Topside Sounder Ionograms Research Articles

Guided radio-wave propagation in the equatorial ionosphere according to the topside sounding onboard Interkosmos-19

In addition to normal vertical-incident ionogram traces, strongly remote (up to 2000km) traces of HF-radio-signal reflections observed on topside-sounder ionograms of the Interkosmos-19 satellite obtained in the equatorial ionosphere are presented. Such traces are connected with waveguides (ducts). These waveguides are field-aligned irregularities of the ionospheric plasma with electron density depletions of a few percent and cross-field dimension of a few to several kilometers. Ray tracing confirms this supposition and allows an estimate of typical waveguide parameters: diameter ≤10–15km and amplitude |ΔN/N|≥10%, where N is the electron density. The waveguide traces usually start at the cutoff frequencies of the main traces. However, sometimes they begin at much lower frequencies which indicates the satellite was transitioning through an equatorial plasma bubble during the recording of the ionogram. The X-mode of ducted echoes is more distinct then the O-mode. Only one ducted trace is usually observed on the Interkosmos-19 ionograms; a second conjugate trace is rarely recorded. The same is true for combination modes which is a combination of an oblique-incidence and guided propagation. Waveguides are observed at all heights of Interkosmos-19 (500–1000km) inside the equatorial anomaly region (from −40° to +40° Dip). Waveguides are usually associated with other irregularities of various sizes in the equatorial ionosphere, some of which cause additional traces and spread F on the topside-sounding ionograms. Ducted-echo characteristics observed with Interkosmos-19 are different from those observed earlier with the Alouette and ISIS satellites. This difference is discussed. It is shown that the ionospheric plasma irregularities responsible for the waveguides are observed much more often during nighttime than during daytime.

Online data base of satellite sounder and insitu measurements covering two solar cycles

Accurate descriptions of the solar cycle variations of ionospheric parameters are an important goal of ionospheric modeling. Reliable predictions of these variations are of essential importance for almost all applications of ionospheric models. Unfortunately there are very few global data sources that cover a solar cycle or more. In an effort to expand the solar cycle coverage of data readily available for ionospheric modeling, we have processed a large number of satellite data sets from the sixties, seventies, and early eighties and have made them online accessible as part of NSSDC's ftp archive (http://nssdcftp.gsfc.nasa.gov/spacecraft data/) and it's ATMOWeb retrieval and plotting system (http://nssdc.gsfc.nasa.gov/atmoweb/). We report about two data restoration efforts supported through NASA's Applied Information Systems Research Program (AISRP). The first project deals with insitu data from a large number of US, Canadian, Japanese and German satellites that measured ionospheric densities and temperatures from 1964 to 1983. The accumulated data base includes data from the BE-B, DME-A, AE-B, Alouette 2, ISIS 1, 2, OGO-6, AEROS A, AE-C, -D, -E, Hinotori, ISS-b and DE-2 satellite missions. The second project involves the production of digital topside sounder ionograms from the ISIS 1 and 2 satellites and their subsequent inversion to produce electron-density profiles. Approximately 340,000 ionograms are available from NSSDC as of July 2002. An automatic topside ionogram scaler with true height algorithm (TOPIST) was developed as part of this project and is now being used to obtain electron density profiles from these ionograms. Providing global coverage over more than two solar cycles the database established by this two projects is a valuable asset for improvements of the International Reference Ionosphere model and for ionospheric research.

Proton cyclotron echoes at the ƒQ3 resonance

We report here new observations on topside sounder ionograms from the Canadian ISIS II satellite of proton cyclotron echoes associated with the ƒQ3 resonance. Proton cyclotron echoes were first observed on swept frequency ionograms from the Canadian Alouette II satellite. The echoes occur on the ionograms at nearly constant apparent ranges which correspond to the proton cyclotron period at the satellite. The echoes were first found to be associated with the fundamental resonance of the electron cyclotron frequency, ƒH, and later reported at the 3ƒH and 4ƒH resonances. The new observations were made on ionograms when the satellite was located in regions with low dip angles less than 8°. This paper will present and discuss the features of these new proton cyclotron echoes, such as the presence often of two echo images, and possible generation mechanisms. A comparison with various ray‐tracing calculations by Muldrew (1972a, b) suggests that these proton cyclotron echoes may occur when the particular waves responsible for the ƒQ3, 3ƒH and 4ƒH resonances interact with protons energized by the satellite antenna.

Proton cyclotron echoes at 3ƒH and 4ƒH resonances

A new class of proton cyclotron echoes observed on topside sounder ionograms is presented; these echoes occur at frequencies above the electron plasma frequency and at the third and fourth harmonics of the electron cyclotron frequency, 3ƒH and 4ƒH. Some features are noted such as the dependence on low dip angle values. It is suggested that the 3ƒH and 4ƒH proton cyclotron echoes may be generated by wave‐particle interaction between the electrostatic waves and the protons that are both energized by the dipole antenna on Alouette II, ISIS I, or ISIS II.

Proton cyclotron echoes and spurs observed on Alouette II and ISIS II

Proton cyclotron echoes and spurs observed on the topside sounder ionograms on the Canadian Alouette and ISIS satellites are phenomena related to the proton cyclotron frequency. They appear on the ionograms at apparent ranges which lead to a frequency close to the proton cyclotron frequency; the frequency is calculated by taking the reciprocal of the time elapsed between the transmission of the sounder pulse and the reception of the signal at the satellite. Most proton cyclotron echoes and spurs occur at frequencies below the electron plasma frequency, ƒN; the echoes occur predominantly at frequencies slightly above the electron cyclotron frequency, ƒH, while the spurs occur predominantly when a harmonic of ƒH is near ƒN. Over 1000 ionograms exhibiting proton cyclotron echoes and/or spurs were examined and were classified depending on apparent range and shape of the echoes or spurs. The major classifications were echoes only, spurs only, and spur‐echo combinations. The subclassifications include one new type of spur which occurs at frequencies above ƒN. The classifications also reveal the absence of echoes with apparent ranges less than those corresponding to the calculated proton gyroperiod in the vicinity of the satellite. These results are compared with theories for the generation mechanism for proton cyclotron echoes and spurs.

Field‐aligned electron density irregularities near 500 km: Equator to polar cap topside sounder observations

Striking evidence, in addition to spread F, for field‐aligned electron density irregularities is commonly observed on postsunset Alouette 2 topside sounder ionograms recorded near perigee (500 km). This evidence is provided by distinctive signal returns from sounder‐generated Z mode waves. At low latitudes these waves become guided in wave ducts caused by field‐aligned electron density irregularities and give rise to strong long‐duration echoes. At high latitudes, extending well into the polar cap, transmitted Z mode waves (and stimulated electrostatic waves at the plasma frequency and trapped ordinary mode echoes) produce a series of vertical bars on the ionogram display as the satellite traverses discrete field‐aligned density structures. Within 15° dip latitude of the dip equator, Z mode wave ducts were encountered on about one third of the ionograms sampled; beyond 75° invariant latitude, vertical bar phenomena were encountered on more than three fourths of the ionograms sampled.

Auroral kilometric radiation: Wave modes, harmonics, and source region electron density structures

A number of dramatic changes are observed in the characteristics of auroral kilometric radiation (AKR) as the source region plasma to gyro‐frequency ratio fN/fH varies from 0.1 to 1.3. Most notable of these is a change from right hand polarized extraordinary (X) mode AKR dominance to left hand ordinary (O) mode AKR dominance as fN/fH varies from smaller to larger values. In addition to X and O mode AKR, Z (the slow branch of the X mode) and whistler (W) mode are also observed. The Z mode is observed over all fN/fH values, is often confined to the frequency region between fN and fH, and is typically slightly less intense than the O mode. The W mode is confined to frequencies less than fH/2, suggesting that it is the result of field‐aligned ducted signals reaching the satellite from a source at lower altitudes. Harmonic AKR bands, which appear to be of natural rather than instrumental origin and appear to be associated with the O as well as the X mode, are commonly observed. The second harmonic X mode appears to be due to propagating signals, whereas the higher harmonics appear to be confined to low‐density source regions. The second harmonic associated with the O mode is observed mainly at frequencies above the ambient 2fH value when fN/fH is large and below 2fH when fN/fH is small. The observations were obtained from some 200 ionograms from 12 passes of the ISIS 1 satellite through AKR source regions. Inferences concerning wave mode identification are based on comparisons of the observed AKR frequencies with sounder‐induced plasma resonances and wave cutoffs of ionospheric reflection traces. Electron density contours from the satellite altitude down to the altitude of the F layer ionization peak were obtained from the sounder data on each pass. Source encounters were made on nearly all of these passes, and both the minimum AKR source altitudes and the corresponding fN/fH values (which correspond to maximum values) were determined. The deduced (fN/fH) max is always less than 0.4 and is typically less than 0.2 during the generation of X mode AKR. For O mode AKR, on the other hand, (fN/fH) max approaches 0.9. The corresponding AKR source region minimum altitudes extend down to 2400 km. The (fN/fH) max values are in excellent agreement with previously published results of the maximum instability temporal growth rates, obtained from the AKR Doppler‐shifted cyclotron mechanism, as a function of fN/fH. The latitudinal extent of the low‐density, i.e., Ne < 100 cm−3, AKR source regions ranged from a few degrees to more than 20° along the satellite orbit (88° inclination). Within the wide density depletions, i.e., those that extended over many degrees, there were no large enhancements of the ambient fN/fH value. These results, which were limited by the ∼100 km spatial resolution corresponding to the spacing between topside sounder ionograms, were substantiated on a much finer spatial scale (∼10 km for Ne or 1/10 km for ΔNe) using in situ data from the on‐board Langmuir probe which were available for 10 of the 12 passes. The observed lack of large density enhancements within AKR source region density cavities provides additional confidence that the observed intense AKR is cyclotron X mode radiation rather than plasma frequency O mode radiation. The observed Ne enhancements, however, were larger than those required to support a feedback mechanism in the source region.

The signature of auroral kilometric radiation on Isis 1 ionograms

Auroral kilometric radiation (AKR) appears on the Isis 1 topside sounder ionograms as intense noise bands between the electron cyclotron frequency and 700 kHz. A variable gap occurs between the cyclotron frequency and the lowest AKR frequency. As Isis 1 traverses the source region, the gap narrows, and the AKR signals at higher frequencies weaken. This signature suggests that the AKR waves are generated directly in the extraordinary mode at frequencies just above the local cutoff frequency and that the radiation is initially perpendicular to the magnetic field.

Seasonal and longitudinal variations in the occurrence frequency of magnetospheric ionization ducts

Seasonal effects in the occurrence frequency of conjugate echoes on top side sounder ionograms recorded at nine telemetry stations, located at various longitudes, are studied by using Alouette 2, Isis 1, and Isis 2 satellite data. The combined data from all telemetry stations (2.8 × 105 ionograms) show minimums in the percentage of ionograms containing conjugate echoes at the equinoxes (2%) and maximums in the percentage of ionograms containing conjugate echoes at the solstices (about 7%). However, data recorded at a single telemetry station may show a maximum at only one of the solstices depending upon the longitude of the station. For example, data recorded at Quito, Ecuador (281°E), show only a December solstice maximum, whereas data recorded at Singapore (104°E) show only a June solstice maximum. A geometric model based on these results and on the observation that conjugate echoes are observed mainly during the nighttime hours is presented. According to this model, the conjugate ducts tend to be created when the angle between the geomagnetic axis and the normal to the plane of the ecliptic is maximum, that is, when the conjugate points of magnetic field lines are most asymmetric with respect to the sun-earth line. Magnetospheric currents flowing from one hemisphere to the other along geomagnetic field lines are suggested as the cause of the field-aligned ionization irregularities which produce the observed conjugate echoes on top side sounder ionograms.

Proton cyclotron frequency phenomena in the topside ionosphere

Proton cyclotron echoes and spurs are phenomena related to the proton cyclotron frequency discovered on topside sounder ionograms from Canadian Alouette satellites. The echoes and spurs appears on the ionograms at apparent ranges which lead to a frequency close to the proton cyclotron frequency; the frequency is obtained by taking the reciprocal of the time elapsed between the transmission of the sounder pulse and the reception of the signal at the satellite. Aloutte II and ISIS and II ionograms for about sixty satellite passes were scaled to study the charateristics of these phenomena. Generally, proton cyclotron echoes and spurs occured on the ionograms at frequencies below the electron plasma frequency f N , the echoes predominantly slightly above the electron cyclotron frequency f H and the spurs just below f N . They appeared most often when a harmonic of the electron cyclotron frequency nf H (n = 1, 2, 3, 4) was approximately equal to one of the other characteristic frequencies, that is: (1) nf H ≈ f N , (2) nf H ≈ f z S, the frequency of the Z wave at the heght of the satellite, and (3) nf H ≈ f T , the upper hybrid resonance frequency. Proton cyclotron echoes, spurs and protein cyclotron wave patterns have many features in common in addition to their fundamental relationship with the proton cyclotron frequency. The echoes and spurs are observed most often when when nf H overlaps one of the other characteristic frequencies, that is: nf H ≈ f N , nf H ≈ f z S, and nf H ≈ f T . The proton cyclotron wave pattern is observed under the first of the three conditions. It appears that the occurence of the phenomena is related to the plama conditions, the geographic location not being important in itself except that reflects different plasma conditions. Although proton cyclotron echoes and spurs were observed more often near the geomagnetic equator, consistent with the results of Matuura and Nishizaki,(8) they still observed at high latitudes even near the north geomagnetic pole. The echoes and spurs occur at frequencies below f N , the echoes predominantly slightly above f H and the spurs just below f N . Generally it is easy to distinguish between the two since usually they appear separately or, if together, often an echo would terminate and a spur begin at a slightly different apparent range. But it is not always easy since sometimes it appeared that a proton cyclotron echo and a spur formed a continuous trace, suggesting that perhaps they may be different manifestations of the same phenomenon. Work is continuing in an attemp to understand the origin of proton cyclotron echoes, spurs, and proton cyclotron wave patterns.

A lunar effect in the occurrence of conjugate echoes on topside sounder ionograms

Radio waves, transmitted by the Alouette 2 and Isis 1 satellites, can become trapped in magnetic field aligned ionization ducts, which can extend between the northern and southern hemispheres of the earth. In a set of data containing 1.8 × 105 ionograms recorded at five low-latitude telemetry stations from December 1965 to November 1971, conjugate echoes were observed on 4.5% of the ionograms. The frequency of occurrence of these echoes has a lunar semimonthly oscillation about the 4.5% average with an amplitude of (0.63 ± 0.13)% and with maximums at lunar ages of 4.9 and 16.9 hours (6 and 21 days after new moon). This oscillation occurs in independent blocks of data at very nearly the same lunar ages. The effect appears to depend on longitude, since it is found to be strongest in the data recorded at the American telemetry stations, intermediate at the African and Hawaiian stations, and weakest at the Asian and Australian stations. This effect is strongest for data recorded at the Quito telemetry station, where for data recorded between December 1965 and December 1966 a lunar semimonthly oscillation of amplitude (3.0 ± 0.2)% was found in the percentage occurrence of conjugate echoes. This semimonthly variation probably indicates that lunar tidal induced electric fields in the dynamo region have some influence in the production or duration of conjugate ducts; it is surprising, however, that no lunar diurnal or semidiurnal variation in occurrence frequency of conjugate ducts was found.

Reduction of Topside Ionograms for Field-aligned Propagation Paths

FIELD-ALIGNED reflexion traces are one of the common features of topside sounder ionograms when the sounder height is large (> 1,000 km) at mid and low latitudes and they usually have greater ranges, at a given frequency, than the vertical incidence reflexion traces. They are considered to be caused by the propagation of radio waves along field-aligned ionization ducts in the topside ionosphere or exosphere1,2.

Abnormal Latitudinal Distributions of the F2 Layer Critical Frequency

THIS letter deals with the unusual behaviour of the latitudinal distribution of NmF2 obtained from topside sounder iono-grams recorded at low latitudes. The anomalous decrease in the critical frequency at the dip equator compared with that at middle latitudes is well known as the “geomagnetic anomaly”1,2. The minimum at the equator is accompanied by crests near ±25°−30° magnetic dip. Many observers have noted that this behaviour is a daytime phenomenon for geomagnetically quiet conditions and for all levels of solar activity. King et al. (unpublished) have analysed the Alouette-I data and studied various aspects of the anomaly and Eccles and King3 have summarized the results of analyses of equatorial ionospheric data carried out by various workers. In the investigations reported in this letter, latitudinal variations of JFoF2* using Alouette-I satellite data are studied and some abnormalities contrary to earlier observations are noted. We have found a few cases where the equatorial anomaly is absent near local noon. These occurred during quiet geomagnetic conditions at a period of low solar activity. We shall discuss two such abnormal cases with respect to their morphological peculiarities only; a detailed study of this behaviour will be presented in a later publication.

A review of topside sounder studies of the equatorial ionosphere

The structure of the equatorial ionosphere as derived from topside sounder data is reviewed. Shortly after sunrise a single ionization crest occurs above the magnetic equator at all heights, but later during the day an equatorial anomaly develops in the form of an arch along which the ionization is enhanced and which lies along different magnetic field lines at different local times. The arch intersects the F2-layer peak at the latitudes where the well-known equatorial anomaly crests occur. The anomaly develops after about 0800 LMT in the Asian sector, but there is some evidence which suggests that the anomaly may start to develop several hours later in the American sector. The anomaly starts to decay after about 1500 LMT in the Asian sector, but only after about 2000 LMT in the American sector. The behavior during this decay phase, however, has not yet been fully established. During the night hours marked spread-F echoes are observed on the low-latitude topside sounder ionograms.

Electron densities less than 100 electron cm-3in the topside ionosphere

Data obtained from Alouette II topside-sounder ionograms have been used to study the regions of occurrence of low electron number density, N ≤ 100 electron cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-3</sup> , from December 1965 to April 1968. The study investigates the diurnal, seasonal, and spatial variations in the height and frequency of occurrence of the low electron density regions below 3000 km, within the limits imposed by the Alouette II orbit. In the northern-hemisphere winter near sunspot minimum, low electron densities were observed at all local times. A few examples were found in the summer (northem hemisphere) but only for local times around midnight and in the region of the main (midlatitude) trough. Most of the low electron densities were observed at geomagnetic latitudes greater than 65° near noon and greater than 55° near midnight, although they were observed at geomagnetic latitudes as low as 45° in the southern hemisphere near midnight. The low height cutoff of occurrence of N ≤ 100 electron cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-3</sup> showed a diurnal variation, with heights of 1400 km at night and about 2400 km near noon. This diurnal effect shifted upward in height during years of increased solar activity. The observations are shown to be consistent with the removal of light ions over regions in which the magnetic field lines are swept back into the tail of the magnetosphere.

Topside sounder observations of the equatorial anomaly in the 75° W longitude zone

The study of the equatorial ionosphere, by means of topside sounder ionograms. provides new insight to the structure of the equatorial anomaly. During a particular stage in the development of the anomaly, a ledge of ionization exists above the peak of the F-layer. The ledge is located along the earth's magnetic field lines between the regions of high electron density on both sides of the magnetic equator known as the equatorial anomaly. In this paper the development and decay of the anomaly is depicted by contours of constant plasma frequency in north-south vertical cross-sections of the equatorial ionosphere observed at different times of day. After sunrise the contours above the peak of the F-layer develop a dome-like shape, with the apex of the dome centered on the magnetic equator. The dome-like shape becomes more pronounced and the height of each plasma frequency contour increases until mid-afternoon. During the afternoon the ledge develops, located progressively along higher field lines. The contours for the region above the ledge retain the dome-like shape, whereas the contours below the ledge tend to be flat or to decrease in height near the magnetic equator. The regions of anomalously high electron density on both sides of the magnetic equator occur where the ledge merges with the peak of the F-layer. The ledge and the anomaly are observed until about local midnight. The presence of the dome-like contours is interpreted as evidence that ionization is lifted over the equator early in the day. The ledge is interpreted as the upper boundary of ionization which has been redistributed along the field. The movement of ionization along the field and away from the equator creates the regions of high electron density on either side of the equator. An examination of the planetary magnetic indices ( K p ) suggests that the observed variations in diurnal behaviour of the equatorial ionosphere are associated with variations in magnetic activity.