! The Sun as a star! Structure of the Sun! The Solar Cycle! Solar Activity! Solar Wind! Observing the Sun The Sun & Solar Activity
The Sun in Perspective
Planck s Law for Black Body Radiation ν = c / λ I ν (T) I λ (T) Figure modified from Eric W. Weisstein
Wien s Displacement Law Setting the derivative of Planck s Law with respect to λ (wavelength) equal to zero, we determine the peak wavelength with respect to temperature. λ = b / T, where b = 2.898 x 10 6 nm/k
Solar Spectrum, Variability, and Atmospheric Absorption
Source: Wikipedia - Kelvinsong Solar Structure
Solar Structure " Core r < 0.3 R S " Radiative Zone 0.3 R S < r < 0.7 R S " Convective Zone r > 0.7 R S " The Photosphere -- visible surface of the sun " The Solar Atmosphere R S ~ 6.96 x 10 8 m ~ 110 x R Earth
Core Nuclear Fusion 1 H + 1 H # 2 H + e + + ν e e - + e + # 2 γ 2 H + 1 H # 3 He + γ Proton-Proton Chain 3 He + 3 He # 4 He + 1 H + 1 H Overall Reaction: 4 1 H + 2e - # 4 He + 2ν e + 6γ ΔE = [4(1.007825u) 4.002603u]*[931MeV/u] ΔE = 26.7 MeV
" Radiative Zone: 0.3 R S < r < 0.7 R S Solar Structure The radiative zone is a region of highly ionized gas. There the energy transport is primarily by photon diffusion, where the energy is passed randomly from atom to atom via absorption and emission. Radiative Zone Image modified from: UCB's Center for Science Education
Solar Structure " Tachocline: ~0.7 R S (.04 R s in width) The transition region of the Sun between the radiative interior and the differentially rotating outer convective zone. Contains large shear flows and plays a role in solar magnetic field reversals
Solar Structure " Convective Zone: 0.7 R S < r < 1 R S Here many atomic absorption processes occur, but emission does not. Since photon radiation does not continue outward, steep temperature gradients are established which lead to convective currents.
Solar Structure " The Photosphere -- Here the plasma becomes transparent to the optical spectrum, allowing for the escape of most of the electromagnetic energy reaching that layer. Hence, the Photosphere is the visible surface of the sun. The depth past which the gas begins to get so dense that we can not see through it, but evidence of the convection zone are visible as granules.
Solar Structure " The Solar Atmosphere " The Photosphere -- Base of the Solar Atmosphere " The Chromosphere (~2000km thick) " The Transition Zone " The Corona " The Heliosphere
The Solar Atmosphere: Chromosphere More visually transparent than the photosphere beneath, the chromosphere is best seen in the Hα emission line at 656 nm (red). It extends ~2000km above the photosphere and is also much hotter!
The Solar Atmosphere: Transition Region Hotter still is the Corona beyond, and the mysterious region that lies between it and the chromosphere is aptly called the transition region. The transition is many fold, it is where gravity stops dominating the shape of the Sun, and dynamic processes take over. It is also the region where magnetic fields start to dominate the shape and motion of structures. Image from TRACE
The Solar Atmosphere: Corona So diffuse it is only visible during eclipse, but over 1 million Kelvin. Highly ionized, the charged particles travel along magnetic field from the sun, creating the visible Helmut streamers. Plasma in the corona moves on average ~ 1 million miles/hr.
The Solar Atmosphere: The Heliosphere As the Solar Atmosphere expands out into the solar system it is eventually stopped by the average solar wind of all other stars in the solar system -- in essence, this is the edge of the Sun s atmosphere. The extent of the Sun s influence is defined as the Heliosphere.
! The Sun as a star! Structure of the Sun! The Solar Cycle! Solar Activity! Solar Wind! Observing the Sun The Sun & Solar Activity
Solar Activity The Solar Cycle " The Sun has an internally generated magnetic field that reverses about every 11 years (~22 years for magnetic cycle) " First noticed through the variation in the number of sunspots " Later recognized by the level of energetic activity on the surface and its impact on the Earth
Solar Activity The Solar Cycle # of Sunspots
Solar Activity The Solar Cycle: Magnetic Reversals Dipole field internal to the sun, complicated by shearing the rotation rates at different radii.
Solar Activity The Solar Cycle: Magnetic Reversals The Sun s surface also differentially rotates, with the equator rotating faster than the poles, the magnetic field of the Sun above the surface is dragged along by the plasma.
http://svs.gsfc.nasa.gov/search/series/solardynamo.html The Solar Cycle: Magnetic Reversals The Sun s surface also differentially rotates, with the equator rotating faster than the poles, the magnetic field of the Sun above the surface is dragged along by the plasma.
Solar Activity The Solar Cycle: Magnetic Reversals
Solar Activity 1 Sun Spots " Cool dark regions of intense magnetic field: " Come in pairs of opposite polarity " Rotate along with the surface of the sun " Last from a few days to weeks " Increase in numbers before magnetic reversal " Decrease in latitude of formation before magnetic reversal
1 Sun Spots Solar Activity
1 Sun Spots Solar Activity
1 Sun Spots Solar Activity
Solar Activity 2 Solar Flares " Large release of energy: " Generate X-rays and sometimes even gamma rays " Magnetic loops colliding together " 10,000-100,000 km in size
2 Solar Flares Solar Activity
3 Coronal Holes Solar Activity " Release fast moving plasma continuously into space: " No magnetic confinement " Dark Areas in X-ray images " Large during solar minimum, smaller closer to solar max
3 Coronal Holes Solar Activity
Solar Activity 4 Coronal Mass Ejections " Very large release of energy and energetic particles: " Generates X-rays " Releases very energetic ions " Larger than the Sun EAS 4360/6360 6:27
Solar Activity 4 Coronal Mass Ejections EAS 4360/6360 6:28
Solar Activity 4 Coronal Mass Ejections " Very large release of energy and energetic particles: " Generates X-rays " Releases very energetic ions " Larger than the Sun EAS 4360/6360 6:29
Solar Wind at Solar Minimum and Max EAS 4360/6360 6:30
The Solar Wind McComas, D. J. et al., GRL, 1998
Solar Wind Propagation At Solar Minimum the Sun s magnetic field is very dipolar, and the solar wind carries the magnetic field radially outward, creating a neutral current sheet.
Solar Wind Propagation Average Solar Wind Conditions at 1 AU (Earth orbital distance): Density of the Solar Wind ~ 5-10 particles/cm 3 Velocity of the Solar Wind ~450 km/s Temperature of the Solar Wind ~ 10 6 K Magnetic Field Strength ~ 6 nt Magnetic Field Orientation ~ variable
Solar Wind Propagation Eugene Parker -- Parker Spiral However, there are two additional effects, the rotation of the Sun, and the fact that the magnetic moment is not perfectly aligned with the rotation axis. These two effects create a spiral with ripples, or ballerina skirt effect. Compliments of the WSO
Solar Wind Propagation Movie compliments of University of Alberta
Solar Activity Propagation
Solar Wind Propagation Solar activity can also generally be tracked along the predicted Parker Spiral. Take these two flare events on Oct. 25th and 30th.
Discussion: Homework & Reading Reminder: HW assignment has 2 components: 1. 1. The first is to read the article by Shadia Rifia Habbal and Richard Woo. (2004) The solar wind and the Sun-Earth link, Astronomy and Geophysics, vol. 45, pp. 4.38-4.43. Describe three topics/questions raised in the paper that you thought were interesting. 2. Read the article by JW Cirtain (2013) Energy release in the solar corona from spatially resolved magnetic braids, Nature, vol. 493, pp. 501-3. Summarize in about a page how the observations were made and why they are important! This will be due on Monday, January 30.
Potential Project Topics! Solar wind comet interactions! Observing the heliosphere/heliospheric current sheet! Solar wind interaction with unmagnetized objects! Substorm evolution/signatures at Earth! Evolution of Earth s radiation belts during storms! Aurora of the outer planets! Planetary satellites embedded in large-scale magnetospheres! Shielding against radiation hazards in space! Plasma properties applied to advanced propulsion! What are you interested in learning more about?!