1. CSI 769/ASTR 769 Topics in Space Weather Fall 2005 Lecture 02 Sep. 6, 2005 Structure of the Sun and its Atmosphere Reading: Gombosi, “Physics of Space Environment”, Chap.11, P211-235 Aschwanden, “Physics of the Solar Corona” Chap. 1, P1-36 Chap. 2, P37-66 Chap. 3, P67-116

2. Stratified Structure of the Sun Inside the Sun Atmosphere (3) Convection Zone (2) Radiative Zone (1) Core (4) Corona (3) Transition Region between Corona and Chromosphere (2) Chromosphere (1) Photosphere

3. Inside the Sun (1) Core depth: 0 – 0.25 Rs Temperature: 20 Million Kelvin Density: 150 g/cm 3 Energy generation through nuclear fusion process 4 1 H  4 He + 2e + + 2ν + 26.73 Mev (2) Radiative Zone depth: 0.25 – 0.70 Rs Temperature: 7 MK to 2 MK Density: 20 g/cm 3 to 0.2 g/cm 3 Energy transport region through radiation transfer, or photon diffusion; conduction is negligible; no convection

4. Inside the Sun (cont.) (3) Convection Zone Depth: 0.70 – 1.00 Rs Temperature: ~ 2 MK to 0.06 MK Density: 0.2 g/cm 3 – 10 -7 g/cm 3 Opacity increase: at 2 MK, opacity increases as heavy ions (e.g., C, N, O, Ca, Fe) starts to hold electrons from fully ionized state. As a result, energy transfer through radiation is less efficient, and temperature gradient increases Convection occurs: when the temperature gradient becomes sufficient large, and larger than that in the adiabatic condition (dT/dr) rad ~ -K T Ls (E11.18, Gombosi) (dT/dr) ad ~ M(r)/r 2 (E11.19, Gombosi)

5. Inside the Sun (cont.) Also see Figure 11.3 in Gombosi (P. 218)

6. Solar Atmosphere: hydrostatic model Fig. 1.19, Aschwanden (P. 24) Also see Fig. 11.11, Gombosi (P.228)

7. Photosphere Surface of the Sun seen in visible wavelength (4000 – 7000 Å) Thickness: a few hundred kilometers Temperature: ~5700 K Density: 10 19 to 10 16 particle/cm 3

8. Chromosphere A layer above the photosphere, transparent to broadband visible light, but can be seen in spectral lines, e.g., Hα line at 6563 Å Thickness: 2000 km in hydrostatic model ~5000 km in reality due to irregularity Temperature: 6000 K plateau, up to 20000 K Density: 10 16 to 10 10 particle/cm 3

9. Transition Region A very thin and irregular interface layer separating Chromosphere and the much hotter corona Thickness: about 50 km only assuming homogeneous Temperature: 20,000 K to 1000,000 K (or 1 MK) Density: 10 10 to 10 9 particle/cm 3 Can’t be seen in visible light or Hα line, but in UV light from ions, e.g, C IV (at 0.1 MK), O IV, Si IV

10. Corona Extended outer atmosphere of the Sun Thickness: Rs and extended into heliosphere Temperature: 1 MK to 2 MK Density: 10 9 to 10 7 particle/cm 3 Difficult to be seen in visible light, nor in UV from light ions, (C,O) Seen in EUV from heavy ions, e.g., Fe X Seen in X-rays

11. Solar Irradiance Spectrum The effective temperature of the Sun is 5770 K Black-body in visible light and longer wavelength Line-blanketing in UV light Excessive emission in EUV and X-ray from TR and corona Also see Figure 11.10, Gombosi (P. 227)

12. Solar Spectrum versus Solar Structure V (visible, 4000 Å – 7000 Å) and IR (Infrared, 7000 Å – 10000 Å From Photosphere, the largest component of solar irradiance UV (Ultraviolet, 1200 Å – 4000 Å) Mainly from chromosphere EUV (300 Å – 1200 Å) Mainly from transition region XUV (100 Å – 300 Å) and Soft X-ray (< 100 Å) Mainly from Corona

13. Solar Irradiance Spectrum Figure 1.25, Aschwanden (P.34)

14. Spectrum lines: absorption and emission Absorption lines in photosphere and chromosphere Emission lines in Transition region and corona (4000 Å – 7000 Å) (300 Å – 600 Å)

15. Spectrum Lines (cont.)

16. Absorption versus Emission (cont.)

17. Features in Photosphere Sunspot: umbra/penumbra Faculae Granule Supergranule Magnetogram

18. Photosphere: Sunspot Observed in continuum visible light as Galileo did

19. Photosphere: Sunspot (cont.) Sunspots show two main structures: 1.Umbra: a central dark region, 2.Penumbra: surrounding region of a less darker zone SOHO/MDI 2004/10/24

20. Photosphere: Sunspot (cont.) Been noticed in ancient time Since 1700, systematic record of sunspot number Sunspot was found to be a magnetic feature in 1930 Big Sunspot is about half the normal brightness. B = σT 4,Or T ~ B ¼ ( Stefan-Boltzman Law) Tspot/Tsun=(Bspot/Bsun) 1/4 =(0.5) 1/4 = 0.84 Tsun = 5700 K Tspot = 5700 * 0.84 = 4788 K Sunspot is about 1000 K cooler than surrounding

21. Photosphere: Sunspot Sunspot is in pressure balance because of internal magnetic pressure Pe = Pi + Pmag Pe: external thermal pressure Pe = N K Te N: particle density K: Boltzmann;s constant Te: external temperature Pi: internal thermal pressure Pi= N K Te Pmag: magnetic pressure inside sunspot Pmag = B 2 /8π B: magnetic field strength in the sunspot

22. Photosphere: Faculae Faculae bright lanes near the sunspot make the visible Sun brighter, e.g., whole disk slightly brighter at the sunspot maximum than that at the minimum Associated with small concentration of magnetic bundles between granules

23. Photosphere: Granules Granules Small (about 1000 km across) cellular features Cover the entire Sun except for areas of sunspots They are the tops of convection cells where hot fluid (bubble) rises up from the interior They cools and then sinks inward along the dark lane Individual granules last for only about 20 minutes Flow speed can reach 7 km/s

24. Photosphere: Granules (cntl.) Granules Exp. a movie of granules

25. Photosphere: Supergranules (ctnl.) Supergranules much larger version of granules (about 35,000 km across) Cover the entire Sun They lasts for a day to two They have flow speed of about 0.5 km/s Best seen in the measurement of the “Doppler shift”

26. Chromosphere Plage Filament/prominence Chromospheric network

27. Chromosphere: Plage Plage (beach in French) Bright patches surrounding sunspots that are best seen in Hα Associated with concentration of magnetic fields

28. Chromosphere: Filament/Prominence (cont.) Filament/Prominence Dense clouds of chromospheric material suspended in the corona by loops of magnetic field Filaments and prominences are the same thing Prominences, as bright emission feature, are seen projecting out above the limb of the Sun, Filaments as dark absorption feature, are seen projecting on the disk of the Sun,

29. Chromosphere: Filament/Prominence (cont.) Filament/Prominence They can be as small as several thousand km They can be as large as one Rs long, or 700,000 km They can remain in a quiet or quiescent state for days or weeks They can also erupt and rise off of the Sun over the course of a few minutes or hours

30. Chromosphere: Filament/Prominence (cont.) Filament/Prominence Exp. Movie of eruption, so called granddady prominence

31. Chromosphere: Network Chromospheric Network web-like pattern mostly seen in red line of Hα (at 6563 Å) and UV line of Ca II K (at 3934 Å) The network outlines the supergranule cells and is due to the presence of bundles of magnetic field lines that are concentrated there by the fluid motions in the supergranules

32. Chromosphere: complex structure (cont.) Magnetized, Highly inhomogeneous, highly dynamic Figure 1.17 Aschwanden (P.22)

33. Transition Region Image: S VI (933 Å) at 200,000 K (SOHO/SUMER) May 12/13 1996 composite Image 9256 raster image, Each with 3 s exposure Collected in eight alternating horizontal scan across the Sun

34. Transition Region (cont.) Image: C IV (1548 Å) at 100,000 K (SOHO/SUMER) Outline the top of chromosphere

35. Corona Large scale coronal structures Coronal loops Physical properties in corona

36. Coronal: Large Scale Structure 1. Coronal holes 2. Active regions 3. Quiet sun regions X-ray Corona > 2 MK Continuum 05/08/92 YOHKOH SXT

37. Corona: Coronal Holes Coronal holes Regions where the corona is dark A coronal hole is dark because plasma density is low there

38. Corona: Coronal Holes(cont.) Coronal holes Coronal holes are associated with “open” magnetic field lines Particles easily flow away along the “open” field lines “open” field lines are caused by a large surface region with unipolar magnetic field, which are often found in the polar regions. Also see Figure 1.14, Aschwanden (P. 18)

39. Corona: Active Region Coronal active region: Consists of bright loops with enhanced plasma density and temperature They are associated with photospheric sunspot

40. Corona: Active Region (cont.) Active region loops trace magnetic field lines that are selectively heated (a)Active Region (b) Sunspots (c)3-D coronal magnetic model (d) side-view of the model

41. Corona: Active Region (cont.) Coronal loop Evolution: from TRACE 171 Å, Fe IX/Fe X, 1.0 MK

42. Corona: Quiet Sun Region Quiet Sun regions Generally, regions outside coronal holes and active regions Properties, such as density and temperature, in-between the coronal holes and active regions Many transient bright points associated with small magnetic dipoles. From SOHO/EIT 195 Å band Fe XII, 1.5 MK Nov. 10, 1997

43. Coronal Properties Plasma state: Elements H/He are fully ionized, and heavy elements are highly ionized (Fe X, Fe XIV). Magnetized plasma

44. Coronal Property: Low β For a magnetized plasma, plasma β is defined as β Ξ gas pressure / magnetic pressure In CGS unit, P th =nKT, and P B =B 2 /8π β = 8πnKT/ B 2 If β >> 1, gas pressure dominates, flows control B If β << 1, magnetic pressure dominates, B control plasma flow Therefore, the coronal structure is determined by magnetic field distribution.

45. Plasma β in solar atmopshere Figure 1.22, Aschwanden (P.29)

46. Corona Property: Loop Thermal conduction is very efficient along the magnetic field line Isothermal plasma along a loop Thermal conduction is inhibited across magnetic field lines Charged particles are tied to magnetic field lines in gyro-motion, preventing particle diffusion across magnetic field lines. Multi-temperature loops are mixed Differential emission measure DEM: dEM/dT

47. Coronal Property: loop (cont.) Hydrostatic Model of Corona (Chap. 3, Aschwanden) dP/ds – ρ g =0 Momentum Equation E H – E R – E C = 0 Energy equation E H : Heating rate E R : Radiative loss rate E C : conductive loss rate

48. Bremsstrahlung Emission Bremsstrahlung emission (in German meaning "braking radiation") the radiation is produced as the electrons are deflected in the Coulomb field of the ions. Bremsstrahlung emission

49. End

50. Limb darkening effect in Photosphere Central region looks brighter than that close the limbs

51. Limb darkening effect in Photosphere (ctnl.) Consider the emission from the same line-of-sight depth but from different positions of the Sun The large the angle θ, the short the penetrating depth along the radial direction we see deeper at the center, and see shallower close the limb. Because the deeper part is hotter and thus looks brighter, while close to the limb looks darker.

52. Scale Height of Atmosphere Density drops exponentially in atmosphere scale height is the distance that density drops by a factor of e (or 2.718, the natural log): N / N 0 = e (-h/H) where N is the particle density, N 0 density at surface, h the height above the surface, and H the scale height Scale Height: H = kT/µg where K Boltzmann constant (1.38 X 10 -16 erg/K), T atmosphere temperature, g the solar gravity at surface (g=GMs/Rs 2 ), and µ the mean particle mass (~1.0 X 10 -24 g in the Chromosphere and Corona)