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$\textstyle \parbox{10cm}{
	      \lq\lq Hold fast to dreams, for if dreams die, life is a broken winged
	      bird that cannot fly.'' --Langston Hughes}\\

This thesis deals with the new information that can be retrieved by systematic and elaborate analysis of the multi-station image data from ALIS. The physics in this thesis mainly covers new findings about HF-pump enhanced airglow in 6300 Å. The thesis also presents work on determination of primary electron specra derived from the altitude variation of aurora.

Chapter 2 a short introduction to auroral and airglow physics is given in. The focus is specifically on the spectral regions in which ALIS currently make observations. The strength with multi-station imaging is the possibility of obtaining estimates of the three-dimensional distribution of imaged emissions. Chapter 3 contains a tutorial introduction to tomographic inversion for ground-based auroral studies. The possibilities and limitations of the tomographic inversion is investigated in depth. It finds that the ground-based setup cannot give retrievals of the emisssions with what can be considered tomographic resolution but that the retrieval gives good enough resolution for auroral studies. In order to obtain accurate results of the emission distribution, there are strict requirements on the calibrations of the cameras. Chapter 4 describes and evaluates a method to meet the requirements on the geometrical calibrations of the optics. Chapter 5 describes the factors that have to be taken into account in the modelling of the imaging process. The introduction is ended in Chapter 6 with a short outlook to possible future work and experiments.

Paper I

Paper I presents the resolution and noise sensitivity of tomographic reconstructions from ground-based multistation imaging of aurora with the auroral large imaging system (ALIS). A full model simulation of the ALIS system is developed for evaluation of the resolution and noise sensitivity of tomographic reconstruction procedures. The results show that for ALIS typical auroral distributions can be retrieved with relative errors in the range 0.05-0.1 from measurements with typical noise levels. A general method to estimate resolution in a tomographic imaging system is developed and used to give estimates of the horizontal and vertical resolution. The current limitations and future perspective of ground-based tomographic inversion are briefly discussed. The paper outlines a method for retrieving feasible auroral reconstructions from a few image projections with variable noise level.

Paper II

Paper II compares two methods for estimating characteristics of primary auroral electron spectra and uses them to describe an auroral event. One method uses the spectral information in the images and the other method is based on the inversion of the $ N_2^+1NG$ $ 4287$ Å altitude distribution. With the second method ALIS can currently give estimates of the primary electron distribution with medium time resolution (10 s).

The auroral event, a passage of an eastward-moving fold in a pre-existing auroral arc, is analysed and characteristics of the precipitating electrons show regions with different fluxes. A soft region that has previously been found inside the fold appears to belong to a wider region of soft precipitation that emerges as the arc activates.

Paper III

Paper III reports about simultaneous measurements of enhanced airglow at 6300 Å and plasma temperatures during an experiment 16th February 1999. The electron gas was heated by transmitting a powerful high frequency electromagnetic pump wave from the EISCAT-Heating facility into the ionospheric F region. With the ALIS multi-station imaging it was possible to determine the height and position of the enhanced airglow volume by triangulattion. A tomography-like inversion method was used to estimate the size and shape of the enhanced airglow volume. The shape was found to be roughly spherical but variations were large. Further, the airglow enhancement is correlated with large pump-induced electron temperature enhancements of up to 250%. These temperature enhancements extend several hundred kilometres above and several tens of kilometres below the airglow cloud.

Paper IV

Paper IV compares the optical and radar data from the ionospheric heating experiment in Tromsø during February 1999 with different theoretical models. By analysing the characteristics based on the models of thermal response of the ionosphere and the atmospheric optical emissions we can draw conclusions about the mechanism of the interaction between the HF-pump wave and the ionospheric plasma. The detailed spatial and temporal characteristics of the red line airglow show that the thermal theory is unable to explain the observations.

Paper V

Paper V presents the first estimates of the three-dimensional volume emission rate of enhanced $ O(^1D)$ 6300 Å airglow caused by HF radio wave pumping. The region of enhanced airglow was imaged by three ALIS stations. The tomography-like inversion used in Paper III is refined to obtain estimates of the volume emission of the airglow. The altitude of maximum emission was found to be around 235 $ \pm$ 5 km with typical horizontal and vertical scale sizes of 20 km. The shape of the $ O(^1D)$ excitation rate varied from flatish to elongated along the magnetic field. The altitude of maximum emission is found to be approximately 10 km below the altitude of the enhanced ion line and 15 km above the altitude of maximum electron temperature. Significant deviations were found when the measured altitude and temporal variation of the 6300 Å emission were compared with emissions predicted by the thermal theory. The 6300 Å emission from excitation of the high energy tail is about a factor of 4 too large compared with what is observed. This shows that the electron distribution that is the source of $ O(^1D)$ excitation is sub-thermal.

Images of the initial excitation show speckled and patch structures that change to a simpler shape with a smaller region that contains most of the excitation within 30-40 s.

Paper VI

Paper VI presents Monte Carlo simulations of the electron energy distribution for a low ionized plasma interacting with the F-region neutral gas. The results show that the electon distribution is not Maxwellian in the energy range relevant for excitation of $ O(^1D)$ and other atomic states that emit light in the visible spectra. It is found that a depletion between 10 and 80 % should exist in the electron distribution above 2 eV. The depletion is due to electron excitation of the vibrational states of $ N_2$. The Monte Carlo model is a micro-physical energy transfer model that gives good agreement with EISCAT UHF measurements of electron cooling during HF radio wave heating experiments.

Some implications for excitation mechanisms relevant to artificial airglow are derived. Further it is predicted that a weak (-5 dB) and wide (1 MHz) peak between $ \pm1-2$ MHz from the ion-line in the EISCAT VHF spectra should be a consequence of the modification in the electron distribution.

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copyright Björn Gustavsson 2000-10-24