The dynamic F2-layer over Arecibo

The idealized ‘servo’ model of the ionospheric F2-layer, developed by Rishbeth, ganguly and Walker (1978), is used to simulate the observed behaviour of the daytime F2-peak at Arecibo for sunspot minimum. Taking the east-west electric field to be given by the observed plasma drift velocity perpendicular to the magnetic field, the theoretical equations are integrated using a trial-and-error approach to match the observed values of field-parallel plasma velocity, and the height and electron density of the F2-peak. From the calculation is determined empirically the meridional pressure-gradient force associated with the meridional neutral-air wind. The local time variation during the day is found to be consistent with the semidiurnal variation given by the MSIS atmospheric model of Hedinet al. (1977a, b), though with a phase shift that varies with season; on some days in the fall the pressure-gradient force displays a strong equatorward ‘surge’ in the evening. The values of F2-layer loss and diffusion coefficients needed to match the data are broadly consistent with the MSIS model. The analysis thus validates the MSIS model by way of ionospheric parameters quite independent of the data from which MSIS was originally derived.     

Some results on mean tidal structure and day-to-day variability over Arecibo

A long series of monthly 36 h observations, plus a series of observations at the same local time each day over two months at Arecibo, have been analyzed for tidal structure and variability. Some of the results are as follows. (1) A diurnal tide with vertical structure similar to that of the S_1,1 mode dominates the wind field up to heights of the order of 110 km over Arecibo. A semidiurnal oscillation dominates above that height. For non-winter conditions the semidiurnal oscillation in the 80–200 km region closely resembles the S_2,2 mode, though there is the possibility of contributions from higher order modes in the 100–120 km region. (2) Larger semidiurnal amplitudes are observed in the lower thermosphere for winter conditions. The data appears roughly consistent with Bernard's (1979) hypothesis that S_2,2, S_2,4 and S_2,5 modes are thermally excited, with the S_2,4 and S_2,5 modes out of phase in the meteor region in summer and in phase in winter. (3) The day-to-day variability of the tides is at least half the amplitude of the mean oscillations. The maximum wind variability was observed to occur in the 100–110 km region where the diurnal tide is strongly dissipated. (4) The day-to-day deviations in the wind and temperature oscillations from a long term mean at one local time tend to be wave-like structures which are generally correlated from day to day. The structures tend to move upwards, i.e., appear at a later local time, from day to day.     

A servo-model of the night-time F2-layer above St. Santin

A modified form of the ionospheric servo-model is used to describe the night-time F2-layer above St. Santin. Data taken by the incoherent scatter radar on nine nights in 1974–1977 were used to determine the height profiles of electron density, electron and ion temperature and electric field. The servo-model was then used to compute the theoretical variation through the night of the height of the F2 peak and the field-aligned plasma velocity, using gas concentrations and horizontal pressure gradients derived from the MSIS79 atmospheric model. On magnetically quiet nights these calculated values agreed closely with the observations. On disturbed nights, however, the calculations and observations began to diverge an hour or so after the onset of a substorm. The divergence could be explained by an enhanced southward wind.     

Seasonal variations of equatorial night-time thermospheric meridional winds

Using h'F data at two equatorial stations, night-time equatorial thermospheric meridional winds have been deduced for a period of two years to study their seasonal characteristics. It has been found that the thermospheric wind shows trans-equatorial flow from summer to winter hemisphere. During equinoxes the flow is mainly equatorward with a reversal to poleward direction around midnight hours. The abatement and reversal of equatorward wind which is weaker in summer compared to equinoxes is attributed to Midnight Temperature Maximum (MTM). The results of the present investigation are compared with those at other equatorial stations and also with the empirical model of Hedin et al. (1991).     

Ionospheric F2-kegion storms at midlatitudes starting in the daytime and the night-time

Disturbances in the F2-region during geomagnetic storms have been studied statistically, using foF2 data for two midlatitude stations in Japan. It is found

• 1.(i) that during the initial stage of the storm foF2 increases on the average for storms starting in the daytime and decreases for storms starting in the night-time, in all seasons,
• 2.(ii) foF2 decreases in summer and at the equinoxes and increases in winter irrespective of the local time of the storm onset.
• 3.(iii) disturbances of foF2 in winter for storms starting in the night-time are larger than those for storms starting in the daytime. Greater positive disturbances occur on the average for strong geomagnetic storms.

     

Multi-instrument observations of mesospheric motions over Arecibo: comparisons and interpretations

The MF/HF partial-reflection technique of observing the mesosphere and lower thermosphere has been employed for more than two decades to measure motions, but there has never been complete agreement as to what motions were being detected. This paper reports on observations made during a major international campaign—AIDA '89—that was initiated with the objective of resolving this question.The partial-reflection system employed was an Imaging Doppler Interferometer operating at 3.175 MHz, but it stands here as a prototype for all MF/HF partial-reflection radar systems: its raw data were analyzed both in its own basic mode, derived on the assumption that it sees wind-borne multiple scattering centers and in modes adopted by other interferometric and ‘spaced antenna’ systems. The motions thus revealed are compared here with those found by what we consider to be more certain measurers of winds: an incoherent-scatteer radar at heights of 65–95 km, a meteor-wind radar at heights of 80–100 km and a Fabry-Perot interferometer measuring 0(¹S) emissions near a height of 97 km.Comparisons of the different sets of observations oblige us to conclude that

• 1.(1) MF/HF partial-reflection systems may be expected to give a good representation of ambient winds up to a height of about 80 km;
• 2.(2) they fail to give a consistently reliable measurement of the ambient winds above a height of about 80 km
• 3.(3) they yield, at the greater heights, what appears in our data to be some convolution of the horizontal phase velocities of atmospheric gravity waves, with the wave spectrum having been modified by passage through the underlying wind system and containing, on occasion, locally generated Kelvin-Helmholtz waves; and
• 4.(4) when the underlying winds change, the local wave spectrum will change in response and, in MF/HF partial-reflection measurements, will give the appearance of a changing local wind: if the underlying winds undergo tidal changes, the wave spectrum will undergo tide-like changes that will masquerade as true tidal winds.

These results are, of course, limited to a single site over a limited period of observation. Nevertheless, taken at face value they suggest that current methods of data reduction are inappropriate for partial-reflection velocities at heights above 80 km and that new methods of data reduction, perhaps extending certain older methods that have been applied successfully in the past to total-reflection measurements, should be employed in their place if the full potential of the MF/HF partial-reflecton technique is to be realized.     

Propagation characteristics of night-time whistlers in the region of equatorial anomaly

In this paper results of simultaneous multi-station observations of night-time whistlers obtained at very low latitudes for the last three years are presented. The propagation mechanism of these whistlers is also discussed. Some records can be shown to have the characteristics of ducted propagation by calculating the dispersion of the whistlers along a ducted path and by noticing the feature of strong intensity, as well as the existence of three-hop echoes. Whistlers in some other records, however, can be shown to be propagating along a non-ducted path by numerical ray-tracing, in which a non-dipole field (IGRF) is used and a negative horizontal gradient of ionization in the equatorial ionosphere is taken into account. The agreement of the results of calculation with observed facts seems to confirm the existence of a definite whistler path at very low latitudes. Finally, based on analysis of typical events, the effects of magnetic storms on the parameters of low latitude whistlers due to variations in the equatorial ionosphere are also discussed.     

A study of the meridional structure of the equatorial red arcs in the night-time ionosphere from the ISIS-II satellite

Equatorial 6300 Å arcs observed by the ISIS—II satellite close to the magnetic equator over the African and Asian zones are studied for night-time conditions from 21:00 h to 02:00 h local time in the summer and spring of 1972–1974 and 1976, respectively. Case studies of the arcs have been made for quiet geomagnetic conditions and for minor storms. Sometimes very intense arcs with intensities of 1–2 kR are observed. Arcs of moderate intensities (300–400 R) are observed during geomagnetically disturbed periods. It is confirmed that these intensities can be fully accounted for theoretically by the dissociative recombination of molecular oxygen ions. Since the emission intensities are found to be sensitive to the geomagnetic activity, the influence of the latter has been taken into account and discussed.Equatorial spread-F (ESF)/bubble conditions are usually present at these local times. The data presented here show a correlation between the 6300 Å emission rate at one of the anomaly crests, the gradient in h (the lowest scaled real height from topside ionosonde trace) and the existence of ESF and gravity waves. This correlation is consistent with the scenario put forward by Maruyama and Matuura that the occurrence of ESF requires a symmetrical electron density distribution around the magnetic equator, so that a transequatorial wind causes an asymmetry and inhibits the formation of ESF.For the ISIS data we conclude that where strong transequatorial winds exist the 6300 Å emission rate at one of the anomaly crests is very large and there is a steep gradient in h. When these winds are weak, the 6300 Å emission is low and the gradient in h is also small. In the latter case, gravity waves of wavelength 200–400 km were present as well, which suggests that ESF is promoted by the existence of gravity waves. However, the magnetic disturbance level was higher during these orbits, which offers another source of gravity waves.     

Interannual fluctuations of the stratospheric temperature over the north polar region

The monthly means for the years 1964–1991 of 30 hPa temperatures over the North Pole and averaged over the 70–90°N region are analyzed. A multiple regression model is used to find long-term monthly trends and possible linear associations between these temperatures and the QBO, ENSO, and the 11-yr solar cycle. The model's residuals are examined for detection of other periodic interannual fluctuations in Arctic temperatures.It is found that the interannual variations of temperature at 30 hPa over the Arctic are a superposition of the oscillations due to the QBO, ENSO, the 11-yr solar cycle, and approximate 6-yr periodic fluctuations of unknown origin. The QBO, ENSO, and the solar cycle effects in the Arctic temperature explain about 35% of the total variance of the temperature monthly anomalies. In winter, the QBO, ENSO, and the 11-yr solar cycle signals in the temperature data depend on the phase of the equatorial QBO. The polar vortex seems to be warmer (colder) than normal when the West (East) phase of the equatorial QBO in a period of high solar activity. The monthly temperature trends over the Arctic show seasonal variations with positive trends in February and March. The year-round trends (sum of the monthly trends) are about −0.5 K per decade.     

Simultaneous observations of the enhanced plasma line and of the reflected HF wave at Arecibo

A suitably devised pulsing sequence of the powerful HF transmissions radiated towards the ionosphere below the penetration frequency made it possible to separate the attenuation of the reflected HF wave due to thermal and to ponderomotive type parametric instabilities. The separation was possible on account of the different growth times of the thermal and ponderomotive type parametric instabilities. At the same time the power in the 430 MHz enhanced plasma line was recorded.The results show that previously accepted explanations of the overshoot in the 430 MHz enhanced plasma line for 5.3 MHz HF transmissions are invalid. A strong overshoot was observed but the attenuation of the powerful HF wave was too small to be observed. For 3.175 MHz HF transmissions substantial attenuation was observed on both time scales.     

Model studies of the D-region negative-ion composition during day-time and night-time

A model for the negative-ion composition of the D-region is constructed, based on gas-phase reactions and photodissociation and photodetachment processes. The height distributions of negative-ion species, electrons and total positive ions for day-time and night-time are examined using time-dependent solutions of their continuity equations which incorporate simultaneous solutions for the minor neutral constituents involved. A distinction is drawn between the two isomers of NO₃⁻ and particular attention is paid to the influence of possible chemical and photodestruction of the more stable form of this ion.     

Validity of the estimates of night-time meridional winds made from bottomside ionograms

Meridional wind estimated from the bottomside ionograms making use of other thermospheric and ionospheric data from a unique rocket experiment from SHAR, has been compared with the direct, in situ measurement of the same. The agreement is found to be well within the experimental errors.     

Vertical propagation characteristics of internal gravity waves around the mesopause observed by the Arecibo UHF radar

A high resolution wind observation of the mesosphere and lower thermosphere (73–95 km) was conducted with the aid of the high power UHF Doppler radar at Arecibo (18.4°N, 66.8°W). Zonal wind velocities were continuously observed during day-time hours on 1–15 August 1980. We discuss here the observed wind fluctuations with periods of 1–4 h in the light of internal gravity waves. The phase propagation associated with these fluctuations is, on average, shown to be downward, indicating an upward energy flux. A space-time spectral analysis shows that waves with vertical wavelengths shorter than 10 km disappear around the mesopause (about 85km), while those with longer vertical wavelengths exist throughout the observational height. This result is explained in terms of wave absorption at a critical layer where the mean zonal wind has a westerly shear with height. This feature is consistent with the behavior expected for internal gravity waves around the summer mesopause in order to explain general circulation models.     

Simultaneous observations of E and F region electric field fluctuations at the magnetic equator

Simultaneous daytime observations of E region horizontal irregularity drift velocities in the equatorial electrojet and F region vertical plasma drifts were made on a few magnetically quiet days at the magnetic equatorial station of Trivandrum (dip 0.5°N). Measurements of the electrojet irregularity velocities by VHF backscatter radar and the F region vertical plasma drifts by HF Doppier radar are used to deduce the daytime East-West electric fields in the E and F regions, respectively. The fluctuating components of the electric fields are separated and subjected to power spectral analysis. The E and F region electric field fluctuations are found to be well correlated; the estimated correlation coefficient is in the range of 0.52–0.8. The fluctuation amplitudes are of the order of 15% over the background for the E region and 25% for the F region. The spectral analysis reveals dominant components in the range of 30–90 min with F region components stronger than those of the E region by a factor of about 1.5 on the average. The F region electric fields during daytime being coupled from the low latitude E region, the good correlation observed between the E and F region perturbations suggests that the electric fields in the E region at low and equatorial latitudes are coherent for the temporal scales of the order of few tens of minutes. The spectral characteristics are such that the commonly occurring medium scale gravity waves could possibly be the source for the observed fluctuations in the E and F region electric fields.     

Role of vertical winds on the Rayleigh-Taylor mode instabilities of the night-time equatorial ionosphere

In view of the recent observations on the presence of vertical winds in the equatorial ionosphere in the evening and night-time, the role of vertical winds in the Rayleigh-Taylor (R-T) mode instability has been re-examined. The mathematical treatment of Chiu and Straus, earlier developd for a case of horizontal winds, is extended to evaluate the role of vertical winds in causing the R-T mode instability. It is shown that the vertical (downward) winds of small magnitude have a very significant effect on the instability growth rate in the. F-region. A downward wind of l m s⁻¹ can cause the same growth rate as a 200 m s⁻¹ eastward wind at 260 km altitude. Furthermore, a downward wind of 16m s⁻¹ at 300 km can be as effective as that due to the gravitational drift itself. Similarly, an upward wind can inhibit the instability on the bottomside of the F-region. It appears that the polarity of the vertical winds (upward or downward) at the base of the F-layer plays an important role in the growth of the R-T mode plasma instability in the equatorial ionosphere.     

Observed and calculated F2 peak heights and derived meridional winds at mid-latitudes over a full solar cycle

The peak height of the F2 layer, hmF2, has been calculated using the ‘servo’ model of Rishbeth et al. [(1978), J. atmos. terr. Phys. 40, 767], combined with the hedin et al. [(1988), J. geophys. Res. 93, 9959] neutral wind model. The results are compared with observed values at noon and midnight derived from ionosonde measurements at two mid-latitude stations, Boulder and Wallops Island, over a full solar cycle. The reduced height of the F2 layer, zmF2, is also computed for the same period using the observed hmF2 values and the MSIS-86 model. Day-night, seasonal, and solar cycle variations in zmF2 are attributed to neutral composition changes and winds. Anomalously low values of hmF2 and zmF2 during summer both at solar minimum and during the solar cycle maximum in magnetic activity may be associated with increases in the molecular to atomic ion concentration ratio. Under these circumstances the F2 peak may lie significantly below the O⁺ peak height calculated by the servo model. Neutral meridional winds at Wallops Island are derived from the servo model using the observed hmF2 values and the calculated O⁺ ‘balance height’. It is shown that if the anomalously low hmF2 values are used, unrealistically large poleward winds are derived, which are inconsistent with both theory and observations made using other techniques. For most conditions the F2 peak is clearly an O⁺ peak, and daily mean winds at hmF2 derived from the servo model are consistent with the hedin et al. (1988) wind model. Unexpectedly, the results do not show an abrupt transition in the thermospheric circulation at the equinoxes. Diurnal curves of the servo model winds reveal a larger day-night difference at solar minimum than at solar maximum.     

The maintenance of the night-time ionosphere at mid-latitudes. I. The ionosphere above Malvern

The total rate of recombination in the night-time ionosphere above Malvern (at L = 2.6) was estimated using a model atmosphere, and the results were compared with the observed rate of change of total electron content to determine the net influx of plasma. Horizontal transport under the influence of electric fields was an important factor on a time-scale of an hour or less but when averaged throughout the night made little contribution. The main influx of plasma was a downward diffusion from the protonosphere, especially before midnight. The average downward flux increased steadily as the protonosphere filled after a magnetic storm, with a saturation time of at least 8 days.     

First low-power VHF radar observations of tropospheric,stratospheric and mesospheric winds and turbulence at the Arecibo Observatory

First VHF radar measurements with height resolution of 300 m and angular resolution of 1.7° were carried out in low latitudes at the Arecibo Observatory, Puerto Rico. A short outline is given of the experimental set-up which consisted of a 160W average power radar-transceiver and a self-contained digital radar control and data acquisition unit. The new VHF feed system of the Arecibo dish is described shortly. Reliable radar echoes were detected from the troposphere, lower stratosphere and from some heights in the mesosphere, indicating that the described VHF radar is capable of proper investigations of dynamical processes in the low latitude middle atmosphere. The angular dependence of aspect sensitive tropospheric and stratospheric turbulence structures was measured to be 1.5–2.5 dB degree⁻¹. Echoes from the mesosphere indicate a patchy structure of turbulence. The analysis of the signal-to-noise ratio shows considerably high reflectivity in the upper troposphere, which can be caused by high-reaching tropical cumulus convection. Wind profiles measured with the VHF radar between 7.5 and 19.5 km with a height resolution of 300m are very similar to radiosonde wind profiles. Mesospheric VHF radar winds are roughly consistent in amplitude with tidal winds.