Solar Radiation

• Source– Hertzprung Russel Diagram– Solar structure– Sunspots– Solar spectrum 1

• Interaction with Earth’s atmosphere– Solar spectrum– Atmospheric Impact – Aurora

Solar Radiation Sun - typical main sequence dwarf starRo = 6.95 x 105 km, Mo = 2 x 1030 kg71% H, 26.5% He, 2.5% heavier metalsMean earth sun distance 1.496 x 108 km

Core out to 0.25 Ro Temperature ~ 107 K Hot enough to cause fusion of H in to He provides energy for the sun.

Energy transfer by radiation, temperature ~ 106 K, ~0.7 Ro

Final ~0.3 Ro energy transfer by convection violent small scale (103 km) mixing process granulation

Photosphere ~ 1000 km thick, dominant source of solar radiation - relatively continuous emission - determines sun's blackbody temperature, 5800 K. Sunspots here.

Chromosphere - 5000 - 10,000 km thick, temperature ~ 105 - 106 K. Radiation from emission lines from H, He, Ca.

Corona - region above chromosphere, extending out for several solar diameters, temperature ~ 106 K. Prominences, Solar flares appear here. Origin of solar wind.

Most solar energy in the continuum is from non-quantized electronic transitions (free - free, bound - free transitions)

Sunspots• Properties

– Dark regions in photosphere with diameters of 103 – 105 km,

~3000 K, cooler than photosphere by nearly 50%.– Never cover more than 0.2% of solar surface and persist ~ a

week.– Number 1 to > 50 in 11 year cycle minimum to minimum.

• Causes– Variations in magnetic activity from interactions between

convection, solar rotation, and magnetic field. – Have magnetic fields up to 4000 Gauss. – Polarity in sunspot pairs reverses every solar cycle so complete

sunspot cycle is sometimes said to be 22 years.

Sunspots • Associated disturbances

– Faculae: Enhanced photospheric emission surrounding sunspots precede and follow sunspots.

– Plages of Flocculi: Bright areas in chromosphere, precede and follow sunspots, occasionally brighten to solar flares.

– Solar flares - intense (minutes to hours) eruptive phenomena, accompanied by rapid enhancement of emission of ionizing radiation, EUV (Extreme UV) and X-rays, cause terrestrial magnetic disturbances and radio fadeouts.

– Prominences - filament like protuberance extending from chromosphere, photospheric eruptions causing large stable clouds of gas in upper part of chromosphere. More occur at solar maximum.

– Corona is more irregular at sunspot maximum.– Measurable solar variability is limited to < 240 nm. At larger

wavelengths the variations are less than instrumental uncertainty.

http://www.pmodwrc.ch/tsi/composite/pics/org_comp2_d41_62_1009.png

http://blog.ltc.arizona.edu/azmasternaturalist/Sunspot%20cycle.JPG

http://solarscience.msfc.nasa.gov/images/ssn_predict_l.gif

http://solarscience.msfc.nasa.gov/images/ssn_predict_l.gif

Measurable solar variability is limited to < 240 nm. At larger wavelengths the variations are less than instrumental uncertainty.

Solar spectrum• X-rays, < 1 nm. Variations of > 1000

observed 0.1 - 0.8 nm over a solar cycle or during solar flares.

• Extreme UV measurements, SOHO• Below 140 nm emission from

chromosphere and corona dominant. Lyman alpha,120 nm, carries more energy than rest of spectra < 100 nm during quiet sun periods.

• As increases from 120 to 300 nm the solar radiance source moves from chromosphere to photosphere, and solar variations diminish to undetectability.

• Fraunhoffer lines a cold gas between a source and observer will absorb radiation ( > 200 nm). A hot gas will emit radiation ( < 200 nm).

Fraunhoffer Lines

• First observed by Wollatson (1802). Fraunhoffer independently discovered them in 1814 and studied them thoroughly.

• The continuous emission of photosphere is interrupted by selective absorption and re-emission in upper photosphere. Seen in visible and IR spectra only as absorption lines. Lines at < 185 nm appear in emission.

• Fraunhoffer mapped over 570 lines. Later these lines were associated with atoms/molecules in the upper solar atmosphere.

Na O2Fe

H – Balmer series

Atmospheric absorption of solar

radiation

• Only photons corresponding to changes in discrete energy states are absorbed absorption appears as a series of discrete lines, or absorption bands.

• Atmospheric molecules are characterized by discrete – Electronic transitions, ionization, disassociation

• Relatively high energies, several eV <= UV, few transitions in the visible.

• These transitions induce photochemistry.• Less than 1% of solar photons at <= UV.

– Vibrational and rotational energy states. • Lower energy so appear for >= IR.• Primary impacts on thermal structure of the atmosphere, so

important for meterorology, and for radiative heat balance.

Solar radiation photochemistry, < 4 m

• Photochemical consequences– X-rays ionization of

oxygen/nitrogen, D region thermosphere.

– 122 nm, Lyman α, absorbed by O2 in mesosphere.

– 100-175 nm, Schumann-Runge continuum, absorption by O2 in mesosphere.

– 175-200 nm, Schumann-Runge bands, absorption by O2 in mesosphere.

– 200-242 nm, Herzberg continuum, absorption by O2 in stratsophere.

– 242-310 nm O3 Hartley band, absorption by O3 in stratosphere leading to formation of O(1d).

– 310-400 nm, O3 Huggins bands, absorption by O3 in stratosphere/troposphere leading to formation of O(3P).

– Penetration of solar radiation into the earth's atmosphere depends on atmospheric composition and absorption bands.

Solar radiation thermal impacts, < 4 m

• Thermal consequences– Thermosphere and upper

mesosphere: absorption of UV by O2, O, N2, and N.

– Lower mesosphere and stratosphere: absorption of UV by O3. As ozone diminishes IR radiation from CO2 causes cooling to the mesopause. The presence of ozone in the stratosphere forms the tropopause, where the convectively forced layer of the atmosphere stops.

• Absorbed energy is converted to thermal energy through chemical reactions in the presence of a third body. When photodissasociated O atoms recombine to O2 and O3, the excess energy is carried away by a third body as kinetic energy, or temperature increase.

• This heating by absorption is balanced by cooling through IR emission from CO2 (15 m), O3 (9.6 m), and H2O (80 m).

– Troposphere, temperature controlled more by thermodynamics and dynamics than by radiation.

Primary atmospheric molecules involved in

absorption of radiation

• Atmospheric molecules are characterized by discrete rotational and vibrational energy states in addition to the electronic transitions.

• Transitions– Electronic – Ionization,

dissasociation – UV some visible.

– Vibrational – IR, far IR

– Rotational – far IR, microwave

• The lower energy states significantly impact terrestrial radiation, which peaks in the IR.

Bohren and Clothiaux, 2006

Aurora

• Caused by interaction of solar wind with oxygen and nitrogen in the thermosphere.

Solar Wind Animation

Courtesy: http://www.douglas-davis.com/energy.htm

Solar Wind Animation

comet

Aurora nearAurora far – Hut PointAurora all sky

Aurora

• Caused by interaction of solar wind with oxygen and nitrogen in the thermosphere.

• Color indicates level of electronic excitation. – Green (most common) or brownish red – Oxygen

returning to ground state. – Red – Nitrogen returning to ground state.– Blue – Nitrogen regaining an electron after ionization.

• Auroral pictures– aurora near,  aurora far – Hut Point,    aurora all sky,