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- ItemOn the longitudinal structure of the transient day-to-day variation of the semidiurnal tide in the mid-latitude lower thermosphere - I. Winter season(München : European Geopyhsical Union, 2001) Merzlyakov, E.G.; Portnyagin, Yu.I.; Jacobi, C.; Mitchell, N.J.; Muller, H.G.; Manson, A.H.; Fachrutdinova, A.N.; Singer, W.; Hoffmann, P.The longitudinal structure of the day-to-day variations of semidiurnal tide amplitudes is analysed based on coordinated mesosphere/lower thermosphere wind measurements at several stations during three winter campaigns. Possible excitation sources of these variations are discussed. Special attention is given to a nonlinear interaction between the semidiurnal tide and the day-to-day mean wind variations. Data processing includes the S-transform analysis which takes into account transient behaviour of secondary waves. It is shown that strong tidal modulations appear during a stratospheric warming and may be caused by aperiodic mean wind variations during this event.
- ItemSome anomalies of mesosphere/lower thermosphere parameters during the recent solar minimum(Göttingen : Copernicus, 2011) Jacobi, C.; Hoffmann, P.; Placke, M.; Stober, G.The recent solar minimum has been characterized by an anomalous strong decrease of thermospheric density since 2005. Here we analyze anomalies of mesosphere/lower thermosphere parameters possibly connected with this effect. In particular, nighttime mean LF reflection heights measured at Collm, Germany, show a very strong decrease after 2005, indicating a density decrease. This decrease is also visible in mean meteor heights measured with VHF meteor radar at Collm. This density decrease is accompanied by an increase of gravity wave (GW) amplitudes in the upper mesosphere and a decrease in the lower thermosphere. On the decadal scale, GWs are negatively correlated with the background zonal wind, but this correlation is modulated in the course of the solar cycle, indicating the combined effect of GW filtering and density decrease.
- ItemDistortion of meteor count rates due to cosmic radio noise and atmospheric particularities(Göttingen : Copernicus, 2010) Stober, G.; Jacobi, C.; Keuer, D.The determination of the meteoroid flux is still a scientifically challenging task. This paper focusses on the impact of extraterrestrial noise sources as well as atmospheric phenomena on the observation of specular meteor echoes. The effect of cosmic radio noise on the meteor detection process is estimated by computing the relative difference between radio loud and radio quiet areas and comparing the monthly averaged meteor flux for fixed signal-to-noise ratios or fixed electron line density measurements. Related to the cosmic radio noise is the influence of D-layer absorption or interference with sporadic E-layers, which can lead to apparent day-to-day variation of the meteor flux of 15-20%. © 2010 Author(s).
- ItemEmerging pattern of global change in the upper atmosphere and ionosphere(München : European Geopyhsical Union, 2008) Laštovička, J.; Akmaev, R.A.; Beig, G.; Bremer, J.; Emmert, J.T.; Jacobi, C.; Jarvis, M.J.; Nedoluha, G.; Portnyagin, Yu. I.In the upper atmosphere, greenhouse gases produce a cooling effect, instead of a warming effect. Increases in greenhouse gas concentrations are expected to induce substantial changes in the mesosphere, thermosphere, and ionosphere, including a thermal contraction of these layers. In this article we construct for the first time a pattern of the observed long-term global change in the upper atmosphere, based on trend studies of various parameters. The picture we obtain is qualitative, and contains several gaps and a few discrepancies, but the overall pattern of observed long-term changes throughout the upper atmosphere is consistent with model predictions of the effect of greenhouse gas increases. Together with the large body of lower atmospheric trend research, our synthesis indicates that anthropogenic emissions of greenhouse gases are affecting the atmosphere at nearly all altitudes between ground and space.
- ItemConnection between the length of day and wind measurements in the mesosphere and lower thermosphere at mid- and high latitudes(Göttingen : Copernicus GmbH, 2019) Wilhelm, S.; Stober, G.; Matthias, V.; Jacobi, C.; J, Murphy, D.This work presents a connection between the density variation within the mesosphere and lower thermosphere (MLT) and changes in the intensity of solar radiation. On a seasonal timescale, these changes take place due to the revolution of the Earth around the Sun. While the Earth, during the northern-hemispheric (NH) winter, is closer to the Sun, the upper mesosphere expands due to an increased radiation intensity, which results in changes in density at these heights. These density variations, i.e., a vertical redistribution of atmospheric mass, have an effect on the rotation rate of Earth's upper atmosphere owing to angular momentum conservation. In order to test this effect, we applied a theoretical model, which shows a decrease in the atmospheric rotation speed of about ĝ1/44 m sĝ'1 at a latitude of 45ĝ in the case of a density change of 1 % between 70 and 100 km. To support this statement, we compare the wind variability obtained from meteor radar (MR) and Microwave Limb Sounder (MLS) satellite observations with fluctuations in the length of a day (LOD). Changes in the LOD on timescales of a year and less are primarily driven by tropospheric large-scale geophysical processes and their impact on the Earth's rotation. A global increase in lower-atmospheric eastward-directed winds leads, due to friction with the Earth's surface, to an acceleration of the Earth's rotation by up to a few milliseconds per rotation. The LOD shows an increase during northern winter and decreases during summer, which corresponds to changes in the MLT density due to the Earth-Sun movement. Within the MLT the mean zonal wind shows similar fluctuations to the LOD on annual scales as well as longer time series, which are connected to the seasonal wind regime as well as to density changes excited by variations in the solar radiation. A direct correlation between the local measured winds and the LOD on shorter timescales cannot clearly be identified, due to stronger influences of other natural oscillations on the wind. Further, we show that, even after removing the seasonal and 11-year solar cycle variations, the mean zonal wind and the LOD are connected by analyzing long-term tendencies for the years 2005-2016. © Author(s) 2019.
- ItemGlobal distribution of the migrating terdiurnal tide seen in sporadic e occurrence frequencies obtained from gps radio occultations(Dordrecht [u.a.] : Springer, 2014) Fytterer, T.; Arras, C.; Hoffmann, P.; Jacobi, C.Global Positioning System radio occultation measurements by FORMOsa SATellite mission-3/Constellation Observing System for Meteorology, Ionosphere and Climate satellites were used to analyse the characteristics of the 8-h oscillation in sporadic E (E S ) layers. Six-year averages based on the 3-monthly mean zonal means from December 2006 to November 2012 were constructed for the amplitude of the terdiurnal oscillation in the occurrence frequency of E S . A global distribution from 60° S to 60° N is given, revealing two peaks above 100 km during solstice with one maximum at low and midlatitudes (approximately 10° to 40°) in each hemisphere. During equinox, the global distribution is marked by two dominant peaks centred at midlatitudes, while an additional weak maximum is located at very low southern latitudes. The seasonal characteristics around 110 km reveal large values during equinox at low and midlatitudes (<40° N), while further peaks occur in April at >40° S and in July near 30° S. The pattern around 90 km is dominated by a broad peak between 20° and 30° S from March to September. Comparisons with the terdiurnal oscillation in the neutral atmosphere derived from zonal wind and vertical zonal wind shear simulated with a circulation model of the middle atmosphere, as well as with satellite observations of the terdiurnal tide in temperature, fit quite well for the results above 100 km, but do not show agreement for lower altitudes.
- ItemRetrieving horizontally resolved wind fields using multi-static meteor radar observations(Göttingen : Copernicus GmbH, 2018) Stober, G.; Chau, J.L.; Vierinen, J.; Jacobi, C.; Wilhelm, S.Recently, the MMARIA (Multi-static, Multi-frequency Agile Radar for Investigations of the Atmosphere) concept of a multi-static VHF meteor radar network to derive horizontally resolved wind fields in the mesosphere-lower thermosphere was introduced. Here we present preliminary results of the MMARIA network above Eastern Germany using two transmitters located at Juliusruh and Collm, and five receiving links: two monostatic and three multi-static. The observations are complemented during a one-week campaign, with a couple of addition continuous-wave coded transmitters, making a total of seven multi-static links. In order to access the kinematic properties of non-homogenous wind fields, we developed a wind retrieval algorithm that applies regularization to determine the non-linear wind field in the altitude range of 82-98 km. The potential of such observations and the new retrieval to investigate gravity waves with horizontal scales between 50-200 km is presented and discussed. In particular, it is demonstrated that horizonal wavelength spectra of gravity waves can be obtained from the new data set. © Author(s) 2018.