Ly-α Brightness Research Articles

Seasonal Variation of Saturn's Lyα Brightness

We examine Saturn’s nonauroral (dayglow) emissions at Lyα observed by the Cassini/Ultraviolet Imaging Spectrograph (UVIS) instrument from 2003 until 2017, to constrain meridional and seasonal trends in the upper atmosphere. We separate viewing geometry effects from trends driven by atmospheric properties, by applying a multivariate regression to the observed emissions. The Lyα dayglow brightnesses depend on the incident solar flux, solar incidence angle, emission angle, and observed latitude. The emissions across latitudes and seasons show a strong dependence with solar incidence angle, typical of resonantly scattered solar flux and consistent with no internal source such as electroglow. We observe a bulge in Lyα brightnesses that shifts with the summer season from the southern to the northern hemisphere. We estimate atomic hydrogen optical depths above the methane homopause level for dayside disk observations (2004–2016) by comparing observed Lyα emissions to a radiative transfer model. We model emissions from resonantly scattered solar flux and a smaller but significant contribution by scattered photons from the interplanetary hydrogen (IPH) background. During the northern summer, inferred hydrogen optical depths steeply decrease with latitude toward the winter hemisphere from a northern hemisphere bulge, as predicted by a 2D seasonal photochemical model. The southern hemisphere mirrors this trend during its summer. However, inferred optical depths show substantially more temporal variation between 2004 and 2016 than predicted by the photochemical model. We benchmark our brightness values by comparing observed IPH Lyα emissions from Cassini/UVIS in 2006 with a model of the IPH emissions. Cassini/UVIS observations agree well with the modeled IPH background.

The Enigmatic Abundance of Atomic Hydrogen in Saturn’s Upper Atmosphere

A planet’s Lyα emission is sensitive to its thermospheric structure. Here we report joint Hubble Space Telescope and Cassini cross-calibration observations of the Saturn Lyα emission made 2 weeks before the Cassini grand finale. To investigate the long-term Saturn Lyα airglow observed by different ultraviolet instruments, we cross-correlate their calibration, finding that while the official Cassini/UVIS sensitivity should be lowered by ∼75%, the Voyager 1/UVS sensitivities should be enhanced by ∼20% at the Lyα channels. This comparison also allowed us to discover a permanent feature of the Saturn disk Lyα brightness that appears at all longitudes as a brightness excess (Lyα bulge) of ∼30% (∼12σ) extending over the latitude range ∼5°–35° N compared to the regions at equator and ∼60° N. This feature is confirmed by three distinct instruments between 1980 and 2017 in the Saturn north hemisphere. To analyze the Lyα observations, we use a radiation transfer model of resonant scattering of solar and interplanetary Lyα photons and a latitude-dependent photochemistry model of the upper atmosphere constrained by occultation and remote-sensing observations. For each latitude, we show that the Lyα observations are sensitive to the temperature profile in the upper stratosphere and lower thermosphere, thus providing useful information in a region of the atmosphere that is difficult to probe by other means. In the Saturn Lyα bulge region, at latitudes between ∼5° and ∼35°, the observed brightening and line broadening support seasonal effects, variation of the temperature vertical profile, and potential superthermal atoms that require confirmation.

New Horizons Detection of the Local Galactic Lyman-α Background

Abstract Since 2007 the Alice spectrograph on the New Horizons (NH) spacecraft has been used to periodically observe the Lyman-α (Lyα) emissions of the interplanetary medium (IPM), which mostly result from resonant scattering of solar Lyα emissions by interstellar hydrogen atoms passing through the solar system. Three observations of IPM Lyα along a single great circle were made during the NH cruise to Pluto, and these have been supplemented by observations along six great circles (spread over the sky at 30° intervals), acquired one month before and one day after the NH flyby of Pluto, and on a further five occasions since then, out to just over 47 au from the Sun. These data indicate a distant Lyα background of 43 ± 3 Rayleigh brightness (equivalent to 56 ± 4 nW m−2 sr−1), which is present in all directions (i.e., not only in the upstream direction, as previously reported). This result is found independently by: (1) the falloff with distance from the Sun of the IPM Lyα brightness observed by NH–Alice in several directions on the sky, and (2) the residual between the observed brightness and a model brightness accounting for the resonantly scattered solar Lyα component alone. The repeated observations show that this distant Lyα background is constant and uniform over the sky, and represents the local Galactic Lyα background. The observations show no strong correlation with the cloud structure of the local IPM. The observed brightness constrains the absorption coefficient of interstellar dust at Lyα to 0.2 ± 0.01 kpc−1.

Absolute calibration of the Lyman-α measurement apparatus at DIII-D.

The LLAMA (Lyman-Alpha Measurement Apparatus) diagnostic was recently installed on the DIII-D tokamak [Rosenthal et al., Rev. Sci. Instrum. (submitted) (2020)]. LLAMA is a pinhole camera system with a narrow band Bragg mirror, a bandpass interference filter, and an absolute extreme ultraviolet photodiode detector array, which measures the Ly-α brightness in the toroidal direction on the inboard, high field side (HFS) and outboard, low field side (LFS). This contribution presents a setup and a procedure for an absolute calibration near the Ly-α line at 121.6nm. The LLAMA in-vacuum components are designed as a compact, transferable setup that can be mounted in an ex situ vacuum enclosure that is equipped with an absolutely calibrated Ly-α source. The spectral purity and stability of the Ly-α source are characterized using a vacuum ultraviolet spectrometer, while the Ly-α source brightness is measured by a NIST-calibrated photodiode. The non-uniform nature of the Ly-α source emission was overcome by performing a calibration procedure that scans the Ly-α source position and employs a numerical optimization to determine the emission pattern. Nominal and measured calibration factors are determined and compared, showing agreement within their uncertainties. A first conversion of the measured signal obtained from DIII-D indicates that the Ly-α brightness on the HFS and LFS is on the order of 1020 Ph sr-1m-2s-1. The established calibration setup and procedure will be regularly used to re-calibrate the LLAMA during DIII-D vents to monitor possible degradation of optical components and detectors.

A 1D Lyman-alpha profile camera for plasma edge neutral studies on the DIII-D tokamak.

A one dimensional, absolutely calibrated pinhole camera system was installed on the DIII-D tokamak to measure edge Lyman-alpha (Ly-α) emission from hydrogen isotopes, which can be used to infer neutral density and ionization rate profiles. The system is composed of two cameras, each providing a toroidal fan of 20 lines of sight, viewing the plasma edge on the inboard and outboard side of DIII-D. The cameras' views lie in a horizontal plane 77cm below the midplane. At its tangency radius, each channel provides a radial resolution of ∼2cm full width at half maximum (FWHM) with a total coverage of 22cm. Each camera consists of a rectangular pinhole, Ly-α reflective mirror, narrow-band Ly-α transmission filter, and a 20 channel AXUV photodetector. The combined mirror and transmission filter have a FWHM of 5nm, centered near the Ly-α wavelength of 121.6nm and is capable of rejecting significant, parasitic carbon-III (C-III) emission from intrinsic plasma impurities. To provide a high spatial resolution measurement in a compact footprint, the camera utilizes advanced engineering and manufacturing techniques including 3D printing, high stability mirror mounts, and a novel alignment procedure. Absolutely calibrated, spatially resolved Ly-α brightness measurements utilize a bright, isolated line with low parasitic surface reflections and enable quantitative comparison to modeling to study divertor neutral leakage, main chamber fueling, and radial particle transport.

Water Production Rate of C/2020 F3 (NEOWISE) from SOHO/SWAN over Its Active Apparition

Abstract C/2020 F3 (NEOWISE) was discovered in images from the Near Earth Object program of the Wide-Field Infrared Survey Explorer (NEOWISE) taken on 2020 March 27 and has become the Great Comet of 2020. The Solar Wind ANisotropies (SWAN) camera on the Solar and Heliospheric Observatory (SOHO) spacecraft, located in a halo orbit around the Earth–Sun L1 Lagrange point, makes daily full-sky images of hydrogen Lyα. Water production rates were determined from the SWAN hydrogen Lyα brightness and spatial distribution of the comet measured over a 4 month period of time on either side of the comet’s perihelion on 2020 July 3. The water production rate in s−1 was moderately asymmetric around perihelion and varied with the heliocentric distance, r, in au as (6.9 ± 0.5) × 1028 r −2.5±0.2 and (10.1 ± 0.5) × 1028 r −3.5±0.1 before and after perihelion, respectively. This is consistent with the comet having been through the planetary region of the solar system on one or more previous apparitions. Water production rates as large as 5.27 × 1030 s−1 were determined shortly after perihelion, once the comet was outside the solar avoidance area of SWAN, when the comet was 0.324 au from the Sun.

Effect of the non-uniform solar chromospheric Lyα radiation on determining the coronal H I outflow velocity

We derived maps of the solar wind outflow velocity of coronal neutral hydrogen atoms at solar minimum in the altitude range 1.5–4.0 R⊙. We applied the Doppler dimming technique to coronagraphic observations in the UV H I Lyα line at 121.6 nm. The technique exploits the intensity reduction in the coronal line with increasing velocities of the outflowing plasma to determine the solar wind velocity by iterative modelling. The Lyα line intensity is sensitive to the wind outflow velocity and also depends on the physical properties of coronal particles and underlying chromospheric emission. Measurements of irradiance by the chromospheric Lyα radiation in the corona are required for a rigorous application of the Doppler dimming technique, but they are not provided by past and current instrumentations. A correlation function between the H I 121.6 nm and He II 30.4 nm line intensities was used to construct Carrington rotation maps of the non-uniform solar chromospheric Lyα radiation and thus to compute the Lyα line irradiance throughout the outer corona. Approximations concerning the temperature of the scattering H I atoms and exciting solar disc radiation were also adopted to significantly reduce the computational time and obtain a faster procedure for a quick-look data analysis of future coronagraphic observations. The effect of the chromospheric Lyα brightness distribution on the resulting H I outflow velocities was quantified. In particular, we found that the usual uniform-disc approximation systematically leads to an overestimated velocity in the polar and mid-latitude coronal regions up to a maximum of about 50−60 km s−1 closer to the Sun. This difference decreases at higher altitudes, where an increasingly larger chromospheric portion, including both brighter and darker disc features, contributes to illuminate the solar corona, and the non-uniform radiation condition progressively approaches the uniform-disc approximation.

CONSTRAINING THE LIFETIME AND OPENING ANGLE OF QUASARS USING FLUORESCENT Lyα EMISSION: THE CASE OF Q0420–388

ABSTRACT A toy model is developed to understand how the spatial distribution of fluorescent emitters in the vicinity of bright quasars could be affected by the geometry of the quasar bi-conical radiation field and by its lifetime. The model is then applied to the distribution of high-equivalent-width Lyα emitters (with rest-frame equivalent widths above 100 Å, threshold used in, e.g., Trainor & Steidel) identified in a deep narrow-band 36 × 36 arcmin2 image centered on the luminous quasar Q0420–388. These emitters are found near the edge of the field and show some evidence of an azimuthal asymmetry on the sky of the type expected if the quasar is radiating in a bipolar cone. If these sources are being fluorescently illuminated by the quasar, the two most distant objects require a lifetime of at least 15 Myr for an opening angle of 60° or more, increasing to more than 40 Myr if the opening angle is reduced to a minimum of 30°. However, some other expected signatures of boosted fluorescence are not seen at the current survey limits, e.g., a fall off in Lyα brightness, or equivalent width, with distance. Furthermore, to have most of the Lyα emission of the two distant sources to be fluorescently boosted would require the quasar to have been significantly brighter in the past. This suggests that these particular sources may not be fluorescent, invalidating the above lifetime constraints. This would cast doubt on the use of this relatively low equivalent width threshold and thus also on the lifetime analysis in Trainor and Steidel.

Jupiter’s hydrogen bulge: A Cassini perspective

We present observations of H Ly-α and H2 emissions on the body of Jupiter obtained during the Cassini flyby in late 2000 and early 2001. The H Ly-α emission is highly organised by System III longitude and latitude, peaking at a brightness of 22 kR between 90 and 120° longitude. This is known as the H Ly-α ‘bulge’. These observations add to a number of previous studies, showing that the feature is very long-lived, present over several decades. We show that there is a strong correlation between the prevailing solar H Ly-α flux (measured at Earth) and the peak brightness of the H Ly-α bulge at Jupiter, which supports the notion that it is primarily driven by solar resonance scatter. The H Ly-α brightness distribution is not aligned with the jovigraphic equator, but is approximately aligned with the particle drift equator of some, but not all, major Jupiter magnetic field models. On the time scale of days, the bulge region appears twice as variable as the non-bulge region. We propose that the electron recombination of H3+ is an important reaction for the generation of the H Ly-α bulge, which requires an enhancement of soft electrons at the location of the bulge. We derive an equatorial H3+ lifetime of 1.6 ± 0.4 h and a corresponding column averaged electron density of 1.7 × 109 m−3.

Ly[formula omitted]@Pluto

The Alice instrument on New Horizons will perform several observations of Pluto’s far-ultraviolet (FUV) airglow emissions during its July 2015 flyby. While Pluto’s atmosphere is dominated by N2, simulations suggest that the brightest airglow signal at Pluto will actually be due to Lyman alpha (Lyα) emissions of atomic hydrogen. This is because H atoms, produced at lower altitudes due to the photolysis of CH4 and other hydrocarbons, rise up above the homopause to become an important constituent of the atmosphere at high altitudes, and are able to scatter the very bright Lyα lines from the Sun and the interplanetary medium (IPM). The IPM Lyα signal at Earth is very much less than direct solar Lyα, but IPM Lyα falls off much more slowly than r-2, so that at Pluto’s distance from the Sun the two sources are of comparable strength. Detailed simulations of its Lyα emissions indicate that Pluto will appear dark against the IPM background, but that enough contrast exists for the useful extraction of H densities from the Alice observations. As viewed on approach (or from the inner solar system), the Lyα brightness of the disk of Pluto is expected to be ∼30R, against an IPM background of ∼90R.

Comet Hale-Bopp (C/1995 O1): UVSTAR-FUV Spectroscopy from the Space Shuttle

We report here on unique post-perihelion (2.3 AU) measurements of Comet Hale-Bopp in the FUV-range (950–1250 A) by means of the UVSTAR spectrometer from the space shuttle with the main purpose of searching for argon and other FUV emitters. New methods for separating the strong airglow emission at shuttle altitudes are here discussed in detail. Due to our low resolution (15 A) and S/N ratio the possible rocket-borne detection of argon near perihelion (0.9 AU) could not be confirmed. New species as N2 are suspected but difficult to separate from the strong airglow emission at shuttle altitudes. From the Lyα brightness (1.30± 0.08 kRy) a water production rate Q = 5.9 ± 0.4 × 1029 molecules s−1 could be derived and compared with other post-perihelion observations.

Constraints on Jupiter's hydrogen corona from Galileo UVS observations

During Galileo orbits C10 and E11, the Ultraviolet Spectrometer (UVS) obtained H Lyα scans at low latitudes across the disk of Jupiter from a phase angle of 90–109° and a distance of 18–21 jovian radii. Jupiter's nightside Lyα brightness was about 0.5kR, approximately half as bright as interplanetary space. From the nightside to the dayside, the Lyα brightness rose to a peak of ∼15kR near the sub-solar limb and then dropped sharply to interplanetary brightness levels. On orbit C10 three disk scans were obtained (near 30°N latitude, 20°S latitude, and along the equator), while a single equatorial scan was made on orbit E11. The steep falloff in Lyα brightness at the sub-solar point provides a strong constraint on the distribution of atomic hydrogen in the upper atmosphere. All disk scans are reasonably well fit by a resonance line radiative transfer model which assumes a total H column density of ∼1017cm−2 and a temperature profile consistent with the Galileo probe results with an exospheric temperature of 940K. However, we find that the inclusion of a small amount of “hot” hydrogen, with an abundance described by a Chapman profile, can substantially improve the fit to the UVS limb scan data. Exactly such a hot hydrogen component is expected in the upper atmosphere of Jupiter, and has been previously invoked to explain observations of the jovian Lyα line profile made from Earth orbit. The best fits to the UVS data occur with a hot H column density that is 0.1% of the ambient H column, with an effective temperature of 25,000K and a topside scale height of 1000km (in our empirical representation, the hot H scale height is unrelated to the hot H temperature). Other combinations of column abundance and the effective temperature can also provide good fits to the data. As long as the hot H is located above the homopause, there is no strong dependence on the height of the Chapman profile.

Results from UVCS and LASCO Observation of the Sungrazing Comet C/2000 C6

During 2000 February 9-10, the Large Angle Spectrometric Coronagraph and Ultraviolet Coronal Spectrometer (UVCS) instruments aboard the Solar and Heliospheric Observatory observed comet C/2000 C6, a member of the Kreutz family of sungrazing comets. A tail nearly 0.5 R☉ in length was detected in Lyα emission. UVCS was able to observe the comet at four heights as it approached the Sun. A jump in the Lyα brightness between 5.71 and 4.56 R☉ suggests that the nucleus fragmented, exposing more area to solar illumination and increasing the outgassing rate. We interpret the Lyα luminosity in terms of the outgassing rate and use this to estimate the diameter of the nucleus. The Lyα emission fades as H I is ionized, providing estimates of the density in the coronal streamer encountered by the comet.

Scattered Lyα Radiation around Sources before Cosmological Reionization

The spectra of the first galaxies and quasars in the universe should be strongly absorbed shortward of their rest-frame Lyα wavelength by neutral hydrogen (H I) in the intervening intergalactic medium. However, the Lyα line photons emitted by these sources are not eliminated but rather scatter until they redshift out of resonance and escape owing to the Hubble expansion of the surrounding intergalactic H I. We calculate the resulting brightness distribution and the spectral shape of the diffuse Lyα line emission around high-redshift sources before the intergalactic medium was reionized. Typically, the Lyα photons emitted by a source at zs ~ 10 scatter over a characteristic angular radius of ~15'' around the source and compose a line that is broadened and redshifted by ~103 km s-1 relative to the source. The scattered photons are highly polarized. Detection of the diffuse Lyα halos around high-redshift sources would provide a unique tool for probing the neutral intergalactic medium before the epoch of reionization. On sufficiently large scales, where the Hubble flow is smooth and the gas is neutral, the Lyα brightness distribution can be used to determine the cosmological mass densities of baryons and matter.

An unusual change in the Jovian Lyman‐alpha bulge

A very unusual change is reported in the morphology of the Jovian Ly‐α bulge, a region of enhanced Ly‐α emission near the Jovian spin equator at a fixed system III magnetic longitude (λIII) that has persisted since its discovery in the late 1970s. Observations from 1979‐March 1989 consistently showed Ly‐α emission that peaked near λIII∼100° then dropped and remained constant in the nonbulge region from λIII∼200‐360°. During this time the bulge had a full width half maximum (FWHM) of ∼100‐130°. In September 1989 and March and May 1991 this nominal morphology was substantially different: the Ly‐α brightness showed a much broader peak (FWHM ∼200°) centered near 40° meaning that the bulge encompassed nearly the entire equatorial region of the planet and the previously flat nonbulge region was virtually nonexistent. These observations show a fundamental change in the morphology of the Ly‐α bulge which was not observed in the previous 11 years. Present theories for the bulge invoke asymmetric processes or densities in the Jovian ionosphere. The change in bulge morphology may imply a significant change in the pattern of large‐scale magnetospheric convection, in turn implying a possible change in longitudinal density structure of the Io plasma torus between March and Sept 1989. The possibility that the observed change in bulge morphology is related to long‐term solar activity also cannot be ruled out.

A differential measurement of the Lyman alpha emission from the giant planets using IUE

The intensity of the resonantly scattered Ly-α line of the gian planets depends on the scattering column length of atomic hydrogen above the methane layer and on the incident solar flux. We have obtained measurements of the Ly-α brightness of Jupiter and Saturn on December 19, 1979, with a time difference of 111 minutes, which is only slightly longer than the additional travel time for solar photons scattered at Saturn compared to those from Jupiter. This observational technique eliminates two major uncertainties — the use of different instruments and solar variability — affecting previous determinations of the relative brightness of the planets. The measured ratio of the brightness of the subsolar points is 3.0 ± 0.4 which agrees well with the ratio of the incident solar flux of 3.4. This implies approximately equal scattering column lengths of H on both planets.

Radiation Pressure in Stellar Atmospheres with Application to Solar Spicules

Radiation pressure due to absorption in spectral lines can play an important role in the equilibrium conditions in the outer portions of the stellar atmosphere. In particular, it is suggested that spicules ejected from the upper solar chromosphere as well as other fine structural features of the chromosphere may very likely be a result of radiation pressure from Ly-a. Numerical computation of the radial radiation pressure due to Ly-α in the chromospheric model of Vernazza et al. (1973) gives a maximum outward pressure gradient due to radiation that is 0.06 of the gravitational pressure gradient. The model chromosphere of Vernazza et al. is a spherically symmetric model that reproduces the average Sun Ly-α intensity and profile. Supergranule borders (network), where spicules arise, are known to have a Ly-α brightness some ten times the average Sun brightness. It is suggested, therefore, that in the network areas the pressure gradient due to radiation is comparable to (and oppositely directed to) that due to gravitation and that in local areas of unusual brightness the net pressure gradient is outwards. An estimate of the expected outwards velocity of spicule material driven by radiation pressure was made by equating the kinetic energy flux (½JϱV3) to the Ly-α energy flux. This gives values of v that are comparable to observed spicule velocities. Also, the maximum outwards radiation pressure occurs in the upper chromosphere where the material density in the model of Vernazza et al. is close to the value 4 × 10−14 g cm−3, which is about a factor of two less than the currently accepted value for spicules. Increased density in the network areas could easily remove this minor difference.