Structure of the Sun:Interior,

and Atmosphere

CSI 662 / ASTR 769 Lect. 02, January 30 Spring 2007

References: •Tascione 2.0-2.2, P15-P18•Aschwanden 1.1-1.8, P1-P30•Gombosi 11.1 – 11.6, P213-P230 •NASA/MSFC Solar Physics at http://solarscience.msfc.nasa.gov/

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/cm3

• Energy generation: through nuclear fusion process41H 4He + 2e+ + 2ν + 26.73 Mev

Inside the Sun: Radiative Zone(2) Radiative Zone

•depth: 0.25 – 0.70 Rs•Temperature: 7 MK to 2 MK•Density: 20 g/cm3 to 0.2 g/cm3 •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/cm3 – 10-7 g/cm3

• 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 11.2.2, 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: 1019 to 1016 particle/cm3

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 20000 K at the top• Density: 1016 to 1010 particle/cm3

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: 1010 to 109 particle/cm3

• 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: 109 to 107 particle/cm3

• 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 = σT4 ,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 byPB = B2/8π 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: PlagePlage (beach in French)•Bright patches surrounding sunspots that are best seen in Hα•Associated with concentration of magnetic fields

Chromosphere: Filament/ProminenceFilament/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/ProminenceFilament/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/ProminenceProminence Eruption•e.g., so called granddady prominence in 1945

Transition RegionImage: 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

Coronal: Different Structures1. Coronal holes 2. Active regions 3. Quiet sun regions

X-ray Corona > 2 MKContinuum

05/08/92

YOHKOHSXT

Corona: Coronal HolesCoronal holes •Regions where the corona is dark•A coronal hole is dark

•plasma density is low•open magnetic field line

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

Corona: Active RegionActive 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 RegionQuiet 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, PG=nKT, and PB=B2/8π

β = 8πnKT/ B2

If β << 1, magnetic pressure dominates, B control plasma flowIf 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