MT4510 Solar Theory Thomas Neukirch

Sun Facts Earth Sun Radius 6 400 km 696 000 km 109 x Earth’s radius Volume 1.1 x 1012 km3 1.41 x 1018 km3 1 300 000 x Earth’s volume Mass 6 x 1024 kg 2 x 1030 kg 333 000 x Earth’s mass Density 5 500 kg/m3 1 400 kg/m3 1/4 x Earth’s density

Sun Facts Earth Sun Consistency Solid core: iron Electrically charged gas core: hydrogen Rotation rate 1 day 25 days - equator 33 days - poles ‘differential rotation’ Distance from Earth 149 000 000 km 23 000 x Earth’s radius Luminosity 4 x 1026 W 4 x 1025 light bulbs

Sun’s Nuclear Core Fusion of hydrogen in core produces gamma rays

Solar Interior Core Radiative Zone Convection Zone Photosphere (surface)

Helioseimology Used to study the interior of the Sun It is the study of resonant wave modes of oscillation of the Sun Modes of oscillation are visible manifestations of trapped standing sound waves Strongest periods ~ 5 mins

Solar Rotation Rate Relative rotation rates of material in the Sun - determined using helioseismology Red material – fastest Dark blue – slowest Interior: red moves ~ 4% faster than outer layers. Surface: red (equator) moves ~ 3,000 mph faster than blue (poles).

Solar Atmosphere Photosphere Chromosphere Corona

Observing Sun’s Atmosphere

Photosphere T ~ 6600-4300 K ρ ~ 10-8th of water P ~ 10-2th of atmosphere H ~ 100 km Visible light images reveal sunspots Magnetograms reveal surface magnetic fields Field into Sun – black Field out of Sun - white

Chromosphere 4300 K < T < 106 K ρ ~ 10-8-10-14 less than water P ~ not much! H ~ 2500 km Observed in many wavelengths, e.g., Ca II K (393.3 nm) H alpha (656.3 nm)

Corona T > 106 K (low corona) ρ < 10-14 less than water P ~ even less! H - to Earth & beyond! Observed in EUV (T~106 K) Soft X-ray (T> 2x106 K) White light

Corona X-ray bright point Coronal hole Coronal loops Active region

Coronal Heating Problem

Solar Cycle Solar Maximum Solar Minimum

Sunspots Magnetic field strength ~ 2-3x103 G Umbra Penumbra Magnetic field strength ~ 2-3x103 G Umbra – (almost) vertical field Penumbra – (more) horizontal field Number of sunspots varies over 11 year cycle

Magnetic field responsible for solar activity ! Sunspots = strong magnetic field Emission in all wavelengths correlates well with strong magnetic field regions Magnetic field dominates solar atmosphere (magnetic energy density >> pressure)

Prominences Situated in corona Cool, dense plasma confined in vertical sheets Lifetimes: days-months Size: 2x105 x 5x104 x 6x103 km3 (LxHxW) ρ: 10-12 kg cm-3 T: 5-10 x103 K

Solar Flares Sudden, impulsive, intense, large-scale, heating events Very energetic: from 1022 - 1025 J Lifetimes: hours

Coronal Mass Ejections (CMEs) Very large events that eject mass from the Sun Energy: 1025 J Mass: 1015 kg Lifetime: hours Usually related to a prominence eruption

Solar Wind Ions and electrons continually stream out along magnetic field lines Solar wind: Strong above poles (open field) Weak about equator (closed field)

Sun-Earth Connection Solar wind continually buffets Earth’s magnetic field CME’s can distort Earth’s magnetic field and cause Auroras

The Sun on 10 Feb 2008 EIT 304 EIT 171 EIT 195 EIT 284 MDI CONT MDI MAG LASCO C2 LASCO C3

Solar Theory Course § Topic Importance 1 Maxwell’s equations and magnetic fields Outline structure of events 2 MHD equations Describe behaviour of Sun 3 Magnetic reconnection and magnetic energy Flares, CMEs, coronal heating, solar wind 4 MHD equilibria Coronal arcades and loop structures, prominences 5 MHD waves Helio/coronal-seismology, coronal heating, solar wind 6 Solar wind Sun/Earth connection

The Sun (Summary)