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.     

Experimental studies of daytime LF radio propagation and their implications for D-region electron densities

Measurements are presented of interference phenomena in amplitude and phase of VLF and LF signals along propagation paths from central England to the Norwegian Sea. The data are interpreted by means of the ‘wave-hop’ propagation theory, incorporating full wave evaluation of ionospheric reflection coefficients with realistic D-region models. No published electron density profiles are found which completely satisfy the experimental data, but modified profiles are presented which provide a better fit to the observations.     

Calibration of electron densities for the EISCAT UHF radar

The accuracy of electron densities in routinely analysed EISCAT data is discussed and comparisons are made between the EISCAT measurements and those determined by rockets and an ionosonde.     

Eiscat observations of the ionospheric D-region during auroral radio absorption events

Electron densities in the D-region have been observed with EISCAT during energetic electron precipitation events. Sample results are presented which demonstrate the value of the technique in studying variations of electron density with fine temporal and spatial resolution. Different types of absorption event can be characterized in terms of the changes in the incoming electron spectrum inferred from profiles of electron density. We contrast the D-region behaviour of night- and day-time events in terms of precipitating spectrum and absorption profile. A softening of the electron spectrum during the course of a morning event is clearly seen.     

Spread-F-like irregularities observed by the Jicamarca radar during the day-time

Until now the presence of F-region irregularities responsible for spread-F (sp-F) traces in ionograms has been considered as a purely night-time phenomenon extending sporadically to the early morning hours. We herein report that, on two occasions (26 March 1974 and 1 February 1984) similar irregularities were observed between 1400 and 1600 hours local time with the Jicamarca radar. These irregularities caused enhancements in the power of the radar echo of as much as two orders of magnitude, were found over a region of a few hundred kilometers on the topside of the F-region extending from around 600 to 1000 km altitude, and persisted for 1–2 h. The irregularities were aspect sensitive (aligned with the magnetic field) and produced echoes with a fading rate of the order of one to a few seconds. The background zonal electric field, inferred from the vertical drift velocity, was fairly constant in altitude, with values smaller than 0.1 mV m⁻¹. During the duration of the events, zonal components of both signs occurred, with the component passing through zero several times. We have no information on the vertical component of E. These irregularities could not be observed with ground-based ionosondes, since they are on the topside of the F-region. They may be related to fossil bubbles that are responsible for HF ducting observed by satellites.     

D-region electron density diurnal changes observed in association with the PCA event of July 1974

Well defined diurnal phase and amplitude deviations of very low frequency radio signals propagation were detected during the solar proton event that occurred from 3 to 8 July 1974. Phase advances observed on signals transmitted from GBR (U.K.), received at Atibaia (Brazil) and at Inubo Radio Observatory (Japan), were compared with simultaneous NWC (Australia) transmissions received at Atibaia, and at Syowa Station (Antarctica). Emphasis was given to the propagation paths NWC-Atibaia and NWC-Syowa, because both propagate completely in the southern hemisphere, crossing regions where the Earth's magnetic field behaves anomalously. The comparison allowed the determination of parameters typical of the D-region at a given height in the lower ionosphere, for geomagnetic regions defined through the McLlwain parameter(L). An exponential model was adopted to fit the vertical distribution of the diurnal electron density. The experimental results showed a delayed contribution to the total ionization observed which was attributed to a slow precipitation of energetic particles in the South Atlantic Geomagnetic Anomaly region.     

D-region signatures of substorm growth phase and onset observed by EISCAT

High resolution electron density measurements by EISCAT during the pre-onset phase and onset of an auroral absorption substorm are used to investigate the characteristics of electron precipitation during these substorm phases. The development of the pre-onset phase is the result of a uniform increase of electron fluxes with energies of a few tens of keV, with no particularly hard component. The absorption spike observed at substorm onset contains fine structure when investigated at 10 s resolution, indicating a rapid hardening of the precipitating spectrum at the onset.     

On the linear electron loss process in the D-region

Chakrabarty's mechanism for the linear electron loss process in the middle D-region is shown to be incorrect. Possibilities of explaining the linear electron loss process are briefly discussed.     

Incoherent scatter spectral measurements of the summertime high-latitude D-region with the EISCAT UHF radar

Measurements of incoherent scatter spectra from the auroral D-region were obtained during the summer of 1985 using a sophisticated pulse-to-pulse correlation technique with the EISCAT UHF radar. The spectral width variations with altitude are interpreted in terms of ion-neutral collision frequency, neutral temperature, mean positive ion mass and negative ion number density. Close agreement with predictions of currently available atmospheric models is obtained, except for a narrow layer around 86 km altitude. This layer showed evidence of increased positive ion mass for most of the experiment, and for short intervals indicated a mean ion mass close to 200 a.m.u. It is suggested that the layer is composed of proton hydrates in the vicinity of a structured noctilucent cloud, and that the index of hydration is occasionally large.     

EISCAT incoherent scatter radar observations and model studies of day to twilight variations in the D-region during the PCA event of August 1989

An intense solar proton event causing enhanced ionization in the ionospheric D-region occurred on 12 August 1989. The event was partially observed during three successive nights by the EISCAT UHF incoherent scatter radar at Ramfjordmoen near Tromsa, Norway. Ion production rates calculated from GOES-7 satellite measurements of proton flux and a detailed ion chemistry model of the D-region are used together with the radar data to deduce electron concentration, negative ion to electron concentration ratio, mean ion mass and neutral temperature in the height region from 70 to 90 km, at selected times which correspond to the maximum and minimum solar elevations occurring during the radar observations. The quantitative interpretation of EISCAT data as physical parameters is discussed. The obtained temperature values are compared with nearly simultaneous temperature measurements at Andøya based on lidar technique.     

Incoherent scatter radar measurements of electron density and ion velocity during an isolated substorm

A study of the average pattern of F-region plasma densities and velocities measured by the Chatanika incoherent scatter radar has previously suggested that the main ionospheric F-region trough is formed in the evening sector by the westward transport of plasma under the influence of convective electric fields. This paper examines the role of convective electric fields on the electron density profile and the formation of the F-region density trough for a particular night. Incoherent scatter radar data from Chatanika are presented.On 25 May 1972 an isolated substorm occurred near 0900 UT after a long period of magnetic quiet. The substorm was manifested at Chatanika, in the evening sector, by a small positive bay and a concurrent onset of westward motion of plasma associated with a rapid decrease in the F-layer electron density in the region of the moving plasma. Analysis of plasma densities and velocities during this event indicates that

• 1.(1) temporal changes of plasma motion are associated with changes in the convective electric field pattern in response to substorm activity
• 2.(2) the electric field pattern created a north-south gradient in the F-layer electron density which is interpreted as the formation of the ionospheric trough near its equatorward edge, and
• 3.(3) large scale electron density fluctuations were observed in the evening sector resulting from westward travelling density variations originating in the midnight sector.

The study emphasizes the complexity, and difficulty in interpretation, of single station auroral zone measurements of the F-region ionosphere.     

On the origin of dusk radar scatter events

The possibility of the generation of decameter scale ionospheric plasma density irregularities, that must be responsible for dusk scatter, by the plasma gradient drift instability (GDI) at F-region altitudes is considered. It is shown that the dusk scatter could be produced by the ion density perturbations which appear as a result of the development of the GDI produced by the maximum westward plasma drift in the region poleward of the trough minimum. Possible reasons for the appearance of growth of the GDI waves as a result of the development of the trough plasma GDI during or just after sunset in the F-region are discussed. It is shown that, if the GDI begins after sunset, then the influence of the drift velocity shear results in the action of the GDI during 1–2 hours after sunset, which is close to the duration of dusk scatter.     

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.     

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.     

Poker Flat MST radar observations of high latitude neutral winds at the mesopause during and after solar proton events

Observations with the Poker Flat, Alaska, MST radar during and after solar proton events in 1982 and 1984 suggest that winds in the altitude range of ~ 80–90 km were altered as a consequence of the influx of energetic charged particles and large electric fields at high latitudes. The atmospheric changes accompanying these events appear to result in a reduction of the semidiurnal tide and an enhancement in the diurnal tide. It is suggested that these changes could result from the alteration of the local tidal heating distribution produced by the particle precipitation, either through changes in the local ozone distribution or as a result of mesospheric Joule heating.     

Beam-generated electrostatic electron waves: relevance to EISCAT VHF radar measurements

The generation by an electron beam of (slowly) growing electrostatic waves, detectable with the EISCAT VHF radar, is investigated. A set of graphs is derived from which it is possible to estimate, by interpolation, frequency, growth rate and resonant beam velocity for most situations of interest. The dependence of the mentioned variables on propagation direction, beam temperature and density and on ionospheric density over the part of parameter space that is of practical interest in the auroral and subauroral ionosphere is presented and discussed.     

Evaluation of electron and ion densities in the middle atmosphere from rocket-borne blunt probes

Detailed consideration has been given to the determination of electron number densities from conductivity data gathered by rocket-borne blunt probes in the middle atmosphere, and the intercomparison of these electron densities with those derived from other diagnostics. A definition of the difficulty of electron density determination from rocket-borne probes is presented. Also, the procedures for the determination of ion densities from blunt probe data in the middle atmosphere are critically evaluated. General aspects of particle collection by supersonic probes are compared with those of subsonic probes. It is noted that strong (× 10) compression regions will form in front of supersonic probes at altitudes up to 100 km, and the altered electron attachment rates could significantly affect indicated electron and negative ion concentrations. A summary of new analysis for determining electron densities from negative conductivities taken with a subsonic blunt probe is presented and the analysis is applied to data on several days where intercomparisons are possible. Blunt probe data from 31 January 1972 and 5 December 1972 (WI),² and 2 October 1975 and 29 September 1977 (WSMR)³ are reduced to predict electron density profiles. In the region of intercomparison, there is general agreement in the electron density predictions. The indications of electron density at altitudes below 70 km are new, and predict a region of moderately enhanced densities down to 45 km.     

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.     

Modelling of the electron density profile in the lowest part of ionosphere D-region on the basis of radiowave absorption data: 2. Seasonal variations

A new model of the lowest part of the D-region is obtained by a trial-and-error inversion method. Its basic feature is a step-like transition between 55 and 70 km which is not depicted in most ionospheric models. The seasonal differences of this are considered to be quite important: the bottom of the ionsphere is found to be lower in summer and spring, the gradient of the profile below the CR-layer is stronger in winter, and a well defined ‘valley’ exists around 70 km in spring. By simulating the ionospheric response to a, solar flare (SID-effect) in summer and in winter, an attempt was made to verify the obtained seasonal peculiarities of the quiet ionosphere.