Deriving column-integrated thermospheric temperature with the N₂ Lyman–Birge–Hopfield (2,0) band

This paper presents a new technique to derive thermospheric temperature from space-based disk observations of far ultraviolet airglow. The technique, guided by findings from principal component analysis of synthetic daytime Lyman–Birge–Hopfield (LBH) disk emissions, uses a ratio of the emissions in two spectral channels that together span the LBH (2,0) band to determine the change in band shape with respect to a change in the rotational temperature of N2. The two-channel-ratio approach limits representativeness and measurement error by only requiring measurement of the relative magnitudes between two spectral channels and not radiometrically calibrated intensities, simplifying the forward model from a full radiative transfer model to a vibrational–rotational band model. It is shown that the derived temperature should be interpreted as a column-integrated property as opposed to a temperature at a specified altitude without utilization of a priori information of the thermospheric temperature profile. The two-channel-ratio approach is demonstrated using NASA GOLD Level 1C disk emission data for the period of 2–8 November 2018 during which a moderate geomagnetic storm has occurred. Due to the lack of independent thermospheric temperature observations, the efficacy of the approach is validated through comparisons of the column-integrated temperature derived from GOLD Level 1C data with the GOLD Level 2 temperature product as well as temperatures from first principle and empirical models. The storm-time thermospheric response manifested in the column-integrated temperature is also shown to corroborate well with hemispherically integrated Joule heating rates, ESA SWARM mass density at 460 km, and GOLD Level 2 column O/N2 ratio.

The First Detection of CH2CN in a Protoplanetary Disk

We report the first detection of the molecule cyanomethyl, CH2CN, in a protoplanetary disk. Until now, CH2CN had only been observed at earlier evolutionary stages, in the molecular clouds TMC-1, Sgr2, and L483, in the prestellar core L1544, and toward the protostar L1527. We detect six transitions of ortho-CH2CN toward the disk around nearby T Tauri star TW Hya. An excitation analysis reveals that the disk-averaged column density, N , for ortho-CH2CN is (6.3 ± 0.5) × 1012 cm−2, which is rescaled to reflect a 3:1 ortho-para ratio, resulting in a total column density, N tot, of (8.4 ± 0.7) × 1012 cm−2. We calculate a disk-average rotational temperature, T rot = 40 ± 5 K, while a radially resolved analysis shows that T rot remains relatively constant across the radius of the disk. This high rotation temperature suggests that in a static disk and if vertical mixing can be neglected, CH2CN is largely formed through gas-phase reactions in the upper layers of the disk, rather than solid-state reactions on the surface of grains in the disk midplane. The integrated intensity radial profiles show a ring structure consistent with molecules such as CN and DCN. We note that this is also consistent with previous lower-resolution observations of centrally peaked CH3CN emission toward the TW Hya disks, since the observed emission gap disappears when convolving our observations with a larger beam size. We obtain a CH2CN/CH3CN ratio ranging between 4 and 10. This high CH2CN/CH3CN is reproduced in a representative chemical model of the TW Hya disk that employs standard static disk chemistry model assumptions, i.e., without any additional tuning.

On the relationship of energetic particle precipitation and mesopause temperature

Energetic particle precipitation (EPP) has the potential to change the neutral atmospheric temperature in the mesopause region. However, recent results are inconsistent, leaving the mechanism and the actual effect still unresolved. In this study we have searched for electron precipitation events and investigated a possible correlation between D-region electron density enhancements and simultaneous neutral temperature changes. The rotational temperature of the excited hydroxyl (OH) molecules is retrieved from the infrared spectrum of the OH airglow. The electron density is monitored by the European Incoherent Scatter Scientific Association (EISCAT) Svalbard Radar. We use all available experiments from the International Polar Year (IPY) in 2007–2008 until February 2019. Particle precipitation events are characterized by rapid increases in electron density by a factor of 4 at an altitude range of 80–95 km, which overlaps with the nominal altitude of the infrared OH airglow layer. The OH airglow measurements and the electron density measurements are co-located. Six of the 10 analysed electron precipitation events are associated with a temperature decrease of 10–20 K. Four events were related to a temperature change of less than 10 K. We interpret the results in terms of the change in the chemical composition in the mesosphere. Due to EPP ionization the population of excited OH at the top of the airglow layer may decrease. As a consequence, the airglow peak height changes and the temperatures are probed at lower altitudes. The observed change in temperature thus depends on the behaviour of the vertical temperature profile within the airglow layer. This is in agreement with conclusions of earlier studies but is, for the first time, constructed from electron precipitation measurements as opposed to proxies. The EPP-related temperature change recovers very fast, typically within less than 60 min. We therefore further conclude that this type of EPP event reaching the mesopause region would only have a significant impact on the longer-term heat balance in the mesosphere if the lifetime of the precipitation was much longer than that of an EPP event (30–60 min) found in this study.

Modeling the infrared cascade spectra of small PAHs: the 11.2 μm band

The profile of the 11.2 μm feature of the infrared (IR) cascade emission spectra of polycyclic aromatic hydrocarbon (PAH) molecules is investigated using a vibrational anharmonic method. Several factors are found to affect the profile including: the energy of the initially absorbed ultraviolet (UV) photon, the density of vibrational states, the anharmonic nature of the vibrational modes, the relative intensities of the vibrational modes, the rotational temperature of the molecule, and blending with nearby features. Each of these factors is explored independently and influence either the red or blue wing of the 11.2 μm feature. The majority impact solely the red wing, with the only factor altering the blue wing being the rotational temperature.

Deriving column-integrated thermospheric temperature with the N₂ Lyman–Birge–Hopfield (2,0) band

This paper presents a new technique to derive thermospheric temperature from space-based disk observations of far ultraviolet airglow. The technique, guided by findings from principal component analysis of synthetic daytime LBH disk emissions, uses a ratio of the emissions in two spectral channels that together span the Lyman–Birge–Hopfield (LBH) (2,0) band to determine the change in band shape with respect to a change in the rotational temperature of N2. The benefits of the two-channel ratio approach include an elimination of representativeness error as absolute LBH intensities are not required in the derivation procedure and a reduced impact of systematic measurement error caused by variations in the instrumental performance across the LBH band system as a fully resolved system is also not required. It is shown that the derived temperature should, in general, be interpreted as a column-integrated property as opposed to a temperature at a specified altitude without utilization of a priori information of the thermospheric temperature profile. The two-channel ratio approach is demonstrated using NASA GOLD Level 1C disk emission data for the period of 2–8 November 2018 during which a small geomagnetic storm has occurred. Due to the lack of independent thermospheric temperature observations, the efficacy of the approach is validated through comparisons of the column-integrated temperature derived from GOLD Level 1C data with version 2 of the GOLD Level 2 temperature product as well as temperatures from first principle and empirical models. The storm-time thermospheric response manifested in the column-integrated temperature is also shown to corroborate well with hemispherically integrated Joule heating rates, ESA SWARM mass density at 460 km, and GOLD Level 2 column O / N2 ratio.

Coherent and Non-Coherent Components of Mesoscale Variations of Hydroxyl Rotational Temperature near the Mesopause.

<p>Mesoscale variations of the rotational temperature of excited hydroxyl (OH*) are studied at altitudes 85 – 90  km using the data of spectral measurements of nightglow emission at Russian observatories Zvenigorod (56 ° N, 37°E.) in years 2004  –  2016, Tory (52 ° N, 103°E) in  2012  –  2017 and Maimaga (63° N,  130° E) in  2014 - 2019. The filtering of mesoscale variations was made by calculations of the differences between the measured values of OH* rotational temperature separated with time intervals of <em>dt</em> ~ 0.5 - 2 hr. Comparisons of monthly variances of the temperature differences for various <em>dt</em> allow us to estimate coherent and non-coherent in time components of the mesoscale temperature perturbations. The first component can be associated with mesoscale waves near the mesopause. The non-coherent component may be produced by instrument errors and atmospheric turbulence. The results allow us correcting the observed mesoscale temperature variances at all listed sites for contributions of instrumental and turbulent errors. Seasonal and interannual changes in the coherent component of mesoscale variances of the temperature at the observational sites are studied, which may reflect respective changes in the intensity of mesoscale internal gravity waves in the mesosphere and lower thermosphere region.</p><p>     The analysis of nightglows data was supported by the grant #19-35-90130 of the Russian Foundation for Basic Research. Hydroxyl nightglow data at the Tory site were obtained with the equipment of the Center for Common Use «Angara» http://ckp-rf.ru/ckp/3056/ at the ISTP SB RAS within budgetary funding from the Basic Research Program (Project 0278-2021-0003). Data of the “Geomodel” Resource Center of Saint-Petersburg State University were used.</p>