Lower Solar Atmosphere: Photosphere and Chromosphere

CSI 769-001/PHYS 590-001 Solar Atmosphere Fall 2004 Lecture 05 Sep. 29, 2004 Lower Solar Atmosphere: Photosphere and Chromosphere http://science.nasa.gov/ssl/pad/solar/

Layered Structure of the Sun (4) Corona (3) Transition Region between Corona and Chromosphere (2) Chromosphere (1) Photosphere Visible Solar Atmosphere surface (3) Convection Zone (2) Radiative Zone (1) Core Inside the Sun

Inside the Sun • Core • depth: 0 – 0.25 Rs • Temperature: 20 Million Kelvin • Density: 150 g/cm3 • Energy creation region of the Sun, • through nuclear fusion process • that converts H  He and releases the nuclear energy (mass of 4H < 1He, and E=MC2 • (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 purely through radiation, or photon diffusion; no convection and conduction is negligible

Inside the Sun • (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 sufficient large, and larger than that in the adiabatic condition; unstable as boiling water in a pan • Overshooting: the rising bubble overshoots at the top of the convection zone because of kinematic momentum • Tachocline: the Sun’s magnetic field is possibly generated in the thin interface, called Tachocline, between convection zone and radiative zone

Inside the Sun: Temperature Dist.

Inside the Sun: Density Dist.

Granual

Sunspot (ctnl.) • A phenomenological description of something we see • Been noticed in ancient time • Since 1700, systematic record of sunspot number • Sunspot was found to be a magnetic feature in 1930 • Sunspot is half the brightness. • L ~ Te4 ,Or Te ~ L 1/4 • Tspot/Tsun=(Lspot/Lsun)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

Sunspot: balancing (ctnl.) • Pext = Pint + Pmag • Pext: external thermal pressure • Pext = Next * K *Text • N: particle density • K: Boltzmann;s constant • T: temperature • Pint: internal thermal pressure • Pint= Nint * K * Tint • Pmag: magnetic pressure inside sunspot • Pmag = B2/8π • B: magnetic field strength in the sunspot • Pext > P int, because Text > T int, but B helps the balance

Sunspot: structure (ctnl.) • Sunspots show two main structures: • Umbra: a central dark region, • Penumbra: surrounding region of a less darker and filamentary zone SOHO/MDI 2004/10/24

Mangetic Field of Sunspot Observed by a magnetogram

Magnetic Field: Measurement (ctnl.) • Zeeman effect • The splitting of a spectral line because the presence of magnetic field. • The electrons moving along different magnetic direction may have different energy

Magnetic Field: Measurement (ctnl.) • A magnetograph measures the wavelength shift of certain spectral lines; the shift is proportional to the wavelength as • Δλ = 4.7 x 10-13 g λ2 H • λ: wavelength • g: Lande factor • H: magnetic field strength • Reference: Zirin (1966), P. 368

Lower Solar Atmosphere: Photosphere and Chromosphere