Jeffrey M. Forbes

Variability of the ionosphere

Abstract Hourly foF2 data from over 100 ionosonde stations during 1967–89 are examined to quantify F-region ionospheric variability, and to assess to what degree the observed variability may be attributed to various sources, i.e., solar ionizing flux, meteorological influences, and changing solar wind conditions. Our findings are as follows. Under quiet geomagnetic conditions ( K p σ ( σ is the standard deviation) variability of N max about the mean is approx. ±25–35% at ‘high frequencies’ (periods of a few hours to 1–2 days) and approx. ±15–20% at ‘low frequencies’ (periods approx. 2–30 days), at all latitudes. These values provide a reasonable average estimate of ionospheric variability mainly due to “meteorological influences” at these frequencies. Changes in N max due to variations in solar photon flux, are, on the average, small in comparison at these frequencies. Under quiet conditions for high-frequency oscillations, N max is most variable at anomaly peak latitudes. This may reflect the sensitivity of anomaly peak densities to day-to-day variations in F-region winds and electric fields driven by the E-region wind dynamo. Ionospheric variability increases with magnetic activity at all latitudes and for both low and high frequency ranges, and the slopes of all curves increase with latitude. Thus, the responsiveness of the ionosphere to increased magnetic activity increases as one progresses from lower to higher latitudes. For the 25% most disturbed conditions ( K p >4), the average 1- σ variability of N max about the mean ranges from approx. ±35% (equator) to approx. ±45% (anomaly peak) to approx. ±55% (high-latitudes) for high frequencies, and from approx. ±25% (equator) to approx. ±45% (high-latitudes) at low frequencies. Some estimates are also provided on N max variability connected with annual, semiannual and 11-year solar cycle variations.

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.

Monthly simulations of the solar semidiurnal tide in the mesosphere and lower thermosphere

Abstract Monthly simulations of the solar semidiurnal tide in the 80–100 km height regime are presented. These calculations benefit from the recent heating rates provided by Groves G. V. (1982a,b) (J. atmos. terr. Phys. 44, 111; 44, 281), the zonally-averaged wind, temperature and pressure fields developed for the new COSPAR international reference atmosphere [ Labitzke K. , Barnett J. J. and Edwards B . (1985) Handbook for MAP 16, 318], and eddy diffusivities determined from gravity wave saturation climatologies and used by Garcia R. R. and Solomon S. (1985) (J. geophys. Res. 90, 3850) to simulate oxygen photochemistry and transport in the mesosphere and lower thermosphere. Some of the main characteristics of the observed semidiurnal tide at middle and high latitudes are reproduced in our simulations: larger amplitudes in winter months than in summer months, and the bi-modal behavior of the phase with summer-like and winter-like months separated by a quick transition around the two equinoxes. The phase transition is also more rapid in the spring, consistent with observations. The wavelengths are also longer in summer than in winter, at least below 95 km (whereas in July and August the simulations exhibit some discrepancies above this altitude), similar to the observational data. Semidiurnal amplitudes are generally smaller and the phases more seasonally symmetric at middle and low latitudes, as compared with the tidal structures above about 50° latitude. In addition, hemispheric differences in the mean zonal wind result in marked asymmetries in tidal behavior between the Arctic and Antarctic regions, and suggest that a comparative study of tide, gravity wave and mean flow interactions in the Arctic and Antarctic mesosphere and lower thermosphere would be fruitful.

First results from the meteor radar at South Pole: A large 12‐hour oscillation with zonal wavenumber one

The first mesopause-region (ca. 92±5 km) wind measurements from the meteor radar at Amundsen-Scott Station at South Pole are described. Measurements are made along four orthogonal azimuth directions approximately 2° from the geographic South Pole. A large (±20 ms−1) oscillation in the northward wind is observed, with 12-hour period and zonal wavenumber one. A similar wave was observed during August 1–13, 1992 at South Pole by Hernandez et al. (1993) using optical methods. The predominant semidiurnal tide in the atmosphere is migrating with the apparent motion of the sun, with s=2. The s=1 oscillation is interpreted here to result from the nonlinear interaction between the migrating semidiurnal tide and a stationary wave with s=1. The present mechanism represents an alternative to the gravity-wave driven ‘pseudotide’ theory put forth by Walterscheid et al. (1986) to explain the occurrence of unexpectedly large semidiurnal tidal oscillations at high latitudes.

Mars Global Surveyor radio science electron density profiles : Neutral atmosphere implications

The Mars Global Surveyor (MGS) Radio Sci- ence (RS) experiment permits retrieval of electron density prof iles versus height (∼90-200 km) from occultation mea- surements. An initial set of electron profiles is examined spanning high northern latitudes, early morning solar local times and high solar zenith angles (78 to 81 ◦ ) near aphelion. Sampling for these 32-profiles is well distributed over longi- tude. The height of the photochemically driven ionospheric peak is observed to respond to the background neutral den- sity structure, with a mean height during this season at this location of ∼134.4 km. Strong wave-3 oscillations about this mean are clearly observed as a function of longitude, and correspond to neutral density variations measured by the MGS Accelerometer (ACC) experiment. The wave-3 tidal pattern implicated by both the RS and ACC datasets is consistent with a semi-diurnal wave frequency. Clearly, the height of the martian dayside ionospheric peak is a sensitive indicator of the state of the underlying Mars atmosphere. This ionospheric peak height can be used as a proxy of the longitude specific non-migrating tidal variations present in the Mars lower thermosphere.

Quasi 16-day oscillation in the ionosphere

A quasi 16-day oscillation is discovered in the E- and F-regions of the equatorial ionosphere during January/February 1979, apparently connected with the upward penetration of a free Rossby mode excited in the winter stratosphere. The observed effects are interpreted in terms of electric fields induced by the ionospheric wind dynamo.

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.

Density and Winds in the Thermosphere Deduced from Accelerometer Data

With the emergence and increased use of highly accurate accelerometers for geodetic satellite missions, a new opportunity has arisen to study nonconservative forces acting on a number of satellites with high temporal resolution. As the number of these satellite missions increases, so does our ability to determine the spatial characteristics and time response of total density and winds in the thermosphere. This paper focuses on the derivation and methodology of inferring density and winds from along-track and cross-track accelerometer measurements, with themain goal of determining the feasibility of this data set. The principal sources of error such as solar radiation pressure, the unknown coefficients of drag and lift, instrument precision and biases, and unaccounted-for winds are discussed in the context of both density and winds. In the context of our treatment of errors, density errors are generally less than 15%, whereas wind-speed errors are more substantial. Finally, comparisons of results to existing empirical models (i.e., horizontal wind model 93) and to self-consistent numerical models (i.e., thermosphere–ionosphere electrodynamic general circulation model) are provided. Comparisons of results to ion drift velocities (as measured by Defense Meteorological Satellite Program) are also provided.

Numerical investigation of the propagation of the quasi‐two‐day wave into the lower thermosphere

We present results of a series of numerical experiments for January conditions using a linearized spectral model which includes realistic mean winds and dissipation. These experiments were designed to characterize the propagation characteristics of the (3,0) mixed Rossby-gravity mode through the middle atmosphere and into the lower thermosphere. Our results suggest that the wave magnitude is extremely sensitive to the zonal mean winds assumed in the calculations. In particular, our results suggest that a comparatively weak eastward stratomesospheric jet during northern hemisphere winter can account for a shift in the resonant frequency of the quasi-two-day wave response. Further, the inclusion of a realistic lower thermospheric jet in the summer lower thermosphere sets up a reflecting layer and an associated enhancement of the summer mesospheric wave signature. These features are in keeping with meteor and partial reflection drift radar observations.