Ultraviolet Spectrograph Research Articles

The Association of Cusp‐Aligned Arcs With Plasma in the Magnetotail Implies a Closed Magnetosphere

AbstractWe investigate a 15‐day period in October 2011. Auroral observations by the Special Sensor Ultraviolet Spectrographic Imager instrument onboard the Defense Meteorological Satellite Program F16, F17, and F18 spacecraft indicate that the polar regions were covered by weak cusp‐aligned arc (CAA) emissions whenever the interplanetary magnetic field (IMF) clock angle was small, |θ| < 45°, which amounted to 30% of the time. Simultaneous observations of ions and electrons in the tail by the Cluster C4 and Geotail spacecraft showed that during these intervals dense (≈1 cm−3) plasma was observed, even as far from the equatorial plane of the tail as |ZGSE| ≈ 13 RE. The ions had a pitch angle distribution peaking parallel and antiparallel to the magnetic field and the electrons had pitch angles that peaked perpendicular to the field. We interpret the counter‐streaming ions and double loss‐cone electrons as evidence that the plasma was trapped on closed field lines, and acted as a source for the CAA emission across the polar regions. This suggests that the magnetosphere was almost entirely closed during these periods. We further argue that the closure occurred as a consequence of dual‐lobe reconnection. Our finding forces a significant re‐evaluation of the magnetic topology of the magnetosphere during periods of northwards IMF.

Interhemispheric Conjugacy of Multiple Transpolar Arcs

AbstractMultiple transpolar arcs appearing simultaneously in the polar cap have gained much interest in recent years. By analyzing Defense Meteorological Satellite Program Special Sensor Ultraviolet Spectrographic Imagers data, we report for the first time, that less than half of the multiple arc events occur simultaneously in both hemispheres. In 60% of the cases, multiple arcs appear in only one hemisphere. There is a clear difference in interplanetary magnetic field (IMF) conditions for those two groups. Conjugate multiple arcs appear on average during stronger northward IMF and smaller IMF clock angles than non‐conjugate multiple arcs. Only non‐conjugate multiple arcs show a dependence on IMF BX. They form in the northern (southern) hemisphere during negative (positive) BX. An IMF BX induced interhemispheric asymmetry in the magnetospheric field line topology might explain why multiple arcs appear sometimes in only one hemisphere.

Seasonal, Latitudinal, and Longitudinal Trends in Nighttime Ozone Vertical Structure on Mars From MAVEN/IUVS Stellar Occultations

AbstractStellar occultation measurements conducted by the Mars Atmosphere and Volatile and EvolutioN/Imaging UltraViolet Spectrograph instrument were able to make vertically resolved measurements of ozone density in the middle atmosphere of Mars that offered good coverage with respect to latitude, longitude, and local time. These measurements were used to identify systematic variations in the vertical structure of ozone with longitude that could be distinguished from general trends in the evolution of ozone with respect to season and latitude. A total of 583 individual nightside occultations between Martian years 32 and 36 were analyzed, of which 224 were confirmed to have ozone, all found between Ls = 15°–165°. Close to aphelion (Ls = 60°–90°), peak ozone densities between 30 and 40 km altitude were observed to be within error of model predictions at all measured latitudes, but diverged from model predictions before and after this time. At low latitudes, seasonal changes were seen to have the greatest effect on the observed vertical structure of ozone, with detached ozone layer densities at altitudes above 30 km usually varying within approximately a factor of two along a given latitudinal band at a given time of year. Nonetheless, evidence of a persistent regional enhancement of ozone abundance was observed over equatorial latitudes during the aphelion season, spanning a longitude range of approximately 50°–130°E longitude. Planetary waves were clearly observed at higher Southern latitudes during Southern winter, often resulting in order of magnitude variations in ozone density with longitude.

Formation and Motion of Horse Collar Aurora Events

AbstractThe polar cap can become teardrop shaped through the poleward expansion of the dusk and dawn sectors of the auroral oval, to form what is called horse collar aurora (HCA). The formation of HCA has been linked to dual‐lobe reconnection (DLR) where magnetic flux is closed at the dayside magnetopause. A prolonged period of northward IMF is required for the formation of HCA. HCA have previously been identified in UV images captured by the Special Sensor Ultraviolet Spectrographic Imager (SSUSI) instrument on‐board the Defense Meteorological Satellite Program (DMSP) spacecraft F16, F17 and F18. Events that have concurrent 630.0 nm all‐sky camera (ASC) data from the Redline Geospace Observatory (REGO) Resolute Bay site are now studied in more detail, making use of the higher cadence of the ASC images compared to DMSP/SSUSI. 11 HCA events are studied and classified based on the IMF conditions at the end of the event. Five of the events were found to end via a southward turning of the IMF, two end with positive By dominated IMF and four with negative By dominance. Under positive (negative) By the arcs move duskward (dawnward) in the northern hemisphere with the opposite true in the southern hemisphere. Under a southward turning the arcs move equatorward. One event is of particular interest as it occurred while there was a transpolar arc (TPA) also present. Understanding the evolution of HCA will allow DLR to be studied in more detail.

Upgrades to the MAVEN Echelle Data Reduction Pipeline: New Calibration Standard and Improved Faint Emission Detection Algorithm at Lyman‐α

AbstractThe Mars Atmosphere and Volatile Evolution (MAVEN) mission instrument suite includes an ultraviolet echelle spectrograph with high‐spectral resolution designed to resolve D and H Lyman‐α emissions. The high‐spectral resolution mode was previously characterized in the lab and in the cruise phase to Mars and had been calibrated using observations and models of interplanetary hydrogen Lyman‐α emissions. This work presents improved characterizations of the high‐spectral resolution mode using in‐orbit observations that allow for more robust detections of the faint D Lyman‐α emission line. Additionally, the instrument was re‐calibrated using simultaneous and comparable observations made with the Hubble Space Telescope high‐spectral resolution instrument. Comparisons to Lyman‐α observations made with the low‐resolution UV channel on the spectrometer, that had been calibrated with stars, showed consistency in the brightness values for measurements obtained at similar observational conditions. The combined upgrades to the faint‐emission fitting and new calibration techniques of the MAVEN echelle channel have resulted in an improved data‐reduction pipeline with favorable implications for the science utility of D and H Lyman‐α emissions.

Shock shaping? Nebular spectroscopy of nova V906 Carinae

ABSTRACT V906 Carinae was one of the best observed novae of recent times. It was a prolific dust producer and harboured shocks in the early evolving ejecta outflow. Here, we take a close look at the consequences of these early interactions through study of high-resolution Ultraviolet and Visual Echelle spectrograph spectroscopy of the nebular stage and extrapolate backwards to investigate how the final structure may have formed. A study of ejecta geometry and shaping history of the structure of the shell is undertaken following a spectral line $\rm {\small SHAPE}$ model fit. A search for spectral tracers of shocks in the nova ejecta is undertaken and an analysis of the ionized environment. Temperature, density, and abundance analyses of the evolving nova shell are presented.

Detecting the Auroral Oval through CSES-01 Electric Field Measurements in the Ionosphere

We present the results of a systematic study of the ionospheric electric field in the Auroral Oval (AO) region in the southern hemisphere. We exploit one year of electric field measurements taken by the Electric Field Detector (EFD) on board the Chinese Seismo-Electromagnetic Satellite-01 (CSES-01), flying at around 500 km altitude in a sun-synchronous orbit. We exploit the high temporal resolution of the EFD to devise a new technique for the detection of CSES-01 crossing of the AO using electric field measurements only. This new technique combines a Median-Weighted Local Variance Measure with Fast Iterative Filtering to automatically isolate high levels of electromagnetic activity caused by, e.g., particle precipitation and Field Aligned Currents (FACs) at auroral latitudes. We validate this new method against other standard proxies, such as the single-FAC product from the Swarm mission and the auroral radiance emission measured by the Special Sensor Ultraviolet Spectrographic Imager (SSUSI) units on board the Defense Meteorological Satellite Program (DMSP) constellation. Furthermore, we identify ∼3000 orbits (out of a dataset of ∼10,000) where CSES-01 crosses the AO boundary under conditions of high geomagnetic activity. This dataset represents the first step in the systematic study of the auroral electric field, with many potential applications to space weather, thanks to the large amount of continuous observations of the ionosphere by CSES-01 and the forthcoming CSES-02 mission.

Enhanced C2H2 Absorption Within Jupiter's Southern Auroral Oval From Juno UVS Observations

AbstractReflected sunlight observations from the Ultraviolet Spectrograph (UVS) on the Juno spacecraft were used to study the distribution of acetylene (C2H2) at Jupiter's south pole. We find that the shape of the C2H2 absorption feature varies significantly across the polar region, and this can be used to infer spatial variability in the C2H2 abundance. There is a localized region of enhanced C2H2 absorption which coincides with the location of Jupiter's southern polar aurora; the C2H2 abundance poleward of the auroral oval is a factor of 3 higher than adjacent quiescent, non‐auroral longitudes. This builds on previous infrared studies, which found enhanced C2H2 abundances within the northern auroral oval. This suggests that Jupiter's upper‐atmosphere chemistry is being strongly influenced by the influx of charged auroral particles and demonstrates the necessity of developing ion‐neutral photochemical models of Jupiter's polar regions.

Solar Irradiance Spectra from the Compact SOLSTICE (CSOL) Experiment: Instrument Design, FUV Calibration, Measurements, and Comparison of the 2018 Rocket Flight

The Compact SOLSTICE, a compact far and mid ultraviolet (FUV and MUV) spectrograph, flew on a sounding rocket on 18 June 2018 to validate and potentially calibrate the SOLar STellar Irradiance Comparison Experiment (SOLSTICE) onboard the Solar Radiation Climate Experiment (SORCE) spacecraft. This article reports the instrument design, the calibration of the FUV channel, and the FUV irradiance measurements. Irradiance measurements are compared to SOLSTICE showing agreement within the combined instrumental uncertainties at most wavelengths, including the H Lyman-alpha emission at 121.6 nm. Some unexplained differences in line ratios between 130.5 nm and 147.5 nm are observed.

Dayside Temperature Maps of the Upper Mesosphere and Lower Thermosphere of Mars Retrieved From MAVEN IUVS Observations of O I 297.2 nm Emission

AbstractWe present temperature maps derived from number density retrievals of carbon dioxide (CO2) for the upper mesosphere and lower thermosphere of Mars using limb observations from the Imaging Ultraviolet Spectrograph (IUVS) aboard NASA's Mars Atmosphere and Volatile EvolutioN (MAVEN) spacecraft. We retrieve CO2 densities using O(1S) metastable atoms that radiatively relax by emitting photons at 297.2 nm, producing a double‐peaked emission profile detectable by IUVS. Retrieved CO2 densities are used to derive altitude profiles of temperature as a function of latitude, longitude, local time, season, dust activity, and solar activity. CO2 density and temperature profiles retrieved using the O I 297.2 nm emission feature presented herein extend previous IUVS retrievals from 130–170 km down to 80 km. We validate retrieved CO2 densities and derived temperatures using coincident measurements and corresponding data products produced by MAVEN IUVS, as available. Analysis of this comprehensive data set, which spans Mars years 32–36, shows (a) a consistently well‐defined mesopause at approximately 120 km, (b) warming at high pressures (typically below ∼100 km) for a variety of geophysical conditions, (c) asymmetry in temperatures at dawn and dusk with respect to latitude during different seasons (warmer temperatures at dawn during northern hemisphere autumn/winter and cooler temperatures at dusk during spring/summer), and (d) longitudinal waves with a dominant wave‐3 component in both the upper mesosphere and lower thermosphere, with lower (80–90 km) and upper (135–145 km) atmospheric waves about 65° out of phase.

The thermal history of the intergalactic medium at 3.9 ≤ z ≤ 4.3

Abstract A new determination of the temperature of the intergalactic medium (IGM) over $3.9 \leq z \leq 4.3$ is presented. We applied the curvature method on a sample of 10 high-resolution quasar spectra from the Ultraviolet and Visual Echelle Spectrograph on the VLT/ESO. We measured the temperature at mean density by determining the temperature at the characteristic overdensity, which is tight function of the absolute curvature irrespective of $\unicode{x03B3}$ . Under the assumption of fiducial value of $\unicode{x03B3} = 1.4$ , we determined the values of temperatures at mean density $T_{0} = 7893^{+1417}_{-1226}$ K and $T_{0} = 8153^{+1224}_{-993}$ K for redshift range of $3.9 \leq z \leq 4.1$ and $4.1 \leq z \leq 4.3$ , respectively. Even though the results show no strong temperature evolution over the studied redshift range, our measurements are consistent with an IGM thermal history that includes a contribution from He ii reionisation.

Middle and near ultraviolet spectrograph of the Scientific Experimental system in Near SpacE (SENSE)

Middle and near ultraviolet spectrograph of the Scientific Experimental system in Near SpacE (SENSE)

Ganymede's UV Reflectance From Juno‐UVS Data

AbstractDuring the Juno orbit 34 Ganymede encounter, the ultraviolet spectrograph mapped UV sunlight reflected by Ganymede from a closest approach altitude of 1,044 km, allowing us to study spatial variations in Ganymede's far ultraviolet reflectance at higher resolution than has previously been possible. We find that a characteristic signature of water ice seen around 165 nm in laboratory spectra is absent over much of the observed area, but is detectable in the north high latitude region. We suggest that the spectral difference between the high latitudes and other icy regions, such as Tros crater, may be explained by the presence of additional UV‐absorbing contaminants such as NH3 at lower latitudes. We also note a decrease in the relative reflectance of the high latitude regions at wavelengths >190 nm, which may be the start of a previously observed ozone absorption feature.

Transmission spectroscopy of WASP-7 b with UVES

Context. Transmission spectroscopy is a prime technique to study the chemical composition and structure of exoplanetary atmospheres. Strong excess absorption signals have been detected in the optical Na i D1, 2 Fraunhofer lines during transits of hot Jupiters, which are attributed to the planetary atmospheres and allow us to constrain their structure. Aims. We study the atmosphere of WASP-7 b by means of high-resolution transit spectroscopy in the sodium lines. Methods. We analyzed a spectral transit time series of 89 high-resolution spectra of the hot Jupiter WASP-7 b that was observed using the Ultraviolet and Visual Echelle Spectrograph (UVES). We used the telluric lines for an accurate alignment of the spectra and carried out a telluric correction with molecfit. Stellar magnetic activity was monitored by investigating chromospheric lines such as the Ca ii H and K, and hydrogen Hα lines. Finally, we obtained transmission spectra and light curves for various lines. Results. The star shows no identifiable flares and, if any, marginal changes in activity during our observing run. The sodium transmission spectra and corresponding light curves clearly show signs of the Rossiter-McLaughlin effect and the stellar center-to-limb variation that we modeled using synthetic spectra. A statistically significant, narrow absorption feature with a line contrast of 0.50 ± 0.06% (at ~8.3σ level) and a full width at half maximum of 0.13 ± 0.02 Å is detected at the location of the Na i D2 line. For the Na i D1 line signal, we derived a line contrast of 0.13 ± 0.04% (at ~3.2σ level), which we consider a tentative detection. In addition, we provide upper limits for absorption by the hydrogen Balmer lines (Hα, Hβ, and Hγ), K i λ7699 Å, Ca ii H and K, and infra-red triplet lines.

Balloon-borne FIREBall-2 ultraviolet spectrograph stray light control based on nonsequential reverse modeling of on-sky data

We present a comprehensive stray light analysis and mitigation strategy for the FIREBall-2 ultraviolet balloon telescope. Using nonsequential optical modeling, we identified the most problematic stray light paths, which impacted telescope performance during the 2018 flight campaign. After confirming the correspondence between the simulation results and postflight calibration measurements of stray light contributions, a system of baffles was designed to minimize stray light contamination. The baffles were fabricated and coated to maximize stray light collection ability. Once completed, the baffles will be integrated into FIREBall-2 and tested for performance preceding the upcoming flight campaign. Given our analysis results, we anticipate a substantial reduction in the effects of stray light.

LRO‐LAMP Survey of Lunar South Pole Cold Traps: Implication for the Presence of Condensed H2O

AbstractWe employ nighttime observations from the Lyman Alpha Mapping Project (LAMP) ultraviolet (UV) spectrograph onboard the Lunar Reconnaissance Orbiter to investigate the presence of condensed water ice within lunar south pole cold traps. This study incorporates LAMP observations between 2009 and 2016, which more than doubles the number of observations from previous LAMP Permanently Shadowed Regions (PSRs) studies. In this study, we focus on cold traps within five south polar craters: Faustini, Shoemaker, Haworth, Cabeus, and Amundsen. An unnamed cold trap region adjacent to Haworth and Shoemaker was also investigated. We present On‐band (148–162 nm), Edge‐band (164–173 nm), and Off‐band (175–190 nm) albedo maps at 4‐km spatial resolution and far‐UV albedo spectra of cold traps at 2‐nm spectral resolution. We find Off‐band to On‐band albedo ratios (the UV wavelength ranges with high and low H2O ice reflectivity, respectively) within cold traps that are consistent with previous PSR studies. Spectral intimate mixture models considering 5 μm H2O grains and a range of regolith grain sizes (20–74 μm) with porosities between 0.3 and 0.75 yields abundances of ∼0.9–4.9 wt.%, where increasing the range of H2O and regolith grain sizes may increase the range of water abundance. We further find generally lower albedos within the cold trap regions than at the surrounding non‐cold trap regions across all wavelengths, consistent with increased regolith porosity within cold traps as suggested by Gladstone et al. (2012), https://doi.org/10.1029/2011JE003913.

Thermal Structure of the Martian Upper Mesosphere/Lower Thermosphere From MAVEN/IUVS Stellar Occultations

AbstractWe report the first detailed study of the diurnal thermal structure of upper mesosphere/lower thermosphere (∼80 to 160 km) of Mars from stellar occultation observations by the Imaging Ultraviolet Spectrograph (IUVS) aboard the NASA Mars Atmosphere and Volatile EvolutioN (MAVEN) spacecraft. Due to stray light contamination, analyses of this data set to date have been confined to the nighttime events. This study makes use of a revised algorithm for removal of stray light from occultation spectra to retrieve the dayside events as well. The dayside is observed to be warmer than the nightside, with the maximum day/night difference of ∼30 K in the lower thermosphere, ∼20 K around the mesopause, with little diurnal variations at lower altitudes. This is consistent with the radiative time constant which is of the order of 1 Mars day in the to Pa region. The data also shows that the regions at pressure less than Pa are under strong solar control with no prominent migrating tidal signatures. In contrast, on Earth, the radiative time constant near the mesopause is ∼10 Earth days and the temperature variations due to tides are quite large. The Mars Climate Database shows a diurnal trend opposite to the data in the mesosphere, with the dayside mesopause predicted to be cooler than the nightside by ∼10 K along with signatures of a vertically propagating tide. The IUVS data set provides an unprecedented constraint on the structure of the Martian mesosphere.

Thermospheric Conditions Associated With the Loss of 40 Starlink Satellites

AbstractWe analyzed far ultraviolet data from Defense Meteorological Satellite Program (DMSP)/Special Sensor Ultraviolet Spectrographic Imager (SSUSI) and Thermosphere Ionosphere Mesosphere Energetics and Dynamics (TIMED)/Global Ultraviolet Imager (GUVI) and found significant changes in the thermospheric density and composition during the 3–5 February 2022 storm when 40 Starlink satellites started to re‐enter the atmosphere associated with increased neutral drag at an altitude around 210 km. The standard NRLMSISE‐00 model predicts only ∼5% increase in neutral density at 210 km. TIMED/GUVI observations showed a clear increase in the thermospheric N2/O column density ratio and an increase in the nitric oxide (NO) column density, indicating high thermospheric density, and temperature. DMSP SSUSI 130.4 nm limb data showed a clear increase in neutral density in the low latitude region (−30°–30°). On the duskside (17:32 LT), the density increase (compared to the pre‐storm density) is around 11%–18% at 210 km and around 18%–26% at 520 km. On the dawn‐side (05:19 LT), the density increase is more significant, 40%–59% at 210 km and ∼300% at 520 km. There are a few reasons for the dawn‐dusk asymmetry in the relative density increase: (a) the pre‐storm background density or heat capacity is lower on the dawn‐side than that on the duskside, (b) stronger auroral heating was observed on the dawnside based on analysis of auroral images, (c) the thermosphere density enhancement partially recovered when the disturbance co‐rotated with the Earth from dawn to dusk side due to enhanced infrared cooling by NO. Note that the neutral density increase is expected to be much higher at high latitudes where the neutral disturbances are initiated.

Dust depletion of metals from local to distant galaxies

Large fractions of metals are missing from the observable gas-phase in the interstellar medium (ISM) because they are incorporated into dust grains. This phenomenon is called dust depletion. It is important to study the depletion of metals into dust grains in the ISM to investigate the origin and evolution of metals and cosmic dust. We characterize the dust depletion of several metals from the Milky Way to distant galaxies. We collected measurements of ISM metal column densities from absorption-line spectroscopy in the literature, and in addition, we determined Ti and Ni column densities from a sample of 70 damped Lyman-α absorbers (DLAs) toward quasars that were observed at high spectral resolution with the Very Large Telescope (VLT) Ultraviolet and Visual Echelle Spectrograph (UVES). We used relative ISM abundances to estimate the dust depletion of 18 metals (C, P, O, Cl, Kr, S, Ge, Mg, Si, Cu, Co, Mn, Cr, Ni, Al, Ti, Zn, and Fe) for different environments (the Milky Way, the Magellanic Clouds, and DLAs toward quasars and towards gamma-ray bursts). We observed overall linear relations between the depletion of each metal and the overall strength of the dust depletion, which we traced with the observed [Zn/Fe]. The slope of these dust depletion sequences correlates with the condensation temperature of the various elements, that is, the more refractory elements show steeper depletion sequences. In the neutral ISM of the Magellanic Clouds, small deviations from linearity are observed as an overabundance of the α-elements Ti, Mg, S, and an underabundance of Mn, including for metal-rich systems. The Ti, Mg, and Mn deviations completely disappear when we assume that all systems in our sample of OB stars observed toward the Magellanic Clouds have an α-element enhancement and Mn underabundance, regardless of their metallicity. This may imply that the Magellanic Clouds have recently been enriched in α-elements, potentially through recent bursts of star formation. We also observe an S overabundance in all local galaxies, which is an effect of ionization due to the contribution of their H ii regions to the measured S ii column densities. The observed strong correlations of the depletion sequences of the metals all the way from low-metallicity quasi-stellar object DLAs to the Milky Way suggest that cosmic dust has a common origin, regardless of the star formation history, which, in contrast, varies significantly between these different galaxies. This supports the importance of grain growth in the ISM as a significant process of dust production.

Formula omitted] cooling in the jovian aurora: Juno remote sensing observations and modeling

Cooling of Jupiter's auroral thermosphere by H3+ radiation to space is one of the processes controlling the energy balance in the auroral upper atmosphere. The UltraViolet Spectrograph (UVS) and the Jupiter InfraRed Auroral Mapper (JIRAM) on board Juno have observed the Jovian polar aurora from its polar orbit since August 2016. The UVS instrument measures the H2 Lyman and Werner bands whose brightness is a proxy of the precipitated auroral electron flux. The 3.3–3.6 μm spectral window of the JIRAM L-band imager maps the H3+ thermal radiance with unprecedented spatial resolution. Comparison of concurrent observations indicates that the morphological features are similar in the two spectral regions but differences are also observed in the spatial intensity contrast. We compare the total (direct and indirect) particle heating rate and the cooling by H3+ radiation derived from four pairs of simultaneous UVS and JIRAM images. The total auroral cooling power H3+ is in the range 2–4 terawatts in both hemispheres. In all cases, the H3+ cooling in the aurora is found to range between 0.45 and 0.67 time less than the particle collisional heating. In a second step, we use the ultraviolet H2 brightness and FUV color ratio to derive the characteristics of the electron precipitation and model the H3+ radiance for each UVS map pixel. The comparison of the H3+ modeled radiance map with the JIRAM observations shows general good agreement with some local differences. The four spatially integrated H3+ cooling power from the model are in very good agreement with the JIRAM values. These results are important constraints for global magnetosphere-ionosphere-atmosphere models.