\section{Introduction}
The Sun is a very important object for all life on Earth.  The nuclear
reactions which occur in its core produce the energy which plants and
animals need to survive.  We schedule our lives around the rising and
setting of the Sun in the sky.  During the summer, the Sun is higher
in the sky and thus warms us more than during the winter, when the Sun
stays low in the sky.  But the Sun's effect on Earth is even more 
complicated than these simple examples.

The Sun is the nearest star to us, which is both an advantage and a
disadvantage for astronomers who study stars.  Since the Sun is very close,
we know much more about the Sun than we know about other distant stars.
This complicates the picture quite a bit since we need to better understand
the physics going in the Sun in order to comprehend all our detailed 
observations.  This difference makes the job of solar astronomers
more difficult than the job of stellar astronomers.  It's a case of having
too much and too detailed data.

One of the things we know best about the Sun is the different layers
of gas which make it up.  Figure 1 is a skematic of the layers of the
Sun's interior and atmosphere.  Label the layers as your TA describes each one.
You should be able to find the tempertures of these layers in your textbook.

\begin{figure}[h]
\centering
\leavevmode
\epsfxsize=0.45\textwidth
\epsfbox{SUN/sun.eps}
\caption{Layers of the Sun}
\end{figure}

\begin{itemize}
\item{{\it Goals:}  to discuss the layers of the sun and solar phenomena; to use these concepts in conjunction with pictures to deduce characteristics of solar
flares, prominences, sunspots, and solar rotation}
\item{{\it Materials:}  Sun pictures, ruler, calculator}
\end{itemize}

\section{Layers of the Sun}
The interior of the Sun is made up of three layers:  the core, the radiative zone, and the convective zone.  The $core$ of the Sun is very hot and dense.
This is the only place in the Sun where the temperature and pressure are high 
enough to support nuclear reactions.  The $radiative~zone$ is the region of the sun where the energy is transported through the process of radiation.  Basically, the photons 
generated by the core are absorbed and emitted by the atoms found in the
radiative zone like cars in stop and go traffic.  This is a very slow process. 
The $convective~zone$ is the
region of the Sun where energy is transported by rising ``bubbles'' of material.
This is the same phenomenon that takes place when you boil a pot of water.  
The hot bubbles rise to the top, cool, and fall back down.  This gives the
the surface of the Sun a granular look.  Granules are bright regions surrounded by darker narrow regions.  These granules cover the entire surface of the Sun.

The atmosphere of the Sun is also comprised of three layers:  the photosphere,
the chromosphere, and the corona.  The $photosphere$ is a thin layer that forms
the visible surface of the Sun.  This layer acts as a kind of insulation, and 
helps the Sun retain some of its
heat and slow its consumption of fuel in the core.  The $chromosphere$ is the Sun's lower atmosphere.  This layer can only be seen during a solar eclipse 
since the photosphere is so bright.  The $corona$ is the outer atmosphere of the
Sun.  It is very hot, but has a very low density, so this layer can only be
seen during a solar eclipse.  More information on the layers of the Sun can be found in your textbook.

\section{Sunspots}
Sunspots appear as dark spots on the photoshere (surface) of the Sun.  They last from a few days to over a month.  Their average size is about the size of the Earth, although they have been observed to be over twice the sixe of the Earth!  Sunspots are commonly found in pairs.  How do these spots form?

The formation of sunspots is attributed to the Sun's {\it differential~rotation}.  
The Sun is a ball of gas, and therefore does not rotate like the Earth or any other solid object.  The Sun's equator rotates faster than its poles.  It takes
roughly 25 days for something to travel once around the equator, but about 35
days for it to travel once around the north or south pole.  This differential rotation acts to twist up the magnetic field lines inside the sun.  At times,
the lines can get so twisted that they pop out of the photosphere.  Figure 2 illustrates this concept.
When a magnetic field loop pops out, the places where it leaves and re-enters
the photosphere are cooler than the rest of the sun's surface.  These cool
places appear darker, and therefore are called ``sunspots''.
\begin{figure}[h]
\centering
\leavevmode
\epsfxsize=1\textwidth
\epsfbox{SUN/Sun_fig2.eps}
\caption{Sunspots are a result of the Sun's differential rotation.}
\end{figure}


The number of sunspots rises and falls over an 11 year period.  This is the amount of time it takes for the magnetic lines to tangle up and then become 
untangled again.  This is called the $Solar~Cycle$.  Look in your textbook for more information on sunspots and the solar cycle.

\section{Solar Phenomenon}
The Sun is a very exciting place.  All sorts of activity and eruptions take
place in it and around it.  We will now briefly discuss a few of these interesting phenomema.  You will be analyzing pictures of prominences and flares
during your lab. 

Prominences are huge loops of glowing gas protruding from the chromosphere.  Charged particles spiral around the magnetic field lines that loop out over the surface of the Sun, and therefore we see bright loops above the Sun's surface.
Very energetic prominences can break free from the magnetic field lines and 
shoot out into space.

Flares are brief but bright eruptions of hot gas in the Sun's atmosphere. These
eruptions occur near sunspot groups and are associated with the Sun's 
intertwined magnetic field lines.  A large flare can release as much energy as
10 billion megatons of TNT!  The charged particles that flares emit can disrupt
communication systems here on Earth.

Another result of charged particles bombarding the Earth is the Northern Lights.
When the particles reach the Earth, they latch on to the Earth's magnetic field
lines.  These lines enter the Earth's atmosphere near the poles.  The charged
particles from the sun then excite the molecules in Earth's atmosphere and cause them to glow.  Your textbook will have more fascinating information about these
solar phenomema. 


\section{Lab Stations}
You will go through a series of stations and answer 
questions about the images that you see.  Use the information that you have
learned from lectures and your book to give explanations for the different 
phenomena that you see at each station.  You can use your 
own insight and knowledge of basic physics and astronomy to obtain important 
information about the phenomena that we see on the Sun, just as solar 
astronomers do.  Don't forget to {\bf SHOW ALL OF YOUR WORK}.

One note of caution about the images that you see: the colors of the pictures
(especially those taken by SOHO) are {\it not} true colors, but are simply
colors used by the observatories' image processing teams to best enhance the 
features shown in the image.  

\bigskip

\noindent
{\bf Station 1:} Here are two images of the Sun taken by the SOHO
satellite several days apart (the exact times are at the top of the
image).  {\it (8 points)}

\begin{itemize}

\item{Look at the sunspot group just below center of the Sun in
{\bf image 1}, and then note that it has rotated to the western
(right-hand) limb of the Sun in {\bf image 2}.  Since the sunspot
group has moved from center to limb, you then know that the Sun has
rotated by one quarter of a turn ($90^\circ$).}

\vspace*{1cm}

\item{Determine the precise time difference between the images.  Use this
information plus the fact that the Sun has turned by 90 degrees in
that time to determine the rotation rate of the Sun.  If the Sun turns
by 90 degrees in time $t$, it would complete one revolution of 360
degrees in how much time?}

\vspace*{4cm}

\item{Does this match the rotation rate given in your textbook or in
lecture?  Show your work. }

\end{itemize}


\noindent
{\bf Station 2:} Here are two different images of the Sun: the one on
the left is a photo of the Sun taken in the near-infrared at Kitt Peak
National Observatory, and the one on the right is a ``magnetogram'' (a
picture of the magnetic field distribution on the surface of the Sun)
taken at about the same time.  (Note that black and white areas
represent regions with different {\it polarities}, like the north and
south poles of a bar magnet.) {\it (7 points)}

\begin{itemize}

\item{What do you notice about the location of {\it sunspots} in the 
photo and the location of the {\it strongest magnetic fields}, shown by the 
brightest or darkest colors in the magnetogram?}

\vspace*{2cm}

\item{Based on your above answer, what causes sunspots to form?  Why are they dark? }

\end{itemize}

\vspace*{3cm}

\noindent
{\bf Station 3:} Here is a picture of the {\it corona} of the Sun, taken by
the SOHO satellite in the extreme ultraviolet.  (An image of the Sun has been
superimposed at the center of the image.  The black ring surrounding it is a 
result of image processing and is not real.)  {\it (10 points)}

\begin{itemize}
\item{Measure the diameter of the Sun, then measure the
minimum extent of the corona (diagonally from upper left to lower
right).}

\vspace*{1cm}

\item{If the photospheric diameter of the Sun is 1.4 million
kilometers (1.4 x $10^6$ km), how big is the corona? (HINT: use unit conversion!)}

\vspace*{3cm}

\item{How many times larger than the Earth is the corona? (Earth diameter=12,500 km)}

\vspace*{3cm}
\end{itemize}


\noindent
{\bf Station 4:} Look on the back wall of the classroom at the poster of a group of sunspots.  The {\it typical} size of a sunspot is 15,000 km.  The average width of the state of New Mexico is 350 miles.  How many New Mexicos can fit across a typical sunspot? (1 mile = 1.61 km)  {\it (7 points)}

\vspace*{3cm}

\noindent
{\bf Station 5:} This image shows a time-series of exposures by the SOHO
satellite showing an {\it eruptive prominence}.  {\it (16 points)} 

\begin{itemize}
\item{As in station 4, measure the diameter of the Sun and then measure
the distance of the top of the prominence from the edge of the Sun in
the first (earliest) image.  Then measure the distance of the top of
the prominence from the edge of the Sun in the last image.}

\vspace*{1cm}

\item{Convert these values into real distances based on the linear
scale of the images.  Remember the diameter of the Sun is 1.4 x $10^6$
kilometers.}

\vspace*{3cm}

\item{The velocity of an object is the distance it travels in a certain amount of time (vel=dist/time).  Find the velocity of the prominence by subtracting the two distances and dividing the answer by the amount of time between the two images.}

\vspace*{3cm}

\item{If the Earth is 150 x $10^6$ kilometers from the Sun, how many $days$ would it
take this ejected material to reach the Earth? }

\vspace*{2cm}
\end{itemize}


\noindent
{\bf Station 6:} This is a plot of where sunspots tend to occur on the Sun as 
a function of {\it latitude} on the Sun and time.  What do you notice about 
the distribution sunspots?  How long does it take the pattern to repeat?  What
does this length of time correspond to? {\it (6 points)}

\vspace*{15cm}

\noindent
{\small
Note:  Kitt Peak Vacuum Telescope images are courtesy of KPNO/NOAO.  
SOHO Extreme Ultraviolet Imaging Telescope images courtesy of the
SOHO/EIT consortium.  SOHO Michelson Doppler Imager images courtesy
of the SOHO/MDI consortium.  SOHO is a project of international cooperation
between the European Space Agency (ESA) and NASA.}


\section{Questions}

\noindent
1) List the six layers of the Sun (in order) and
give their temperatures. {\it (6 points)}

\vskip 3in

\noindent
2) What causes the Northern (and Southern) Lights, also known as ``Aurorae''?
{\it (5 points)}

\vskip 3in

\section{Summary}

({\it 35 points}) Please summarize the important concepts discussed in this lab.   
\begin{itemize}
\item{Discuss the different types of phenomena and structures you looked at in the lab}
\item{Explain how you can understand what causes a phenomenon to occur by looking at the right kind of data}
\end{itemize}
  
\noindent
Use complete sentences and, proofread your summary before turning it in.

%\end{document}
