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 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 > gm, - Height 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.

Sheared arcade model: (DeVore et al.)

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

Quasi-Separatrix Layer 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.