Class 4 - Sunlight on Atmospheres

Read Chapter 2 of Atmospheres by Goody & Walker - pp17-43

Types of Photon Reactions

List the ways that solar photons can interact with an atmosphere.






After photo-reactions atoms & molecules can de-excite (i.e. radiate) or combine to form a new molecule or recombine (ion+electon -> neutral). The net result - PHOTOCHEMISTRY - is the distribution of molecules, atoms, ions and electrons of the atmosphere. E.g. for oxygen chemistry alone we have....

Before delving into the details of photochemistry - let's look at the source of photons - THE SUN.


Solar Radiation & Variability

Electromagnetic spectrum (Energy of photon inversely proportional to wavelength)

Thermal radiation laws:

  1. Weins Law - wavelength of maximum emission inversely proportional to T of emitting body
  2. Stephan-Boltzmann Law - Power emitted is proportional to T4

The "surface" of the Sun - photosphere - has a temperature of 5785K

Note this is a log-log plot

Here is a log-linear plot.... emphasizing the UV

... and a linear-linear plot

Earth's albedo = 0.39 = reflectivity at visible wavelengths

Note units.... Total flux of sunlight = 1368 / (distance in AU)2 W m2

But - while most of the energy flux from the Sun is in the visible part of the spectrum, it is the shorter wavelengths - UV and X-rays - that cause all the photochemistry.

Sun at different wavelengths - interactive

Solar images from High Altitude Observatory

Xrays on top, visible sunspots in the middle and the corona (eclipse or coronagraph pictures in visible light) on the bottom. This figure shows the huge variations in activity that occur over the Sun's 11-year solar activity cycle.

The Sun not only is a source of photons - but also of particles - both the ambient solar wind and bursts of energetic particles or cosmic rays. The corona is the source of the solar wind - protons and electrons that stream from the Sun. At the Earth the solar wind has a density of ~10 particles /cc moving at a speed of ~3-400 km/s. Space Weather is the impact on the Earth environment (particularly the space environment of the upper atmosphere and beyond) of the variations in the solar output of both.

For today's space weather go to the Space Environment Center, Boulder

So - variability at short wavelengths is obvious from the images in UV and x-rays - but to model the atmosphere we need to QUANTIFY the variability. First - how do we measure the variability at these short wavelengths? With difficulty...but that's where LASP comes in....

Measuring solar variability SIM and Sorce at LASP



Over longer periods of time....

How does the variation in sunspot number (easy to see and measure) relate to variation in UV flux (the quanity that matters for atmospheric photochemisty)? This is a tricky issue - we have to develop "proxies" - quantities we can measure now and compare with the current UV output of the Sun and then go back to historic measurements (e.g. from rocks, icecores, tree logs, etc) of these proxies.