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4201 The Sun: observations

The Sun

Only star that we can see close up: fortunately it is a fairly typical.

Optically, we see a fairly bland surface, broken up by sunspots (and in this case, Mercury!)

Credit & Copyright: Rick Scott and Joe Orman

Mass≈ 2 × 10³⁰ kg
Radius ≈ 7 × 10⁸ m
Density ≈ 1400 kg m^-3 (≈1.4 × H₂O)
Surface temp ≈ 5785 K
Luminosity = 3.9 × 10²⁶ W
Elemental Abundances (by no of atoms)
1. H, 10⁶ atoms, or approx 90%
2. He, 10⁵ atoms, or 9%
3. All "metals" 1%, including:
  1. C, 350 atoms
  2. N, 110 atoms
  3. O, 670 atoms
  4. Mg, 34 atoms
  5. Si, 35 atoms
  6. S, 16 atoms
  7. Fe, 25 atoms

When we split the light up into a spectrum, we can see the sun in different ways

What do we mean by taking the sun using the light of various atoms? This is what the solar spectrum looks like: dark lines correspond to various specific elements, and they have precisely known wavelengths.

SO (e.g.) we can pick out Hydrogen (H), which picks up the prominences very clearly

Credit & Copyright: Ralph Encarnacion

The Sun

A prominence erupts from the surface of the sun: note there is very little material actually in these

But we can look at the sun in different ways, so we can see how structure varies.

This is helium light: a "rosti" picture. Note the sunspots

This is in X-rays: note the hot X-rays come from the cool sunspots!

and this looks at the mag field: Dark is a N pole, light is a S pole

And this looks at the sun at 3 different temperatures: Red at 2 million, green at 1.5 million, and blue at 1 million degrees C.

Credit: SOHO - EIT Consortium, ESA, NASA

Solar Dynamics Observatory

Many wavelengths simmultanously

Channel name	Primary ion(s)	Region of atmosphere^*	Char. log(T)
white light	continuum	photosphere	3.7
1700Å	continuum	temperature minimum, photosphere	3.7
304Å^f	He II	chromosphere, transition region	4.7
1600Å^f	C IV+cont.	transition region + upper photosphere	5.0
171Å^f	Fe IX	quiet corona, upper transition region	5.8
193Å^f	Fe XII, XXIV	corona and hot flare plasma	6.1, 7.3
211Å^f	Fe XIV	active-region corona	6.3
335Å^f	Fe XVI	active-region corona	6.4
94Å^f	Fe XVIII	flaring regions (partial readout possible)	6.8
131Å^f	Fe VIII, XX, XXIII	flaring regions (partial readout possible)	5.6, 7.0, 7.2

"Surface"

No surface in any normal sense,

Photosphere is at a fairly uniform 5800 °C. Light coming out of interior of sun will scatter ∼ 10²⁸ times (and take ∼10⁶ years) : the surface is the position of last scatter, after which the light takes 7 minutes to reach earth.
Chromosphere: ∼ solar atmosphere. Thickness ∼ 10⁴ km, Optically thin, so can only be seen directly during a solar eclipse. Hotter than the photosphere.
Corona: very diffuse, very hot (1000000°C)
Core: all nuclear reactions go on in core of sun

Sunspots

Dark areas (umbra) surrounded by less dark (penumbra).
Dark is relative: temperature in umbra ∼ 3800 K

Strong magnetic fields: typical solar mag field ∼ .01 T = 100 gauss
In sunspot, can be ∼ .4 T ∼ 4000 gauss (say about 1/4 that in MRI machine)

Can take magnetogram: look at sun in three (or more) nearby wavelengths. By subtracting these, can get "magnetic field" picture of the sun.

Can get "magnetic field" picture of the sun. Usually in pairs (or larger groups) with opposite magnetic polarity.

Allow solar rotational period to be deduced:
P ∼ 27.3 days

However, period varies from P ∼ 25 days at equator
P ∼ 27 at 40⁰
P ∼ 30 at poles
Lifetime of large sunspots ∼ 2-3 months

Show irregular 2 × 11 year cycle:

few emerge near 40⁰ (with,say N mag poles leading S)
more emerge in lower latitudes, forming large groups
number drops off, and they only appear close to equator

back to 1) few emerge near 40⁰ (with now S mag poles leading N)

Cycle is irregular, sometimes very few spots.
Maunder minimum (long period of very few spots) coincided with "little ice age" in Europe (1550-1700)

From High Altitude Observatory

Granulation of solar surface: Hi-res pictures of sun (usually in H line) show "granules" of about 700 km radius surrounded by darker (i.e. cooler) "lanes" Lifetime ≈ few minutes.

Convection cells: large masses of hot gas well up, release energy by radiation, cooling down and sink back down.
Vertical speeds ≈ 100 m/s
c/f hot coffee.

Super-granulation: much larger structures (≈30000 km) surrounded by regions of more intense mag fields.

Spicules: "flames" of hot gas, mixed with mag fields, which appear at edge of supergranules
500 km across, extend 10,000 km above chromosphere.
Speeds ≈ 20 km/s
Life of 15 mins

Prominences:

huge (≈ 10⁵ km) masses of hot gas, which extend from photosphere up into the corona. Can be very long-lived "quiescent" last weeks, "active" last hours.

Prominences look very dramatic, but are actually quite benign: this shows a prominence erupting on the sun. Note that we can see these in H-light: invisible in optical since that is BB radiation and is swamped by photosphere

This shows a huge prominence erupting from the sun's surface

Credit: Canadain Space Agency

Corona

Corona: much hotter,

Can be studied by choosing a suitable X-ray (not masked by photosphere): e.g. O_IV shows temperature of corona at 700,000 K
See complex structure, which partly reflects super-granules
Also can be seen in optical, by looking at forbidden lines (so low pressure) of highly ionised (so high temp) atoms.

Hard to see corona from earth, but magnetic field concentrates matter. Coronal loop is about 50000 km high

TRACE

The corona changes according to the stage of the sunspot cycle: this is at a minimum

and this is close to a maximum: note the huge streamers of particles blowing out

Image Credit: High Altitude Observatory, NCAR

and this is a "hole" in the corona

Aurora

The magnetic field interactions with the gas (plasma) trigger effects on earth.

This is a normal looking aurora (where is it?)

But we can see these from space as well

And we will always get N and S auroras

Auroras Over Both Earth Poles Credit: Polar VIS, JPL, NASA

Where do aurora come from?

This is a tube of magnetic flux on the sun: it confines particles. When it collapses, it squirts out the particles

which travel through space
Image credit: Goddard Space Flight Center .

and cause a magnetic storm/aurora here on earth

The colours arise because of different interactions in the atmosphere
Diagram from University of Alaska Geophysical Institute

And while we are about it, Jupiter also has aurora: this is on the night side of the planet as seen from the Galileo space probe

and Saturn
Saturn Aurora Credit: J. Trauger (JPL), NASA

Now lets look at other stars