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Introduction to Space Weather Jie Zhang CSI 662 / PHYS 660 Spring, 2012 Copyright © The Sun: Magnetic Structure Feb. 16, 2012.

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Presentation on theme: "Introduction to Space Weather Jie Zhang CSI 662 / PHYS 660 Spring, 2012 Copyright © The Sun: Magnetic Structure Feb. 16, 2012."— Presentation transcript:

1 Introduction to Space Weather Jie Zhang CSI 662 / PHYS 660 Spring, 2012 Copyright © The Sun: Magnetic Structure Feb. 16, 2012

2 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

3 CH3: Magnetic Structure CSI 662 / PHYS 660 Feb. 07, 2012 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

4 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)

5 CH 3.1. Solar Corona --Highly Structured-- X-ray image of Corona

6 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

7 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

8 Extremely Unstable Frequent explosive energy releases resulting in flares, CMEs and filament eruptions Energy is stored in current-carrying magnetic field lines CH 3.1. Solar Corona

9 The nature of magnetic field determines the property of solar corona Structured Hot Explosive CH 3.2 Corona Magnetic Field

10 Feb. 2, 2008 http://www.lmsal.com/forecast/ind ex.html Coronal hole: Unipolar magnetic field Open magnetic field lines Reduced plasma density Reduced plasma temperature CH 3.2 Corona Magnetic Field

11 Magnetic field energy (KAL 2.1.4, P21-22) Magnetic pressure force (KAL 3.3.1) Magnetic tension force (KAL 3.3.2) Plasma 5 - Magnetohydrostatics

12 Plasma β (KAL P58) Plasma 5 - Magnetohydrostatics

13 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

14 Plasma β

15 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 CH 3.3 Active Region B Field TRACE (Credit: NASA)

16 Schrijver & Derosa, 2003 Active region: Strong magnetic field Closed magnetic field loops Enhanced plasma density Enhanced plasma temperature CH 3.3 Active Region B Field

17 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 Helical Field Structure

18 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

19 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

20 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

21 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

22 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 2 nd 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.

23 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

24 Hydrodynamic heating mechanisms Acoustic wave transfer, and shock dissipation CH 3.4 Coronal Heating

25 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

26 Direct current heating mechanisms: microflare heating Current sheet --- magnetic reconnection (turbulent heating, wave heating) CH 3.4 Coronal Heating

27 The End

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