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The Low Solar Corona and the Stars N The low solar corona and the stars n Hardi Peter , Wendy, Carlos & John Ker Kiepenheuer-Institut für Sonnenphysik 11.8.1999 Freiburg Sun Solar eclipse, ¾ energetics ¾ the transition region ¾ heating the corona α Coronae Borealis ¾ stellar coronae “There is more to the solar corona Why the corona? than physics and mathematics.” ¾ astrophysical interest in general Jeff Linsky heating of the corona is is one of the 10 most interesting questions in astronomy! ¾ solar-terrestrial relations: strongest variability in UV: <160 nm from corona/TR! coronal mass ejections (CME): - satellite disruptions - safety of astronauts and air travel geomagnetic disturbances -GPS - radio transmission - Oil pipelines - power supply ¾ other astrophysical objects accretion disks of young stars: stellar and planetary evolution … 1 KIS Drawing vs. photography Hardi Peter 18. July 1860 Spain, Desierto, Spain, Drawing after eclipse, 40 s exposure, Warren de la Rue Angelo Secchi from: Secchi / Schellen: Die Sonne, 1872 KIS CMEs: now and then …. Hardi Peter SOHO / Lasco C3 20.4.1998 (with Mars and Saturn...) compare: drawing of G. Tempel of the corona during an eclipse 18.7.1860 2 KIS The “global” corona: minimum of solar activity Hardi Peter coronal hole (magnetically open) “Quiet Sun” prominences “helmet streamer” “polar plumes” Solar eclipse, 3. Nov. 1994, Putre, Chile, High Altitude Observatory / NCAR KIS The corona is structured by the magnetic field Hardi Peter 1. magnetic fields in the photosphere (“solar surface”) J Zeeman-effect 2. potential field extrapolation (or better) 3. compare to structures in the corona Î “hairy ball” coronal holes: magnetically open quiet Sun: magnetically closed Solar eclipse, 30.Juni 1973, Serge Koutchmy Potential field extrapolation: Altschuler at al. (1977) Solar Physics 51, 345 3 KIS The cycle of activity of the Sun Hardi Peter MinimumSun in white light Maximum y r o t a v r e s b O r a l o S r a e B g i 29.5.1996 28.3.2001 B sunspot number 11-years cycle of the Sun: monthly smoothed ¾ sunspot number (since 1843) ¾ magnetic polarity (since 1908) ¾ magnetic activity driving mechanism: time [years] D magnetic field generating dynamo KIS The solar corona: minimum vs. maximum Hardi Peter Minimum Maximum ¾ “simple” dipolar structure ¾ complex magnetic structure ¾ few active regions (sunspots) ¾ many active regions ¾ prominent coronal holes ¾ almost no coronal holes ¾ “helmet streamer” only at equator ¾ “helmet streamer” at all latitudes 18. 3. 1988, Philippines 16. 2. 1980, India High Altitude Observatory - NCAR 4 KIS The solar corona: minimum vs. maximum Hardi Peter Minimum Maximum KIS The solar X-ray corona during the cycle Hardi Peter 1993 1995 minimum 100 x brighter ! 1991 maximum Yohkoh Soft X-ray Telescope (SXT), X-ray Emission at ≈1 nm, ≈2· 106 K 5 Chromospheric network: magnetic structure and flows and structure magnetic network: Chromospheric solar Y [arcsec] Dutch Open Telescope, La Palma Photospheric magneto-convectionGranulation / 350 400 450 500 550 600 ≈ which are brighter than their surrounding their than brighter are which tubes, flux magnetic small points: bright G-band (Ø granulation in G-Band observation 12. Sept. 1999 (Sütterlin & Rutten) 38 000 km x 25 000 km, km, 000 km 25 000 x 38 0 50 100 150 200 ...well,the ultimate energy source is the fusionin the center ofthe Sun... ≈ 1000 km) km) 1000 solar X [arcsec] X solar ≈ 430 nm 430 ≈ 27 min ≈ network boundaries (yellow) ¾ ¾ supergranulation 2400 km x 1400 km, km, 1400 x km 2400 SOHO / MDI, 23.2.1996 / MDI, SOHO magnetic field is is field magnetic define flows transported to the transported boundaries supergranulation boundaries magnetogram (b/w image) (radiation-MHD) granulation photospheric flux tube embedded in of 2D-simulation a flows (arrows) ≈ 18 min 18 Hardi Peter Hardi Peter KIS KIS Steiner et al. (1997) ApJ 495, 468 6 KIS Transition region: emission patterns Hardi Peter see also transition region above chromospheric network Feldman et al. (2003), ESA SP-1274: (!) network built up by bright structures "Images of the Solar Upper Atmosphere from SUMER on SOHO". (?) loops across network-boundaries (?) low loops across cells SUMER / SOHO C III (97.7 nm) ≈80 000 K 28.1.1996 Peter (2001) A&A 374,1108 KIS Magnetic loops in the low corona Hardi Peter emission lines of Fe IX / X (17.1 nm) 6 ~ ≈ 10 K R ~ 0.1 ~ 0.1 9. November 2000 ~ 70 000 km 000 ~ 70 be careful: light ≠ magnetic field Transition Region And Coronal Explorer (TRACE), NASA 7 considerations on the energetics FSW = 0 Fq = 0.1 FH Frad = Fq = FH FH KIS Hardi Peter KIS An “old” 1D temperature structure Hardi Peter following Vernazza et al. (1981) ApJS 45, 635 T, N T, 8 KIS Energy budget in the quiet Corona Hardi Peter magnetically closed magnetically open FSW = 0 FSW = 0.9 FH Fq = 0.1 FH Fq = 0.1 FH Frad = Fq = FH Frad = Fq = 0.1 FH F FH H radiation ≈ 100 % of energy input radiation ≈ 10 % of energy input following Holzer et al. (1997) assume the same energy input into open and closed regions: almost ALL emission we see on the disk outside coronal holes originates from magnetically closed structures (loops) ! KIS Temperature in a static corona Hardi Peter Heating at the coronal base FH T 2 2 Fq FH = 4π rH fH = 4π R f0 TC 2 typical: f0 = 100 W/m inner part: r height r R < r < rH R H ¾ Equilibrium of “heated aluminum pipe” heating and heat conduction: 2 4π r fq = Fq = − FH dT conductive flux: f = −κ T 5/2 q 0 d r boundary condition: 2/7 T (r = R ) << T ⎛−7 f r R ⎞ C T = ⎜ 0 H ⎟ ⇒ C ⎜ 2 κ r /R ⎟ Integration: R → rH ⎝ 0 H ⎠ 9 KIS The corona: a thermostat Hardi Peter 1. thermal conductivity: 5/2 ¾ increased heating: J T-Anstieg fW ∝ T J effective heat conduction 2/7 TC ∝ f0 J only small T-increase ¾ similar for decreased heating 2. solar wind ¾ magnetically open regions: 90% of the energy into acceleration ¾ more heating J even higher losses due to acceteration J less energy for heating changing the hating rate f 2 6 0 f0 [ W/m ] TC [10 K] by some orders of magnitude leads to small changes 17600 5.0 of the peak temperature 150 1.0 I like Sun of the corona 0.29 0.5 following Leer (1998) KIS Where does this apply? Hardi Peter FH T Fq TC m 0 k 00 00 R rH height r “old” 1D 7 picture “heated aluminum pipe” (Unsöld ~1960) 10 0 0 00 km 5000 km large hot (106K) coronal loops small cool (105K) coronal loops 10 KIS The coronal base pressure Hardi Peter ¾ dump heat in the corona FH FH log p radiation is not very T efficient in the corona (106K) T Fq ¾ heat conduction Fq transports energy down log p ¾ energy is radiated in the low transition region Frad and upper chromosphere height r radiation depends on increase the heating rate: particle density more has to be radiated higher base pressure pressure: p ~ Frad transition region moves to lower height ! pcorona ~ FH The “details” might change (e.g. spatial distribution of heating) but the basic concept remains valid! investigating the transition region KIS Hardi Peter 11 KIS An “old” 1D temperature structure Hardi Peter UV continua (C I, S I, Si I) following Vernazza et al. (1981) ApJS 45, 635 T, N T, EUV emission lines from transition region and low corona and neutral lines & continua from the chromosphere KIS Solar and Heliospheric Observatory / SUMER Hardi Peter EUV-Spectrograph SUMER Solar Ultraviolet Measurements of Emitted Radiation spatial resolution: 2” (1” pixel) (1500 km) spectral resolution: λ/∆λ ≈ 30 000 wavelength range: 50 – 155 nm covering temperatures on the Sun: 5000 – 106 K ¾ dynamics and structure of the transition region from the chromosphere to the corona ¾ accuracy for Doppler shifts: ~ 2 km/s 12 st KIS SUMER: spectral range (1 order) Hardi Peter Wilhelm (2000) KIS Full spectral frame and spectral windows Hardi Peter C II O VI O VI C II O VI O I full frame: 1024 spectral pixels ≈ 44 Å (1st order) spectral window: st often 50 spectr. pxl ≈ 2 Å (1 order) 10 s (or 25, 512, …) exposure time Problem: sometimes windows not wide enough (telemetry…) Images by raster procedure 13 KIS Doppler shifts in the transition region Hardi Peter 10 ≈ 105 K ≈ 6.5⋅105 K 5 0 9 4 0 , 6 1 5 J p A ) 9 5 9 9 Doppler shift [ km/s ] 1 ( r e t e 10 P SUMER quiet Sun Doppler shifts (along equator) coronal holes ¾ low temperatures: T < 3⋅105 K: redshifts ¾ “coronal” temperatures: ¾ high temperatures: T > 4⋅105 K: blueshifts T > 6⋅105 K: blueshifts ¾ Doppler-shifts: flows ??? ¾ coronal hole outflows: (sound-) waves ??? base of solar wind KIS TR Doppler shift as a function of temperature Hardi Peter ¾ basically shows quiet Sun network line shifts ¾ similar for active region line shifts (Teriaca et al. 1999, 8 A&A 349, 636) 4 1 1 , 2 2 5 J p A ) 9 9 9 1 ( e g d u J & r e t e P SUMER mean quiet Sun Doppler shifts at disk center 14 KIS Scatter of line shifts Hardi Peter 0 9 4 , 6 1 5 J p A ) 9 9 9 1 ( r e t e P SUMER Questions to answer... (1) How can the persistent net line shift be produced at all ? (2) How to get redshifts below 5⋅105 K, but blueshifts above ? (3) What causes the large scatter of line shifts ? Understanding line shifts I : single structure KIS a Hardi Peter asymmetric heating: flows Doppler shift as a function of temperature corona asymmetric heating line formation temperature T [K] shock (red) 106 K → 4·105 K higher 105 K density 104 K Doppler shift [km/s] [km/s] shift Doppler photosphere ← (blue) line formation temperature log (T [K]) more or less like that, i.e.
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