Download PHY2505S Atmospheric Radiation & Remote Sensing Lecture 3 23

PHY2505S Atmospheric Radiation & Remote Sensing
Lecture 4
23/1/03
The Solar Radiation Source
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
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
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,
T
= 2GmpM/3kR= 1.56 x 107K
mp=1.67 x 10-27 kg
Temperature structure
Liou, Figure 2.1, 2.2
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
http://www.coseti.org/highspec.htm
Solar absorption spectrum
Lines
Due to
Wavelengths
A-band
O2
7594 - 7621
B - (band)
O2
6867 - 6884
C
H
6563
a - (band)
O2
6276 - 6287
D - 1, 2
Na
5896 & 5890
E
Fe
5270
b - 1, 2
Mg
5184 & 5173
c
Fe
4958
F
H
4861
d
Fe
4668
e
Fe
4384
f
H
4340
G
Fe & Ca
4308
Solar constant
• Calculate radiance in the direction of the sun
• Flux normal to the beam is
F=IsDW =
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?
Ground-based (long) method
Instrument measures
I=Io
e-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
Z
Io
I
q
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
Diurnal variation
Insolation, Q =  F (t )dt  S  ro 
r
t
 ro 
 S 
r

2 sunset
 cosq o t dt
sunrise
2 H
 sin  sin d  cos cosd cosh w
dh
H
2
S  ro 
  sin  sin dH  cos cos d sin H 
pr
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|
Daily mean insolation (Q/24 hours)
Eqinoxes
Solar
declination, d
Liou, Figure 2.8
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
Satellite results
http://www.ngdc.noaa.gov/stp/SOLAR/IRRADIANCE/irrad.html
•
•
•
Offsets between instruments
Solar maxima, minima
Smallscale variability
Offsets between instruments
.
SSM/UARS Active Cavity Radiometer Irradiance Monitor (ACRIM)
principle
The
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
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/
Sunspot cycle
The sunspot number has been seen to vary with a period from maximim to
maximum of ~11 years.
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.
Any theory to explain sunspots must also explain the butterfly effect of their motion:
..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,
and changes to the
solar wind
flares
Solar and Heliospheric Observatory
http://sohowww.nascom.nasa.gov/
L1 point - an uninterrupted view of the sun
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
Magnetohydrodynamics
Solar activity is thought to be due to
interaction between the sun’s magnetic
field, solar rotation rate, and convection
SOHO movies