J. F. Helmboldt, T. J. W. Lazio, H. T. Intema, K. F. Dymond
We have used a relatively long, contiguous VHF observation of a bright cosmic
radio source (Cygnus A) with the Very Large Array (VLA) through the nighttime,
midlatitude ionosphere to demonstrate the phenomena observable with this
instrument. In a companion paper, we showed that the VLA can detect
fluctuations in total electron content (TEC) with amplitudes of <0.001 TECU and
can measure TEC gradients with a precision of about 0.0002 TECU/km. We detail
two complementary techniques for producing spectral analysis of these TEC
gradient measurements. The first is able to track individual waves with
wavelengths of about half the size of the array (~20 km) or more. This
technique was successful in detecting and characterizing many medium-scale
traveling ionospheric disturbances (MSTIDs) seen intermittently throughout the
night and has been partially validated using concurrent GPS measurements.
Smaller waves are also seen with this technique at nearly all times, many of
which move in similar directions as the detected MSTIDs. The second technique
allows for the detection and statistical description of the properties of
groups of waves moving in similar directions with wavelengths as small as 5 km.
Combining the results of both spectral techniques, we found a class of
intermediate and small scale waves which are likely the quasi-periodic (QP)
echoes that have been observed to occur within sporadic-E (Es) layers. We find
two distinct populations of these waves. The members of one population are
coincident in time with MSTIDs and are consistent with being generated within
Es layers by the E-F coupling instability. The other population seems more
influenced by the neutral wind, similar to the predominant types of QP echoes
found by the Sporadic-E Experiments over Kyushu (Fukao et al. 1998; Yamamoto et
al. 2005).
View original:
http://arxiv.org/abs/1201.3874
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