General Properties Absolute visual magnitude M V = 4.83 Central temperature = 15 million 0 K X = 0.73, Y = 0.25, Z = 0.02 Initial abundances: Age: ~ 4.52 Gyr Spectral type G2V Effective (surface) temperature = K Absolute bolometric magnitude M bol = 4.76 The Sun

The Sun’s Interior Structure Temp, density and pressure decr. outward Energy generation via nuclear fusion Energy transport via radiation Energy transport via convection Flow of energy Photosphere

Interior Structure of the Sun Site of main energy production dL/dr r/R

Interior Structure of the Sun Mass composition Mass fraction r/R H11H 4 2 He 3 2 He (x100)

The Sun’s Atmosphere Only visible during solar eclipses Apparent surface of the sun Heat Flow Solar interior Temp. incr. inward

Granulation … is the visible consequence of convection

Sun Spots (I)

Sun Spots (II) VisibleUltraviolet Cooler regions of the photosphere (T ≈ 4000 K). Active Regions

Solar Activity, seen in soft X-rays

Magnetic Fields in Sun Spots Magnetic fields on the photosphere can be measured through the Zeeman effect → Sun Spots are related to magnetic activity on the photosphere

Sun Spots (III) Related to magnetic activity. Magnetic field in sun spots is about 1000 times stronger than average. In sun spots, magnetic field lines emerge out of the photosphere. Magnetic North Poles Magnetic South Poles

Magnetic Loops Magnetic field lines Mass ejection from the sun often follow magnetic field loops.

The Solar Cycle 11-year cycle Reversal of magnetic polarity After 11 years, North/South order of leading/trailing sun spots is reversed => Total solar cycle = 22 years

The Sun’s Magnetic Cycle After 11 years, the magnetic field pattern becomes so complex that the field structure is re-arranged. → New magnetic field structure is similar to the original one, but reversed! → New 11-year cycle starts with reversed magnetic-field orientation

The Solar Cycle (II) Maunder Butterfly Diagram Sun spot cycle starts out with spots at higher latitudes on the sun Evolve to lower latitudes (towards the equator) throughout the cycle.

The Maunder Minimum The sun spot number also fluctuates on much longer time scales: Historical data indicate a very quiet phase of the sun, ~ 1650 – 1700: The Maunder Minimum

Prominences Looped Prominences: gas ejected from the sun’s photosphere, flowing along magnetic loops

Eruptive Prominences (Ultraviolet images) Extreme events, called coronal mass ejections (CMEs) and solar flares, can significantly influence Earth’s magnetic field structure and cause northern lights (aurora borealis). coronal mass ejections (CMEs) solar flares

Eruptive Prominences (Ultraviolet images)

Solar Aurora Sound waves produced by a solar flare ~ 5 minutes Coronal mass ejections

The Chromosphere Chromospheric structures visible in H  emission Region of sun’s atmosphere just above the photosphere. T: 4400 K → 25,000 K n: cm -3 → 10 7 cm -3 Absorption and emission lines from singly ionized metals (He II, Fe II, Si II, Cr II, Ca II)

The Chromosphere (II) Spicules = filaments of hot gas, visible in H  emission. Streams extend up to ~ 10,000 km above photosphere

The Transition Zone h ~ 2300 – 2600 km above photosphere Transition from moderate to high ionization T ~ 25,000 → 10 6 K n ~ 10 7 → 10 5 cm -3 Observe selective heights in (UV) emission lines of ionized metals Ly  → ~ 20,000 K CIII 977 → ~ 90,000 K O VI 1032 → ~ 300,000 K Mg X 625 → ~ 1,400,000 K

The Solar Corona Very hot (T ≥ 10 6 K), low-density (n ≤ 10 5 cm -3 ) gas

Active vs. Quiescent Corona Near Solar Maximum Near Solar Minimum

Coronal Holes X-ray images of the sun reveal coronal holes. These arise at the foot points of open field lines and are the origin of the solar wind.