The Auroral Large Imaging System (ALIS) was proposed in 1989 by Åke Steen as a joint Scandinavian ground-based nework of automated auroral imaging stations. The primary scientic objective was in the field of auroral physics, but it was soon realised that ALIS could be used in other fields, for example, studies of Polar Stratospheric Clouds (PSC), meteors, as well as other atmospheric phenomena. This report describes the design, operation and scientic results from a Swedish prototype of ALIS consisting of six unmanned remote-controlled stations located in a grid of about 50 km in northern Sweden. Each station is equipped with a sensitive high-resolution (1024 x 1024 pixels) unintensified monochromatic CCDimager. A six-position filter-wheel for narrow-band interference filters facilitates absolute spectroscopic measurements of, for example, auroral and airglow emissions. Overlapping fields-of-view resulting from the station baseline of about 50 km combined with the station field-of-view of 50° to 60°, enable triangulation as well as tomographic methods to be employed for obtaining altitude information of the observed phenomena. ALIS was probably one of the first instruments to take advantage of unintensi- fied (i.e. no image-intensifier) scientific-grade CCDs as detectors for spectroscopic imaging studies with multiple stations of faint phenomena such as aurora, airglow, etc. This makes absolute calibration a task that is as important as it is dificult. Although ALIS was primarily designed for auroral studies, the majority of the scientific results so far have, quite unexpectedly, been obtained from observations of HF pump-enhanced airglow (recently renamed Radio-Induced Aurora). ALIS made the first unambiguous observation of this phenomena at high-latitudes and the first tomography-like inversion of height profiles of the airglow regions. The scientific results so far include tomographic estimates of the auroral electron spectra, coordinated observations with satellite and radar, as well as studies of polar stratospheric clouds. An ALIS imager also participated in a joint project that produced the first ground-based daytime auroral images. Recently ALIS made spectroscopic observations of a Leonid meteor-trail and preliminary analysis indicates the possible detection of water in the Leonid.

<p>This thesis treats interaction between electromagnetic waves and localized plasma oscillations. Two specific physical systems are considered, namely artificially excited magnetic field-aligned irregularities (striations) and naturally excited lower hybrid solitary structures (LHSS). Striations are mainly density depletions of a few percent that are observed when a powerful electromagnetic wave, a pump wave, is launched into the ionosphere. The striations are formed by upper hybrid (UH) oscillations that are localized in the depletion where they are generated by the linear conversion of the pump field on the density gradients. However, the localization is not complete as the UH oscillation can convert to a propagating electromagnetic Z mode wave. This process, termed Z mode leakage, causes damping of the localized UH oscillation. The Z mode leakage is investigated and the theory predicts non-Lorentzian skewed shapes of the resonances for the emitted Z mode radiation. Further, the interaction between individual striations facilitated by the Z mode leakage is investigated. The LHSS are observed by spacecraft in the ionosphere and magnetosphere as localized waves in the lower hybrid (LH) frequency range that coincides with density cavities. The localized waves are immersed in non-localized wave activity. The excitation of localized waves with frequencies below LH frequency is modelled by scattering of electromagnetic magnetosonic (MS) waves off a preexisting density cavity. It is shown analytically that an incident MS wave with frequency less than the minimum LH frequency inside the cavity is focused to localized waves with left-handed rotating wave front. In addition, the theory is shown to be consistent with observations by the Freja satellite. For frequencies between the minimum LH frequency inside the cavity and the ambient LH frequency, the MS wave is instead mode converted and excites pressure driven LH oscillations. This process is studied in a simplified geometry.</p>

This thesis treats interaction between electromagnetic waves and localized plasma oscillations. Two specific physical systems are considered, namely artificially excited magnetic field-aligned irregularities (striations) and naturally excited lower hybrid solitary structures (LHSS). Striations are mainly density depletions of a few percent that are observed when a powerful electromagnetic wave, a pump wave, is launched into the ionosphere. The striations are formed by upper hybrid (UH) oscillations that are localized in the depletion where they are generated by the linear conversion of the pump field on the density gradients. However, the localization is not complete as the UH oscillation can convert to a propagating electromagnetic Z mode wave. This process, termed Z mode leakage, causes damping of the localized UH oscillation. The Z mode leakage is investigated and the theory predicts non-Lorentzian skewed shapes of the resonances for the emitted Z mode radiation. Further, the interaction between individual striations facilitated by the Z mode leakage is investigated. The LHSS are observed by spacecraft in the ionosphere and magnetosphere as localized waves in the lower hybrid (LH) frequency range that coincides with density cavities. The localized waves are immersed in non-localized wave activity. The excitation of localized waves with frequencies below LH frequency is modelled by scattering of electromagnetic magnetosonic (MS) waves off a preexisting density cavity. It is shown analytically that an incident MS wave with frequency less than the minimum LH frequency inside the cavity is focused to localized waves with left-handed rotating wave front. In addition, the theory is shown to be consistent with observations by the Freja satellite. For frequencies between the minimum LH frequency inside the cavity and the ambient LH frequency, the MS wave is instead mode converted and excites pressure driven LH oscillations. This process is studied in a simplified geometry.