D-region electron densities observed by incoherent-scatter radar during auroral-absorption spike events

Electron density profiles in the night-time auroral ionosphere were obtained with the incoherent-scatter radar at Chatanika, Alaska, during short duration precipitation events characterized by riometer data as spike events. The measurements show exceptionally large electron densities in the D-region during spike events, the electron density typically exceeding 10⁶ cm³ at 90 km altitude for a short time. The existence of a steep horizontal gradient, particularly on the poleward edge of the event, is inferred. The altitude and thickness of the absorbing layer are deduced. It is shown that 20–40 keV electrons make the greatest contribution to an absorption spike and that the spectrum of electrons producing such an event is probably softer than that producing a more slowly varying absorption peak. These absorption layers are too high for their altitudes to be measured by the technique of multi-frequency riometry.     

One full-day radar measurement of lower stratospheric winds over Jicamarca

The lower stratospheric wind field is measured simultaneously at several altitudes for 24 h on 23–24 May 1974 using the Jicamarca radar in Peru. Maximum entropy spectral analysis is used to estimate echo power spectra from the measured autocorrelation functions which are truncated beyond the maximum time lag of 6.65 s. The fluctuating wind velocity is resolved into steady and tidal components. A vertical shear of 2–3 m s⁻¹ km⁻¹ is observed throughout the time of observation in the zonal wind. The mean zonal wind is in good agreement with the Lima-Callao radiosonde data obtained in the same month. A very large diurnal component, of amplitude 1–5 m s⁻¹ is observed in the zonal wind in the lower stratosphere. However it appears that a semidiurnal oscillation below the tropopause is predominant.     

Morning sector electron precipitation events observed by incoherent scatter radar

Two auroral zone electron precipitation events in the morning sector have been studied in detail using the UHF incoherent scatter radar in northern Scandinavia. The electron density profiles are interpreted in terms of the incoming spectrum of energetic electrons, and it is shown that the spectrum is most energetic at the maximum of the event and softens subsequently. The observations cannot be explained by simple gradient-curvature drift of trapped electrons. It is shown, further, that events appearing to be fresh substorms in magnetometer and riometer data may be no more than intensifications of continuing activity. During a pulsation event the incoming electron spectrum was modulated in energy as well as in intensity. The height and thickness of the resulting radio-absorption layers are derived.     

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.     

Phase velocity studies of 34-cm E-region irregularities observed at Millstone Hill

Phase velocity observations at E-region heights made with the Millstone Hill 440 MHz radar find no evidence of an ion acoustic limiting speed for phase speeds observed near 0° magnetic aspect angle. Under most circumstances the phase speed increases steadily with increasing backscattered power amplitude. For a 34cm volume backscatter cross-section, σ_v, less than ∼5 × 10⁻¹³ m⁻¹, the phase speed is at or below the usual ion acoustic speed in the E-region (350m/s), and increases only slowly with the observed backscattered power amplitude (∼50 m/s per 10dB). At higher power levels, the phase speed exceeds 350 m/s, reaching values in excess of 750 m/s at times, and increases more rapidly with backscattered power (∼200 m/s per 10dB). Phase velocity/time maps observed over a 3° span of latitude suggest that many features of the phase speeds observed are directly related to changes in the ambient convection electric field in the E-region due to changing activity conditions or the effects of superimposed magnetospheric pulsations.     

Seasonal variations of day-time ionisation flows inferred from a comparison of calculated and observed NmF2

This paper reports on a comparison of calculated and observed monthly mean day-time ionospheric F2-peak density (NmF2) at a chain of stations from Japan to Australia for both solar minimum (1976) and solar maximum (1980). Nm values are calculated using the MSIS model for the observed peak heights (hmF2) and a simplified version of the continuity equation for day-time equilibrium conditions. The observed NmF2 values are always higher than the calculated ones in winter. This implies that a substantial downward flow of ionisation from above into the winter ionosphere is induced by the strongly poleward winter neutral wind which drives the ionisation down the field lines, lowering the peak height hmF2. In summer, winds are smaller, and the fluxes are more upward in comparison to winter. The seasonal variation of the ionisation fluxes and neutral winds are estimated for solar minimum, and compared with results of detailed calculations.     

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.     

Long period wind oscillations observed by the Kyoto meteor radar and comparison of the quasi-2-day wave with Adelaide HF radar observations

We have detected wind oscillations with periods ranging from 1.4 to 20 days at 80–110 km altitude using Kyoto meteor radar observations made in 1983–1985. Among these oscillations, the quasi-2-day wave is repeatedly enhanced in summer and autumn. We found that the period of the quasi-2-day wave ranges from 52 to 55 h in summer, and becomes as short as 46 to 48 h in autumn in 1983 and 1984. The change in the wave period seems to coincide with a decrease in the amplitude of the zonal mean wind. A quasi-2-day wave event was simultaneously observed in January 1984 at Kyoto (35° N, 136°E) and Adelaide (35° S, 138° E), which are located at conjugate points relative to the geographic equator. Amplitudes of the meridional component at Adelaide are approximately four times larger than those observed at Kyoto. Comparison observations clearly show that the meridional component is in phase and the zonal component is out of phase, respectively, implying antisymmetry of the quasi-2-day wave between the northern and southern hemispheres. Relative phase progressions with height are similar between the Kyoto and Adelaide results for both meridional and zonal components, and indicate the presence of an upward energy propagating wave with a vertical wavelength of about 100 km.     

HF scattering induced by field-aligned irregularities in the equatorial E-region during total solar eclipses

In this paper, results and analyses of solar eclipse effects on the lower ionosphere are presented. After the first contact of the total eclipse on 16 February 1980, an absorption increment of 12 dB was observed. At the same time, the frequency of amplitude fading increased largely and Doppler frequency shift disturbances appeared. The calculation of signal strength is carried out by means of Booker's scattering theory, supposing an outer scale T_o = 1000 m and an inner scale T_i = 5 m, of space scale spectrum of field-aligned irregularities in the equatorial E-region. The calculated results agree fairly well with observations. Results showed that, because of the formation of lower ionospheric field-aligned irregularities in the course of the obscuration of solar local ionization source, radio wave scattering was strengthened.     

Observations of 3-m auroral irregularities during the ERRRIS campaigns

In the late winter of 1988 and 1989, three NASA sounding rockets were flown through the auroral electrojet from ESRANGE (Sweden) as part of the E-region Rocket-Radar Instability Study (ERRIS). Many ground-based instruments supported these flights, including the EISCAT, STARE, and CUPRI radars, as well as all-sky cameras, riometers, and magnetometers. In this paper we summarize the observations of the Cornell University Portable Radar Interferometer (CUPRI), which detected coherent backscatter from 3-m irregularities in the auroral E-region. Twenty hours of power spectra and interferometry data are available, and, during the 1989 campaign, three weeks of nearly continuous Range-Time-Intensity (RTI) and first moment data were recorded.     

Atmospheric dynamics observed during the Energy Budget Campaign

Using data of several meteor radar experiments during the Energy Budget Campaign, a synthesis of the main components of the upper mesospheric wind field is presented. Tides, long period waves and prevailing wind, as well as small scale fluctuations, are studied. The prominent feature of this comparison relates to the semidiurnal tide variability.     

Some further results on the lower stratospheric winds and waves over Jicamarca

The VHF radar at Jicamarca, Peru (12.0°S, 76.9°W) was used to probe the tropical lower stratosphere on 3–4 October and 6–8 December 1977. Velocity data obtained during these experiments exhibit features indicative of winds and waves in the stratosphere as follows:

• 1.(1) The amplitude and phase of a diurnal oscillation in the observed horizontal wind compare well with theoretically predicted values of the solar diurnal tide. An observed semidiurnal oscillation differs considerably from theoretical values, although minimal phase variation with height is a fundamental property of both theory and observation. The observed vertical wind oscillations are larger than theoretical values of vertical tidal components, although the data is consistent with recent rocket observations.
• 2.(2) Dominant velocity oscillations with periods near the Brunt-Väisälä period are frequently observed.
• 3.(3) Downward phase progression of the westerly regime of the quasi-biennial wind oscillation is observed during the course of the two observations. A long-period oscillation with a period exceeding two days also appears to be superposed on the quasi-biennial oscillation.
• 4.(4) Systematic differences are found between horizontal winds measured by the radar and those measured by rawinsondes launched from the Lima-Callao airport some 30 km west of Jicamarca.

     

Dominant vertical scales of gravity waves in the middle atmosphere observed with the MU radar and rocketsondes

We have simultaneously observed wind motions in the altitude range of 5–90 km by means of the MU radar, rocketsondes and radiosondes. Dominant vertical scales of wind fluctuations due to gravity waves were 2–5 km in the lower stratosphere, about 5–15 km in the upper stratosphere and longer than 15 km in the mesosphere. The increase in the vertical scale with altitude is interpreted in terms of the saturation of upward propagating gravity waves. In the stratosphere, the observed vertical wavenumber spectra showed smaller amplitudes and more gradual slopes than the model values. Furthermore, the wind velocity variance in the stratosphere increases exponentially with an e-folding height of about 9 km, implying that the gravity waves were not fully saturated. On the other hand, the spectra in the upper stratosphere and mesosphere agreed fairly well with the model spectra. The variance in the mesosphere seems to cease increase of the wave amplitudes and agrees reasonably well with the model value.     

Stationary large-scale irregularities of the ionosphere

Ionospheric absorption measurements (Al method) made in the course of eight voyages by Soviet and other research vessels indicate that the global distribution of absorption shows a distinctive regional structure. Areas of abnormally high absorption in the neighbourhood of the equator have been located in the Pacific near the west coast of South America and in the Indian Ocean. The west Mediterranean area also shows abnormally high absorption. In some cases these areas of high absorption appear to coincide with areas of low nocturnal F-region electron density.     

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.     

Preliminary results from a study of the F-region heating during an intense aurora observed by EISCAT

Observations of large time variations in the ionospheric F-region temperature derived from EISCAT are compared with simultaneous observations of the E- and F-region plasma densities. The observations suggest that the F-region may be heated by current driven instabilities generated during intense precipitation of auroral electrons.     

Structures responsible for rapid fading of medium frequency radio reflections from the day-time E-layer

The steerable beam Bribie Island radar (152°E, 27°S) operating at a frequency of 1.98 MHz was used to obtain data relevant to reflection conditions near 100 km altitude on 7 days during June–October 1982. The rapid signal fading commonly observed is primarily due to transient reflectors with lifetimes of a few seconds, often seen up to angles of 20° from the zenith. Longer lived moving reflectors (presumed to be sporadic-E clouds) also play a part. Certain properties of the transient reflectors are consistent with a turbulent generation mechanism. However, any theory of their origin must explain why, for about a third of the time, they tend to occur preferentially to the north and east of the observing site. A direct comparison of velocities using Doppler and spaced antenna drifts methods shows reasonable agreement when the data is averaged over quarter hour periods. However, conclusions by previous workers, on the basis of observations of motions of diffraction patterns, that the ionospheric structure responsible for the diffraction pattern observed on the ground is undulations of the isoionic contours by gravity waves, is not supported by a detailed analysis of the data.