Far-ultraviolet Research Articles

Imaging and photometric studies of NGC 1316 (Fornax A) using Astrosat/UVIT

We present imaging and photometric studies of the radio galaxy NGC 1316 (Fornax~A) using high spatial resolution near-ultraviolet (NUV) and far-ultraviolet (FUV) imaging telescopes of the first Indian multi-wavelength space observatory {\it AstroSat}. The residual maps of UV emission obtained from the subtraction of smooth models witness peculiar features within the central few kpc (1-2 kpc) region. The spatial correspondence between the radio emission maps and FUV imaging study reveal that the UV emitting sources are displaced away from the centre by the AGN outburst (radio jet). The presence of rims and clumpy structures in the outskirt of this galaxy delineate that the galaxy has acquired a large fraction of gas through merger-like events and is still in the process of settling. The estimates of the star formation rates (SFR) using FUV and NUV luminosities are found to be 0.15 M$_\odot$yr$^{-1}$ and 0.36 M$_\odot$yr$^{-1}$, respectively, and provide the lower limit due to the screen effect. The estimated lower rates of SFR in this galaxy probably represent its quenching due to the AGN driven outflows emanating from the central engine of NGC 1316.

Comparing the Inner and Outer Star-forming Complexes in the Nearby Spiral Galaxies NGC 628, NGC 5457, and NGC 6946 Using UVIT Observations

Abstract We present a far-UV (FUV) study of the star-forming complexes (SFCs) in three nearby galaxies using the Ultraviolet Imaging Telescope. The galaxies are close to face-on and show significant outer disk star formation. Two of them are isolated (NGC 628 and NGC 6946), and one is interacting with distant companions (NGC 5457). We compared the properties of the SFCs inside and outside the optical radius (R 25). We estimated the sizes, star formation rates (SFRs), metallicities, and Toomre Q parameter of the SFCs. We find that the outer disk SFCs are at least 10 times smaller in area than those in the inner disk. The SFRs per unit area (ΣSFR) in both regions have similar mean values, but the outer SFCs have a much smaller range of ΣSFR. They are also metal-poor compared to the inner disk SFCs. The FUV emission is well correlated with the neutral hydrogen gas (H i) distribution and detected within and near several H i holes. Our estimation of the Q parameter in the outer disks of the two isolated galaxies suggests that their outer disks are stable (Q > 1). However, their FUV images indicate that there is ongoing star formation in these regions. This suggests that there may be some nonluminous mass or dark matter in their outer disks, which increases the disk surface density and supports the formation of local gravitational instabilities. In the interacting galaxy, NGC 5457, the baryonic surface density is sufficient (Q < 1) to trigger local disk instabilities in the outer disk.

The Hubble PanCET program: long-term chromospheric evolution and flaring activity of the M dwarf host GJ 3470

Neptune-size exoplanets seem particularly sensitive to atmospheric evaporation, making it essential to characterize the stellar high-energy radiation that drives this mechanism. This is particularly important with M dwarfs, which emit a large and variable fraction of their luminosity in the ultraviolet and can display strong flaring behavior. The warm Neptune GJ 3470b, hosted by an M2 dwarf, was found to harbor a giant exosphere of neutral hydrogen thanks to three transits observed with the Hubble Space Telescope Imaging Spectrograph (HST/STIS). Here we report on three additional transit observations from the Panchromatic Comparative Exoplanet Treasury program, obtained with the HST Cosmic Origin Spectrograph. These data confirm the absorption signature from GJ 3470b’s exosphere in the stellar Lyman-α line and demonstrate its stability over time. No planetary signatures are detected in other stellar lines, setting a 3σ limit on GJ 3470b’s far-ultraviolet (FUV) radius at 1.3 times its Roche lobe radius. We detect three flares from GJ 3470. They show different spectral energy distributions but peak consistently in the Si III line, which traces intermediate-temperature layers in the transition region. These layers appear to play a particular role in GJ 3470’s activity as emission lines that form at lower or higher temperatures than Si III evolved differently over the long term. Based on the measured emission lines, we derive synthetic X-ray and extreme-ultraviolet (X+EUV, or XUV) spectra for the six observed quiescent phases, covering one year, as well as for the three flaring episodes. Our results suggest that most of GJ 3470’s quiescent high-energy emission comes from the EUV domain, with flares amplifying the FUV emission more strongly. The neutral hydrogen photoionization lifetimes and mass loss derived for GJ 3470b show little variation over the epochs, in agreement with the stability of the exosphere. Simulations informed by our XUV spectra are required to understand the atmospheric structure and evolution of GJ 3470b and the role played by evaporation in the formation of the hot-Neptune desert.

Local Volume Late-Type Galaxies with an Extreme Ratio of Hα-to-FUV Star-Formation Rates

Abstract Using a currently most representative sample of 477 late-type galaxies within 11 Mpc of the Milky Way with measured star-formation rates (SFRs) from the far ultraviolet (FUV) and Hα emission line fluxes, we select galaxies with the extreme ratios: SFR(Hα)/SFR(FUV) > 2 and SFR(Hα)/SFR(FUV) < 1/20. Each subsample amounts to ∼5% of the total number and consists of dwarf galaxies with the stellar masses M*/M⊙ = (5.5 − 9.5) dex. In spite of a huge difference in their SFR(Hα) activity on a scale of ∼10 Myr, the temporarily “excited” and temporarily “quiescent” galaxies follow one and the same relation between SFR(FUV) and M* on a scale of ∼100 Myr. Their average specific star-formation rate log [SFR(FUV)/M*] = −10.1 ± 0.1 (yr−1) coinsides with the Hubble parameter log (H0) = −10.14 (yr−1). On a scale of t ∼ 10 Myr, variations of SFR have a moderate flash amplitude of less than 1 order above the main-sequence and a fading amplitude to 2 orders below the average level. In general, both temporarily excited and temporarily quiescent galaxies have almost similar gas fractions as normal main-sequence galaxies, being able to maintain the current rate of star-formation on another Hubble time scale. Ranging the galaxies according to the density contrast produced by the nearest massive neighbor exhibits only a low average excess of SFR caused by tidal interactions.

Attenuated Total Reflection-Far-Ultraviolet Spectroscopy and Quantum Chemical Calculations of the Electronic Structure of the Top Surface and Bulk of Polyethylenes with Different Crystallinities.

In this study, we explored the electronic structure of the surfaces of polyethylene samples having different crystallinities using attenuated total reflection (ATR) far-ultraviolet (FUV) spectroscopy and quantum chemical calculations. Specifically, the ATR-FUV spectra of five types of high-density polyethylene (HDPE), six types of linear low-density PE (LLDPE), and seven types of low-density PE (LDPE) were obtained. All the spectra contained an intense band near 156 nm and a broad band between 180 and 190 nm. Transmission spectra were obtained for the thin-film (30 µm) PE samples between 165 and 250 nm. In this region, the HDPE films show very low-intensity bands. In contrast, the transmission spectra of the LLDPE and LDPE samples yielded weak-to-medium and medium-intensity bands around 180-190 nm, respectively. In addition, to understand the differences in the absorption spectra among the PEs observed, we simulated the spectra of n-pentane as a PE crystal model using time-dependent density functional theory and found that the common intense band at 156 nm is due to the σ (C(2p)-H)→Rydberg 3s, 3p transition. The absorption bands near 180-190 nm may correspond to aggregates of numerous molecular chains in the amorphous parts of the LLDPE and LDPE samples.

Optimization of the deposition parameters of MgF2/LaF3 narrowband reflective FUV multilayers

The development of efficient dielectric coatings in the far UV (FUV) is demanded for upcoming space instrumentation, such as LUMOS (LUVOIR Ultraviolet Multi-Object Spectrograph) in LUVOIR mission, among other applications. Multilayers (MLs) based on MgF2 and LaF3 have been developed in the last decades for the 157-nm and 193-nm lithography, demonstrating excellent optical properties. Yet, the deposition procedure to obtain coatings with optimal performance has not been fully detailed in the open literature, such as the dependence of ML performance with deposition and post-deposition temperature. This research investigates the effect of the substrate deposition temperature of MgF2/LaF3 ML coatings prepared by thermal evaporation on FUV reflectance, stress, roughness, as well as the performance of the coatings and their evolution over time. The relatively higher expansion coefficient of these two fluorides in comparison with fused silica substrates results in a large tensile stress for coatings deposited at high temperature and later cooled down to room temperature. Such stress may result in coating cracking and delamination. A compromise deposition temperature of ∼240°C and 13 bilayers was found for optimal ML reflectance peaked at ∼160 nm before cracks are generated. Above this deposition temperature, stress increased, which resulted in an extension of the cracked area and in a slight roughness increase and FUV reflectance decrease. MLs that were deposited at room temperature and later annealed resulted in a similar reflectance and stress to those of hot-deposited coatings for a given temperature.

The ALPINE-ALMA [CII] survey

Star formation rate (SFR) measurements at z > 4 have relied mostly on the rest-frame far-ultraviolet (FUV) observations. The corrections for dust attenuation based on the IRX-β relation are highly uncertain and are still debated in the literature. Hence, rest-frame far-infrared (FIR) observations are necessary to constrain the dust-obscured component of the SFR. In this paper, we exploit the rest-frame FIR continuum observations collected by the ALMA Large Program to INvestigate [CII] at Early times (ALPINE) to directly constrain the obscured SFR in galaxies at 4.4 < z < 5.9. We used stacks of continuum images to measure average infrared luminosities taking both detected and undetected sources into account. Based on these measurements, we measured the position of the main sequence of star-forming galaxies and the specific SFR (sSFR) at z ∼ 4.5 and z ∼ 5.5. We find that the main sequence and sSFR do not significantly evolve between z ∼ 4.5 and z ∼ 5.5, as opposed to lower redshifts. We developed a method to derive the obscured SFR density (SFRD) using the stellar masses or FUV-magnitudes as a proxy of FIR fluxes measured on the stacks and combining them with the galaxy stellar mass functions and FUV luminosity functions from the literature. We obtain consistent results independent of the chosen proxy. We find that the obscured fraction of SFRD is decreasing with increasing redshift, but even at z ∼ 5.5 it constitutes around 61% of the total SFRD.

FUV and NIR size of the HI selected low surface brightness galaxies

How low surface brightness galaxies (LSBGs) form stars and assemble stellar mass is one of the most important questions related to understanding the LSBG population. We select a sample of 381 HI bright LSBGs with both far ultraviolet (FUV) and near infrared (NIR) observations to investigate the star formation rate (SFR) and stellar mass scales, and the growth mode. We measure the FUV and NIR radii of our sample, which represent the star-forming and stellar mass distribution scales respectively. We also compare the FUV and H band radius-stellar mass relation with archival data, to identify the SFR and stellar mass structure difference between the LSBG population and other galaxies. Since galaxy HI mass has a tight correlation with the HI radius, we can also compare the HI and FUV radii to understand the distribution of HI gas and star formation activities. Our results show that most of the HI selected LSBGs have extended star formation structure. The stellar mass distribution of LSBGs may have a similar structure to disk galaxies at the same stellar mass bins, but the star-forming activity of LSBGs happens at a larger radius than the high surface density galaxies, which may help to identify the LSBG sample from the wide-field deep u band image survey. The HI is also distributed at larger radii, implying a steeper (or not) Kennicutt-Schmidt relation for LSBGs.

Hubble Space Telescope Observations of the Old Pulsar PSR J0108–1431*

Abstract We present results of optical-UV observations of the 200 Myr old rotation-powered radio pulsar J0108−1431 with the Hubble Space Telescope. We found a putative candidate for the far-UV (FUV) pulsar counterpart, with the flux density f ν = 9.0 ± 3.2 nJy at λ = 1528 Å. The pulsar was not detected, however, at longer wavelengths, with 3σ upper limits of 52, 37, and 87 nJy at λ = 4326, 3355, and 2366 Å, respectively. Assuming that the pulsar counterpart was indeed detected in FUV, and the previously reported marginal U and B detections with the Very Large Telescope were real, the optical-UV spectrum of the pulsar can be described by a power-law model with a nearly flat f ν spectrum. Similar to younger pulsars detected in the optical, the slope of the nonthermal spectrum steepens in the X-ray range. The pulsar’s luminosity in the 1500–6000 Å wavelength range, L ∼ 1.2 × 1027(d/210 pc)2 erg s−1, corresponds to a high efficiency of conversion of pulsar rotation energy-loss rate to the optical-UV radiation, , depending on somewhat uncertain values of distance and spectral slope. The brightness temperature of the bulk neutron star surface does not exceed 59,000 K (3σ upper bound), as seen by a distant observer. If we assume that the FUV flux is dominated by a thermal component, then the surface temperature can be in the range of 27,000–55,000 K, Requiring a heating mechanism to operate in old neutron stars.

Long-slit cross-dispersion spectroscopy for Hyperion UV space telescope

Molecular clouds are a crucial stage in the lifecycle of a star, and the far ultraviolet (FUV) spectral range is a prime observation band. Hyperion is an FUV space telescope that investigates the birth clouds of stars using a high-resolution spectrometer. To meet the scientific requirements, we developed and evaluated a spectrometer that covers the 140.5 to 164.5 nm wavelength range with a spectral resolution higher than 30,000. We employed on-axis and on-plane dispersive optic layouts to control the aberration from a large aspect ratio slit (10 arcmin × 2.5 arcsec, aspect ratio R = 240). The cross-dispersion isolates three orders from the échelle grating (n = − 19, −18, and −17), and the subsequent two-mirror freeform imaging optics form a two-dimensional spectral distribution on a 50 mm × 50 mm detector array. The geometrical and spectral performances of this innovative design are evaluated.

A detailed study of X-ray cavities in the intracluster environment of the cool core cluster Abell 3017

ABSTRACT We present a detailed analysis of a cool-core galaxy cluster Abell 3017, at a redshift of z = 0.219, which has been identified to be merging with its companion cluster Abell 3016. This study has made use of X-ray (Chandra), ultraviolet (UV) [Galaxy Evolution Explorer(GALEX)], optical [European Southern Observatory (ESO)/very large telescope (VLT)], mid-infrared [(Wide-field Infrared Survey Explorer (WISE)], and radio upgraded Giant Metrewave radio telescope (uGMRT) observations of this cluster. Using various image processing techniques, such as unsharp masking, 2D fits using β models, contour binning and the use of surface brightness profiles, we show the existence of a pair of X-ray cavities, at a projected distance of ∼20 arcsec (70 kpc) and ∼16 arcsec (57 kpc), respectively, from the core of Abell 3017. We also detect an excess of X-ray emission located at ∼25 arcsec (88 kpc) south of the centre of Abell 3017, is likely due to the bulk motions in the intracluster medium either by gas sloshing or ram-pressure striping due to a merger. We find that the radio lobes are responsible for the observed X-ray cavities detected in this system. The lower values of mid-infrared WISE colour [W1–W2] and [W2–W3] imply that the central BCG of Abell 3017 is a star-forming galaxy. The current star formation rate of the central BCG, estimated from the H α and GALEX far-ultraviolet (FUV) luminosities, are equal to be ∼5.06 ± 0.78 and ∼9.20 ± 0.81 $\rm M_{\odot }$ yr−1, respectively. We detect, for the first time, a radio phoenix ∼150 kpc away from the radio core, with a spectral index of (α ≤ −1.8). We also report the detection of $\rm ~Pa\,\alpha$ emission in this cluster using ESO VLT SINFONI imaging data.

Multi-instrument analysis of far-ultraviolet aurora in the southern hemisphere of comet 67P/Churyumov-Gerasimenko

Aims.We aim to determine whether dissociative excitation of cometary neutrals by electron impact is the major source of far-ultraviolet (FUV) emissions at comet 67P/Churyumov-Gerasimenko in the southern hemisphere at large heliocentric distances, both during quiet conditions and impacts of corotating interaction regions observed in the summer of 2016.Methods.We combined multiple datasets from the Rosetta mission through a multi-instrument analysis to complete the first forward modelling of FUV emissions in the southern hemisphere of comet 67P and compared modelled brightnesses to observations with the Alice FUV imaging spectrograph. We modelled the brightness of OI1356, OI1304, Lyman-β, CI1657, and CII1335 emissions, which are associated with the dissociation products of the four major neutral species in the coma: CO2, H2O, CO, and O2. The suprathermal electron population was probed by the Ion and Electron Sensor of the Rosetta Plasma Consortium and the neutral column density was constrained by several instruments: the Rosetta Orbiter Spectrometer for Ion and Neutral Analysis (ROSINA), the Microwave Instrument for the Rosetta Orbiter and the Visual InfraRed Thermal Imaging Spectrometer.Results.The modelled and observed brightnesses of the FUV emission lines agree closely when viewing nadir and dissociative excitation by electron impact is shown to be the dominant source of emissions away from perihelion. The CII1335 emissions are shown to be consistent with the volume mixing ratio of CO derived from ROSINA. When viewing the limb during the impacts of corotating interaction regions, the model reproduces brightnesses of OI1356 and CI1657 well, but resonance scattering in the extended coma may contribute significantly to the observed Lyman-βand OI1304 emissions. The correlation between variations in the suprathermal electron flux and the observed FUV line brightnesses when viewing the comet’s limb suggests electrons are accelerated on large scales and that they originate in the solar wind. This means that the FUV emissions are auroral in nature.

The Thermospheric Column O/N2 Ratio.

More than 2 decades ago, D. J. Strickland and colleagues proposed use of the O/N2 column number density ratio as a new geophysical quantity to interpret thermospheric processes recorded in far ultraviolet (FUV) images of the Earth. This concept has enabled multiple advances in understanding the global behavior of Earth's thermosphere. Nevertheless, confusion remains about the conceptual meaning of the column density ratio, and in the application of this integral quantity. This is so even though it is now a key thermospheric measurement made by current and planned far ultraviolet remote sensing missions in pursuit of new understanding of thermospheric processes and variability. The intent here is to review the historical context of the O/N2 column density ratio, clarify its physical meaning, and resolve misunderstandings evident in the literature. Simple examples elucidate its original derivation for extracting column O/N2 ratios from measurements of the OI 135.6 nm/N2 Lyman-Birge-Hopfield (LBH) emission based on an algorithmic synthesis of model precomputations. These are organized in the form of a table lookup of column density ratio as a function of observed radiance ratios. To accommodate generalized solar-geophysical and viewing conditions, the table required to specify the number of needed parameters becomes large. Proposed as an alternative is a simplified, first principles approach to obtaining the column density ratio from the emission ratio. This new methodology is now being applied successfully to FUV measurements made from onboard the Ionospheric CONnection satellite and will be applied retrospectively to the Global Ultraviolet Imager data.

Photoevaporative Dispersal of Protoplanetary Disks around Evolving Intermediate-mass Stars

Abstract We aim to understand the effect of stellar evolution on the evolution of protoplanetary disks. We focus in particular on the disk evolution around intermediate-mass (IM) stars, which evolve more rapidly than low-mass ones. We numerically solve the long-term evolution of disks around 0.5–5 M ☉ stars considering viscous accretion and photoevaporation (PE) driven by stellar far-ultraviolet (FUV), extreme-ultraviolet (EUV), and X-ray emission. We also take stellar evolution into account and consider the time evolution of the PE rate. We find that the FUV, EUV, and X-ray luminosities of IM stars evolve by orders of magnitude within a few million years, along with the time evolution of stellar structure, stellar effective temperature, or accretion rate. Therefore, the PE rate also evolves with time by orders of magnitude, and we conclude that stellar evolution is crucial for the disk evolution around IM stars.

Semiempirical Modeling of the Atmospheres of the M Dwarf Exoplanet Hosts GJ 832 and GJ 581

Abstract Stellar ultraviolet (UV) radiation drives photochemistry, and extreme-ultraviolet (EUV) radiation drives mass loss in exoplanet atmospheres. However, the UV flux is partly unobservable due to interstellar absorption, particularly in the EUV range (100–912 Å). It is therefore necessary to reconstruct the unobservable spectra in order to characterize the radiation environment of exoplanets. In the present work, we use a radiative transfer code SSRPM to build one-dimensional semiempirical models of two M dwarf exoplanet hosts, GJ 832 and GJ 581, and synthesize their spectra. SSRPM is equipped with an extensive atomic and molecular database and full-NLTE capabilities. We use observations in the visible, ultraviolet, and X-ray ranges to constrain atmospheric structures of the modeled stars. The synthesized integrated EUV fluxes are found to be in good agreement with other reconstruction techniques, but the spectral energy distributions disagree significantly across the EUV range. More than two-thirds of the EUV flux is formed above 105 K. We find that the far-ultraviolet (FUV) continuum contributes 42%–54% of the entire FUV flux between 1450 and 1700 Å. The comparison of stellar structures of GJ 832 and GJ 581 suggests that GJ 832 is a more magnetically active star, which is corroborated by other activity indicators.

A possible far-ultraviolet flux-dependent core mass function in NGC 6357

Context. NGC 6357 is a galactic star-forming complex (d ~ 1.7 kpc) composed of several HII regions, a few young stellar clusters, and giant molecular clouds. In particular, the HII regions G353.2+0.9, G353.1+0.6, and G353.2+0.7 are associated with three young clusters; the most prominent of these, Pismis 24, contains some of the most massive stars known. Aims. We aim to derive the properties of the densest compact gas structures (cores) in the region as well as the effects of an intense far-ultraviolet (FUV) radiation field on their global properties. Methods. We mapped the NGC 6357 region at 450 and 850 μm with SCUBA-2 and in the CO(3–2) line with HARP at the JCMT. We also made use of the Herschel Hi-GAL data at 70 and 160 μm. We used the algorithm Gaussclumps to retrieve the compact cores embedded in the diffuse sub-millimetre emission and constructed their spectral energy distribution from 70 to 850 μm, from which we derived mass and temperature. We divided the observed area into an ‘active’ region (i.e. the eastern half, which is exposed to the FUV radiation from the more massive members of the three clusters) and a ‘quiescent’ region (i.e. the western half, which is less affected by FUV radiation). We compared the core mass functions and the temperature distributions in the two areas to look for any differences that could be due to the different levels of FUV radiation. Results. We retrieved 686 dense cores, 411 in the active region and 275 in the quiescent region, with an estimated mass completeness limit of ~5 M⊙. We also attempted to select a sample of pre-stellar cores based on cross-correlation with 70 μm emission and red WISE point sources, which unfortunately is biased due to distance, emission at 70 μm from the dust on the surface of the cores that is heated by the FUV radiation, and saturation in the WISE bands. Most of the cores above the mass completeness limit are likely to be gravitationally bound. The fraction of gas in dense cores is very low, 1.4%. We found a mass-size relation log(M∕M⊙) ~ a × log(D∕arcsec), with a in the range 2.0–2.4, depending on the precise selection of the sample. The temperature distributions in the two sub-regions are clearly different, peaking at ~25 K in the quiescent region and at ~35 K in the active region. The core mass functions are different as well, at a 2σ level, consistent with a Salpeter initial mass function in the quiescent region and flatter than that in the active region. The dense cores lying close to the HII regions are consistent with pre-existing cores being gradually engulfed by a photon dominated region and photoevaporating. A comparison of the obtained distribution of core masses with those derived from simulations of cloud-cloud collisions yields no conclusive evidence of ongoing cloud-cloud collisions. Conclusions. We attribute the different global properties of dense cores in the two sub-regions to the influence of the FUV radiation field.

On the use of directional derivatives of SuperDARN convection flow for estimating open-closed field line boundary during the intervals of substorm activity on 14 October 2001

The maximum peak of the directional derivative of the SuperDARN convection flow has been proposed as a proxy for the open/closed magnetic field line boundary (OCB) and this has been evaluated using data from the ultraviolet imagers collected during the steady southward Interplanetary Magnetic Field (IMF) condition. However, the technique ignores the contribution of ionospheric vorticity in determining the peak of the directional derivative of SuperDARN convection flow. In this paper these effects have been considered when identifying the OCB in the night sector, calculated using the directional derivatives of SuperDARN convection flow during the active substorm interval which occurred on October 14, 2001. Statistical comparison of the latitudes of the OCB, as estimated from the peaks of the directional derivative of the SuperDARN convection flow, determined with and without considering the relative maxima and minima vorticity, and as provided by images taken by the two Far Ultraviolet (FUV) Spectral Imagers onboard the IMAGE spacecraft (SI-12 and SI-13) indicates that the modified approach of the directional derivative of the SuperDARN convection flow may be used as an OCB proxy during active geomagnetic activity.

ATR-far-ultraviolet spectroscopy in the condensed phase—The present status and future perspectives

Far-ultraviolet (FUV) spectroscopy in the region of 140–200 nm of condensed-phase has received keen interest as a new electronic spectroscopy. The introduction of the attenuated total reflection (ATR) technique to the FUV region has opened a new avenue for FUV spectroscopy of liquids and solids. ATR-FUV spectroscopy enables the study of electronic structures and transitions of most types of molecules. It also holds great promise for a variety of applications, i.e., from the application to basic sciences to practical applications. In this review, the characteristics and advantages of ATR-FUV spectroscopy in the condensed phase are described first; then, a brief historical overview is provided. Next, the ATR-FUV spectroscopy instrumentation is outlined. After these introductory parts, a variety of AFT-FUV spectroscopy applications are introduced, starting from applications to investigations of electronic structure and transitions of alkanes, graphenes, and polymers. Then, time-resolved ATR-FUV spectroscopy is discussed. The applications to materials research, such as the research on inorganic semiconductors and ionic liquids, follow. In the last part, the FUV spectroscopy perspective is emphasized.

Auroral ionospheric E region parameters obtained from satellite- based far-ultraviolet and ground-based ionosonde observations – effects of proton precipitation

Abstract. Coincident auroral far-ultraviolet (FUV) and ground-based ionosonde observations are compared for the purpose of determining whether auroral FUV remote sensing algorithms that assume pure electron precipitation are biased in the presence of proton precipitation. Auroral particle transport and optical emission models, such as the Boltzmann 3-Constituent (B3C) model, predict that maximum E region electron density (NmE) values derived from auroral Lyman–Birge–Hopfield (LBH) emissions, assuming electron precipitation, will be biased by up to ∼20 % (high) for pure proton aurora, while comparisons between LBH radiances and radiances derived from in situ particle flux observations (i.e., Knight et al., 2008, 2012) indicate that the bias associated with proton aurora should be much larger. Surprisingly, in the comparisons with ionosonde observations described here, no bias associated with proton aurora is found in FUV-derived auroral NmE, which means that auroral FUV remote sensing methods for NmE are more accurate in the presence of proton precipitation than was suggested in the aforementioned earlier works. Possible explanations for the discrepancy with the earlier results are discussed.

Construction of a far-ultraviolet all-sky map from an incomplete survey: application of a deep learning algorithm

ABSTRACT We constructed a far-ultraviolet (FUV) all-sky map based on observations from the Far Ultraviolet Imaging Spectrograph (FIMS) aboard the Korean microsatellite Science and Technology SATellite-1. For the ${\sim}20{{\ \rm per\ cent}}$ of the sky not covered by FIMS observations, predictions from a deep artificial neural network were used. Seven data sets were chosen for input parameters, including five all-sky maps of H α, E(B − V), N(H i), and two X-ray bands, with Galactic longitudes and latitudes. 70 ${{\ \rm per\ cent}}$ of the pixels of the observed FIMS data set were randomly selected for training as target parameters and the remaining 30 ${{\ \rm per\ cent}}$ were used for validation. A simple four-layer neural network architecture, which consisted of three convolution layers and a dense layer at the end, was adopted, with an individual activation function for each convolution layer; each convolution layer was followed by a dropout layer. The predicted FUV intensities exhibited good agreement with Galaxy Evolution Explorer observations made in a similar FUV wavelength band for high Galactic latitudes. As a sample application of the constructed map, a dust scattering simulation was conducted with model optical parameters and a Galactic dust model for a region that included observed and predicted pixels. Overall, FUV intensities in the observed and predicted regions were reproduced well.