Transcript The magnetic nature of solar flares

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.
1.
2.
3.
1.
2.
Observational Feature:
Eruption of CME and
prominence.
An arcade of rising soft
x-ray loops.
Ha ribbons at their feet
separated each other.
Behavior of magnetic
field:
Twisted and sheared
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
- g-ray
- LF radio
- soft x-ray
- visible
- EUV
Energy of solar flare:
- ~ 3 x 1025 joules.
- magnetic storage: large enough
2
- magnetic energy:
Bn
 2 dV
- magnetic field:
B = Bph + Bcor
Bcor: the source of flare energy
- Aly (1990): The energy (W) of any 3D closed force-free field
Wpot < W < Wopen
- magnetic helicity (conserved):
H   B  AdV
- Really free energy: WFREE = W – Wlin fff
Requirement for solar eruption (Model, Theory, …)
Must produce explosive mass acceleration:
- Velocity > 1,000 km/s, mass > 1025 gm,
- Height < 1.5 R, W > 1032 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.