Dark Energy in the Solar Corona

H.S. Hudson, UCB & SPRC

LMSAL, January 31, 2002

Dark energy in the solar corona

• Not the "dark energy" of the cosmos, but B!

• MHD equilibrium

• Imperturbable compact event

• Imperturbable cusp

• Two CMEs

• Blow-out of LPS

The corona is surprisingly stable.

What is the nature of this equilibrium?

The corona exhibits a "stick-slip" variability, as shown on any GOES plot

The open fraction of coronal field lines (v. roughly the area of the coronal holes) remains approximately constant

A. There isn't much braiding observed in quiet-Sun magnetic fields (K. Schrijver, 2001)

B. Predictions of the solar wind are happy with a “potentialfield” source-surface model

REMARKS

C. So how does “energy build-up” happen?

Imperturbability on a compact scale

Flares often involve multiple loops that don’t appear to touch.

One loop brightening (pressure pulse) usually doesn’t affect its neighbors geometrically.

Sorry not to have any good examples readily at hand.

Imperturbability on a large scale

The same principle applies - often the distant corona ignores what looks to us like a titanic explosion.

Excellent example from June 6-7, 2000 (see APOD).

A beautiful coronal cusp resulting from a flare/CME on June 6, 2000

The GOES time history for June 6-7, 2000

Before… After

500 DN 100 DN

15:36:23 minus 14:52:27 UT

Remember the movie!

Measurementgeometry

Measurement results

Photosphere

“SourceSurface”

Solar wind: radial field lines

Corona: force-free field

Minimal model for a CME: initial conditions

Photosphere

“SourceSurface”

Minimal model for a CME: a flare has happened!

FF FF

Radial field

Minimal model of a CME

• A force-free coronal equilibrium interfaces to a radial field in the solar wind, initially.

• A flare happens and a CME opens a large new swathe of field lines.

• The MHD equilibrium prior should adjust to a new equilibrium state outside the CME.

• We don’t see this happening.

How “massive” is the corona?

1. Total magnetic energy = 1034 ergs, hence a major CME is a 1% effect.

2. A major CME occupies 1 steradian hence a major CME is a 10% effect.

3. In any case there should be a local effect on the equilibrium.

Seemingly identical interplanetaryCMEs can have very different coronal origins.

Impulsive flareNorth of equatorFilamentDimming

Giant arcadeSouth of equatorFilamentNo dimming

Ulysses ICME observations (Gosling et al., 1994)

1 2

Inferences

• The solar-wind structures resulting from a CME may have little to do with the coronal structure causing it.

• The energy involved is hard to trace, because the dominant term is the coronal B field, especially if it is low

An exception to prove the rule:the disruption of a post-flare

arcade - v. unusual

FFIs first...

Dark energy in the solar corona

• Apparently great perturbations can occur without disrupting the basic equilibrium of the corona.

• Great perturbations often do not permanently displace nearby coronal structures.

• Extremely different solar perturbations can cause nearly identical ICMEs.

• The bottom line: coronal MHD equilibrium is not easy to understand intuitively.

Once again, I’ve managed to give atalk, the main point of which is thatI don’t understand what is going on...