Tuesday, 10 May 2011
PhD thesis: Solitary Waves and Enhanced Ion Lines
This thesis addresses solitary waves and their significance for auroral particle acceleration, coronal heating and incoherent scatter radar spectra. Solitary waves are formed due to a balance of nonlinear and dispersive effects. There are several non-linearities present in ideal magnetohydrodynamics (MHD) and dispersion can be introduced by including the Hall term in the generalised Ohm's law. The resulting system of equations comprise the classical ideal MHD waves, whistlers, drift waves and solitary wave solutions. The latter reside in distinct regions of the phase space spanned by the speed and the angle (to the magnetic field) of the propagating wave. Within each region, qualitatively similar solitary structures are found. In the limit of neglected electron intertia, the solitary wave solutions are confined to two regions of slow and fast waves, respectively. The slow (fast) structures are associated with density compressions (rarefactions) and positive (negative) electric potentials. Such negative potentials are shown to accelerate electrons in the auroral region (solar corona) to tens (hundreds) of keV. The positive electric potentials could accelerate solar wind ions to velocities of 300–800 km/s. The structure widths perpendicular to the magnetic field are in the Earth's magnetosphere (solar corona) of the order of 1–100 km (m). This thesis also addresses a type of incoherent scatter radar spectra, where the ion line exhibits a spectrally uniform power enhancement with the up- and downshifted shoulder and the spectral region in between enhanced simultaneously and equally. The power enhancements are one order of magnitude above the thermal level and are often localised to an altitude range of less than 20 km at or close to the ionospheric F-region peak. The observations are well-described by a model of ion-acoustic solitary waves propagating transversely across the radar beam. Two cases of localised ion line enhancements are shown to occur in conjunction with auroral arcs drifting through the radar beam. The arc passages are associated with large gradients in ion temperature, which are shown to generate sufficiently high velocity shears to give rise to growing Kelvin-Helmholtz instabilities. The observed ion-line enhancements are interpreted in the light of the low-frequency turbulence associated with these instabilities.
Ekeberg, J., Solitary Waves and Enhanced Incoherent Scatter Ion Lines, IRF Scientific Report, URI: urn:nbn:se:umu:diva-42955, ISBN 978-91-977255-7-6, IRF, Umeå University, Sweden. (permanent url)