	%\subsection{Bremmstrahlung}

%As an charged particle passes close to a nucleus it is scattered sharply resulting in a rapid change in velocity and a photon being emitted. This can be thought as `leaving behind' a photon from the cloud of virtual particles surrounding the incident particle\cite{part-int-notes}. See figure \ref{bremmstrahlung}.

% FEYNMANN DIAGRAM FOR BREMSSTRAHLUNG

%This is more pronounced in light particles since the acceleration is much greater, it is also more prounced when scattering off heavier nucleus since the acclerating force is greater. The change in energy per unit length due to Bremsstrahlung is as follows,

%$$-\left(\frac{dE}{dx}\right)_{\textrm{Brems}} = \frac{4nZ^2\alpha^3(\hbar c)^2}{m^2}T\ln{\frac{183}{Z^{1/3}}}$$

%where $Z$ is the atomic mass number, $\alpha$ is the fine structure constant, $m$ is the charged particle's mass and $T$ is the charged particle's kinetic energy.

	%\subsection{Cerenkov light}

%If the velocity of the incident particle is faster than the phase velocity of light in a dielectric (i.e. $v > c/\sqrt{\epsilon}$) then it 

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%The Earth's magnetic field significantly affects the path of cosmic rays which reach sea-level. There is, for example, a strong relationship between the flux of secondary cosmic rays at sea-level and geomagnetic latitude which shows a dip of around 10\% as one moves towards the equator\cite{latitude-cr}. It also means that we cannot easily deduce the direction of a cosmic ray without the use of a computer model\cite{thin-cosmic-rain}

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%Extending the energy spectrum in Figure \ref{primary-energies-extended} to higher energies we see two interesting features. One is known as the `knee' and manifests as a kink in the energy distribution at around $10^{16}$eV, the other is known as the `ankle' and appears just under $10^{19}$eV.

%\begin{figure*}
%\begin{center}
%\includegraphics[scale=0.95]{../pics/knee-ankle}
%\caption{Primary cosmic ray energy spectrum extended to show the `knee' and `ankle'\cite{phys-rev:cr}}
%\label{primary-energies-extended}
%\end{center}
%\end{figure*}

%Protons with energies of around $10^{18}$GeV have their paths curved by the galactic magnetic field into a radius of $10^{21}$cm, much smaller than the galactic diameter of $10^{23}$. As energies increase past $10^{21}$GeV the radius of the paths increase beyond the diameter of our galaxy and are no longer confined to our galactic disc\cite{thin-cosmic-rain}. For this reason we expect an anisotropic distribution of particles

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\subsection{The effect of our Sun}

The only cosmic rays to orginate from within our solar system are attributed to the Sun\cite{physrev:cr}. Typical energies are low at around 10--100keV meaning most of the solar cosmic rays are absorbed in the atmosphere. Only during strong solar flares do we obtain the rare solar cosmic rays of energies more than the 400MeV needed to produce pions in the atmosphere which then decay to the muons which are most commonly observed in cosmic ray experiments.

The Sun radiates \emph{solar wind}, an expanding magnetised plasma which varies in strength according to solar activity. This plasma `sweeps' away many of the lower energy cosmic rays originating from outside of our solar system resulting in a lower flux of cosmic rays below 10GeV. A particularily strong solar wind causes a noticeable dip in cosmic ray observations, known as the \emph{Forbush decrease}. This decrease tends to follow the 11 year sunspot cycle although is subject to less predictable solar flares.
