T. L. Killeen

Revised global model of thermosphere winds using satellite and ground-based observations

Thermospheric wind data obtained from the Atmosphere Explorer E and Dynamics Explorer 2 satellites have been combined with wind data for the lower and upper thermosphere from ground-based incoherent scatter radar and Fabry-Perot optical interferometers to generate a revision (HWM90) of the HWM87 empirical model and extend its applicability to 100 km. Comparison of the various data sets with the aid of the model shows in general remarkable agreement, particularly at mid and low latitudes. The ground-based data allow modeling of seasonal/diurnal variations, which are most distinct at mid latitudes. While solar activity variations are now included, they are found to be small and not always very clearly delineated by the current data. They are most obvious at the higher latitudes. The model describes the transition from predominately diurnal variations in the upper thermosphere to semidiurnal variations in the lower thermosphere and a transition from summer to winter flow above 140 km to winter to summer flow below. Significant altitude gradients in the wind are found to extend to 300 km at some local times and pose complications for interpretation of Fabry-Perot observations.

Diurnal nonmigrating tides from TIMED Doppler Interferometer wind data : Monthly climatologies and seasonal variations

[1] TIMED Doppler Interferometer (TIDI) measurements of zonal and meridional winds in the mesosphere/lower thermosphere are analyzed for diurnal nonmigrating tides (June 2002 to June 2005). Climatologies of monthly mean amplitudes and phases for seven tidal components are presented at altitudes between 85 and 105 km and latitudes between 45°S and 45°N (westward propagating wave numbers 2, 3, and 4; the standing diurnal tide; and eastward propagating wave numbers 1, 2, and 3). The observed seasonal variations agree well with 1991-1994 UARS results at 95 km. Comparisons between the TIDI results and global scale wave model (GSWM) and thermosphere-ionosphere-mesosphere-electrodynamics general circulation model (TIME-GCM) tidal predictions indicate that the large eastward propagating wave number 3 amplitude is driven by tropical tropospheric latent heat release alone. In contrast, latent heating and planetary wave/ migrating tidal interactions are equally important to westward 2 and standing diurnal tidal forcing. There is good quantitative agreement between TIDI and the model predictions during equinox, but the latter tend to underestimate the westward 2 and standing diurnal tide during solstice. Neither model reproduces the observed seasonal variations of the eastward propagating components.

Geomagnetic storm effects in the low‐ to middle‐latitude upper thermosphere

In this paper, we use data from the Dynamics Explorer 2 (DE 2) satellite and a theoretical simulation made by using the National Center for Atmospheric Research thermosphere/ionosphere general circulation model (NCAR-TIGCM) to study storm-induced changes in the structure of the upper thermosphere in the low- to middle-latitude (20°-40°N) region of the winter hemisphere. Our principal results are as follows: (1) The winds associated with the diurnal tide weaken during geomagnetic storms, causing primarily zonally oriented changes in the evening sector, few changes in the middle of the afternoon, a combination of zonal and meridional changes in the late morning region, and mainly meridional changes early in the morning. (2) Decreases in the magnitudes of the horizontal winds associated with the diurnal tide lead to a net downward tendency in the vertical winds blowing through a constant pressure surface. (3) Because of these changes in the vertical wind, there is an increase in compressional heating (or a decrease in cooling through expansion), and thus temperatures in the low- to middle-latitudes of the winter hemisphere increase. (4) Densities of all neutral species increase on a constant height surface, but the pattern of changes in the O/N2 ratio is not well ordered on these surfaces. (5) The pattern of changes in the O/N2 ratio is better ordered on constant pressure surfaces. The increases in this ratio on constant pressure surfaces in the low- to middle-latitude, winter hemisphere are caused by a more downward tendency in the vertical winds that blow through the constant pressure surfaces. Nitrogen-poor air is then advected downward through the pressure surface, increasing the O/N2 ratio. (6) The daytime geographical distribution of the modeled increases in the O/N2 ratio on a constant pressure surface in the low- to middle-latitudes of the winter hemisphere correspond very closely with those of increases in the modeled electron densities at the F2 peak.

A theoretical study of thermospheric composition perturbations during an impulsive geomagnetic storm

The compositional response of the neutral thermosphere to an impulsive geomagnetic storm has been investigated using a numerical simulation made with the National Center for Atmospheric Research thermospheric general circulation model (NCAR-TGCM). Calculated time-dependent changes in neutral thermospheric composition have been studied, together with detailed neutral parcel trajectories and other diagnostic information from the model, to gain a greater understanding of the physical mechanisms responsible for composition variability during geomagnetic storms and, in particular, to investigate the causes of the positive and negative ionospheric storm effects. The following principal results were obtained from this study. (1) Calculated perturbations in thermospheric composition following the onset of an impulsive geomagnetic storm were found to be in good qualitative agreement with the previous experimental statistical study of storm time thermospheric morphology by Prolss (1981). (2) During the initial (onset) phase of the simulated storm, upward vertical winds occurred in the auroral zone and downward winds occurred in the central magnetic polar cap. (3) The largest perturbations in mass mixing ratio of nitrogen (ΨN2) at F region altitudes were found to be associated with parcels of neutral gas that travelled through the cusp region and with parcels that were trapped within the auroral zone for a long time. (4) Storm time enhancements in ΨN2 were found to occur in the midnight and early morning sectors both within and equatorward of the auroral zone, and these were determined to be associated with the advective effects of the large antisunward polar cap neutral winds. The physical size of the region of enhanced ΨN2 values was determined to be related to the time taken for individual parcels of air to corotate out of the large day-to-night thermospheric wind jet, and thus to the magnitudes of the antisunward polar cap winds. (5) Decreases in ΨN2 were found to occur in regions of downward vertical winds and in regions into which relatively nitrogen-poor air was advected by the horizontal wind. (6) Calculated compositional perturbations following the onset of the storm were found to propagate to mid-latitudes due to the advective effects of equatorward winds, leading to the mid-latitude positive ionospheric storm effects. This last result is in disagreement with the earlier study of Rishbeth et al. (1985), who found the compositional disturbance to be largely restricted to the auroral zone on a constant-pressure surface. However, it is consistent with the more recent study undertaken by Fuller-Rowell et al. (1991), who demonstrated that composition changes were seen well beyond the auroral oval.

Ionospheric storm effects at subauroral latitudes: A case study

An attempt is made to classify ionospheric storm effects at subauroral latitudes according to their presumed origin. The storm of December 7/8, 1982, serves as an example. It is investigated using ionosonde, electron content, and DE 2 satellite data. The following effects are distinguished: (1) positive storm effects caused by traveling atmospheric disturbances, (2) positive storm effects caused by changes in the large-scale thermospheric wind circulation, (3) positive storm effects caused by the expansion of the polar ionization enhancement, (4) negative storm effects caused by perturbations of the neutral gas composition, and (5) negative storm effects caused by the equatorward displacement of the trough region.

Large enhancements in the O/N2 ratio in the evening sector of the winter hemisphere during geomagnetic storms

In this paper, we have looked for enhancements of the O/N2 ratio in data measured by the Dynamics Explorer 2 (DE 2) satellite in the middle latitudes of the winter hemisphere, based on a prediction that was made by the National Center for Atmospheric Research thermosphere/tonosphere general circulation model (NCAR-TIGCM) that such increases occur. The NCAR-TIGCM predicts that these enhancements should be seen throughout the low latitude region and in many middle latitude locations, but that the enhancements in O/N2 are particularly strong in the middle-latitude, evening-to-midnight sector of the winter hemisphere. When this prediction was used to look for these effects in DE 2 NACS (neutral atmosphere composition spectrometer) data, large enhancements in the O/N 2 ratio (-50 to 90%) were seen. These enhancements were observed during the main phase of a storm that occurred on November 24, 1982, and were seen in the same region of the winter hemisphere predicted by the NCAR-TIGCM. They are partially the result of the depletion of N 2 and, as electron loss is dependent on dissociative recombination at F 2 altitudes, they have implications for electron densities in this area. Parcel trajectories, which have been followed through the NCAR-TIGCM history file for this event, show that large O/N2 enhancements occur in this limited region in the winter hemisphere for two reasons. First, these parcels of air are decelerated by the antisunward edge of the ion convection pattern; individual parcels converge and subsidence occurs. Thus molecular-nitrogen-poor air is brought from higher to lower heights. Because neutral parcels that are found a little poleward of the equatorial edge of the eveningside convection pattern are swept inward toward the center of the auroral oval, the enhancements occur only in a very limited range of latitudes. Second, nitrogen-poor air is transported from regions close to the magnetic pole in the winter hemisphere. During geomagnetic storms, enhanced meridional winds are driven by the increased pressure-gradient force that is associated with intensified Joule heating in the auroral oval. These pressure-driven winds decrease rapidly on the dayside beyond the auroral oval where the parcels originate, limiting the region into which the parcels can be transported. Thus these two processes drive values of O/N2 in a limited region of the winter hemisphere, and reinforce only in the evening sector, causing large changes in this region.

Upper thermosphere winds and temperatures in the geomagnetic polar cap : solar cycle, geomagnetic activity, and interplanetary magnetic field dependencies

Ground-based Fabry-Perot interferometers located at Thule, Greenland (76.5°N, 69.0°W, Λ=86°) and at Sondre Stromfjord, Greenland (67.0°N, 50.9°W, Λ=74°) have monitored the upper thermospheric (∼240-km altitude) neutral wind and temperature over the northern hemisphere geomagnetic polar cap since 1983 and 1985, respectively. The thermospheric observations are obtained by determining the Doppler characteristics of the (O I) 15,867-K (630.0-nm) emission of atomic oxygen. The instruments operate on a routine, automatic, (mostly) untended basis during the winter observing seasons, with data coverage limited only by cloud cover and (occasional) instrument failures. This unique database of geomagnetic polar cap measurements now extends over the complete range of solar activity. We present an analysis of the measurements made between 1985 (near solar minimum) and 1991 (near solar maximum), as part of a long-term study of geomagnetic polar cap thermospheric climatology. The measurements from a total of 902 nights of observations are compared with the predictions of two semiempirical models: the vector spherical harmonic (VSH) model of Killeen et al. (1987) and the horizontal wind model (HWM) of Hedin et al. (1991). The results are also analyzed using calculations of thermospheric momentum forcing terms from the thermosphere-ionosphere general circulation model (TIGCM) of the National Center for Atmospheric Research (NCAR). The experimental results show that upper thermospheric winds in the geomagnetic polar cap have a fundamental diurnal character, with typical wind speeds of about 200 m s−1 at solar minimum, rising to up to about 800 m s−1 at solar maximum, depending on geomagnetic activity level. These winds generally blow in the antisunward direction, but are interrupted by episodes of modified wind velocity and altered direction often associated with changes in the orientation of the interplanetary magnetic field (IMF). The central polar cap (>∼80 magnetic latitude) antisunward wind speed is found to be a strong function of both solar and geomagnetic activity. The polar cap temperatures show variations in both solar and geomagnetic activity, with temperatures near 800 K for low Kp and F10.7 and greater than about 2000 K for high Kp and F10.7. The observed temperatures are significantly greater than those predicted by the mass spectrometer/incoherent scatter model for high activity conditions. Theoretical analysis based on the NCAR TIGCM indicates that the antisunward upper thermospheric winds, driven by upstream ion drag, basically “coast” across the polar cap. The relatively small changes in wind velocity and direction within the polar cap are induced by a combination of forcing terms of commensurate magnitude, including the nonlinear advection term, the Coriolis term, and the pressure gradient force term. The polar cap thermospheric thermal balance is dominated by horizontal advection, and adiabatic and thermal conduction terms.

A theoretical and empirical study of the response of the high latitude thermosphere to the sense of the “Y” component of the interplanetary magnetic field

Abstract The strength and direction of the Interplanetary Magnetic Field (IMF) controls the transfer of solar wind momentum and energy to the high latitude thermosphere in a direct fashion. The sense of “ Y” component of the IMF (BY) creates a significant asymmetry of the magnetospheric convection pattern as mapped onto the high latitude thermosphere and ionosphere. The resulting response of the polar thermospheric winds during periods when BY is either positive or negative is quite distinct, with pronounced changes in the relative strength of thermospheric winds in the dusk-dawn parts of the polar cap and in the dawn part of the auroral oval. In a study of four periods when there was a clear signature of BY, observed by the ISEE-3 satellite, with observations of polar winds and electric fields from the Dynamics Explorer-2 satellite and with wind observations by a ground-based Fabry-Perot interferometer located in Kiruna, Northern Sweden, it is possible to explain features of the high latitude thermospheric circulation using three dimensional global models including BY dependent, asymmetric, polar convection fields. Ground-based Fabry-Perot interferometers often observe anomalously low zonal wind velocities in the (Northern) dawn auroral oval during periods of extremely high geomagnetic activity when BY is positive. Conversely, for BY negative, there is an early transition from westward to southward and eastward winds in the evening auroral oval (excluding the effects of auroral substorms), and extremely large eastward (sunward) winds may be driven in the auroral oval after magnetic midnight. These observations are matched by the observation of strong anti-sunward polar-cap wind jets from the DE-2 satellite, on the dusk side with BY negative, and on the dawn side with BY positive.

The high latitude circulation and temperature structure of the thermosphere near solstice

Abstract The neutral gas temperature and circulation of the thermosphere are calculated for December solstice conditions near solar cycle maximum using NCARs thermospheric general circulation model (TGCM). High-latitude heat and momentum sources significantly alter the basic solar-driven circulation during solstice. At F -region heights, the increased ion density in the summer hemisphere results in a larger ion drag momentum source for the neutral gas than in the winter hemisphere. As a result there are larger wind velocities and a greater tendency for the neutral gas to follow the magnetospheric convection pattern in the summer hemisphere than in the winter hemisphere. There is about three times more Joule heating in the summer than the winter hemisphere for moderate levels of geomagnetic activity due to the greater electrical conductivity in the summer E -region ionosphere. The results of several TGCM runs are used to show that at F -region heights it is possible to linearly combine the solar-driven and high-latitude driven solutions to obtain the total temperature structure and circulation to within 10–20%. In the lower thermosphere, however, non-linear terms cause significant departures and a linear superposition of fields is not valid. The F -region winds at high latitudes calculated by the TGCM are also compared to the meridional wind derived from measurements by the Fabry-Perot Interferometer (FPI) and the zonal wind derived from measurements by the Wind and Temperature Spectrometer (WATS) instruments onboard the Dynamics Explorer ( DE −2) satellite for a summer and a winter day. For both examples, the observed and modeled wind patterns are in qualitative agreement, indicating a dominant control of high latitude winds by ion drag. The magnitude of the calculated winds (400–500 m s −1 ) for the assumed 60 kV cross-tail potential, however, is smaller than that of the measured winds (500–800 m s −1 ). This suggests the need for an increased ion drag momentum source in the model calculations due to enhanced electron densities, higher ion drift velocities, or some combination that needs to be further denned from the DE −2 satellite measurements.

A comparison of wind observations of the upper thermosphere from the dynamics explorer satellite with the predictions of a global time-dependent model

Abstract Seven polar passes of the NASA Dynamics Explorer 2 (DE-2) satellite during October and early December 1981 have been used to examine the high-latitude circulation in the upper thermosphere. Vector winds along the satellite track are derived by appropriate merging of the data from the remote-sensing Fabry-Perot interferometer (meridional wind) and the in situ wind and temperature spectrometer (zonal wind) and are compared with the predictions of a three-dimensional, time-dependent, global model of the thermosphere. Major features of the experimental winds, such as the mean day to night circulation caused by solar u.v. and e.u.v. heating, augmented by magnetospheric processes at high latitude and the sharp boundaries and flow reversals imposed on thermospheric winds by momentum transfer (ion drag) from the magnetosphere, are qualitatively explained by a version of the global model using a semi-empirical global model of polar electric fields (Volland Model 2 or Heppner Model A) and a model of global electron density which excludes the effects of high-latitude geomagnetic processes. A second version of the global dynamic model includes a theoretical model of the high-latitude ionosphere which is self-consistent and reflects the enhancement of ionization due to magnetospheric phenomena acting in addition to solar e.u.v. photo-ionization, including the interactive processes which occur between ionization and high latitude ion convection and thermospheric winds. This second dynamical model shows an improved comparison with the structure and magnitude of polar cap and auroral oval winds at times of other than extremely low geomagnetic activity, when the first model appears a better match. An improved empirical description of the complex magnetospheric processes exciting the thermosphere in the vicinity of the dayside polar cusp and an empirical description of storm-time electric fields will be required for a quantitative explanation of the polar thermospheric winds during geomagnetic substorm events.