PHY2505S Atmospheric Radiation & Remote Sensing Lecture 4 23/1/03 The Solar Radiation Source

1. PHY2505SAtmospheric Radiation & Remote SensingLecture 423/1/03 The Solar Radiation Source

2. The solar radiation source • The sun – our nearest star • Geophysical parameters • Temperature structure & composition • Photosphere, chronosphere, corona • Solar constant • Measurement • Diurnal & latitudinal variation • Satellite measurements • Solar variability • Sunspots • Solar flares • Prominences • Magnetohydrodynamics • SOHO movies

3. The Sun – our nearest star Solar radius = 695,990 km = 432,470 mi = 109 Earth radii Solar mass = 1.989 1030 kg = 4.376 1030 lb = 333,000 Earth masses Solar luminosity (energy output of the Sun) = 3.846 x1033 erg/s =3.846 x 1026W Surface temperature = 5770 ºK = 10,400 ºF Surface density = 2.07 10-7 g/cm3 = 1.6 x10-4 Air density Surface composition = 70% H, 28% He, 2% (C, N, O, ...) by mass Central temperature = 15,600,000 ºK = 28,000,000 ºF Central density = 150 g/cm3 = 8 × Gold density Central composition = 35% H, 63% He, 2% (C, N, O, ...) by mass Solar age = 4.57 109 yr From NASA Marshall Solar Physics:http://science.msfc.nasa.gov/ssl/pad/solar/default.htm

4. The Sun – our nearest star 300,000 times closer to Earth than next nearest star Energy: nuclear fusion 4 1H + 2 e --> 4He + 2 neutrinos + 6 photons Producing 26 MeV = 26 x 106 eV 0.3% hydrogen mass converted to energy 5% of solar mass converted to energy Temperature in the core Can estimate temperature in the core by PROTON: Thermal energy = gravitational energy 3/2kT =GmpM/R, mp=1.67 x 10-27 kg T = 2GmpM/3kR= 1.56 x 107K

5. Temperature structure Liou, Figure 2.1, 2.2

6. Photosphere, chronosphere, corona • Photosphere: • Visible light from thin layer 400km thick, surface at temperature~5800K, continuous radiation • UV continuum (1% of solar outpur) • Chronosphere & corona: • EUV, 120> l > 30nm • solar spectral lines

7. Solar absorption spectrum http://www.coseti.org/highspec.htm

8. Solar constant • Calculate radiance in the direction of the sun • Flux normal to the beam is F=IsDW = sT4DW/p = 5.67e-8 x (5800)4 x 6.8e-5/ p = 4363/p = 1388.8K This is the solar constant, S How do we measure this?

9. Z Io q I Ground-based (long) method Instrument measures I=Ioe-krz/cosq Plot ln(I) = ln (Io) – krzsecq Extrapolate back to secq=0 to give Io Integrate over l Multiply by DW “Long“ method as takes 2-3 hours of measurement to calculate Io Errors:large zenith angle non-homogeneity multiple scattering opaque regions of atmosphere

10. Variability due to orbit F(t)=S (ro/r)2 cos qo ro=mean distance qo=solar zenith angle Eccentricity, e= 0.017 Major axis ~ro(1+e) Minor axis~ ro(1-e) Variation =((1+e)/(1-e))2 ~7% Solar zenith angle cos qo = sin y sin d + cos y cos d cos h Where y= latitude d = solar declination h= hour angle Solar noon, h=0 Each hour h=+15 degrees Liou Figures 2.5 & 2.6

11. Diurnal variation Insolation, Q = where angular velocity of the Earth, w =dh/dt H = half solar day (radians) cos H=-tan y tan d If y=0 (equator) or d=0 (equinoxes) then cos H =0 and the length of the solar day is 12 hours The latitude of the polar night H=0: y=90-|d|

12. Daily mean insolation (Q/24 hours) Eqinoxes Solar declination, d Liou, Figure 2.8

13. Satellite measurements of S • NIMBUS-716 Nov 78-13 Dec 93 • Solar Maximum Mission (SMM) 16 Feb 80-01 Jun 89 • Earth Radiation Budget Satellite (ERBS)25 Oct 84-21 Dec 94 • NOAA-9 23 Jan 85-20 Dec 89 and 10 Oct 86-01 Apr 87 • Upper Atmospheric Research Satellite (UARS) 5 Oct 91-30 Sep 94 • Measured total solar irradiance, S, with radiometers equally sensitive across the full spectral range (EUV to far IR) • Typically 60 min orbit, with 35 min view of the sun

14. Satellite results • http://www.ngdc.noaa.gov/stp/SOLAR/IRRADIANCE/irrad.html • Offsets between instruments • Solar maxima, minima • Smallscale variability

15. Offsets between instruments .SSM/UARS Active Cavity Radiometer Irradiance Monitor (ACRIM) Theprinciple of measuring total solar irradiance is that the heating effect of irradiant flux on a detector is compared with that of electrical power dissipated in a heating element in intimate thermal contact with the detector. An accurate knowledge of the effective absorptance of the detector for the irradiant flux, the area over which the detector is illuminated and the electrical heating power facilitates the accurate measurement of irradiant fluxes on an absolute basis in the International System of Units. The total solar irradiance data, expressed in Watt per square meter at the instrument, are calculated based on the equation: S = K(Pref-Pobs)+E where S is the calculated irradiance, Pref and Pobs are the cavity electrical heating powers during the reference and observational phase of the measurements. K is the standard detector constant of proportionality which contains instrument parameters, such as the area of the primary aperture, effective cavity absorptance for solar irradiance, cavity reflectance for solar irradiance, and reflectance of solar radiation by the cavity field of view. E summarizes small terms due to small departures from instrument equilibrium. Corrections for temperature dependence, solar viewing angle, Sun-satellite distance and relative velocity, and sensor degradation From http://www.ngdc.noaa.gov/stp/SOLAR/IRRADIANCE/uars.html

16. Sunspots Sunspots have been observed for centuries. Early question was whether the dark blobs seen on the visible disc of the sun were planets passing across the disc or “clouds”. Galileo’s 1610 observations showed a foreshortening of the images over some days from which he interpreted correctly that the blobs must be on the surface of the sun http://www.exploratorium.edu/sunspots http://science.msfc.nasa.gov/ssl/pad/solar/

17. Sunspot cycle The "sunspot number" = the sum of the number of individual sunspotsand ten times the number of groups. Since most sunspot groups have, on average, about ten spots, this formula for counting sunspots gives reliable numbers even when the observing conditions are less than ideal and small spots are hard to see. The sunspot number has been seen to vary with a period from maximim to maximum of ~11 years. Any theory to explain sunspots must also explain the butterfly effect of their motion:

18. ..And other observed variability in the sun The total solar output (solar constant ) variation is found to correlate with the sunspot maximum and minimum cycle Solar activity is also seen in the form of prominences, flares and changes to the solar wind

19. Solar and Heliospheric Observatoryhttp://sohowww.nascom.nasa.gov/L1 point - an uninterrupted view of the sun

20. Prominences and flares • Prominences: huge clouds of relatively cool, dense plasma suspended in the Sun's hot, tenuous corona • Flares: enormous explosions in the surface of the sun, ejecting energy and matter – post flare loops are shown above http://science.msfc.nasa.gov/ssl/pad/solar/loops.htm

21. Magnetohydrodynamics Solar activity is thought to be due to interaction between the sun’s magnetic field, solar rotation rate, and convection

22. SOHO movies