Electrical discharges in planetary upper atmospheres: thermal and chemical effects

Parra Rojas, Francisco C.
Francisco J. Gordillo Vázquez, Alejandro Luque Estepa
Solar System Department
Thursday, 18 June, 2015 - 11:00
Salón de Actos

A one-dimensional electrochemical model is developed to describe, in a self-consistent way, the response of the Earth mesosphere to different types of lightning discharges between 50-87 km of altitude. This model is applied to the case of sprite halos, one of the most common types of Transient Luminous Events (TLE). We have studied the time-altitude evolution of more than 20 chemical species. Our model predicts an increase of up to 70 cm−3 in the electron density from ambient electron density values between 55-81 km of altitude in the +CG lightning cases and a negligible mesospheric electron density perturbation in the -CG lightning case. For all the +CG and some -CG (200 kAkm) cases considered, the model also shows an enhancement of several orders of magnitude in the concentration of ground state negative (O− , O− , NO− ) and positive (O+ , O+ ) ions and electronically excited positive ions such as N+ (A2 Πu ) responsible for the N+ Meinel emissions and N+ (B2 Σ+ ). On the other hand, for the first positive group of N2 the calculated emission brightness exceeds 1 MR for a halo of 100 km of diameter at an altitude of 77 km for all the CG lightning discharges studied (except for the -CG case with 100 kAkm current peak) and for relatively lower altitudes when +CG lightning are considered. Moreover, the calculated concentration of the metastables N2 (A3 Σ+ ) and O(1 D) exhibit an enormous enhancement (of more than ten orders of magnitude) over their ambient values that, for +CG, remains high (5-7 orders of magnitude above ambient values) for long times (up to 500 s), below 55 km.

We have studied laboratory low pressure (0.1 mbar ≤ p ≤ 2 mbar) glow air discharges by optical emission  spectroscopy to discuss several spectroscopic techniques that could be implemented by field  spectrographs, depending on the available spectral resolution, to experimentally quantify the gas  temperature associated to TLEs occurring at different altitudes including blue jets, giant blue jets and sprites. Laboratory air plasmas have been analysed from the near UV (300 nm) to the near IR (1060 nm) with high (up to 0.01 nm) and low (2 nm) spectral resolution commercial grating spectrographs and by an inhouse intensified CCD grating spectrograph that we have recently developed for TLE spectral diagnostic surveys with 0.45 nm spectral resolution. We discuss the results of lab tests and comment on the convenience of using one or another technique for rotational (gas) temperature determination depending on the altitude and available spectral resolution. Moreover, we compare available low resolution (3 nm ≤ ∆λ ≤ 7 nm) N2 1PG field recorded sprite spectra at 53 km (1 mbar), and resulting vibrational distribution function (VDF), with 1 mbar laboratory glow discharge spectrum (∆λ = 2 nm) and synthetic sprite spectra from models. We found that while the relative population of N2 (B 3 Πg , v = 2 − 7) in sprites and laboratory produced air glow plasmas are similar, the N2 (B 3 Πg , v = 1) vibrational level in sprites is more efficiently populated (in agreement with model predictions) than in laboratory air glow plasmas at similar pressures.

Concerning sprites, a one-dimensional self-consistent model has also been developed to study the chemical and thermal effects of a single sprite streamer in the Earth mesosphere. We have used sprite streamer profiles with three different driving current durations (5 ms, 50 ms and 100 ms) between 50 and 80 km of altitude and considering a kinetic scheme of air with 20 chemical species. Our model predicts strong increases in practically all the concentrations of the species studied at the moment of the streamer head passage. Moreover, their densities remain high during the streamer afterglow phase. The electron concentration can reach values of up to 108 cm−3 in the three cases analysed. The sprite model also predicts an important enhancement, of several orders of magnitude above ambient values, of nitrogen oxides (NOx and N2 O) and the considered metastables molecular species (N2 (A3 Σ+ ), O2 (a1 ∆g ), O2 (b1 Σ+ )). Metastables are capable of storing energy for relatively long time (hundreds of seconds). On the other hand, we found that the 4.26 μm IR emission brightness of CO2 can reach 10 GR at low altitudes (< 65 km) for the cases of intermediate (50 ms) and long (100 ms) driving currents. These results suggest the possibility of detecting sprite IR emissions from space with the appropiate instrumentation. Moreover, according to our model, the Meinel emission brightness of N+ could also reach 1 MR below 50 km. Finally, we found that the thermal impact of sprites in the Earth mesosphere is proportional to the driving current duration. This produces variations of up to 30 K (in the extreme case of a 100 ms driving current) at low altitudes (< 55 km) and at about 10 seconds after the streamer head.

Finally, we have also studied the chemical effects of intracloud lightnings with different Charge Moment Change (CMC) and with different ambient electron density profiles on the lower ionosphere of Saturn. We have developed a self-consistent kinetic model that allows us to estimate the time- and altitude-dependence of the electric field and the chemical species included in our model as well as photon emissions. We have tested two ambient electron density profiles on Saturn and found that the conservative estimation of lightning CMC = 105 C km could lead to faint halos and possibly sprites if the base of the ionosphere is located at 1000 km of altitude over the 1-bar reference level. If the base of the ionosphere is at 600 km above the 1-bar level, then only the extreme case of 106 C km could produce considerable ionization, halos and possibly sprites. We also found that (H+3) ions are rapidly produced from the parent (H2+) ions through the reaction (H2+) + H2 → (H3+) + H, so that (H+3) becomes the dominant ion in all the cases considered. The resulting light emissions, mainly in the blue and ultraviolet spectral region, are below the detection threshold of Cassini.