Sun's Atmosphere Lecture Notes PDF

Atmosphere of the Sun  Sun does not rotate as one mass Breakdown in rigid rotation near base of convection zone Different rotation rate with depth at different latitudes https:/...

Atmosphere of the Sun  Sun does not rotate as one mass Breakdown in rigid rotation near base of convection zone Different rotation rate with depth at different latitudes https://space.com https://space.com  The Sun has a magnetic field, generated in the convection zone The convection zone contains bands of flow which vary with time => The Sun's magnetic field varies with time Solar activity and the solar cycle Photosphere of Sun often shows small dark features which rotate with the Sun - Sunspots Sunspots come and go (the largest may last 2-3 months) and their average numbers vary with time Sunspot Galileo’s drawings Solar Space Telescope Sunspots are regions where the Sun's magnetic field is so strong that it slows down convection Also get regions of weaker field where convection is stronger and the photosphere is hotter – faculae Magnetic field breaks through photosphere in sunspots Sunspots often occur in pairs or multiples (N & S magnetic points) Sunspot pairs (or multiples) linked by loops of magnetic field above the photosphere Image courtesy of Dr. Dan Brown Sunspots first emerge at ~30°N/S after solar minimum Number of sunspots increases towards solar maximum and location moved down in latitude In declining phase of cycle sunspot location moves towards equator – fields cancel  How active is the Sun today? Sunspots can affect the Earth Although the fraction of the solar disk occupied by sunspots is exceedingly small, the magnetic perturbations associated with them can affect the upper layers of Earth’s atmosphere. Best example: Maunder minimum, an almost total absence of spots between 1640 and 1715. Its occurrence correlates with historical records and tree ring evidence of unusually cold winters and heavy snow falls. Tree rings are narrower in cold winters and show an unusually high concentration of radioactive C14 produced high in the atmosphere by cosmic radiation from the galaxy. Connection between solar and terrestrial magnetic fields... Photosphere Limb darkening The limb (the "edge") of the Sun appears darker than the centre of the disc – why? Sun gets hotter with increasing depth Looking through photosphere – like looking through fog Look at limb – view photo-sphere at an angle so can only see into relatively cool layers Look at centre of disc - see deeper bright, hot layers Chromosphere ~ 2000 km deep Temperature ~ 6000 K at base, increases with height to 20000 K Lower density Glows red (Hydrogen- emission) Chromosphere is hotter than Photosphere! Heated by sound waves from photosphere? NASA SDO http://sdo.gsfc.nasa.gov ⚫ Plasma jets discovered in Chromospheric Spicules 1877 (Secchi), fibrils on disc ⚫ At least 100 000 present at any time ⚫ Provide significant mass and energy flux to the atmosphere ⚫ What is the generation mechanism? ⚫ The mechanism should act along the spicule https://www.nasa.gov/solar-system/scientists-uncover-origins-of-the-suns-swirling-spicules/ channel (macrospicules reach heights of 20 000 km) Filaments and Prominences Two names for the same thing On the disc of the Sun: Dark channels – Filaments Relatively cool, dense gas suspended above the chromosphere by loops of magnetic field Made up of chromospheric material Off the limb of the Sun: See the loop of material against the sky Bright suspended loops – Prominences CME observed by SDO/AIA, STEREO, and SoHO Solar maxima – ~ 3 CMEs every day Solar minima – ~ 1 CME every 5 days  Eclipses and the corona http://www.eclipse.org.uk/eclipse/0212006/ During total solar eclipses, the moon blocks out light from the photosphere Faint bright structures can be seen around the Sun – the Corona Extends visibly out to 2-3 solar radii Simple calculation – how hot does the corona have to be to support itself out to 2-3 RSun? About 2 million K... The photosphere is at ~5800 K, interstellar space is cold – how can the corona be so hot?  http://solar.physics.montana.edu/YPOP/Spotlight/SunInfo/Transregion.html Very sharp rise in temperature into corona Corona very hot Temperatures (and differences in temperatures between ions and electrons) vary across the corona Causes of coronal heating not yet fully understood  Active regions, coronal structure and solar activity http://sohowww.estec.esa.nl/ Photosphere Magnetic field Top of transition Corona (photosphere) region (80000 K) 1.3x106 K Hot loops overlie sunspot groups – Active regions Strong magnetic fields link all parts of an active region Connection between strong magnetic field and hot corona Corona Corona 2x106 K 1.6x106 K Coronal loops Why does temperature increase with height? Why is the corona so hot? Corona can't be heated from photosphere (cooler) Heated from somewhere hotter? Energy carried as tension in magnetic field, released in chromosphere as magnetic waves, dissipated in corona as heat We do not know the details yet  Questions What are sunspots and how do they evolve? Describe the chromosphere and its features How do we explain the heating of the solar atmosphere?  Post-Session Activity Could Solar Storms Destroy Civilization? Solar Flares & Coronal Mass Ejections How active is the Sun today?