References of "Stewart, A Ian F"
     in
Bookmark and Share    
See detailScience highlights from MAVEN/IUVS after two years in Mars orbit
Schneider, Nicholas M.; Deighan, Justin; Stiepen, Arnaud ULg et al

in AAS/Division for Planetary Sciences Meeting Abstracts (2016, October 01)

The broad capabilities of the Imaging UltraViolet Spectrograph on the MAVEN mission are enabling new science ranging from Mars' lower atmosphere up though the escaping corona. After two years in Mars ... [more ▼]

The broad capabilities of the Imaging UltraViolet Spectrograph on the MAVEN mission are enabling new science ranging from Mars' lower atmosphere up though the escaping corona. After two years in Mars orbit, the instrument has yielded insights on the Mars dayglow, season cycles, nightglow, aurora, meteor showers, clouds, solar-planetary interactions and atmospheric escape. In this presentation we will highlight several new discoveries. First, IUVS has observed a third type of aurora not previously seen at Mars, indicative of a new kind of solar-planet interaction for non-magnetized planets. Second, spatial mapping of nitric oxide nightglow reveals regions of atmospheric downwelling necessitating substantial changes to global atmospheric circulation models. Finally, a new high-spatial-resolution UV imaging mode allows unprecedented determinations of Mars' low-altitude ozone, as well as detection of clouds from nadir to limb. [less ▲]

Detailed reference viewed: 9 (1 ULg)
See detailMeasurements of D and H in the Upper Atmosphere of Mars with the MAVEN IUVS Echelle Channel
Clarke, John T.; Mayyasi, Majd A.; Bhattacharyya, Dolon et al

in AAS/Division for Planetary Sciences Meeting Abstracts (2016, October 01)

The enhanced ratio of D/H in the martian atmosphere has long been interpreted to provide evidence for the historic loss rate of water into space, with the enhancement resulting from the mass ratio of the ... [more ▼]

The enhanced ratio of D/H in the martian atmosphere has long been interpreted to provide evidence for the historic loss rate of water into space, with the enhancement resulting from the mass ratio of the species. Understanding the dependence of the enrichment of D on the loss rate of water requires that one understand the controlling factors, including all sources of exospheric hydrogen and the present-day transport of D and H into the upper atmosphere. Toward this end the MAVEN mission has included an echelle channel in the IUVS instrument that resolves the D and H Lyman alpha emissions produced by resonant scattering of bright solar emission. In this mode it has now been possible to measure the upper atmospheric D and H abundances over most of a martian year. The results are surprising, with larger than expected changes in the D and H abundances and a changing ratio of D/H with a strong seasonal dependence. This talk will present the D and H data to date (Mars will be near perihelion by the time of the DPS meeting) and discuss the interpretation of the large changes that have been observed. [less ▲]

Detailed reference viewed: 4 (1 ULg)
See detailH Escape Rates Inferred from MAVEN/IUVS Observations of the Mars Hydrogen Corona
Chaffin, Michael S.; Chaufray, Jean-Yves; Deighan, Justin et al

in AAS/Division for Planetary Sciences Meeting Abstracts (2016, October 01)

H escape oxidizes and dessicates the Mars atmosphere and surface, providing a key control on the present-day chemistry and long-term evolution of the planet. Recently, large variations in the escape rate ... [more ▼]

H escape oxidizes and dessicates the Mars atmosphere and surface, providing a key control on the present-day chemistry and long-term evolution of the planet. Recently, large variations in the escape rate of H as a function of season have been reported by several studies, making continued observation of the variation a high priority. We present escape rates derived from Mars Atmosphere and Volatile EvolutioN (MAVEN) mission Imaging UltraViolet Spectrograph (IUVS) observations of the extended atmosphere of Mars at H Lyman alpha (121.6 nm), which must be interpreted with a coupled density/radiative transfer model owing to the optically thick nature of the emission and the small fraction of H present in the corona on escaping trajectories. We recover densities, temperatures, and escape rates under the assumption of spherical symmetry for multiple periods across MAVEN's mission so far, beginning in December 2014 (escape rates ~4e8/cm2/s). We describe the observed variation and compare it with previously observed seasonal variation in retrieved H escape rates, providing a necessary input for future photochemical modeling studies and estimates of water loss from Mars over its history. [less ▲]

Detailed reference viewed: 4 (1 ULg)
See detailLimb Observations of Solar Scattered Light by the Imaging Ultraviolet Spectrograph on MAVEN: New Constraints on Martian Mesospheric Cloud Variability
Stevens, Michael H.; Siskind, David E.; Evans, Scott et al

in AAS/Division for Planetary Sciences Meeting Abstracts (2016, October 01)

The Imaging Ultraviolet Spectrograph (IUVS) on NASA's Mars Atmosphere and Volatile Evolution (MAVEN) mission observed the Martian upper atmosphere in late 2015 (Ls ~ 70) and early 2016 (Ls ~ 150 ... [more ▼]

The Imaging Ultraviolet Spectrograph (IUVS) on NASA's Mars Atmosphere and Volatile Evolution (MAVEN) mission observed the Martian upper atmosphere in late 2015 (Ls ~ 70) and early 2016 (Ls ~ 150). Although designed to measure the dayglow between 90-200 km IUVS also scans the limb down to 60 km, where solar scattered light dominates the mid-ultraviolet (MUV) signal. Occasionally, this MUV light shows enhanced scattering between 60-90 km indicating the presence of aerosols in the mesosphere. We quantify the solar scattering for each daylight scan obtained between October and December, 2015 and between April and June, 2016. We then identify over 100 scans of enhanced scattering between 60-90 km and assemble them both geographically and diurnally. The geographical distribution of the enhancements in 2015 is preferentially located near the equator, consistent with previous observations of mesospheric clouds for this part of the season. A wave three pattern in equatorial cloud occurrence suggests forcing from a non-migrating tide, possibly linked to the longitudinal variation of Mars surface topography. At the same time, there are indications of a diurnal variation such that the clouds seen in 2015 and 2016 are preferentially observed in the early morning, between 0600-0900 local solar time. This suggests an important role for a migrating temperature tide controlling the formation of Martian mesospheric clouds. [less ▲]

Detailed reference viewed: 2 (1 ULg)
See detailStructure and variability of the Martian upper atmosphere: Ultraviolet dayglow observations by MAVEN/IUVS
Deighan, Justin; Jain, Sonal K.; Lo, Daniel Y. et al

in AAS/Division for Planetary Sciences Meeting Abstracts (2015, November 01)

Mars has been studied extensively at ultraviolet wavelengths starting from Mariner 6 and 7, Mariner 9, and more recently by SPICAM aboard Mars Express. The results from these measurements reveal a large ... [more ▼]

Mars has been studied extensively at ultraviolet wavelengths starting from Mariner 6 and 7, Mariner 9, and more recently by SPICAM aboard Mars Express. The results from these measurements reveal a large variability in the composition and structure of the Martian upper atmosphere. However, due to the lack of simultaneous measurements of energy input (such as solar electromagnetic and particle flux), and limitations in the observation geometry and data itself, this variability is still not fully understood.We report a comprehensive study of Mars dayglow observations by the Imaging Ultraviolet Spectrograph (IUVS) aboard the Mars Atmosphere and Volatile EvolutioN (MAVEN) satellite, focusing on vertical and global upper atmospheric structure and seasonal variability. The dayglow emission spectra show features similar to previous UV measurements at Mars. IUVS has detected a second, low-altitude peak in the emission profile of OI 297.2 nm, confirming the prediction that the absorption of solar Lyman alpha emission is an important energy source there. We find a significant drop in thermospheric scale height and temperature between Ls = 218° and Ls = 337 - 352°, attributed primarily to the decrease in solar activity and increase in heliocentric distance. The CO[SUB]2[/SUB][SUP]+[/SUP] UVD peak intensity is well correlated with simultaneous observations of solar 17 - 22 nm irradiance at Mars by Extreme Ultraviolet Monitor (EUVM) aboard MAVEN. Variations of the derived CO[SUB]2[/SUB] density also exhibit significant persistent global structure with longitudinal wavenumbers 1, 2 and 3 in a fixed local solar time frame, pointing to non-migrating atmospheric tides driven by diurnal solar heating. We will present and discuss the variability in Martian UV dayglow, its dependence on solar EUV flux, and the importance of IUVS observations in our current understanding of Mars’ thermosphere. [less ▲]

Detailed reference viewed: 9 (1 ULg)
Full Text
Peer Reviewed
See detailCassini UVIS observations of Titan nightglow spectra
Ajello, Joseph M.; West, Robert A.; Gustin, Jacques ULg et al

in Journal of Geophysical Research. Space Physics (2012), 117

In this paper we present the first nightside EUV and FUV airglow limb spectra of Titan showing molecular emissions. The Cassini Ultraviolet Imaging Spectrograph (UVIS) observed photon emissions of Titan's ... [more ▼]

In this paper we present the first nightside EUV and FUV airglow limb spectra of Titan showing molecular emissions. The Cassini Ultraviolet Imaging Spectrograph (UVIS) observed photon emissions of Titan's day and night limb-airglow and disk-airglow on multiple occasions, including during an eclipse observation. The 71 airglow observations analyzed in this paper show EUV (600-1150 Å) and FUV (1150-1900 Å) atomic multiplet lines and band emissions arising from either photoelectron induced fluorescence and solar photo-fragmentation of molecular nitrogen (N[SUB]2[/SUB]) or excitation by magnetosphere plasma. The altitude of the peak UV emissions on the limb during daylight occurred inside the thermosphere at the altitude of the topside ionosphere (near 1000 km altitude). However, at night on the limb, a subset of emission features, much weaker in intensity, arise in the atmosphere with two different geometries. First, there is a twilight photoelectron-excited glow that persists with solar depression angle up to 25-30 degrees past the terminator, until the solar XUV shadow height passes the altitude of the topside ionosphere (1000-1200 km). The UV twilight glow spectrum is similar to the dayglow but weaker in intensity. Second, beyond 120° solar zenith angle, when the upper atmosphere of Titan is in total XUV darkness, there is indication of weak and sporadic nightside UV airglow emissions excited by magnetosphere plasma collisions with ambient thermosphere gas, with similar N[SUB]2[/SUB] excited features as above in the daylight or twilight glow over an extended altitude range. [less ▲]

Detailed reference viewed: 27 (3 ULg)
Full Text
Peer Reviewed
See detailThe production of Titan's ultraviolet nitrogen airglow
Stevens, Michael H; Gustin, Jacques ULg; Ajello, Joseph M et al

in Journal of Geophysical Research. Space Physics (2011), 116

The Cassini Ultraviolet Imaging Spectrograph (UVIS) observed Titan's dayside limb in the extreme ultraviolet (EUV) and far ultraviolet (FUV) on 22 June 2009 from a mean distance of 23 Titan radii. These ... [more ▼]

The Cassini Ultraviolet Imaging Spectrograph (UVIS) observed Titan's dayside limb in the extreme ultraviolet (EUV) and far ultraviolet (FUV) on 22 June 2009 from a mean distance of 23 Titan radii. These high-quality observations reveal the same EUV and FUV emissions arising from photoelectron excitation and photofragmentation of molecular nitrogen (N[SUB]2[/SUB]) as found on Earth. We investigate both of these solar driven processes with a terrestrial airglow model adapted to Titan and find that total predicted radiances for the two brightest N[SUB]2[/SUB] band systems agree with the observed peak radiances to within 5%. Using N[SUB]2[/SUB] densities constrained from in situ observations by the Ion Neutral Mass Spectrometer on Cassini, the altitude of the observed limb peak of the EUV and FUV emission bands is between 840 and 1060 km and generally consistent with model predictions. We find no evidence for carbon emissions in Titan's FUV airglow in contrast to previous Titan airglow studies using UVIS data. In their place, we identify several vibrational bands from the N[SUB]2[/SUB] Vegard-Kaplan system arising from photoelectron impact with predicted peak radiances in agreement with observations. These Titan UV airglow observations are therefore comprised of emissions arising only from solar processes on N[SUB]2[/SUB] with no detectable magnetospheric contribution. Weaker EUV Carroll-Yoshino N[SUB]2[/SUB] bands within the v′ = 3, 4, and 6 progressions between 870 and 1020 Å are underpredicted by about a factor of five while the (0,1) band near 980 Å is overpredicted by about a factor of three. [less ▲]

Detailed reference viewed: 31 (2 ULg)
Full Text
Peer Reviewed
See detailThe auroral footprint of Enceladus on Saturn
Pryor, Wayne R; Rymer, Abigail M; Mitchell, Donald G et al

in Nature (2011), 472

Although there are substantial differences between the magnetospheres of Jupiter and Saturn, it has been suggested that cryovolcanic activity at Enceladus could lead to electrodynamic coupling between ... [more ▼]

Although there are substantial differences between the magnetospheres of Jupiter and Saturn, it has been suggested that cryovolcanic activity at Enceladus could lead to electrodynamic coupling between Enceladus and Saturn like that which links Jupiter with Io, Europa and Ganymede. Powerful field-aligned electron beams associated with the Io-Jupiter coupling, for example, create an auroral footprint in Jupiter's ionosphere. Auroral ultraviolet emission associated with Enceladus-Saturn coupling is anticipated to be just a few tenths of a kilorayleigh (ref. 12), about an order of magnitude dimmer than Io's footprint and below the observable threshold, consistent with its non-detection. Here we report the detection of magnetic-field-aligned ion and electron beams (offset several moon radii downstream from Enceladus) with sufficient power to stimulate detectable aurora, and the subsequent discovery of Enceladus-associated aurora in a few per cent of the scans of the moon's footprint. The footprint varies in emission magnitude more than can plausibly be explained by changes in magnetospheric parameters--and as such is probably indicative of variable plume activity. [less ▲]

Detailed reference viewed: 24 (5 ULg)
Full Text
Peer Reviewed
See detailCassini UVIS observations of Jupiter's auroral variability
Pryor, Wayne R.; Stewart, A. Ian F.; Esposito, Larry W. et al

in Icarus: International Journal of Solar System Studies (2005), 178(2), 312-326

The Cassini spacecraft Ultraviolet Imaging Spectrograph (UVIS) obtained observations of Jupiter's auroral emissions in H-2 band systems and H Lyman-alpha from day 275 of 2000 (October 1), to day 81 of ... [more ▼]

The Cassini spacecraft Ultraviolet Imaging Spectrograph (UVIS) obtained observations of Jupiter's auroral emissions in H-2 band systems and H Lyman-alpha from day 275 of 2000 (October 1), to day 81 of 2001 (March 22). Much of the globally integrated auroral variability measured with UVIS can be explained simply in terms of the rotation of Jupiter's main auroral arcs with the planet. These arcs were also imaged by the Space Telescope Imaging Spectrograph (STIS) on Hubble Space Telescope (HST). However, several brightening events were seen by UVIS in which the global auroral output increased by a factor of 2-4. These events persisted over a number of hours and in one case can clearly be tied to a large solar coronal mass ejection event. The auroral UV emissions from these bursts also correspond to hectometric radio emission (0.5-16 MHz) increases reported by the Galileo Plasma Wave Spectrometer (PWS) and Cassim Radio and Plasma Wave Spectrometer (RPWS) experiments. In general, the hectometric radio data vary differently with longitude than the UV data because of radio wave beaming effects. The 2 largest events in the UVIS data were on 2000 day 280 (October 6) and on 2000 days 325-326 (November 20-21). The global brightening events on November 20-21 are compared with corresponding data on the interplanetary magnetic field, solar wind conditions, and energetic particle environment. ACE (Advanced Composition Explorer) solar wind data was numerically propagated from the Earth to Jupiter with an MHD code and compared to the observed event. A second class of brief auroral brightening events seen in HST (and probably UVIS) data that last for similar to 2 min is associated with aurora] flares inside the main auroral ovals. On January 8, 2001, from 18:45-19:35 UT UVIS H-2 band emissions from the north polar region varied quasiperiodically. The varying emissions, probably due to amoral flares inside the main auroral oval, are correlated with low-frequency quasiperiodic radio bursts in the 0.6-5 kHz Galileo PWS data. (c) 2005 Elsevier Inc. All rights reserved. [less ▲]

Detailed reference viewed: 133 (12 ULg)