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From eclipse drawings to the coronagraph and spectroscopy 1 Hardi Peter Kiepenheuer-Institut für Sonnenphysik Freiburg solar eclipse, 11.8.1999, Wendy Carlos and John Kern What we might see on Wednesday… 2 for before Antalyathe– eclipse: Linker, Mikic et al. Theeclipse coronaprediction one rotation 2 March 2006 global MHD model based on observed magnetogram N N HAO / MLSO Lasco C2 / SOHO 1 3 Why the Sun? ¾ it is our Star ¾ nearest and only star where we might resolve the basic processes on their characteristic temporal and spatial scales ¾ the Sun is our ultimate source of energy — influences the whole heliosphere ¾ solar activity can cause failure of space based and terrestrial technical systems ¾ the Sun influences the Earth’s climate ¾ ¾ ¾ ¾ the Sun is a huge plasma laboratory dynamics of a magnetized plasma how do stars and galaxies produce magnetic fields? basic physical questions: e.g. the (solved) neutrino problem ¾ the Sun’s past: how was the Sun formed and why do we have (9) planets? ¾ the Sun’s future: how will the habitable zone change in the future? ¾ change of paradigms: so far: classification (partly) according to instrumental capabilities visible UV EUV/X-ray particles — — — — photosphere chromosphere corona heliosphere ¬ but: strong coupling between various atmospheric structures ! Challenge: close interaction between modeling and observations: models are needed for interpretation of increasingly complex models ... and the Sun (and its corona) is fascinating 4 fine loops in the corona amateur photography Eclipse drawing, 1860 the solar corona modern coronagraph 2 Outline 5 1. From eclipse drawings to the coronagraph and spectroscopy 2. The solar atmosphere and magnetic field 3. Modern observational techniques 4. Coronal heating and energetics 5. Closed magnetic structures – loops 6. Open magnetic structures – coronal holes and the solar wind 7. Stellar coronae 8. The microstate of the solar corona and the solar wind 9. Space weather and solar–terrestrial relations 10. Structures, waves and turbulence in the heliosphere 6 the solar chromosphere, just before totality 3 First eclipse photo: Berkowski, Königsberg, 28.7.1851 7 daguerrotype Real adventures … 1898 in India 8 4 Fundamental Physics: eclipse 1919 Drawing vs. photography 9 10 18. July 1860 Spain, drawing following the eclipse, Warren de la Rue Desierto, Spain, 40 s exposure time, Angelo Secchi 5 Solar eclipse 18.7.1860: more drawings G. Tempel F. A. Oom von Feilitzsch E.W. Murray F.Galton C. von Wallenberg 11 from: C.A. Ranyard (1879), Mem. Roy. Astron. Soc. 41, 520, Kap. 44. 12 … astrophysics … –– spectroscopy Spectrum screen prism (today: reflective grating) slit normal lamp: solid state (Wolfram-spiral) black body continuum hot solid body („black body radiator“) –– continuous spectrum 6 13 … astrophysics … –– spectroscopy spectrum screen prism slit street lamp at pedestrian crossing: Sodium-lamp hot solid body („black body radiator“) –– continuous spectrum „emission line spectrum“ radiating gas –– emission lines 14 … astrophysics … –– spectroscopy spectrum screen prism slit Na gas in front of continuous radiator hot solid body („black body radiator“) –– continuous spectrum „absorption lines" radiating gas –– emission lines gas absorbing radiation –– absorption lines 7 15 Fingerprints of the elements solar spectrum Fraunhofer lines KH G Ca [Fe] F Hβ E D Fe Na BA C Hα terrestrial spectra of gases H Na Fe Ca Mg 16 A new element: Helium absorption lines in the photosphere Ca II KH [Fe] G Hβ Fe I F E solar eclipse 1868: Janssen and Lockyer identify the up to then unknown „D3 line“ Æ new element: Helium Hα C Na I D 1 2 3 discovered in Earth's atmosphere not before 1895 (!) emission line spectrum of the chromosphere during a solar eclipse 8 17 Emission line spectrum of the chromosphere ¾ slit perpendicular to limb ¾ take spectrum just when moon covers photosphere but NOT chromosphere only a short instance flash spectrum chromosphere: flash spectrum wavelength Ca II K Ca II H "normal" photospheric spectrum with absorption lines 18 Even more new elements ? emission lines in planetary nebula Nebulium ?? … green line: 530.3 nm yellow line: 569.4 nm red line: 637.4 nm ⇔ ⇔ ⇔ Fe13+ Ca14+ Fe9+ first attributed to „Coronium“ Chemistry: ~1900: periodic system of the elements: no room for "normal" new element ! SOHO / Lasco C1 / green line more than 100 „corona lines“ identified in the visible: The strongest ones: atomic physics: systematic investigation of spectra of atoms and ions astrophysics: systematic comparison to results of atomic physics Bengt Edlén (Lund) & Walter Grotrian (Potsdam) (1933 – 1942) Æ lines from highly ionized elements 9 The corona is hot !!! temperature of the corona: – ca 106 K – more than 100 x hotter than surface 19 ~ 1 000 000 K energy required to heat corona: flux ~ 100 W/m² at photosphere no problem! (less than 10-6 of luminosity) Problem: How does a cold body heats a warmer body ?? ~6000 Grad ~ 5777 K (surface) (corona) contradiction to laws of thermodynamics? energy transport e.g. through waves e.g. microwave oven in the solar corona: magnetic field is the "energy agent" magneto-acoustic waves induced currents Prominences: cool structures in the hot corona 20 Prominence observed on Skylab, 1973, He II (304 Å) also visible during eclipses… Grande Daddy prominence 4. June 1946, HAO, Hα amateur shot Franky Dubois 2.9.1999 10 21 Prominences and filaments on the disk: filament. absorption of light from the photosphere full Sun in Hα (Meudon) above the limb: prominence. „grande curtain“ Prominences through spectroscopy outside eclipses 22 1860s observations: – use prime focus of single lens (stray light) – f ≈ 2 m diameter of solar image: ~20 mm procedure to observe above the limb (following Secchi/Schellen, 1872) 1. place slit parallel to limb 2. now slowly increase width of slit up to ¼ to ½ mm 3. take care that the slit "is not touching the disk" ≈ ¼ mm this works only for strong isolated lines e.g. Hα ("slitless spectrograph") today: narrow band filtergraphs: – visible: Lyot filter, Fabry-Perot and Michelson interferometers – VUV: FPI (?) – EUV-X-ray: foils, multi-layer coating 11 Drawings of structures above the limb 23 using spectroscopic methods outside eclipses by Etienne Trouvelot (1827-95) he was one of the most skilled scientific artists of the pre-photographic era The magnetic field structuring the corona 1. 2. 3. 25 magnetic field map of the photosphere (“solar surface”) J Zeeman effect potential field extrapolation (or better) compare to structures in the corona solar eclipse, 30.June 1973, photograph by Serge Koutchmy potential field extrapolation: Altschuler at al. (1977) Solar Physics 51, 345 12 26 The activity cycle of the Sun the Sun in white light maximum 29.5.1996 28.3.2001 Big Bear Solar Observatory minimum 11 year cycle of the Sun: Sunspot number monthly smoothed ¾ sunspot number ¾ magnetic polarity ¾ magnetic activity (since 1843) (since 1908) basic mechanism: D dynamo generating magnetic field Year The corona: maximum vs. minimum Minimum Maximum ¾ ¾ ¾ ¾ ¾ ¾ ¾ ¾ “simple” dipolar structure few active regions (sunspots) prominent coronal holes “helmet streamer” only at equator 18. 3. 1988, Philippines 27 complex magnetic structure many active regions almost no coronal holes “helmet streamer” at all latitudes High Altitude Observatory - NCAR 16. 2. 1980, India 13 28 The X-ray corona in the solar cycle 1993 1995 minimum 100 x brighter 1991 maximum Yohkoh Soft X-ray Telescope (SXT), X-ray emission at about 1 nm The corona outside eclipses: Lyot's coronagraph 29 essentials of the coronagraph: get stray light down to < 10-7 – 10-8 !! occulting surface: light from solar disk reflected out of window Lyot stop: Lyot dot: blocks light diffracted at entrance pupil / edges of objective stops light from secondary reflections within objective Lyot (1932), Z. Astrophys. 5, 73 exit windows elongated tube to protect lens from dust objective – single lens for minimal number of scattering surfaces – easy to take out for frequent cleaning achromatic field lens: images objective objective: images onto Lyot stop corona occulting surface image plane of the Sun Diaphragm Lyot dot Lyot stop /screen image plane of objective / entrance pupil image of corona 14 What is seen during an eclipse? limb ¾ (K) continuum corona 1 intensity relative to disk center 30 - no absorption lines solar disk - polarised: free electron scattering ¾ Fraunhofer corona “normal” sky clear blue sky -6 10 -8 F-corona ¾ Line corona total eclipse L-corona 1 - not polarized: dusk scattering – Zodiac light… K-corona 10 - absorption lines visible 5 distance from disk center in solar radii - emission lines: e.g.: “green coronal line” - emission of atoms / ions: new elements? helium, coronium The Zodiac light 31 15 32 The corona is hot! ¾ intensity scale height: 0.1 R (around 1900) ¾ K-corona: free electron scattering: thermal speed of electrons: most narrow spectral features: 6 nm (Waldmeier 1941) 6 nm @ 500 nm ⇔ 4000 km/s ⇔ 600·103 K (electron temperature) ¾ Emission lines of highly ionised species (Edlén & Grotian 1939-41) green line: Fe XIV (530.3 nm) yellow line: Ca XV (569.4 nm) red line: Fe X J (637.4 nm) these ions exist only at > 106 K ¾ L-corona: line width of emission lines: green line: 0.08 nm 0.08 nm @ 530 nm ⇔ 45 km/s ⇔ 4·106 K (ion temperature) 33 A static heat conduction corona: temperature heating at the “base” of the corona: T TC below base: R < r < rC R rC ¾ equilibrium of heat conduction and heating: "heated aluminum pipe" conductive heat flux: BC: T ( r = R ) << TC Integration in: R → rC height r ⇒ following Unsöld (~1960) typically: f0 = 100 W/m2 16 Why 106 K ? – a coronal thermostat 34 ¾ thermal conductivity: fW ∝ T 5/2 ¾ more heating J T-increase J more efficient heat conduction TC ∝ f 0 2 / 7 J only small net T-increase ¾ same for less heating… changing the heating rate f0 by orders of magnitude results only in a small change of the coronal temperature f0 [ W/m2 ] 17600 370 0.29 TC [106 K] 5.0 3.0 0.5 I “solar like” (Leer 1998) ¾ solar wind ¾ magnetically open regions: 90% of the energy input powers solar wind ¾ more heating J even more losses due to wind J less energy to heat corona 35 Pressure of a static heat conduction corona heat flux in a fully ionized plasma (Spitzer 1956): idea: carry away energy to Sun and infinity by heat conduction (spherical coordinates): T TC R rC r solve hydrostatic equilibrium in outer part: pressure at infinity does not vanish ! 17 36 Can a static corona exist ? Compare pressure of static corona to interstellar medium (ISM) ISM: TISM ≈ 100 K nISM ≈ 10 cm-3 -3 p* ≈ 1000 K cm =Γ The Sun: TC ≈ 106 K nC ≈ 108 cm-3 rC ≈ 2 R¯ p pC pC ≈ 1014 K cm-3 Γ ≈ 10-5 pISM pISM p∞ p*∞ ≈ 109 K cm-3 À p*ISM Corona has to expand! coronal wind thermally driven pressure (gradient) driven rC Rheliosphere r However: Stars with cool outer atmospheres TC ≈ 104 K p∞ < pISM no wind !! Continuous corpuscular radiation: solar wind 37 tails of comets: α two-part structure: – dust tail: controlled through radiation pressure – ion tail (→ polarized light) interaction of ionized particles in cometary tail with solar wind com et Æ solar wind comet Hale-Bopp (Biermann ~1941) angle α: wind speed ~ 1000 km/s solar wind at Earth: Sun velocity: 400 – 800 km/s density: 1 – 10 protons / cm3 temperature: 100 000 K 18 38 Observing the solar atmosphere photosphere transition region chromosphere corona 1024 106 radio: 70 – 900 GHz temperature [K] 1022 EUVlines 105 104 Hα wing “normal” ground-based observations Hα core Ca II K1 K2 (600-1600 Å) K3 1020 1018 He IR particle density [m-3] X-rays 1018 EUV-Continua (>912 Å) 100 1000 height above τ5000=1 [km] 10000 From the upper chromosphere to the hot corona 39 full Sun scans of CDS / SOHO 19 Summary / lessons learnt 40 ~ 1850: first systematic "modern" eclipse observations ~ 1870: introduction of spectroscopy into coronal physics ~ 1930: invention of coronagraph ~ 1940: coronal lines are from highly ionized species the corona ~106 K ~ 1970: first advanced X-ray observations ¾ the corona is magnetically structured ¾ the appearance of the corona changes with solar activity cycle ¾ 106 K is "quite natural": heat conduction acts as thermostat ¾ a static hot corona cannot exist expansion ¾ appearance the solar atmosphere changes dramatically with temperature From eclipse drawings to the coronagraph and spectroscopy 20