Optical Emission Excitation Research Articles

Efficiencies of emission excitation and electron density formation in auroras

It is shown that the efficiencies of excitation of optical emissions λ = 391.4, 557.7, and 630.0 nm and the efficiency of formation of the total electron density weakly depend on the shape of the energy spectrum of precipitating electrons and are mainly determined by the values of the average energy of the electron flux.

EISCAT observations of pump‐enhanced plasma temperature and optical emission excitation rate as a function of power flux

We analyze optical emissions and enhanced electron temperatures induced by high power HF radio waves as a function of power flux using the EISCAT heater with a range of effective radiated powers. The UHF radar was used to measure the electron temperatures and densities. The Digital All Sky Imager was used to record the 630.0 nm optical emission intensities. We quantify the HF flux loss due to self‐absorption in the D‐region (typically 3–11 dB) and refraction in the F‐region to determine the flux which reaches the upper‐hybrid resonance height. We find a quasi‐linear relationship between the HF flux and both the temperature enhancement and the optical emission excitation rate with a threshold at ∼37.5 μWm−2. On average ∼70% of the HF flux at the upper‐hybrid resonance height goes in to heating the electrons for fluxes above the threshold compared to ∼40% for fluxes below the threshold.

Modelling of optical emissions enhanced by the HF pumping of the ionospheric F-region

Abstract. Strong enhancement of the optical emissions with excitation threshold from 1.96 eV (630.0 nm from O(1D)) up to 18.75 eV (427.8 nm from N2+(1NG)) have been observed during experiments of the ionosphere modification by high power HF radio waves. Analysis of the optical emission ratios showed clearly that a significant part of the ionospheric electrons have to be accelerated to energies above 30 eV and more in the region where the HF radio wave effectively interacts with the ionospheric plasma. The Monte-Carlo model of electron transport and the optical emission model were used to study the dependence of the optical emission intensity on the acceleration electron parameters. We obtained the following results from analysis of the enhanced intensities of the four optical emissions (630.0, 557.7, 844.6 and 427.8 nm) observed in the EISCAT heating experiment on 10 March 2002. The 630.0 emission with an excitation threshold of 1.96 eV is formed predominately by the thermal electrons, where the accelerated electrons play a minor role in the excitation of this emission. In order to explain the experimentally observed intensity ratios, the accelerated electrons must gain energies of more than 60 eV. For accelerated electrons with a power law energy dependence, the efficiency of the optical emission excitation depends on the exponent defining the shape of the electron spectra. However, an agreement with the observed emission intensities is achieved for exponent values not less than zero. Moreover, increasing the exponent to higher values does not affect the emission intensity ratios.

A survey of ELF and VLF research on lightning‐ionosphere interactions and causative discharges

Extremely low frequency (ELF) and very low frequency (VLF) observations have formed the cornerstone of measurement and interpretation of effects of lightning discharges on the overlying upper atmospheric regions, as well as near‐Earth space. ELF (0.3–3 kHz) and VLF (3–30 kHz) wave energy released by lightning discharges is often the agent of modification of the lower ionospheric medium that results in the conductivity changes and the excitation of optical emissions that constitute transient luminous events (TLEs). In addition, the resultant ionospheric changes are best (and often uniquely) observable as perturbations of subionospherically propagating VLF signals. In fact, some of the earliest evidence for direct disturbances of the lower ionosphere in association with lightning discharges was obtained in the course of the study of such VLF perturbations. Measurements of the detailed ELF and VLF waveforms of parent lightning discharges that produce TLEs and terrestrial gamma ray flashes (TGFs) have also been very fruitful, often revealing properties of such discharges that maximize ionospheric effects, such as generation of intense electromagnetic pulses (EMPs) or removal of large quantities of charge. In this paper, we provide a review of the development of ELF and VLF measurements, both from a historical point of view and from the point of view of their relationship to optical and other observations of ionospheric effects of lightning discharges.

Sprites produced by quasi‐electrostatic heating and ionization in the lower ionosphere

Quasi‐electrostatic (QE) fields that temporarily exist at high altitudes following the sudden removal (e.g., by a lightning discharge) of thundercloud charge at low altitudes lead to ambient electron heating (up to ∼5 eV average energy), ionization of neutrals, and excitation of optical emissions in the mesosphere/lower ionosphere. Model calculations predict the possibility of significant (several orders of magnitude) modification of the lower ionospheric conductivity in the form of depletions of electron density due to dissociative attachment to O2 molecules and/or in the form of enhancements of electron density due to breakdown ionization. Results indicate that the optical emission intensities of the 1st positive band of N2 corresponding to fast (∼ 1 ms) removal of 100–300 C of thundercloud charge from 10 km altitude are in good agreement with observations of the upper part (“head” and “hair” [Sentman et al., 1995]) of the sprites. The typical region of brightest optical emission has horizontal and vertical dimensions ∼10 km, centered at altitudes 70 km and is interpreted as the head of the sprite. The model also shows the formation of low intensity glow (“hair”) above this region due to the excitation of optical emissions at altitudes ∼ 85 km during ∼ 500 μs at the initial stage of the lightning discharge. Comparison of the optical emission intensities of the 1st and 2nd positive bands of N2, Meinel and 1st negative bands of , and 1st negative band of demonstrates that the 1st positive band of N2 is the dominating optical emission in the altitude range around ∼70 km, which accounts for the observed red color of sprites, in excellent agreement with recent spectroscopic observations of sprites. Results indicate that the optical emission levels are predominantly defined by the lightning discharge duration and the conductivity properties of the atmosphere/lower ionosphere (i.e., relaxation time of electric field in the conducting medium). The model demonstrates that for low ambient conductivities the lightning discharge duration can be significantly extended with no loss in production of optical emissions. The peak intensity of optical emissions is determined primarily by the value of the removed thundercloud charge and its altitude. The preexisting inhomogeneities in the mesospheric conductivity and the neutral density may contribute to the formation of a vertically striated fine structure of sprites and explain why sprites often repeatedly occur in the same place in the sky as well as their clustering. Comparison of the model results for different types of lightning discharges indicates that positive cloud to ground discharges lead to the largest electric fields and optical emissions at ionospheric altitudes since they are associated with the removal of larger amounts of charge from higher altitudes.

The interaction with the lower ionosphere of electromagnetic pulses from lightning: Excitation of optical emissions

A self consistent and fully kinetic simulation of the interaction of lightning radiated electromagnetic (EM) pulses with the nighttime lower ionosphere indicates that optical emissions observable with conventional instruments would be excited. For example, emissions of the 1st and 2nd positive bands of N2 occur at rates reaching 7 × 107 and 107 cm−3 s−1 respectively at ∼92 km altitude for a lightning discharge with an electric field E100 = 20 V/m (normalized to a 100 km distance). The maximum height integrated intensities of these emissions are 4 × 107 and 6 × 106 R respectively, lasting for ∼50 µs.

Fluorescence in the dissociative excitation of sulphur dioxide by electron impact

Electron-impact excitation of sulphur dioxide at energies > ca. 10 eV results in optical emission from the A3Π and B3Π states of the SO radical. The threshold energy for production of the SO(A3Π) was found to be 10.6 ± 0.5 eV, a value similar to the minimum thermochemical energy requirements. Optical emission excitation functions for the SO(A) state show that in the energy region 10–20 eV an important excitation channel involves the triplet manifold of sulphur dioxide, although there is still a significant contribution from singlet excitation, particularly at higher energies. The SO(A) state is quenched by molecular SO2 with near-unit collisional efficiency, with an estimated rate constant of 8.4 × 10–11 cm3 molecule–1 s–1. Detailed studies on production of SO(B) could not be undertaken owing to considerable spectral interference from molecular sulphur dioxide emission.

Molecular emission in the electron-impact excitation of sulphur dioxide

Electron-impact excitation of sulphur dioxide at energies greater than the threshold energy (3.85 ± 0.4 eV) results in optical emission mainly from the B1B1 state of sulphur dioxide. Optical emission excitation functions have been determined at several wavelengths and demonstrate that the primary excitation process(es) is dominated by production of optically forbidden triplet states. The observed emission arises from intersystem crossing from these triplet states into the fluorescing B1B1 state.

Methoxyl radical fluorescence in the electron impact excitation of dimethylether and related compounds

Electron impact excitation of dimethylether, dimethoxymethane and methyl formate at energies above ca. 8 eV leads to production of electronically excited methoxyl radicals, which fluoresce in the 300–450 nm region. Optical emission excitation functions for these methoxyl radicals have been measured. The threshold energy for production of excited methoxyl from dimethylether is 8.4 ± 0.4 eV. At energies only a little above threshold optically forbidden excitation processes are largely responsible for production of the excited methoxyl radicals, whereas at energies above 20 eV optically allowed processes dominate the excitation. In the case of dimethylether, non-linear pressure effects in the energy region close to threshold indicate that a triplet state is produced which may be capable of undergoing collisionally assisted predissociation at mTorr pressures.

Auroral excitation of optical emissions of atomic and molecular oxygen

Rocket measurements of the O I ‘green line’ (¹S‐¹D) at 5577 Å and the O2 (0, 0) atmospheric band at 7620 Å were made in a steady IBC II+ aurora, simultaneously with measurements of N2 emissions and the auroral electron flux. An empirical model based on these data demonstrates that the principal excitation source of O2(b¹Σg+) is energy transfer from O(¹D), with direct electron impact of O2 contributing less than 5%. The altitude profile of the green line emission, taking into account quenching of O(¹S) by O(³P) near the lower border of the aurora, resembles what would be produced by electron impact excitation of O2 but would require a dissociative excitation cross section of 10−16 cm². None of the other known O(¹S) excitation mechanisms are capable of producing the observed emission rate.