PHY2505S Atmospheric Radiation

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• PHY2505S Atmospheric Radiation Remote
  SensingLecture 423/1/03
• The Solar Radiation Source

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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

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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

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The Sun our nearest star

• 300,000 times closer to Earth than next
  nearest star
• Energy nuclear fusion
• 4 1H 2 e --gt 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, mp1.67 x 10-27 kg
• T 2GmpM/3kR 1.56 x 107K

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Temperature structure

• Liou, Figure 2.1, 2.2

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Photosphere, chronosphere, corona

• Photosphere
• Visible light from thin layer 400km thick,
  surface at temperature5800K, continuous
  radiation
• UV continuum (1 of solar outpur)
• Chronosphere corona
• EUV, 120gt l gt 30nm
• solar spectral lines

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Solar absorption spectrum
http//www.coseti.org/highspec.htm
 
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Solar constant

• Calculate radiance in the direction of the sun
• Flux normal to the beam is
• FIsDW sT4 DW/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?

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Ground-based (long) method

• Instrument measures IIoe-krz/cosq
• Plot ln(I) ln (Io) krzsecq
• Extrapolate back to secq0 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

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Variability due to orbit
F(t)S (ro/r)2 cos qo romean
distance qosolar zenith angle Eccentricity, e
0.017 Major axis ro(1e) Minor axis
ro(1-e) Variation ((1e)/(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, h0 Each
hour h15 degrees
Liou Figures 2.5 2.6
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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 y0 (equator) or d0 (equinoxes) then cos H 0
  and the length of the solar day is 12 hours
• The latitude of the polar night H0 y90-d

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Daily mean insolation (Q/24 hours)
Eqinoxes Solar declination, d

• Liou, Figure 2.8

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Satellite measurements of S

• NIMBUS-7 16 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

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Satellite results

• http//www.ngdc.noaa.gov/stp/SOLAR/IRRADIANCE/i
  rrad.html
• Offsets between instruments
• Solar maxima, minima
• Smallscale variability

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Offsets between instruments

• . SSM/UARS Active Cavity Radiometer
  Irradiance Monitor (ACRIM)
• The principle 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

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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. Galileos
  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//scien
ce.msfc.nasa.gov/ssl/pad/solar/
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Sunspot cycle

• 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

The "sunspot number" the sum of the number of
individual sunspots and 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.
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..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
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Solar and Heliospheric Observatoryhttp//sohowww.
nascom.nasa.gov/L1 point - an uninterrupted view
of the sun
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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

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Magnetohydrodynamics

• Solar activity is thought to be due to
  interaction between the suns magnetic field,
  solar rotation rate, and convection

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SOHO movies