The Interior Structure of the Sun Data for one of many model calculations of the Sun center Temperature 1.57 10 7 K Pressure 2.34 10 16 N m -2 Density 1.53 10 5 kg m -3 Hydrogen 0.3397 Helium 0.6405
The Abundance of Elements in the Suns Interior The original composition of the Sun is still unaltered at large radii Helium-3 is an intermediate element in the pp-chain, the main energy producing chain of the present day Sun The abundance of helium-3 indicates the outer region of hydrogen burning
Temperature, Pressure and Luminosity Profiles The largest contribution to nuclear energy production occurs at about 0.1 solar radius Here the Suns luminosity increase is the largest
Density and Interior Mass About 90% of the solar mass are contained within roughly a half solar radius Integration of the interior mass gradient yields the solar mass function dm r = 4 π r 2 ρ dr
Convection and Radiation Zones The condition for the onset of convection was given by d T dr act > d T dr ad Assuming a monatomic gas we can derive d ln P d ln T < 2.5 The opacity becomes large enough at large radii to prevent radiation
The Photosphere The region from which photons originate is called photosphere The change from optically thick to thin occurs in only about 600 km Optical depth and opacity are wavelength dependent The base of the photosphere is arbitrarily defined as 100 km below the level where the optical depth at a wavelength of 500 nm is unity At this depth T 9400 K The temperature reaches a minimum about 525 km above this depth with a value of T 4400 K About 10-7 hydrogen atoms are H - ions which contribute mainly to the continuum blackbody radiation in the infrared and visible spectrum
Solar Granulation The base of the photosphere appears granulated into brighter and darker regions The regions have spatial extent of about 700 km and a lifetime of about five to ten minutes This lifetime is about the time a convective eddy needs to rise and fall
Differential Rotation One full rotation of the Sun takes 25.38 (sidereal) days at the equator and up to 36 days at the poles The solar radius at which radiation and convection zones border (the base of the convection zone) is called the tachocline where large shear forces result likely in the generation of the solar magnetic field tachocline
The Chromosphere The chromosphere is about 10,000 times less dense as compared to the photosphere The luminosity is also about 10-4 of that of the photosphere The temperature rises from 4400 K to about 10,000 K In this region absorption lines also of heavier elements can be found, like ionized He II, Fe II, Si II, Cr II, Ca II
The Transition Region In this region the temperature rises to about 10 5 K New absorption lines of highly ionized atoms appear C III at 90,000 K O VI at 300,000 K Mg X at 1.4 10 6 K log ρ / g cm -3
The Corona Defining three regions depending on the type of radiation K corona from 1 to 2.3 R - continuous photospheric white light scattered by free electrons (Doppler broadening of lines) F corona beyond 2.3 R - scattering of photospheric light by dust (Fraunhofer lines visible) E corona - emission from highly ionized atoms
Coronal Holes The X-ray image from Hinode shows non-uniform emission Spots of increased X-ray emission exist a few hours Coronal holes correspond to regions of open magnetic field lines Bright X-ray regions to closed field lines Dipolar magnetic field strength at the surface is about 10-4 T Solar wind is generated by open magnetic field lines where charged particles can escape
The Solar Cycle Sunspots where first observed by Galileo Sunspots exist for about a month Their number are observed to follow an eleven year cycle of maxima and minima We are presently recovering from a long minimum The latitude of sunspots also follow a pattern (butterfly plot) Sunspots consist of an inner part umbra and an outer filament-like penumbra Often a large pair with smaller sunspots is observed Magnetic fields can be measured via the Zeeman effect Galileo Galilei (1564-1642)
The Butterfly Plot
Maunder Minimum Larger time-scale sunspot cycles have been observed historically From 1645-1715 very few sunspots were observed This sunspot minimum coincides with the small Ice-Age in Europe Carbon-14 production in the atmosphere through cosmic rays increased significantly during this period
The Solar Magnetic Field Sunspots have a bipolar character, the lead sun spot always has the same polarity and the following spots the opposite polarity The lead spot has opposite polarity on North and South hemisphere The cycle reverses after eleven years A full cycle actually lasts for 22 years
Solar Flares Sunspots can be connected through solar flares up to 100,000 km long and lasting seconds to more than an hour Disturbance in the magnetic field can locally produce a sheet of current in the highly conducting plasma The resistance of the plasma leads to heating up to 10 7 K Electromagnetic radiation is emitted from the flare Released energy 10 17 J to 10 25 J
Solar Prominence Solar Prominences are similar in nature to flares They are build by ionized gas forming often an arc Eruptive prominences are ejection of gas into space and last for a few hours
Coronal Mass Ejection About one CME per day when averaged over 11-year sunspot cycle with maxima about 3.5 CME per day Ejection of 5 10 12 kg to 5 10 13 kg with speeds between 400 km/s and 1000 km/s CMEs can be considered as a magnetic bubble
Coronal Mass Ejection Electrons get accelerated along magnetic field lines attaining speeds larger than the solar wind A shock front can form, which leads to the force ejecting gas into space SOHO
Video by SOHO Solar and Heliospheric Observatory SOHO
Magnetic Dynamo Theory Horace Babcock proposed a magneto dynamo model to describe the solar cycle (it cannot describe all features of solar activity) The gas differential rotation drags field lines from dipolar shape into a shape with a toroidal component Turbulent convection zones twist the field lines creating strong magnetic fields, called ropes The magnetic ropes rise to the Suns surface appearing as sunspot groups
Magnetic Dynamo Theory In this picture, the initial little twisting occurs at higher latitudes during sunspot minimum Continuing twisting leads to sunspots also forming at lower latitudes At the equator, opposite polarities cancel each other leading to less sunspots The equatorial cancellation leads after 11 years to a new, reversed cycle