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THE SUN INTRODUCTION General stellar category of type G – “a yellow runt” Monotonously common in MWG – billions of stars like our Sun Remarkably stable Nothing else in the Universe (other than the Earth) is more immediately important to us The Sun is the origin of virtually all the energy that sustains life The Sun is the source of our weather The Sun is the “Rosetta stone of Astrophysics” Galileo discovered sunspots over 4 centuries ago Surge of international interest about Sun over past 20 years Before it was “solar dermatology;” now it’s really Astrophysics Best resolution today shows solar structures that are 50 miles across Nearly a dozen major space-based observatories throughout Earth – almost all launched since mid-1990s New initiatives to understand space weather – the effects created by billions of tons of plasma that can erupt from the Sun and cause magneto-electrical squalls throughout Solar System Goal: end-to-end modeling – a comprehensive view of what happens all the way from the interior of the Sun to the Earth’s upper atmosphere SOLAR FLARES AND CMEs Two types of explosive solar events impact Earthlings most severely: SOLAR FLARE – small area above the solar surface suddenly roars to tens of millions of degrees, throwing off a surge of radiation that can cause communication blackouts, disable satellites, or kill a spacewalking astronaut CME (CORONAL MASS EJECTION) – billions of tons of charged particles escape from the sun’s halo, the corona, at millions of miles per hour. When these behemoth clouds slam into the Earth’s protective magnetosphere, they squash magnetic field lines and dump trillions of watts of power into the Earth’s upper atmosphere. This can overload powerlines, causing massive blackouts, and destroy delicate instruments on anything in Earth’s orbit. October 2003 – flare and CME occurred together (4th most powerful flare ever observed) Back-to-back CMEs smacked the Earth. Atmosphere was so electrically charged – Northern Lights were seen as far south as the Mediterranean. Little damage was done due to advanced warning system. However, in 1989, a fierce CME struck the earth and blew out Hydro-Quebec’s power grid, leaving almost 7 million people without electricity, and a multi-million dollar damage bill. SUN IS PLASMA All of the Sun is gas 70% Hydrogen, 20% helium and 2% heavier elements Sun is a madhouse of electromagnetic activity On the Sun, almost everything is electrically conductive because there aren’t very many intact neutral atoms Overwhelming thermal and radiation energies excite electrons to the point at which they pop off their atoms, creating a seething stew of positively charged nuclei and free negative electrons – this gaseous mix that conducts electricity as good as copper wire is called PLASMA. Like any electricaly charged object, plasma produces magnetic fields when it moves As those fields shift, they induce more currents, which in turn produce more fields This tangle of plasma and electric and magnetic effects determines the forms of nearly everything in or above the Sun, such as the bright coronal loops and the dark areas we call sunspots. Everything we see as solar activity is a magnetic field being acted upon by plasma and vise-versa. Both are forever in motion. NUCLEAR FUSION IN THE CORE Local gas and dust drifted together, drawn by gravity Mass of sphere gets larger and gigantic pressure in center Fusion reaction is sparked – hydrogen nuclei come together to create helium Resulting helium nuclei are slightly less massive than the component hydrogen nuclei The difference is converted o energy E=mc2 Most of the energy is gamma rays It takes thousands of years for a single photon to travel ½ million miles to the surface By that time, it has shed so much energy (due to the numerous collisions) that it emerges as visible light. In 1920s, Sir Arthur Eddington and others were convinced that the source of the Sun’s power was some subatomic phenomenon requiring enormous heat. This idea was ridiculed as outlandish. By the 1950s, the fusion model was convincingly verified. SOLAR EXPLORATION Solar and Heliospheric Observatory (SOHO) a satellite run jointly by NASA and the European Space Agency Breakthroughs have been made on all solar fronts New puzzles constantly revealed The “ceaseless dance between plasma and magnetic fields Everything on Sun influences everything else – and it all has a direct effect on the Earth “The Sun is the only astronomical object that critically matters to humankind” WHAT INTERIOR MECHANISMS PRODUCE THE SUN’S MIGHTY MAGNETIC DYNAMO? The magnetic field drives virtually everything on the Sun Sun has opposite north and south magnetic poles like the Earth The Earth’s field is produced by motion of the molten iron in the outer core The Sun’s field is produced by the internal motion of plasma Until recently, impossible to see beneath the photosphere Helioseismology developed – an ultrasound scan on the Sun’s interior Global Oscillation Network Group (GONG) – worldwide array of observation stations positioned 60degrees apart to observe the Sun 24-hours a day The idea came about in the 60s – observed that solar surface throbbed with rhythmic oscillations like the skin of a drum Discovery: Much faster rotation at equator (4400 mph) than at poles (545 mph) The “shear zone” (a.k.a. – tacholine) between the regions of varying rotations is what is believed to cause the magnetic field. The internal motion stretches and twists the north-south magnetic field lines, wrapping them around the Sun thus adding energy (imagine how energy is stored in a stretched rubber band) Sometimes the powerful twisted field lines become buoyant and rise above the photosphere – these manifest as loops, prominences and sunspots, which are observed on the surface of the Sun WHY DO SUNSPOTS FLUCTUATE IN 11-YEAR CYCLES? WHAT EFFECT DOES THIS HAVE ON TERRESTRIAL CLIMATE? Sunspots can range in diameter from 1,500 miles wide to several times the width of the Earth. Sunspots are visible because the bundled field lines impede the flow of convection The center of the sunspot, the umbra, is 1000+ degrees cooler than the surrounding 10,000-degree photosphere. Chinese observed sunspots in first century Sunspots observed by telescope in 17th century German astronomer, Samuel Heinrich Schwabe, began a systematic count in 1826 In 1843, he reported that their number goes from minimum to maximum and back in about a decade. 1915 – American astronomer, George Ellery Hale (Mt. Wilson Observatory, California) – sunspots occur in pairs roughly parallel to the Sun’s equator, and each half of a pair has the opposite magnetic polarity. All spot pairs in N-Hem. Of Sun have same orientation, and all spot pairs in S-Hem. have opposite orientation. Every 11 years, on the average, the Sun reverses its overall magnetic polarity One complete cycle lasts an average of 22 years No one completely understands the entire process. The Earth’s field also reverses – the most recent reversal was 780,000 years ago. There is evidence that sunspot cycles have direct consequences for human life MAUNDER MINIMUM (1645-1715) no sunspots appeared on the solar face at all Later studies done with tree rings dated from that period – significantly more Carbon-14 Higher amounts of cosmic radiation reached the earth during that time (a magnetically active sun reduces the amt. of cosmic radiation we receive) Although, more sunspots mean a more luminous (brighter) Sun – sunspot areas are extra bright – the magnetism causes extra-bright areas called faculae Investigation of another low sunspot period followed (1460-1550) These extended minimums (1400-1850) is called the LITTLE ICE AGE The Thames River and the lagoons of Venice regularly froze Sunspot activity has been high over the past century (Earth’s temperatures have climbed) The greater luminosity only accounts for ½ the global temperature increase (pre-1940) and much less than that for the following years up to the present. We still need to understand the structure of magnetic fields under sunspots to better predict these cycles. HOW IS IT POSSIBLE THAT THE CORONA IS TYPICALLY 100s OF TIMES HOTTER THAN THE EARTH’S SURFACE? The corona is only visible during a total solar eclipse The chromosphere is only one-millionth as dense as the photosphere The corona is one-hundredth as dense as the chromosphere The photosphere is 5700 degrees C; the corona is 10,000 degrees C. Shouldn’t the layer farthest from the center of the sun be the coolest? This defies logic! Researchers think that this is due to magnetic reconnection – or “splicing” of magnetic field lines that causes energy to be released. The “magnetic carpet” just underneath the surface. The tightly wound magnetic field lines come together and release an enormous amount of energy, which accounts for the high temperature. WHAT EXPLAINS FLARES AND THE CORONAL MASS EJECTIONS THAT ARE RESPONSIBLE FOR ELECTRICAL TEMPESTS ON EARTH? HOW CAN THESE STORMS BE PREDICTED? The corona produces the most powerful particle accelerators in the solar system – flares and CMEs Flares expel much of their energy as x-rays and are generated when electrical currents are suddenly released as one or more magnetic field loops in the corona become strained to the breaking point and snap into a new shape This radiation reaches the earth in 8 minutes and can disrupt radio communications and navigation systems. A small percentage of flares hurl out fast-moving, high-energy protons that can cripple satellites. CMEs often follow flares These massive eruptions of plasma can also occur on their own CMEs take 1-3 days to reach Earth They smash into the Earth’s magnetosphere and deform it – and if conditions right – produce a multimillion ampere ring current in the belts of charged particles that continually circle the Earth. CMEs are even more threatening to communication satellites than flares and can take out power grids causing power failures. The trigger mechanism for CMEs is not known, therefore not possible to predict With SOHO we can see these storms leaving the Sun The magnetic polarity of a CME can change during its journey If the polarity is opposite that of the Earth’s, it does the most damage on impact We can predict with 80-percent accuracy if they will hit the Earth The worst storms often come in the waning years after the solar maximum. The most recent solar maximum ended in 2001; the strongest x-ray flare ever observed was in Nov. 2003 Sun Statistics Mass (kg) 1.989e+30 Mass (Earth = 1) 332,830 Equatorial radius (km) 695,000 Equatorial radius (Earth = 1) 108.97 Mean density (gm/cm^3) 1.410 Rotational period (days) 25-36* Escape velocity (km/sec) 618.02 Luminosity (ergs/sec) 3.827e33 Magnitude (Vo) -26.8 Mean surface temperature 6,000°C Age (billion years) 4.5 Principal chemistry Hydrogen 92.1% Helium 7.8% Oxygen 0.061% Carbon 0.030% Nitrogen 0.0084% Neon 0.0076% Iron 0.0037% Silicon 0.0031% Magnesium 0.0024% Sulfur 0.0015% All others 0.0015% The Sun's period of rotation at the surface varies from approximately 25 days at the equator to 36 days at the poles. Deep down, below the convective zone, everything appears to rotate with a period of 27 days.