Lecture 20: Solar Variability

Lecture 20: Solar Variability

1. Sunspots and solar radiation

   • High sunspot abundance = strong solar emissions.
     • From the Sun's polar regions and bright margins of sunspots.
   • Sunspots
     • Temporary features visible on the Sun's surface.
     • Caused by slightly lower temperatures that reflect concentrations of magnetic flux inhibiting convection.
     • The brighter area around sunspots represent solar flares that are responsible for altering the total radiation emitted by the sun.
     • Occur over an 11-year cycle (known as the solar cycle).
       • Defined by astronomer Rudolf Wolf.
         • Cycle 1 is between 1755-1766.
         • We are currently reaching the end of cycle 24.

2. Sunspot cycles

   • Cause 0.1% variability in total irradiance output.
   • Vary over time.
   • First observed by Spörer and Maunder during the Maunder Minimum (1645-1715).
     • The Maunder Minimum was a period of low sunspot activity.

3. The radiation spectrum

   • There are changes in the spectrum of radiation emitted during these cycles.
     • Extreme Ultra Violet radiation (EUV) peaks at the solar maxima, increasing by 10%.

4. Long-term sunspot cycles

   • The solar index has been constructed to cover the last 12,000 years.
     • Data used in climate models.

5. Cosmogenic isotopes

   • Cosmic rays were first visualised using cloud chambers, which display ionising particles.
   • The radiocarbon timescale is calibrated using dendrochronology (by lining up tree rings).
     • Ice core records also contain evidence supporting solar variability in the form of cosmogenic isotopes beryllium-10 and carbon-14.
   • Carbon-14 production varied with the solar index throughout history.
   • Given that cosmic particles are required to form carbon-14, having a weaker magnetic field leads to faster carbon-14 production.
     • Weaker magnetic field = weaker solar shielding = more bombardment by cosmic particles = faster carbon-14 production.
     • This is largely controlled by a 420-year cycle.
   • Carbon-14 cycle
     • 1. Carbon-14 forms in the upper atmosphere when free neutrons collide with nitrogen-14 (a cosmic particle).
     • 2. Because it is an unstable isotope, nitrogen-14 undergoes radioactive decay and forms carbon-14.
       • This occurs over ~15,000 years.
       • Beta particles are emitted during decay.
     • 3. Carbon-14 is oxidised into carbon(-14) dioxide which mixes with carbon(-12) dioxide through the atmosphere and hydrosphere.
       • It is then incorporated into the biosphere (e.g. via photosynthesis).
       • Organisms cannot ingest neat carbon-14.
     • 4. Carbon-14 is incorporated into plants. When they die, carbon-14 begins to decay back into nitrogen.
       • So scientists measure how much C-14 remains in an object to estimate its age.

6. Climate

   • The 'hockey stick curve' shows temperatures over the last millennium (1000 years).
     • Temperature records are reflected in shell increments and tree rings.
     • Temperature records also partly line up with solar variability records (as temperature is also affected by other factors).

7. The Little Ice Age

   • Took place approximately between 1500 and 1900.
   • A period of high Icelandic sea ice coverage.