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\subsection {Observations of High Frequency Waves in the Magnetosheath}
\label{MSH3:ss4}

% Written by J. S. Pickett - version 1  12 Mar., 2004
% file = MSH3s411.tex


\subsubsection {Electromagnetic waves}\label{MSH3:ss4:emwaves} %-----------------------------------

\subsubsection {Lion roars}\label{MSH3:ss4:emwaves:lionroars} %-----------------------------------------------

Lion roars are intense, short-duration, narrow-band packets of whistler mode waves 
observed in Earth’'s magnetosheath and first reported by \cite{Smith67a} using data 
from OGO 1.  The average mean frequency of these waves is ~ 100 Hz (0.25 - 0.5 fce) 
with typical amplitudes of 0.1 nT and burst durations of 1-2 s 
[\cite{Smith76a}].  A more detailed study of lion roars found that these emissions 
were most often observed in the inner region of the sunward magnetosheath, and the 
distribution of their intensity at 200 Hz was highest near the subsolar magnetopause 
[\cite{Rodriguez85a}].  \cite{Tsurutani82a} found that lion roars often coincide 
with minima of the magnetic field strength and maxima of plasma density, indicative 
of mirror mode structures.  However, using large amounts of data collected by the 
Geotail spacecraft, \cite{Zhang98a} found that only 30\% of the lion roars were 
associated with mirror modes.  Lion roars are most probably generated by unstable 
anisotropic electron distributions, when the perpendicular temperature is larger 
than the parallel temperature.  In the mirror mode structures, the number of resonant 
electrons is larger for a given anisotropy [\cite{Smith76a}].  

Early studies of lion roars in the magnetosheath found that they typically propagate 
with wave vectors at angles less than $30\,^{\circ}$ relative to the magnetic field 
[\cite{Smith76a}].  \cite{Baumjohann99a} found the wave vectors very close to parallel 
to the magnetic field, taking into account measurements of nearly monochromatic magnetic 
wave packets detected by the Equator-S spacecraft in the minima of the magnetic field.  
Some of the lion roars contained in the Geotail study [\cite{Zhang98a}] were found to 
have wave vectors near $90\,^{\circ}$, suggesting they were downstream propagating whistler 
waves from the bow shock.  \cite{Zhang98a} also found that although the majority of 
lion roars that were detected by Geotail were propagating in one single direction, one 
class of lion roars were found to be propagating in two directions simultaneously, 
suggesting the local plasma as the source.

The multipoint measurements of Cluster are ideally suited for determining the 
location of the source region and characteristics of the magnetosheath lion roars.  
\cite{Maksimovic01a}, using data obtained in the duskside magnetosheath from the 
Cluster STAFF Spectrum Analyzer, found that close to the magnetopause, lion roars 
in deep magnetic troughs are observed to propagate simultaneously in two directions, 
both parallel and anti-parallel to the magnetic field.  This suggests that the Cluster 
spacecraft were in the source region of the lion roars.  Far from the magnetopause, 
the waves were found to propagate in only one direction, roughly anti-parallel to 
the magnetic field.  In addition \cite{Maksimovic01a} found that the lion roars were 
propagating at angles of $30\,^{\circ}$ to $50\,^{\circ}$ from the local magnetic field direction, which 
is inconsistent with whistler mode waves which would normally be Landau damped in a 
bi-Maxwellian plamsa.  This differs from many of the earlier works which found much 
smaller angles, and may be due to the sampling and bandwidth characteristics of the 
STAFF-SA instrument.  It is also possible that this angular difference has a physical 
explanation that will become apparent through future statistical studies of lion roars.

Since the work of \cite{Maksimovic01a} was an introductory study of lion roars using 
the Cluster fleet, there is still much to be learned in the future.  The primary 
contribution that Cluster can make to the knowledge already gained on the 
characteristics of lion roars is to perform a statistical study of direction of 
propagation using the multi-spacecraft measurements of STAFF-SA obtained in various 
regions of the magnetosheath.  These measurements will then be annotated with 
information obtained from various Cluster instruments, such as FGM to determine 
whether the lion roars are observed within mirror mode structures, or such as PEACE 
to determine whether an electron temperature anistropy exists for those cases of 
lion roars observed to be propragating in two directions.  In addition, the very 
high time resolution waveform data of the Wideband (WBD) plasma wave receiver should 
help in this regard.  Figure \ref{fig:MSH3:ss4:lionroars} shows a sample of WBD 
data obtained on April 30, 2002 in the magnetosheath on all four spacecraft
(top four panels) using the magnetic search 
coil sensors.  The data are plotted as frequency (in kHz) on the vertical axis
vs. time (in UT) on the horizontal axis, with color indicating power
spectral density (in relative dB).  An example of the wave packets 
associated with the approximately 200 Hz lion roars 
observed on Cluster 4 starting at 21:26:50.56 UT is seen in the 
bottom panel.  Similar lion roar wave packets 
can be observed on the other three spacecraft.  Since WBD has time resolution of 35 
$\mu$s and time of measurement accuracies to 10 $\mu$s on all four 
spacecraft, cross-spacecraft correlations of these waveforms will be performed, 
thereby obtaining information on the propagation of these lion roar wave packets 
from the source region.  A similar correlation analysis has been successfully 
carried out for the chorus emission region in the inner magnetosphere 
[\cite{Santolik03a}].
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
\begin{figure}
\begin{center}
\includegraphics[width=1.0\textwidth]{lionroars}
\caption{\small Lion roars observed in the magnetosheath by the Cluster 
WBD plasma wave receiver using the magnetic search coil sensor
on each of the four Cluster spacecraft (panels 1 through 4) on April 30, 2002.
The bottom panel shows two typical lion roar wave packets from Cluster 4.
Cross-spacecraft correlation can be carried out to find the location of the
source region of the lion roars.}\label{fig:MSH3:ss4:lionroars}
\end{center}
\end{figure}
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\bibliography{MSH3s411.bib}

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