Journey of solar energy from core to surface takes hundreds of thousands of years Radiative zone: photons constantly collide with electrons, get thrown off in a random direction Convective zone: the temperature is low enough so that the plasma absorbs photons and convection takes over as the principal means for transporting energy.

Radiative zone: No energy is produced here, but the huge power generated in the core is carried outwards by photons, whose average energy slowly decreases from X-ray into UV as the temperature and density slowly decline.

Radiative Zone: Energy gradually leaks out of the radiative zone in the form of randomly scattering photons.

Convective zone: While photons are still wending their ways outwards through this relatively low density region, the only way all of the luminosity can be carried out is if blobs of hot plasma flow outward and colder blobs sink inward.

Convection: material heated from below expands, becomes buoyant, and rises into a cooler region. It gives heat to material in this new region, cools, contracts, and sinks back to lower hotter layers.

granules Convective Zone: Convection (rising hot gas) takes energy to the surface.

Photosphere: “Surface” of Sun ~ 6,000 K

Granules: the top of the convection zone Granules: the top of the convection zone. At the centers of granules hot solar gas rise and rapidly radiate heat into space; the gas then is diverted horizontally, and sinks back into the Sun in the darker inter-granular lanes.

How dense is it? Earth atmosphere at sea level = 1 Relative density Photosphere – Chromosphere boundary 10-4 Lower photosphere/granules 10-3 Water 103 Lead 104 Core of Sun 105 White dwarf Neutron star 109 1017 1 tablespoon of neutron star matter would weigh 100,000,000,000 lbs!

Why does the Sun, a ball of gas and plasma, have a “surface”?

Photosphere: ~400 km deep region At a depth of 400 km, a photon can go about 200 km before it hits an atom. At 200 km, about 1/2 the photons that are moving outward manage to make it all the way out. At 0 km, nearly all of them manage to make it into outer space.

Chromosphere: Middle layer of solar atmosphere ~ 104–105 K

Corona: Outermost layer of solar atmosphere ~1 million K

Solar wind: A flow of charged particles from the surface of the Sun

How do we know what is happening inside the Sun?

We learn about the inside of the Sun by … making mathematical models. observing solar vibrations. observing solar neutrinos.

Helio-seismology: Patterns of vibration on the surface of the Sun tell us what the interior of the Sun is like. This is very similar to how seismology tells us what the interior of the Earth is like. Download a movie to illustrate solar oscillations from: http://science.nasa.gov/ssl/pad/solar/p_modes.htm

Data on solar vibrations agree with mathematical models of solar interior. Download a movie to illustrate solar oscillations from: http://science.nasa.gov/ssl/pad/solar/p_modes.htm

Neutrinos interact very weakly with matter. created in the core during fusion immediately escape from the Sun. Observations of these solar neutrinos can tell us what’s happening in the core. John Updike poem - permissions needed ?

Solar neutrino problem: Early searches for solar neutrinos failed to find the predicted number.

Solar neutrino problem: More recent observations find the right number of neutrinos, but some have changed form. There are three types of neutrinos; the early searches could only detect one of these forms.

What causes solar activity?

Solar activity is like “weather” Sunspots Solar flares Solar prominences All are related to magnetic fields.

Sunspots … Regions with strong magnetic fields Cooler than other parts of the Sun’s surface (4,000 K) and thus not as bright as their surroundings. Recall: Cooler objects emit less radiation per surface area.

Sun Spots Discovered by Galileo Galilei. Sun's surface sprinkled with small dark regions - sunspots. Sunspots are darker because they are cooler by 1000 to 1500 K than the rest of the photosphere. Spots can last a few days or as long as a few months.  Galileo used the longer-lasting sunspots to map the rotation patterns of the Sun. Sunspots number varies in a cycle with an average period of 11 years. Cycle starts with minimum and most of them are at around 35° from the solar equator. At solar maximum (number peaked), about 5.5 years later, most of the sunspots are within just 5° of the solar equator.

How do we know they have strong magnetic fields? A magnetic field can cause spectral lines to be split into several components (the process is called the “Zeeman effect”). The details of the splitting depends upon the strength and geometry of the magnetic field. Examination of spectral lines from the surface of the Sun shows that the magnetic field varies with location on the Sun. This same effect is important in MRI (magnetic resonance imaging) done in medical tests.

Outside a sunspot, a single spectral line Zeeman Effect Outside a sunspot, a single spectral line The strong magnetic field inside a sunspot splits that line into multiple lines We can measure magnetic fields in sunspots by observing the splitting of spectral lines

Charged particles spiral along magnetic field lines.

Loops of bright gas often connect sunspot pairs, leading one spot to have one magnetic polarity and the other the opposite polarity.

Magnetically Linked Spots

solar flare: a violent explosion in the sun’s atmosphere Magnetic activity causes solar flares that send bursts of X-rays and charged particles into space.

Magnetic activity also causes solar prominences that erupt high above the Sun’s surface.

The corona appears bright in X-ray photos in places where magnetic fields trap hot gas. Wave_fade.mpg photosphere:chromosphere:corona, optical:UV:X-ray http://www.lmsal.com/YPOP/FilmFestival/index.html

How does solar activity affect humans? An active region on the Sun that produced a powerful flare shown over a period of about four hours. The glowing gas flowing around the relatively stable magnetic field loops above the Sun's photosphere has a temperature of over ten million degrees Celsius. These flows occurred after violently unstable magnetic reconnection events above the Sun produced the flare.

Charged particles streaming from the Sun can disrupt electrical power grids, disable communications satellites, and cause aurora (“northern and southern lights”).

Coronal mass ejections send bursts of energetic charged particles out through the solar system.

How does solar activity vary with time?

The number of sunspots rises and falls in 11-year cycles.

The sun rotates faster at its equator than away from the equator. The sunspot cycle arises from the winding and twisting of the Sun’s magnetic field produced by this differential rotation.

Active Sun: Coronal Loops Coronal loops are tracing magnetic field lines in the corona Coronal loops are rooted in the photosphere, one end with positive polarity, and the other end negative polarity