NO emission at the limb is 32 kR, but it is highly variable with limb intensities as large as 440 kR observed at low latitude and values below 5 kR seen at northern midlatitudes. No systematic dependence of the brightness with latitude is observed, but the mean altitude of the emission maximum statistically drops with increasing latitude between 6° and 72°N. Typical observed limb profiles are compared with simulations based on a one-dimensional chemical-diffusive atmospheric model. From model fits to observed profiles, we find that the downward flux of N atoms at 130 km typically varies between 1 × 10[SUP]8[/SUP] to 4 × 10[SUP]9[/SUP] atoms cm[SUP]-2[/SUP] s[SUP]-1[/SUP]. Comparisons of observed airglow topside scale heights with modeled profiles smoothed by the instrumental field of view indicate that the observations are compatible with a downward flow of O and N atoms by molecular and turbulent transport above the peak of emission. The K coefficient deduced from comparisons to limb profiles is less than that determined from the observations made with the Pioneer Venus UV spectrometer at low latitude during periods of high solar activity. [less ▲]

Morphology and dynamics of Venus oxygen airglow from Venus Express/Visible and Infrared Thermal Imaging Spectrometer observations

in Journal of Geophysical Research. Planets (2008), 113

Images obtained by the Visible and Infrared Thermal Imaging Spectrometer (VIRTIS)-M channel instrument onboard Venus Express have been used to retrieve maps and apparent motions of the O[SUB]2[/SUB] ([SUP ... [more ▼]

Images obtained by the Visible and Infrared Thermal Imaging Spectrometer (VIRTIS)-M channel instrument onboard Venus Express have been used to retrieve maps and apparent motions of the O[SUB]2[/SUB] ([SUP]1[/SUP]Delta) infrared nightglow on Venus at 1.27 mum. The nightglow distribution is highly inhomogeneous with the regions of brightest emission generally located at low latitudes near the midnight meridian. Unexpectedly some orbits show also intense airglow activity over the south polar region. The spatially resolved airglow is spectacularly variable not only in its morphology and intensity but also in the apparent motions of the airglow small- and large-scale structures. Visual tracking of the bright features allowed to obtain mean zonal and meridional motions related to the subsolar to antisolar circulation. The zonal velocity is dominated by an intense prograde jet (contrary to the retrograde planetary rotation) from dawn to midnight extending up to 22 hours in local time with lower velocities and reversed sign from dusk. Typical zonal velocities range between +60 (prograde) to -50 (retrograde) m/s, whereas most meridional velocities range from -20 (poleward) to +100 m/s (equatorward) with an average meridional circulation of +20 m/s toward low latitudes. The brightest small-scale (~100 km) features appear correlated with locations of apparent convergence which may be a signature of compression and downwelling, whereas this is not evident for the large-scale structures suggesting slow subsidence over large areas mixed with horizontal motions. We argue that part of the tracked motions are representative of real motions at the mesosphere over an altitude range of 95-107 km. [less ▲]

Observational evidence of a localized magnetic anomaly near Jupiter’s North Pole

Conference (2008, April 18)

We have analyzed more than 1000 HST/ACS images of Jupiter’s ultraviolet auroral emission in the northern hemisphere. A systematic planet center finding algorithm made it possible to infer reliable and ... [more ▼]

We have analyzed more than 1000 HST/ACS images of Jupiter’s ultraviolet auroral emission in the northern hemisphere. A systematic planet center finding algorithm made it possible to infer reliable and consistent jovicentric location of the auroral footprints of Io, Europa and Ganymede. These footprints form reference contours which provide an absolute magnetic mapping from the ionosphere of Jupiter to the equatorial plane, independent of any magnetic field model. So far, the VIP4 magnetic field model is the most accurate in terms of fitting the auroral emissions. However, it cannot reproduce the distorted shape of the satellites UV footpaths in the “kink region” in the northern ionosphere between S3 longitudes 80 ̊-150 ̊. We show that the model is significantly improved by decreasing the VIP4 surface magnetic field in the kink region and by adding a localized dipolar perturbation field beneath the surface. [less ▲]

Jupiter’s diffuse auroral emissions - Comparison of HST and Galileo data

Conference (2008, April 18)

Based on an extensive HST FUV image database obtained between 1997 and 2007, we have studied the morphology and brightness of the equatorward diffuse auroral emissions in both Jovian hemispheres. The ... [more ▼]

Based on an extensive HST FUV image database obtained between 1997 and 2007, we have studied the morphology and brightness of the equatorward diffuse auroral emissions in both Jovian hemispheres. The emissions are wider and brighter on the dusk side than on the dawn and they often form multiple discrete arcs parallel to the main oval. What could be the origin of these equatorward diffuse emissions and their local time variations is still unclear. Galileo observations have shown changes in the electron pitch angle distributions between the inner and middle magnetosphere of Jupiter (10 to 17 RJ ) which could be associated with auroral emissions, without the need of field aligned currents. We derive the electron precipitation flux for the first time in a global scale, based on Galileo electron measurements between 10 and 17 RJ . We magnetically map this region in the ionosphere and compare the derived energy flux with the brightness of the diffuse emissions. We discuss the possibility that the energetic particle distribution in the middle magnetosphere could account for the multiple structured equatorward diffuse emissions and their local time variations. [less ▲]

Jupiter’s diffuse auroral emissions - Comparison of HST and Galileo data,

Conference (2008, April)

Observational evidence of a localized magnetic anomaly near Jupiter’s North Pole

Conference (2008, April)

The Venus ultraviolet oxygen dayglow and aurora: Model comparison with observations

in Planetary and Space Science (2008), 56

We compare the intensity of the OI 130.4 and 135.6 nm emissions calculated using the soft electron precipitation measured on board the Pioneer Venus (PV) Orbiter with the auroral brightness observed with ... [more ▼]

We compare the intensity of the OI 130.4 and 135.6 nm emissions calculated using the soft electron precipitation measured on board the Pioneer Venus (PV) Orbiter with the auroral brightness observed with the ultraviolet spectrometer (OUVS) on board the PV. For this purpose, we use a new electron transport model based on a Monte Carlo implementation of the Boltzmann equation and a multi-stream radiative transfer model to calculate the effects of multiple scattering on the intensity field of the 130.4-nm triplet. We show that the consideration of the enhancement of the emergent 130.4-nm to the 135.6-nm intensity by multiple scattering in the optically thick Venus atmosphere increases the auroral 130.4/135.6 ratio by a factor of about 3. We find agreement with the mean 130.4/135.6 ratio observed with PV-OUVS using the typical suprathermal electron energy spectrum reported from PV in situ measurements showing a characteristic energy of about 14 eV. To account for the average OI auroral emissions, the required precipitated energy flux is 2×10[SUP]-3[/SUP] mW m[SUP]-2[/SUP], that is about 30% of the measured suprathermal night-side soft electron spectrum used as a reference. The calculated brightness of the CO Cameron bands is about twice as large as the weak observed emission, but within the error bars of the observations and the uncertainties of the dissociative excitation cross-section of CO[SUB]2[/SUB]. The electron transport model, coupled with calculations of excitation processes is also applied to an analysis of the FUV oxygen day airglow observations made with PV-OUVS and the Hopkins Ultraviolet Telescope (HUT) spectrograph. Comparisons indicate that the model accounts for both the disc-averaged intensities observed with the HUT spectrograph, the limb scans and the 130.4-nm images obtained with PV-OUVS. The relative contribution of resonance scattering of the solar line and photoelectron impact to the excitation of the 130.4-nm triplet depends on the altitude, but is globally dominated by resonance scattering. The intensity of the 130.4-nm dayglow emission does not vary proportionally with the O density in the lower thermosphere, but provides nevertheless a useful tool to remotely probe the atomic oxygen density and its variations. [less ▲]

Monte Carlo model of electron transport for the calculation of Mars dayglow emissions

in Journal of Geophysical Research. Planets (2008), 113

A model of the photoelectron collision-induced component of the Mars dayglow using recent cross sections and solar flux is described. The calculation of the photoelectron source of excitation is based on ... [more ▼]

A model of the photoelectron collision-induced component of the Mars dayglow using recent cross sections and solar flux is described. The calculation of the photoelectron source of excitation is based on a stochastic solution of the Boltzmann equation using the direct simulation Monte Carlo method. The neutral atmosphere is taken from outputs of a global circulation model, and recent inelastic collision cross sections are adopted. The calculated vertical profiles of the CO Cameron bands and CO[SUB]2[/SUB] [SUP]+[/SUP] doublet emissions integrated along the line of sight compare well with the Spectroscopy for the Investigation of the Characteristics of the Atmosphere of Mars (SPICAM) limb profiles observed with the SPICAM spectrograph on board Mars Express made at Ls = 166° during the summer season at northern midlatitudes. The comparison shows agreement to within the uncertainties of the excitation cross sections. Seasonal changes in the brightness and the altitude of the emission peaks are predicted with intensity variations in the range 15-20%. [less ▲]

Discontinuity in Jupiter's main auroral oval

in Journal of Geophysical Research. Space Physics (2008), 113

On the basis of a series of FUV Hubble Space Telescope images obtained between 1997 and 2007 it is shown that there is a segment of the main auroral oval where the emission drops significantly from a few ... [more ▼]

On the basis of a series of FUV Hubble Space Telescope images obtained between 1997 and 2007 it is shown that there is a segment of the main auroral oval where the emission drops significantly from a few hundreds to a few tens of kiloRayleigh, forming a discontinuity in the oval. It is shown that the discontinuity is present in both hemispheres and confined in magnetic local time. Its equatorial source is located in the prenoon and early noon sector. The main auroral oval is associated with the ionosphere-magnetosphere coupling current system which is related to the breakdown of corotation in the middle magnetosphere. Necessary for the electron precipitation in the ionosphere and the formation of the main auroral oval is the presence of upward field-aligned currents, carried by downward moving electrons. Field-aligned currents inferred by Pioneer, Voyager and Galileo in situ observations in the near equatorial plane showed evidence of reduced or/and downward field-aligned currents in the prenoon and early afternoon sector, the location of the equatorial source of the discontinuity. Additionally, we estimate the precipitation energy flux in the ionosphere, for a typical reduced upward field-aligned current value at that region, which is found to be within the range of the observed brightness of the discontinuity. Field aligned current distributions in the ionosphere based on magnetohydrodynamic simulations of the interaction between the solar wind and the magnetosphere have predicted a region of downward currents implying a discontinuity at the main auroral oval emission, in very good agreement with the HST observations presented in this work. [less ▲]