1. Introduction to Space Weather
The Sun:
Solar Corona
Sep. 24, 2009
CSI 662 / PHYS 660
Fall, 2009
Jie Zhang
Copyright ©

2. Roadmap Part 1: The Sun Part 1: The Sun Part 2: The Heliosphere
The Structure of the Sun: Interior and Atmosphere
Solar Magnetism: Sunspots, Solar Cycle, and Solar Dynamo
Solar Corona: Magnetic Structure, Active Regions, Coronal Heating
Major Solar Activities: Flares and Coronal Mass Ejections (and outer corona)
Part 1: The Sun
Part 2: The Heliosphere
Part 3: The Magnetosphere
Part 4: The Ionsophere
Part 5: Space Weather Effects

3. Solar Corona References: Kallenrode: Chap. 2.1, Chap. 3
CSI 662 / PHYS September
Solar Corona
References:
Kallenrode: Chap. 2.1, Chap. 3
Aschwanden: Chap.1, Chap. 5.1, 5.2, 5.3

4. Plasma Physics 1. Magnetic Energy Kallenrode: Chap 2.1.4
2. Magnetic Pressure and Tension
Kallenrode: Chap 3.3
3. Plasma β
Kallenrode: Chap 3.2
4. Potential Field and Force Free Field
Aschwanden: Chap. 5.2, 5.3

5. Corona: Highly Structured
Active regions
Coronal holes
Quiet sun regions
These features are distinct
They are all organized by magnetic field lines in the corona
X-ray image of Corona

6. Corona: Highly Structured
An active region is composed of numerous thin thread 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 flows along the magnetic field
TRACE (Credit: NASA)

7. Corona: Highly Energetic
It is extremely hot, ~ 1 million degree plasma
coronal heating due to mechanic energy of magnetic field
It is extremely unstable
Frequent explosive flares and CMEs
Spontaneous release of energy stored in the magnetic field

8. Why? Magnetic field has (1) energy, (2) pressure and (3) tension
Just like material fluid has energy, pressure and tension
In corona, magnetic pressure force dominates the plasma pressure force, the low-β plasma. Thus, the topology of magnetic field determines the structure of the corona

9. Magnetic Energy
Kallenrode: Chap , P21-22

10. Magnetic Pressure and Tensor
Kallenrode: Chap , P60-62
Kallenrode: Chap , P63-66

11. Plasma β Kallenrode: Chap. 3.2, P58
Low β plasma, plasma structure is dominated by the topology of the magnetic field, e.g., corona
High β plasma, plasma drags the magnetic field along with the movement, e.g, convection, solar wind advection

12. Plasma β

13. Coronal Magnetic Field
Active region:
Strong magnetic field
Closed magnetic field loops
Enhanced plasma density
Enhanced plasma temperature
Magnetogam image, coronal loops, extrapolated coronal magnetic field
Schrijver & Derosa, 2003

14. Coronal Magnetic Field
Coronal hole:
Unipolar magnetic field
Open magnetic field lines
Reduced plasma density
Reduced plasma temperature
Synoptic calculation
Surface magnetogram used data assimilation to take into account the evolution on the backside of the Sun.
Feb. 2, 2008

15. Dipole Field
Aschwanden 5.2.2, P

16. Potential Field Model

17. Force-free Field Model

18. Helical Structure
Helical magnetic field in the region close to the magnetic polarity inversion line (neutral line)
Helical structure supports the filament material
Complex magnetic field above the neutral line leads to magnetic instability, causing solar flares and CMEs

19. Coronal Heating See Aschwanden Chap. 9
Coronal heating should be from mechanic energy, since thermal energy is impossible
There are many different heating mechanism:
hydrodynamic (wave) heating mechanisms
magnetic (wave) heating mechanisms
direct current 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. On the other hand, simple total energy argument is not sufficient for deciding which mechanism is actually operating.

20. Coronal Heating Coronal heating process has three basic elements
Generation of mechanical energy
Transport of mechanical energy
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

21. Coronal Heating Hydrodynamic heating mechanisms
Acoustic wave transfer, and shock dissipation

22. 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

23. Coronal Heating Direct current heating mechanisms: microflare heating
Current sheet magnetic reconnection
(turbulent heating, wave heating)

24. The End