Seismic emissions from solar flares
thesisposted on 15.01.2017, 23:29 by Ionescu, Diana
The Sun is the centre and the master of our solar system. Its power gives us light and warmth and life, but its moods can be fearsome. Sunspots, solar flares, coronal mass ejections (CME) and the plethora of other complex magnetic activities that follow the 11-year rhythm of the solar cycle can mean devastation for our technologies, from satellites, to power grids, to telecommunications. Pity the unfortunate astronaut caught unprotected in space during a solar storm! Flares are the most powerful and most dangerous events on the Sun, spewing huge quantities of radiation and particles into space, often accompanied by a CME, sometimes to pummel the Earth. Only our magnetic shield, the magnetosphere, protects us. All means must be brought to bear when studying the Sun's activity, including observations in many wavelengths from terrestrial and space-borne observatories, and the relatively new science of local helioseismology for peering beneath the surface, the photosphere. Especially during the last decade, that of Solar Cycle 23, advances in theory and technology have opened the Sun's magnetic active regions to our deeper seismic gaze. Cycle 23 was closely mapped by the Solar and Heliospheric Observatory (SOHO) orbiting between the Earth and the Sun. A fascinating discovery made with SOHO is that flares high in the Sun's corona not only project their power outwards into space, but also downwards to the photosphere, where they can cause powerful sunquakes. This thesis is devoted to sunquakes: their discovery, their characteristics, and their physics. How and why are they formed, and why do many flares not excite them? How much energy is required to produce them? And how can this unique resource of a sudden localized seismic source be used to better probe the Sun's interior? In so-doing, I present the most detailed survey yet of sunquakes and their physics. Specifically, we have used the local helioseismic technique known as helioseismic holography to detect sunquakes by imaging their acoustic sources rather than their harder-to-see expanding ripples. Nearly all known sunquakes, more than a dozen, have been discovered by us in this way. This is sufficient to begin a survey to discover which types of flares excite quakes and which do not. Using a wide range of observations over several wavelengths, we explore the effects of such features as flare area, energy, height, and spectral hardness. We have also applied a 1D radiation hydrodynamics simulation code RADYN to synthesizing the mechanisms which might create quakes. Both our survey and our simulations favour ``back-warming'', whereby the low chromosphere is suddenly heated by the flare. This energy is then quickly transferred to the adjacent photosphere producing a quake. Because seismic emission from solar flares presents by far the most localized seismic sources in the solar environment, both spatially and temporally, and flares being the only seismic generators whose operation is open to direct view, this phenomenon offers an especially opportune control facility for 21st-century helioseismology.