Transcript Acceleration of Coronal Mass Ejection In Long Rising Solar

Introduction to Space Weather
The Sun: Magnetic Structure
Feb. 16, 2012
Jie Zhang
Copyright ©
CSI 662 / PHYS 660
Spring, 2012
Roadmap
•Part 1: Sun
•Part 2: Heliosphere
•Part 3: Magnetosphere
•Part 4: Ionosphere
•Part 5: Space Weather
Effects
CH1: Structure
CH2: Magnetism and Dynamo
CH3: Magnetic Structure
CH4: Solar Eruptions
CSI 662 / PHYS 660
Feb. 07, 2012
CH3: Magnetic Structure
3.1. Solar Corona
3.2. Coronal Magnetic Field
3.3. Active Region Magnetic Field
3.4. Coronal Heating
Plasma-3: Magnetohydrokinematics:
•Magnetic Induction Equation
•Frozen-in Effect
•Magnetic Reynolds Number, Magnetic Diffusivity, Ohmic Dissipation
Time
Plasma-4: MHD Dynamo
Plasma-5: Magnetohydrostatics: Magnetic Energy, Pressure and Tension,
Plasma Beta
Plasma-6: Potential Field, Force-Free Field
CH3: Magnetic Structure
References and Reading Assignment:
•KAL CH 3.3
(on Magnetohydrostatics)
•ASCH CH 5.2
(on Potential Field)
•ASCH CH 5.3
(on Force-Free Field)
CH 3.1. Solar Corona
--Highly Structured--
X-ray image of Corona
CH 3.1. Solar Corona
•Highly structured, not simply gravitationally stratified
as in a typical atmosphere
•Three distinct regions
•Active regions
•Coronal holes
•Quiet sun regions
•They are all well organized by magnetic field lines
CH 3.1. Solar Corona
•Highly extended
•Extended to millions of km
•implying a large scale height
•Scale height h = kT/mg
•Implying a high temperature
•The hot corona
•First recognized in 1939 by identifying spectral lines
5303Å as Fe XIV (green line) and 6374 Å as Fe X (red
line)
•These highly ionized Fe must be at a temperature of
millions of degrees
•These lines have been observed since 1869, but
thought from element “coronium”
CH 3.1. Solar Corona
•Extremely Unstable
•Frequent explosive energy releases resulting in
flares, CMEs and filament eruptions
•Energy is stored in current-carrying magnetic
field lines
CH 3.2 Corona Magnetic Field
•The nature of magnetic field determines the
property of solar corona
•Structured
•Hot
•Explosive
CH 3.2 Corona Magnetic Field
Coronal hole:
• Unipolar
magnetic field
• Open magnetic
field lines
• Reduced plasma
density
• Reduced plasma
temperature
Feb. 2, 2008
http://www.lmsal
.com/forecast/ind
ex.html
Plasma 5 - Magnetohydrostatics
•Magnetic field energy (KAL 2.1.4, P21-22)
B2
EB 
2 0
•Magnetic pressure force (KAL 3.3.1)
B2
PB 
2 0

1
F  PB  
B 2
2 0
•Magnetic tension force (KAL 3.3.2)


1 
F
( B  ) B
2 0
Plasma 5 - Magnetohydrostatics
•Plasma β (KAL P58)

Pg
PB
Pg  nkT
2
B
PB 
2 0
Plasma β
•Low β plasma: magnetic pressure dominates the plasma
thermal pressure
• plasma structure is dominated by the topology of the
magnetic field, e.g., corona
•High β plasma: thermal pressure force dominates the
magnetic pressure force
• plasma motion drags the magnetic field, e.g, convection
zone, solar wind
Plasma β
CH 3.3 Active Region B Field
•An active region is composed of
numerous thin threads of
magnetic flux tubes which are
anchored in the photosphere and
filled with hot plasmas.
•The thin threads are largely
independent of one another
•Plasma can only flow along the
magnetic field lines
TRACE (Credit: NASA)
CH 3.3 Active Region B Field
Active region:
• Strong magnetic
field
• Closed magnetic
field loops
• Enhanced plasma
density
• Enhanced plasma
temperature
Schrijver &
Derosa, 2003
Helical Field Structure
• Helical magnetic field lines occur in the region very close to
the magnetic polarity inversion line (neutral line)
• Helical structure supports the filament material
• Helical structure, often called flux rope, may cause
macroscopic magnetic instability, resulting in CMEs
Plasma 6 – Potential Field
versus Force Free Field
•Potential Field (ASCH CH 5.2)
•The simplest magnetic field configuration
•Current is zero everywhere
•The magnetic system has a minimum energy
•For example: a dipole magnetic field
•For example: global magnetic field of the corona

j 0

 B  0

B  
 2  0
Plasma 6 – Potential Field
versus Force Free Field
•Potential Field Source Surface Model (PFSS)
•Potential magnetic field in the global corona can be
calculated from the so-called potential field source
surface model.
•Solving the Laplace equation of magnetic potential
  0
2
Plasma 6 – Potential Field
versus Force Free Field
•Force Free Field (ASCH CH 5.3)
•Realistic magnetic field in solar active regions
•Current is not zero
•The magnetic system has free energy
•However, the force has to be zero for the equilibrium
•For example: current sheet
•For example: current channel or flux rope

j 0


  B  B
Plasma 6 – Potential Field
versus Force Free Field
•α factor
•A measure of strength of electric current
•Thus, a measure of coronal free magnetic energy, or
non-potentiality
•Linear force free model (LFFF), α is constant
everywhere
•Non linear force free model (NLFFF), α is not a constant

j 0


  B  B
CH 3.4 Coronal Heating
• Refer to Aschwanden Chap. 9
•Coronal heating should be from mechanic energy
•Thermal energy from the photosphere is impossible
•Energy can not be conducted from low temperature to high
temperature, against the 2nd law of thermal dynamics
•Many mechanical heating mechanisms are proposed
•hydrodynamic (wave) heating mechanisms
•magnetic (wave) heating mechanisms
•direct current heating mechanisms: microflares
•Because the coronal energy budget is only a tiny fraction
(~0.01%) of the Sun’s total output, each mechanism is able to
deliver the total energy required for coronal heating.
CH 3.4 Coronal Heating
• A coronal heating process has three basic elements
1. Generation of mechanical energy
2. Transport of mechanical energy
3. Dissipation of the energy
• Near-universal agreement that energy is produced by the
turbulent fluid motion of the Sun’s outer convective zone
• But proposed mechanisms differ in energy transport and
dissipation
CH 3.4 Coronal Heating
• Hydrodynamic heating mechanisms
•Acoustic wave transfer, and shock dissipation
CH 3.4 Coronal Heating
• Magnetic or magneto-hydrodynamic heating mechanisms
•Slow mode MHD wave
--- shock dissipation
•Longitudinal MHD tube wave --- shock dissipation
•Fast mode MHD wave
--- Landau damping
•Alfven waves
--- mode-coupling
•
--- resonance heating
•
--- compressional viscous heating
•
--- turbulent heating
•
--- Landau damping
•Magneto-acoustic surface wave --- mode-coupling
•
--- phase-mixing
•
--- resonant absorption
CH 3.4 Coronal Heating
• Direct current heating mechanisms: microflare heating
•Current sheet --- magnetic reconnection
•
(turbulent heating, wave heating)
The End