PHY2505S Atmospheric Radiation & Remote SensingLecture 423/1/03

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 fusion4 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 coreCan 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

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

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

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Solar constant

• Calculate radiance in the direction of the sun• Flux normal to the beam is

F=Is= T4 = 5.67e-8 x (5800)4 x 6.8e-5/ = 4363/= 1388.8K

This is the solar constant, S

How do we measure this?

Ground-based (long) method

Instrument measures I=Ioe-kz/cos

Plot ln(I) = ln (Io) – kzsecExtrapolate back to secto giveIntegrate overMultiply by

“Long“ method as takes 2-3 hours of measurementto calculate Io

Errors: large zenith anglenon-homogeneity multiple scatteringopaque regions of atmosphere

ZIo I

Variability due to orbit

Liou Figures 2.5 & 2.6

F(t)=S (ro/r)2 cos o

ro=mean distanceo=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 anglecos o = sin sin + cos cos cos h

Where = latituded = solar declinationh= hour angle

Solar noon, h=0Each hour h=+15 degrees

Diurnal variation

Insolation, Q =

where angular velocity of the Earth, =dh/dtH = half solar day (radians)cos H=-tan tan

If (equator) or(equinoxes) then cos Hand the length of the solar day is 12 hours

The latitude of the polar night H=0: =90-||

HHr

rS

dh

r

rS

dttr

rSdttF

o

H

H

o

sunset

sunriseo

o

t

sincoscossinsin

coshcoscossinsin

cos)(

2

2

2

Daily mean insolation (Q/24 hours)

Liou, Figure 2.8

Eqinoxes

Solar declination,

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)

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

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.

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.

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

..And other observed variability in the sun

Solar and Heliospheric Observatoryhttp://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