Structure of the Sun: Interior, and Atmosphere CSI 662 / ASTR 769 Lect. 02, January 30 Spring 2007 References: Tascione , P15-P18 Aschwanden , P1-P30 Gombosi 11.1 – 11.6, P213-P230 NASA/MSFC Solar Physics at

Maxwell’s Equations References: Tascione 1-1, P1--P2 –Integration format Aschwanden 5.1.1, P176 –Differentiation format

Stratified Structure of the Sun Gravitational stratification: caused by the gravitational force, which always points toward the center of the gas ball

Stratified Structure of the Sun Interior Atmosphere (3) Convection Zone (2) Radiative Zone (1) Core (4) Corona (3) Transition Region (2) Chromosphere (1) Photosphere Surface

Inside the Sun: Core (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 Mev

Inside the Sun: Radiative Zone (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

Inside the Sun: Convection Zone (3) Convection Zone Depth: 0.70 – 1.00 Rs Temperature: ~ 2 MK to 0.06 MK Density: 0.2 g/cm 3 – 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 so large that it drives convection by buoyancy force Reference: Gombosi , P216-P217

Inside the Sun: Convection Zone Evidence of convection seen as granules in the photosphere

Inside the Sun: Convection Zone Numerical calculation shows that temperature decreases rapidly in the convection zone

Atmosphere Layered, complex, and dynamic atmosphere

Atmosphere Temperature and Density Profiles

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

Chromosphere A layer above the photosphere, transparent to broadband visible light, but can be seen in spectral lines Hα line at 6563 Å (Hydrogen spectral line between level 3 to level 2, first line in Balmer Series) Thickness: 2000 km in hydrostatic model ~5000 km in reality due to irregularity Temperature: 6000 K plateau, up to K at the top Density: to particle/cm 3

Transition Region A very thin and irregular interface layer separating Chromosphere and the much hotter corona Thickness: about 50 km assuming homogeneous Temperature: 20,000 K to 1000,000 K (or 1 MK) Density: 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

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 (171 Å) Seen in X-rays (1 – 8 Å) Eclipse reveals the corona

Solar Irradiance Spectrum The effective temperature of the Sun is 5770 K Continuum black-body radiation in visible light and longer wavelength Excessive continuum and line emission in EUV and X-ray from corona and transition region

Spectral Lines Lyman series, Lα = 912 Å Balmer series, Hα = 6563 Å Bohr’s atomic model Emission: electron de- excitation from high to low orbit Absorption: electron excitation from low to high orbit

Absorption and Emission Lines Absorption lines in photosphere and chromosphere Emission lines in Transition region and corona (4000 Å – 7000 Å) (300 Å – 600 Å)

Features in Photosphere Sunspot: umbra/penumbra Magnetogram Granules, Supergranule

Observed in visible light as Galileo did Photosphere: sunspot 1.Umbra: a central dark region, 2.Penumbra: surrounding region of a less darker zone

Sunspot is darker because it is cooler Big Sunspot is about half of the normal brightness. B = σT 4,or T ~ B ¼ ( Stefan-Boltzman Law for blackbody) 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 photosphere Photosphere: sunspot

Magnetic Pressure In 1930s, it was realized that sunspot is a magnetic feature Magnetic field has pressure, describe by P B = B 2 /8π where B: magnetic field strength (Gauss) Pmag is equivalent to magnetic energy density Gas pressure: Pg = N K T N: particle density K: Boltzmann;s constant T: gas temperature Sunspot’ internal pressure is the gas pressure combined with the magnetic pressure, in balance with the external gas pressure Lower gas pressure means lower temperature

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 Photosphere: Granules

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” Photosphere: Supergranules

Chromosphere Mainly seen in Hα line Plage Filament/Prominence

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

Chromosphere: Filament/Prominence 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,

Chromosphere: Filament/Prominence 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

Chromosphere: Filament/Prominence Prominence Eruption e.g., so called granddady prominence in 1945

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

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

Corona: Coronal Holes Coronal holes Regions where the corona is dark A coronal hole is dark plasma density is low open magnetic field line

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

Corona: Active Region 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

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

Corona: Magnetized Plasma Plasma state: Elements H and He are fully ionized Heavy elements are partially ionized (Fe). Magnetized plasma: magnetic pressure dominates the gas thermal pressure, a low β plasma

Plasma β is defined as β Ξ gas pressure / magnetic pressure In CGS unit, P G =nKT, and P B =B 2 /8π β = 8πnKT/ B 2 If β << 1, magnetic pressure dominates, B control plasma flow If β >> 1, gas pressure dominates, flows control B Therefore, the coronal structure is determined by magnetic field distribution. Corona: Magnetized Plasma

Plasma β in solar atmopshere Figure 1.22, Aschwanden (P.29) Low β in corona High β in photosphere and solar wind

The End