The Sun Magnetic Fields and Charged Particles Magnetic fields Charged particles are affected by magnetic fields They are forced to follow magnetic field lines

Plasmas At ordinary temperatures, atomic nuclei attract electrons to make them neutral At extreme temperatures, the electrons pop loose, and the electrons can run off freely This is a plasma Plasmas are excellent conductors They can generate magnetic fields

Atomic spectra Suppose you take an atom and bump the electrons up (by heating it, for example). What type of light comes out? Spectrum consists of emission lines Exactly which lines tells you the kind of atom, like a fingerprint

Suppose you take an atom and shine a bright white light on it. What does the spectrum of the light look like when it comes out? Initial spectrum is continuous Atom absorbs certain very specific frequencies The same frequencies you saw before. Atomic spectra

Kirchoff’s Laws Hot thick solid, liquid, or gas – Continuous spectrum Thin gas – bright line spectrum Thin gas with hotter thick gas behind it – dark line spectrum

Continuous Spectrum: Hot, thick solid, liquid, or gas Bright Line Spectrum: Hot, thin gas Dark Line Spectrum: Cooler gas in front of hot, thick solid, liquid, or gas Kirchoff’s Laws

Stellar Spectra Star A Spectrum Hydrogen Spectrum From this spectrum you can tell this star A) Is made of hydrogen B) Is made of gas C) Is hot on inside, cooler on outside D) All of the above

Star B Spectrum Hydrogen Spectrum Stellar Spectra This star is made of: A) Hydrogen B) Something besides hydrogen C) Part hydrogen, part something else

The Sun’s Spectrum Hundreds of lines Scores of Elements

Composition of a Typical Star (Sun) Hydrogen Helium Oxygen Carbon Iron Neon Sulfur Nitrogen Silicon Etc. Know These!

The Sun: Basic Facts 109  Earth diameter 333,000  Earth mass Density: a bit higher than water Rotates once every 25 days Differential rotation March 1, 2016 from SDO

The Sun: How We Know What We Know Exterior Spectra Temperature Motion Chemical Composition Magnetic Fields Ground-based telescopes Eclipses Space telescopes Solar Wind Interior Gravity Magnetic Fields Neutrinos Helioseismology

Solar Dedicated Spacecraft Solar Dynamics Observatory SOHO Ulysses STEREO

Photosphere The layer we see Sunspots Chromosphere Where the dark lines are made Corona Prominences Coronal Holes Flares Solar Wind Outer Layers of the Sun

The Photosphere The point at which gas is thick enough that we can’t see through it This is a little shallower towards the edge Limb darkening Here we see shallow and cool Here we see deep and hot

Granulation The Sun’s surface is not uniformly bright Hot regions of upwelling gas Cooler regions of falling gas Convection near the surface

Granulation Granulation changes over time

Earth Granulation Circulation of gases in outer layer Hot plasma rising, cool plasma falling – convection Underneath, evidence of circulation on a larger scale Supergranules larger than the Earth

Chromosphere A cooler layer just outside the photosphere Light from photosphere lines absorbed from this layer Bright line spectrum Visible during eclipses Red color from spectral line of hydrogen

Spicules Spicules – bright small jets that reach from the photosphere up through the chromosphere Last about 5 minutes Cause not understood Thousands on the Sun at any given time

Corona Thin, extremely hot region Visible during eclipses If you took a spectrum of the corona, it would be A) A bright line spectrum B) A dark line spectrum C) A continuous spectrum

Corona: Temperature High temperature not well understood Heated by magnetic fields Like a whip Low density – has little total heat

Sunspots Relatively cool regions on a brighter, hotter background Large magnetic fields What causes them: Mag. Field sticks out Plasma can’t flow horizontally Prevents convection from working Hot gas can’t rise Temperature cooler

Sunspots - Structure Umbra Penumbra

Sunspots often come in groups Pairs are common North with south Larger groups are not uncommon

The Solar Cycle The number of sunspots changes over time Approximately an eleven year cycle: Butterfly diagram Sunspots first appear far from the equator Over time, they become more numerous, and move towards the equator Then they all disappear

The Solar Cycle Sun’s North/South magnetic pole reverses each cycle Not all cycles are created equal! , very low level, “Maunder Minimum” Corresponded to “Little Ice Age” in Europe We just finished an unusually quiet and long Solar Minimum Level of sunspots/magnetic activity affects all other solar activity

The Solar Cycle

Flares Explosive release of magnetic energy on surface of the Sun Typically reach through photosphere, chromosphere, to corona Burst of energy, especially X-rays Can cause coronal mass ejections

Prominences Loops of magnetic fields sticking high above the Sun’s surface

Prominences

Charged particles following magnetic lines from inside the Sun Show movie

Coronal Mass Ejections Flares can expel gas at high velocities from the Sun Similarly, prominences can become “disconnected” High temperature gas expelled at high velocity Up to half the speed of light These events are called coronal mass ejections They can cause satellite malfunction and communication disruptions on Earth They pose a major danger to astronauts beyond the Earth’s magnetosphere

Coronal Mass Ejections

Coronal Holes Most gas in the corona is trapped by the Sun’s magnetic fields “Coronal holes” allow some to escape to space This creates the solar wind

The Solar Wind Particles flowing outward from Sun travel to Earth and beyond Contains hydrogen, helium, etc. Magnetic fields Earth funnel them to Earth’s magnetic poles Causes the Aurora Borealis and Aurora Australis