M. E. Hagan

Control of equatorial ionospheric morphology by atmospheric tides

[1] A newly discovered 1000-km scale longitudinal variation in ionospheric densities is an unexpected and heretofore unexplained phenomenon. Here we show that ionospheric densities vary with the strength of nonmigrating, diurnal atmospheric tides that are, in turn, driven mainly by weather in the tropics. A strong connection between tropospheric and ionospheric conditions is unexpected, as these upward propagating tides are damped far below the peak in ionospheric density. The observations can be explained by consideration of the dynamo interaction of the tides with the lower ionosphere (E-layer) in daytime. The influence of persistent tropical rainstorms is therefore an important new consideration for space weather. Citation: Immel, T. J., E. Sagawa, S. L. England, S. B. Henderson, M. E. Hagan, S. B. Mende, H. U. Frey, C. M. Swenson, and L. J. Paxton (2006), Control of equatorial ionospheric morphology by atmospheric tides, Geophys. Res. Lett., 33, L15108, doi:10.1029/2006GL026161. [2] The ionosphere is the region of highest plasma density in Earth’s space environment. It is a dynamic environment supporting a host of plasma instability processes, with important implications for global communications and geo-location applications. Produced by the ionization of the neutral atmosphere by solar x-ray and UV radiation, the uppermost ionospheric layer has the highest plasma density with a peak around 350–400 km altitude and primarily consists of O + ions. This is called the F-layer and it is considered to be a collisionless environment such that the charged particles interact only weakly with the neutral atmosphere, lingering long after sunset. The E-layer is composed of molecular ions and is located between 100–150 km where collisions between ions and neutrals are much more frequent, with the result that the layer recombines and is reduced in density a hundredfold soon after sunset [Rees ,1 989;Heelis, 2004]. The respective altitude regimes of these two layers are commonly called the E- and F-regions. [3] The ionosphere glows as O + ions recombine to an excited state of atomic oxygen (O I) at a rate proportional to

GSWM-98: Results for migrating solar tides

We report on new global-scale wave model (GSWM) predictions for the migrating solar tide in the troposphere, stratosphere, mesosphere and lower thermosphere. The model revision, hereafter GSWM-98, includes an updated gravity wave (GW) stress parameterization and modifications to the background atmosphere based on 6-year monthly averaged Upper Atmosphere Research Satellite (UARS) climatologies. UARS Halogen Occultation Experiment and Microwave Limb Sounder ozone data are used to define the strato-mesospheric tidal source, while GSWM-98 background winds are based on UARS High Resolution Doppler Interferometer (HRDI) zonal mean zonal wind data. We quantify and interpret differences between previous diurnal and semidiurnal predictions, hereafter GSWM-95, and GSWM-98 results. The revised GW stress parameterization accounts for the most profound changes and leads to seasonal variability predictions that are consistent with diurnal amplitudes observed in the upper mesosphere and lower thermosphere. Unresolved differences between HRDI and other wind climatologies significantly affect MLT tidal predictions.

On modeling migrating solar tides

Recent updates and extensions to a steady-state two-dimensional linearized model of global-scale atmospheric waves have facilitated improved calculations of the subset of those waves which are subharmonics of a solar day and propagate with the apparent motion of the sun. The model improvements are briefly described and some updated predictions of the migrating solar diurnal component are highlighted. The latter represent the first numerical modeling effort to examine the seasonal variability of the migrating diurnal harmonic as it propagates into the mesosphere and lower thermosphere.

Long‐term variability in the solar diurnal tide observed by HRDI and simulated by the GSWM

Observations of the mesosphere and lower thermosphere winds obtained by the High Resolution Doppler Imager (HRDI) on the Upper Atmosphere Research Satellite (UARS) during 1991 to 1995 reveal a semiannual variation in the amplitude of the (1,1) diurnal tide. The global-scale wave model (GSWM) represents the first numerical modeling attempt at simulating this seasonal variability, and a preliminary comparison of the GSWM tidal results with HRDI measurements is presented. The results of the comparison and of numerical tests point to some vital and unresolved questions regarding tidal dissipation and tropospheric forcing. In addition to the seasonal variability, HRDI has revealed a strong interannual modulation of the diurnal tide with amplitudes observed to change by nearly a factor of 2 from 1992 to 1994.

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.

Modeling diurnal tidal variability with the National Center for Atmospheric Research thermosphere-ionosphere-mesosphere-electrodynamics general circulation model

We used the National Center for Atmospheric Research thermosphere-ionosphere-mesosphere-electrodynamics general circulation model (TIME-GCM) to calculate the variability of the migrating diurnal tide in a January through December 1993 simulation. While TIME-GCM captures the salient features of the latitudinal, altitudinal, and seasonal variability of the migrating diurnal tide, upper mesospheric meridional wind amplitudes are somewhat smaller than those that have been observed from the ground and space. The discrepancies may be attributable to unresolved uncertainties in tidal forcing and/or dissipation in the TIME-GCM. However, our diagnostic simulation suggests that the nonlinear interactions between the migrating diurnal tide and stationary planetary wave 1 produce measurable nonmigrating diurnal tidal components that modulate migrating diurnal tidal amplitudes and account for significant variability in the upper mesosphere and lower thermosphere.

Migrating thermospheric tides

The capabilities of the global-scale wave model (GSWM) [Hagan et al., 1995, 1999] are extended to include migrating thermospheric solar tides. The GSWM thermospheric tidal forcing parameterization is based on neutral gas heating calculated from first principles in the National Center for Atmospheric Research (NCAR) thermosphere/ionosphere electrodynamics general circulation model (TIE-GCM). This is the first time that a physics-based thermospheric forcing scheme has been used in a model like GSWM. Previous two-dimensional steady state linear tidal models used exospheric temperature measurements to calibrate upper atmospheric tidal forcing. New GSWM results illustrate thermospheric tidal responses that are largely consistent with tides in the TIE-GCM. Diurnal temperature amplitudes increase with increasing solar activity, but there is no analogous diurnal wind response. The thermospheric semidiurnal tide is much weaker than the diurnal tide. Semidiurnal temperature perturbations peak in the lower thermosphere where the semidiurnal forcing maximizes. The new in situ results must be combined with the GSWM upward propagating tide in the lower thermosphere, because the upward propagating components dominate the semidiurnal response throughout the region and the diurnal response below ∼130 km. In situ forcing accounts for most of the diurnal response aloft. Our preliminary evaluation of the GSWM thermospheric predictions is inconclusive. More extensive evaluations are necessary to make a firm assessment of whether the model captures the salient features of the seasonal and solar cycle variability of thermospheric tides.

Comparative effects of migrating solar sources on tidal signatures in the middle and upper atmosphere

A steady state two-dimensional linearized model that extends from the ground into the thermosphere and captures the salient features of migrating diurnal and semidiurnal tidal components is used to investigate the comparative importance of the principal sources of these waves. The results, which have previously gone unreported in the literature, demonstrate the nonnegligible effects of atmospheric absorption of solar radiation at infrared (in the troposphere) and ultraviolet (in the stratosphere) wavelengths on mesospheric and lower thermospheric semidiurnal and diurnal tidal fields, respectively. In addition, latent heat release associated with cloudiness or rainfall in the troposphere is shown to be another plausible source of semidiurnal variability in the upper atmosphere. The important effects of these sources on the dynamics of the mesosphere and lower thermosphere emphasize the need to include realistic parameterizations of migrating tides at the lower boundaries of middle and upper atmospheric general circulation models. The results of this investigation also suggest that updated parameterizations of tropospheric tidal forcing are needed to further current understanding of tidal variability in the upper atmosphere.

Quasi 16-day oscillation in the mesosphere and lower thermosphere

A quasi-16-day wave in the mesosphere and lower thermosphere is investigated through analyses of radar data during January/February 1979 and through numerical simulations for various background wind conditions. Previous workers have examined about 19 days of tropospheric and stratospheric data during January 10–28, 1979, and present conflicting evidence as to whether a large westward propagating wavenumber 1 oscillation observed during this period can be identified in terms of the second symmetric Rossby normal mode of zonal wavenumber 1, commonly referred to as the “16-day wave.” In the present work we have applied spectral analysis techniques to meridional and zonal winds near 95 km altitude obtained from radar measurements over Obninsk, Russia (54°N, 38°E) and Saskatoon, Canada (52°N, 107°W). These data reveal oscillations of the order of ±10 m s−1 with a period near 16 days as well as waves with periods near 5 and 10 days. These periodicities all correspond to expected resonant frequencies of atmospheric disturbances associated with westward propagating free Rossby modes of zonal wavenumber 1. Numerical simulations are performed which demonstrate that the 95-km measurements of the 16-day wave are consistent with upward extension of the oscillation determined from the tropospheric and stratospheric data. Noteworthy features of the model in terms of its applicability in the mesosphere/lower thermosphere regime are explicit inclusion of eddy and molecular diffusion of heat and momentum and realistic distributions of mean winds, especially between 80 and 100 km. The latter include a westerly wind regime above the summer easterly mesospheric jet, thus providing a ducting channel enabling interhemispheric penetration of the winter planetary wave disturbance. This serves to explain the appearance of a quasi-16-day wave recently reported in the high-latitude summer mesopause (Williams and Avery, 1992). However, the efficiency of this interhemispheric coupling may be reduced by gravity wave stress. No significant penetration of the 16-day oscillation above about 100 km is predicted by the model. Reported signatures of a 16-day periodicity in ionospheric data therefore require modulation of tidal or gravity wave accessibility to the thermosphere, or perhaps in situ excitation.

Effect of atmospheric tides on the morphology of the quiet time, postsunset equatorial ionospheric anomaly

longitudinal wave number four pattern in the magnetic latitude and concentration of the F region peak ion density when measured at a fixed local time. In a new comparison of two data sets with observations made by the OGO 4 satellite, this pattern is seen to be persistent over many days around equinox during magnetically quiet conditions close to solar maximum but can be dominated by other processes such as cross-equator winds during other periods. It is found that the longitudinal variability is created by a processes occurring in the dayside ionosphere. A longitudinal modulation of the dayside equatorial fountainisthemostlikelydrivingmechanism.ThroughcomparisonwithGWSM-02model,it isshownthatthepredictedmodulationofthedaysidethermosphericwindsandtemperaturesat E region altitudes created by non-migrating diurnal tides can explain the modulation in the dayside equatorial fountain. This result highlights the importance of understanding the temporal variability of tropospheric weather systems on our understanding and possible predictability of the development of the F region ionosphere. It may also provide a possible further means of testing our understanding of atmospheric tides on a global scale.