Presentation on theme: "The magnetic nature of solar flares Paper by E.R. Priest & T.G. Forbes Review presented by Hui Song."— Presentation transcript:
The magnetic nature of solar flares Paper by E.R. Priest & T.G. Forbes Review presented by Hui Song
Introduction The paper review: 1. The overall scenario of solar flare; 2. Why does the eruption or flares occur? 3. How is the energy released? – reconnection theory
The scenario of flares Solar Flares: A sudden brightening in the solar atmosphere and involving substantial mass motions and particle acceleration. A sudden brightening in the solar atmosphere and involving substantial mass motions and particle acceleration.
Observational Feature: 1. Eruption of CME and prominence. 2. An arcade of rising soft x-ray loops. 3. H ribbons at their feet separated each other. Behavior of magnetic field: 1. Twisted and sheared 2. Reconnected > erupted > fast particles and flares loops
Stages in solar flares: - Preflare phase; - Impulsive phase; - Gradual phase; Emission at different wavelength: - Microwave, - Hard x-ray - -ray - LF radio - soft x-ray - visible - EUV
Energy of solar flare: - ~ 3 x 10 25 joules. - magnetic storage: large enough - magnetic energy: - magnetic field: B = B ph + B cor B cor : the source of flare energy - Aly (1990): The energy (W) of any 3D closed force-free field W pot < W < W open - magnetic helicity (conserved): - Really free energy: W FREE = W – W lin fff
Requirement for solar eruption (Model, Theory, …) Must produce explosive mass acceleration: - Velocity > 1,000 km/s, mass > 10 25 gm, - Height 10 32 ergs Must open field - solar corona has infinite volume Must drive field quasi-statically - corona magnetically dominated: β << 1
Why does the eruption occur? Instability and non-equilibrium: - Separation between footpoints is too large. - Presence of a prominence. -- critical height - Shearing the footpoints of an arcade of loops. >> formation of current sheet >> reconnection.
Eruption Models: - The flux-rope catastrophic model: - The break-out model: - The sheared arcade model: First model: reconnection does not necessarily trigger a catastrophe eruption. Last two models: require magnetic reconnection to trigger the eruption;
The flux-rope catastrophic model: - Converging photospheric flow or flux emergence >> sheared arcade field - sources at ±λ approach each other >> h decrease until a catastrophe point is reached >> prominence erupts. - Reconnection in a current sheet below prominence >> eruption continues.
Breakout Model: - Sheared dipolar prominence field with neighboring flux systems; - Multipolar field with coronal null point; - Shearing of prominence flux causes overlying field to expand outward; - Overlying field encounters neighboring flux system and reconnects with it; - Reconnection removes the overlying field, allowing sheared field to expand further outward.
How is the energy released? Reconnection theory: - the breaking and topological rearrangement of magnetic field lines. 2D reconnection models: - Sweet-Parker (1958, 1957): - Petschek (1964): - Almost-uniform (1986): - Non-uniform (1990): 4 different types in 3D reconnection: - Spine reconnection; - Fan reconnection; - Separator reconnection; - Quasi-Separatrix Layer Reconnection;
Null point: where the magnetic field vanishes; field lines break and & reconnect. Spine field line: an isolated field line approaches (or leaves) the null point. Fan surface: a set of field lines leave (or approach) the null. Separator: The intersection field line of two null points’ fan surfaces, which links one null to another. Separatrix surfaces: separate the volume into topologically different regions which intersect each other in a separator.
3D Rec. at null points Spine Reconnection Fan Reconnection
Conclusion An eruption takes place when energy is stored in the coronal magnetic field. The energy is released by the lose of stable magnetic equilibrium. The different eruption mechanism: 2D and 3D. The eruption forms a current sheet, which undergoes reconnections. The different types of magnetic reconnections.