G. Hernandez

An update to the Horizontal Wind Model (HWM): The quiet time thermosphere

The Horizontal Wind Model (HWM) has been updated in the thermosphere with new observations and formulation changes. These new data are ground-based 630 nm Fabry-Perot Interferometer (FPI) measurements in the equatorial and polar regions, as well as cross-track winds from the Gravity Field and Steady State Ocean Circulation Explorer (GOCE) satellite. The GOCE wind observations provide valuable wind data in the twilight regions. The ground-based FPI measurements fill latitudinal data gaps in the prior observational database. Construction of this reference model also provides the opportunity to compare these new measurements. The resulting update (HWM14) provides an improved time-dependent, observationally based, global empirical specification of the upper atmospheric general circulation patterns and migrating tides. In basic agreement with existing accepted theoretical knowledge of the thermosphere general circulation, additional calculations indicate that the empirical wind specifications are self-consistent with climatological ionosphere plasma distribution and electric field patterns.

Optical measurements of winds and kinetic temperatures in the upper atmosphere

A development history is presented for the optical probing of the structure of the upper atmosphere, noting the contributions made by both ground and spaceborne optical sensors, and the role of advancements in thermospheric theory. The significance of optical techniques to current understanding of the upper atmosphere is discussed in the context of the new COSPAR International Reference Atmosphere-1986. While much of the work performed to date has involved the analysis of data obtained by individual observatories, the ability to effectively constrain theoretical models with such single-station data has become more limited with increasing model sophistication.

Antarctic high-latitude mesospheric dynamics

Abstract The ground-based dynamical investigations of the mesosphere made at high austral latitudes from South Pole (90°S) and Scott Base (78°S, 165°E) are reviewed and discussed. The observations at these two locations show that atmospheric oscillations are reduced to zonal wavenumber zero and wavenumber one as the rotational pole is approached. These results have been theoretically explained as a natural extension of the solution of Laplaces Equation in the region near the rotational axis. The unambiguous confirmation of the theoretical results is possible because of the simultaneous measurement capability by the ground-based experiment of the wind field, kinetic temperature and emission rate.

Noctilucent cloud observed in late April at South Pole Station: Temperature anomaly or meteoritic debris?

A sunlit cloud was observed near the horizon at South Pole Station (90°S), four months after summer solstice in 1992, at a solar depression angle of 14.6°. The angular location of the transition from sunlit to twilit cloud in the photograph establishes the cloud height at about 80 km. The cloud extended horizontally at least from 81° to 85°S and from 40°W to 20°E. The probable origin of this cloud by formation of water-ice crystals near the mesopause suggests that mesospheric temperatures occasionally deviate by at least 70 K from their climatological means. An alternative explanation that the cloud was a debris cloud formed by disintegration of a reentering man-made satellite is ruled out. A debris cloud from an entering meteoroid is a possible explanation but would be unprecedented. Hydroxyl airglow emissions at the south pole in May give a frequency of 1% for mesopause temperatures below 155 K, consistent with the rarity of out-of-season sightings of noctilucent clouds in Antarctica.

Mesospheric standing waves near South Pole

Optical measurements of mesospheric winds near South Pole, during August 1996, show the presence of large-scale standing waves with periods of 51.3-hr and 107-hr, respectively. In addition, a 11.6-hr period zonal wavenumber-one oscillation with a westward phase progression was also observed. The direction of the observed mean wind has changed nearly 60° westward from the typical wind direction observed during the same month in previous years. The direction of the presently observed mean wind is nearly parallel to wind direction of the 51.3-hr standing oscillation, suggesting an interaction between the mean wind field and the wave during this observation. Based on the simultaneous observation at Scott Base of an uncommon wave with 11.53-hr periodicity with short (∼20 km) vertical wavelength of propagation, the 11.6-hr wave observed from South Pole can be identified as a zonal wavenumber one inertio-gravity mode of oscillation. Because of the transience of this wave and its closeness to the inertial period, we also discuss another approach to the interpretation of such waves as possible forced oscillations.

Mapping the wind in the polar thermosphere a case study within the CEDAR Program

The thermosphere is that region of neutral atmosphere in which atmospheric constituents are gravitationally bound to the Earth but are barometrically distributed according to their molecular or atomic weights. Unlike the lower atmosphere, mixing processes a reweak, which allows each constituent gas to behave independently. The thermosphere begins at about 100-km-altitude and extends up to 500 km or beyond. The temperature increases with height throughout the layer, which is a stabilizing influence.

Upper thermospheric temperatures at South Pole

Abstract Six years of upper thermospheric temperatures measured using 630 nm emission in the vicinity of South Pole Station (75° geomagnetic latitude) have been analyzed both on a daily and hourly averaged basis to compare observed behavior with that predicted using the MSIS86 semi-empirical atmospheric model. Daily averaged temperatures are in good general agreement, but the response of the model to geomagnetic activity appears to be too weak to match the observed changes near the peak of the solar cycle. Observations at time of low solar activity showed that temperatures tend to be lower than predicted; on the other hand, temperatures were higher than predicted at high activity. Observed diurnal temperature variations are very similar in amplitude and phase compared to those predicted by MSIS86. Observed mean temperature gradients indicated that the maximum temperature in the polar cap could well be 200K higher than the maximum seen at South Pole Station and at a position geomagnetically higher in latitude. The highest observed temperatures are found on the geomagnetic nightside associated with the exit of the cross-polar thermospheric jet towards lower latitudes and the lowest temperatures are observed on the geomagnetic dayside in the region of inflow to the polar cap. No signature of cusp heating was found.

Thermospheric dynamics in the southern polar region

The dynamical behaviour of the upper thermosphere in the southern polar region has been investigated for a range of solar and geomagnetic activity and as a function of season, using a series of simulations by a coupled thermosphere - ionosphere model. The predicted behaviour in the southern winter period can be compared with recent ground-based Fabry-Perot interferometer observations of the OI 1D emission from the Amundsen-Scott station at the South Pole.