P. L. Dyson

A model of the vertical distribution of the electron concentration in the ionosphere and its application to oblique propagation studies

An empirical model of the vertical distribution of the electron concentration of the ionosphere based on quasi-parabolic segments is presented (QPS model). The model is quite flexible and any number of ionospheric layers, valleys and ‘ledges’ can be incorporated. Nevertheless, it has been found that options of an E layer, E-F valley, F1 layer and F2 layer provide models of sufficient flexibility to give a good description of true height electron concentration profiles determined from vertical incidence ionograms. In this instance, the basic input parameters required are the same as those used by the CCIR models. The QPS models main advantage is that it can be used to obtain analytical solutions to ray parameters for propagation in spherically stratified ionospheres when the Earths magnetic field is neglected. An expression for the power of radio pulses backscattered from the Earths surface is also derived. This expression enables several effects, including those of the ionosphere, antennas and pulse length, to be readily examined. It is used, together with the QPS model, to illustrate the effects of the ionosphere and antennas on backscatter signals.

Thermospheric neutral winds at southern mid‐latitudes: A comparison of optical and ionosonde hmF2 methods

During the first 6 days of March 1995, measurements of the ionospheric electron density were made with a digisonde, and thermospheric winds were measured with a Fabry-Perot interferometer. This was a period of low solar activity and moderate to high magnetic activity. The ionograms have been scaled and the traces inverted to obtain the electron density profile and the peak height of the F2 layer (hmF2). Modeling has been employed to derive equivalent thermospheric neutral winds at hmF2. The derived neutral winds are in very good agreement with the measured optical winds most of the time. The winds follow a strong diurnal pattern with poleward winds during the day, weak winds near dawn and dusk, and strong equatorward winds peaking near local midnight. On most nights the peak equatorward wind speed was around 200 m s−1, but on March 1 it did not exceed 110 m s−1. For these magnetic and solar activity conditions the wind at the F2 peak altitude (∼350 km) from the HWM93 empirical wind model[Hedin et al., 1996] did not exceed 90 m s−1 at any time but was in generally good agreement with the hmF2 wind during the day and with both measured winds on the nights of March 1 and 2. The good agreement between the optical and hmF2 winds was obtained by using the recommended Burnside factor of 1.7 to multiply the O+-O collision frequency, but better agreement was obtained either by using a Burnside factor of 2.0 or by increasing the atomic oxygen density by 20%. Recent suggestions of much lower Burnside factors could be tolerated only if there were large systematic errors in the measurements or large electric fields.

Behavior of the ionosphere and thermosphere at a southern midlatitude station during magnetic storms in early March 1995

During the first six days of March 1995, measurements of the ionospheric electron density were made near Melbourne, Australia, with a digisonde and thermospheric winds, temperatures, and 6300-A emission rates were measured with a Fabry-Perot interferometer. The ionograms were inverted to obtain electron density profiles and peak heights of the F2 layer (h m F 2 ). This paper compares modeled and measured electron densities, airglow emission rates, and neutral temperatures. The measured peak electron density shows strong negative effects from magnetic storms and rapid recovery to normal levels afterward. The model daytime peak density is in good agreement with the measurements on the undisturbed days and also shows negative phases at the appropriate times. However, the model negative effects are not as strong as the measured negative effects. A new algorithm is introduced to bring the measured and modeled peak electron density into better agreement by adjusting the exospheric Tn and atomic oxygen density in the MSIS model. The modified Tn at 300 km altitude agrees reasonably well with both the standard MSIS model and the measured Tn before midnight local time. However, the modified Tn, like the measured Tn, has a tendency to increase after about 2200 LT. At night, adjustments to the MSIS model exospheric temperature and atomic oxygen density are generally small. There is reasonably good agreement between the relative variations of the measured and modeled emission rates. However, there is much more variability in the model intensities than in the measured intensities as a result of rapid movements in the height of the F2 layer.

Fabry-Perot spectrometer observations of the auroral oval/polar cap boundary above Mawson, Antarctica

Abstract Zenith observations of the oxygen λ1630 nm auroral/airglow emission (produced at an altitude of ∼220 to ∼250 km) were obtained with the Mawson Fabry-Perot Spectrometer (FPS) during three ‘zenith direction only’ observing campaigns in 1993. The data show many instances of strong (50 to 100 m s −1 ) upwellings in the vertical wind, when the auroral oval is located equatorward of the zenith. Our data appear consistent with the existence of a region of upwelling up to ∼ 4° poleward of the poleward boundary of the visible auroral oval, rather than short duration, explosive heating events. The upwellings are probably the vertical component of wind shear produced by reversal of the zonal thermospheric winds, which occurs near the poleward boundary of the visible auroral oval. Zenith temperature was also seen to increase when the oval was equatorward of Mawson, showing rises of up to 300 K or more. However, this increase is at times unrelated to the upwellings, and seems to be caused by the expansion of the warm polar cap over the observing site. On a number of nights the boundary between the polar cap and the auroral oval was observed to pass over our site several times, occasionally showing a quasi-periodic expansion and contraction. We speculate that this quasi-periodic movement may be related to periodic auroral activity that is known to generate large-scale gravity waves.

Thermospheric vertical winds above Mawson, Antarctica

Abstract High-resolution (R ≅ 200,000) night-time Fabry-Perot spectra of the λ630 run OI thermospheric airglow emission obtained from Mawson, Antarctica, during the austral winter of 1992 have been used to provide estimates of thermospheric temperature and neutral wind. In this paper, we report on characteristics of the resulting vertical wind estimates. The final data set comprised 103 observing nights between 18 March and 20 September for a total of 2096 useable zenith spectra. Because the instrumental reference rest wavelength of the 7630 nm emission is not known, this experiment was only capable of measuring vertical wind variations relative to an arbitrarily-chosen reference velocity. For this work, we established our reference by requiring that the nightly mean vertical wind be zero. We have divided our data into two subsets, corresponding to days of quiet and moderately disturbed magnetic activity. In both cases, the mean daily variation in vertical wind velocity was dominated by its diurnal component with downward winds (relative to our chosen zero, and negative) appearing prior to local magnetic midnight and upward (positive) winds after. Averaged over all days of low magnetic activity, the diurnal range was −2.6 to 3.0 m s−1 whilst for days of moderate activity, it was −6.8 to 4.1 m s−1. Superimposed on this mean vertical wind are fluctuations at shorter periods than our sampling interval of 1 h. We estimate the distributions of vertical velocities prevailing during our observations to be characterized by one-sigma halfwidths of 14.3 and 18.7 m s−1 for low and moderate magnetic activity, respectively. This experiment has yielded generally smaller estimates of vertical wind speeds than those reported by previous authors from auroral latitudes for similar levels of magnetic activity. Comparison is also made with the UCL thermospheric general circulation model.

Further observations of the thermospheric vertical wind at the auroral oval/polar cap boundary

Abstract We present further observations of thermospheric winds and temperatures, derived from observations of the λ630 nm oxygen auroral/airglow emission (from ∼240 km altitude), obtained with a Fabry-Perot spectrometer (FPS) at Mawson station, Antarctica (L = 9). We report further instances of large upward zenith winds, with velocities up to ∼80ms−1, often associated with increases in temperature of up to 200 K. These upward winds are mostly seen around 21 Z, which is when Mawson station passes under the poleward edge of the (discrete) auroral oval during intervals of low to moderate geomagnetic activity, and are yet further examples of the type of event presented in an earlier paper (Innis et al., 1996). We also find intervals, up to several hours in length, when the temperatures measured South and East of the station can be several hundred degrees higher than those measured in the North and West directions, which we ascribe to the expansion of the hot polar cap into the South and East viewing directions. We have observed examples of seemingly anomalous wind measurements in the South and East directions during these times, which appear to be related to the presence of an upward wind in the observing volume of the FPS. Our observations suggest, however, that the upward wind would cause only a small perturbation (relative to the horizontal component) of the high-latitude thermospheric neutral wind field. The size of the zenith wind events (up to ∼120 m s−1) seen in our observations at Mawson are comparable with the amplitude of oscillations seen in gravity waves propagating at ∼300 km altitude over the polar cap from the nightside to the dayside, detected by Johnson et al. (1995) from Dynamics Explorer 2 observations. This similarity and the fact that we see the upward winds at the night-time auroral oval/polar cap boundary suggest that the origin of the upward winds may be intrinsically linked to the processes that generate these gravity waves.

Ionospheric convection at casey, a southern polar cap station

A digital ionosonde (Digisonde Portable Sounder 4) located at Casey, Antarctica (66.3°S, 110.5°E, −80.8° corrected geomagnetic latitude) has been operational since early 1993 and has accumulated 3 years of plasma drift measurements, providing an excellent data set for studying the characteristics of ionospheric convection flow at a southern polar cap station. The purpose of this study is to investigate the influence of the IMF on the F region ionospheric convection over Casey and to compare it to the Heppner-Maynard satellite-derived electric field models. We find clear dependencies in the drift on the sign and strength of the IMF By and Bz components and with Kp. Antisunward flow dominates during Bz south conditions, turning to have a sunward component around noon when Bz is northward. The By component causes the entire convection system to rotate and distorts the dayside flow in the proximity of the throat, with a dawnward (duskward) component for By negative (positive). Comparison with the Bz south Heppner-Maynard BC, DE, and A patterns is favorable at most times, although we predict a rounder, more dominant dusk (dawn) cell and a smaller crescent-shaped dawn (dusk) cell for By 0). There is a dependence on Kp when Bz is south in both the model and the drifts, flow directions becoming more antisunward and velocities becoming higher on the dayside as Kp increases. This implies the polar cap is expanding under conditions of enhanced reconnection. When Bz is north, the F region drift agreement with the BCP(P) and DEP(P) models is excellent on the dawn (dusk) side for By 0) but diverges on the opposite side as the pattern flow lines twist sunward. Separation of the drifts into Bz weakly ( 3 nT) northward cases did not reveal any appreciable difference in the observed drift velocities.

The association between ionospheric and geomagnetic pulsations in the Pc3–4 range at mid-latitudes

Abstract The response of the ionosphere to hydromagnetic waves in the Pc3–4 range has been investigated using an induction magnetometer and a digital ionosonde at Beveridge ( L = 2.1), Australia. Doppler shift measurements on four spaced radio frequencies have been made to monitor simultaneously the response of the E - and F -regions. E -region fluctuations correlated with Pc3 activity have been observed for the first time. Excellent correlation is sometimes observed between magnetic pulsations and both E - and F -region fluctuations. The lack of correlation more often observed is probably caused by ionospheric irregularities which produce RF interference effects in the Doppler measurements. Nine events exhibiting particularly high correlation have been analyzed in detail. The phase differences between the magnetic and ionospheric data varied from event to event, such that different events are consistent with different theoretical mechanisms. This indicates the complex nature of the interaction of hydromagnetic waves with the ionosphere at mid-latitudes.

Topside irregularities in the equatorial ionosphere

This paper is a brief review of ionospheric irregularities in the equatorial topside ionosphere. Results from topside sounders, direct measurement satellites, and the Jicamarca incoherent scatter radar are discussed. Scintillation observations and theories of irregularities are not discussed in detail as these are the subject of other review papers. Many of the phenomena detected in the topside ionosphere are related to bottomside irregularities, commonly known as spread-F. These include aspect-sensitive scattering observed on topside sounders, significant concentrations of Fe+, electrostatic turbulence and the topside irregularities detected by the Jicamarca radar. Satellite measurements show that the irregularities in electron concentration have amplitudes which increase almost linearly with wave-length over the range 70m to 3km. Duct irregularities detected by the topside sounders and some wavelike irregularity structures detected occasionally by direct measurement satellites may be separate from the general spread-F phenomenon although this has not definitely been established.

Automatic fitting of quasi-parabolic segments to ionospheric profiles with application to ground range estimation for single-station location

Abstract A method of automatically fitting a quasi-parabolic segment (QPS) model to measured profiles of electron concentration is described. The model consists of many smoothly joined quasi-parabolic segments. The base of the ionosphere is represented by a quasi-linear segment. This model has the advantage of possessing analytic solutions for ray quantities in a spherical ionosphere (neglecting the effect of the Earths magnetic field). It is also able to represent measured vertical ionospheric profiles to within a few percent in electron concentration. The usefulness of the model is illustrated by an application to the single-station location (SSL) of a distant radio transmitter from angle-of-elevation measurements. The no magnetic field results can represent the ordinary mode and approximate results for the extraordinary mode can be obtained using a frequency scaling technique.