• Welcome back!
• Homework #4 on web later today
• Exams to be handed back in section this week or early next week
• Drop date 3/31 (next Friday)

• The Sun's atmosphere
• Photosphere
• Chromosphere
• Corona
• Sunspots
• caused by magnetic field
• The Sun's energy source
• E = mc²

• The sun is a ball of gas, no solid surface
• Solar atmosphere is what we can see at optical wavelengths
• From bottom to top:
• Photosphere
• Chromosphere
• Corona
• From coldest to hottest:
• Photosphere
• Chromosphere
• Corona

• What we see when we look at the Sun (with appropriate filters) NEVER LOOK DIRECTLY AT THE SUN
• Photosphere is a blackbody
• Recall: blackbodies aren't really black, they just don't reflect light
• Blackbodies emit light
• The hotter a blackbody, the higher the energy of the light it emits (shorter wavelength)
• hotter blackbodies emit more light
• Average temperature of photosphere is 5800 K
• Sun looks darker toward the edges (limb darkening)
• Caused by temperature drop with altitude in photosphere
• Association: Earth's lowest atmospheric layer (troposphere) also gets colder with altitude
• Close-up view of photosphere shows granules
• Each granule is ~1000 km across
• Granules caused by convection (lava lamp effect)

Movie

• Chromosphere is hotter than photosphere
• 4400-25,000 K
• Has an emission line spectrum
• Primary line is at H-a at 656.3 nm (red)
• Much lower density than photosphere
• Much less bright
• only visible during eclipse or with special filters
• Temperature increases with altitude in chromosphere
• association: stratosphere

• Outermost part of solar atmosphere
• Least dense part of solar atmosphere
• Made up of highly ionized atoms
• Beginning of solar wind
• About as bright as the full moon
• Visible with coronagraph or during lunar eclipse
• Temperature increases with altitude
• cause of increase unknown
• Corona emits X-rays

• Look like flecks of dirt in on the picture
• Caused by cool spots in photosphere
• recall cooler blackbodies emit less light
• Galileo observed sunspots and the rotation of the sun
• Sun rotates every ~4 weeks
• Equator rotates faster than poles!
• Differential rotation
• Number of sunspots varies on an 11 year cycle
• Even though sunspots are cooler, the sun as a whole is more active when there are more sunspots
• Number of sunspots may effect the weather!

• Fact: sunspots are sites of enhanced magnetic fields
• Fact: sun's magnetic field reverses poles every 22 years
• Fact: sunspots appear near poles and move to equator
• Theory: Babcock's Magnetic Dynamo
• Sun's magnetic field threads through photosphere
• Differential rotation winds field up
• Convection kinks field, sometimes forcing it out of photosphere
• Sunspots happen where magnetic field leaves photosphere
• Kinks undone by differential rotation, moving pairs of sunspots apart
• Sunspots that congregate at pole, canceling out and then reversing the Sun's total magnetic field
• Total cycle takes 22 years, covering 2 cycles of sunspot activity

• Chromosphere has Plages and Filaments near sunspots
• Filaments when viewed from the side are called solar prominences (see famous Skylab picture)
• prominence follows magnetic field lines
• Solar flares are short-lived, high energy eruptions
• Causes burst in solar wind which effects magnetosphere, which disrupts radios, satellites

• Short answer: Nuclear power (E = mc²)
• Pressure in center of Sun is so high, protons (hydrogen nuclei) are pushed together to form Helium
• Helium nucleus has less mass than 4 protons
• Missing mass is converted to energy as predicted by Einstein's equation

• Compare chemical burning: 10^-19 joule
• Nuclear burning is 10 billion times more efficient than chemical burning

• The Sun's atmosphere
• Photosphere
• Chromosphere
• Corona
• Sunspots
• caused by magnetic field
• 11 year solar cycle
• magnetic field reverses every 2 solar cycles (22 years)
• The Sun's energy source
E = mc²