Solar Resonance Fluorescence Research Articles

A physico-chemical model to study the ion density distribution in the inner coma of comet C/2016 R2 (Pan-STARRS)

ABSTRACT The recent observations show that comet C/2016 R2 (Pan-Starrs) has a unique and peculiar composition when compared with several other comets observed at 2.8 au heliocentric distance. Assuming solar resonance fluorescence is the only excitation source, the observed ionic emission intensity ratios are used to constrain the corresponding neutral abundances in this comet. We developed a physico-chemical model to study the ion density distribution in the inner coma of this comet by accounting for photon and electron impact ionization of neutrals, charge exchange and proton transfer reactions between ions and neutrals, and electron–ion thermal recombination reactions. Our calculations show that CO2+ and CO+ are the major ions in the inner coma, and close to the surface of nucleus CH3OH+, CH3OH2+, and O2+ are also important ions. By considering various excitation sources, we also studied the emission mechanisms of different excited states of CO+, CO2+, N2+, and H2O+. We found that the photon and electron impact ionization and excitation of corresponding neutrals significantly contribute to the observed ionic emissions for radial distances smaller than 300 km and at larger distances, solar resonance fluorescence is the major excitation source. Our modelled ion emission intensity ratios are consistent with the ground-based observations. Based on the modelled emission processes, we suggest that the observed ion emission intensity ratios can be used to derive the neutral composition in the cometary coma only when the ion densities are significantly controlled by photon and photoelectron impact ionization of neutrals rather than by the ion–neutral chemistry.

Detection of the Nitric Oxide Dayglow on Mars by MAVEN/IUVS

AbstractWe report the first remote observation of nitric oxide (NO) densities on Mars. The Imaging Ultraviolet Spectrograph (IUVS) on NASA's Mars Atmosphere and Volatile Evolution (MAVEN) satellite observes NO γ band solar resonance fluorescence between 213.0 and 225.5 nm. We invert an average dayglow limb radiance profile to retrieve a number density profile between 80 and 130 km. The retrieved IUVS NO number density at 117 km is 5 times smaller than those measured by Viking mass spectrometers over 40 years ago but consistent with photochemical model results within the IUVS statistical uncertainty. These observations may therefore help to reconcile a longstanding problem in our understanding of NO photochemistry in the Martian upper atmosphere. We also report the first detection of the CO+ First Negative bands in the Martian dayglow near 219 nm.

Spatial Heterodyne Imager for Mesospheric Radicals on STPSat‐1

The Spatial Heterodyne Imager for Mesospheric Radicals (SHIMMER) was a high‐resolution, near ultraviolet spectrometer that imaged the Earth's limb for 2.5 years between March 2007 and October 2009. The instrument used the Spatial Heterodyne Spectroscopy technique for the first time on a satellite and successfully demonstrated its capabilities. SHIMMER measured the solar resonance fluorescence of the OH A2Σ+‐X2Π (0, 0) band around 309 nm, which allows the retrieval of mesospheric OH density profiles. It also measured the Rayleigh scattered background from the clear atmosphere and solar scattering from polar mesospheric cloud particles. We present details on the SHIMMER mission, the payload design, and the data analysis. A comparison between SHIMMER and concurrent Microwave Limb Sounder OH data shows good agreement between 60 and 90 km altitude for several latitudes and seasons. We also find good agreement of the SHIMMER OH densities and standard photochemical model calculations between 60 and 80 km. We find no evidence of a 25%–35% mesospheric OH deficit, previously reported using Middle Atmosphere High‐Resolution Spectrograph Investigation (MAHRSI) OH data. However, independent analysis of Rayleigh scattered background signals observed by SHIMMER and MAHRSI under similar lighting conditions revealed that MAHRSI radiances are systematically smaller than SHIMMER radiances by 24%. Although this difference is well outside of the combined uncertainties for both experiments, the agreement of SHIMMER OH with Microwave Limb Sounder OH and standard photochemistry results, together with our Rayleigh scattering comparison, suggests an unidentified MAHRSI calibration problem that effectively eliminates the mesospheric OH deficit reported using MAHRSI observations.

OH A2Σ+ –X2Π band ratios observed in the mesosphere by OSIRIS

In this study, we present spectra of the mesospheric OH A2Σ+ –X2Π band system, including the 0–0, 1–1, and 1–0 bands, as observed by OSIRIS (Optical Spectrograph and Infrared Imaging System). Spectral components due to Rayleigh-scattered sunlight, lower thermospheric dayglow emission features, and baffle scatter have been removed to isolate the OH emission signature. The observed spectra are compared with model spectra assembled using rotational emission rate factors for solar resonance fluorescence (g-factors) plus prompt emission of the OH A2Σ+ –X2Π band system from solar Lyman-α photodissociation of water. The observed band ratios are in good agreement with the model values. The altitude variation of the 0–0 band, relative to the 1–1 band, is in agreement with model predictions based on vibrational energy transfer from OH A2Σ+ ν′ = 1 to OH A2Σ+ ν′ = 0. This detailed understanding of the OH A2Σ+ –X2Π system is critical for the successful application of OH observations to the determination of mesospheric OH densities and water vapor concentrations.PACS Nos.: 33.20.Lg, 33.20.Tp, 33.70.Fd, 92.60.hc, 92.60.hw

Optical Spectrograph and Infra‐Red Imaging System (OSIRIS) observations of mesospheric OH A2Σ+‐X2Π 0‐0 and 1‐1 band resonance emissions

Although only a minor species, the OH molecule plays an important role in the photochemical control of mesospheric ozone density and has been the target of a number of observational programs, principally through the OH A2Σ+‐X2Π 0‐0 band emission at 308 nm. This emission band arises from solar resonance fluorescence excitation of OH X2Π ground state molecules, and its observation is complicated by the presence of an underlying atmospheric Rayleigh scattering spectrum. We show that the OH A2Σ+‐X2Π 0‐0 band emission has been reliably and routinely detected with the moderately low, 0.9 nm, spectral resolution Optical Spectrograph and Infra‐Red Imaging System (OSIRIS) limb scanning spectrograph. Changes in upper mesospheric water vapor observed by the Halogen Occultation Experiment (HALOE) are readily detected as changes in the OH density profiles seen by OSIRIS. Altitude profiles of OH density in the middle and upper mesosphere are in good agreement with model results that incorporate coordinated HALOE water vapor measurements. The agreement is within the HALOE and OSIRIS error limits when the recommended standard reaction rates are assumed. Conversely, model calculations of OH density using the revised reaction rates proposed to explain the Middle Atmosphere High‐Resolution Spectrograph Investigation (MAHRSI) OH profiles typically fall outside the observed OSIRIS error limits. The OSIRIS results suggest that the probable difference between the observed and modeled OH densities is less than 15% from 55 to 80 km. Diurnal and seasonal OH variations observed by OSIRIS are in good agreement with model predictions. These successful comparisons suggest that the OSIRIS OH database, spanning more than 4 years of operation and broad ranges of latitude and local time, can contribute significantly to studies of OH photochemistry and upper mesospheric water vapor.

Robust monolithic ultraviolet interferometer for the SHIMMER instrument on STPSat-1.

We describe the design, fabrication, and testing of a monolithic interferometer consisting entirely of optically contacted fused-silica optical elements that are assembled, adjusted, and permanently bonded in place. The interferometer is part of a spatial heterodyne spectrometer (SHS) [SHIMMER (Spatial Heterodyne Imager for Mesospheric Radicals)] that will be used for near-ultraviolet high-spectral-resolution limb imaging of OH solar resonance fluorescence from low Earth orbit aboard the satellite STPSat-1 scheduled for launch in 2006. The stability of the monolith coupled with the relaxed tolerances on optical quality and alignment inherent to SHS make this new instrument extremely robust and especially attractive for applications in harsh environments.

Calculated OH A2Σ+−X2Π (0,0) band rotational emission rate factors for solar resonance fluorescence

The Middle Atmosphere High‐Resolution Spectrograph Investigation (MAHRSI) measured vertical density profiles of hydroxyl (OH) by observing A2Σ+−X2Π (0,0) solar resonance fluorescence in the mesosphere near 309 nm [Conway et al., this issue]. Interpretation of the MAHRSI results requires calculation of the OH(0,0) rotational emission rate factors (g factors) which describe the molecule's fluorescent scattering efficiency. This calculation relies heavily on a solar irradiance measurement with high enough spectral resolution to identify the amount of excitation for each rotational transition (resolving power >3.1×105). Recent high‐resolution ground‐based measurements of the solar irradiance and more accurate absolute solar irradiance observations allow for much more accurate g factors. We find that the relative intensities of calculated OH (0,0;) rotational g factors are in excellent agreement with MAHRSI spectra. The calculated OH (0,0) band g factor is found to be 3.51×10−4 s−1 at 200 K, in good agreement with the most recent work on the subject.

Middle Atmosphere High Resolution Spectrograph Investigation

The Middle Atmosphere High Resolution Spectrograph Investigation (MAHRSI) was developed specifically to measure the vertical density profiles of hydroxyl (OH) and nitric oxide (NO) in the middle atmosphere from space. MAHRSI was launched on its first flight in November 1994 on the CRISTA‐SPAS satellite that was deployed and retrieved by the space shuttle. The instrument measured the radiance profiles of ultraviolet solar resonance fluorescence on the Earth's limb with a spectral resolving power of 15,600 at a wavelength of 308 nm and 7200 at 215 nm. The instantaneous height of the field of view projected to the tangent point was about 300 m. OH limb radiance measurements were made between altitudes of 90 and 30 km, and each limb scan extended over a horizontal distance of 1200 km. For NO a limb scan extended between altitudes of 140 and 76 km and over a horizontal distance 700 km. Observations were made from 52°S latitude to 62°N latitude. The OH measurements have been inverted to provide the first global maps of the vertical distribution of OH between 90 and 50 km. The data show a detailed history of the morning formation of a strongly peaked layer of OH at an altitude of 68 km. This layer was produced by solar photodissociation of a thin layer of water vapor peaked at 65 km extending between 30°S and 35°N observed contemporaneously by the Halogen Occultation Experiment (HALOE) on the Upper Atmosphere Research Satellite. MAHRSI was successfully flown for a second time in August 1997 under conditions that extended the geographical coverage to 72°N latitude and local solar time coverage through the afternoon hours. This paper provides a detailed description of the experiment and instrumentation, of the algorithms used to reduce the spectral data and perform the inversions, and presents examples of key results from the 1994 flight.

Polar cap optical observations of topside (>900 km) molecular nitrogen ions

The Ballistic Missile Defense Organization's (BMDO) Midcourse Space Experiment (MSX) has obtained the first optical observations of molecular ions at very high altitudes above the northern polar cap. Spectra over geomagnetic latitudes ∼ 80°N, at 1300 MLT of the N2+ 1st Negative Bands are identified with a total band intensity of 2.5 kR and an implied number density of 10³ ions cm−3 at 900 km. Additional N2+ transitions from the Meinel bands were also observed. No other permitted optical emissions were observed above 450 km indicating that the source must be solar resonance fluorescence of the N2+ ion. The presence of heavy molecular ions at high altitudes is an indication of the upward flow of ions from the ionosphere into the magnetosphere. These optical observations suggest a new technique for the study of the global structure and temporal variation of plasma energization and transport between the ionosphere and magnetosphere in the polar regions.

On the role of solar EUV, photoelectrons, and auroral electrons in the chemistry of C(1D) and the production of C I 1931 Å in the cometary coma: A case for comet 1P/Halley

The observations of C I 1931 Å emission in the ultraviolet spectra of comets show that a large fraction of carbon atoms in the cometary coma are produced in the metastable 1D excited state. A coupled‐chemistry transport model is employed to examine in detail the various processes of production and loss of the C(1D) atoms in the coma of comet 1P/Halley. Different mechanisms forming CI 1931 Å line are subsequently studied. The impacts of solar UV‐EUV photons, photoelectrons, and auroral electrons of solar wind origin, on the chemistry of C(1D) and the C I 1931 Å emission are evaluated. Dissociation of CO by solar photon and electron impact are the dominant mechanisms of the production of C(1D). Contrary to the findings of the earlier works, the dissociative e‐CO+ recombination is not a significant source of C(1D) in the inner (<104 km) coma. Quenching of C(1D) by H2O is the main loss mechanism of C(1D) in the inner most coma (<103 km), while radiative deexcitation dominates at larger radial distances. The density of C(1D) in the inner coma is found to be controlled predominantly by the CO and H2O density distribution. Deep in the coma (<300 km) the photodissociative excitation of CO is the major source of 1931 Å emission, while beyond 500 km the resonant scattering of solar radiation by C(1D) atoms dominates. The contribution of the solar resonance fluorescence to the 1931 Å IUE slit‐averaged intensity is about 80% in the normal and auroral conditions. Results of the model case studies performed to assess the impact of changing C(1D) + H2O quenching rate coefficient, C(1D) to C(3P) production ratio, neutral and electron temperatures, and extended CO source are discussed. The modeled intensity of carbon 1931 Å emission is found to be a factor of 3–6 smaller compared to the IUE‐observed intensity.

MAHRSI observations of nitric oxide in the mesosphere and lower thermosphere

In November, 1994, the Middle Atmosphere High Resolution Spectrograph Investigation (MAHRSI) observed the distribution of NO between the altitudes of 76 and 140 km by measuring limb intensity profiles of solar resonance fluorescence in the NO A ²Σ → X ²II (1,0) γ band near 215 nm. The observations were made from the ASTRO‐SPAS (Shuttle Pallet Satellite) spacecraft which was deployed and retrieved by the space shuttle. The data provided a seven hour snapshot of lower thermospheric and mesospheric NO from sunrise near 48°S to sunset near 61° N latitude following a period of low solar and high geomagnetic activity. Inferred peak lower thermospheric NO densities ranged from 3 × 107 cm−3 near the equator to 2 × 108 cm−3 at high northern latitudes, roughly consistent with previous observations for the same conditions. Individual vertical density profiles showed substantial structure and large orbit to orbit variations, suggesting that the distribution of mesospheric and lower thermospheric NO is dynamically influenced.

Satellite measurements of hydroxyl in the mesosphere

The global distribution of hydroxyl (OH) in the middle atmosphere was recently measured by the Middle Atmosphere High Resolution Spectrograph Investigation (MAHRSI) on a satellite deployed and retrieved by the space shuttle. During 75 orbits, MAHRSI acquired 1800 daytime limb scans of the OH ultraviolet solar resonance fluorescence intensity. Each limb scan extends over the altitude region from 30 to 90 km and across 10° of latitude between 53°S and 63°N. OH number densities were retrieved using a Twomey regularization scheme constrained by the smoothness of the retrieved profile. Results provide a detailed description of the diurnal variation of mesospheric OH. Midmorning OH densities had a well defined peak of about 6 ×106 cm³ near 70 km, a broad minimum centered near 64 km, and rose to about 1 × 107 cm³ at 50 km. This profile is in substantial disagreement with photochemical model predictions [Summers et al., this issue]. The observations are compared with the two previous measurements.

OH prompt emission in comet IRAS-Araki-Alcock (1983 VII)

In a cometary atmosphere the photodissociation of water by solar ultraviolet radiation creates a small fraction of OH radicals in the A 2 Σ + state in highly excited rotational levels which quickly decay to OH( X 2 Π) through spontaneous emission, producing a spectrum distinct from that of OH solar resonance fluorescence. J. L. Bertaux (1986, Astron. Astrophys. 160, L7–L10) has suggested that this photodissociative OH fluorescence, called prompt emission, can provide a direct measure of the water density distribution within ∼100 km of the cometary nucleus, but the observation requires very high spatial resolution. We examine the ultraviolet spectra of two comets obtained by the International Ultraviolet Explorer satellite, which differ significantly only with respect to the effective spatial resolution, to search for the presence of OH prompt emission. A comparison of the spectra of comets IRAS-Araki-Alcock (1983 VII) and Tuttle (1980 XIII), at comparable heliocentric distance and velocity, but at geocentric distances of 0.032–0.048 and 0.50 AU, respectively, in the wavelength region 2700–3200 Å shows evidence for OH prompt emission in Comet IRAS-Araki-Alcock. The water production rate obtained from the prompt emission reflects an instantaneous value, and can be compared to that derived from OH solar resonance fluorescence, which gives an average over the lifetimes of the parent and daughter molecules. Future observations of comets with the Hubble Space Telescope should make possible routine determinations of the spatial distribution of water near the nucleus.