Topside Ionograms Research Articles

Enhancing the ISIS‐I Topside Digital Ionogram Database

AbstractSelected original analog telemetry tapes from three of the topside‐sounder satellites of the International Satellites for Ionospheric Studies (ISIS) program, namely Alouette 2, ISIS I, and ISIS II, were used in an earlier project to produce more than ½ million digital topside ionograms; the resulting digital topside ionograms from ISIS II were used to produce more than 86,000 globally distributed vertical topside ionospheric electron density profiles Ne(h) that cover a time span of more than a solar cycle. These Ne(h) were produced using the Topside Ionogram Scaler with True height algorithm auto‐scaling software. Before attempting to automatically process Alouette‐2 or ISIS‐I ionograms, a data‐enhancement project was initiated so as to increase the number of ionograms suitable for manual scaling and to increase the auto‐processing success rate. These enhancements were mainly to correct problems that often occurred during the analog‐to‐digital conversion of the original telemetry tapes. Here we illustrate the improvements made to the ISIS‐I digital topside ionograms and compare Ne values at the satellite altitude and Ne(h) profiles, based on the manual scaling of selected ionograms, to both the auto‐scaled values and the predictions of the International Reference Ionosphere 2016 model. The results indicate the need to improve the available auto‐processing software for the new ISIS‐I digital ionograms and that International Reference Ionosphere 2016 predicts midlatitude winter topside Ne values that are too high in the late morning and at noon but too low in the early morning.

High‐latitude topside ionospheric vertical electron density profile changes in response to large magnetic storms

AbstractLarge magnetic‐storm‐induced changes were detected in high‐latitude topside vertical electron density profiles Ne(h) in a database of profiles and digital topside ionograms, from the International Satellites for Ionospheric Studies (ISIS) program, that enabled Ne(h) profiles to be obtained in nearly the same region of space before, during, and after a major magnetic storm (Dst < −100 nT). Storms where Ne(h) profiles were available in the high‐latitude Northern Hemisphere had better coverage of solar wind parameters than storms with available Ne(h) profiles in the high‐latitude Southern Hemisphere. Large Ne(h) changes were observed during all storms, with enhancements and depletions sometimes near a factor of 10 and 0.1, respectively, but with substantial differences in the responses in the two hemispheres. Large spatial and/or temporal Ne(h) changes were often observed during Dst minimum and during the storm recovery phase. The storm‐induced Ne(h) changes were the most pronounced and consistent in the Northern Hemisphere in that large enhancements were observed during winter nighttime and large depletions during winter and spring daytime. The limited available cases suggested that these Northern Hemisphere enhancements increased with increases of the time‐shifted solar wind velocity v, magnetic field B, and with more negative values of the B components except for the highest common altitude (1100 km) of the profiles. There was also some evidence suggesting that the Northern Hemisphere depletions were related to changes in the solar wind parameters. Southern Hemisphere storm‐induced enhancements and depletions were typically considerably less with depletions observed during summer nighttime conditions and enhancements during summer daytime and fall nighttime conditions.

Improving the automatic inversion of digital Alouette/ISIS ionogram reflection traces into topside electron density profiles

The topside sounders of the International Satellites for Ionospheric Studies (ISIS) program were designed as analog systems. The resulting ionograms were displayed on 35 mm film for analysis by visual inspection. Each of these satellites, launched between 1962 and 1971, produced data for 10 to 20 years. A number of the original telemetry tapes from this large data set have been converted directly into digital records. Software, known as the Topside Ionogram Scalar With True‐Height (TOPIST) algorithm, has been produced and used for the automatic inversion of the ionogram reflection traces on more than 100,000 ISIS‐2 digital topside ionograms into topside vertical electron density profiles Ne(h). Here we present some topside ionospheric solar cycle variations deduced from the TOPIST database to illustrate the scientific benefit of improving and expanding the topside ionospheric Ne(h) database. The profile improvements will be based on improvements in the TOPIST software motivated by direct comparisons between TOPIST profiles and profiles produced by manual scaling in the early days of the ISIS program. The database expansion will be based on new software designed to overcome limitations in the original digital topside ionogram database caused by difficulties encountered during the analog‐to‐digital conversion process in the detection of the ionogram frame sync pulse and/or the frequency markers. This improved and expanded TOPIST topside Ne(h) database will greatly enhance investigations into both short‐ and long‐term ionospheric changes, e.g., the observed topside ionospheric responses to magnetic storms, induced by interplanetary magnetic clouds, and solar cycle variations, respectively.

New satellite mission with old data: Rescuing a unique data set

We review efforts to save a unique data set and scientific results based on the rescued data. The goal of the project was to produce Alouette 2, ISIS 1, and ISIS 2 digital topside ionograms from selected original seven‐track analog telemetry tapes. This project was initiated to preserve a significant portion of 60 satellite years of analog data, collected between 1962 and 1990, in digital form before the tapes were discarded. More than 1/2 million digital topside ionograms are now available for downloading at http://nssdcftp.gsfc.nasa.gov and for browsing and plotting at http://cdaweb.gsfc.nasa.gov. We illustrate data products, discuss analysis programs, review scientific results based on the digital data, and recognize those who made the project possible. The scientific results include evidence of extremely low altitude ionospheric peak densities at high latitudes, improved and new ionospheric models including one connecting the F2 topside ionosphere and the plasmasphere, transionospheric HF propagation investigations, and new interpretations of sounder‐stimulated plasma emissions that have challenged theorists for decades. The homepage for the ISIS project is at http://nssdc.gsfc.nasa.gov/space/isis/isis-status.html.

The Poynting vector applied to the complex refractive index in a hot plasma near the electron cyclotron frequency and to the cyclotron resonance observed on topside ionograms

The electron cyclotron resonance is the only fundamental resonance observed on topside ionograms that, up to the present, has not been understood. There is a solution to the hot plasma dispersion equations for the wave frequency f near the cyclotron frequency fH when the wave number k makes a small angle to the Earth's magnetic induction B. For this case, the magnitude of the imaginary part of k is more than half the real part. This means that the group velocity, dω/dk, where ω = 2πf, is complex. The real part of dω/dk is too large to explain the cyclotron resonance observations. In addition to the wave number, the dispersion relations for a hot magnetoplasma can be used to obtain the electric and magnetic fields of the cyclotron wave and hence the Poynting vector. Dividing the Poynting energy flux by the energy density of the wavefield, where the energy density is the sum of the electric, magnetic, and electron energy densities, gives the Poynting velocity VP, which is a good approximation to the actual wave energy velocity for f near fH. Cyclotron waves are strongly damped but can exist several milliseconds as observed. The waves radiate from the plane containing B and the dipole antenna L. The waves with f > fH beat with those with f < fH to produce the beat frequency observed on fixed‐frequency ionograms.

A global study of nighttime midlatitude topside spread echoes

Range spreading is frequently evident in midlatitude topside ionograms from the ISS‐B satellite. These ionograms provide a geographically unbiased perspective for the first global investigation of nighttime spread echo events that focuses solely on the midlatitude domain. The topside echo occurrence probability data show significant longitudinal variations, along with very large winter/equinox asymmetries in several geographic regions. The spread echoes are divided into two types and found to have different geophysical dependences. Small‐scale events that appear as spreading in one or two consecutive ionograms have a marked seasonal dependence on magnetic declination and no obvious correlation with auroral indices. Larger‐scale events in which the spreading is observed over spatial domains of 1000 km or more have less dependence on magnetic declination but are anticorrelated with the AE index in periods 6–8 hours prior to their appearance. The declination dependence for the small‐scale events suggests that the irregularities are most prevalent when the E region conductivity is low at both ends of the flux tube. This in turn suggests that electric fields mapping from the E region up to F region altitudes are likely involved in their generation. The correlation of the larger‐scale events with depressed AE signatures suggests that traveling ionospheric disturbances caused by auroral heating are not the most likely cause of these large features at midlatitudes.

Ionization ledge structures observed in the equatorial anomaly region by using PPS system on-board the Ohzora (EXOS-C) satellite

To verify an additional ionization layer predicted in the equatorial ionosphere, topside ionograms obtained by the Planetary Plasma Sounder (PPS) system on-board the Ohzora (EXOS-C) satellite were analyzed. Based on the analysis of the PPS data of 8 passes in March and 11 passes in May, 1987, the ionization ledge observed in the local noon time period shows similar nature as it has been theoretically predicted for the F3 layer by Balan and Bailey (1995). It was noted that some peaks of the ledge structure were located on the field line of higher latitude region than the field line of the crest of the equatorial anomaly.

Automated diagnostics for resonance signature recognition on IMAGE/RPI plasmagrams

The Radio Plasma Imager (RPI) aboard the IMAGE spacecraft probes plasma at both far and near ranges by means of radio sounding. The RPI plasmagrams, similar in their concept to the ground‐based and topside ionograms, contain not only a variety of signatures pertaining to the remote plasma structures and boundaries, but also a suite of the local plasma resonances stimulated by the RPI radio transmissions. Detection and interpretation of the resonance signatures is a valuable diagnostic tool providing the actual electron density and magnetic field strength at the spacecraft location, which are needed for the accurate processing of the remote sensing information on the plasmagrams. The high volume of the RPI sounding data demanded the development of automated techniques for routine interpretation of the plasmagrams. This paper discusses a new method for the detection and interpretation of the resonance signatures in the RPI plasmagrams that employs pattern recognition techniques to localize the signatures and identifies them in relation to model‐based resonances.

Topside Ionogram Scaler With True Height Algorithm (TOPIST): Automated processing of ISIS topside ionograms

The United States/Canadian ISIS‐1 and ISIS‐2 satellites collected several million topside ionograms in the 1960s and 1970s with a multinational network of ground stations that provided good global coverage. However, processing of these ionograms into electron density profiles required time‐consuming manual scaling of the traces from the analog ionograms, and as a result, only a few percent of the ionograms had been processed into electron density profiles. In recent years an effort began to digitize the analog recordings to prepare the ionograms for computerized analysis. As of November 2002, approximately 390,000 ISIS‐1 and ISIS‐2 digital topside‐sounder ionograms have been produced. The Topside Ionogram Scaler With True Height Algorithm (TOPIST) program was developed for the automated scaling of the echo traces and for the inversion of these traces into topside electron density profiles. The program is based on the techniques that have been successfully applied in the analysis of ground‐based Digisonde ionograms. The TOPIST software also includes an “editing option” for manual scaling of the more difficult ionograms, which could not be scaled during the automated TOPIST run. TOPIST is now successfully scaling ∼60% of the ISIS ionograms, and the electron density profiles are available through the online archive of the National Space Science Data Center at ftp://nssdcftp.gsfc.nasa.gov/spacecraft_data/isis/topside_sounder. This data restoration effort is producing a unique global database of topside electron densities over more than one solar cycle, which will be of particular importance for improvements of topside ionosphere models, especially the International Reference Ionosphere.

Recent advances in topside profile modeling

A parameterized model for topside profile was developed based on the Epstein function approximation. Using the Intercosmos-19 database, model parameters were obtained for different geophysical conditions, including strong-magnetic storms. In some specific conditions the F3 layer was observed on topside ionograms. A physical explanation is proposed as well as results based on modeling approach. Topside ionograms from the sounder on MIR Space Station were studied. Some exotic cases are presented including oblique propagation, station position under the peak height etc. Peak height global distribution is described as well as neutral wind parameters derived from the topside peak height values.

Modelling bottom and topside electron density and TEC with profile data from topside ionograms

Abstract The paper describes the technique that has been implemented to model the electron density distribution above and below the F2 peak making use of only the profiles obtained from the INTERCOSMOS-19 topside ionograms. Each single profile from the satellite height to the ionosphere peak has been fitted by a semi-Epstein layer function of the type used in the DGR model with shape factor variable with altitude. The topside above the satellite height has been extrapolated to match given values of plasmaspheric electron densities to obtain the full topside profile. The bottomside electron density has been calculated by using the maximum electron density and its altitude estimated from the topside ionogram as input for a modified version of the DGR derived profiler that uses model values for the foF1 and foE layers of the ionosphere. Total electron content has also been calculated. Longitudinal cross sections of vertical profiles from latitudes 50° N to 50° S latitude are shown for low and high geomagnetic activity. These cross sections indicate the equatorial anomaly effect and the changes of the shape of low latitude topside ionosphere during geomagnetic active periods. These results and the potentiality of the technique are discussed.

MIR Space Station Topside Sounding near Taiwan

First results from ionospheric topside sounding data received at Taiwan National Central University are reported. The sounding was accomplished from the topside sounder on the Russian MIR. The topside ionograms and calculated vertical profiles of electron concentration were compared with corresponding bottomside ionograms and profiles. The total composed vertical profiles were obtained. A very good correspondence between the topside and bottomside ionospheric data, both for critical frequency and peak height was achieved and a total composite vertical profile was thus obtained. The scaled local plasma (ordinary cutoff) and critical frequencies, as well as topside profiles along the station pass, permit the position and shape of the northern crest of the equatorial anomaly to be determined. The low orbit of MIR space station gives the unique possibility to check the peak height models of the ionosphere due to ability of in-situ and remote sounding measurements simultaneously, especially in periods when the station is inside the peak.

Radio sounding in space: magnetosphere and topside ionosphere

Modern sounding techniques have been developed for the space-borne exploration of Earth's magnetosphere and topside ionosphere. Two new satellite instruments will use the advanced techniques of the ground-based Digisondes. The Radio Plasma Imager (RPI), a low-frequency sounder with 500-m dipole antennas designed to sweep from 3 kHz to 3 MHz, will be part of NASA's IMAGE mission to be launched in February 2000 into an elliptical orbit with an altitude at apogee of 7 R e. While in the magnetospheric cavity, RPI will receive echoes from the magnetopause and the plasmasphere and will measure the direct response of the magnetosphere's configuration to changes in the solar wind. With three orthogonal dipole antennas (two 500-m tip-to-tip antennas in the spin plane used for transmission and reception, one 20-m antenna along the spin axis for reception only) the arrival angle of returning echoes can be determined with high accuracy. The other instrument is the TOPside Automated Sounder (TOPAS), which was originally conceived for the Ukrainian WARNING mission with a launch date in 2001. Using one antenna for transmission and three orthogonal 10-m antennas for reception, TOPAS will be able to determine the arrival angle of ionospheric echoes and their wave polarization. It will then be possible to automatically scale the topside ionograms and calculate the electron density profiles in real time. Operating as a high-frequency radar, TOPAS will for the first time measure topside plasma velocities by tracking the motions of plasma irregularities.

OEDIPUS‐C topside sounding of a structured auroral E region

The Observations of Electric‐field Distributions in the Ionospheric Plasma—A Unique Strategy C (OEDIPUS‐C) rocket payload was launched from Poker Flat, Alaska, into an evening aurora at 0638 UT, on November 7, 1995. The payload included a tethered HF transmitter‐receiver pair which acted as a topside sounder. The bistatic (two‐point) configuration allowed an in situ calibration of the radiated power. The conditions in the magnetosphere and ionosphere during the experiment were monitored by a ground‐based network of instruments and by instruments on the GOES 7 satellite in a geosynchronous orbit. In this paper we present results of the data analysis of topside ionograms that were obtained during the down‐leg flight of OEDIPUS‐C (OC). The relatively low altitudes through which OC carried out topside sounding make the resulting ionograms a novel data set. Ionospheric reflections of the 10‐W transmissions were detected at payload heights between 780 and 160 km on the down leg. Near apogee at 824 km, extremely low electron densities (∼100 cm−3) were observed. The monotonic rise in electron density at the payload from apogee to reentry clearly showed that there was no ionospheric F layer peak. The topside‐sounding echoes came from all heights between the payload and the E layer peak around 100 km altitude. Strong X‐mode ionospheric reflections plus strong O‐mode ground reflections were observed. OC thus has provided a close‐hand view of a thick, highly structured, auroral E layer sounded at small ranges. The RF signal was efficiently guided along the magnetic field aligned density depletions that were located at the equatorward edges of auroral arcs. Large pulse‐to‐pulse variations in the amplitude of the ionospheric reflection are not explained by ducting in the geometric‐optics sense.

Topside sounder proton‐cyclotron echo generation from a plasma memory process and electron Bernstein‐wave propagation

Evidence is presented for the radiation of an electron Bernstein wave from a radio frequency pulse simulated by proton‐cyclotron motion. The protons are energized by an ionospheric topside sounder transmitter pulse and take part in a plasma memory process that simulates the original transmitter pulse. These echoes appear on topside ionograms as thin traces. At frequencies below the electron cyclotron frequency fH, the proton echo delay is a multiple of the proton‐cyclotron period tp. At frequencies above fH, there can be an additional delay to tp depending on two factors: 1) the antenna orientation and 2) the closeness of the proton echo frequency f to fH. A model is presented here in which protons passing within a few tens of centimeters of the antenna are energized and modulated by the RF pulse by an amount depending on the RF phase. At multiples of tp later, these protons return to almost the same field line they were on at the time of the RF pulse, although they may have moved some tens of meters parallel or antiparallel to the field line. At multiples of tp, ntp, the protons are thus located in a plane defined by the linear dipole antenna (73 m long tip to tip) at the time of the pulse and the direction of the Earth's magnetic field B. At ntp the protons produce a field which simulates the original field of the RF pulse, and, if wave propagation in the plasma is possible, a pulse is produced which propagates to the new position of the antenna which has moved in the plasma at the satellite velocity Vs. For f > fH, the perpendicularly propagating electron Bernstein waves are responsible for the propagation. No such Bernstein waves exist for f < fH, and proton echoes can be observed at these frequencies if Vs is approximately in the plane of the energized protons at ntp so that wave propagation is not necessary to produce a signal that appears as an echo. For the second factor above, as the wave frequency approaches the electron cyclotron frequency, the group velocity of the Bernstein waves approaches zero, and consequently the propagation time to the antenna increases. This change in group velocity explains the observed increase in tp, which can be as much as several percent.

Response of topside radio sounding signals to small-scale field-aligned ionospheric irregularities

Abstract Small-scale ionospheric irregularities produce numerous observed effects during high-frequency radio wave sounding from a satellite. Scattering of z-mode waves radiated by an ionosonde, which results in the occurrence of characteristic traces on ionograms, are among the most complex of them. In the present paper the factors determining the power characteristics of such signals are considered. These are the size and shape of the surface of orthogonality for a family of z-mode waves radiated from a satellite, features of the radiating and propagating radio waves in a non-uniform magnetized plasma, and the details of the scattering process. Simple but adequate physical models for each of these factors are presented and a numerical calculation algorithm of the power of the z-mode scattered signals received by the satellite sounder receiver has been constructed. Values for this calculated quantity are presented as a function of time delay, sounding frequency, and magnetic latitude. A synthesis of topside ionograms showing z-mode scattered traces has been performed. The results support new diagnostic methods of detecting small-scale ionospheric irregularities using topside-sounder z-mode signal returns and the concept that the sounder radiation can, under certain conditions, stimulate artificial plasma turbulence. In addition, it was shown that z-mode scattering may contribute to the resonance observed at the plasma frequency on topside ionograms.

Seismoionospheric fountain-effect as analogue of active space experiment

Space investigation includes modelling and initiation of natural processes in ionospheric-magnetospheric plasma by active experimental methods with artificial influence as a specific part. For the first time we present here the possibility of triggering the unique equatorial ionosphere phenomenon known as fountain-effect by lithospheric processes before an earthquake. Anomalous influence of earthquake preparation processes with epicenter close to magnetic equator on latitudinal distribution of f oF - F-region penetration frequencies and spread-F is discussed and considered by us as an analogue of an active experiment. Using Alouette-1 data it is shown that approximately one day before the earthquake f oF2 dependence on magnetic latitude looks like a double-crest curve with minimum close to epicenter. Simul taneously oblique and spread traces appeared at topside ionograms. The effect is evident during evening hours. Such kind of f oF2-dependence is unusual and is not observed during the preceding days, nor some days later. So, a possible reason for the observations might be connected with generation of an anomalous electric field near the epicenter during earthquake preparation stage, which initiates a phenomenon similar to the natural fountain-effect.

Tunneling modes in ionospheric waveguides

The importance of tunneling rays in ionospheric structures is examined by considering VHF transequatorial propagation in field‐aligned equatorial plasma bubbles. It is found that tunneling rays can cause a significant decrease in the amount of VHF power capable of being guided by bubbles, therefore indicating that allowance should be made for tunneling modes when modeling this sort of propagation. While the power loss associated with tunneling rays decreases with a decrease of frequency, it increases with a decrease in the radius of curvature of a guide's cross section. Consequently, the effect of these modes on HF ducting within guides of small cross section, similar to those observed on topside ionograms, is also investigated. Results from this analysis also indicate a substantial amount of power loss due to tunneling modes, which must be accounted for in any detailed analysis of experimental HF ducting results.

The global distribution of ionospheric small-scale irregularities from topside sounding data

This paper examines the global distribution of electron density irregularities with scales of the order of several tens to hundreds of meters in the ionosphere by using topside sounder data from the COSMOS-1809 satellite obtained in May–June and December 1987. The diffuse traces of Z-waves on topside ionograms in a frequency band just below the upper hybrid resonance are used for diagnostics. These traces are attributed to the scattering of sounder-generated ordinary and slow extraordinary mode waves.

N(h) profile data at World Data Centers

Availability of electron density profiles obtained from ground-based and topside ionograms and incoherent scatter radar observations is surveyed. World Data Centers A, B2, and C1 are included in the survey.