<p>Waves are a fundamental feature in many parts of physics, since they transport energy without transporting matter. This is the case also in space physics. Waves are responsible for energy transport both between different parts of space and between different particles in the space plasma. They are also useful for diagnostics of the space plasma itself. The present thesis considers two different parts of the large subject of space plasma waves: Lower hybrid cavities (LHCs) and simple-pole particle distribution functions.</p><p>The LHCs are localised density depletions that have been observed by several spacecraft. They have increased wave activity in the lower hybrid frequency range, and was previously found on altitudes up to 1750 km. New observations by the Viking and Cluster satellites show that they are common magnetospheric features, at least up to an altitude of 35,000 km. Theoretical results, assuming a cylindrically symmetric density depletion, show that even though the density depletion may decrease slowly with increasing radial distance, and thus be essentially infinite in extent, there is a maximum distance within which a trapped mode, with given wave number <i>k</i><i>z</i> parallel to the geomagnetic field, may propagate. Furthermore, there is a local relation between the plasma density gradient and the lowest possible frequency that the trapped waves can have, for any monotonic density and given <i>k</i><i>z</i>. The combined theoretical and observational results indicate that the length of the cavities is larger than the width by a factor of at least 200.</p><p>Simple-pole particle distribution functions are introduced because they can model high velocity tails of the particle distribution in a way that is not possible to do with Maxwellian distribution functions. These distributions also simplify the calculations. This gives new possibilities for the physical understanding, as well as the numerical calculations, of the dispersion relations of real space plasmas. The dispersion relations of plasmas described by simple-pole distributions are examined, both for unmagnetised and for magnetised plasmas. These examples show how particle populations with the same density and mean particle energy, but with somewhat different distribution functions, have different wave propagation properties that should be observable by existing spacecraft.</p>

Waves are a fundamental feature in many parts of physics, since they transport energy without transporting matter. This is the case also in space physics. Waves are responsible for energy transport both between different parts of space and between different particles in the space plasma. They are also useful for diagnostics of the space plasma itself. The present thesis considers two different parts of the large subject of space plasma waves: Lower hybrid cavities (LHCs) and simple-pole particle distribution functions. The LHCs are localised density depletions that have been observed by several spacecraft. They have increased wave activity in the lower hybrid frequency range, and was previously found on altitudes up to 1750 km. New observations by the Viking and Cluster satellites show that they are common magnetospheric features, at least up to an altitude of 35,000 km. Theoretical results, assuming a cylindrically symmetric density depletion, show that even though the density depletion may decrease slowly with increasing radial distance, and thus be essentially infinite in extent, there is a maximum distance within which a trapped mode, with given wave number kz parallel to the geomagnetic field, may propagate. Furthermore, there is a local relation between the plasma density gradient and the lowest possible frequency that the trapped waves can have, for any monotonic density and given kz. The combined theoretical and observational results indicate that the length of the cavities is larger than the width by a factor of at least 200. Simple-pole particle distribution functions are introduced because they can model high velocity tails of the particle distribution in a way that is not possible to do with Maxwellian distribution functions. These distributions also simplify the calculations. This gives new possibilities for the physical understanding, as well as the numerical calculations, of the dispersion relations of real space plasmas. The dispersion relations of plasmas described by simple-pole distributions are examined, both for unmagnetised and for magnetised plasmas. These examples show how particle populations with the same density and mean particle energy, but with somewhat different distribution functions, have different wave propagation properties that should be observable by existing spacecraft.

<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.