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No description available.

No description available.

The ubiquitous nature of magnetohydrodynamic (MHD) waves and oscillations resolved in magnetic structures in the solar atmosphere is beginning to be documented. To date, the majority of the waves predicted by theory, i.e. the theory of (MHD) waves in a cylindrical, magnetic waveguide, have been observed in the solar corona. These oscillatory events appear to be driven by continuous motions present in the lower solar atmosphere, e.g. p-modes, granular buffeting, vortex motions. The amount of energy available from these drivers is extremely large and could make up a significant portion of the coronal heating budget. Further, in recent years, waves in the solar atmosphere have also begun to be exploited as magneto-seismological tools to measure plasma parameters that are hard to measure using direct methods. This makes study of the waves in the solar atmosphere of vital importance. Here we investigate the influence of geometric and dynamical effects on the eigenfrequencies and eigenfunctions of MHD oscillations supported by magnetic waveguides in solar atmospheric plasmas. The concept of magnetic waveguides is ideal for modelling the waves and oscillations that are observed in magnetic solar structures. Previous modelling efforts have studied simplified solar plasma waveguides but have provided a number of useful, initial magneto-seismological tools. We aim to generalise, further develop and extend these models to include more realistic and applicable loop geometries, i.e. elliptical loop cross-section and density stratification in an elliptically curved loop. We study how the stage of emergency is relevant for loop oscillations. In addition to this we provide the crucial first investigations into how dynamic plasma behaviour in the magnetic waveguides influences the oscillations. As coronal loops are observed to be cooling due to radiation, we investigate the influence of the radiative cooling of the background plasma on the oscillations. This latter topic is a very important and widely applicable development of MHD wave theory. It is found that if we are to obtain more accurate estimates of plasma parameters from the current cohort of magneto-seismological tools, then taking into account seemingly second order effects, e.g geometry and dynamism, will be more than essential. The second order, geometry (in terms of mathematics) effects are shown to cause important, measurable deviations from the first order theory and ignoring them can lead to substantial errors in the estimates of the plasma quantities (e.g. coronal magnetic field strength). With regards to dynamic plasmas, we find a number of very interesting and important results. The first is that the radiative cooling of a plasma also provides another vital and competing method for the damping of oscillations within the plasma (in addition to the previously suggested methods, e.g. resonant absorption, thermal conduction, etc.). The damping can be attributed to changes in temperature and density of the plasma. The majority of the 1 MK plasma in coronal loops is observed to be radiatively cooling, so the damping due to the cooling process could be a dominant method of wave damping in the solar atmosphere. Secondly, we also demonstrate three new magnetoseismological tools that can be used to measure cooling timescales in the corona. The inclusion of the plasma dynamism will also be necessary when calculating the contribution from waves to the heating of the coronal plasma. This novel investigation in to dynamic plasmas can also be applied to wave studies in a wide range of astrophysical plasma scenarios, as all plasmas experience some level of radiative cooling.

A test particle approach is used to study two distinct plasma physics situations. In the first case, the collisionless response of protons to cold plasma fast Alfven waves propagating in a non-uniform magnetic field configuration (specifically, a two-dimensional X-point field) is studied. The field perturbations associated with the waves, which are assumed to be azimuthally-symmetric and invariant in the direction orthogonal to the X-point plane, are exact solutions of the linearized ideal magnetohydrodynamic (MHD) equations. The protons are initially Maxwellian, at temperatures that are consistent with the cold plasma approximation. Two kinds of wave solution are invoked: global perturbations, with inward- and outward-propagating components; and purely inward-propagating waves, localised in distance from the X-point null, the wave electric field E having a preferred direction. In both cases the protons are effectively heated in the direction parallel to the magnetic field, although the parallel velocity distribution is generally non-Maxwellian and some protons are accelerated to highly suprathermal energies. This heating and acceleration can be attributed to the fact that protons undergoing E x B drifts due to the presence of the wave are subject to an effective force in the direction parallel to B. The localised wave solution produces more effective proton heating than the global solution, and successive wave pulses have a synergistic effect. This process, which could play a role in both solar coronal heating and late-phase heating in solar flares, is effective for all ion species, but has a negligible direct effect on electrons. However, both electrons and heavy ions would be expected to acquire a temperature similar to that of the protons on collisional timescales. In the second case the same approach is used to study the collisional transport of impurity ions (carbon, mainly, although tungsten ions are also simulated) in spherical tokamak (ST) plasmas with transonic and subsonic toroidal flows. The efficacy of this approach is demonstrated by reproduscing the results of classical transport theory in the large aspect ratio limit. The equilibrium parameters used in the ST modelling are similar to those of plasmas in the MAST experiment. The effects on impurity ion confinement of both counter-current and co-current rotation are determined. Various majority ion density and temperature profiles, approximating measured profiles in rotating and non-rotating MAST plasmas, are used in the modelling. It is shown that transonic rotation (both counter-current and co-current) has the effect of reducing substantially the confinement time of the impurity ions. This effect arises primarily because the impurity ions, displaced by the centrifugal force to the low-field region of the tokamak, are subject to a collisional diffusivity that is greater than the flux surface-averaged value of this quantity. for a given set of plasma profiles, the carbon ions are found to be significantly less well-confined in co-rotating plasmas than in counter-rotating plasmas, although the difference in confinement time between co- and counter-rotation lessens as the mass of the impurity increases. In the case of carbon ions the poloidal distribution of losses exhibits a pronounced up/down asymmetry that is consistent with the direction of the net vertical drift of the impurity ions. Increasing the mass of the impurity ion is also found to significantly decrease the confinement time in the rotating cases, though the confinement time for the case of a stationary plasma is increased. Such studies of impurity transport within tokamaks are important because it is desirable to expel impurity ions from the plasma to avoid both dilution of the fuel ions and unacceptable radiation losses from the plasma.

Οι ηλιακές παρατηρήσεις υποβαθμίζονται εξαιτίας του φαινομένου του seeing το οποίο σχετίζεται με τις διαταραχές της γήινης ατμόσφαιρας. Για να μελετήσουμε τις λεπτές δομές της ηλιακής επιφάνειας είναι απαραίτητο να εφαρμόσουμε τεχνικές που ενισχύουν τις εικόνες και βελτιώνουν την ορατότητα τους. Επιπλέον, πολλοί από αυτούς τους σχηματισμούς βρίσκονται «κρυμμένοι» μέσα στο υπόβαθρο και πρέπει να διαχωριστούν από αυτό. Σε αυτήν τη διατριβή περιγράφονται τεχνικές που χρησιμοποιούνται για να βελτιωθεί η ορατότητα ηλιακών σχηματισμών διαφορετικών κατηγοριών όπως κηλίδες, λεπτοί σχηματισμοί κοντά στο χείλος και βόμβες Ellerman. Εκμεταλλευόμενοι την ανάλυση πολλαπλής διακριτικής ικανότητας που παρέχουν τα κυματίδια, εφαρμόζουμε αυτό το είδος ανάλυσης σε εικόνες από την ηλιακή επιφάνεια. Ο αλγόριθμος κυματιδίων à trous χρησιμοποιήθηκε ως ο πιο κατάλληλος μετασχηματισμός κυματιδίων. Οι εικόνες ενισχύθηκαν επιτυχώς και οι συγκεκριμένες τεχνικές αποδείχτηκαν πραγματικά αποδοτικές. Αυτές οι τεχνικές μπορεί να βασίστηκαν στον μετασχηματισμό à trous αλλά τροποποιήθηκαν κατάλληλα, ανάλογα με την συγκεκριμένη εφαρμογή. Θα πρέπει να διευκρινίσουμε ότι οι τεχνικές στις οποίες αναφερόμαστε εξαρτώνται κατά πολύ από το πρόβλημα που έχουμε να αντιμετωπίσουμε. Από την άλλη μεριά, οι ηλιακές παρατηρήσεις είναι χρονικές ακολουθίες εικόνων. Έτσι έχουμε να αντιμετωπίσουμε την ανάλυση χρονικών σειρών δεδομένου ότι θέλουμε να μελετήσουμε την χρονική συμπεριφορά και την εξέλιξη των παρατηρούμενων ηλιακών δομών. Αναλύοντας μια χρονική ακολουθία στο χρονο-συχνοτικό χώρο, μπορεί κανείς να προσδιορίσει τον κύριο τρόπο μεταβλητότητας αλλά και πως αυτοί οι τρόποι αλλάζουν με τον χρόνο. Εκμεταλλευόμαστε αυτή την ιδιότητα των κυματιδίων για να εξετάσουμε την χρονική μεταβολή της περιόδου των ταλαντώσεων της σκιάς των κηλίδων χρησιμοποιώντας παρατηρήσεις της ηλιακής ατμόσφαιρας οι οποίες έχουν ληφθεί από γήινα τηλεσκόπια. Χρησιμοποιούμε αυτό το σήμα του πραγματικού κόσμου για να δοκιμάσουμε τις δυνατότητες διαφορετικών κυματιδίων και για να δούμε τι προβλήματα ανακύπτουν αναλύοντας ένα τέτοιο σήμα. Στην μελέτη μας χρησιμοποιούμε τον συνεχή μετασχηματισμό κυματιδίων και τον αλγόριθμο à trous ως detrending εργαλείο. vii Αναφερόμαστε επιγραμματικά σε προηγούμενες μελέτες οι οποίες βασίζονται στις κλασσικές μεθόδους ανάλυσης σήματος ώστε να μπορέσουμε να καταλάβουμε τι μπορούν να προσφέρουν τα κυματίδια στην ηλιακή φυσική. Τα αποτελέσματα μπορούν να συνοψιστούν στα ακόλουθα: τα κυματίδια είναι ένα ισχυρότατο εργαλείο της ανάλυσης σήματος ακόμη και για τους ηλιακούς φυσικούς και θα μας απασχολήσουν πολύ στο μέλλον και στην μελέτη εικόνων της ηλιακής επιφάνειας. / Solar observations are usually degraded due to seeing effects related to the turbulence of earth’s atmosphere. In order to study the fine structure of the solar surface it is necessary to apply techniques that enhance the images and improve the visibility of fine structures. Moreover many of those structures are embedded in the background and we need separate them from it. In this thesis we describe techniques that are used in order to improve the visibility of several categories of solar features like sunspots, fine structures near the limb and Ellerman bombs. Taking advantage of the multiresolution analysis that provides wavelets, we perform that kind of analysis to images from solar atmosphere. The à trous wavelet algorithm has been used as the proper wavelet transform. The images were successfully enhanced and those techniques have been proved really efficient. Those enhancement techniques were based to the à trous wavelet transform but they were modified properly depending on the specific application. We must clarify that the discussed techniques depend by far on the problem we have to deal with. On the other hand, solar observations are time sequences of images. So we have to deal with time series analysis provided that one wants to study the temporal behavior and evolution of the observed solar structures. By decomposing a time series into time-frequency space, one is able to determine both the dominant mode of variability and how those modes vary in time. We take advantage of this property of the wavelet analysis in order to examine the temporal variation of the period of the umbral oscillations using ground–based observations of the solar atmosphere. We use this real-life signal in order to test the capabilities of different wavelets and to see the problems that arise analyzing such a signal. In our study we use the continuous wavelet transform in order to perform the analysis and the “à trous” algorithm as a detrending tool. We make slightly reference to previous works based on classical methods of signal analysis just to be able to understand what wavelets can offer in solar physics. The results can be summarized as follows: wavelets are a powerful tool for signal analysis even for solar scientists and will preoccupy us a lot in the future and for the study of the solar surface.

Η εργασία αυτή ασχολείται κατά πρώτον με την στατιστική επεξεργασία πεντάλεπτων μετρήσεων φυσικού φωτισμού κατά τη διάρκεια ενός έτους και την παραγωγή διαγραμμάτων μέσων ωριαίων μηνιαίων τιμών για κάθε τύπο καιρού και για κάθε μέγεθος (ολικός φωτισμός, διάχυτος φωτισμός, ολική ηλιακή ακτινοβολία, διάχυτη ηλιακή ακτινοβολία) και κατά δεύτερον με τη δικτύωση μελλοντικών σταθμών στην Αττική (με ενσύρματο ή ασύρματο τρόπο) που μετρούν το φυσικό φωτισμό. / --