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Introduction to Solar Physics

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Introduction to Solar Physics Structure of lectures I Introduction and overview Core and interior: energy generation and standard solar model Introduction to Solar radiation and spectrum Solar Physics Solar spectrum Radiative transfer Sami K. Solanki Formation of absorption and emission lines Convection: The convection zone and IMPRS lectures granulation etc. January 2005 Solar oscillations and helioseismology Solar rotation Structure of lectures II Structure of lectures III: next time The solar atmosphere: structure Explosive and eruptive phenomena Photosphere Flares Chromosphere Transition Region CMEs Corona Explosive events Solar wind and heliosphere Sun-Earth connection The magnetic field CMEs and space weather Zeeman effect and Unno-Rachkovsky equations Longer term variability and climate Magnetic elements and sunspots Chromospheric and coronal magnetic field Solar-stellar connection The solar cycle Activity-rotation relationship Coronal heating Sunspots vs. starspots MHD equations & dynamo: see lectures by Ferriz Mas The Sun: a brief overview The Sun, our star The Sun is a normal star: middle aged (4.5 Gyr) main sequence star of spectral type G2 The Sun is a special star: it is the only star on which we can resolve the spatial scales on which fundamental processes take place. The Sun is a special star: it provides almost all the energy to the Earth The Sun is a special star: it provides us with a unique laboratory in which to learn about various branches of physics. 1 The Sun: Overview The Sun: a few numbers 30 Mass = 1.99 10 kg ( = 1 M) Average density = 1.4 g/cm3 26 Luminosity = 3.84 10 W ( = 1 L) Effective temperature = 5777 K (G2 V) Core temperature = 15 106 K Surface gravitational acceleration g = 274 m/s2 Age = 4.55 109 years (from meteorite isotopes) Radius = 6.96 105 km Distance = 1 AU = 1.496 (+/-0.025) 108 km 1 arc sec = 722±12 km on solar surface (elliptical Earth orbit) Rotation period = 27 days at equator (sidereal, i.e. as seen from Earth; Carrington rotation) The Sun’s Structure The solar surface Solar interior: Everything below Since solar material does not exhibit a phase transition (e.g. the Sun’s (optical) from solid or liquid to gaseous as for the Earth), a standard surface way to define the solar surface is through its radiation. Divided into The photons travelling from the core outwards make a hydrogen-burning random walk, since they are repeatedly absorbed and core, radiative and reemitted. The mean free path increases rapidly with radial convective zones distance from the solar core (as the density and opacity Solar atmosphere: decrease). Directly observable A point is reached where the average mean free path part of the Sun. becomes so large that the photons escape from the Sun. Divided into This point is defined as the solar surface. It corresponds to photosphere, optical depth τ = 1. Its height depends on λ. chromosphere, corona, Often τ = 1 at λ=5000 Å is used as a standard for the solar heliosphere surface. Wide range of physical parameters The Sun presents a wide variety of physical phenomena and processes, between solar core and corona. Solar physics in E.g. Gas density varies by ≈ 30 orders of magnitude, temperature by 4 orders, relevant time scales from 10-10 sec relation to other to 10 Gyr branches of physics ÎDifferent observational and theoretical techniques needed to study different parts of Sun, e.g. helioseismology & nuclear physics for interior, polarimetry & MHD for magnetism, etc. 2 Solar Physics in Relation to Other Fields The Sun as a plasma physics lab. Sun-Earth relations: climate, space weather Turbulence, Cosmic rays, local dynamos interstellar medium Plasma physics Solar planets, extrasolar planets Sun Atomic/molecular Cool stars: activity, physics structure & evolution Fundamental physics: Neutrinos, Gravitation Solar Tests of Gravitational Physics The Sun and particle physics Curved light path in solar gravitational field Î Test The fact that the rate of neutrinos measured by the of General Relativity Homestake 37Cl detector is only 1/3 of that predicted Red shift of solar spectral lines Î Test of EEP by standard solar models was for > 30 years one of the major unsolved problems of physics. Oblate shape of Sun Î Quadrupol moment of solar Possible resolutions: gravitational field: Test of Brans-Dicke theory (R. Mecheri) Standard solar model is wrong Neutrino physics is incomplete Comparison of solar evolution models with Recent findings from SNO and Superkamiokande: observations Î Limits on evolution of fundamental Problem lies with the neutrino physics constants ÎStandard model of particle physics needs to be Polarization of solar spectral lines: Tests revised gravitational birefringence Î Tests of equivalence Nobel prize 2002 for R. Davies for discovery of the principle & alternative theories of gravity (O. Preuss) principle & alternative theories of gravity (O. Preuss) solar neutrino problem. Solar physics and astrophysics Which stars have magnetic fields or show magnetic activity? Phenomena happening all over the universe can be Best studied star: Sun best studied at close distances, where the relevant F, G, K & M stars (outer physical processes can be spatially resolved (same convection zones) show for solar planets). magnetic activity & have E.g. magnetic activity, is present on innumerable <B> fields of G-kG. stars, in accretion disks, in jets, in the interstellar Early type stars: Ap, Bp, medium, etc., but the relevant spatial scales can (kG-100kG), Be (100G) generally not be resolved. The Sun provides a key. White dwarfs have B ≈ Radio astronomy, radiative transfer, kG-109 G, no activity spectropolarimetry, asteroseismolgy, etc. are Not on diagram: pulsars techniques first developed to study the Sun 3 The Sun compared with active stars Sun, Earth and planets Solar output affects the magnetospheres and atmospheres of planets Solar energy is responsible for providing a habitable environment on Earth Solar evolution and liquid water on Mars... The Sun’s core In the Sun’s core mass is turned into energy. Nuclear reactions burn The solar interior 7x1011 kg/s of hydrogen into helium. Inside the core the particle density and temperature are so high, that individual protons ram into each other at sufficient speed to overcome the Coulomb barrier, forming heavier He atoms and releasing energy Nuclear reactions in cores of stars Nuclear reactions of pp-chain p=proton Sun gains practically all its energy from the reaction d=deuterium 4p → α + 2e+ + 2ν = 4He + 2e+ + 2ν α=Helium Two basic routes γ=radiation p-p chain: yields about 99% of energy in Sun ν=neutrino 2nd reaction CNO cycle : 1% of energy released in present day Sun 2 reaction (but dominant form of energy release in hotter stars) replaces step 3 of st Both chains yield a total energy Q of 26.7 MeV, 1st reaction rd nd mainly in the form of γ-radiation Qγ (which is 3 reaction replaces steps 2+3 of 2 reaction absorbed and heats the gas) and neutrinos Qν Branching ratios: (which escapes from the Sun). 1st vs. 2nd + 3rd 87 : 13 2nd vs. 3rd → 13 : 0.015 4 Nuclear reactions of CNO-cycle Temperature dependence of pp- chain and CNO cycle C, N and O act only as catalysts: Basically the same things happens as with proton p-p chain in cool main- chain. sequence stars CNO cycle in hot main- sequence stars Triple alpha process in red giants: 3He → C Solar neutrinos Solar neutrino spectrum Neutrinos, ν, are produced at various stages of the pp-chain. Continua: Neutrinos are also produced by the reaction: p(p e- , ν)d , so- Continua: 2 called pep reaction. Being a 3-body reaction it is too rare to number/(cm2 s contribute to the energy, but does contribute to the number MeV) of ν. of ν. Lines: number/(cm2 s) Bars at top & shading: sensitivity of different materials to ν Solar neutrinos II Results of various neutrino Since 1968 the Homestake 37Cl experiment has experiments given a value of 2.1 ± 0.3 snu (1snu =1 ν / 1036 target atoms) Standard solar models predict: 7±2 snu ÎSolar Neutrino Problem! In 1980s & 90s water based Kamiokande and larger Superkamiokande detectors found that approximately half the rare, high energy 8B ν were missing. 71Ga experiments (GALLEX at Gran Sasso and SAGE in Russia) showed that the neutrino flux was too low, even including the p(p,e+ ν)d neutrinos. 5 Solar neutrinos III Solar Neutrinos IV Sensitivity of H2O, Possible solutions to solar neutrino problem: 71Ga and 37Cl to ν increases ~ Standard solar model is incorrect (5-10% lower exponentially with temperature in core gives neutrino flux consistent increasing energy. with Homestake detector). ÎHomestake 37Cl detector and (Super-) Kamiokande Neutrino physics is incomplete (i.e. the standard see mainly high-energy ν from rare β+-decay of 8B. model of particle physics is wrong!) Branching ratios between the various chains: central Nuclear physics describing the pp-chain is 37 for predicting exact ν-flux detectable by Cl & H2O incorrect Branching ratios depend very sensitively on T(r=0), Nuclear physics describing interaction between while total ν-flux depends only linearly on luminosity. 37 71 neutrino and Cl is incorrect (Kamiokande & Even Ga experiments sensitve largely to high 71 energy ν. Ga showed that this wasn’t the problem) Resolution of neutrino problem Resolution of neutrino problem II SNO (Sudbury Neutrino Observatory) in Sudbury, Canada uses D2O and can detect not just the Lesson learnt: neutrinos have a multiple electron neutrino, but also µ and τ neutrinos personality problem (J. Bahcall) ÎThe neutrinos aren’t missing, e- neutrinos produced Other lesson learnt: the “dirty” and difficult in the Sun just convert into µ and τ neutrinos Other lesson learnt: the “dirty” and difficult ÎThe problem lies with the neutrino physics.
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