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%sp check complete 1240 pm 2/25/2013
% post defense: updated and sp check complete 8:28 am on 3/25/2013

\Chapter{\uppercase{Introduction}} \label{Intro}
\indent The Earth's magnetosphere is a complex plasma physics system driven by the solar wind.  
 Figure (\ref{fig:magnetosphere}) shows a schematic diagram detailing the major regions and systems that form the Earth's magnetosphere.  
The regions highlighted with colors in Figure (\ref{fig:magnetosphere}) are associated with the nightside auroral zone.  
Electric fields, currents, and plasma trapped on magnetic field lines within the magnetosphere, form the fundamental nonlinear feedback mechanisms that control the dynamics within the magnetospheric system. 

A systems approach can be used to study individual subsystems and the interaction between different interconnected regions of the magnetosphere with the ionosphere.  
Figure (\ref{fig:MI_coupling}) illustrates the subsystems that form the electrodynamic coupling between the magnetosphere and the ionosphere.  
A key subsystem in the magnetospheric-ionospheric system is field aligned current.  
As shown in Figure (\ref{fig:magnetosphere}), field aligned current (in red) connects plasma from the plasmasheet (in blue) in the distant magnetosphere to the auroral oval (in green) at ionospheric altitudes.  
Currents flowing perpendicular to the mean magnetic field within the ionosphere close the magnetospheric-ionospheric (MI) circuit by connecting the upward and downward field aligned currents.  
Energy from the magnetosphere is dissipated into the ionosphere through Joule heating ($\mathbf{J}\cdot\mathbf{E}_\perp$), collisional ionization, and heating, that can be significantly enhanced by auroral particle precipitation.  %Poynting flux ($\mathbf{E}\times\mathbf{B}$),  
Energy dissipation in the ionosphere is an important function within the whole magnetospheric system.  

\begin{figure}[!t]
 \begin{centering}
\includegraphics[keepaspectratio=true, width=5.5in]{figures/ch1_intro/magnetosphere_diagram.pdf}
\caption{A diagram from \citet{Kelley:2009} showing the whole magnetospheric system.  
The regions and subsystems associated with the nightside auroral zone are highlighted with colors.
Precipitating electrons flow along the geomagnetic field and carry field aligned current into the auroral zone.  }
\label{fig:magnetosphere}
 \end{centering}
\end{figure}


\begin{figure}[!t]
 \begin{centering}
\includegraphics[keepaspectratio=true, width=5.0in]{figures/ch1_intro/MI-coupling.pdf}
\caption{Figure 1 from \citet{Baum:1982} of individual subsystems that form electrodynamic coupling between magnetospheric and the ionosphere.  
The arrows indicate the direction of flow and in most cases are bi-directional.  }
\label{fig:MI_coupling}
 \end{centering}
\end{figure}


% One of the key systems is the coupling between the magnetosphere and the ionosphere.  


\section{The Aurora}
The aurora is an important subsystem within the magnetosphere-ionosphere system. 
Visible light is a key observable associated with the aurora and is emitted through atomic de-excitation, at visible or sub-visual intensities.  
Visible light commonly observed in an auroral display is at red or green wavelengths, and results from forbidden transitions of atomic oxygen molecules.
Figure (\ref{fig:O_levels}) from \citet{Roach:Smith:1967} shows the energy levels and transition times for atomic oxygen. 
These forbidden transitions have excited life-times of the order of seconds, rather than atomic transition that obey the selection rules with life-times of the order of micro- or nano-second.    
The green line at 5577\AA \, is typically associated with auroral displays and has a metastable $\mathrm{{^1}S \rightarrow {^1}D}$ transition with a life-time of 0.74 seconds.
The light associated with this transition will be seen at altitudes above 100 km, because the excited atom is more likely to collide before it emits light at lower altitudes \citep{Paschmann:2002}.  
The red line is associated with auroral displays at higher altitudes and corresponds to the $\mathrm{{^1}D \rightarrow {^3}P}$ transition that has wavelengths of 6330\AA \, or 6364\AA, depending on which orbital the electron transitions to.
The red light exhibits less structure because of the longer transition time of approximately 110 s and is observed at altitudes above 200 km \citep{Paschmann:2002}. 
This long transition time allows the excited species to be influenced by plasma drifts, that can further diffuse structure.  
 
 \begin{figure}[!t]
 \begin{centering}
\includegraphics[keepaspectratio=true, width=3.0in]{figures/ch1_intro/energy_O.pdf}
\caption{A diagram from \citet{Roach:Smith:1967} showing the energy levels of atomic oxygen which are responsible for red and green auroral light emission.  
It is noted that the transition lifetimes are shown for these forbidden transitions.
The green light is observed at 5577\AA \, with a life time of 0.74 s. 
The red light emission is at 6330\AA \, or 6364\AA, and has a lifetime of 110 s.     }
\label{fig:O_levels}
 \end{centering}
\end{figure}

Aurorae are generally divided into two major classes based on the particle acceleration mechanism: discrete aurora and diffuse aurora.  
Diffuse aurora is typically not associated with an acceleration mechanism, rather plasmasheet ions and electrons are pitch angle scattered into the loss cone \citep{Kennel:Petschek:1966}.  
Although the mechanisms which cause diffuse aurora are important in the context of magnetospheric physics, the focus of this dissertation will be on discrete aurora (for a current review of diffuse aurora, see \citet{Lessard:2012}). 
Discrete aurora is caused by a parallel electric field at altitudes of 2000-12000 km  above the ionosphere that accelerate plasmasheet electrons.
These precipitating plasmasheet electrons reside on closed magnetic field lines. 
 %mention altitude  
U-shaped electrostatic potential structures have been found to be consistent with electric field observations.  

Visually, discrete aurora are comprised of structures such as auroral arcs, sheets, and rays.  
An emission feature in the electron spectrograms called the inverted-V is a characteristic upside-down V shape that has been associated with U-shaped electrostatic potentials and visible aurora.  
There are other types of discrete aurora with differing acceleration mechanisms; most notably, aurora produced by electron acceleration through parallel electric fields generated by inertial \Alfven waves.  
There are also other electron spectral features such as suprathermal bursts which are strongly field-aligned precipitation at energies typically below 1 keV.  
% Electrons causing the discrete aurora are thought to originate in the plasmasheet on closed magnetic field lines, which forms a connection to the distant outer magnetosphere.  
Discrete aurora are also typically associated with upward field-aligned currents and these currents are carried by precipitating, accelerated, plasmasheet electrons.      
``Black aurora'', or regions devoid of visible light and electron flux have been associated with the return, downward field-aligned current.  

The auroral region still has many open questions, with one of chief the questions regarding the mechanism(s) that are responsible for the generation of parallel electric fields. 
Despite many theories presented in the literature, at the current time this question remains unresolved \citep{Borovsky1993, Karlsson:2012}. % a topic of active research at the current time.


\section{Closure of Field-Aligned Current in the Ionosphere}
%auroral - have increased particle precipit
A connection exists between ionospheric currents, ionospheric conductivity, that can be significantly enhanced by particle precipitation, and electric fields.  
The effect of these interconnected elements is expressed through Ohm's law, as shown in the lower portion of Figure (\ref{fig:MI_coupling}).  
The current closure region is located where field aligned current from the magnetosphere closes because plasma collisions with neutral atoms become sufficiently significant to allow particles to drift across magnetic field lines, thereby breaking the frozen-in approximation.  
Ohm's law can be used to describe plasma drift perpendicular to the mean magnetic field.    
The perpendicular current can be related to the electric field by, 
\begin{equation}
\begin{align}
\mathbf{j}_{\perp}(z) & = |q_{i}|n_{i}(z)\mathbf{U_{\perp i}} - |q_{e}|n_{e}(z)\mathbf{U_{\perp e}} \\  
& = \mathbf{j}_{P}(z) + \mathbf{j}_{H}(z) \\
& =  \sigma_{P}(z)\mathbf{E}_{\perp} - \sigma_{H}(z)(\mathbf{E}_{\perp}\times\mathbf{\hat{B}})
\end{align}
\label{eq:hall_ped_current}
\end{equation}
where $\mathbf{U_{\perp s}}$ is the perpendicular bulk velocity for ions or electrons,  $\mathbf{E}_{\perp}$ is the electric field perpendicular to the mean magnetic field, and $\mathbf{\hat{B}}$ is the magnetic field unit vector \citep{Paschmann:2002}.
A derivation of the Hall and Pedersen current is presented in Appendix \ref{ch:app1}.  
The Pedersen conductivity is defined as, 
\begin{equation}
\sigma_{P}(z) = e^{2} n_{e} \left(\frac{\nu_{in}}{m_{i}\left(\nu^{2}_{in}+\Omega^{2}_{i}\right)} +\frac{\nu_{en}}{m_{e}\left(\nu^{2}_{en}+\Omega^{2}_{e}\right)}\right)
\label{eq:S_Ped_1}
\end{equation}
and the Hall conductivity, 
\begin{equation}
\sigma_{H}(z) = e^{2} n_{e} \left(\frac{\Omega_{i}}{m_{i}\left(\nu^{2}_{in}+\Omega^{2}_{i}\right)} - \frac{\Omega_{e}}{m_{e}\left(\nu^{2}_{en}+\Omega^{2}_{e}\right)}\right)
\label{eq:S_Hall_1}
\end{equation}
where $\mathrm{\Omega_{i}}$ and $\mathrm{\Omega_{e}}$ are the ion and electron cyclotron frequency, respectively \citep{Paschmann:2002}.  
The Hall and Pedersen conductivities are altitude dependent.  
A key point to note from equation (\ref{eq:hall_ped_current}) is that the perpendicular electric field determines the direction of the Hall and Pedersen current.  
Pederson currents ($\mathbf{j}_{P}$ in eq. \ref{eq:hall_ped_current}) flow parallel to $\mathbf{E}_\perp$; whereas, Hall currents ($\mathbf{j}_{H}$ in eq. \ref{eq:hall_ped_current}) flow perpendicular to both $\mathbf{E}_\perp$ and $\mathbf{\hat{B}}$.  
Over an auroral arc, the enhancement of electrons deposition by particle precipitation can increase the local electron density by orders of magnitude compared to the background electron density; this effect can significantly increase the Hall and Pedersen conductivities within an arc.  

% [mention that this is the same form as in the modeling chapter, functions of altitude]
% [role of particle precipitation in enhancing n_e suddenly]

% Within the lower altitude ionosphere, where the perpendicular current can flow, collisions with neutrals become sufficiently significant to allow particles to move across field lines, breaking the frozen-in approximation.  
To quantify how collisions with atmospheric neutral atoms allows electrons and ions to break off their orbits in the auroral ionosphere, the variable $\mathrm{\kappa}$ can be defined as the ratio of the cyclotron frequency over the collisional frequency, that is, $\mathrm{\kappa_{i} = \Omega_{i}/\nu_{in}}$ and $\mathrm{\kappa_{e} = \Omega_{e}/\nu_{en}}$ for the ions and electrons, respectively  \citep{Kelley:2009}.
As shown in Figure (\ref{fig:kappa}a) , when $\mathrm{\kappa_{i}}, \kappa_{e} \ll 1}$ the ions and electrons will experience strong collisions and effectively be decoupled from the magnetic field.  
In this limiting case, the Hall and Pedersen conductivities will asymptote to the strongly collisional limit, $\sigma \approx \mathrm{e^2 {n_e}/ \nu_{in} m_{i}}$.
In the limit that the plasma is strongly magnetized, $\mathrm{\kappa_{i}}, \kappa_{e} \gg 1}$, then the Hall and Pedersen conductivities go to zero. % and do not contribute to any perpendicular current.  
This situation is consistent with ions and electron traveling on $\mathbf{E\times B}$ drift orbits, as shown in Figure (\ref{fig:kappa}b).
No perpendicular current is set up because the $\mathbf{E\times B}$ drifts are independent of charge; therefore, the ions and electrons move in the same direction with no bulk velocity difference. %being set up.    
The final case is the situation where $\mathrm{\kappa \approx 1}$ and is schematically represented in Figure (\ref{fig:kappa}c). 
In this situation, there is a balance between collisions and the $\mathbf{E\times B}$ drift motion for the ions and the electrons.  
This sets up significant perpendicular drifts of the ions relative to the electrons, which translates into a significant perpendicular closure current.  
It can be shown that the conductivities peak when the condition that $\mathrm{\kappa = 1}$ is satisfied for the ions and the electrons, respectively.  
This situation can be interpreted as the largest drift between ions and electrons.  

 \begin{figure}[!t]
 \begin{centering}
\includegraphics[keepaspectratio=true, width=4.0in]{figures/ch1_intro/kappa_diagram.pdf}
\caption{Figure 2.4 from \citet{Kelley:2009} that details the different cases of particles moving perpendicular across the geomagnetic field.  
Case (a), is the strong collisional case in which the plasma constituents would have many collisions with neutral atmosphere, resulting a low level of magnetization.  
Case (b), is when the plasma is strongly magnetized.  
This situation would result in $\mathbf{E\times B}$ drift motion, which would not set up a perpendicular current.  
Case (c), is the situation that happens at the peak of the conductivity in the ionosphere where there is a balance between collisional effects and the plasma remaining tied to the magnetic field lines.  
This case results in the most significant perpendicular drifts. }
\label{fig:kappa}
 \end{centering}
\end{figure}

% One of the open questions in both MI-coupling and auroral physics is the closure of field aligned currents within the ionosphere.  
% Currents can be related to the electric field via:  % across perpendicular to the $mathbf{E}\times\mathbf{B}$ drift.   %ions flow off ExB drift orbits, 
% , $\sigma_{H}(z)$ and $\sigma_{P}(z)$ are the electron Hall and Pederson conductivities, respectively, which are defined as follows:
% \begin{equation}
%  \sigma_{P}(z) = \frac{n_{e}e^{2}}{m_{e}}\frac{\nu_{e}}{\nu_{e}^{2}+\Omega_{e}^2},\:  \sigma_{H}(z) = \frac{n_{e}e^{2}}{m_{e}}\frac{\Omega_{e}}{\nu_{e}^{2}+\Omega_{e}^2}
% \label{eq:sigma}
% \end{equation}
% where $n_{e}$ is the electron density, $\nu_{e}$ is the electron collisional frequency, $\Omega_{e}$ is the cyclotron frequency defined as $\Omega_{e} = {e B(z)}\,{m_{e}^{-1}}$ and $B(z)$ is the local magnetic field that is an explicit function of the altitude $z$.  
% Equation \ref{eq:hall_ped_current} describes currents that form along channels of conductivity perpendicular to the mean magnetic field, but in two different orientations relative to $\mathbf{E}_\perp$.  
% As can be seen in equation \ref{eq:sigma}, collisions also are inversely related to the conductivity in the ionosphere.

%may want to add more in the appendix about the polarization electric field

The key equation that connects the field aligned current with the perpendicular closure current is the continuity of charge equation, 
\begin{equation}
 \frac{\partial \rho}{\partial t} + \nabla\cdot\mathbf{j} =  \frac{\partial \rho}{\partial t}+ \frac{\partial j_{\parallel}(z)}{\partial z} + \nabla_{\perp}\cdot\mathbf{j}_{\perp}(z) = 0
\label{eq:curr_cont}
\end{equation}
In a steady state current configuration (${\partial \rho}/{\partial t} =0$), the current continuity equation can be solved to yield,
\begin{equation}
\frac{\partial j_{\parallel}(z)}{\partial z} =  - \nabla_{\perp}\cdot\mathbf{j}_{\perp}(z)
\label{eq:perp_curr_fa_curr}
\end{equation}
which connections the divergence of the perpendicular current to vertical gradients in the field aligned current.  
Equations (\ref{eq:S_Ped_1}), (\ref{eq:S_Hall_1}), and (\ref{eq:perp_curr_fa_curr}) are explicit functions of altitude ($z$). 
% These are referred to as ``Birkeland'' or ``field aligned current'' arcs \citep{Marklund:1984}.  

% First, if there was a sufficiently fast movement of space charge such that $\mathrm{\partial \rho/\partial t =0}$ remains valid and a lack of input field-aligned current ($\mathrm{j_{\parallel} = 0}$ ), a significant charge separation can set up a polarization electric field.  
% % The large gradient in space charge generates an large electric field, which can balance out the Hall and Pedersen conductivities.  
% Observationally the parameter of interest is $\mathrm{E_{A} - E_{o}}$, which is the difference between the electric field within the arc and the ambient (convective) electric field outside of the arc \citep{Paschmann:2002}
% This type of configuration is consistent with what been termed a ``polarization arc'' \citep{Marklund:1984} and . 


A simplifying assumption that is commonly invoked is to integrate out the altitude dependence in the conductivities \citep{Fejer:1953}, yielding the height integrated conductivities or conductance,
\begin{equation}
 \Sigma_{i} = \int \sigma_{i}(z) dz
\label{eq:height_pederson} 
\end{equation}
where i corresponds to either the Hall or Pedersen conductivity.  
The current continuity equation, (equation \ref{eq:curr_cont}) can be integrated to get an expression for the field-aligned current in terms of the divergence of the electric fields and the conductances. 
If it is assumed that the perpendicular electric fields independent of altitude, then the continuity of current equation can be simplified to, 
% If a significant assumption is applied that there is no altitude dependence in the perpendicular electric fields, 
% \begin{equation}
% % \begin{array}{1}
% j_{\parallel} = \int{-\nabla\cdot\mathbf{j}_{\perp}(z)}  dz = \int{\nabla_{\perp}\cdot\left(\sigma_{P}(z)\mathbf{E}_{\perp}\right) - \nabla_{\perp}\cdot\left(\sigma_{H}(z)(\mathbf{E}_{\perp}\times\mathbf{\hat{B}})\right) dz} 
% % = \Sigma_{P}\nabla_{\perp}\cdot\mathbf{E}_{\perp} - \Sigma_{H}\nabla_{\perp}\cdot(\mathbf{E}_{\perp}\times\mathbf{\hat{B}})
% % \end{array}
% \label{eq:j_par}
% \end{equation}
% If a significant assumption is applied that there is no altitude dependence in the perpendicular electric fields, these expression can be further simplified (\ref{eq:j_par}) to, 
\begin{equation}
 j_{\parallel} = \nabla_{\perp}\cdot\left(\Sigma_{P}\mathbf{E}_{\perp}\right) - \nabla_{\perp}\cdot\left(\Sigma_{H}(\mathbf{E}_{\perp}\times\mathbf{\hat{B}})\right)
\label{eq:j_par_HI} %j parallel height integrated
\end{equation}
where $\Sigma_{H}$ and $\Sigma_{P}$ are the height integrated Hall and Pederson conductivities, respectively.  
If $\mathrm{\nabla_{\perp}\times E_{\perp} = 0}$ is further assumed, then equation (\ref{eq:j_par_HI}) can be simplified to,
\begin{equation} \label{eq:j_par_simp}
 j_{\parallel} = \left(\nabla_{\perp}\Sigma_{P}\right)\cdot \mathbf{E}_{\perp}
+\Sigma_{P} \left(\nabla_{\perp}\cdot\mathbf{E}_{\perp} \right)
 - \left(\nabla_{\perp}\Sigma_{H}\right)\cdot\left(\mathbf{E}_{\perp}\times\mathbf{\hat{B}}\right)
\end{equation}
and if the perpendicular gradients in the Hall or Pedersen conductances equal zero, then the closure current is solely composed of the Pedersen current \citep{Paschmann:2002}.  
Equations (\ref{eq:j_par_HI}) is the key equation that dictates the closure of field-aligned current within the ionosphere for a steady-state situation.  

%what about frozen in flux
One key assumption in the derivation of the Hall and Pedersen currents is that electric fields map down from the magnetosphere into the ionosphere.  
That electric fields can be mapped is because it is often assumed that magnetic field lines are equipotentials.   
This condition is supported by the very high parallel conductivity ($\sigma_{\parallel}$) that exists along the magnetic field.  
As shown in Appendix \ref{app2:parallel}, the continuity of current equation can be solved to obtain a condition that $\mathrm{E_{\parallel}\sigma_{\parallel} =1}$ \citep{Kelley:2009}.
The consequences of this equation are two fold.  
First, as \citet{Kelley:2009} suggests if the conductivity is infinite along the field line, then the potential drop between two altitudes is zero, and the potential difference between two field lines is a constant. % (shown in \ref{ch:app1}).  
The second consequence is a result of the limit of nearly infinite conductivity that is generally valid between the magnetosphere into the ionosphere because the plasma along the field lines is collisionless.  
However, as will be discussed in the Chapter \ref{ch2:background}, the idea of parallel electric fields causing auroral electron acceleration was contentious because it suggested there were regions of finite parallel conductivity, that contradicted the plasma being collisionless between the magnetosphere and the ionosphere.     
Discrete aurora exist because of the generation of a parallel electric field.    
% Parallel electric fields  supported being s collisionless plasma.  
In the auroral-ionosphere, if the frozen-in condition is violated by collisions, it would be expected that the electric field equipotential contours would also break at these altitudes.  
Therefore, it is significant to assume $\mathrm{E_{\perp}}$ maps through the ionosphere.   
However, in some situations magnetospheric electric fields have been observed at altitudes below the ionosphere \citep{Kelley:2009, Mozer_Serlin:1969}.  
% [mention the magnetospheric electric fields mapping to the ionosphere]

% The assumption that there is no altitude dependence in the electric field is 
% [magnetic field mapping]
% In application, particularly in global magnetospheric models, the current closure region is typically modeled as a thin sheet with a specified height integrated Pederson conductivity to act as a dissipative mechanism.  


Many questions still remain regarding the structure of the current closure region and the effects this region has within the magnetospheric-ionospheric system as a whole.  
One of the key questions that is not well understood: what is the configuration of the current structure in the region where field aligned current closes?
% One of the open questions regarding the region where field aligned currents close is the configuration of the current structure.
Closely related is the question: what is the fraction of Hall or Pedersen current that closes the field-aligned current?  
Based on observed or inferred closure configuration, what are the consequences for the dissipation of energy within the ionosphere through the Pedersen current?  
In a model of the auroral ionosphere by \citet{Mallinckrodt:1985}, it was shown that the altitudinal and latitudinal structure of the current closure region strongly depended on the boundary electric fields, currents, and conductivities.  

Observationally there have been few in situ observations within the low altitude auroral ionosphere.  
Incoherent scatter radar observations can be made of the current closure region; however, such observations have limited capability particularly in deducing the current structure within this region.  
To the author's knowledge, the only recent in situ observation that has focused on the low altitude ionosphere have been from the JOULE II sounding rocket missions \citep{Sangalli:2009}.  
Additional multi-point in situ observations can better enable our ability to deduce the current closure structure in the ionosphere. 

% Another phenomena that would also benefit from {\it in-situ} observations in the current closure region is plasma density holes or auroral ionospheric cavities (AIC).  
% Plasma density holes are regions of evacuated ionospheric electron density that may be associated with downward field aligned current.  
% Plasma density holes have been well observed by incoherent scatter radar (\citep{Doe:1993}), but \citet{Marklund:1982} is one of the only sounding rocket observation that has been attributed to observing plasma density holes.  
% Observations from a sounding rocket equipped to observe currents could make observations to correlate more directly plasma density holes with downward field aligned currents.    

\section{The ACES Sounding Rocket Mission} 
\begin{figure}[!t]
 \begin{centering}
\includegraphics[keepaspectratio=true, width=5.0in]{figures/ch1_intro/aces_setup.pdf}
\caption{Schematic diagram of the ACES mission configuration including the magnetic field geometry and the currents to be measured.  }
\label{aces_setup}
 \end{centering}
\end{figure}

% Multi-point, {\it in-situ} observations of the current closure region would allow many of these open questions to be investigated with new detail.  
% Moreover, the low altitude of the current closure region makes it especially accessible to sounding rockets.  
The Auroral Current and Electrodynamics Structure (ACES) sounding rocket mission was devised to make new observations that would examine the current structure, energy transfer and the electrodynamics within the current closure region.
Figure (\ref{aces_setup}) is a schematic diagram detailing the design of the mission to investigate the current structure  of the upward and downward field aligned currents, along with cross field perpendicular closure current.   
Two payloads containing similar instrument packages were flown along nearly conjugate magnetic footpoints.  %simultaneous observation  
A high altitude payload (hereafter referred to as ACES High) was designed to fly at a higher altitude to measure the input electrodynamic and plasma parameters onto the current closure region.
A low altitude payload (hereafter referred to as ACES Low) was designed to fly through the current closure region to make in-situ observations of the plasma parameters and electrodynamics within this region.  
The longitudinal dimension (out of the plane of the Figure (\ref{aces_setup})) was kept small so that a thin sheet approximation could be applied and the payloads crossed magnetic field lines mapped to similar footpoints nearly simultaneously to constrain the spatial-temporal ambiguity inherent of in situ observation. 
Data obtained from these payloads, for conjugate footpoints, allows for comparison between observations at different altitudes. 

% The goal of the ACES mission is in-situ measurements of the current closure region and understand current closure in the large MI system.  
% In the next section, a literature review of selected papers will focus on auroral particle measurements, as well as, electrodynamics within the auroral and MI system.  
% A procedure and results section will follow detailing the work that has been done thus far on data acquired from the ACES payloads.  
% Finally, a future work section will focus on the future direction of this work, in part, to fulfill the requirements for a doctorate in physics at the University of Iowa.
  
This dissertation will examine plasma and electrodynamic observations above and within the current region using data from the ACES sounding rocket mission.  
% A brief introduction to the auroral zone and current closure within the ionosphere has been presented.  
Chapter \ref{ch2:background} is a review of auroral physics and field-aligned currents, with an emphasis on auroral zone electric field measurements and precipitating electrons.  
The experimental configuration that was flown on the ACES mission and processing of that data are presented in Chapter \ref{ch3:instr_process}.  
The main results and scientific analyses are contained in Chapter \ref{ch:results}, which follow very closely to results that were presented in \citet{Kaeppler_et_al:2012}.  
The development of a model of the auroral ionosphere that was used to assist in the interpretation of the results is discussed in Chapter \ref{ch:modeling}.  
Finally, a summary and some potential directions for future research in this topic are presented in Chapter \ref{ch6:summary}.  






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