Lecture 8: The Sun

Don't Panic

Sections

Introduction

We will first consider the Sun as it is today. Later we will consider its birth, evolution, and death.

This image must have been taken in kind of a neutral filter. So I think it approximates what you would see if you just toned down the sunlight to a tolerable level without re-mixing the colors. The background is fake.

Note the Sun is darker toward the edges where you are not seeing so directly into the hotter brighter layers.

Credit: NSO/AURA/NSF.

We are just considering a snapshot in its lifetime, but its a snapshot that is roughly valid for most of it's 10 or 11 Gyr lifetime as a nuclear burning object. (For nuclear burning see below.)

The Sun is currently 4.6 Gyr old.

IAWL Lecture 10: Solar System Formation

MAIN SEQUENCE STAR

Stars on the MAIN SEQUENCE burn hydrogen (in a nuclear sense: see below) in their cores and are fairly stable and unchanging.

The MAIN SEQUENCE phase of a star is the longest phase of a star's nuclear burning lifetime.

The Sun is a typical star. It is a G star out of the OBAFGKM classification of stars. The stars get smaller, colder, longer-lived, and more abundant as one goes from O to M.

But instead of relabeling the classes, they just re-ordered the labels to get a photosphere temperature sequence in decreasing order.

OBAFGKM can be remembered from the mnemonic ``O be a fine girl/guy kiss me'' (Shu-164) which is sometimes the only sensible thing to say.]

Sun's

Sun

spectroscopy

Some Basic Facts

The Sun is a giant ball of hot gas.

It is rotating, but not all at the same speed. It is not a solid and so can rotate DIFFERENTIALLY.

On the surface its period at the equator is 25 days and near the poles is 36 days. The deep interior seems to rotate as if rigid and has a period of 27 days as is known from helioseismology which otherwise we won't discuss (Ni-122).

Question:

Iron (Fe).
Helium (He).
Hydrogen (H).

Answer 3 is right.

         H, hydrogen  70.7 +/- 1.8 %,

         He, helium   27.4 +/- 2.1 %, and

         metals       1.89 +/- 0.17 % .

         Metals
         in astro-jargon are everything which is not H or He.

         The deep interior (i.e., the core)
         of the Sun and other stars is richer in He because of ongoing
         nuclear fusion
         which is discussed below and in
IAWL Lecture 22:  The Main Sequence Life of Stars.

         The H and He abundances are approximately accurate throughout the
observable universe,
         except in those minor components:
         planets, asteroids, dust, etc.

         The abundances of metals
         vary wildly from about 4 % down to 0.1 % or even much lower, but
         never 0
         (HI-414).

         The ratios of the metals among themselves often
         vary LESS wildly.

         The leading metals in decreasing order of solar surface abundance
         by number are oxygen (O),  carbon (C), neon (Ne),
         nitrogen (N), magnesium (Mg), silicon (Si),
         iron (Fe), and sulfur (S)
         (Cox-28--29).

ROCKY BODIES

IAWL Lecture 10: Solar System Formation

The protons and neutrons make up nearly all the mass and they are bound by the strong nuclear force.

Electrons surround the nucleus in quantum mechanical oribits (or orbitals) and are bound to the nucleus by the electric force. They make up only a tiny bit of the mass, but give the atom it's usual size scale of 10**-10 m.

But energy can unbind the electrons and this happens completely in hot stellar interiors.]

This ionized state of matter is called a PLASMA.

But the Sun's surface is only partially ionized.

Here are some basic solar parameters (i.e., controlling variables).

Basic solar parameters.

______________________________________________________________________

Center and Photosphere Values for the Sun
______________________________________________________________________

 R (m)           T (K)       Density (g/cm**3)    P (Earth atmospheres)
______________________________________________________________________

    0         15.7*10**6       150                 2.33*10**11


 6.96*10**8     5770         2.18*10**(-7)           0.827

________________________________________________________________________

Cox-342

As one can see there are vast center to surface variations.

The surface temperature by Wien's law means that the Sun (approximated as a blackbody) has its maximum in emission at


  Lambda_max = 2898 micron-K / 5770 K

            = approximately 0.50 microns 
                 which is in the visible range 0.4--0.7 microns.
  
                 The peak is in green light, in fact, but 
                 the Sun's human-sensitivity-weighted average color
                 is yellow or yellow-white.

In particular the PRESSURE at the Sun's center is huge as it must be to hold up the Sun against the force of gravity trying to collapse it.

Recall only the pressure force can support against gravity for objects more massive than a small asteroid.

Question:

Gas pressure of ions and electrons. Essentially their banging around to provide a force similar to air molecules in the Earth's atmosphere.
Radiation pressure provided by the EMR.
Solid pressure such as provided by the Earth's crust, mantle, and core.
1 and 2.
2 and 3.

MAIN SEQUENCE STARS

Cl-163--165

I didn't expect you to know that, but now you do---even more importantly so do I.

The gas and EMR pressure are both strongly temperature dependent.

As temperature increases, they increase; as temperature decreases they decrease.

If the temperature of the Sun were to drop to zero, the Sun would collapse to become a WHITE DWARF which is a star of Earth size, of enormous density, held up by a quantum mechanical degenerate electron pressure.

Becoming a WHITE DWARF is the ultimate fate of the Sun when it runs of out hydrogen and helium fuel as we will discuss in Intro Astro Lecture 9: The Life of the Sun.

Solar Luminosity and the Solar Constant

The Sun is hot; space is cold. The temperature of most of space (i.e., the temperature of the EMR that pervades spaces) is T=2.726 K (

HI-481

Heat energy always flows from hot to cold spontaneously.

Thus the Sun (and all other stars) are constantly losing heat energy. They radiate it away as EMR mainly.

The luminosity of the Sun is 3.845x10**26 watts (Cox-12). Compare that to a 100-watt light bulb.

How much of that energy do we get at Earth?

The solar constant.

The solar constant is a vital number for life on Earth.

It is a key number for the Sun-supplied energy that we need to keep warm and for photosynthesis.

Of course, it is not absolutely constant---its name is a bit of a misnomer.

The solar constant is the power per unit area at the top of the Earth's atmosphere. The mean value is about 1366.5 W/m**2. During a full solar sunspot cycle the value varies by only by 0.1 percent from maximum and minmum when the sort turn variations have been smoothed away. The near constancy of the solar constant is good for life on Earth.

Credit: NASA.

In addition to the periodic variation of the solar constant there are also probably secular variations.

The most important one is a long-term increase in the solar constant due to a long-term increase in the Sun's luminosity (WB-106).

Just after formation, the Sun was probably about 30 % less bright than now.

About 3.5 Gyr from now the Sun will probably be about 30 % brighter than now.

This long-term increase is only theoretical, but it seems as certain as pure theory can be.

Alas, it spells the doom of complex life on Earth. But that's a story for another day. See the discussion in IAWL Lecture 11: The Earth.

Nuclear Fusion in the Sun

Since the Sun is emitting

EMR

The resupply comes from nuclear fusion of hydrogen (H) to helium (He) in the core of the Sun which extends from the center to about 0.2 R_Sun.

Astrophysicists usually call nuclear fusion BURNING with it understood that chemical burning is not meant.

The Sun's core is hot (T=15.7*10**6 K) and dense (density=150 g/cm**3) (Cox-342).

Under such conditions, the matter is completely ionized: i.e., there are no electrons bound to nuclei and both nuclei and electrons bounce around as free particles. As we said above, this state of matter is called a PLASMA.

A TINY BIT OF NUCLEAR PHYSICS

Now nuclei are made out of positively charged PROTONS and neutral NEUTRONS.

PROTONS and NEUTRONS are both categorized as NUCLEONS.

The number of protons determines the chemical species.

Species with the same number of protons and different numbers of neutrons are isotopes of each other.

The hydrogen nucleus usually just consists of a single proton. A proton and neutron nucleus is a heavy hydrogen: a deutron.

Nuclei are held together against the electrostatic repulsion of the protons by the strong nuclear force.

The strong nuclear force is a very strong force, but it is very short range. It acts only over a distance of 10**(-15) meters or less. 10**(-15) meters is 10**5 times smaller than an atom size.

The size scale of nuclei is about 10**(-15) to 10**(-14) meters.

Now H nuclei strongly repel by the electrostatic force because they are like-charged particles.

In stars only in the cores is it sufficiently hot and dense that the electrostatic repulsion can be overcome and the H nuclei can collide closely enough that the strong nuclear force can bind them (i.e., fuse them) releasing nuclear binding energy as heat.

Hydrogen nuclear fusion or burning to deuterons.

The deuteron is a reactive nucleus compared to ordinary hydrogen and it burns He-3 (two protons and one neutron in the nucleus) comparatively quickly.

But the final product in stellar hydrogen burning is the very stable He-4 nucleus.

There are several H-to-He-4 burning processes in stars.

The dominant one in stars less than 1.5 M_Sun is the proton-proton (PP or PPI) chain (HI-343; Cl-369).

The PP chain.

The rest mass of the products is 0.7 % less than the rest mass of the reactants (CK-262).

Now rest mass is a form of energy and energy is conserved overall.

The missing rest mass was transformed into the emitted heat energy (i.e., kinetic energy of the products and EMR).

rest mass energy

Einstein equation

 

      E = mc**2

      and thus 1 kilogram of rest mass in energy terms is 

      1 kg * (3*10**8 m/s)**2 = 9*10**16 joules

                              = about 2.5*10**10 KW-hours

                                ( 1 kW-hr = 3.6*10**6 J )

                              = about 20 megatons of TNT

                                ( 1 megaton TNT = about 4*10**15 J )

The Sun's H burning lifetime.

The limitation is that the Sun can only do nuclear burning in the core region where it is hottest and densest. Thus only a fraction of its H can be burnt. In its old age it will be able to burn He, but again only a fraction and only comparatively briefly. (Core He burning is expected to last about 2 Gyr [CK-313]).

The Sun's lifetime before collapse to a white dwarf is only 10 or 11 Gyr. It has already used up 4.6 Gyr.

The Sun's nuclear burning is STABLE.

It is not going to do a thermonuclear runaway and blow up like a giant bomb. This happens with some white dwarfs, but not to MAIN SEQUENCE STARS.

Stability of a mechanical system.

The Sun's H burning stability.

This steady input of nuclear energy in the core of the Sun allows the Sun to have a steady output of EMR which is good for life on Earth.

Down here on Earth we would like to have STABLE HYDROGEN BURNING or, as it is called, CONTROLLED FUSION.

The most feasible plan is to burn deuterons (a isotope of the hydrogen nucleus with one neutron: i.e., heavy hydrogen) and tritons (a isotope of the hydrogen nucleus with 2 neutrons: i.e., heavier hydrogen) to helium and create electrical power. (HRW-1109; KB-239).

HRW-1109

Deuterons are essentially limitless since 1 in every 6700 hydrogens is a deuteron (HRW-1109). Tritons can be created in the fusion process itself (KB-240).

En-528

PLASMAS

The tactic is to control the PLASMAS with magnetic fields rather than solids which, of course, would melt and also cool the plasma.

The fusion dream has been with us since circa 1950 and seems good for another 50 years.

Experimentally, CONTROLLED FUSION has been done, of course. But whether practical energy generation is possible is still uncertain.

If it works, then limitless, safe, relatively clean energy.

CONTROLLED FUSION is safe because a fusion reactor has no chance of having becoming uncontrolled: its always on the verge of turning off: in fact, that's the problem so far: the fusion reactors are pretty much off.

Fusion reactor technology in itself is NOT nuclear weapons proliferation concern: such reactors do NOT directly connect to bomb manufacture.

The energy is relatively clean in that the nuclear wastes it produces have relatively short half-lives, and so just burying the stuff in the ground for a few centuries is a reasonable procedure.

Can it work? Maybe, but I'm losing faith.

The Outward Flow of Energy

The center of the Sun is hot. Space is cold: the pervasive CMB component has T=2.726 K (

HI-481

So in a general sense we know that thermal energy will flow from hot to cold spontaneously.

But how in detail?

Inside the Sun.

Well from the center to about 0.71 R_Sun, the dominant energy transfer process is RADIATIVE TRANSFER.

In a simplified view, one can picture photons executing a RANDOM WALK in which they fly along straight lines between matter interactions.

The interactions often actually destroy the photons, but others are created in the same place feeding off the energy of the destroyed ones. The created ones fly off in random directions.

Despite the random photon flight directions, there is a net flow outward for two reasons:

The density decreases outward, and so in the outward direction the flights are longer. This creates a bias toward outward flow.
Each layer has less thermal energy than the next inward layer, and so it can radiate less energy inward than the next inward layer can radiate outward.
Again there is a bias toward outward flow. This bias ultimately traces back to energy streaming off into space from the photosphere and not returning.

In the Sun, the convective zone extends from about 0.71 R_Sun to the photosphere (Cox-342). Of course, radiative transfer goes on in the CONVECTIVE ZONE too.

Convection is a universally important, macroscopic heat transfer process. It occurs in:

most stars including the Sun.
the interior of the Earth. It is the driver of the plate tectonics of the crust as we'll see later.
in the Earth's atmosphere.

Question:

convection

The freezing of water.
The boiling of water.
The churning of butter.

Answer 2 is right.

Why and how does convection occur?

Say you have a cold upper surface and hot lower surface to a fluid layer. Gravity points downward.

A blob of fluid at the bottom absorbs heat from the bottom.
The heat raises it's temperature and causes the blob to expand and become less dense.
When it becomes less dense than the surrounding it tries to float upward.
This is just the buoyancy force effect (which is actually a fluid pressure effect). It is familiar from playing in the pool.
If the blob rises and cools sufficiently it contracts, becomes more dense and sinks. The fluid is then STABLE against convection.
But if it doesn't cool sufficiently, it can keep expanding and rising in a runaway fashion.
Eventually the blob reaches the cool top and possibly breaks up and releases its heat to the top. The possibly broken-up, cooled blob sinks back to the bottom and starts over again.
Convection forms a cycle of heat transfer from bottom to top.

Convection happens whenever the TEMPERATURE GRADIENT becomes sufficiently steep and the insulation is a fluid---or at least sufficiently fluid-like as inside the Earth's mantle as is discussed in IAWL Lecture 11: The Earth.

Question:

The parts of the solid arn't free to move relative to each other.
There are no heat flows through solids.
Solids are perfect insulators.

Answer 1 is right.

[Answer 3 is wrong, but if true implies answer 2, but answer 2 does not imply answer 3 since there may be no heat flows since there never are any temperature gradients. Just a little exercise in logic.]

Convection is a chaotic, turbulent process. Thus it is very hard to calculate its behavior in detail.

The full calculation requires three-dimensional hydrodynamics which is still difficult even with supercomputers. Frequently, one gets the wrong answer.

Dealing with convection is one of the difficult and uncertain parts of astrophysics. For example, our understanding of stellar evolution is we think quite good, but uncertainty about convection is one of the weak links.

In the Sun, the gas relatively near the photosphere is partially neutral, and thus more opaque to photons. Thus there is a higher insulation barrier for heat flow and the temperature gradient tries to steepen.

CONVECTION is the upshot.

At the photosphere, the Sun becomes sufficiently transparent that some photons can just escape to space.

The escaping photons are how the blobs of hot gas deposit their heat. Then they can break up (?) and sink as cold gas.

We see the hot blobs at the photosphere as GRANULES.

The Photosphere

The PHOTOSPHERE is where the Sun becomes transparent: i.e., where at least about half photons can escape to infinity.

It is the layer of the Sun that is usually called the surface. But actually the Sun extends outward without a sharp break at all.

The PHOTOSPHERE is the first of the outer layers of the sun.

A cartoon of the outer layers of the Sun.

The PHOTOSPHERE is about 500 km in thickness: this is probably partially a definition since there are no sharp boundaries???.

The temperature in the photosphere temperature varies, but is all about 6000 K.

A blackbody curve fitted to the photospheric EMR emission gives a temperature of 5777 K (Cox-341).

Cooler gas in the PHOTOSPHERE, above the main point of emission, creates the absorption lines which we discussed in IAWL Lecture 7: Spectra.

The convective blobs that reach up into the PHOTOSPHERE are called GRANULES because they look granular.

The GRANULES are brighter than their surroundings (which look like dark lanes) because the GRANULES are hotter.

Recall if you just tone down all parts of a bright image equally, the less bright parts can become dark.

The darker surroundings of the GRANULES is the sinking convective gas.

The granules are hot rising convective cells that break up after about 10 minutes: they are about 1000 km in size scale (Se-148; Cox-364).

Credit: T. Rimmele/NSO/AURA/NSF.

Between the granules the gas sinks.

Sunspots are somewhat colder (about 4000 K) than the surroundings (about 6000 K) and so appear dark. The are regions where magnetic field lines plunge or into the sun: thus they often come in pairs.

One can see the granulation off the spots. The granules are the tops of hot convective currents. They break up in about 10 minutes and have a size scale of about 1000 km (Cox-364; Se-148).

Credit: ?

The Chromosphere

The CHROMOSPHERE is a low density layer above the photosphere that is about 10,000 km thick.

The radius of the photosphere is 700,000 km.

So the CHROMOSPHERE is still a thinnish layer of the Sun.

The CHROMOSPHERE is overall much hotter than the photosphere: temperature rises from a low of 4000 K to 100,000 K at the top. It's low density and high temperature make it emit an EMISSION LINE SPECTRUM.

Chromo means color and the name probably arises from the pink color the CHROMOSPHERE would show to the unaided eye.

But, the CHROMOSPHERE is probably never seen by the unaided eye under ordinary conditions. However, solar prominences which we discuss below are chromospheric in color (Se-160) and are visible during total solar eclipses.

Nowadays, the CHROMOSPHERE is often observed from space through narrow filters centered on emission lines where it is bright.

The SOHO probe (Solar and Heliospheric Observatory) shows some good extreme UV images from the 0.0304 micron line of once-ionized He. This is the strongest once-ionized He line.

A Sun image in He II 0.0304 micron emission line. The shown color is, of course, false.

We are seeing the upper chromosphere/lower transition region, not the photosphere. There is a nice PROMINENCE.

The arc shape is because the 60,000 K plasma follows magnetic field lines.

Credit: NASA: SOHO. SOHO is the Solar and Heliospheric Observatory.

The Corona

Above the chromosphere is the CORONA.

Question:

never.
at sunset and sunrise.
during total solar eclipses.
during annular solar eclipses.

Answer 3 is right.

The CORONA is that milky white, tenuous, wispy gas seen around the Sun in total solar eclipses.

The wispy structure is because the ions tend to spiral around the magentic field lines of the Sun. This is the effect of the magnetic force. We discuss magnetic fields further below.

The corona is a hot (of order 1,000,000 K), very lower density layer of gas surrounding the Sun. It is milky white and appears whispy because the ionized atoms stream along magnetic field lines.

Credit: ?

The CORONA reaches from the chromosphere outward until it makes a transition into the solar wind. There is no sharp transition. The corona can be traced out to 30 R_Sun (0.14 AU) (Se-151) which is still well within Mercury's mean distance to the Sun of 0.38709893 AU (Cox-294).

The CORONA'S temperature is of order 10**6 K, and so it is much hotter than the photosphere and chromosphere. But it is so dilute that it radiates much less than the photosphere.

It can be seen from Earth and space at non-eclipse times by masking out the Sun.

This 2000may03 picture also shows some planets and the Pleiades. I've no idea what wavelength band is used. Probably optical.

The ``disks'' on the planets are an artifact of over-exposure. The dark band is the arm holding the mask.

The planet positions for the image are shown here:

Credit: NASA: SOHO mission.

Why are the CHROMOSPHERE and CORONA hotter than the photosphere?

Some mechanism pumps heat to them and it is not EMR from the photosphere.

The most popular idea is that somehow magnetic field energy generated in the interior is then dumped as thermal energy above the photosphere.

There is certainly much more elaboration in the theory, but the a definitive answer is not yet in.

The Solar Wind

The corona extends into the SOLAR WIND.

This expanding stream of protons (ionized hydrogen atoms) and electrons and other particles coming off the corona.

We will not go into the causes of the solar wind: they may arise from magnetic effects????, but the instructor admits to plain ignorance on the subject.

The solar wind extends probably out to of order 100 AU and then runs into the interstellar medium at a shock surface called the HELIOPAUSE. The wind eventually merges with the interstellar medium.

No probe has yet reached the HELIOPAUSE, but radio emission from it probably has been detected by the Voyager probes launched in 1977 which are now at of order 50 AU from the Sun ( The HELIOPAUSE: adapted from JPL press release, 1993may26).

Strictly speaking, the HELIOPAUSE is a purely theoretical entity, but there are no doubts that it exists.

The solar wind mostly comes off from coronal holes: places where the Sun's magnetic field lines DO NOT close trapping the particles on closed loops (Ni-130).

Recall charged particles have a strong tendency to spiral around magnetic field lines, and thus if the those field lines return to the Sun, the particles have difficulty escaping to infinity.

The Sun's magnetic field is essentially dipolar like the Earth's and a bar magnet. It switches polarity every 11 years on average for a total solar cycle of 22 years on average. (Dipole means two poles: a north and a south pole.)

A cartoon of dipole magnetic fields of the Sun and a bar magnet.

Additionally, there are complex magnetic field structures that are time variant.

There are permanent coronal holes at the Sun's axial poles (Ni-130) which are also it's magnetc poles or close to them (???). Holes can occur at other latitudes in a time dependent fashion (Ni-131).

Coronal holes and the solar wind.

The images are from about 2 days apart. The rotation of the Sun is clear. Coronal holes seem to be magnetic field free areas or areas of outwardly open magnetic field lines in the corona that allow more free-streaming solar wind sort of like the nozzle of the hose whipped around.

Credit: NASA.

The diagram is not well captioned. I'm guessing that the image is in the X-ray (and hence false color). IMF probably stands for something magnetic field. I assume that all these speeds were measured at the circular orbit of Ulysses, but I can't track that information right now.

Credit: NASA

Ze-287

The mass loss rate by the solar wind isn't large: it's only about 2*10**9 kg/s (Se-152, but note the values need some correction.)

If the rate kept steady---which it won't---how long until the Sun is exhausted?

First, let us convert to solar masses lost per year.

  2*10**9 kg/s x (1 M_Sun / 2*10**30 kg) x (3*10**7 s / 1 year)

  =  about 3*10**(-14) M_Sun/yr  ,

Then from the ordinary exhaustion formula

   Amount/Rate =  1 M_Sun / 3*10**(-14) M_Sun/yr

               = about 3*10**13 yr  = 3*10**4 Gyr .

Question:

3*10**4 Gyr.
1 Gyr.
10 Gyr.

Answer 3 is right.

Since the Sun's lifetime is only about 10 Gyr, the Sun will not lose significant mass because of the current solar wind.

In the post-main-sequence life it will have stronger solar winds and will probably end up with only about 70 % of its current mass when it becomes a white dwarf star (CK-329).

The Earth is protected from the solar wind mostly by the Earth's magnetic field. Basically a distorted dipole field which forms what is called the magnetosphere---although it isn't spherical.

A cartoon of the solar wind interacting with the Earth's magnetosphere.

The solar wind particles mostly can't force their way to the Earth. The Earth's magnetic field tends to make them slide around the magnetosphere.

The protection by the magnetosphere is probably necessary. The solar wind probably blew away part of Mars' atmosphere (Se-480).

The solar wind particles can also act as dangerous IONIZING RADIATION for life and electronic systems. The magnetosphere largely protects astronauts and satellites, but large solar storms (see below) can cause solar wind particles to penetrate the magnetosphere and be more dangerous than ordinarily. See UCAR's Effects at Earth of Space Weather Events.

Some particles do get trapped in the reservoirs in the magnetosphere. These reservoirs are called the Van Allen Belts. They are donut-shaped or toroidal and there are 3 of them (PF-99): the inner 3rd belt was discovered circa 2000.

A very, very crude diagram of the inner magnetosphere of the Earth.

Some of the solar wind particles can spiral into the Earth's atmosphere near the poles. The tend to come down in a ring called an AURORAL RING.

In fact, during strong gusts of solar wind (e.g., coronal mass ejections: see below), the particles can spiral in at lower latitudes.

The collisions of these particles (electrons mainly) with air, excites air molecules (i.e., gives them internal energy).

When the air molecules de-excite they emit light. This is same process as in a neon light. The result is the AURORA.

Question:

Never.
Frequently.
Rarely.

Answer 3 is right.

My senior colleague at UNLV Lon Spight remembers seeing an aurora in the late 1950's, but I think they must have been seen more recently than that.

the University of Alaska, Fairbanks, Aurora site

The aurora in Arizona was associated with a coronal mass ejection from the Sun. Coronal mass ejections are massive gusts of the solar wind often accompanying solar flares.

The image is long-exposure as one can see from the finite star images. Long-exposure means that the aurora is brighter than the eye would have seen.

Credit: Adam Block/NOAO/AURA/NSF.

Hydrogen, oxygen (O_2), and nitrogen in various forms (N_2, N_2+, N I, N II) provide most of the aurora colors according to this spectrum.

Credit: National Oceanic and Atmospheric Administration/Department of Commerce: Image ID: wea01029, Historic NWS Collection; Credit: Collection of Dr. Herbert Kroehl, NGDC.

The image is long-exposure as one can see from the finite star images. Long-exposure means that aurora is brighter than the eye would have seen.

Credit: National Oceanic and Atmospheric Administration/Department of Commerce: Image ID: wea02007, Historic NWS Collection; Location: Kangaroo Island, South Australia; Photographer: David Miller; Credit: National Geophysical Data Center.

Credit: National Oceanic and Atmospheric Administration/Department of Commerce: Image ID: wea01013, Historic NWS Collection; Location: Anchorage, Alaska; Photo Date: 1977; Photographer: Doctor Yohsuke Kamide, Nagoya University; Credit: Collection of Dr. Herbert Kroehl, NGDC.

Credit: National Oceanic and Atmospheric Administration/Department of Commerce: Image ID: wea01034, Historic NWS Collection.

Solar Activity and Magnetic Fields

Solar activity is sort of like solar ``weather.'' But unlike the Earth's weather, solar ``weather'' is dominated by magnetic effects. But we won't go into the causes of solar activity which are not fully understood and are certainly too complex for our course.

The Sun has an overall magnetic field that is dipolar with a north and south pole like a bar magnet and like the Earth (Ni-130).

A cartoon of dipole magnetic fields of the Sun and a bar magnet.

The polarity of the Sun's field reverses every 11 years on average for an overall cycle of 22 years on average (HI-300; FMW-296). The reversals occur at the solar minima of the sunspot cycle ????.

HI-300

What causes the magnetic field of the Sun?

Well the Sun is a plasma in its interior: i.e., all the particles are charged. It is also rotating differentially and has convection.

Somehow, in way that is not fully understood yet, large electric currents must form. Electric currents generate a magnetic field: this is just a fundamental fact. And this must be what happens in the Sun.

The process of generating a magnetic field this way is called the DYNAMO EFFECT (Se-157).

In addition to the main dipole magnetic field structure their are smaller time varying structures associated with sunspots mainly.

Sunspots

Sunspots have probably been occasionally noticed going back into prehistory.

Certainly, they were observed and recorded for for thousands of years in China (SRJ-357).

You shouldn't be paranoid about catching glimpses of the Sun: we do this all the time. But the damage over a lifetime may be cumulative, and so one should avoid viewing the Sun.]

GALILEO

This image must have been taken in kind of a neutral filter. So I think it approximates what you would see if you just toned down the sunlight to tolerably level without re-mixing the colors. The background is fake.

Note the Sun is darker toward the edges where you are not seeing so directly into the hotter brighter layers.

Credit: NSO/AURA/NSF.

Sunspots are typically tens of thousands of kilometers in size scale and are typically twice the diameter of the Earth (Ni-123; Se-154).

Sunspots are transient and may last a week or so.

They tend to occur in groups of up to 100 members and the groups can last 2 months or more (Se-155).

Sunspots occur in the photosphere and are colder than the surrounding photosphere. Their temperature at center is about 4000 K (Se-155), whereas the photosphere temperature is usually 6000 K.

Because of the lower temperature, sunspots radiate less than the surrounding photosphere and appear dark in comparison in toned down images.

The dark inner region of a sunspot is called the UMBRA and the less dark border the PENUMBRA. This is a different usage of ``umbra'' and ``penumbra'' than in eclipse phenomena (Se-155).

Sunspots are somewhat colder (about 4000 K) than the surroundings (about 6000 K) and so appear dark. The are regions where magnetic field lines plunge into or rise out of the sun: thus they often come in pairs.

One can see the granulation off the spots. The granules are the tops of hot convective currents. They break up in about 10 minutes and have a size scale of about 1000 km (Cox-364). Se-148).

Credit: ?

Sunspots are comparatively cold because they are have strong magnetic fields that seem to suppress convection that is otherwise heating the photosphere.

Sunspots are essentially a magnetic phenomenon.

Frequently sunspots occur in pairs. One member being a north pole and the other a south pole.

A cartoon of a sunspot pair.

Sunspots obey in 22-year cycle divided into two 11-year subcycles. The time lengths are averages. Subcycles as low as 8 years and as high as 16 years are known (FMW-296).

Note the Maunder minimum. This is the period 1645--1715 where there were few sunspots. This time seems to have corresponded to a world-wide cold spell (sometimes called the Little Ice Age) (Ze-293). There may be a connection.

Fa-248--249

Credit: NASA.

At mid-subcycle there are typically of order 100 and the record is 254 (HI-299): this is the solar maximum.

HI-299--300

Well for one thing the overall dipole magnetic field of the Sun reverses at the solar minima.

So if the 1st sub-cycle as the north magnetic pole at/near?? the Sun's north axial pole and the south magnetic pole at/near?? the Sun's south axial pole, then in the next subcycle the magnetic poles reverse.

Also the polarities of the sunspot pairs tend to change, but this is best explained diagramatically.

Sunspot tendencies during subcycles.

The latitude evolution is illustrated by a butterfly diagram that plots sunspot latitude versus time.

This diagram shows the abundance of sunpots at different solar latitudes as a function of time.

The shape of the pattern gives rise to the name butterfly diagram.

Credit: NASA.

There are theories of sunspot formation. But we won't discuss them here.

In fact the theories are inadequate in that the sunspot cycle cannot be predicted.

If you cannot predict a basic fact of a phenomenon, then clearly you don't really know what is happening.

Associated with sunspots are other solar activities: dark filaments, prominences, flares, and coronal mass ejections.

Dark Filaments and Prominences

In the vicinity of sunspots are DARK FILAMENTS: comparatively dark narrow sinuous string-like regions on the Sun's photosphere---but I can't find a good image of one.

They stretch for up to 100,000 km and are locations of zero magnetic field between regions of opposite magnetic field lines (PF-186).

PROMINENCES

PF-186

PROMINENCES

The arc shape is determined by magnetic field lines. The charged particles spiral around the field lines tracing the arc.

The special strong time-varying magnetic field structure that shapes a PROMINENCE must be determined by internal currents and energy sources in the Sun---and that is all I'm going to say about cause.

PROMINENCES in the visible are pink or red from H alpha emission and resemble chromospheric conditions. They are of order 100 times denser than the corona and have temperatures of order 10,000 K (Ni-126).

They are only seen by the unaided eye during total solar eclipses.

A QUIESCENT PROMINENCE can arise in hours and last weeks or months (Se-160).

One of the most spectacular prominences ever seen. From Skylab, 1973dec19. The image is in the UV and hence is false color.

Credit: NASA. I've lost track of the download site, but the image appears in Se-161.

There are also ERUPTIVE PROMINENCE that occur on the order of hours (???) and eject matter into space.

An eruptive prominence is not the same as a coronal mass ejection: it is probably much less powerful. The blob can still make it to Earth. This is an extreme ultraviolet image from the SOHO probe and so the color is false.

Credit: NASA.

Flares

FLARES are enormous explosions that are much stronger than eruptive prominences.

FLARES are believed to be caused by MAGNETIC RECONNECTION. This is when tangled magnetic field lines quite suddenly become unstable and reform in a simpler pattern.

When they do this they somehow dump magnetic field energy as thermal energy, EMR, and kinetic energy.

The analogy is often made that MAGNETIC RECONNECTION is like an elastic band snapping: potential energy stored in the stretched configuration is suddenly released as kinetic energy.

The flare is being observed in the H alpha line (i.e., the red line emission of hydrogen).

Credit: NASA.


   10**25 J = 2.5 * 10**9 megatons

              ( 1 megaton TNT = about 4*10**15 J )

and temperatures can reach

        5*10**6 K  which is much hotter than the chromosphere or photosphere.

Coronal Mass Ejections

CORONAL MASS EJECTIONS are huge bubbles of coronal gas ejected from the Sun within a few hours.

Magnetic effects seem responsible for CORONAL MASS EJECTIONS---and that is all we'll say about cause.

CORONAL MASS EJECTIONS often accompany flares or eruptive prominences, but can occur in the absence of either.

This is a 2003feb18 image. The caption gives no information on wavelength, but I'd guess the visible band since we see stars and the corona looks white. The Sun is masked.

Credit: NASA: SOHO mission.

CORONAL MASS EJECTION

NASA site

If CORONAL MASS EJECTIONS are aimed at the Earth, we can get those solar wind gusts, magnetic storms, and strong aurora mentioned in the solar wind section above.

The image is not to scale.

Coronal mass ejections are the biggest kind of solar wind effect on the Earth. They can cause severe magnetic storms on Earth.

Credit: NASA.