The Ever-Changing Invisible Sun
1. Seeing the Invisible Corona

A. Total solar eclipse - brief and rare visible coronal light one millionth as bright as photosphere Corona then seen edge - on B. Coronagraph = telescope with occulting disk, works best in space also edge-on view. C. Radio or X-rays - see corona across visible disk of Sun because photosphere radiates weakly at these wavelengths


2. The Corona is a Million Degrees Hot

Click her to view an X-ray image of the Sun, taken by the Yohkoh Solar Mission Satellite

A. How do you know Corona is so hot?
"Forbidden" emission lines of highly ionized elements - ones with lots of electrons removed at high temperatures (Coronium story).
Doppler broadening of spectral lines High temperature holds Corona up against Sun's gravity Radio and X-ray radiation of million degree gas
B. How can Corona be so hot?
Heat does not flow from cold to hot Radiation passes right through tenous Corona Motions might heat Corona - sound waves Magnetism (changing) might heat the Corona Magnetism shapes and contrains the corona Merging of oppositely directed magnetism makes energy


3. Loops and Holes in Corona

A. Coronal Loops
Seen all over the Sun by X-ray telescopes Magnetic loops in Corona are rooted in sunspots of opposite magnetic polarity Hot million-degree gas consists of charged particle that cannot cross magnetic fields - they move along them (Magnetism acts as barrier to moving charges)
B. Coronal Holes
Always present north and south poles of the Sun Dark areas in X-ray images - low density and low temperature Coronal holes are wide open to space


4. Dynamic Ever-Changing Corona

Pictures are misleading - frozen snapshots of changing Corona Coronal loops can eject mass going out to space, or disappear inside Sun. Coronal holes grow and shrink.


5. The Eternal Solar Wind

Solar wind first inferred from comet ion trails Solar wind consists of charged particles (electrons and protons mostly) and magnetic fields - a perpetual ceaseless flow Solar wind is hot Corona expanding into space Solar wind has two components:
1. High-speed = 700-800 km/sec
2. Low-speed = 300-400 km/sec
High speed wind squirts out of polar coronal holes
SOHO - what heats Corona?
what accelerates high-speed wind?
where does slow-speed wind come from?
(See Unsolved Mysteries article - Part 1 and Scientific American Article)


6. Ulysses Spacecraft

First one over poles of Sun Slingshot from Jupiter High-speed wind from polar coronal holes Low speed wind from Sun's equatorial regions.


7. Edge of the Solar System

Voyager spacecraft detects edge at 116 to 177 AU where
1AU = Earth - Sun distance
| <----- <---------------
| radio signal o Sun
| Voyager
edge detects solar eruption
it goes by, then radio
signal from edge
Heliosphere = region where solar wind dominates behavior of matter, charged particles.