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.     

The application of a new method for determining irregularity parameters from topside ionograms

A method is presented for determining the amplitude and vertical wavelength of ionospheric irregularities which produce distortions or perturbations in topside ionogram traces. By way of illustration we discuss the application of the method to several examples which give irregularity amplitudes ranging from 0.1% to 10% and vertical wavelengths from 40–300 km. From the examination of 2 years data obtained by the ISIS II topside sounder it was found that irregularities readily analyzed using this method were confined to the equatorial region. They are detected at approximately 1000 km altitude and above, however, they may exist below these altitudes, since it is shown that topside sounders are more sensitive to irregularities in regions where the ionospheric scale height is large. Their characteristics are consistent with their being part of the field-aligned or duct irregularity phenomenon and their occurrence is consistent with the production mechanism suggested by Cole.     

Theoretical calculations on the changes in the ionospheric composition and temperature profiles during storms

The changes in the ionospheric composition and temperature profiles, in the altitude range of 120–1000 km, due to different mechanisms currently considered important during storms, are estimated quantitatively for a low latitude station, Delhi, for moderate solar activity conditions using the computer method of Stubbe. The theoretical results reported here are discussed in the light of the available ion composition and temperature variations observed at low latitudes during storms with satellite data in the topside ionosphere. The results are presented for the three atomic ions O⁺, H⁺ and He⁺ which are important in the F-region and topside ionsophere. It is found that all the three atomic ions increase or decrease in phase with the change in the concentration of n(O) when there is no change in total neutral density. When the change in the exospheric temperature T_∞ with its consequent change in neutral composition and an additional storm time increase in N₂ by a factor of 2 is considered, O⁺ is found to increase in the topside and decrease in the bottomside ionosphere, whereas H⁺ and He⁺ decrease all throughout except for a small increase in He⁺ above 800 km during day. The effect of eastward electric field or southward (equatorward) winds during the day is to increase all three ions in the topside ionosphere and to decrease them in the bottomside ionosphere and vice versa for westward fields or northward (poleward) winds. At night, O⁺ shows the same type of behaviour as for day, while He⁺ shows an increase above 900 km and a decrease below that height for eastward fields or soutward winds and H⁺ shows an oscillating behaviour.Electron and ion temperature (T_e and T_i) during the day shows anticorrelation with the change in the electron concentration N_e (equal to total ion concentration), whereas at night it does not show any significant change except for the case of change in T_∞ and N₂.     

Ionospheric plasma modification in the vicinity of a spacecraft by powerful radio pulses in topside sounding

A mechanism of strong turbulence is proposed for interpretation of the resonances observed by a wide-band receiver during topside sounding. The turbulence is created in the vicinity of the spacecraft due to the striction modulation instability. Experimental results obtained with the aid of a wide-band receiver on board the Intercosmos-19 satellite are discussed in terms of strong wave-wave and/or wave-particle interaction, namely electron acceleration in Langmuir cavitons and non-linear generation of the electron Langmuir plasma waves and Tonks-Dattner resonances.     

The mid-latitude trough in the electron concentration of the ionospheric F-layer: a review of observations and modelling

Experimental observations and theoretical modelling of the terrestrial mid-latitude trough are reviewed. The mid-latitude trough is considered as an F-layer phenomenon, and its relationships to the lightion trough in the topside ionosphere and to the plasmapause are discussed. The observed morphology of the mid-latitude trough is summarised. Recent evidence on plasma temperatures in the trough is examined. The physical processes that may be important in the trough region are listed. Large-scale computational models that include some of those processes are described and the results compared with observations. Deficiencies in the models and possible future developments are mentioned.     

Topside and intelhemispheric ion flows in the mid-latitude plasmasphere

A modelling study has been carried out of field-aligned ion flows in the topside ionospheres of conjugate hemispheres under solstice conditions at mid to low latitudes. In the model calculations coupled time-dependent O⁺, H⁺ and electron continuity, momentum and heat balance equations are solved along dipole magnetic field lines at L = 1.5 and 3.0 Sunspot medium and sunspot minimum atmospheric conditions are considered.It has been found that thermal coupling between conjugate hemispheres gives rise to strong flows of O⁺ in the topside ionosphere of the summer hemisphere that are directed upwards at conjugate sunrise and directed downwards at conjugate sunset. At conjugate sunrise in the winter hemisphere there is a small upward-directed signature in the O⁺ field-aligned flux; there is no observable signature in the O⁺ field-aligned flux in the winter hemisphere at conjugate sunset. There are strong upward and downward flows of O⁺ at local sunrise and local sunset, respectively, in both the summer and winter hemispheres.At both L = 1.5 and 3.0 the 24 h time-integrated interhemispheric H⁺ flux is in the direction summer hemisphere to winter hemisphere. At L = 1.5 its magnitude is in good agreement with the magnitude of the 24 h time-integrated plasma (O⁺ + H⁺) field-aligned flux at 1000 km altitude; there are no such agreements at L = 3.0.A study of the roles played by the individual terms of the O⁺ momentum equation has demonstrated the complex structure of momentum balance. Certain of the terms may be orders of magnitude greater than the combined total of the individual terms, i.e. the O⁺ field-aligned flux.     

Mid-latitude frequency spread and its association with small scale ionospheric stratifications

A variety of ground based radio techniques have provided new information relating to the nature of mid-latitude F-region irregularities responsible for frequency spreading on ionograms. Firstly, an analysis of ionograms covering a restricted frequency band indicates that frequency spreading is primarily caused by duplicate traces which are often unresolved in group path on standard ionograms. Furthermore, where angle of arrival information is available, the duplicate traces are shown to represent reflections from markedly different directions and the spread in critical frequencies is therefore indicative of a horizontal gradient in the peak electron density over a scale size of the order of many tens of kilometres. Secondly, the individual duplicate traces themselves are shown to comprise quasi-horizontal trace (QHT) segments which are unresolved on conventional ionograms and contribute to the diffuse appearance of spread-F traces on those ionograms. Difficulties in attributing these observations to the widely held view that scattering from small scale structures is the causative mechanism are discussed.     

Comparison of total electron content calculated using the IRI with observations in China

Values of total electron content (TEC) calculated using the International Reference Ionosphere (IRI-86 and IRI-90) are compared with the observations at Xinxiang based on the Faraday rotation measurement. It is found that the IRI gives acceptable values with respect to the observations during low solar activity. Generally the IRI-90 is better than the IRI-86 and the URSI coefficients are better than the CCIR coefficients in the calculation of TEC. Making use of the foF2 and M(3000)F₂ calculated using the Asia Oceania Region F₂-layer mapping (AOR) instead of using the CCIR or the URSI coefficients, the IRI gives more accurate TEC values. In October-April during high solar activity, however, the IRI obviously underestimates TEC in the daytime, which could be due to an improper topside electron density profile.     

Small scale stratification of the mid-latitude F-region

lonograms recorded over limited frequency and height ranges have been used to reveal systematic structures on both main and range spread traces for a mid-latitude region. This fine structure has apparently not been reported previously. When disturbances are present ionogram traces are often made up of quasi-horizontal segments, suggesting that the F2-layer consists of stratified regions spaced a few kilometres apart vertically. It is proposed that if in fact stratifications are present, they may be produced by instabilities created by the presence of internal gravity waves. Interference effects on ionogram traces are also reported. These are shown to result from the presence of overlapping traces. The existence of these quasi-horizontal trace segments and interference effects is shown to contribute to the diffuseness of a range spread mid-latitude ionogram. Thus a mid-latitude range spread ionogram often appears diffuse even though the spread may consist primarily of a number of duplicate traces produced by specular reflections from a range of off-vertical angles.     

Mid-latitude range spread and travelling ionospheric disturbances

This paper first discusses some early results (most hitherto unpublished) on off-vertical reflections which result from tilted isoionic contours associated with the passage of travelling ionospheric disturbances (TIDs). Azimuth of arrival and zenith angle information on these returning signals is discussed together with the role of these signals in producing both resolved and unresolved range spread on ionograms. Some ray tracing through a model ionosphere which incorporates wavelike structures is shown to predict fixed-frequency patterns (on recordings of virtual height vs. time) of converging and diverging satellite traces similar to those observed in practice. New experimental evidence is then presented to suggest (from the N(h) analyses performed) that TID wavetrains of several cycles are effective in producing spread traces on ionograms by specular reflections from the tilted surfaces of each cycle of these wavetrains. Ionograms from a modern ionosonde show that range spread consists primarily of discrete satellite traces even though at times these traces overlap and vary in intensity as a function of frequency thus creating a diffuse range spread ionogram.     

Equatorial spread-F: recent results and outstanding problems

Recent results and outstanding problems in the field of equatorial spread-F are reviewed. The discussion is organized about four distinct wavelength regimes: long (≳ 20 km). intermediate (20 km–100 m). transitional (100 m–10 m) and short (≲ 10 m). The intermediate waves are best understood, although the enhanced turbulent power near 1 km wavelength needs explanation, as does the saturation mechanism itself. The role of shear flow and irregularity seeding by ‘geophysical noise’ in the neutral atmosphere form important future research topics at the longest scales. Identification of drift waves in the transitional wavelength range has been a major new step in explaining the full spread-F phenomenon. This drift wave turbulence in the transition regime will be actively studied in the near future, particularly with regard to its role in determining the saturated amplitude of the intermediate waves, as well as in the anomalous diffusion of plasma at high altitudes. Although waves as short as 11 cm have been unambiguously detected and linear theories exist, the origin and amplitude spectra of these short wavelength waves inside topside plumes remain in some doubt and should also be examined in more detail.     

Refilling process in the plasmasphere and its relation to magnetic activity

When geomagnetic activity is moderate, the geosynchronous orbit crosses the plasmasphere bulge region in which the variations of plasma density from day to day can therefore be detected by geosynchronous satellites. The plasma density was measured by the Relaxation Sounder onboard ESA's GEOS-2 satellite. Variations of plasma density reflect the combined effects of refilling of particles from the ionosphere and loss of plasma by convection. The saturation level of the electron density at the geo-synchronous orbit and the refilling rate under different conditions of geomagnetic activity have been obtained and are found to be 70.5 cm⁻³ and 7–25 cm⁻³ day⁻¹, respectively. In this paper the refilling morphology and the relationship between the refilling process and magnetic activity (Dst index) are analysed. The refilling rate or refilling time constant inferred from the data, either directly on fairly well-defined refilling events, or indirectly through a simple model, are found to compare reasonably well with the refilling time constant expected by theory. The observed correlation of refilling rate with Dst index is interpreted as resulting from the modification of the composition of the topside ionosphere occurring after intense storms.     

Absolute electron density measurements in the equatorial ionosphere

Accurate measurement of the electron density profile and its variations is crucial to further progress in understanding the physics of the disturbed equatorial ionosphere. To accomplish this, a plasma frequency probe was included in the payload complement of two rockets flown during the CONDOR rocket campaign conducted from Peru in March 1983. In this paper we present density profiles of the disturbed equatorial ionosphere from a night-time flight in which spread-F conditions were present and from a day-time flight during strong electrojet conditions. Results from both flights are in excellent agreement with simultaneous radar data in that the regions of highly disturbed plasma coincide with the radar signatures. The spread-F rocket penetrated a topside depletion during both the upleg and downleg. The electrojet measurements showed a profile peaking at 1.3 × 10⁵cm⁻³ at 106 km, with large scale fluctuations having amplitudes of roughly 10 % seen only on the upward gradient in electron density. This is in agreement with plasma instability theory. We further show that simultaneous measurements by fixed-bias Langmuir probes, when normalized at a single point to the altitude profile of electron density, are inadequate to correctly parameterize the observed enhancements and depletions.     

The morphology of dayside Birkeland currents

Intense (10⁵ A) electric currents flow into and from the Earth's two polar ionospheres near magnetic noon. These currents, called Birkeland or magnetic field-aligned currents, are the agent by which momentum couples from the flowing solar wind plasma to drive plasma motions in the high latitude ionosphere. Coupling is strongest when the interplanetary magnetic field (IMF) has a southward component and when this occurs there exist two principal regions of Birkeland current near magnetic noon called the region 1 and the cusp systems. We present a simple model bringing theoretical order to the many patterns proposed previously for the morphology of these dayside Birkeland currents as observed by orbiting satellites in the topside polar ionosphere. Specifically we show that the cusp Birkeland current system is not a latitudinally separate region but is instead the extension in longitude of the region 1 Birkeland current from either dawn or dusk; which particular one depends on the sign of the east-west (Y) component of the IMF. The presence of an IMF Y-component therefore leads to two region 1 current systems near magnetic noon, with the poleward one being that previously called the ‘cusp’ system.     

Modifications in the nighttime low-latitude ionosphere after southward turning of the IMF

Sharp decreases in Φ_oF2 are found to occur frequently in the nighttime low-latitude ionosphere after southward turning of the IMF B_z component, especially under isolated B_z turnings, i.e. when the IMF has been northward for at least 6 h before its turning. These decreases occur simultaneously (within a 1-h time interval) with the B_z turning. The effect is observed both when a substorm or a magnetic storm begins after B_z has turned southward, and when a noticeable substorm does not occur. The effect is more pronounced after midnight and a maximum at 03 LT. Short-term (with scale times of about 1 h) variations of Φ_oF2 and h_mF2 for B_z southward turning are analysed using a large amount of ground-based and topside sounding data. The decreases in Φ_oF2 are shown to occur at first over the magnetic equator and then, during the second hour after the turning, at the crests of the equatorial anomaly. The ionosphere returns to its undisturbed state, on average, in 4–5 h (if other disturbing agents do not arise). These decreases are suggested to be caused by modifications in the electric field in the low-latitude ionosphere associated with B_z southward turning.     

Large-scale irregularities of electron concentration in the southern magnetic anomaly area according to Intercosmos 19 satellite data

Using Intercosmos 19 satellite topside sounding data, a type of complex ionogram for which the lowest frequency of the radio-wave which has passed through the ionosphere is smaller than the greatest frequency of the radio-wave reflected from the ionosphere is considered. (Under normal conditions these frequencies are identically equal.) A mechanism is suggested by which radio-waves transmitted by the satellite propagate over 3000 km in the topside ionosphere in the presence of inclined large-scale plasma structures, which can explain the main features of such ionograms.The space-time distribution of this phenomenon on a global scale is analysed. It is shown that it manifests itself mainly in the local winter, in the daytime and in the Southern Hemisphere. It is hypothesized that these large-scale irregularities are formed in the vicinity of the South Atlantic magnetic anomaly and then move westward.     

A theoretical study of the distribution of ionization in the high-latitude ionosphere and the plasmasphere: first results on the mid-latitude trough and the light-ion trough

A model of the O⁺ and H⁺ distributions in the plasmasphere and high-latitude ionosphere is described and first results are presented. The O⁺ and H⁺ continuity and momentum equations are solved from the F-region to the equatorial plane in the inner plasmasphere, and to an altitude of 1400 km in the outer plasmasphere and high-latitude ionosphere. Account is taken of high-latitude convection, departure from corotation inside the plasmasphere, and neutral air winds. The neutral air winds are consistent with the assumed convection pattern. For equinox and magnetically quiet conditions the calculations show that a mid-latitude trough in F-layer electron concentration is present from 1600 to 0600 LT and the trough may occur either inside or outside the plasmasphere. The movement of the trough in this period is from higher to lower latitudes and is in qualitative agreement with AE-C and ESRO-4 data. A light-ion trough feature is apparent in the H⁺ distribution in the topside ionosphere at all local times. During the day the upward H⁺ flow increases with latitude to produce the light-ion trough. At night the H⁺ trough may be directly produced by the occurrence of the mid-latitude O⁺ trough. The relationships between the position of the plasmapause and the trough are discussed. Also discussed are the influence of particle ionization in the auroral zone and the effect of the neutral air wind.     

Application of a simplified theory of ELF propagation to a simplified worldwide model of the ionosphere

The approximate theory of ELF propagation in the Earth-ionosphere transmission line described by Booker (1980) is applied to a simplified worldwide model of the D- and E-regions, and of the Earth's magnetic field. At 1000 Hz by day, reflection is primarily from the gradient on the underside of the D-region. At 300 Hz by day, reflection is primarily from the D-region at low latitudes, but it is from the E-region at high latitudes. Below 100 Hz by day, reflection is primarily from the gradient on the underside of the E-region at all latitudes. By night, reflection from the gradient on the topside of the E-region is important. There is then a resonant frequency (~300 Hz) at which the optical thickness of the E-region for the whistler mode is half a wavelength. At the Schumann resonant frequency in the Earth-ionosphere cavity (~8Hz) the nocturnal E-region is almost completely transparent for the whistler mode and is semi-transparent for the Alfvén mode. Reflection then takes place from the F-region. ELF propagation in the Earth-ionosphere transmission line by night is quite dependent on the magnitude of the drop in ionization density between the E- and F-regions. Nocturnal propagation at ELF therefore depends significantly on an ionospheric feature whose magnitude and variability are not well understood. A comparison is made with results based on the computer program of the United States Naval Ocean Systems Center.     

Small-scale ionospheric structures associated with mid-latitude spread-F

Slant-F traces on ionograms recorded by a modern ionosonde in a sunspot-minimum period have revealed the existence of field-aligned irregularities at times of spread-F occurrence. This appears to be the first investigation in a mid-latitude region around 36° (geomagnetic) to detect these irregularities at F₂-region heights using an ionosonde. Although such traces were observed frequently near sunspot minimum they were seldom recorded for periods close to sunspot maximum. Also, for a specific spread-F event in August 1989, both the ionograms from the modern ionosonde and scintillations of 150 MHz transmissions from a Transit satellite indicate the existence in the ionosphere of periodic structures (period around 11 min). The scintillation recording also included rapidly fading signals indicative of small-scale structures. The satellite had a path close to the magnetic meridian which passed through the recording station (Brisbane, Australia). Because of the enhanced signal fluctuations in the scintillation recording on this occasion it seems likely (with the support of other evidence on the ionograms) that the small-scale structures present were field-aligned.     

A comparison of mid-latitude and equatorial-latitude spread-F characteristics

Experimental evidence using a fast-swept-gain technique on an ionosonde is presented to support the idea that mid-latitude spread-F irregularities are large-scale wave-like structures. Also,diurnal and annual distributions of spread-F occurrence at an equatorial station at times of low sunspot activity are shown to be similar to those found for mid-latitude stations. The sunspot-cycle variation of post-midnight spread-F occurrence is also found to be similar in the two latitude regions. The similarity of certain spread-F characteristics at both mid- and equatorial-latitude regions is discussed. An attempt is made to reconcile current spread-F models for these two latitude regions by proposing that the primary spread-F structures for equatorial regions are large-scale wave-like structures. It is further proposed that the small-scale plasma instabilities have a role of modifying the traces resulting from specular reflections from the large-scale structures.