CSI /PHYS Solar Atmosphere Fall 2004 Lecture 05 Sep. 29, 2004 Lower Solar Atmosphere: Photosphere and Chromosphere

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

Inside the Sun (1)Core depth: 0 – 0.25 Rs Temperature: 20 Million Kelvin Density: 150 g/cm 3 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=MC 2 (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 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/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 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.

Solar Atmosphere: hydrostatic model Textbook, Figure 1.1, P.2

Solar Atmosphere (1) Photosphere Nominal surface of the Sun seen in visible wavelength (4000 – 7000 Å) Thickness: about 100 km only Temperature: 5700 K Density: to particle/cm 3 (2) 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 K Density: to particle/cm 3

Solar Atmosphere (ctnl.) (1) 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: 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 (2) Corona Extended outer atmosphere of the Sun Thickness: several Rs and continuously extended Temperature: 1 MK to 2 MK Density: 10 9 to 10 7 particle/cm 3 Can’t be seen in visible light, nor in UV from light ions (C,O), but in EUV from heavy ions, e.g., Fe X, or X-ray

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 erg/K), T atmosphere temperature, g the solar gravity at surface (g=GMs/Rs 2 ), and µ the mean particle mass (~1.0 X g in the Chromosphere and Corona)

Features in Photosphere Sunspot: umbra/penumbra Faculae Granuals Supergraduals

Features in Photosphere (cntl.) 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 graduals

Features in Photosphere (cntl.) 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 that is in a constant convection state 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

Features in Photosphere (cntl.) Granules Exp. a movie of granules

Features in Photosphere (ctnl.) Granules and Faculae

Features in Photosphere (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”

Features in the Chromosphere Chromospheric network Plage Filament/prominence Spicules

Features in Chromosphere (ctnl.) 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 supergradule cells and is due to the presence of bundles of magnetic field lines that are concentrated there by the fluid motions in the supergranules

Features in Chromosphere (ctnl.) Plage (beach in French) Bright patches surrounding sunspots that are best seen in Hα Also associated with concentration of magnetic fields and form a part of the network of the chromosphere

Features in Chromosphere (ctnl.) Filament/Prominence Dense clouds of chromospheric material suspended above the surface of the Sun 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,

Features in Chromosphere (ctnl.) Filament/Prominence : absorption and emission feature

Features in Chromosphere (ctnl.) 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

Features in Chromosphere (ctnl.) Filament/Prominence Exp. Movie of eruption, so called granddady prominence

Features in Chromosphere (ctnl.) Filament/Prominence: Exp. Movie, a modern observation EIT 304 Å Emission from He II At about 0.08 MK

Features in Chromosphere (ctnl.) Spicules Small, jet-like eruptions seen through the chromosphere network They appear as short dark streaks in the Hα image They last a few minutes in the ejection process at speeds of 20 to 30 km/s They rise up about a few thousand km above the base BBSO Hα

Chromosphere structure Highly inhomogeneous, very structured Highly dynamic, not in hydrostatic equilibrium Height from about 5000 to 10,000 km, not at 2000 km as calculated from the model assuming hydrostatic equilibrium Highly magnetized, e.g., magnetic canopy

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

Limb darkening effect in Photosphere (ctnl.) Consider the emission from the same (radial) depth but from different positions of the Sun The large the angle θ, the longer the real path, because of the project effect, or the factor of secθ.

Limb darkening effect in Photosphere (ctnl.) Because the inner part is hotter than the outer part, the absorption dominates the emission outwards Therefore, the longer the path, the larger the absorption, which leads to the limb darkening effect Another way is that, because of the project effect, in average, 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.