[1] Thermospheric neutral density and composition exhibit a strong seasonal variation, with maxima near the equinoxes, a primary minimum during northern hemisphere summer, and a secondary minimum during southern hemisphere summer. This pattern of variation is described by thermospheric empirical models. However, the mechanisms are not well understood. The annual insolation variation due to the Sun-Earth distance can cause an annual variation, large-scale interhemispheric circulation can cause a global semiannual variation, and geomagnetic activity can also have a small contribution to the semiannual amplitude. However, simulations by the National Center for Atmospheric Research (NCAR) Thermosphere-Ionosphere-Electrodynamics General Circulation Model (TIE-GCM) indicates that these seasonal effects do not fully account for the observed annual/semiannual amplitude, primarily because of the lack of a minimum during northern hemisphere summer. A candidate for causing this variation is a change in composition, driven by eddy mixing in the mesopause region. Other observations and model studies suggest that eddy diffusion in the mesopause region has a strong seasonal variation, with eddy diffusion larger during solstices than equinoxes, and stronger turbulence in summer than in winter. A seasonal variation of eddy diffusion compatible with this description is obtained. Simulations show that when this function is imposed at the lower boundary of the TIE-GCM, neutral density variation consistent with satellite drag data and O/N 2 consistent with measurements by TIMED/GUVI, are obtained. These model-data comparisons and analyses indicate that turbulent mixing originated from the lower atmosphere may contribute to seasonal variation in the thermosphere, particularly the asymmetry between solstices that cannot be explained by other mechanisms.
Lidar measurements of mesospheric Fe were conducted for 325 h during 75 nights at Urbana, Ill. (40°N, 88°W), in fall 1989 and from spring 1991 through summer 1992. The Fe layer abundance and root‐mean‐square (RMS) width have strong annual variations, with minima in summer. The abundance varied from 3.5 × 109 to 25 × 109 cm−2 with a mean of 10.6 × 109 cm−2, and the RMS width varied from 2.3 to 5.3 km with a mean of 3.4 km. The centroid height of the Fe layer has a strong semiannual variation, with minima at the solstices. The centroid varies from 86.0 to 90.3 km and has a mean of 88.1 km. Sporadic Fe (Fes) layers were present about 27% of the total observation time. The Fe measurements are compared with the extensive Na layer observations obtained during the past decade at Urbana and with common volume observations made simultaneously on 24 nights with a Na temperature lidar. The mean Fe column abundance is approximately twice the mean Na column abundance. The Fe layer centroid height is also on average nearly 4 km lower and the RMS width is approximately 24% narrower than the corresponding Na layer parameters. A chemical model of the mesospheric Fe layer is described and compared to various experimental results. The reaction of Fe with O3 to form FeO on the bottom side of the layer and the subsequent reaction of FeO with CO2 to form FeCO3 appear to be the dominant chemical sinks for Fe. The temperature dependency of the latter reaction may explain the annual variation in the column abundance. The lidar observations and the chemical model calculations suggest that the expected cooling of the mesopause region by approximately 10 K due to the doubling of CO2 and other greenhouse gases during the next century may reduce the mean Fe abundance by as much as 45% and the mean Na abundance by 55%.
[1] The long-term change of thermospheric neutral density has been investigated using satellite drag measurements and through sensitivity studies using upper atmosphere general circulation models. The magnitude of the change has been quantified in both approaches, and the source of the secular change attributed to the concentration changes of greenhouse gases. In this study, we use CO 2 concentration measured at Mauna Loa Observatory and solar variation based on a proxy model to calculate the secular change of thermosphere neutral density for the last three decades, using a global mean upper atmosphere model. Our results show that the average density decrease at 400 km from 1970 to 2000 is 1.7% per decade. To quantify the impact of solar activity on the secular change of neutral density, we also calculated the long-term density change under solar minimum and solar maximum conditions for the same time period. The average trends at 350 km and 450 km are 2.2% per decade and 2.9% per decade for solar minimum conditions, while at solar maximum, they are 0.7% per decade and 0.8% per decade, respectively. These model results are compared to estimates of thermosphere density change derived from satellite drag observations, showing good agreement. In addition, based on a recent forecast of the intensity of solar cycle 24, we predict that the long-term change of thermospheric neutral density from 2006 to the end of solar cycle 24 will be $2.7% per decade at 400 km.
Orbital variation in reflected starlight from exoplanets could eventually be used to detect surface oceans. Exoplanets with rough surfaces, or dominated by atmospheric Rayleigh scattering, should reach peak brightness in full phase, orbital longitude = 180°, whereas ocean planets with transparent atmospheres should reach peak brightness in crescent phase near OL = 30°. Application of Fresnel theory to a planet with no atmosphere covered by a calm ocean predicts a peak polarization fraction of 1 at OL = 74°; however, our model shows that clouds, wind-driven waves, aerosols, absorption, and Rayleigh scattering in the atmosphere and within the water column, dilute the polarization fraction and shift the peak to other OLs. Observing at longer wavelengths reduces the obfuscation of the water polarization signature by Rayleigh scattering but does not mitigate the other effects. Planets with thick Rayleigh scattering atmospheres reach peak polarization near OL = 90°, but clouds and Lambertian surface scattering dilute and shift this peak to smaller OL. A shifted Rayleigh peak might be mistaken for a water signature unless data from multiple wavelength bands are available. Our calculations suggest that polarization alone may not positively identify the presence of an ocean under an Earth-like atmosphere; however polarization adds another dimension which can be used, in combination with unpolarized orbital light curves and contrast ratios, to detect extrasolar oceans, atmospheric water aerosols, and water clouds. Additionally, the presence and direction of the polarization vector could be used to determine planet association with the star, and constrain orbit inclination.
[1] Observations of secular trends in the E and F 1 regions of the ionosphere indicate that electron densities have increased, and that the height of the E-region peak has decreased, during the past several decades. Detection of trends in the upper ionosphere through analysis of F 2 -layer parameters has been more complex and controversial. In order to facilitate observational detection of long-term trends in the ionosphere, simulations were performed using a single-column upper atmosphere model. CO 2 concentrations for the year 2000 and projected for the year 2100 were used to investigate changes of electron densities and the altitudes of ionospheric layers. Results show that increased CO 2 concentration increases electron density in the lower regions of the ionosphere, but decreases electron density in the upper ionosphere. The transition altitude occurs slightly below the F 2 peak altitude (h m F 2 ). The proximity of h m F 2 to the transition altitude may explain why different analyses of long-term trends in F 2 peak density have shown both positive and negative trends. The altitudes of the E, F 1 and F 2 regions all decrease with increased CO 2 concentration.
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