Turbopause Height Research Articles

The possible effect of turbopause on formation of mid-latitude sporadic E layers

Sporadic E (Es) layers are thin enhanced ionization patches. The electron density of Es layer is very high, up to 100 times that of the regular E layer. Therefore, Es layer has a significant effect on radio wave propagation. The formation mechanisms of Es layers is very complicated, so it needs multi-observations and simulations research. Combined observations using an ionosonde, very high frequency (VHF) coherent backscatter radar, VHF broadband backscatter radar, and meteor radar were performed. The height and irregular structure of mid-latitude Es layers observed by ionosonde, VHF coherent backscatter radar and VHF broadband backscatter radar were compared with the zonal and meridional winds observed by meteor radar, the role of wind shear on the formation of Es layer was confirmed, and examples were also found where wind shear theory could not explain and apply to the formation of Es layer. We simulated the formation of Es layers based on wind shear theory. The results of simulations and observations revealed other potential role for Es layer formation besides wind shear. By comparing the height of turbopause with that of Es layer, it was found that the variation of the altitudes of turbopause and Es layer was consistent over different latitude stations, and the effect of the turbopause on the formation of Es layer was proposed. The combined effects of eddy diffusion below and molecular ion collision above turbopause can prevent metal ions from drifting downward and promote their convergence to form thin plasma layers at the turbopause height. The turbopause accumulation effect is considered as a potentially important mechanism for the formation of Es layers besides wind shear theory. The study in the paper is helpful for analysis of physical origin and modeling of Es layer.

Dynamics of the Mesosphere and Lower Thermosphere Based on Results of Observations on the SURA Facility

The results of studies of atmospheric dynamics in the altitude range of 60–130 km are presented based on measurements of the velocity of vertical plasma motion, the temperature and density of the neutral component, and the turbopause height. The measurements were carried via resonant scattering of radio waves by artificial periodic inhomogeneities of the ionospheric plasma. The method is based on ionospheric perturbation by powerful high-frequency radio emission, the creation of periodic inhomogeneities in the field of a standing wave, which forms upon reflection from the ionosphere of a powerful radio wave emitted into the zenith, and the location of inhomogeneities by test radio waves. The parameters of the neutral component, the turbulent velocity, and the height of the turbopause are determined from the relaxation time of the signal scattered by the inhomogeneities after the end of the ionospheric impact. The measurement of the phase of the scattered signal makes it possible to determine the rate of vertical plasma motion. The experiments were carried out on a SURA heating facility (56.15° N; 46.11° E). The paper presents and analyzes the time–altitude dependences of the vertical velocity and temperature, which are largely due to the propagation of atmospheric waves during various natural phenomena. The results of determination of the turbopause height and turbulent velocity are presented.

Change in turbopause altitude at 52 and 70° N

Abstract. The turbopause is the demarcation between atmospheric mixing by turbulence (below) and molecular diffusion (above). When studying concentrations of trace species in the atmosphere, and particularly long-term change, it may be important to understand processes present, together with their temporal evolution that may be responsible for redistribution of atmospheric constituents. The general region of transition between turbulent and molecular mixing coincides with the base of the ionosphere, the lower region in which molecular oxygen is dissociated, and, at high latitude in summer, the coldest part of the whole atmosphere. This study updates previous reports of turbopause altitude, extending the time series by half a decade, and thus shedding new light on the nature of change over solar-cycle timescales. Assuming there is no trend in temperature, at 70° N there is evidence for a summer trend of ∼ 1.6 km decade−1, but for winter and at 52° N there is no significant evidence for change at all. If the temperature at 90 km is estimated using meteor trail data, it is possible to estimate a cooling rate, which, if applied to the turbopause altitude estimation, fails to alter the trend significantly irrespective of season. The observed increase in turbopause height supports a hypothesis of corresponding negative trends in atomic oxygen density, [O]. This supports independent studies of atomic oxygen density, [O], using mid-latitude time series dating from 1975, which show negative trends since 2002.

Long‐term trends in thermospheric neutral temperature and density above Millstone Hill

AbstractIncoherent scatter radar measurements of ionospheric temperature and density collected above Millstone Hill over the years 1976–2013 are analyzed to show the long‐term trends in noontime neutral temperature and neutral O density over the height region 120–500 km. Exospheric temperature cooled by 69.3 ± 6.4 K over the period, an order of magnitude greater than that expected from greenhouse gas action. The O density dropped 0.081 ± 5.6% at 400 km altitude but rose by 36.9 ± 5.0% at 120 km over this period. This trend in density at 400 km agrees with that determined from satellite drag. The increase in density at 120 km counteracts the thermal contraction of the thermosphere expected to be associated with the cooling, resulting in only a small density response at 400 km. The long‐term O density increase at 120 km may be caused by a long‐term descent of the turbopause height of 4.2 km. Such a descent has been documented by a series of rocket mass spectrometer measurements.

Quantitative characteristics of long-term changes in parameters of the upper atmosphere of the earth over the 1966–1992 period

On the basis of the long series of mass-spectrometer measurements conducted in rocket experiments during 1966–1992 period, long-term changes in the structural parameters of the upper atmosphere within the 100–160 km height interval at equatorial, middle, and high latitudes of the Northern Hemisphere of the Earth are quantitatively estimated. At all latitudes statistically significant negative trends are revealed in the turbopause height, temperature of the neutral atmosphere, and atomic oxygen concentration. Since the contribution to the obtained quantitative estimates of the trends caused by long-term solar-induced factors does not exceed 15–20%, a conclusion is drawn on the anthropogenic nature of the changes having occurred in the upper atmosphere.

Semidiurnal thermal tide in the mesopause region over Yakutia

The study is based on measuring fluctuations of the intensity and rotational temperatures of the molecular emissions of hydroxyl OH(6,2) and the first atmospheric band of oxygen O2(0–1), excited at approximately 87 and 95 km, respectively. The measurements are conducted at Maimaga station (63°N, 129.5°E), located 150 km north of Yakutsk. The semidiurnal tide parameters were obtained using the database compiled from 1999 to 2005. The data obtained from October to March were analyzed. The measurements conducted during 214 nights were used to determine the semidiurnal tide parameters. The wave amplitude at the height of the molecular oxygen emission (∼95 km) is 8 K, which is larger than the amplitude at the height of the hydroxyl emission (∼87 km) by approximately 2 K. Except November, the 12-h oscillation at the height of molecular oxygen excitation leads the oscillation at the height of hydroxyl excitation. On average, the phase is ∼5.7 h at the OH emission height and ∼6.4 h at the O2 emission height. We note that an abrupt increase in the tide amplitude in March at the molecular oxygen height can be related to a seasonal decrease in the so-called “wave” turbopause height.

TOMEX: Mesospheric and lower thermospheric diffusivities and instability layers

The Turbulent Oxygen Mixing Experiment (TOMEX), which was carried out at White Sands Missile Range in New Mexico on 26 October 2000, included a rocketborne trimethyl aluminum (TMA) chemical tracer experiment. The subsequent TMA trails provided detailed information about the horizontal neutral wind, turbulence, and diffusivity properties of the atmosphere between approximately 85 and 140 km altitude. Measurements with the University of Illinois Na wind/temperature lidar located at the Starfire Optical Range, NM, provided a detailed time history of the stability properties between 85 and 105‐km altitude, including high‐resolution wind and temperature measurements prior to and during the chemical tracer measurements. The diffusivities estimated from the trail expansion rates have values consistent with the values expected for molecular diffusion above 110‐km altitude and values that are larger than those for molecular diffusion at most altitudes below. Below 103 km, both regions of dynamic and convective instability were found, and the diffusivities are strongly controlled by the instabilities. The unstable regions are well mixed, but the intermediate regions, in some cases, have very small eddy energy dissipation rates. The nearly instantaneous measurements also suggest that eddy diffusion is still important in the height range between 103 km, the nominal turbopause height, and 110 km.

An analysis of tidal and planetary waves in the neutral winds and temperature observed at low‐latitude E region heights

We present an extensive analysis of tidal and planetary waves in the altitude range of 94 to 144 km for the January 20–30, 1993, period using the temperature and winds measured by the Arecibo incoherent scatter radar (ISR). This is the first time that simultaneous observational results for the 6–8, 12, 24 hour tides and a quasi 2‐day planetary wave at E region heights have been reported at tropical latitudes. In order to derive the major oscillations from the mainly daytime data, we fill in the nighttime periods with assumed data values and large error bars when valid measurements are not obtainable and then fit the data using the measured and assumed errors as weight. Simulated results show that such a method is demonstratively better than leaving the nighttime as a gap. The salient features of the tidal analysis results include the following: (1) Despite their large day‐to‐day variabilities in amplitude, the 6–8 hour oscillations are shown to be upward propagating tides. (2) The vertical wavelength of the semidiurnal tide in the zonal wind lengthens from 25 to 110 km for the altitude range from 94 to 135 km while the vertical wavelength of the meridional wind remains constant. The semidiurnal component of the meridional wind experiences little dissipation above 106 km, which is indicative of the turbopause height. (3) The diurnal tide, which has often been assumed to be negligible in earlier observations, can be quite substantial above 110 km. (4) A 2‐day planetary wave dominates the diurnal and semidiurnal tides in the meridional wind between 97 and 108 km. Continuous ISR operation during the January 20–30, 1993, campaign also provides an opportunity to examine the tidal variability both in phase and amplitude.

Investigation of acoustic gravity waves in the upper atmosphere by the artificial periodic inhomogeneities method at Nizhny Novgorod

We present the results of observation of acoustic gravity waves in the height range 60–120 km. AGW were studied by means of plasma velocity measurements. The observations, based upon use of a newly-developed diagnostic technique, were carried out during September 1990–May 1991. The method uses the artificial periodic inhomogeneities (API) which occur in the ionosphere illuminated by high- power HF radio waves. This method, based upon measurements of phases of waves back scattered from API, has a time resolution sufficient to observe short-term variations in the atmosphere. Seasonal variations of daily averaged vertical velocities were obtained. Above the turbopause height upward vertical motions dominated. Wave-like oscillations in the vertical velocity, which are most probably caused by AGW, occurred.

Changes in thermospheric density caused by turbulence variations

Seasonal and geomagnetic density variations in the thermosphere can be explained either by a system of thermally driven winds or, equally well, by changes in the turbulence structure. Observations show that both processes take place, but their relative importance has not yet been established. In this paper, a model incorporating both mechanisms is presented. Whereas seasonal changes may be generated by changes in the turbopause height, geomagnetic perturbations may be associated with a turbulent layer above the homopause region.

Evidence for turbopause control on neutral density variations in the lower thermosphere

Data obtained from in situ neutral composition measurements over the dip equator are used in explaining the altitude variation of relative composition. The turbopause height ( h 1) and the model exospheric temperatures ( T ∞) are used in explaining the altitude variation of [O 2] [N 2] . Although the measured [O 2] [N 2] ratio at any altitude in the lower thermosphere does not show any direct relation to either the exospheric temperature or the turbopause height, it shows a fairly linear relation to the product of h t × T ∞. This is clear evidence for the altitude distribution of the neutral species being controlled by the mutually independent variables, viz. the turbopause height and the exospheric temperature.

Direct and indirect estimates of turbulence around the turbopause

Direct and indirect estimates of turbulence parameters at altitudes 80–120 km are reviewed. It is shown that there are contradictions in data on absolute value, shape, seasonal variations etc. of eddy diffusion coefficient K t revealed or accepted by different authors. The strongest contradiction is in the views on the seasonal variations of turbulence. Possible explanation may be connected with the correct account for mean motions. Data on turbopause height obtained from neutral composition data (Ar, N 2) are discussed. The revealed reverse connection of this height with the temperature at 120 km is discussed in terms of K t dependence on temperature gradient.

Thermospheric model calculations based on turbulence data

Thermospheric model calculations are presented which explain the seasonal compositional changes of helium and argon by the combined effect of seasonal-latitudinal variations of turbulence and global seasonal winds. The observational base of the model calculations is given by empirical data of upper thermospheric densities and by estimates of the turbopause height derived from composition measurements and incoherent scatter temperatures in the lower thermosphere. The results are compared with observations of the seasonal variability of atomic oxygen in the turbopause region.

Semi-theoretical global models of the eddy diffusion coefficient based on satellite data

Long periodic density variations of the thermospheric constituents can be described by combined variations of two parameters: exospheric temperature and turbopause height. Two global models of thermospheric composition relying on exospheric temperature and turbopause height variations were presented by the authors in a previous paper. In this paper the turbopause height changes and their interpretation in terms of the eddy diffusion coefficient are discussed. The determination of the eddy diffusion coefficient is not unique as some assumptions have to be made with respect to the shape of the height profile of the eddy diffusion coefficient. Two models are presented: one for a height independent and one for a height dependent profile of the eddy diffusion coefficient. The models are discussed and compared with other results.

Semi-empirical models of the neutral atmosphere based on turbopause height and exospheric temperature variations

Based on mass spectrometer observations of the satellites Ogo 6 and Esro 4 the long periodic density variations of He, O, N 2 and Ar are interpreted to be a result of combined variations of the exospheric temperature and the turbopause height. Only these two parameters are used in the models. Thus in comparison with previous models the number of parameters needed is significantly reduced. This simplification does not diminish the accuracy of the analytic representation of the observations and provides new physical insight into the processes which govern the dynamic behaviour of the neutral thermosphere. The annual variation of the turbopause height is found to cover the height range between 90 and 115 km and is approximately equal for both the Ogo 6 and the Esro 4 data.

Atmospheric composition in the lower thermosphere

Atomic-to-molecular oxygen concentrations ratio values in upper atmosphere used to determine strength of turbulent mixing in atmosphere, noting definition of turbopause height