Greg Holsclaw

Titan airglow spectra from Cassini Ultraviolet Imaging Spectrograph (UVIS): EUV analysis

u ! X 1 P g ), while the FUV spectrum consists of one (a 1 !g ! X 1 P g ). Both the EUVand FUV spectra contain many N I and N II multiplets that are produced primarily by photodissociative ionization. Spectral intensities of the N2 c4 0 1 P u (v 0 =0 )! X 1 P g (v 00 = 0-2) progression from 950-1010 A u are resolved for the first time. The UVIS observations reveal that the c4 1 P u (0) ! X 1 P (3) Stevens et al. (1994) developed a c4 (0, v 00 ) multiple scattering model for the terrestrial atmosphere and showed that c4 0 (0, 0) should be weak or undetectable near peak photoelectron excitation and that c4 0 (0, 1) should dominate over c4 (0, 0). This result was also inferred at Titan by Stevens (2001), who argued that c4 0 (0, 0) was misidentified at Titan and that two prominent N I multiplets produced primarily by photodissociative ionization (PDI) of N2 were present instead. This meant that the Titan EUV dayglow could be excited exclusively by the Sun. The key EUV emissions that could not be conclusively identified by UVS because of its low spectral resolution (30 A u ) can now be determined by UVIS with its higher spectral resolution (5.6 A u ). (4) Here we present for the first time UVIS EUV and FUV airglow spectra from Titan. We discuss the implica- tions of the spectra to the excitation sources on Titan using models of the EUV airglow. A subsequent paper will provide a model of the FUV airglow.

Variability of D and H in the Martian Upper Atmosphere Observed with the MAVEN IUVS Echelle Channel

The MAVEN IUVS instrument contains an echelle spectrograph channel designed to measure D and H Ly α emissions from the upper atmosphere of Mars. This channel has successfully recorded both emissions, which are produced by resonant scattering of solar emission, over the course of most of a martian year. The fundamental purpose of these measurements is to understand the physical principles underlying the escape of H and D from the upper atmosphere into space, and thereby to relate present-day measurements of an enhanced HDO/H2O ratio in the bulk atmosphere to the water escape history of Mars. Variations in these emissions independent of the solar flux reflect changes in the density and/or temperature of the species in the upper atmosphere. The MAVEN measurements show that the densities of both H and D vary by an order of magnitude over a martian year, and not always in synch with each other. This discovery has relevance to the processes by which H and D escape into space. One needs to understand the controlling factors to be able to extrapolate back in time to determine the water escape history from Mars at times when the atmosphere was thicker, when the solar flux and solar wind were stronger, etc. Further measurements will be able to identify the specific controlling factors for the large changes in H and D, which likely result in large changes in the escape fluxes of both species.

IUVS echelle‐mode observations of interplanetary hydrogen: Standard for calibration and reference for cavity variations between Earth and Mars during MAVEN cruise

The high-resolution echelle mode of the Imaging Ultraviolet Spectrograph (IUVS) instrument on the Mars Atmosphere and Volatile Evolution (MAVEN) mission has been designed to measure D and H Lyman-α emissions from the martian atmosphere to obtain key information about the physical processes by which water escapes into space. Toward this goal, the absolute calibration of the instrument is critical for determining the D and H densities, the D/H ratio, and the escape flux of water. The instrument made observations of interplanetary hydrogen (IPH) along multiple look directions and conducted several post-launch calibration campaigns during cruise as well as during orbit around Mars. The calibration efforts monitored instrument degradation and produced a consistent calibration factor at the hydrogen Lyman-α wavelength (121.567 nm). The instrument was calibrated with the diffuse emission of interplanetary hydrogen (IPH) as a standard candle using measurements and model results from the Solar Wind Anisotropies (SWAN) instrument. Validation of the calibrated instrument was made by (1) comparisons to simultaneous observations of the IPH made with the lower resolution FUV-mode of the IUVS instrument that were independently calibrated using standard stars as well as by (2) comparisons to same-day observations of Mars at hydrogen Lyman-α made with the Hubble Space Telescope that were calculated with a radiative transfer model. The FUV-mode stellar calibrated values and HST-based model results agreed with the echelle SWAN calibrated values to within 6% and 4%, respectively. The calibrated IUVS instrument can be used to interpret emissions of atmospheric species at Mars for insights into water evolution at the planet, as well as observed IPH measurements made during cruise for further insights into dynamics of the inner heliosphere.

Martian Thermospheric Response to an X8.2 Solar Flare on 10 September 2017 as Seen by MAVEN/IUVS

We report the response of the Martian upper atmosphere to a strong X‐class flare on September 10, 2017 as observed by the Imaging Ultraviolet Spectrograph (IUVS) instrument aboard the Mars Atmosphere Volatile EvolutioN (MAVEN) mission. The solar flare peaked at 16:24 hrs UT and IUVS dayglow observations were taken about an hour after the flare peak. Retrieved temperatures from IUVS dayglow observations show a significant increase during the flare orbit, with a mean value of ∼270 K and a maximum value of ∼310 K. The retrieved temperatures during the flare orbit also show a strong latitudinal gradient, indicating that the flare induced heating is limited between low‐ and mid‐latitudes. During this event IUVS observed an ∼70% increase in the observed brightness of CO2+ Ultraviolet Doublet and CO Cameron band emission at 90 km, where high‐energy photons (< 10 nm) deposit most of their energy.

Hydrogen atoms in the inner heliosphere: SWAN‐SOHO and MASCS‐MESSENGER observations

We present here a study made by two instruments, MASCS on MESSENGER and SWAN on SOHO that observed the interplanetary background in 2010. The combination of these two data sets allows us to perform the first study of the distribution of hydrogen atoms inside the Earths orbit. Triangulation of the position of the Maximum Emissivity Region (MER) was performed for the data of the UVVS channel of the MASCS-MESSENGER instrument. We find that the ecliptic longitude of the MER is 253.2° ±2.0°. This is the same value that was found from the analysis of the SWAN-SOHO H cell data obtained in 1996. This strongly suggests that the direction of the interstellar hydrogen wind has not changed between 1996 and 2010. We have also determined the distance of the MER to the Sun. We find that the volume emission rate peaks at 2.37 AU ±0.2 AU from the Sun. This value is a good test for the solar parameters for total H ionization and radiation pressure used in models. Comparison between the two datasets obtained by the UVVS-MASCS channel and SWAN on SOHO allow to derive the intensity between the two spacecraft at peak emission. Based on the SWAN-SOHO calibration, we find an intensity of 80 R ±36 R. This corresponds to a column density of 1540 m−3 AUx2.3 × 1014 m−3 . When divided by the distance between the two spacecraft, we find an average number density of 2300 m−3.

Absolute ultraviolet irradiance of the moon from SORCE SOLSTICE

The Moon has been shown to be an extremely stable radiometric reference for calibration and long-term stability measurements of on-orbit sensors. The majority of previous work has been in the visible part of the spectrum, using ground-based lunar images. The SOLar-STellar Irradiance Comparison Experiment (SOLSTICE) on the SOlar Radiation and Climate Experiment (SORCE) can be used to extend the lunar spectral irradiance dataset to include the 115-300 nm range. SOLSTICE can directly measure both the solar and lunar spectra from orbit, using the same optics and detectors. An observing campaign to map out the dependence on phase angle began in mid 2006, and continues through the present. The geometry of SORCEs orbit is very favorable for lunar observations, and we have measurements of almost the entire 0-180 degree range of phases. In addition to Earth Observing Systems using the Moon for calibration, recent planetary missions have also made ultraviolet observations of the Moon during Earth flyby, and these SOLSTICE measurements can be useful in calibrating their absolute responsivity.

Nitric oxide nightglow and Martian mesospheric circulation from MAVEN/IUVS observations and LMD-MGCM predictions

We report results from a study of nitric oxide nightglow over the northern hemisphere of Mars during winter, the southern hemisphere during fall equinox and equatorial latitudes during summer in the northern hemisphere based on observations of the δ and γ bands between 190 and 270 nm by the Imaging UltraViolet Spectrograph (IUVS) on the MAVEN spacecraft. The emission reveals recombination of N and O atoms dissociated on the dayside of Mars and transported to the nightside. We characterize the brightness (from 0.2 to 30 kR) and altitude (from 40 to 115 km) of the NO nightglow layer, as well as its topside scale height (mean of 11 km). We show the possible impact of atmospheric waves forcing longitudinal variability, associated with an increased brightness by a factor 3 in the 140 - 200∘ longitude region in the northern hemisphere winter and in the -102∘ to -48∘ longitude region at summer. Such impact to the NO nightglow at Mars was not seen before. Quantitative comparison with calculations of the LMD-MGCM (Laboratoire de Meteorologie Dynamique - Global Circulation Model) suggests that the model globally reproduces the trends of the NO nightglow emission and its seasonal variation, but also indicates large discrepancies (up to a factor 50 fainter in the model) in northern winter at low to mid-latitudes. This suggests that the predicted transport is too efficient towards the night winter pole in the thermosphere by ∼20∘ latitude north.

Martian mesospheric cloud observations by IUVS on MAVEN: Thermal tides coupled to the upper atmosphere

We report observations of Martian mesospheric ice clouds and thermospheric scale heights by the Imaging Ultraviolet Spectrograph on NASAs Mars Atmosphere and Volatile Evolution mission. The clouds are observed between 6 AM and 8 AM local time using mid-UV limb observations between 60 and 80 km tangent altitude where ice particles that scatter sunlight can appear as detached layers near the equator. The equatorial longitudinal distribution shows populations of clouds near -110° E and -10° E as well as a population near 90° E, which does not have a clear precedent. The cloud populations indicate a wave 3 pattern near 70 km, which is confirmed by independent mesospheric temperature observations. Scale heights 100 km above the clouds derived from concurrent IUVS observations also reveal a wave 3 longitudinal structure, suggesting that the temperature oscillations enabling the formation of mesospheric clouds couple to the upper atmosphere.

Global Aurora on Mars During the September 2017 Space Weather Event

We report the detection of bright aurora spanning Mars’ nightside during the space weather event occurring in September 2017. The phenomenon was similar to diffuse aurora detected previously at Mars, but 25 times brighter and detectable over the entire visible nightside. The observations were made with the Imaging UltraViolet Spectrograph (IUVS), a remote sensing instrument on the Mars Atmosphere and Volatile EvolutioN (MAVEN) spacecraft orbiting Mars. Images show that the emission was brightest around the limb of the planet, with a fairly uniform faint glow against the disk itself. Spectra identified four molecular emissions associated with aurora, and limb scans show the emission originated from an altitude of ~60 km in the atmosphere. Both are consistent with very high energy particle precipitation. The auroral brightening peaked around 13 September, when the flux of solar energetic electrons and protons both peaked. During the declining phase of the event, faint but statistically significant auroral emissions briefly appeared against the disk of the planet in the form of narrow wisps and small patches. These features are approximately aligned with predicted open field lines in the region of strong crustal magnetic fields in Mars’ southern hemisphere.

Significant Space Weather Impact on the Escape of Hydrogen From Mars

In September 2017, an active region of the Sun produced a series of strong flares and a coronal mass ejection that swept past Mars producing enhanced ionization and heating in the upper atmosphere. Emissions from atmospheric hydrogen Lyman‐α were also enhanced at Mars. Temperatures derived from neutral species scale heights were used in conjunction with the H Lyman‐α observations to simulate the effects of this space weather event on martian hydrogen properties in the exosphere. It was found that hydrogen abundance in the upper atmosphere decreased by ~25%, and that the H escape rate increased by a factor of 5, mainly through an increase in upper atmospheric temperature. This significant escape rate variation is comparable to seasonally observed trends but occurred at much shorter timescales. Such solar events would statistically impact extrapolation of martian water loss over time.