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Ann. Geophys., 24, 2911-2919, 2006
www.ann-geophys.net/24/2911/2006/
© European Geosciences Union 2006
Structured waves near the plasma frequency observed in three auroral rocket flights
M. Samara1,2 and J. LaBelle1
1Dartmouth College, Department of Physics and Astronomy, Hanover, NH, USA
2now at: Southwest Research Institute, San Antonio, TX, USA
Abstract. We present observations of waves at and just above the plasma frequency
(fpe)
from three high frequency electric field experiments on
three recent rockets launched to altitudes of 300–900 km in active aurora.
The predominant observed HF waves just above fpe are narrowband,
short-lived emissions with amplitudes ranging from
<1 mV/m to 20 mV/m, often associated with structured electron density.
The nature of these HF waves, as determined from frequency-time
spectrograms, is highly variable: in some cases, the frequency decreases
monotonically with time as in the "HF-chirps" previously reported
(McAdams and LaBelle, 1999), but
in other cases rising frequencies are observed, or features which
alternately rise and fall in frequency. They exhibit two timescales of amplitude
variation: a short timescale, typically 50–100 ms, associated with individual
discrete features, and a longer timescale associated
with the general decrease in
the amplitudes of the emissions
as the rocket moves away from where the condition f~fpe
holds. The latter timescale ranges from 0.6 to 6.0 s, corresponding to
distances of 2–7 km, assuming the phenomenon to be stationary and using
the rocket velocity to convert time to distance.
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