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

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

Solar Corona References: Kallenrode: Chap. 2.1, Chap. 3 CSI 662 / PHYS 660 September 24 2009 Solar Corona References: Kallenrode: Chap. 2.1, Chap. 3 Aschwanden: Chap.1, Chap. 5.1, 5.2, 5.3

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

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

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)

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

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

Magnetic Energy Kallenrode: Chap. 2.1.4, P21-22

Magnetic Pressure and Tensor Kallenrode: Chap. 3.3.1, P60-62 Kallenrode: Chap. 3.3.2, P63-66

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

Plasma β

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

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 http://www.lmsal.com/forecast/index.html

Dipole Field Aschwanden 5.2.2, P180 - 182

Potential Field Model

Force-free Field Model

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

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.

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

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

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

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

The End