AOS 101 Weather and Climate
Lisha M. Roubert
University of Wisconsin-Madison
Department of Atmospheric & Oceanic Sciences
• The Sun is the star at the center of
the Solar System. It is our source of
energy and heat, which sustains life on
Earth, and controls our climate and
weather.
• It generates energy by fusing hydrogen
nuclei into Helium.
• While it looks small, a hundred earths
could fit inside it. The moon looks larger,
because it’s only 400,000 km from the
earth, as opposed to the sun, which is
150,000,000 km away.
• Due to this distance, the sun looks
relatively serene and constant. However,
it is nothing of the sort!
• Observing the sun closely is
difficult because it is so bright.
Two methods are pinhole
telescopes, and filters.
• A common filter used to study
the solar surface is an H-Alpha
filter. It produces an image like
the one to the right. This can
show convective cells and
plasma filaments.
• Another way to look at
the sun is to photograph
the x-rays coming from
it.
• The image at the left
shows this.
• X-rays tend to show the
outer layers of the sun,
and the complex
magnetic fields.
• From these observations, it
became clear that the sun
isn’t constant. The first
observations of this were
done by the ancient Chinese,
who looked at the sun’s
reflection in mud puddles.
They could see spots come
and go on the sun. These
became called sunspots,
although they are actually
massive arches of plasma
following magnetic fields.
sunspots
• Over time, scientists began to describe a “solar
cycle”. The solar cycle is a process during which the
magnetic field of the sun flips polarity. In other
words, the Sun's north pole becomes its south
pole.
• The changing magnetic field can cause a great
amount of solar activity, increasing the number
of sunspots, solar flares and coronal mass
ejections.
▫ At the solar maximum sunspots and solar flares
occur regularly
▫ 1/2 way through the cycle solar activity
decreases
• During the last solar cycle, there were 805 days
with no sunspots, which is the longest time
without since serious solar study began in 1755.
http://www.windows2universe.org/sun/ac
tivity/sun_mag_field_rotate_tangle.html
• Differential Rotation
▫ The sun rotates faster towards the equator because it is
not a solid body. This difference in rotation speed
causes the magnetic field to wrap around the sun.
• Sunspots are
temporary phenomena on the
surface of the Sun that
appear visibly as dark spots
compared to surrounding regions.
• They are caused by
intense magnetic activity, which
inhibits convection, forming areas of
reduced surface temperature.
• Sunspots expand and contract as
they move across the surface of the
sun and can be as large as
80,000 kilometers in diameter,
making the larger ones visible from
Earth without the aid of a telescope.
• The number of sunspots correlates
with the intensity of solar radiation,
thus affecting the amount of solar
irradiance we receive on Earth.
• The surrounding margins of
sunspots are brighter than the
average, and so are hotter; overall,
more sunspots increase the sun's
solar constant or brightness.
• However, the variation caused by
the sunspot cycle to solar output is
relatively small, on the order of 0.1%
of the solar constant. The effects of
this on Earth’s temperature is
relatively un-measurable.
• Homework: Find two current solar study programs and
describe in a paragraph for each one how they are studying
the sun.
▫ 1 space-based
▫ 1 ground based
• We will have a discussion next time based on this.