| Abstract |
Eighteen years ago researchers accidentally discovered spectacular flashes occurring high up in the atmosphere above intense thunderstorms, which have later become known as sprites. Since, a category of upper atmospheric optical phenomena has been established, including elves, blue jets, and halos, in addition to sprites, collectively termed as transient luminous events (TLEs). Their highly non-linear nature and short lifetimes, from less than 1ms to about 250ms, constitute an impediment to their study. Although their discovery opened a new field of research that has advanced our physical understanding of TLEs, important questions still remain unanswered. In particular, there is need to improve our knowledge on how the mechanisms responsible for TLEs affect the medium hosting them, that is, the upper atmosphere and lower ionosphere. The aim of this thesis is to contribute new knowledge on the physics of TLEs and the electrical coupling processes between the troposphere, where lightning occurs, and the upper atmosphere where the TLEs appear. The approach followed here is mostly experimental, based on the reception and analysis of electromagnetic signals from powerful very low frequency (VLF, 3−30kHz) transmitters around the globe. Their propagation in the Earth-ionosphere waveguide can be affected by perturbations in ionospheric conductivity which might also be caused directly by the electrical coupling of the troposphere and the lower ionosphere during TLE occurrences. High resolution VLF recordings made with state-of-the art receivers in Crete and France simultaneously with TLE and lightning detection observations in France, were analysed to study the effects of lightning-induced electric fields on the lower ionosphere. These investigations led to a number of new findings which are reported in the present thesis. A virtually one-to-one relationship between sprites and early VLF events was established. This constitutes evidence of electron density enhancements in the D region during sprites, corroborating theoretical predictions of lightning-induced air breakdown and ionisation production in the upper atmosphere. The relation between sprites and their causative positive cloud-to-ground (+CG) discharges was examined and revealed a 30% occurrence of long-delayed sprites lagging a preceding discharge by 30 to 220ms. This time lag is much larger than in previous reports, suggesting that long-lasting continuing currents play a key role in the build-up of sprite-causative, strong quasielectrostatic (QE) fields in the upper atmosphere/lower ionosphere. The role of intracloud lightning in the sprite generation process was studied further by using broadband VLF recordings in addition to lightning detection data of intracould discharges. It was found that intracloud lightning does indeed play a role, apparently by influencing the charge redistribution and thus shaping the intensity and duration of the mesospheric QE fields that produce sprites. In particular, the presence of intracloud discharges was found to be decisive in sprite morphology, leading to the appearance of carrot rather than column type sprites, possibly because they cause the impacting QE fields in the upper atmosphere to exceed the breakdown threshold for longer times and larger areas in the former cases than the latter. A D-region model, accounting for reactions between four types of charged particles and involving various electron loss mechanisms, was solved numerically to compute relaxation times for electron density enhancements produced by the action on the ionosphere of lightning-induced QE fields. Model results for different parameters were compared with early VLF event recoveries to infer estimates of sprite-related ionisation increases and the altitudes at which they occur. The model accounted for the observed recovery times from about 20 to more than 250s, if electron density increases of 102 to 104 times the ambient values were taken to occur in the altitude range from 75 to 85km. This endorsed the notion of VLF scattering from relatively large diffuse regions of electron ionisation enhancements located right below the night-time VLF reflection heights, in line also with existing simulations and observations of sprite halos. Studies of sprite-related early VLF perturbations lead to the discovery of a new category of VLF events, termed early/slow, characterised by long onset durations up to 2.5s in contrast to the well-known early/fast events reaching their maximum amplitude in less than 50ms. A new mechanism was proposed to explain the slow growth of these events, postulating a process of gradual secondary ionisation build-up, caused by the effects on sprite-produced electrons of electromagnetic pulses (EMPs) radiated upwards by intracloud lightning channels. This was substantiated by the identification of burst-like VLF sferic activity in conjunction with intracloud discharges occurring in the sprite-producing storms, accompanying the early/slow event amplitude growth. An examination of elve observations, made by ground-based and satellite-borne imagers, along with simultaneous VLF recordings, led to the identification of elve-related early VLF perturbations for the first time. This is long-awaited experimental evidence in favour of ionisation produced in the upper D region by lightning-induced EMPs that also produce elves, which came in support of existing theories and simulation models. Using a chain of high-resolution pulsation magnetometers, the ultra low frequency (ULF, f< 10Hz) response to sprite-producing lightning discharges was studied statistically. ULF signatures of sprite-producing and non sprite-related +CG discharges were compared. No signature was found unique to sprite-causative +CG discharges, in contrast with previous claims based on event studies. Finally, an existing numerical model was upgraded to be capable of simulating VLF scattering from a group of vertical plasma structures corresponding to sprite bodies. Under the action of an incident VLF wave, the scattering system was treated as an assembly of conducting columns divided into sequential radiating segments (dipoles). A novel feature was the inclusion of electromagnetic interaction between the radiating elements and realistic Earth-ionosphere waveguide propagation effects, which enables the simulation of a self-consistent scattering radiation pattern. First tests indicate that this model could be easily adapted to simulate early VLF perturbations at a receiver location.
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Very Low Frequency EM Wave Studies of Transient Luminous Events in the Lower Ionosphere
