Search

The first implementation of NASA’s Total and Spectral Solar Irradiance Sensor (TSIS-1) launched on December 15th, 2017, and was integrated into the International Space Station (ISS) to measure both the total solar irradiance (TSI) and the solar spectral irradiance (SSI). The direct measurement of the SSI is made by the LASP Spectral Irradiance Monitor (SIM) and provides data essential to interpreting how the Earth system responds to solar spectral variability. Extensive advances in TSIS-1 SIM instrument design and new SI-traceable spectral irradiance calibration techniques have resulted in improved absolute accuracy with uncertainties of less than 0.5% over the continuous 200–2400 nm spectral range. Furthermore, improvements in the long-term spectral stability corrections provide lower trend uncertainties in SSI variability measurements. Here we present the early results of the TSIS-1 SIM measurements covering the first 5 years of operations. This time period includes the descending phase of solar cycle 24, the last solar minimum, and the ascending phase of solar cycle 25. The TSIS-1 SIM SSI results are compared to previous measurements both in the absolute scale of the solar spectrum and the time dependence of the SSI variability. The TSIS-1 SIM SSI spectrum shows lower IR irradiance (up to 6% at 2400 nm) and small visible increases (~0.5%) from some previous reference solar spectra. Finally, initial comparisons are made to current NRLSSI2 and SATIRE-S SSI model results and offer opportunities to validate model details both for short-term (solar rotation) spectral variability and, for the first time, the longer-term (near half solar cycle) spectral variability across the solar spectrum from the UV to the IR.

The MgII index is an important proxy for solar activity; in particular it correlates well with the EUV which is important for understanding the upper atmosphere. We compare the measurements from all of the instruments making daily measurements during the most recent solar minimum. After adjustments to the data to account for instrumental effects, we find that there are still some discrepancies between the various time series. The data from the primary channel of the SOLar-STellar Irradiance Comparison Experiment (SOLSTICE) on SORCE requires a correction factor starting in early 2006 in order to bring it into agreement with the redundant SOLSTICE channel and with the other datasets. Once this correction has been applied, all the MgII measurements are in good agreement throughout the solar minimum interval.

The Solar Radiation and Climate Experiment (SORCE) and Total and Spectral Irradiance Sensor (TSIS‐1) conducted an intercomparison for the two Spectral Irradiance Monitors (SIM) spanning 704 days from 23 March 2018 to 25 February 2020 and permitted 554 time‐matched pairs of observations. This comparison was conducted during the extremely quiescent Solar Cycle 24 minimum, so all observed differences and drifts between the two sensors are instrumental in nature. The TSIS‐1 SIM benefitted from advanced calibration capabilities based on SI standards that were not available during the preflight calibration time period of SORCE. For this reason, a revision of the SORCE SIM absolute scale is appropriate. As expected, wavelength dependent differences in absolute agreement are a function of detector sensitivity and local changes in spectral slope. At the time of the comparison SORCE SIM has been on‐orbit for 17 years while TSIS‐1 observations commenced immediately after a 100‐day outgassing and commissioning period. Peak‐to‐peak absolute scale differences are about 12% with a mean fractional difference of 0.7% ± 2.9%. The greatest scale differences occur at the change‐over between the UV and visible photodiodes in the 310 nm region, and a systematic disagreement is present in the 850–1,600 nm range. A multiplicative scale correction factor has been developed to reconcile the TSIS‐1 and SORCE difference with a wavelength dependent error on the mean typically less than 0.01% derived from every matched pair of observations. Key Points: Conduct a comparison of Solar Radiation and Climate Experiment (SORCE) and Total and Spectral Irradiance Sensor (TSIS‐1) Spectral Irradiance Monitors (SIM) over a 705‐day time periodConstruct a multiplicative correction factor that brings the SORCE SIM into agreement with the pre‐flight calibration of TSIS‐1 SIMCharacterize the uncertainties associated with the correction factor—uncertainty was two orders of magnitude less than the correction [ABSTRACT FROM AUTHOR]

Key Points: Comparison of three correction methods applied to SORCE SIM is presentedSolar spectral irradiance trends differ in the ultraviolet We compare solar spectral irradiance (SSI) measurements obtained from the Spectral Irradiance Monitor (SIM) on board the Solar Radiation and Climate Experiment (SORCE), corrected for instrument degradation using three different methods; SIM Version 25, Multiple Same‐Irradiance‐Level and SIM constrained Version 2 (SIMc V2), to quantify their differences and gain understanding about the relative performance of each method. Furthermore, we compare the three data sets to independent SSI observations that include the SORCE SIM successor Total and Spectral Solar Irradiance Sensor SIM and solar irradiance models. While we find agreement between the SSI for the three correction methods for wavelengths in the visible, differences persist for shorter and longer wavelengths, on time scales on the order of the 11‐year solar cycle. The comparisons to independent SSI observations reveal remaining instrument trends in some of the corrected SORCE SIM data. [ABSTRACT FROM AUTHOR]

We report a detailed high resolution rovibrational analysis of the ν[sub 1] fundamental absorption band system of CF[sub 3]I, which appears as a very complex resonance polyad. Pulsed slit jet diode laser spectra (instrumental bandwidth ...25 MHz) of CF[sub 3]I (neat and seeded in He) at rotational temperatures of about 7, 30, and 70 K have been obtained. The low J, K part of the observed transitions can be fitted by a model including the anharmonically coupled levels ν[sub 1](...[sub 0] = 1076.0551cm[sup -1]), 2ν[sup 0, sub 5](...[sub 0]= 1079.6732cm[sup -1]) and ν[sub 3]+3&nu[sup ±3][sub 6](...[sub 0]= 1072.3884cm[sup -1]), and the local Coriolis perturber ν[sub 3]+[sub 3]ν[sup +1][sub 6] (...[sub 0] = 1074.1845cm[sup -1]). The level ν[sub 3]+3ν [sup +3][sub 6] was observed and included for the first time, and an additional anharmonic coupling between the levels ν[sub 3] + 3ν[sup +3][sub 6] and ν[sub 3] + 3 ν[sup -3][sub 6] proved relevant. In order to analyze the higher d, K regions, perturbations arising from 2 ν[sup ±2][sub 5] turned out to be important. In certain regions of the spectra, effects due to nuclear quadrupole interaction are clearly visible. We report new sets of parameters for a low d, K model and for an extended model including the perturber 2 ν[sup ±2][sub 5] . The results are discussed in relation to infrared multiphoton excitation of CF[sub 3]I, infrared absorption in the atmospheric window and intramolecular vibrational redistribution. [ABSTRACT FROM AUTHOR]

The techniques of resonant two-photon ionization (R2PI) and pulsed field ionization (PFI) were used to measure absorption spectra of ortho-chlorotoluene (S1, A 1A1) and of ortho-chlorotoluene+ (D0, X 2A1; the cation ground state) for internally cold molecules in a pulsed nozzle expansion. The adiabatic ionization potential is 71 318±5 cm-1=8.8423±0.0006 eV. Most of the observed low lying torsion–vibrational structure in both S1 and D0 can be assigned using a one-dimensional torsional model plus low frequency vibrational modes whose identity is corroborated by the ab initio normal modes of D0. The intensities of certain weak, forbidden torsion–vibration combination bands in the S1–S0 spectrum are well predicted by a nuclear coordinate dependence of the electric dipole transition moment. The threefold methyl torsional barrier is V3=144.2±2.0 cm-1 in S1 and V3=456.5±2.0 cm-1 in D0. Ab initio calculations at the HF/6-31G* level find V3=481 cm-1 in S0 with the minimum energy conformation pseudo-trans, i.e., with one CH bond lying in the plane of the ring on the opposite side of the chlorine substituent. Spectral band intensities show that the minimum is pseudo-trans in S1 and D0 as well.In both S1 and D0, excitation of either of the two lowest frequency out-of-plane bending modes, ν38 or ν37, leaves the methyl torsional potential essentially unchanged. In S0, S1, and D0, the barrier is substantially larger for ortho-chlorotoluene than for ortho-fluorotoluene, consistent with greater steric repulsion between the 3p chlorine lone pair and the CH bond pairs. The effects of π excitation and π ionization on the barrier are similar in ortho-chlorotoluene and ortho-fluorotoluene. Apparently both chlorine and fluorine are weak π donors that have similar effects on the crucial ring CC bond orders closest to methyl. Both the S1 and D0 spectra reveal several examples of torsion–vibrational coupling which... [ABSTRACT FROM AUTHOR]

In toluenelike molecules with sixfold internal rotation symmetry, three weak, Franck–Condon forbidden, pure torsional transitions m21, m30, and m41 invariably appear in S1–S0 absorption spectra. The intensities are typically 1%–5% of the allowed bands, m00 and m11. Determination of the preferred conformer in S1 as staggered or eclipsed relies on proper assignment of the upper state of the m30 band as 3a‘1 or 3a‘2 under the molecular symmetry group G12. In addition, inferences of the preferred conformer in S0 and D0 (ground state cation) from band intensities of fluorescence and threshold photoionization spectra have also relied on the same assignment. For a set of six molecules having sixfold rotor potentials, including both –CH3 and –CD3 rotors, we present experimental relative intensities for the three forbidden S1–S0 absorption bands. Within an adiabatic electronic representation, we show how a Fourier expansion of the three components of the S1–S0 electric dipole transition moment predicts relative forbidden band intensities in quantitative agreement with experiment. This fixes the assignment of the upper state of the key m30 transition as 3a‘1, establishes the preferred S1 conformer as staggered for all six molecules, and places earlier inferences of the preferred conformers in S0 and D0 on solid ground. © 1995 American Institute of Physics. [ABSTRACT FROM AUTHOR]

The direct absorption spectrum of the A 2A2←X 2B1 transition of jet-cooled OClO is reported in the 21 000–33 000 cm-1 region. As a result of the rotational cooling afforded by the jet, the upper state level structure has been measured and analyzed in the absence of much of the spectral congestion present in the static gas spectrum. This cooling, in addition to the sensitivity and resolution of the interferometric detection, results in accurate determinations of frequencies and relative absorption intensities. All of the vibrational modes are observed in the 2A2 spectrum with unusually large intensities in the even quanta of the asymmetric stretch. A complete reanalysis of the spectrum yields new 2ν3 and 4ν3 energies of 886.8 and 1855.8 cm-1, respectively. The present assignment is supported by relative intensity and anharmonicity comparisons. We have reconsidered the issue of the intense even ν3 activity in terms of a double-minimum potential in the Q3 coordinate adjusted for the new energies of the asymmetric stretch. The results of this fit agree with experimental relative intensity and isotope measurements. The potential involves a C2v barrier height of 1153 cm-1 with a 0+–0- inversion splitting of 127 cm-1. This value is consistent with the 001 energy determined indirectly from anharmonicity-induced Coriolis perturbations present in the low energy (v1,0,0) levels assumed to be caused by (v1-1,2,1) interactions. [ABSTRACT FROM AUTHOR]

This paper presents the A 2A2←X 2B1 direct absorption spectrum of OClO recorded at rotational resolution in a cold molecular jet. Rotational temperatures as low as 30 K are achieved and result in substantial reduction of the inhomogeneous effects that congest the room temperature spectrum. No direct spectroscopic evidence is found for the 2A1←X 2B1 perpendicular transition which was previously thought to be hidden under the stronger A 2A2←X 2B1 parallel transition. All of the modes of the 2A2 state observed display homogeneous rotational linewidth broadening. This broadening, due to the efficient excited state chemistry of the 2A2 state, is measured and modeled for the vibrational bands involving (v1,0,0), (v1,1,0) and (v1,0,2) for v1=4–10. We confirm the lack of rotational state dependence on the predissociation within a given vibrational band by a comparison of the linewidths obtained here at low J and K with those known at high J and K. For a given symmetric stretch band, the linewidths associated with both of the corresponding combinations of ν1 with the bend and asymmetric stretch are always greater than the symmetric stretch alone. Because of this and in view of the substantial drop in frequency of both ν2 and ν3 upon the transition we discuss the possible involvement of these modes with regard to the photoreactivity of the 2A2 state. In particular, from the higher energy lifetime information obtained here and the available photoproduct information, we address the dynamics of OClO in the 2A2 state with respect to couplings to both the 2A1 and 2B2 close-lying excited states. [ABSTRACT FROM AUTHOR]

The current implementation for continuous, long-term solar spectral irradiance (SSI) monitoring is the Total and Spectral Solar Irradiance Sensor (TSIS-1) Spectral Irradiance Monitor (SIM) that began operations from the International Space Station (ISS) in March 2018 and nominally provides an SSI spectrum every 12 h. Advances in both instrument design and spectral irradiance calibration techniques have resulted in the TSIS-1 SIM achieving higher absolute accuracy than its predecessor instrument in the wavelength range (200–2400 nm). A comprehensive detector-based Spectral Radiometer Facility (SRF) was developed in collaboration with the US National Institute for Standards and Technology (NIST) to ensure the ties to spectral SI standards in power and irradiance. Traceability is achieved via direct laser calibration of a focal plane electrical substitution radiometer (ESR) against a cryogenic radiometer in power and also irradiance responsivity via calibrated apertures. The SIM accuracy definition followed an absolute sensor approach based on a full radiometric measurement equation where component-level performance characterizations and calibrations were quantified with an associated uncertainty error budget and verified by independent measurements for each parameter. Unit-level characterizations were completed over the full operational envelope of external driving factors (e.g., pointing and temperature ranges) and were allowed for the independent parameterization of sub-assembly performance for expected operating conditions. Validation and final instrument end-to-end absolute calibration in the Laboratory for Atmospheric and Space Physics (LASP)-SRF achieved low combined standard uncertainty (uc < 0.25%, k = 1) in spectral irradiance. [ABSTRACT FROM AUTHOR]

Airborne observations of ozone, temperature and the spectral actinic photon flux for ozone in the Arctic lower stratosphere April-September 1997 and January-March 2000 allow a connection to be made between the rate of production of translationally hot atoms and molecules via ozone photodissociation and the intermittency of temperature. Seen in the context of non-equilibrium statistical mechanics literature results from molecular dynamics simulations, the observed correlation between the molecular scale production of translationally hot atoms and molecules and the macroscopic fluid mechanical intermittency of temperature may imply a departure from Maxwell-Boltzmann distributions of molecular velocities, with consequences for chemistry, radiative line shapes and turbulence in the atmosphere, arising from overpopulated high velocity tails of the probability distribution functions (PDFs).