The Solar Corona and Solar Wind

Steven R. Cranmer

Harvard-SmithsonianCenter for Astrophysics

The Solar Corona• Plasma at T > 1 million K emits most of its radiation in the UV & X-ray

AIA on SDO

The Solar Wind• The Sun’s magnetic field Is “stretched open” by an outflow of hot plasma

from the surface, which eventually accelerates to speeds of 300–800 km/s

Total solar eclipse, Aug. 1, 2008 (Mongolia)

The Solar Wind

How much of the Sun’s mass is “evaporated” in the wind?

Over nearly all of its ~10 billion year lifetime, it will lose only about 0.1% of its total mass.

(Some stars lose more than half!)

How far out into the solar system does the wind flow?

Shouldn’t the Sun’s strong gravity keep gas from escaping?

At about 100 AU, the wind is stopped by pressure exerted by the interstellar magnetic field.

(Earth: 1 AU, Jupiter: 5 AU, Pluto: 40 AU)

The Solar Wind: History• 1850–1950: Evidence slowly builds for the existence of something

outflowing from the Sun and interacting with the Earth . . .

• solar flares aurora, geomagnetic “storms,” telegraph snafus• comet ion tails point anti-sunward (no matter comet’s motion)

“Celestial battery:”

Transcript between Portland and Boston telegraph stations (1859):Portland: “Please cut off your battery; let us see if we can work with the auroral current alone.”Boston: “I have already done so! How do you receive my writing?”Portland: “Very well indeed – much better than with batteries.”

The Solar Wind: History• 1958: Eugene Parker proposed that the hot corona provides enough gas

pressure to counteract gravity and accelerate a steady outflow.

• 1962: Mariner 2 provided direct confirmation!

Exploring the Wind: 1960s to presentSpace probes have pushed out the boundaries of the “known” solar wind . . .

• Helios 1 & 2: “inner” solar wind (Earth to Mercury).

• Ulysses: “outer” solar wind (Earth to Jupiter, also flew over N/S poles).

• Voyager 1 & 2: far out past Pluto: recently passed the boundary between the solar wind and the interstellar medium.

• CLUSTER: several “formation flying” spacecraft probe time and space variations simultaneously.

However, we still don’t understand how the solar corona is heated to millions of degrees, and thus we don’t know how the solar wind is really accelerated !

There’s no shortage of proposed ideas . . .

Exploring the Wind: now

What do these people do all day?

• Theory: Taking existing basic principles (conservation of mass, energy, momentum) and figuring how they combine to create what we see.

• Observations: For the solar wind, both remote-sensing (with telescopes) and “in situ” detection of particles and fields are available!

Example: ions in “coronal holes”• In 1996, the UVCS instrument on SOHO measured the properties of

oxygen ions for the first time far above the solar surface.

• For me, that began a series of questions & answers that I’m still following!

Following the trail of questions

What do the observed spectral lines tell us about the plasma?

Even though hydrogen temperature is only 1–2 million K, the oxygen ions have temperatures greater than 100 million K !

(the low-density plasma is collisionless)

Why is this important to understanding solar wind acceleration?

Different suggestions for solving the “coronal heating problem” have different predictions for how the heating should depend on the

charge and mass of the particles in the plasma.

Measuring this dependence (at the heights where the wind acceleration happens) helps us narrow down the possibilities.

Following the trail of questions

What physical process can heat the “heavy ions” so strongly?

If there are high-frequency waves propagating along the magnetic field lines, those waves must eventually “damp out”

and transfer their energy to the ions.

Okay, we do see evidence for waves in the corona and solar wind. Are they the right kind of waves?

No.

needed observed

Following the trail of questions

What happens to the plasma when the observed kinds of waves damp out?

If we don’t worry about which particles receive the heat, the

total amount is just right for heating the corona!

If we apply that damping to other stars, it also matches many of

their observed mass loss rates!

But for the Sun, it wants to heat free electrons, not the heavy ions!

But that’s just using a simple theory. What happens when more realistic effects are included?

We’re working on it . . .

Conclusions• The solar corona & solar wind are great “laboratories without walls” for

learning more about magnetic fields, plasmas, and waves.

• Producing better theories of solar wind acceleration will lead to better real-time prediction of “space weather” hazards . . .

• However, we still do not have complete enough observations to be able to choose between competing theories.

For more info: http://www.cfa.harvard.edu/~scranmer/