Introduction to Space Weather The Sun: The Structure Sep. 10, 2009 CSI 662 / PHYS 660 Fall, 2009 Jie Zhang Copyright ©

Roadmap Part 1: The Sun Part 1: The Sun Part 2: The Heliosphere The Structure of the Sun: Interior and Atmosphere Solar Magnetism: Sunspots, Active Regions, Solar Cycle, and Solar Dynamo Solar Corona: Coronal Heating, Magnetic Effects, and Activities Major Solar Activities: Flares and Coronal Mass Ejections Part 1: The Sun Part 2: The Heliosphere Part 3: The Magnetosphere Part 4: The Ionsophere Part 4: Space Weather Effects

The Structure of the Sun: Interior and Atmosphere CSI 662 / PHYS 660 September 10 2009 The Structure of the Sun: Interior and Atmosphere References: Kallenrode: Chap. 6 NASA/MSFC Solar Physics at http://solarscience.msfc.nasa.gov/

The Sun: Basic Facts Distance 1 AU = 1.5 × 108 km Radius: Rs = 696, 000 km Mass: Ms = 1.99 × 1033 kg Density: ρs = 1.91 g/cm3 Luminosity: Ls = 3.86 × 1023 kW Solar Constant: LE = 1380 W/m2 Effective Temperature: Ts = 5780 K Sun from Unaided Eyes Given solar constant, calculate the Sun’s surface effective temperature using Stefan-Boltzmann’s Law (Eq. 6.2) ? F = σ T4 and σ = 5.67 J m-2 s-1 K-1

Stratified Structure of the Sun Gravitational stratification: caused by the spherically symmetric gravitational force, which always points toward the center of the gas ball Density varies by 10 order of magnitude Temperature varies by 3 order of magnitude

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

Core Core Depth: 0 – 0.3 Rs Temperature: 15 MK  7 MK Density: 150 g/cm3

Core Core 90% of H, and 10% of He in particle numbers Energy generation: through nuclear fusion process called PP chain (Proton-Proton chain) 41H  4He + 2e+ + 2ν + 26.73 Mev or, the chain reaction formula: 1H(p,e+υe)2D(p,γ)3He(3He,2pγ)4He + 26.2 Mev Mean free path of particles Photons: a few cm Neutrinos: 7000 AU Thermal Equilibrium maintains the stability of the core

Radiative Zone Radiative Zone depth: 0.30 – 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 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 states. As a result, energy transfer through radiation or photon leaking is less efficient, and temperature gradient increases Convection occurs: when the temperature gradient becomes so large, larger than the adiabatic gradient, the buoyancy force starts to drive the convection

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

Convection Zone Evidence of convection seen as granules in the photosphere

Atmosphere Layered, but complex and dynamic

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 Surface mass density: ~ 10-8 g/cm3 As a comparison, Earth atmosphere density ~ 10-3 g/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 the 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 Corona was revealed by eclipse observations Extended atmosphere of the Sun Thickness: ~ Rs and extended further into the 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) Corona was revealed by eclipse observations

Interior versus Atmosphere Standard solar model explains well the structure of the interior, up to the photosphere Based on the assumption of hydrostatic equilibrium Based on knowledge of radiation transfer, thermal statistics, atomic physics and nuclear physics However, the standard solar model can not explain the existence of chromosphere and corona Due to the existence of magnetic field Magnetohydrostatics and/or magnetohydrodynamics (MHD) should be used as the model, instead of the hydrostatic assumption

Solar Spectrum Continuum black-body radiation in visible light and Infrarad The effective temperature is 5780 K (Wien’s Law Eq. 6.3) Excessive continuum and line emission in EUV and X-ray from corona and transition region

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

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

Features in Photosphere Sunspot: umbra/penumbra Granules, Supergranule

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

Photosphere: sunspot Sunspot is darker because it is cooler Big Sunspot is about half of the normal brightness. F = σT4 ,or T ~ B ¼ (Stefan-Boltzman Law Eq. 6.2) 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 In 1930s, it was realized that sunspot is a magnetic feature Magnetic field has pressure, describe by PB = B2/8π (in CGS unit) (see Eq 3.63 in MKS unit) where B: magnetic field strength (Gauss) Gas pressure: Pg = N K T N: particle number density K: Boltzmann’s constant T: gas temperature

Photosphere: sunspot Sunspot’ internal pressure is the gas pressure combined with the magnetic pressure, in balance with the external gas pressure Pg_in + PB_in = Pg_ext Given a sunspot with a magnetic field of 3000 Gauss, (1) calculate its magnetic pressure ? (2) Calculate the typical gas pressure ? (3) If the plasma density is the same inside and outside the sunspot, what is the temperature of the sunspot? (4) How much darker is the sunspot?

Photosphere: 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: 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” Small magnetic elements outside the sunspot tend to concentrate along the supergranule boundaries

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 Dense clouds of chromospheric material suspended in the corona by the tension force 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 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 12/13 1996 composite Image 9256 raster image, Each with 3 s exposure Collected in eight alternating horizontal scan across the Sun

Corona Coronal holes 2. Active regions 3. Quiet sun regions Best seen in X-rays and EUV 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 above the photospheric sunspots They are formed of closed magnetic loops

Corona: Active Region Active region loops trace magnetic field lines that are selectively heated Active Region Sunspots 3-D coronal magnetic model 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

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