Statistical analysis of thermospheric gravity waves from Fabry-Perot Interferometer measurements of atomic oxygen
Other literature type
Ford, E. A. K.
Aruliah, A. L.
Griffin, E. M.
(issn: 1432-0576, eissn: 1432-0576)
Data from the Fabry-Perot Interferometers at KEOPS (Sweden), Sodankylä
(Finland), and Svalbard (Norway), have been analysed for gravity wave
activity on all the clear nights from 2000 to 2006. A total of 249 nights
were available from KEOPS, 133 from Sodankylä and 185 from the Svalbard
FPI. A Lomb-Scargle analysis was performed on each of these nights to
identify the periods of any wave activity during the night. Comparisons
between many nights of data allow the general characteristics of the waves
that are present in the high latitude upper thermosphere to be determined.
Comparisons were made between the different parameters: the atomic oxygen
intensities, the thermospheric winds and temperatures, and for each
parameter the distribution of frequencies of the waves was determined. No
dependence on the number of waves on geomagnetic activity levels, or
position in the solar cycle, was found. All the FPIs have had different
detectors at various times, producing different time resolutions of the
data, so comparisons between the different years, and between data from
different sites, showed how the time resolution determines which waves are
observed. In addition to the cutoff due to the Nyquist frequency, poor
resolution observations significantly reduce the number of short-period
waves (<1 h period) that may be detected with confidence. The length of
the dataset, which is usually determined by the length of the night, was the
main factor influencing the number of long period waves (>5 h)
detected. Comparisons between the number of gravity waves detected at KEOPS
and Sodankylä over all the seasons showed a similar proportion of waves
to the number of nights used for both sites, as expected since the two sites
are at similar latitudes and therefore locations with respect to the auroral
oval, confirming this as a likely source region. Svalbard showed fewer waves
with short periods than KEOPS data for a season when both had the same time
resolution data. This gives a clear indication of the direction of flow of
the gravity waves, and corroborates that the source is the auroral oval.
This is because the energy is dissipated through heating in each cycle of a
wave, therefore, over a given distance, short period waves lose more energy
than long and dissipate before they reach their target.