Conveners:
John Emmert (jemmert@gmu.edu)
Douglas Drob (douglas.drob@nrl.navy.mil)
2005 Thursday 30 June 0400 - 0600 PM
Neutral winds are an important component of the coupled ionosphere-thermosphere system, and affect the distribution of ionospheric plasma via numerous processes. Experimental and theoretical studies often require accurate estimates of neutral dynamics, but concurrent wind data are generally sparse or not available. In these cases, empirical assimilations of past winds measurement can be used to predict large-scale wind patterns for given geophysical conditions.
Currently, the only global empirical model of thermospheric neutral winds is the Horizontal Wind Model (HWM), which was last updated in 1993. Development of an upgraded empirical model is underway, and the current focus is on improving the performance of HWM in the E and F regions, particularly during geomagnetically disturbed conditions. In this workshop, we discussed large-scale thermospheric wind behavior, the impact of winds on ionospheric properties, and ways to improve empirical wind specification.
Our goal was to get CEDAR community involved in contributing and developing timely strategies for the HWM upgrade. We solicited contributions related to climatological wind results in the E and F regions as they relate to important aspects of winds in ionospheric studies. We were particularly interested in supporting numerical simulations (especially multi-year studies and investigations of systematic storm effects), which can benefit from and contribute to empirical studies. Thirty three scientists attended the workshop including the speakers and conveners. Of those thirty three about half were students. The two-hour workshop consisted of eight short presentations followed by about 45 minutes of open discussion and debate.
Douglas Drob (US Naval Research Laboratory) began the workshop session with a review of the HWM statistical data analysis and assimilation codes, overview of new upper atmospheric wind data sets, and model upgrade strategy.
Geoff Crowley (South West Research Institute) discussed the need for a rigorous data based validations of winds in current TIEGCMs, as well as the utility of updating the current HWM empirical wind model for the purposes of specifying neutral atmospheric inputs in first-principles ionospheric models which lack a theoretical neutral atmosphere. In both instances there is a need to understand the geophysical drivers of the neutral thermospheric and ionospheric climatology. In addition, just as statistical and empirical data analysis can provide valuable insight in to current theoretical thermospheric and ionospheric modeling efforts, TIEGCM modeling efforts can provide valuable information for the development of improved mathematical representations for empirical models, especially synthetic data sets for the development of empirical data assimilation procedures. A series of multi-year TIEGCM runs will be generated to develop and test a new HWM model formulation.
Larisa Goncharenko (Haystack Observatory) presented new data and a climatology of lower thermospheric winds over Millstone Hill. In general, current empirical and tabular climatologies under-estimate the magnitude of the winds over Millstone Hill, but there is relatively consistent agreement with seasonal and tidal phases. In addition, good agreement is seen between the Millstone Hill wind observations and observations made by the NASA-UARS WINDII instrument. The new ISR data set shows differences from earlier Millstone Hill climatologies, including larger wind magnitudes, greater differences between equinoxes, and the occurrence of a distinct westward wind in summer morning. Planning for a continuous 30-day data collection campaign, to be conducted in September 2005, was discussed.
Michael Faivre (Clemson University) presented results from the Arequipa, Peru FPI. This instrument was recently outfitted with a new CCD camera that improved instrument sensitivity. Data processing techniques for estimating the line-of-site wind velocities and temperatures from the circular fringe pattern on the new CCD were presented. This was followed by a presentation of line-of-site wind measurements. The relationship between measured temperatures and wind, as well as divergence and convergence quantities were in the context of the midnight temperature maximum phenomena, other low latitude temperature enhancements, and the terdiurnal tidal oscillation.
Next, Geonhwa Jee (Utah State University) presented results of a theoretical study of the sensitivity of total electron content (TEC) to neutral winds. It was demonstrated that the neutral wind has a significant effect on the mid-latitude ionospheric plasma density both during the day and night. Without meridional neutral winds there would be a factor of 2 increase of TEC during the daytime, and a factor of 4 decrease of TEC during the nighttime, thus highlighting the importance on meridional neutral winds on the maintenance of the mid-latitude nighttime ionosphere. The effect of the zonal wind showed a strong dependence on the magnetic declination angle, and consequently on geographic longitude.
Joe Huba (US Naval Research Laboratory) similarly discussed the influence of wind specifications on modeled ionospheric plasma density profiles. The impact of different neutral wind specifications on calculations of the low- to mid-latitude ionosphere calculated by the SAMI2 model was shown. Changes in the neutral wind specification can significantly modify the calculated width and symmetry of the ionization crests. A neutral wind/ionosphere coupling mechanism was offered to explain observed `brightness waves' associated with the midnight temperature maximum. Model calculations with SAMI2 and TIEGCM using HWM specified winds were unable reproduce or explain the observed phenomena, whereas calculations using the SLIM model (Anderson) in conjunction with SAMI2 showed consistency with the observations.
Bela Fejer (Utah State University) discussed low-latitude winds and electric fields. The coupling of quiet-time thermospheric wind to the electric field varies significantly with latitude, season, and solar flux. Geomagnetic storm generated perturbation winds are more closely coupled to plasma drifts than is the case during quiet conditions. Comparison of thermosphic winds and plasma drift provides important information on ionospheric conductivities, which are otherwise difficult to measure directly.
John Emmert (George Mason University) high-lighted quiet and disturbed wind features seen in recent ground-based and satellite wind measurements that are potentially worth including in the new HWM. These included reformulation and specification of the global disturbance wind patterns, including clear seasonal and solar activity dependence. In addition, the new data indicates greater solar cycle dependence, including coupled seasonal effects during geomagnetically quiet conditions. Additional equinoctial asymmetries in the high-latitude wind patterns were also shown to be of significance. Finally, the known organization of high-latitude winds in geomagnetic coordinates was highlighted and supported using recent high-quality experimental data wind data sets.
In addition to discussion resulting from the items presented above, several main issues relating to the development of the next-generation model were clearly recognized as important to the CEDAR-GEM community in the open discussion that followed. The first was the need to continue collecting high-quality wind data in the lower-thermosphere, particularly over the complete range of possible geophysical conditions, latitudes, longitudes, and local time. Data in the night-time E-region is desperately needed as the number of techniques to do so is currently very limited. The second important issue was related to a proper formulation and representation of low-latitude terdiurnal tidal component, believed to be related to the MTM and other significant ionospheric phenomena. Finally, of great importance to thermospheric modelers, and also to the eventual evolution of HWM into a full fledged operational data assimilation system that can be utilized in combination with current first principles models, is the proper specification and provision of the statistical uncertainty. Such a specification would estimate the natural geophysical variability beyond the slowly varying seasonal and tidal fluctuations of the upper atmospheric winds, and would be determined from the residual noise of the comprehensive data sets after proper estimation and removal of the seasonal, tidal and other low -order variability represented by the HWM.