Disturbed Ionospheric Conditions Research Articles

Observations of unusually broadened HF radar spectra from heater-induced artificial plasma irregularities

[1] Cooperative UK Twin-Located Auroral Sounding System (CUTLASS) HF backscatter targets may be artificially produced using the European Incoherent Scatter (EISCAT) HF ionospheric modification facility at Tromso, Norway. Plasma irregularities created in this way are known to be highly field aligned and usually give rise to radar echoes possessing spectral widths that are much lower than those seen in natural radar auroral data. In this study we present CUTLASS Finland and Iceland observations of heater-induced irregularities; these irregularities gave rise to enhanced spectral width in the Iceland radar backscatter signal, while the width observed by the Finland radar, pointing almost orthogonal to the Iceland beams, remained low. The observations coincided with a period of disturbed ionospheric conditions when faster-flowing natural irregularities were present, poleward of the heated volume, as well as a flow gradient across the heated volume itself. A latitudinal variation of the Doppler velocity measurements was exhibited in both the Finland and Iceland radar observations. We suggest that this flow inhomogeneity, taken together with the geometry of the observing radar beams, would explain not only the broad spectrum of flow components across the heated volume detected by the Iceland radar but also the asymmetry between the spectral widths measured at Iceland and Finland.

Handling cycle slips in GPS data during ionospheric plasma bubble events

[1] During disturbed ionospheric conditions such as the occurrence of plasma bubbles, the phase and amplitude of the electromagnetic waves transmitted by GPS satellites undergo rapid fluctuations called scintillation. When this phenomenon is observed, GPS receivers are more prone to signal tracking interruptions, which prevent continuous measurement of the total electron content (TEC) between a satellite and the receiver. In order to improve TEC monitoring, a study was conducted with the goal of reducing the effects of signal tracking interruptions by correcting for “cycle slips,” an integer number of carrier wavelengths not measured by the receiver during a loss of signal lock. In this paper, we review existing cycle-slip correction methods, showing that the characteristics associated with ionospheric plasma bubbles (rapid ionospheric delay fluctuations, data gaps, increased noise, etc.) prevent reliable correction of cycle slips. Then, a reformulation of the “geometry-free” model conventionally used for ionospheric studies with GPS is presented. Geometric information is used to obtain single-frequency estimates of TEC variations during momentary L2 signal interruptions, which also provides instantaneous cycle-slip correction capabilities. The performance of this approach is assessed using data collected on Okinawa Island in Japan during a plasma bubble event that occurred on 23 March 2004. While an improvement in the continuity of TEC time series is obtained, we question the reliability of any cycle-slip correction technique when discontinuities on both GPS legacy frequencies occur simultaneously for more than a few seconds.

Within-the-hour variability: levels and their probabilities

The study of foF2 data measured every 5-min and of TEC measurements made every 10-min shows that the within-the-hour variability is different in the two parameters. Deciles of this variability for foF2 and for TEC are determined together with the probabilities of exceeding a given level of variability. Furthermore, considering hourly values, it is found that the variability in TEC is like an «intrinsic noise» throughout the day of the order of less than 5% of the hourly value; but at sunrise and often at sunset large values take place. A seasonal dependence is evident. Besides, a within-the-hour variability in foF2 is always present with large values at sunrise or sunset depending on the season, and also during disturbed ionospheric conditions.

Near Earth space plasma monitoring under COST 296

This review paper presents the main achievements of the near Earth space plasma monitoring under COST 296 Action. The outputs of the COST 296 community making data, historical and real-time, standardized and avail- able to the ionospheric community for their research, applications and modeling purposes are presented. The con- tribution of COST 296 with the added value of the validated data made possible a trusted ionospheric monitoring for research and modeling purposes, and it served for testing and improving the algorithms producing real-time data and providing data users measurement uncertainties. These value added data also served for calibration and validation of space-borne sensors. New techniques and parameters have been developed for monitoring the near Earth space plasma, as time dependent 2D maps of vertical total electron content (vTEC), other key ionospheric parameters and activity indices for distinguishing disturbed ionospheric conditions, as well as a technique for im- proving the discrepancies of different mapping services. The dissemination of the above products has been devel- oped by COST 296 participants throughout the websites making them available on-line for real-time applications.

A local ionospheric model for forecasting the critical frequency of the F2 layer during disturbed geomagnetic and ionospheric conditions

Abstract. An ionospheric forecasting empirical local model over Rome (IFELMOR) has been developed to predict the state of the critical frequency of the F2 layer (foF2) during geomagnetic storms and disturbed ionospheric conditions. Hourly measurements of foF2 obtained at the Rome observatory, hourly quiet-time values of foF2 (foF2QT), and the hourly time-weighted accumulation series derived from the geomagnetic planetary index ap (ap(τ)), were considered during the period January 1976–December 2003. Under the assumption that the ionospheric disturbance index log(foF2/foF2QT) is correlated to the integrated geomagnetic index ap(τ), statistically significant regression coefficients are obtained for different months and for different ranges of ap(τ) and used as input to calculate the short-term ionospheric forecasting of foF2. The empirical storm-time ionospheric correction model (STORM) was used to make comparisons with the IFELMOR model. A few comparisons between STORM's performance, IFELMOR's performance, the median measurements and the foF2QT values, were made for significant geomagnetic storm events (ap>150) occurring from 2000 to 2003. The results provided by IFELMOR are satisfactory, in particular, for periods characterized by high geomagnetic activity and very disturbed ionospheric conditions.

Storm time changes in total electron content in ionosphere measured by low orbiting topside sounder

Abstract. A new experimental technique is presented for the determination of the total electron content (TEC) below a low-orbiting satellite. According to this technique TEC can be obtained using the segment of a topside ionogram that only contains the traces of signals reflected from the Earth's surface. Possibilities of this technique were demonstrated using MIR station topside sounding data at the night time for both quiet and disturbed ionospheric conditions, and in particular, during the 14 November 1998 storm. An interesting fact was revealed with the help of this technique: after a series of relatively strong storms the main ionospheric trough on 14 November 1998 was detected at an abnormally low geomagnetic latitude (~43°). During this study some spatial variations of TEC were registered that can be interpreted as a TID-type wave structure.

Geomagnetically correlated autoregression model forshort-term prediction of ionospheric parameters

A new method for short-term prediction of ionospheric parametersis developed by incorporating the cross-correlation between theionospheric characteristic of interest and the Ap index intothe autocorrelation analysis. We consider the hourly timeseries of an ionospheric characteristic as composed of aperiodic component and a random component. The periodiccomponent containing the average diurnal variation is removed by using its relative deviations from the median values (Φ), which in the case of the critical frequency of the F2layer, foF2, has the form: Φ = (foF2 - foF2med)/foF2med. The geomagnetically correlated autoregression model (GCAM) is an extrapolation model based on theweighted past data. The new term in the regression equation expresses linearly the dependence of Φ on magnetic activity byintroducing a synthetic geomagnetic index G, which approximatesthe average dependence of Φ on hourly interpolated Kp.Using parametric expressions of the auto- and cross-correlationfunctions ensures the statistical sufficiency in GCAM; theparameters are then obtained by data fitting. Data from 2years of high solar activity (1981-2) and 2 years of lowsolar activity (1985-6) were used to evaluate the predictionaccuracy of GCAM. The mean square error in per cent of the1-day prediction of foF2 relative to the median shows a largegain of accuracy of GCAM in the first 8-10 h of prediction relative to the median based prediction, a diurnal variation oferrors and a steady offset of the GCAM prediction error fromthe median based prediction error. The GCAM error at the firsthour is lowest, but gradually approaches the median error witha timescale of 8-10 h. A new error estimate, called`prediction efficiency' that is a good indicator of prediction performance during disturbed ionospheric conditions is defined.

Performance of HF modems on high‐latitude paths using multiple frequencies

The high‐latitude HF channel has been measured and characterized in terms of Doppler spread, delay spread, and signal‐to‐noise ratio. The performance of data modems has been determined over a comprehensive range of simulated channel conditions. A comparison is made between the channel measurements and the modem characterizations. Thereby the percentage of time that the modem works satisfactorily (availability), during the measured channel conditions, is determined. Two 75 bits/s modems (compliant with STANAG 4415 and STANAG 4285), a 2400 bits/s modem (compliant with STANAG 4285), Morse code, and voice have been selected for analysis. The two 75 bits/s modems show 60–75% higher availability than the 2400 bits/s modem on the measured paths. The 2400 bits/s modem is degraded in performance during a geomagnetic disturbance whereas the 75 bits/s modems maintain approximately the same performance level. The paper also addresses the necessary frequency pool (at a specific time) required for an HF circuit to achieve the best possible communications availability. For the 75 bits/s modems, independent of path length, a one‐frequency pool is all that is required during summer and winter when the ionospheric conditions are quiet. A four frequency pool is necessary during disturbed ionospheric conditions. For the 2400 bits/s waveform, three to four frequencies are needed on the 200 km west‐east path, and four to six frequencies are required on the north‐south 2000 km path. This applies to all seasons and geomagnetic conditions analyzed.

Enhancing differential GPS using regional ionospheric error models

Distance-dependent errors due to ionospheric refraction complicate ambiguity resolution and limit the accuracy attainable in GPS baseline determination. This paper presents an approach for modelling these errors from the observations of several permanent GPS-stations surrounding the area of interest. Regional ionospheric correction models are produced epoch-by-epoch and satellite-by-satellite. It is shown that after their application, observation residuals, fractional parts of estimated ambiguities, and single-frequency coordinate errors are greatly reduced, thus improving ambiguity resolution even under disturbed ionospheric conditions, with short observation sessions, in the case of baseline lengths of more than 10 km, and with singlefrequency and dual-frequency data.

A long wave transverse electric‐transverse magnetic noise prediction model

This paper describes a computerized physical model that predicts both horizontally and vertically polarized noise in the ELF to LF band (10 Hz to 60 kHz). Since naturally occurring radio noise in this band is produced by lightning and propagates to the receiver via the Earth‐ionosphere waveguide, the model starts with average lightning flash density data from which it calculates radiated power for horizontal and vertical noise. Adjustments are made to the radiated power to account for seasonal and latitudinal differences in the lightning processes. The noise power is then integrated over fairly large geographic areas to formulate horizontal and vertical equivalent noise transmitters. The power radiated from each of these transmitters is propagated to the receiver location using standard anisotropic long wave propagation algorithms and well‐known models of the Earth‐ionosphere waveguide. From the received power the model predicts RMS noise, standard deviation, voltage deviation VD, and the amplitude probability distribution of the noise for both polarizations. Since the model is based on theory, it can also predict these parameters under disturbed ionospheric conditions. The model's generally good agreement with RMS noise data is demonstrated.

Spectral broadening of VLF transmitter signals and sideband structure observed on Aureol 3 satellite at middle latitudes

Electric and magnetic field wave data acquired on Aureol 3 satellite demonstrate the existence of a spectral broadening effect in which VLF transmitter signals from Alpha station (geographic coordinates, 50.5°N, 137°E) in USSR undergo a significant spectral broadening on electric fields as they propagate through the ionosphere up to the spacecraft in the altitude range of 500–2000 km at middle latitudes (L ∼ 2). The spectral broadening phenomena may be divided into two types: (1) spectrally broadened components occurring without any association with ELF/VLF emissions under disturbed ionospheric conditions and (2) spectrally broadened components with predominant sideband structure in association with ELF emissions. Bicoherence computation results suggest a nonlinear mode coupling between the transmitter signal and ELF emission which produces sidebands that are quasi‐electrostatic in nature. However, faint spectral broadened components in both types 1 and 2 may be connected with Doppler shift of quasi‐electrostatic whistler mode waves with a broad spectrum of k near the resonance cone, due to scattering of the transmitter signals from ionospheric irregularities in the F region.

Ionospheric refraction in radio source observations at long radio wavelengths

Ionospheric refraction effects encountered in radio source observations in the 30 to 75 MHz range with the Clark Lake TPT telescope are discussed. It is found that simple calibration procedures are sufficient to provide positions of unknown sources with an accuracy of approximately one arcmin. Observations made near sunrise, or during disturbed ionospheric conditions must be discarded. If no corrections are applied, RMS errors of a few arcmin are to be expected.

The accuracy and resolution of model ionospheres derived from VLF propagation parameters

An inversion technique is outlined for calculating the resolution and gross error associated with D-region electron density profiles obtained by modelling from a given set of VLF radiowave propagation parameters. Two profiles, representing quiet and disturbed ionospheric conditions, are examined in detail and the height resolution and accuracy obtainable from the amount of observational data used in constructing these profiles is determined.

A Laboratory Model for VLF Propagation Experiments

A laboratory model of a section of the earth‐ionosphere has been designed and constructed to provide a means for systematically investigating the effect of ionospheric perturbations on long range VLF paths. This model provides a capability for the investigation of a wide variety of different disturbed ionospheric conditions. For any given configuration, all the pertinent parameters are clearly defined and repeatable. This is in contradistinction to naturally occurring events, for which many parameters that are important for a clear interpretation are either unknown or, at best, poorly known. The design rationale and the construction methods are presented in this paper with initial experimental data obtained from the model for uniform propagation paths. It is shown that these data are in good agreement with theoretical data obtained for the modeled conditions. Data are also presented for propagation paths through a nuclear type ionospheric perturbation. These data exhibit both amplitude loss and phase advance caused by the perturbation.

Vector measurement of the midlatitudeSqionospheric current system

This paper reports the results of two flights of vector magnetometer systems designed to measure the magnitude and direction of the ionospheric current system responsible for the solar quiet daily geomagnetic variation. The measurements were obtained from cesium vapor magnetometers with bias coils flown aboard sounding rockets from Wallops Island, Virginia. Instrumental difficulties on both flights limited the resolution of measurement, but the data indicated that current sheets were penetrated on both flights. These current sheets were observed to be thinner and more intense than those measured by previous investigators, and were confined to the lower edge of the altitude range over which currents have usually been observed. The unusual altitude profiles on both flights and an anomalously large current magnitude with a fluctuating direction observed on the second flight are attributed to localized enhancement of ionospheric electron concentration in sporadic E layers observed in ionograms taken during the flights. Because of disturbed ionospheric conditions and unknown boundary effects due to horizontal localization of the sporadic E layer, it is difficult to evaluate the validity of theoretical conductivity models. However, calculations of the conductivity profile required by the current position and intensity indicate that the observed current structure requires electron concentration enhancement compatible with that experimentally observed in sporadic E layers. Previous theoretical work also predicts a localized enhancement of the Sq current confined to the horizontal extent of the sporadic E layer, although such a large enhancement as was observed is not easily explainable.

VLF amplitude perturbations in the Antarctic due to the nuclear explosion of July 9, 1962

Broadband VLF magnetic tape recordings were obtained at Byrd, South Pole, and Cape Hallet in the Antarctic and onboard the U.S.N.S. ‘Eltanin’ near Valparaiso, Chile, during the Starfish high-altitude nuclear event of July 9, 1962. Amplitudes of selected VLF transmitter signals are presented, confirming the existence of disturbed ionospheric conditions over a wide geographical area. Comparison of signal perturbations on various pairs of paths from common transmitters provides evidence of disturbed conditions at L values greater than 6.

Ionospheric ion‐molecule reaction rates

The current status of laboratory measurements of ion‐molecule reactions of ionospheric relevance is reviewed. Rate constants for the atmospheric loss reactions of He+, O+, and N2+ appear to be established to within about a factor of 2 at 300°K. The laboratory rate constants for O+ and N2+ ion loss are compatible with quiet midlatitude E‐ and F‐region requirements. The He+ loss reactions appear to be higher than is compatible with some He+ concentrations observed; however, the laboratory rate constants have been measured by so many investigators that they are unlikely to be seriously in error. In the D region, on the other hand, many of the important reactions are as yet unmeasured. Measurements of the relevant negative ion reactions with neutrals have only recently been started. No in situ atmospheric negative ion composition studies have as yet been carried out to guide the laboratory investigations. Many positive ion reactions of importance in the D region (as well as in sporadic E layers) remain to be measured. Finally, very little information concerning reactions of excited state ions with neutrals has been obtained. This will be important for certain disturbed ionospheric conditions and possibly for a precise analysis of normal ionospheric conditions, when this is merited.

Derivation of electron-density profiles in the lower ionosphere using radio absorption measurements at multiple frequencies

A new method has been developed permitting the derivation, during disturbed ionospheric conditions, of the electron-density profiles in the lowermost ionosphere. On the basis of simultaneous measurement of ionospheric absorption on several frequencies, and an assumed model of the variation of electron collision frequency with height, the Sen-Wyller formulation of the magnetoionic theory is used to compute the profile best capable of producing the observed absorption values. The method is described, and some of its limitations are discussed. Its use is illustrated by means of representative electron-density profiles derived for the polar-cap absorption events of July 1961. These profiles show that the base of the layer varied in height from 30 to 50 km, and that the region of maximum absorption, for a frequency of 30 Mc/s, occurred systematically at about 60 km.

Upper Atmosphere Temperature Derived from Charged Particle Observations

Abstract Charged Particle temperature results obtained by three different methods (rocket and satellite-borne Langmuir probes, rocket measurements of the altitude dependence of charged particle densities above the F2 peak, and radar incoherent backscatter) are compared with recent models of kinetic gas temperature. From the few sets of charged particle data available for conditions of a quiet sun at temperate latitudes, the comparison favors the concept of temperature equilibrium between electrons and heavy constituents in all but the lower F region of the ionosphere. Additional departures from equilibrium are indicated for disturbed ionospheric conditions and possibly for a quiet sun at auroral latitudes.