Transcript Acceleration of Coronal Mass Ejection In Long Rising Solar

CSI 769/ASTR 769
Lecture 02
Sep. 6, 2005
Topics in Space Weather
Fall 2005
Structure of the Sun
and its Atmosphere
Reading:
•Gombosi, “Physics of Space Environment”, Chap.11, P211-235
•Aschwanden, “Physics of the Solar Corona”
•Chap. 1, P1-36
•Chap. 2, P37-66
•Chap. 3, P67-116
Stratified Structure of the Sun
Atmosphere
(4) Corona
(3) Transition Region between
Corona and Chromosphere
(2) Chromosphere
(1) Photosphere
(3) Convection Zone
Inside the Sun
(2) Radiative Zone
(1) Core
Inside the Sun
(1) Core
• depth: 0 – 0.25 Rs
• Temperature: 20 Million Kelvin
• Density: 150 g/cm3
• Energy generation
• through nuclear fusion process
41H  4He + 2e+ + 2ν + 26.73 Mev
(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 (cont.)
(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
(dT/dr)rad ~ -KTLs (E11.18, Gombosi)
(dT/dr)ad ~ M(r)/r2 (E11.19, Gombosi)
Inside the Sun (cont.)
Also see Figure 11.3 in Gombosi (P. 218)
Solar Atmosphere: hydrostatic model
Fig. 1.19, Aschwanden (P. 24)
Also see Fig. 11.11, Gombosi (P.228)
Photosphere
• Surface of the Sun seen in visible wavelength (4000 –
7000 Å)
• Thickness: a few hundred kilometers
• 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, 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 20000 K
• 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 only 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
• Seen in X-rays
Solar Irradiance Spectrum
•The effective temperature of the Sun is 5770 K
•Black-body in visible light and longer wavelength
•Line-blanketing in UV light
•Excessive emission in EUV and X-ray from TR and corona
•Also see Figure 11.10, Gombosi (P. 227)
Solar Spectrum versus Solar Structure
• V (visible, 4000 Å – 7000 Å) and IR (Infrared, 7000 Å – 10000 Å
• From Photosphere, the largest component of solar irradiance
•UV (Ultraviolet, 1200 Å – 4000 Å)
•Mainly from chromosphere
•EUV (300 Å – 1200 Å)
•Mainly from transition region
•XUV (100 Å – 300 Å) and Soft X-ray (< 100 Å)
•Mainly from Corona
Solar Irradiance Spectrum
Figure 1.25,
Aschwanden (P.34)
Spectrum lines: absorption and emission
(4000 Å – 7000 Å)
•Absorption lines in
photosphere and
chromosphere
•Emission lines in
Transition region and
corona
(300 Å – 600 Å)
Spectrum Lines (cont.)
Absorption versus Emission (cont.)
Features in Photosphere
•Sunspot: umbra/penumbra
•Faculae
•Granule
•Supergranule
•Magnetogram
Photosphere: Sunspot
Observed in continuum
visible light as Galileo did
Photosphere: Sunspot (cont.)
Sunspots show two main structures:
1. Umbra: a central dark region,
2. Penumbra: surrounding region of a less darker zone
SOHO/MDI 2004/10/24
Photosphere: Sunspot (cont.)
•Been noticed in ancient time
•Since 1700, systematic record of sunspot number
•Sunspot was found to be a magnetic feature in 1930
•Big Sunspot is about half the normal brightness.
•B = σT4 ,Or T ~ B ¼ (Stefan-Boltzman Law)
•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: Sunspot
•Sunspot is in pressure balance because of internal magnetic
pressure
• Pe = Pi + Pmag
•Pe: external thermal pressure
•Pe = N K Te
•N: particle density
•K: Boltzmann;s constant
•Te: external temperature
•Pi: internal thermal pressure
•Pi= N K Te
•Pmag: magnetic pressure inside sunspot
•Pmag = B2/8π
•B: magnetic field strength in the sunspot
Photosphere: Faculae
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 granules
Photosphere: Granules
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 (cntl.)
Granules
•Exp. a movie of granules
Photosphere: Supergranules (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”
Chromosphere
Plage
Filament/prominence
Chromospheric network
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 (cont.)
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 (cont.)
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 (cont.)
Filament/Prominence
•Exp. Movie of eruption, so called granddady prominence
Chromosphere: Network
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 supergranule cells and is due to
the presence of bundles of magnetic field lines that are
concentrated there by the fluid motions in the supergranules
Chromosphere: complex structure (cont.)
• Magnetized, Highly inhomogeneous, highly dynamic
Figure 1.17
Aschwanden
(P.22)
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
Transition Region (cont.)
Image: C IV (1548 Å) at
100,000 K
(SOHO/SUMER)
Outline the top of
chromosphere
Corona
• Large scale coronal structures
• Coronal loops
• Physical properties in corona
Coronal: Large Scale Structure
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 because plasma density is low there
Corona: Coronal Holes(cont.)
Coronal holes
•Coronal holes are associated with “open” magnetic field lines
•Particles easily flow away along the “open” field lines
•“open” field lines are caused by a large surface region with
unipolar magnetic field, which are often found in the polar
regions.
Also see Figure 1.14,
Aschwanden (P. 18)
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 (cont.)
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: Active Region (cont.)
Coronal loop Evolution: from TRACE
171 Å, Fe IX/Fe X, 1.0 MK
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
Coronal Properties
•Plasma state: Elements H/He are fully ionized, and heavy
elements are highly ionized (Fe X, Fe XIV).
•Magnetized plasma
Coronal Property: Low β
For a magnetized plasma, plasma β is defined as
β Ξ gas pressure / magnetic pressure
In CGS unit, Pth=nKT, and PB=B2/8π
β = 8πnKT/ B2
If β >> 1, gas pressure dominates, flows control B
If β << 1, magnetic pressure dominates, B control plasma
flow
Therefore, the coronal structure is determined by magnetic
field distribution.
Plasma β in solar atmopshere
Figure 1.22, Aschwanden (P.29)
Corona Property: Loop
•Thermal conduction is very efficient along the magnetic
field line
•Isothermal plasma along a loop
•Thermal conduction is inhibited across magnetic field lines
•Charged particles are tied to magnetic field lines in
gyro-motion, preventing particle diffusion across
magnetic field lines.
•Multi-temperature loops are mixed
•Differential emission measure DEM: dEM/dT
Coronal Property: loop (cont.)
•Hydrostatic Model of Corona (Chap. 3, Aschwanden)
•dP/ds – ρg =0
Momentum Equation
•EH – ER – EC = 0
Energy equation
•EH: Heating rate
•ER: Radiative loss rate
•EC: conductive loss rate
Bremsstrahlung Emission
• Bremsstrahlung emission (in German meaning "braking
radiation")
• the radiation is produced as the electrons are deflected in
the Coulomb field of the ions.
Bremsstrahlung emission
End
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 line-of-sight depth but
from different positions of the Sun
•The large the angle θ, the short the penetrating depth along
the radial direction
•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.
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 / N0 = e(-h/H)
where N is the particle density, N0 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 10-16 erg/K), T
atmosphere temperature, g the solar gravity at surface
(g=GMs/Rs2), and µ the mean particle mass (~1.0 X 10-24 g
in the Chromosphere and Corona)