\setcounter{figure}{0}
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\hspace*{3.0in}Name:{\hrulefill} \\
\hspace*{3.25in}Date:{\hrulefill}

\begin{Large}
\section{\bf The Sun}
\end{Large}

\subsection{Introduction}
The Sun is a very important object for all life on Earth.  The nuclear
reactions that occur in its core produce the energy required by plants and
animals for survival.  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,
and very bright, 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 on in the Sun in order to comprehend all of our detailed 
observations.  This difference makes the job of solar astronomers
in some ways more difficult than the job of stellar astronomers, and in
some ways easier!  It's a case of having
lots of incredibly detailed data. But all of the phenomena associated with
the Sun are occuring on other stars, so understanding the Sun's behavior
provides insights to how other stars might behave.

\begin{figure}[h]
\centerline{{\includegraphics[width=12cm]{convection.pdf}}}
\caption{A diagram of the various layers/components of the Sun, as well
as the appearance and location of other prominent solar features.} 
\label{sunlayers-fig}
\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:}  You will be given a Sun image notebook, a bar magnet
with iron filings and a plastic tray. You will need  paper to write on, a ruler, and a calculator}
\end{itemize}

\subsection{Layers of the Sun}
One of the things we know best about the Sun is its overall structure.
Figure \ref{sunlayers-fig} is a schematic of the layers of the
Sun's interior and atmosphere. 
The interior of the Sun is made up of three distinct regions:  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.

\subsection{Sunspots}
Sunspots appear as dark spots on the photoshere (surface) of the Sun (see 
Figure \ref{spots-fig}).  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 size of the Earth!  Sunspots are commonly found in pairs.  How do these spots form?

\begin{figure}[h]
\centerline{{\includegraphics[width=12cm]{sunspotgroup.pdf}}}
\caption{A large, complex group of sunspots.} 
\label{spots-fig}
\end{figure}

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 
\ref{diffrot-fig} 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]
\centerline{{\includegraphics[width=12cm]{sun_diffrot.pdf}}}
\caption{Sunspots are a result of the Sun's differential rotation.} 
\label{diffrot-fig}
\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.

\subsection{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. 

{\it 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.

{\it 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. 


\subsection{Lab Exercises}
There are two main exercises in this lab. The first part consists of
a series of ``stations'' in a three ring binder where you examine some 
pictures  of the Sun and answer some questions about the images that you see.  In the second exercise you will actually look at the Sun using a special telescope to see some
of the phenomena that were detailed in the images in the first exercise
of this lab. During this lab you will 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. If there is not
sufficient room to write in your answers into this lab, do not hesitate
to use additional sheets of paper. Do not try to squeeze your answers into the 
tiny blank spaces in this lab description if you need more space then
provided! 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
\subsubsection{Exercise \#1: Getting familiar with the Size and Appearance of the Sun}
\noindent
{\bf Station 1:} In this first station we simply present some images of the
Sun to familiarize yourself with what you will be seeing during the
remainder of this lab. Note that this station has no questions that you
have to answer, but you still should take time to familiarize yourself with
the various features visible on/near the Sun, and get comfortable with
the specialized, filtered image shown here.
\begin{itemize}
\item{The first image in this station is a simple ``white light'' picture of the
Sun as it would appear to you if you were to look at it in a telescope
 that was designed for viewing the Sun. Note the dark spots on the surface
of the Sun. These are ``sunspots,'' and are dark because they are cooler
than the rest of the photosphere.}

\item{When we take a very close-up view of the Sun's photosphere we see
that it is broken up into much smaller ``cells.'' This is the ``solar
graunulation,'' and is shown in picture \#2. Note the size of these
granules. These convection cells are about the size of New Mexico!}

\item{To explore what is happening on the Sun more fully requires special
tools. If you have had the spectroscopy lab, you will have seen the spectral
lines of elements. By choosing the right element, we can actually probe
different regions in the Sun's atmosphere. In our first example, we look
at the Sun in the light of the hydrogen atom (``H-alpha''). This is the
red line in the spectrum of hydrogen. If you have a daytime lab, and the
weather is good, you will get to see the Sun just like it appears in
picture \#3. The dark regions in this image is where cool gas is present
(the dark spot at the center is a sunspot). The dark linear, and curved
features are ``prominences,'' and are due to gas caught in the magnetic
field lines of the underlying sunspots. They are above the surface of the
Sun, so they are a little bit cooler than the photosphere, and therefore
darker.}

\item{Picture \#4 shows a ``loop'' prominence located at the edge (or ``limb'') 
of the Sun (the disk of the Sun has been blocked out using a special
telescope called a ``coronograph'' to allow us to see
activity near its limb). If the Sun cooperates, you may be able to see
several of these prominences with the solar telescope.}
\end{itemize}
\noindent

{\bf Station 2:} 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).

\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 ($90^\circ$).}

\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?  {\bf (5 points)}}


\vspace*{3.5cm}
\item{Does this match the rotation rate given in your textbook or in
lecture?  Show your work. {\bf (5 points)}}

\vspace*{3.5cm}
\end{itemize}


\noindent
In the second set of photographs at this station are two different 
images of the Sun: the first one on is a photo of the Sun taken in normal light, and the second 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.)

\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?  {\bf (5 points)}}

\vspace*{3.5cm}

\item{Based on this answer, what do you think causes sunspots to form?  Why do you think they are dark?  {\bf (5 points)}}

\end{itemize}

\vspace*{3.5cm}

\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 picture.  The black ring surrounding it is a 
result of image processing and is not real.)

\begin{itemize}
\item{Determine the diameter of the Sun, then measure the
minimum extent of the corona (diagonally from upper left to lower
right). {\bf (3 points)}}

\vspace*{2.5cm}

\item{If the photospheric diameter of the Sun is 1.4 million
kilometers (1.4 $\times$ $10^6$ km), how big is the corona? (HINT: use unit conversion!) {\bf (7 points)}}

\vspace*{4cm}

\item{How many times larger than the Earth is the corona? (Earth diameter=12,500 km) {\bf (5 points)}}

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

\vspace*{1cm}

\noindent
{\bf Station 4:} This image shows a time-series of exposures by the SOHO
satellite showing an {\it eruptive prominence}. 

\begin{itemize}
\item{As in station 3, 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. {\bf (3 points)}}

\vspace*{3.5cm}

\item{Convert these values into real distances based on the linear
scale of the images.  The diameter of the Sun is 1.4 $\times$ $10^6$
kilometers. {\bf (7 points)}}

\vspace*{3.5cm}

\item{The velocity of an object is the distance it travels in a certain amount 
of time (velocity=distance/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. {\bf (5 points)}}

\vspace*{3.5cm}

\item{In the most severe of solar storms, those that cause flares, and ``coronal
mass ejections'' (and can disrupt communications on Earth), the material ejected
in the prominence (or flare) can reach velocities of 2,000 kilometers per 
second. If the Earth is 150 $\times$ $10^6$ kilometers from the Sun, how long (hours 
or days) would it take for this ejected material to reach the Earth? {\bf (5 points)}}

\vspace*{4cm}
\end{itemize}

\noindent
{\bf Station 5:} This is a plot of where sunspots tend to occur on the Sun as 
a function of {\it latitude} (top plot) and time (bottom plot).  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? {\bf (5 points)}

\vspace*{3cm}


\subsubsection{Exercise \#2: Looking at the Sun}

The Sun is very bright, and looking at it with either the naked eye or any
optical device is dangerous---special precautions are necessary to enable 
you to actually look at the Sun. To make the viewing safe, we must eliminate
99.999\% of the light from the Sun to reduce it to safe levels. In this exercise you will
be using a very special telescope designed for viewing the Sun. This telescope
is equipped with a hydrogen light filter. It only allows a tiny amount of
light through, isolating a single emission line from hydrogen (``H-alpha'').
In your lecture session you will learn about the emission spectrum of 
hydrogen, and in the spectroscopy lab you get to see this red line of hydrogen
using a spectroscope. Several of the pictures in Exercise \#1 were actually
obtained using a similar filter system. This filter system gives us a
unique view of the Sun that allows us to better see certain types of
solar phenomena, especially the ``prominences'' you encountered in Exercise
\#1.
\begin{itemize}
\item{In the ``Solar Observation Worksheet'' below, draw what you see on
and near the Sun as seen through the 
special solar telescope. {\bf (10 points)}}
\end{itemize}
\clearpage

\begin{figure}[h]
\centerline{{\includegraphics[width=13cm]{solarobs.pdf}}}
\end{figure}

{\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.}

\newpage

\hspace*{3.0in}Name:{\hrulefill} \\
\hspace*{3.0in}Date:{\hrulefill}\\

\subsection{Summary}

({\bf 30 points}) 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}
\noindent
\item{List the six layers of the Sun (in order). What are the temperatures of
the core and photosphere.}
\noindent
\item{What causes the Northern (and Southern) Lights, also known as ``Aurorae''?
}
\end{itemize}
  
\noindent
Use complete sentences and proofread your summary before turning it in.

\newpage
\mbox{ }
