Showing papers by "Martin J. Jarvis published in 2005"
TL;DR: In this article, the effect of solar and geomagnetic drivers has been minimized through use of proxy indices (aa index, sunspot number, F10.7 solar flux) to leave a residual which shows ∼20% reduction in both diurnal and semidiurnal amplitudes at midlatitudes since the middle of the century.
Abstract: [1] Climatological change in the tides in the lower thermosphere has been estimated at five different latitudes using wavelet analysis of geomagnetic data series extending back to the beginning of the 20th century. The tidal signature present in the geomagnetic data is a consequence of atmospheric tides in the dynamo region between approximately 120 and 140 km altitude. The diurnal and semidiurnal spectral power have been accumulated through each year. The effect of solar and geomagnetic drivers has been minimized through use of proxy indices (aa index, sunspot number, F10.7 solar flux) to leave a residual which shows ∼20% reduction in both diurnal and semidiurnal amplitudes at midlatitudes since the middle of the century. This midlatitude reduction is qualitatively similar to the decrease observed using wind data from 1964 onward at approximately 90 km altitude over a range of latitudes by Bremer et al. (1997), but quantitatively it is about half the magnitude. At high latitude a similar decrease would be a statistically insignificant measurement because the estimation becomes sensitive to errors in the aa index. At low latitude there is no significant change throughout the century. The observed decrease at midlatitudes appears consistent with that predicted by Ross and Walterscheid (1991) for the dynamo region current as a consequence of global ozone depletion. Reduced tidal heating of the stratosphere during the latter half of the 20th century and the consequent reduction in tidal power propagating up to the lower thermosphere is implicated.
41 citations
TL;DR: In this paper, a medium frequency spaced-antenna radar has been operating at Rothera station, Antarctica for two periods, between 1997-1998 and since 2002, measuring winds in the mesosphere and lower thermosphere.
Abstract: . A medium frequency spaced-antenna radar has been operating at Rothera station, Antarctica (67° S, 68° W) for two periods, between 1997-1998 and since 2002, measuring winds in the mesosphere and lower thermosphere. In this paper monthly mean winds are derived and presented along with three years of radiosonde balloon data for comparison with the HWM-93 model atmosphere and other high latitude southern hemisphere sites. The observed meridional winds are slightly more northwards than those predicted by the model above 80 km in the winter months and below 80 km in summer. In addition, the altitude of the summer time zero crossing of the zonal winds above the westward jet is overestimated by the model by up to 8 km. These data are then merged with the wind climatology obtained from falling sphere measurements made during the PORTA campaign at Rothera in early 1998 and the HWM-93 model atmosphere to generate a complete zonal wind climatology between 0 and 100 km as a benchmark for future studies at Rothera. A westwards (eastwards) maximum of 44 ms-1 at 67 km altitude occurs in mid December (62 ms-1 at 37 km in mid July). The 0 ms-1 wind contour reaches a maximum altitude of 90 km in mid November and a minimum altitude of 18 km in January extending into mid March at 75 km and early October at 76 km.
27 citations
TL;DR: In this article, the first observations at such high southern latitude, the first using a dynasonde for detection, and the first at 28 MHz, were made during January and February 2004.
Abstract: [1] Polar Mesosphere Summer Echoes (PMSE) are a pointer to summer mesospheric conditions and have fuelled the debate over temperature differences between the Antarctic and Arctic mesopause regions. However few PMSE observations have ever been made in Antarctica. We present initial PMSE observations from Halley (76°S, 27°W), taken during January and February 2004. These are the first observations at such high southern latitude, the first using a dynasonde for detection, and the first at 28 MHz. PMSE are frequently observed and exhibit a double maximum in diurnal occurrence in contrast to published observations at similar Arctic latitudes. The PMSE season ends slightly earlier, relative to summer solstice, than in the Arctic. The strength of the PMSE appear similar to PMSE observed in the northern hemisphere; this is consistent with published falling sphere measurements showing similar summer mesopause temperatures in January in the Antarctic as in July in the Arctic.
13 citations
TL;DR: In this article, the authors describe a new polar mesosphere summer echo (PMSE) radar system implementation that uses a snow-buried antenna operated with the Dynasonde at Halley, Antarctica (76°S), the highest southern latitude at which any PMSE measurements have been made.
Abstract: We describe a new polar mesosphere summer echo (PMSE) radar system implementation that uses a snow-buried antenna operated with the dynasonde at Halley, Antarctica (76°S), the highest southern latitude at which any PMSE measurements have been made to date. Two 100 m2 coaxial-collinear antenna arrays were built so that the antenna beams were colocated at 85 km altitude. Operations in PMSE mode began on 20 January 2004, close to the maximum operational frequency of the system at 28 MHz. PMSE signals were observed at the end of the Antarctic summer season in 2004 and subsequently during the austral summer of 2004/2005 at similar altitudes to PMSE observed in the Northern Hemisphere. We compare the sensitivity of the system to the well-known mesosphere-stratosphere-troposphere radar at Poker Flat (65°N, 147°W, 50 MHz) and calculate a volume reflectivity at 28 MHz of η = 2.9 × 10−11 m−1, which is consistent with that determined from Northern Hemisphere radar systems operating at higher frequencies.
1 citations