Neutral Horizontal Wind Research Articles

Seasonal and Interannual Variations of Global Tides in the Mesosphere and Lower Thermosphere Neutral Winds: II. Semidiurnal and Terdiurnal Tides

AbstractIn the mesosphere and lower thermosphere (MLT) region, semidiurnal and terdiurnal tides are dominant at middle and high latitudes and are important for the dynamics and structures. Using the global neutral horizontal wind data in the MLT region observed by TIMED Doppler interferometer from 2002 to 2021, the seasonal and interannual variations of six semidiurnal and terdiurnal tidal components (SW2, SW3, SW4, TW3, TW2 and TW4) are investigated. Particularly, the responses of these tidal components to the stratospheric quasi‐biennial oscillation (SQBO) and the solar cycle are presented. The results indicate that: (a) seasonal and interannual variations of these tidal components are pronounced in their peak regions. The peak region is defined as the region where the ratio of the annual mean amplitude to the maximum annual mean amplitude is larger than 0.8. (b) In their peak regions, migrating tidal components exhibit strong seasonal variations at annual (annual oscillations (AO)), semiannual (semiannual oscillations (SAO)) and terannual (terannual oscillations (TAO)) periods, whereas nonmigrating tidal components only exhibit prominent AO and SAO. (c) In their peak regions, the responses of the annual mean amplitudes of these tidal components to SQBO are negative; the responses of migrating tidal components to solar cycle are negative whereas those of nonmigrating tidal components are positive. (d) For these tidal components, the responses of the amplitudes of seasonal variations to SQBO and solar cycle are weaker than their annual mean amplitudes, and the latitude × altitude response patterns are not always consistent with those of the annual mean amplitudes.

Comparative statistical analysis of neutral wind in mid-latitude mesosphere / lower thermosphere based on meteor radar and Fabry — Perot interferome-ter data

Studies of the upper atmosphere wind are very important both for understanding the mechanism of transformation of exposure energy into heating, movement and chemical activity of the atmosphere, and similar processes of energy transfer from lower to upper atmospheric layers. Instruments and methods for studying the wind velocity and its variations at different height levels usually complement each other in such studies. We perform a comparative analysis of information about the neutral horizontal wind over Eastern Siberia in the winter-spring period obtained by different methods. We observe some features appearing at different heights during geomagnetic storms and sudden stratospheric warming events. We propose a method for comparative statistical analysis of the neutral wind registered at different sites and show its validity. Using this method, we make a quantitative comparison of winds measured over Eastern Siberia in the winter-spring period. The wind measured by the meteor radar and Fabry-Perot interferometer at 90 km has similar direction and absolute velocity. The wind measured by the Fabry—Perot interferometer at 100 km differs from that estimated by the meteor radar only in direction. The wind measured by the Fabry—Perot interferometer at 250 km has a velocity 2.5 times faster and a direction differing by 30–40° from the wind obtained by the meteor radar.

Сравнительный статистический анализ поведения скорости нейтрального ветра в области мезосферы / нижней термосферы средних широт по данным измерений метеорным радаром и интерферометром Фабри — Перо

Studies of the upper atmosphere wind are very important both for understanding the mechanism of transformation of exposure energy into heating, movement and chemical activity of the atmosphere, and similar processes of energy transfer from lower to upper atmospheric layers. Instruments and methods for studying the wind velocity and its variations at different height levels usually complement each other in such studies. We perform a comparative analysis of information about the neutral horizontal wind over Eastern Siberia in the winter-spring period obtained by different methods. We observe some features appearing at different heights during geomagnetic storms and sudden stratospheric warming events. We propose a method for comparative statistical analysis of the neutral wind registered at different sites and show its validity. Using this method, we make a quantitative comparison of winds measured over Eastern Siberia in the winter-spring period. The wind measured by the meteor radar and Fabry — Perot interferometer at 90 km has similar direction and absolute velocity. The wind measured by the Fabry — Perot interferometer at 100 km differs from that estimated by the meteor radar only in direction. The wind measured by the Fabry — Perot interferometer at 250 km has a velocity 2.5 times faster and a direction differing by 30–40° from the wind obtained by the meteor radar.

A preliminary comparison of Na lidar and meteor radar zonal winds during geomagnetic quiet and disturbed conditions

We investigate the possibility that sufficiently large electric fields and/or ionization during geomagnetic disturbed conditions may invalidate the assumptions applied in the retrieval of neutral horizontal winds from meteor and/or lidar measurements. As per our knowledge, the possible errors in the wind estimation have never been reported. In the present case study, we have been using co-located meteor radar and sodium resonance lidar zonal wind measurements over Andenes (69.27°N, 16.04°E) during intense substorms in the declining phase of the January 2005 solar proton event (21–22 January 2005). In total, 14 h of measurements are available for the comparison, which covers both quiet and disturbed conditions. For comparison, the lidar zonal wind measurements are averaged over the same time and altitude as the meteor radar wind measurements. High cross correlations (∼0.8) are found in all height regions. The discrepancies can be explained in light of differences in the observational volumes of the two instruments. Further, we extended the comparison to address the electric field and/or ionization impact on the neutral wind estimation. For the periods of low ionization, the neutral winds estimated with both instruments are quite consistent with each other. During periods of elevated ionization, comparatively large differences are noticed at the highermost altitude, which might be due to the electric field and/or ionization impact on the wind estimation. At present, one event is not sufficient to make any firm conclusion. Further study with more co-located measurements are needed to test the statistical significance of the result.

Neutral wind and density perturbations in the thermosphere created by gravity waves observed by the TIDDBIT sounder

AbstractIn this paper, we study the 10 traveling ionospheric disturbances (TIDs) observed at zobs∼283 km by the TIDDBIT ionospheric sounder on 30 October 2007 at 0400–0700 UT near Wallops Island, USA. These TIDs propagated northwest/northward and were previously found to be secondary gravity waves (GWs) from tropical storm Noel. An instrumented sounding rocket simultaneously measured a large neutral wind peak with a similar azimuth at z ∼ 325 km. Using the measured TID amplitudes and wave vectors from the TIDDBIT system, together with ion‐neutral theory, GW dissipative polarization relations and ray tracing, we determine the GW neutral horizontal wind and density perturbations as a function of altitude from 220 to 380 km. We find that there is a serious discrepancy between the GW dissipative theory and the observations unless the molecular viscosity, μ, decreases with altitude in the middle to upper thermosphere. Assuming that , where is the density, we find using GW dissipative theory that the GWs could have been observed at zobs and that one or more of the GWs could have caused the wind peak at z≃325 km if q ∼ 0.67 for z≥220 km. This implies that the kinematic viscosity, , increases less rapidly with altitude for z≥220 km: . This dependence makes sense because as , the distance between molecules goes to infinity, which implies no molecular collisions and therefore no molecular viscosity μ.

Jupiter Thermospheric General Circulation Model (JTGCM): Global structure and dynamics driven by auroral and Joule heating

A growing multispectral database plus recent Galileo descent measurements are being used to construct a self‐consistent picture of the Jupiter thermosphere/ionosphere system. The proper characterization of Jupiter's upper atmosphere, embedded ionosphere, and auroral features requires the examination of underlying processes, including the feedbacks of energetics, neutral‐ion dynamics, composition, and magnetospheric coupling. A fully 3‐D Jupiter Thermospheric General Circulation Model (JTGCM) has been developed and exercised to address global temperatures, three‐component neutral winds, and neutral‐ion species distributions. The domain of this JTGCM extends from 20‐μbar (capturing hydrocarbon cooling) to 1.0 × 10−4 nbar (including auroral/Joule heating processes). The resulting JTGCM has been fully spun‐up and integrated for ≥40 Jupiter rotations. Results from three JTGCM cases incorporating moderate auroral heating, ion drag, and moderate to strong Joule heating processes are presented. The neutral horizontal winds at ionospheric heights vary from 0.5 km/s to 1.2 km/s, atomic hydrogen is transported equatorward, and auroral exospheric temperatures range from ∼1200–1300 K to above 3000 K, depending on the magnitude of Joule heating. The equatorial temperature profiles from the JTGCM are compared with the measured temperature structure from the Galileo ASI data set. The best fit to the Galileo data implies that the major energy source for maintaining the equatorial temperatures is due to dynamical heating induced by the low‐latitude convergence of the high‐latitude‐driven thermospheric circulation. Overall, the Jupiter thermosphere/ionosphere system is highly variable and is shown to be strongly dependent on magnetospheric coupling which regulates Joule heating.

Theoretical effects of geomagnetic activity on thermospheric tides

The theoretical effects of auroral activity on thermospheric tides during equinox solar cycle minimum are investigated using simulations from the National Center for Atmospheric Research thermosphere‐ionosphere general circulation model. One set of model runs examined the effects of increasing levels of geomagnetic activity on the neutral horizontal winds and temperatures. A second set of model runs examined the generation of diurnal and semidiurnal waves in the neutral horizontal winds and temperatures by solar forcing, auroral forcing, and waves propagating vertically from the lower atmosphere. The model simulations were made for four levels of geomagnetic activity, parameterized principally by the total hemispheric power index and the potential drop across the polar cap. The resulting neutral horizontal wind and temperature fields were examined at geographic latitudes of 17.5°N, 42.5°N, and 67.5°N at 70°W longitude. The modeled response to the level of geomagnetic activity varies with altitude and latitude: the effects tend to maximize at high altitudes and high latitudes and penetrate lower in altitude as geomagnetic activity increases. The simulated mean temperatures increase and the mean winds become more southward and westward at all latitudes with increasing auroral activity. In the upper thermosphere, the model diurnal temperature amplitudes decrease with increasing activity, while the diurnal meridional wind amplitudes increase. The modeled semidiurnal winds are strongly affected by the level of geomagnetic activity, while the semidiurnal temperatures are not. Small but discernible effects are predicted at low latitudes. Analysis of the second set of model simulations focusing on the generation of the tidal waves indicates that the tidal response to auroral activity is largely determined by the interference between the waves due to upward propagating tides and in situ solar forcing and those generated by the auroral momentum and energy sources. Depending on location, the waves forced by these sources may have comparable amplitudes but different phases or may be dominated by one source. Thus at high latitudes the wave response is largely driven by the auroral sources; at low latitudes the upward propagating tides dominate. At midlatitudes, complex tidal structures are predicted, since all sources contribute.

Gravity wave parameters derived from traveling ionospheric disturbances observations in the auroral zone

Large‐scale wavelike fluctuations of ion velocity, as measured by the European incoherent scatter radar along the geomagnetic field line, have been attributed to gravity wave effects. The height‐dependent parameters of the causative gravity waves are calculated, taking into account the neutral horizontal wind and the electric field. The results are compared with the solutions of a dissipative dispersion relation. Much better agreement is achieved for the imaginary part of the vertical wave vector than for its real part. The calculated wave damping is greater than that given by theory. The possible reasons for this are discussed. It is suggested that E × B drift of the ions and vertical neutral winds, which are characteristic features of the auroral zone ionosphere, may contribute to the observed discrepancies.