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Polar vortex evolution during Northern Hemispheric winter 2004/05
(Journal article; Tidsskriftartikkel; Peer reviewed, 2007)
The 16-day planetary waves: multi-MF radar observations from the arctic to equator and comparisons with the HRDI measurements and the GSWM modelling results
(Journal article; Peer reviewed; Tidsskriftartikkel, 2002)
The mesospheric and lower thermospheric (MLT) winds (60–100 km) obtained by multiple MF radars, located from the arctic to equator at Tromsø (70° N, 19° E), Saskatoon (52° N, 107° W), London (43° N, 81° W), Hawaii (21° N, 157° W) and Christmas Island (2° N, 157° W), respectively, are used to study the planetary-scale 16-day waves. Based on the simultaneous observations (1993/1994), the variabilities ...
Periodicities in energy dissipation rates in the auroral mesosphere/lower thermosphere
(Journal article; Peer reviewed; Tidsskriftartikkel, 2003)
It is possible for medium-frequency (MF) radar systems to estimate kinetic energy dissipation rates by measuring signal fading times. Here, we present approximately 5 years of such results from Tromsø (69° N, 19° E) and in particular, investigate the periodicities present at different altitudes in the regime 80 to 100 km. We detect the known annual variation in the mesosphere and the semiannual ...
Testing the hypothesis of the influence of neutral turbulence on the deduction of ambipolar diffusivities from meteor trail expansion
(Journal article; Peer reviewed; Tidsskriftartikkel, 2005-03-30)
Fading times of radar echoes from underdense meteor trails in the upper mesosphere/lower thermosphere are commonly used to determine ambipolar diffusivities and hence ambient temperature. Diffusivities are generally expected to increase exponentially with height through the region from which the meteor trail echoes are obtained, viz., typically 70-110km altitude for a ~30-MHz radar. In practice, ...
Isolated lower mesospheric echoes seen by medium frequency radar at 70º N, 19º E
(Journal article; Peer reviewed; Tidsskriftartikkel, 2006-11-23)
We have noted sporadic instances of strong isolated
reflections of medium frequency (MF) radar waves
from the mesosphere from as low as 50 km altitude and have
devised a set of criteria for isolating these apparently anomalous
echoes from those normally occurring from progressive
partial reflections in the D-region. The object of this study
is to map the occurrences of such echoes facilitating ...
On the possibility of ion-drag to induce dynamic instability in the lower thermosphere neutral gas
(Journal article; Tidsskriftartikkel; Peer reviewed, 2000)
Strong wind shears may result in dynamic instability, often characterised by the Richardson number lying between zero and 0.25. The extent to which electric-field driven ion flow may induce such neutral wind shears is examined. Further, it is proposed that, in the ionosphere, it is possible for electric fields to drive ion winds such that the collisionally induced neutral air response may be comparable ...
An analogy to the Reynolds number for the neutral gas component of a weak plasma
(Journal article; Tidsskriftartikkel; Peer reviewed, 2001)
The Reynolds number Re is used as a metric to assess whether or not a flow may contain turbulence. In a weakly ionised gas with an external electric field imposed, ions exert a drag on the neutral particles. Thus, a component of the neutral motion is attributable to the ion-drag. An analogy to Re has been proposed in which the ion-drag-induced velocity contribution to the neutral motion is used. ...
Climatic trends in E-region critical frequency and virtual height above Tromsø (70 degrees N, 10 degrees E)
(Journal article; Tidsskriftartikkel; Peer reviewed, 2007-11-29)
We have examined the long time series of observations
of E-region virtual height (1948–2006) and critical frequency
(1935-2006) hitherto made by the Tromsø ionosonde
at 70 degrees N, 19 degrees E. Combining a simplistic trend analysis with
a rigorous treatment of errors we identify a negative trend
in critical frequency. While a similar analysis of the virtual
height h'E also suggests a negative ...
Tidal signatures in mesospheric turbulence
(Journal article; Peer reviewed; Tidsskriftartikkel, 2006)
We search for the presence of tidal signatures in high latitude mesospheric turbulence as parameterized by turbulent energy dissipation rate estimated using a medium frequency radar, quantifying our findings with the aid of correlation analyses. A diurnal periodicity is not particularly evident during the winter and spring months but is a striking feature of the summer mesopause. While semidiurnal ...
Vertical and interhemispheric links in the stratosphere-mesosphere as revealed by the day-to-day variability of Aura-MLS temperature data
(Journal article; Tidsskriftartikkel; Peer reviewed, 2009)