Topside Sounder Research Articles

Large scale structures from topside sounding

ISIS 2 orbits at an altitude of about 1400 km and sounds the ionosphere in the range from 0.1 MHz to 20 MHz. On analyzing a sequence of ionograms measured on April 26, 1971, covering a region from about −73° to 88° in geographic latitude along a trajectory that is mostly close to 80°W longitude, at about 1000 UT or about 5 a.m. local time, one finds large‐scale structures in the spatial fluctuations of the electron density above the F peak. The structural features are analyzed by using isoelectronic contours, separating out trends in the data to obtain the spatial fluctuations in the altitude for various levels of electron density. The altitude variations for the various electron density levels strongly correlate, and spatial fluctuation waves are found on the order of 2000 km in scale size. Another example is shown for February 20, 1975, and is for an orbit that is in near conjunction with the Atmosphere Explorer C (AE‐C) satellite. AE‐C is at a much lower altitude, about 250 km, and measures, among many parameters, in situ ionization density. Comparison of the structures observed by ISIS 2 and AE‐C in the electron density fluctuations relates features above and below the F peak; the same spatial wave system is found in the data of both satellites with scale sizes from about 600 km to more than 2000 km.

Longitudinal variability of annual changes in activity of equatorial spread F and plasma bubbles

Global distribution maps of equatorial spread F (ESF) activity for various annual periods were derived from the topside soundings by the Ionosphere Sounding Satellite b (ISS‐b) in 1978–1980. The ESF activity during the northern winter period reveals maximum enhancement at the Atlantic longitudes of large westward geomagnetic declination, and during the northern summer at the Pacific longitudes of large eastward declination. On several orbits passing over the region of the ESF activity enhancement, abrupt depletions of the electron density or equatorial plasma bubbles were observed at the satellite altitude of about 1100 km. It has also been found that the background electron density distributions tend to be symmetric with respect to the magnetic equator in the region of the enhanced ESF activity and asymmetric in the region of the suppressed ESF activity. These observations are taken account of by the influence of a transequatorial thermospheric wind upon the suppression of the Rayleigh‐Taylor type plasma instability.

Uniformly spaced field‐aligned ionization ducts

MF and HF radio wave signals transmitted from topside sounders can often become trapped in magnetic‐field‐aligned ionization ducts located approximately vertically below the satellite. The resulting traces observed on ionograms are called combination mode traces. A number of cases of combination mode traces observed with the ISIS topside sounders were studied for passes over mid‐ and low‐latitude regions which exhibited distinguishable, nearly uniformly spaced traces on the ionograms typical of this type of propagation in the frequency range above 1 MHz. Associated extraordinary vertical and ducted traces were also evident in all cases. The ducted trace is interpreted as due to propagation in a duct encompassing the satellite at the time of measurement, whereas the combination mode traces are interpreted as due to ducts located well below the satellite. The simultaneous presence of a ducted trace and combination mode traces suggests the presence of a parallel system of ducts that are field aligned. On interpreting the ionograms for events studied by ray tracing, it was found that the ducts are separated by as much as 70 km transverse to their field alignment. The data indicate that, at times, the ducts may exist down to the F peak and may extend into the conjugate region as well. A multiplicity of ducts with such large, relatively uniform separations was not previously appreciated, and the mechanism for creating them in this fashion is not clear. One possibility for the even spacing of these irregularities is that they result from acoustic gravity waves at lower altitudes. Possible relationships to other geophysical phenomena, particularly equatorial phenomena, are discussed.

Alouette‐ISIS radio wave studies of the cleft, the auroral zone, and the main trough and of their associated irregularities

The principal high‐latitude ionospheric features such as the cleft, the nighttime auroral zone, the main trough and their associated irregularities, as observed with the Alouette/ISIS topside sounders, are reviewed. These features can be clearly distinguished in Alouette 1 electron density contour plots as a function of latitude and height below 1000 km. Alouette 2 data recorded near apogee (3000 km) show the location of the light‐ion (H+ and He+) trough as a function of local time and show it to be more closely related to the plasmapause than is the main trough. An attempt is made to clarify the location and terminology of troughlike structures. Regions of small‐scale irregularities (tens of meters) are associated with the high‐latitude ionospheric features. These irregularities are responsible for the presence of aspect‐sensitive spread F on topside ionograms. The most intense spread is found in the cleft region. Spread F regions usually have a sharp equatorward boundary. Some recent unpublished work by the author indicates that the spread F boundary for the nighttime auroral zone tends to be located several degrees equatorward of the keV electron precipitation zone and is probably generated indirectly by this precipitation and convected equatorward. In the main trough region spread is sometimes present and other times absent. Some of the individual small‐scale irregularities responsible for spread F can extend along magnetic‐field lines for hundreds of kilometers.

Field-aligned plasma flow in the quiet, mid-latitude ionosphere deduced from topside soundings

A method for quantifying diffusive flows of O + ions in the topside ionosphere from satellite soundings is described. A departure from diffusive equilibrium alters the shape of the plasma scale-height profile near the F2-peak where ion-neutral frictional drag is large. The effect enables the evaluation of , the field-aligned flux of O + ions relative to the neutral oxygen atom gas, using MSIS model values for the neutral thermospheric densities and temperature. Upward flow values are accurate to within about 10%, the largest sources of error being the MSIS prediction for the concentration of oxygen atoms and the plasma temperature gradient deduced from the sounding. Downward flux values are only determined to within 20%. From 60,000 topside soundings, taken at the minimum and rising phase of the solar cycle, a total of 1098 mean scale-height profiles are identified for which no storm sudden commencement had occurred in the previous 12 days and for which K p was less than 2o, each mean profile being an average of about six soundings. A statistical study ofdeduced from these profiles shows the diurnal cycle of O + flow in the quiet, topside ionosphere at mid-latitudes and its seasonal variations. The differences betweenand ion flux observations from incoherent scatter radars are considered using the meridional thermospheric winds predicted by a global, three-dimensional model. The mean interhemispheric flow from summer to winter is compared with predictions by a numerical model of the protonospheric coupling of conjugate ionospheres for up to 6 days following a geomagnetic storm. The observed mean (of order 3 × 10 16 ions day −1 along a flux tube of area 1 m 2 at 1000 km) is larger than predicted for day 6 and the suggested explanation is a decrease in upward flows from the winter, daytime lonosphere between the sixth and twelfth days.

High‐latitude F region irregularities observed simultaneously with ISIS 1 and the Chatanika radar

Simultaneous measurements of the high‐latitude ionosphere were made by the Chatanika radar and the ISIS 1 topside sounder and cylindrical electrostatic probe (CEP) when the satellite was at a height of about 700 km. The variation of enhancements and valleys of electron density with invariant latitude at the satellite, on the scale of a few hundred kilometers, agrees reasonably well with the radar results, even though the satellite displacement changed from almost coincident to about 500 km magnetic west of the radar scan path. These density enhancements comprise a collection of large‐scale (tens of kilometers) irregularities or ‘blobs.’ Evidence for the formation of these blobs was obtained from the sounder. Natural, intense radio noise (hiss) was observed on the sounder ionograms recorded on the poleward side of the auroral zone, indicating the presence of precipitating kiloelectron volt electrons. A few minutes after these ionograms were recorded the radar scanned the same invariant latitude region and a large‐scale ionization enhancement, or ‘blob,’ not present 13 minutes earlier, had developed in the bottomside F layer. ISIS 1 telemetry dropouts resulting from medium‐scale irregularities, and enhanced irregularities down to about 200 m, observed with the CEP, are observed in association with blobs. Scatter signatures observed on the ISIS ionograms may be associated with large‐scale irregularities that can be identified in the radar data. The scattering detected by the sounder is caused by 30‐ to 60‐m wavelength irregularities (small‐scale) that presumably are created by instabilities operating on the edges of the large‐scale irregularities. The small‐scale irregularities, however, are observed on both poleward and equatorward directed gradients. If the gradient‐drift instability is responsible for these irregularities, then they either convect to the point of observation, are due to zonal gradients, or are field‐aligned extensions of irregularities generated at lower heights.

Departures from diffusive equilibrium in the topside F-layer from satellite soundings

A method for the detection of O + ion fluxes from topside soundings is described. The shape of the plasma scale-height profile is altered by such flows only at heights near the F2-peak, where ion-neutral drag is large. Model profiles are used to relate changes in scale height to the ratio ( φ φ L ) where φ is the field-aligned O + flux (relative to the neutral air) and φ L is the limiting value set by frictional drag. Values of ( φ φ L ) can then be determined to within a few per cent from experimental soundings, using the plasma temperature and its gradient (as deduced from the observed profile) and the MSIS model neutral temperature. It was found that 3700 topside profiles show departures from diffusive equilibrium, out of 10,000 used to obtain the global morphology of ( φ φ L ) near the sunspot minimum. Results reveal dynamic ion-flow effects such as the transequatorial breeze and the effects of the polar wind and protonospheric replenishment light-ion flows can be inferred.

Simultaneous wave and particle observations in the auroral kilometric radiation source region

Simultaneous observations from the topside sounder and the soft particle spectrometer onboard the ISIS 1 satellite reveal that very specific conditions on the local electron density and the energetic electron distributions must both occur in the auroral kilometric radiation (AKR) source region. Such regions are associated with inverted V electron precipitation and with depletions in the local electron density. The electron velocity distribution functions obtained near the inverted V peak were found to increase near several keV energy with increasing velocity as required for plasma instability. The electron density observed near the inverted V peak was too high to support AKR for three events investigated, however, and the AKR source was identified with the edge of the inverted V where the density was low (⩽ 30 cm−3) in each case. Whereas this density depletion can extend deep into the ionosphere (∼ 1500 km altitude), the severe depletion associated with the AKR density cavity is restricted to higher altitudes (> 2750 km for an event studied in detail).

Operation and experimental results of the Ionosphere Sounding Satellite-b

The Ionosphere Sounding Satellite-b (ISS-b) launched in February 1978 is successfully performing the mission observations which provide the information of electron distributions in the ionospheric F layer, the radio noise and the plasma parameters in a global scale in order to monitor the electro-magnetic environment of the Earth as well as to promote the efficient use of the radio spectrum. In addition to the brief description of satellite-borne mission apparatuses, this paper describes the operation of the satellite including the pass assignment suitable for measurements of the global distribution of miscellaneous quantities. Experimental results obtained from the following four mission observations are also presented: (1) Topside sounding of the ionosphere (TOP mission) has made it possible to map the ionospheric F layer critical frequency (foF2) on a global basis. Five sets of foF2 global maps, two of which are representative of autumn (centered in October 1978) and winter (centered in January 1979) respectively, have been obtained. (2) Radio noise measurement (RAN) by means of four channel HF receivers has brought to light the global maps of the lightning occurrence at different local time in various seasons. (3) and (4) In situ measurements of the ambient plasma have revealed the global distributions of electron temperature and concentration (RPT mission) and those of ion composition (PIC mission) at about 1000 km altitude above the Earth.

Mini-review on topside sounding

Ionospheric sounders flown on satellites have returned a wealth of global information of the electron density distribution from the <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">F</tex> -region electron peak upwards, not obtainable in any other way. This mini-review outlines the techniques used to obtain topside ionograms and reduce them to electron profiles, indicating the most important differences between ground-based and satellite-based ionograms. An attempt has been made here to indicate the most essential features, with the details provided in the cited references.

Nonlinear Landau damping in the ionosphere

A model is presented to explain the nonresonant waves which give rise to the diffuse resonance observed near 3/2 fH by the Alouette and Isis topside sounders, where fH is the ambient electron cyclotron frequency. These waves are the result of plasma wave instabilities driven by anisotropic electron velocity distributions initiated by the high‐power (400 W) short‐duration (100 µs) sounder pulse. In a strictly linear analysis these instability‐driven waves will decay owing to Landau damping on a time scale much shorter than the observed time duration of the diffuse resonance. Calculations of the nonlinear wave‐particle coupling coefficients, however, indicate that the diffuse resonance wave can be maintained by nonlinear Landau damping of the sounder‐stimulated 2fH wave. This 2fH wave is always observed with a time duration longer than that of the diffuse resonance wave, a finding which indicates that it is available as an energy source for the proposed nonlinear mechanism. The time duration of the diffuse resonance is determined by the transit time of the instability‐generated and nonlinearly maintained diffuse resonance wave from the remote short‐lived hot region back to the antenna. The model is consistent with the Alouette/Isis observations and clearly demonstrates the existence of nonlinear wave‐particle interactions in the ionosphere. Such interactions have been observed in the laboratory and have long been speculated as being important in the energetics of the earth's magnetosphere and ionosphere.

Wave propagation experiments at medium frequencies between two ionospheric satellites, 1, General results

Occasionally the ISIS I and ISIS II spacecraft rendezvous in parts of the world where simultaneous telemetry coverage is feasible. Electromagnetic propagation between ISIS I and ISIS II has been studied using their topside sounders at medium and high frequencies. Above the gyrofrequency, the O and upper‐frequency X modes have been identified often during separations of up to a few thousand kilometers. In a few instances, the lower‐frequency X, or Z, mode has been observed with separations less than 1000 km. This mode is continuous below the gyrofrequency where the whistler mode is also seen. Intersatellite ionograms confirm that the O‐, X‐, and Z‐mode pulses propagate largely as expected on the basis of magnetoionic theory applied to a smoothly varying medium, with little distortion except near cutoff. On the other hand, whistler‐mode pulses are significantly elongated by the medium. Their interpretation is the subject of a companion paper, part 2.

Stimulated plasma waves in the ionosphere

The results of 15 years of plasma resonance observations from rocket‐ and satellite‐borne topside sounders are summarized. The major resonances are observed at the electron plasma frequency ƒN, the electron cyclotron frequency ƒH, the upper hybrid frequency ƒT = (ƒ2N+ƒ2H)1/2, the harmonics 2ƒT and nƒH with n &gt; 1, and two series of resonances that appear between nƒH∶ the ƒQn resonances observed above ƒT (at the maximum frequency of the Bernstein modes) and the ƒDn resonances observed below ƒT (called the diffuse resonances). Most of the resonances can be interpreted in terms of the reception of longitudinal plasma waves stimulated by the sounder pulse. In some cases these waves, which travel with low group velocity (of the order of the electron thermal velocity or less) are received after being reflected in the nearby medium (within a distance of several kilometers). The reflections are due to the extreme sensitivity of the dispersion curves near the resonant frequencies to small changes in electron density (for the ƒN and ƒT resonances) or to small changes in magnetic field strength (for the lower‐order nƒH resonances). In other cases (ƒQn resonances and the higher‐order nƒH resonances) the signal reception is due to the matching of a component of the wave group velocity to the satellite velocity rather than to a wave reflection process. The ƒDn resonances involve a sounder‐stimulated electron temperature anisotropy leading to the Harris instability of the Bernstein modes and a nonlinear coupling between these wave‐modes. Since the interpretation of all of the resonances depends on warm plasma theory, the observations can be used to obtain the electron temperature T⊥ and T∥ (corresponding to electron motions perpendicular and parallel to the ambient magnetic field direction, respectively) as well as the electron density and the magnetic field intensity. These measurements pertain to a large volume (hundreds of meters to kilometers) around the antenna and thus are not seriously affected by spacecraft/plasma interactions. The results of the investigations of artificially stimulated plasma waves by sounder pulses have been applied to the interpretation of naturally occurring wave phenomena such as the (n+1/2)ƒH magnetospheric VLF emissions, the terrestrial kilometric radiation, and the Jovian decametric radiation. Among the major unexplained plasma resonance phenomena are the resonances observed at ƒH and 2ƒT, and the numerous ion effects associated with the electron plasma resonances. The advent of the Space Shuttle with subsatellites will enable active‐controlled experiments to be conducted with highly flexible radio‐frequency instrumentation in order to investigate problems of interest to many geophysical and astrophysical phenomena such as wave mode coupling and the evolution of nonlinear and plasma wave instability processes.

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.

Certain characteristics of trajectories of longitudinal waves in an ionospheric plasma near the resonance at the Langmuir frequency

The refraction of longitudinal waves in an inhomogeneous magnetoactive plasma near the Langmuir frequency is investigated in connection with the topside sounding of the ionosphere. It is shown that in this case the propagation is possible only for forward waves whose wave vectors and group velocity form an acute angle. The possibility of appearance of loop-type trajectories is analyzed under the condition where the plasma frequency is greater than the electron gyrofrequency. The criteria for the existence of different types of trajectories are obtained as functions of the plasma parameters and the wave characteristics. The observation of resonance effects during topside sounding of the ionosphere led to a large number of theoretical studies on their interpretations (for example, see [1-8]). A number of studies had the objective of explaining the large durations of the signal observed at resonance frequencies Wp, u h, u H, and nu H (cop is the Langmuir frequency for the electrons, coh is the upper hybrid frequency, UH is the electron gyrofrequency, and n = 2, 3,...). The interpretation of these durations with the use of refraction of the packets of electrostatic waves in the homogeneous plasma has found extensive use. This interpretation was first proposed by McAfee in [2], where the propagation of plasma waves at u ~ Up was considered. In [2] the possibility of the group trajectories returning in the vicinity of the radiator after propagation in the in_homogeneous ionospheric plasma was analyzed for the case Up > w H. The computations [2] show that two types of group trajectories exist which can intersect the level of the radiator simultaneously. One of the trajectories has the form of a loop and, according to the authors, corresponds to the backward wave; the second trajectory has the usual form and corresponds to the direct wave. In another work of McAfee [3], the refraction of plasma waves is investigated for the case Up WH and Wp w H and in the case COp ~o H is confirmed. The criteria for the appearance of such trajectories are expressed in analytical form as functions of the plasma parameters and the wave characteristics.

Mid-latitude electron density enhancement in the nocturnal topside ionosphere and its longitudinal inequalities.

A study of data from the cylindrical electrostatic probe and the topside sounder aboard the Alouette 2 satellite reveals the existence of nighttime Ne enhancements at mid-latitudes, at altitudes 1, 000-3, 000km. The peaks seem to approach the equator with increasing altitude, and appear to lie on a specific set of geomagnetic field lines. The intensity and location of these nocturnal Ne peaks, as well as the electron density in the entire low- to mid-latitude range, show a marked dependence on geomagnetic longitude. These observations are interpreted in terms of field-aligned flow of thermal plasma from the protonosphere above 3, 000km in the presence of a westward electric field at L≈2.

Coincident observations of ionospheric troughs and the equatorial plasmapause

Explorer 45 traversed the plasmapause (determined approximately via the saturation of the d.c. electric field experiment) at near-equatorial latitudes on field lines which were crossed by Ariel 4 (~600km altitude) near dusk in May 1972 and on field lines which were crossed by Isis II (~1400km altitude) near midnight in December 1971 and January 1972. Many examples were found in which the field line through the near-equatorial plasmapause was traversed by Explorer 45 within one hour local time and one hour universal time of Ariel and Isis crossings of the same L coordinate. For the coincident passes near dusk, the RF electron density probe on Ariel detected electron density depletions near the plasmapause L coordinates when Ariel was in darkness. When the Ariel passes were in sunlight, however, electron depletions were not discernable near the plasmapause field line. On the selected near-midnight passes of Isis II, electron density depressions were typically detected (via the topside sounder) near the plasmapause L coordinate. The dusk Ariel electron density profiles are observed to reflect O + density variations. Even at the high altitude of Isis near midnight, O + is found to be the dominant ion in the trough region whereas H + is dominant at lower latitudes as is evident from the measured electron density scale heights. In neither local time sector was it possible to single out a distinctive topside ionosphere feature as an indicator of the plasmapause field line as identified near the equator. At both local times the equator-determined plasmapause L coordinate showed a tendency to lay equatorward of the trough minimum.

The topside magnetospheric cleft ionosphere observed from the Isis 2 spacecraft

Data from the Isis 2 spacecraft for five orbits in November 1971 are examined in detail. The spacecraft was then in cartwheel configuration, permitting detailed angular and spectral measurements of charged particle fluxes (E > 5 eV), local ion densities and temperatures, local electron densities and temperatures, F region peak electron densities, and 6300- and 5577-Å emissions accurately located at the field line foot. In the November 9 and 10 orbits, highly structured regions of electron flux having average energies of ∼0.7 and ∼0.2 keV are observed. The O+ density at the 1400-km spacecraft altitude generally reflects the electron precipitation, disappearing rapidly at the poleward boundary with He+ becoming dominant in the polar cap. Throughout this region of enhanced O+ densities, strong polar wind flows of H+ are observed. The electron densities obtained with the topside sounder are sometimes higher than the ion densities; this is in part due to the structured nature of the cleft region, but other indirect plasma processes may also be involved. The ion temperature is about 2000°K inside the cleft and rises to about 5000°K at the poleward cleft boundary. The F region peak density shows some structure but no uniquely identifiable cleft response, and the optical emissions generally show a multiple peaked structure, normally with the lowest red/green ratio in the equatorward components. The November 17 and 18 orbits differ in that the electron spectrum precipitation energies are closer to 100 eV, are less structured, and are displaced to higher invariant latitude. The O+ densities are higher, while the light ions are little changed, and O+ is observed poleward of the precipitation region, implying strong convection perpendicular to the auroral oval. The optical red/green ratio is higher for these orbits. A tentative explanation for the different behavior is the passage of a sector boundary between these two dates with the interplanetary field direction changing from ‘away’ to ‘toward.’

The magnetospheric cleft-Ionospheric effects

The fluxes of electrons that precipitate through the day side clefts of the magnetosphere are intense enough and deposit their energy at a high enough altitude to heat and ionize the F layer and top side ionosphere substantially. Relevant synoptic results from bottom side and top side sounder measurements are reviewed, and new data from the Isis satellites are presented. The effects of precipitation through the cleft can be seen in bottom side ionograms, but the diurnal variations that appear in routine monthly median plots are probably due primarily to other effects. As seen from above the F layer by the top side sounders on Alouette and Isis spacecraft, the most obvious effects are an expansion (increase in scale height) of the top side ionosphere due to heating and the presence of small-scale spatial fluctuations in density. The electron density at the F layer peak is often, but not always, increased. The increased electron density at satellite altitudes associated with top side expansion is often seen at a slightly higher latitude than the precipitation, and this displacement in latitude (of ∼1°) is interpreted in terms of an antisunward drift due to magnetospheric convection.

Theory of topside sounder transmission effects on antenna quasistatic sheath impedance

A quasistatic theory has been constructed for predicting low frequency sheath impedance changes produced on spacecraft sounding antennas as a result of intense RF emissions by them. The theory involves calculation of time‐independent sheath profiles around an infinite cylindrical antenna in a collisionless plasma, using a numerical scheme which includes the time‐averaged repelling force on electrons due to a nonlinear effect of the RF near field of the antenna. These sheath profiles then yield values of static sheath capacitance and conductance per unit antenna length. The theory includes a more accurate electron current calculation than in the earlier treatments by the authors. Results are presented for a range of parameters applicable to spacecraft sounding conditions. In most cases, the quasistatic sheath capacitance and conductance both decrease as RF amplitude increases. At large enough RF amplitudes, the capacitance becomes almost independent of antenna potential and Debye ratio.