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Advanced Solar Theory (MT5810) 1. 2. 3. 4. 5. 6. 7. 8. OUTLINE Observational properties of the Sun MHD equations (revision) Induction equation - solutions when Rm <<1, >>1 Magnetic equilibria Waves Solar wind Reconnection Instabilities Books: Solar MHD -- ER Priest Camb. Encyc. of the Sun -- K Lang Nearest Star -- L Golub & J Pasachoff 1.1 History 1. Observations Ancient observations - sunspots (chinese, greeks) - forgotten Galileo (1610) rediscovered sunspots 1851 -- first photograph of corona (eclipse) 1859 -- first solar flare observed 1908 -- sunspots have strong B 1956 -- foundations of MHD (Cowling) 1957 -- satellite observations of interplanetary medium 1980’s & 90’s -- development of MHD -- space observations (Skylab 73-74, Yohkoh 91-05, SOHO 95 - …, TRACE 98 - …, RHESSI 02 - … Future Space Missions STEREO (NASA) April 2006 * Two satellites * _ _ _ _ _ _ _ _ _ _ _ _ _* Solar B (Japan-US-UK) Sept 2006 * Connections surface/corona Solar Dynamics Observatory 2008 * _ _ _ _ _ _ _ _ _ _ _ _* Solar Orbiter (ESA) 2012 *____________ * 1.2 Properties of the SUN Ball of gas held together by gravity ? Tsurface_ _ _ _ _ ? Mass/ s. system _ _ _ _ _ ? Chemical composition _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ ? Radius _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ ? Age _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ Some other properties ? Mean distance from Earth 93 million miles = 150 million km _ _ _ _ _ _ _ _ _ __________ ? Rotation period = 25 days (equator), 37 days (poles) ? Mean density = 1.4 x 103 kg m-3 _ _ _ _ _ _ _ _ _ _ ? Surface gravity = 274 m s-2 _ _ _ _ _ _ _ _ _ _ ? Escape velocity at surface = 618 km s-1 _ _ _ _ _ Overall Structure Interior --> Core, Convection zone __________ __________ Atmosphere --> Photosphere, Chromosphere, Corona Photosphere Temperature 6000 K QuickTime™ and a YUV420 codec decompressor are needed to see this picture. Covered with turbulent convection cells: *_ _ _ _ _ _ _ _ _ _ * (1 Mm) *_ _ _ _ _ _ _ _ _ _ * (15 Mm) ?? why Amazing images [70 km] from La Palma (Canaries) In closeup: points, flowers, ribbons (created by magnetic fields) 1.3 SUNSPOTS Photosphere --> Sunspots Galileo discovered (1610) w. telescope Dark because cool But why cool ? *_ _ _ _ _ _ _ _ _ _ _ _ _ _* ??? why Stunning Image from La Palma Penumbral structure created by B Map of Photospheric Magnetic Field White -- towards you; Black -- away from you Regions around sunspots -bipolar *_ ______* SURPRISES: -1. Intense magnetic fields over whole Sun •_ _ _ _ _ _ _ _ _ _ _ _* ?? why 2. Patterns of sunspots/active regions ?? why QuickTime™ and a YUV420 codec decompressor are needed to see this picture. 1.4 CORONA (Turkey, 1999) Fine structure Magnetic tubes “prominences” SURPRISE (1940) *_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _* ??why Next: Egypt, March 2006 Picture with X-ray telescope: * * Yohkoh A magnetic world Amazingly rich variety of MHD phenomena TRACE (Active region) - from above From side: *_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ * Magnetic field dominates plasma creates intricate structure/ heats ?? how 1.5 Solar Activity On top of the “Quiet Sun” (ie the background photosphere, chromosphere & corona), there are several types of transient phenomena: (i) _ _ _ _ _ _ _ _ _ (ii) _ _ _ _ _ _ _ _ _ (iii) _ _ _ _ _ _ _ _ _ (iv) _ _ _ _ _ _ _ _ _ (ii) Prominences On disc in chromosphere, see as long, thin structures Dark - cutting out light from below * _______ _______ _ _ _ _ _ _ _ _* ??? Sometimes erupt [look like twisted tube] why ?? (iii) Solar Flare Sudden brightening in chromosphere & corona near sunspots *_ _ _ _ _ _ _ _ _ _ _ ___________* How ?? (iv) Coronal Mass Ejections SOHO Coronagraph -- Artificial eclipse Discovered huge ejections of plasma ?? *_ _ _ _ _ _ _* Can reach Earth - communications / satellites 1.6 SOHO(Solar & Heliospheric Observatory) QuickTime™ and a Sorenson Video decompressor are needed to see this picture. Launched 1995. Orbiting Sun at point in phase with Earth Observing Sun continuously for 1st time (ESA/NASA) --> 1st comprehensive view of Sun MANY NEW SURPRISES