Jeff Stanley Morrill

Simultaneous observations of mesospheric gravity waves and sprites generated by a midwestern thunderstorm

from a central excitation region directly above the storm. During the initial stages of the storm outwardly expanding waves possessed a period of � ≈10 min and wavelength � ≈50 km. Over a 1 h interval the waves gradually changed to longer period � ≈11 min and shorter wavelength � ≈40 km. Over the full 2 h observation time, about two dozen bright sprites generated by the underlying thunderstorm were recorded near the center of the outwardly radiating gravity wave pattern. No distinctive OH brightness signatures uniquely associated with the sprites were detected at the level of 2% of the ambient background brightness, establishing an associated upper limit of approximately A T . 0:5 K for a neutral temperature perturbation over the volume of the sprites. The corresponding total thermal energy deposited by the sprite is bounded by these measurements to be less than ∼1 GJ. This value is well above the total energy deposited into the medium by the sprite, estimated by several independent methods to be on the order of ∼1–10 MJ. c

Auroral N2 emissions and the effect of collisional processes on N2 triplet state vibrational populations

Previous model results have shown that the N2 triplet vibrational level populations in the aurora are strongly affected by cascade and quenching by atomic and molecular oxygen. As the aurora penetrates to lower altitudes (less than 100 km) the role of quenching by atomic oxygen becomes less important and processes involving N2 collisions begin to play a more prominent part. We are developing a model which will yield steady state vibrational level populations for both the singlet and triplet valence states of N2. The model currently provides results for the seven low-lying N2 triplet states (A 3Σu+, B 3IIg, W3Δu, B′ 3Σu−, C3IIu, D3Σu+, and E3Σg+). These states are responsible for auroral emissions from the UV (Vegard-Kaplan (VK), second positive (2PG)) through the visible to the infrared (first positive (1PG), infrared afterglow (IRA), Wu-Benesch (WB)). We have included two additional collisional processes in the current model which were not treated previously. These are the intersystem collisional transfer of excitation (ICT) between the B state and the A, W, and B′ states and vibrational redistribution within the A state vibrational manifold, both due to collisions with ground state N2. The present work compares our current model results with those of a previous model, as well as ground, airborne, and rocket observations. The comparison between N2(A) (VK) and N2(B) (1PG) vibrational level populations predicted by our model and a number of auroral observations indicate that the current model achieves a significant improvement in the fit between calculation and observation. In addition, the current model predicts a shift in the band intensity distribution of the 1PG Δν = 3 sequence from the infrared into the visible red at the lower altitudes (less than 90 km) as well as an overall enhancement in the entire 1PG system. Consequently, this provides a possible explanation of a dominate feature of type b aurora, the auroral red lower border.

Time resolved N2 triplet state vibrational populations and emissions associated with red sprites

Abstract The results of a quasi-electrostatic electron heating model were combined with a time dependent N2 vibrational level population model to simulate the spectral distributions and absolute intensities observed in red sprites. The results include both N2 excited state vibrational level populations and time profiles of excited electronic state emission. Due to the long atmospheric paths associated with red sprite observations, atmospheric attenuation has a strong impact on the observed spectrum. We present model results showing the effect of atmospheric attenuation as a function of wavelength for various conditions relevant to sprite observations. In addition, our model results estimate the variation in the relative intensities of a number of specific N2 emissions in sprites (1PG, 2PG, and VK) in response to changes in observational geometry. A recent sprite spectrum, measured from the Wyoming Infrared Observatory (WIRO) on Jelm Mountain, during July, 1996, has been analyzed and includes N2 1PG bands down to v′ = 1. In addition to N2 1PG, our analysis of this spectrum indicates the presence of spectral features which are attributable to N+2 Meinel emission. However, due to the low intensity in the observed spectrum and experimental uncertainties, the presence of the N+2(A2Πu) should be considered preliminary. The importance of both the populations of the lower levels of the N2(B3Πg) and the N2(B3Πg)/N+2(A2Πg) population ratio in the diagnosis of the electron energies present in red sprites is discussed. While the current spectral analysis yields a vibrational distribution of the N2(B3Πg) which requires an average electron energy of only 1–2 eV, model results do indicate that the populations of the lower levels of the N2(B3Πg) will increase with increases in the electron energy primarily due to cascade. Considering the importance of the populations of the lower vibrational levels, we are beginning to analyze additional sprite spectra, measured at higher resolution, which contain further information on the population of B(v = 1).

N2(B3Πg) and N2+(A2Πu) vibrational distributions observed in sprites

Abstract A pair of spectra taken simultaneously by two different ground-based instruments has been analyzed by our group. As with previous observations, the spectra are composed primarily of the N 2 first positive group (1PG) ( B 3 Π g − A 3 Σ u + ). In a previous study, we compared the N 2 ( B ) vibrational distributions from the spectral analysis with those resulting from a time-dependent kinetic model of N 2 triplet excited state populations. Both spectra reflect emission between 50 and 60 km . The higher-altitude spectrum is primarily 1PG but also shows the presence of features which appear to be N 2 + Meinel ( A 2 Π u − X 2 Σ g + ). The lower-altitude spectrum shows little or none of the apparent Meinel emission but has an N 2 ( B ) vibrational distribution similar to ones observed in laboratory afterglows. In this paper we discuss the apparent presence of the Meinel emission and present the observed N 2 ( B ) vibrational distributions.

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.

The (X 2Πg) nsσg 1,3Πg Rydberg states of O2: Spectra, structures, and interactions

The results of a critical reexamination of previous experimental observations of the (X 2Πg) nsσg 1,3Πg Rydberg states of O2 are employed to optimize a coupled-channel Schrodinger-equation (CSE) model describing rovibronic interactions among a number of 1,3Πg Rydberg and valence states. The results of calculations using this CSE model are compared with the experimental energy-level and predissociation linewidth data base. As a result of the optimization process, relevant potential-energy curves and Rydberg–valence, Rydberg–Rydberg, and valence–valence interactions are characterized for 1,3Πg states in the ∼6.5–9.5 eV region above the ground state of O2. The precision of these characterizations and the interpretation of the experimental observations are found to be limited by the quality of the existing data, which exhibit a number of inconsistencies that cannot be reconciled without new, well-characterized experimental studies. In spite of these difficulties, the present CSE model, as currently parametriz...

Near-ultraviolet and blue spectral observations of sprites in the 320–460 nm region: N2 (2PG) emissions

Abstract : A near-ultraviolet (NUV) spectrograph (320-460 nm) was flown on the EXL98 aircraft sprite observation campaign during July 1998. In this wavelength range video rate (60 fields/sec) spectrographic observations found the NUV/blue emissions to be predominantly N2(2PG). The negligible level of N+2 (1NG) present in the spectrum is confirmed by observations of a co-aligned, narrowly filtered 427.8 nm imager and is in agreement with previous ground-based filtered photometer observations. The synthetic spectral fit to the observations indicates a characteristic energy of 1.8 eV, in agreement with our other NUV observations.

Two-Photon REMPI Spectra from a 1 Δ g and b 1 Σ + g to d 1 Π g in O 2

Abstract Well-calibrated, resonance-enhanced multiphoton-ionization (REMPI) spectra of eight bands of d 1 Π g ←a 1 Δ g and four bands of d 1 Π g ←b 1 Σ g + in O2 have been recorded and analyzed. These data provide detailed characterizations of the complex rotational structures in the five lowest vibrational levels (v=0–4) of the d 1 Π g state. The observed complexities in the rovibronic structure of d 1 Π g are the result of strong perturbations between the d 1 Π g Rydberg and the II 1 Π g valence states. Weaker perturbations resulting from interactions between d and the II 1 Δ g valence state also are observed.

Perturbations in the 3sσg 1,3Πg Rydberg states of O2: Bound–bound interactions with the second 1Πg and 1Δg valence states

Existing experimental (2+1) REMPI spectra for transitions into rotationally resolved levels of the 3sσg d 1Πg and 3sσg C 3Πg Rydberg states of O2 have been rotationally analyzed, resulting in the first characterization of rotational perturbations in d(v=1–3) and C(v=2). In addition, the results of this analysis have been interpreted with the aid of a coupled-channel Schrodinger-equation (CSE) model of the interacting electronic states. The identification and characterization of perturbations in the d and C states have allowed the nature of the interactions between the 3sσg 1,3Πg Rydberg states and the II 1Πg valence state to be clarified and a realistic empirical potential-energy curve for the II 1Πg state to be determined. While it is found that first- and second-order interactions with the II 1Πg valence state are responsible for the strongest perturbations observed in d(v=1–3) and C(v=2), additional weak perturbations found in d(v=2 and 3) are shown to result from a second-order interaction with the II...

Analysis of the comparative responses of SMEI and LASCO

Surface-brightness responses of the SOHO-LASCO C3 coronagraph and of the Solar Mass Ejection Imager (SMEI) are compared, using measurements of a selection of bright stars that have been observed in both instruments. Seventeen stars are selected that are brighter than 4.5 magnitudes, are not known variables, and do not have a neighboring bright star. Comparing observations of these determines a scaling relationship between surface-brightness measurements from one instrument to those from the other. We discuss units of surface brightness for the two instruments, and estimate a residual uncertainty for the present scaling relationship.