Variations Of Ionospheric Electron Density Research Articles

Reduction of electron density in the night-time lower ionosphere in response to a thunderstorm

Tropospheric thunderstorms have been reported to disturb the lower ionosphere, at altitudes of 65–90 km. The use of lightning signals from a distant mesoscale storm to probe the lower ionosphere above a small tropospheric thunderstorm reveals a reduction in ionospheric electron density in response to lightning discharges in the small storm. Tropospheric thunderstorms have been reported to disturb the lower ionosphere, at altitudes of 65–90 km, by convective atmospheric gravity waves1,2,3,4,5 and by electric field changes produced by lightning discharges6,7,8,9,10,11,12,13,14,15. Theoretical simulations suggest that lightning electric fields enhance electron attachment to O2 and reduce electron density in the lower ionosphere7,8. Owing to the low electron density in the lower ionosphere, active probing of its electron distribution is difficult16,17, and the various perturbative effects are poorly understood. However, it is now possible to probe the lower ionosphere in a spatially and temporally resolved manner by using remotely detected time waveforms of lightning radio signals4,5,18,19. Here we report such observations of the night-time ionosphere above a small thunderstorm. We find that electron density in the lower ionosphere decreased in response to lightning discharges. The extent of the reduction is closely related in time and space to the rate of lightning discharges, supporting the idea that the enhanced electron attachment is responsible for the reduction. We conclude that ionospheric electron density variations corresponding to lightning discharges should be considered in future simulations of the ionosphere and the initiation of sprite discharges.

High dynamic-range radio-interferometric images at 327 MHz

Radio astronomical imaging using aperture synthesis telescopes requires deconvolution of the point spread function as well as calibration of the instrumental characteristics (primary beam) and foreground (ionospheric/atmospheric) effects. These effects vary in time and also across the field of view, resulting in directionally-dependent (DD), time-varying gains. The primary beam will deviate from the theoretical estimate in real cases at levels that will limit the dynamic range of images if left uncorrected. Ionospheric electron density variations cause time and position variable refraction of sources. At low frequencies and sufficiently high dynamic range this will also defocus the images producing error patterns that vary with position and also with frequency due to the chromatic aberration of synthesis telescopes. Superposition of such residual sidelobes can lead to spurious spectral signals. Field-based ionospheric calibration as well as "peeling" calibration of strong sources leads to images with higher dynamic range and lower spurious signals but will be limited by sensitivity on the necessary short-time scales. The results are improved images although some artifacts remain.

Application of ionospheric tomography inversion to GPS data to study ionospheric electron-density distribution over China

Abstract: Temporal variations of ionospheric electron-density (IED) distributions over China are inversed by applying the computerized ionospheric tomography (CIT) technique to dual-frequency GPS data of the Crustal Movement Observation Network of China (CMONOC) in 2003. Characteristics of the diurnal variations of IED, especially the development and disappearance of the equatorial anomaly structure and the variations of the ionospheric vertical structure, are investigated. The result shows that it is feasible to sound and investigate the temporal-spatial variations of IED by using CIT technique and the high-accuracy GPS data, but the accuracy and reliability are limited by insufficiency of GPS data, reconstruction methods, and CIT models.

Time-dependent 3-D Computerized Ionospheric Tomography with Ground-Based GPS Network and Occultation Observations

This paper illuminates the principle and algorithm of time-dependent three-dimensional computerized ionospheric tomography (CIT) with ground-based GPS network and occultation observations as well as reconstruction results based on real measurements. A comparison of reconstruction results shows that the whole quality of reconstructed electron density image, specially the vertical resolution of electron density structure, is obviously improved by the combination of ground-based GPS beacon measurements with GPS occultation observations. Reconstructed results based on actual observations under conditions of both geomagnetic quiet and storm show that variations of ionospheric electron density with height, latitude, longitude and time can be resolved by means of CIT with ground-based GPS network and occultation observations. Ionospheric negative phase storm effects are revealed by the reconstruction results during the magnetic storm. The study of present paper confirms that time-dependent 3-D CIT with ground-based GPS network and occultation observations is a powerful tool to monitor large-scale ionospheric structures under disturbed conditions.

Flexible Prior Models: Three‐dimensional ionospheric tomography

A three‐dimensional ionospheric reconstruction system is presented that models ionospheric dynamics and accounts for well‐known limitations in the available data by using geometrically transformed prior models based on available models, such as the Parameterized Ionospheric Model (PIM) or the International Reference Ionosphere (IRI), to produce density solutions consistent with available Total Electron Content (TEC) measurement data. It is known that current state of the art prior density models, such as PIM or IRI, can only simulate a statistical mean ionosphere. As a result, sudden variations in ionospheric electron density will not be represented in these models. This paper focuses on a three‐dimensional ionospheric density reconstruction system that uses a set of geometrical transformations to produce Flexible Prior Models (FPM). The Flexible Prior Model process allows means to include flexibility in a prior model in the form of geometrical variations that are not well represented in current state‐of‐the‐art ionospheric prior density models. The updated priors are used as initial solution models for a set of (A, B) Multiplicative Algebraic Reconstruction Technique (ABMART) algorithms, that can easily incorporate additional information and provide a spatially constrained density solution, consistent with the available data and our current knowledge of ionospheric dynamics. The ultimate goal is to create a three‐dimensional adaptive ionospheric electron density reconstruction system that would make it possible to generate real‐time ionospheric maps supported by available TEC data. Such maps would be of significant utility in predicting and correcting the impact of electron density gradients and irregularities on radio waves.

On the 18-day quasi-periodic oscillation in the ionosphere

Abstract. The presence and persistence of an 18-day quasi-periodic oscillation in the ionospheric electron density variations were studied. The data of lower ionosphere (radio-wave absorption at equivalent frequency near 1 MHz), middle and upper ionosphere (critical frequencies f0E and f0F2) for the period 1970–1990 have been used in the analysis. Also, solar and geomagnetic activity data (the sunspot numbers Rz and solar radio flux F10.7 cm, and aN index respectively) were used to compare the time variations of the ionospheric with the solar and geomagnetic activity data. Periodogram, complex demodulation, auto- and cross-correlation analysis have been used. It was found that 18-day quasi-periodic oscillation exists and persists in the temporal variations of the ionospheric parameters under study with high level of correlation and mean period of 18–19 days. The time variation of the amplitude of the 18-day quasi-periodic oscillation in the ionosphere seems to be modulated by the long-term solar cycle variations. Such oscillations exist in some solar and geomagnetic parameters and in the planetary wave activity of the middle atmosphere. The high similarities in the amplitude modulation, long-term amplitude variation, period range between the oscillation of investigated parameters and the global activity of oscillation suggests a possible solar influence on the 18-day quasi-periodic oscillation in the ionosphere.

On the 18-day quasi-periodic oscillation in the ionosphere

The presence and persistence of an 18-day quasi-periodic oscillation in the ionospheric electron density variations were studied. The data of lower ionosphere (radio-wave absorption at equivalent frequency near 1 MHz), middle and upper ionosphere (critical frequencies f0E and f0F2) for the period 1970–1990 have been used in the analysis. Also, solar and geomagnetic activity data (the sunspot numbers Rz and solar radio flux F10.7 cm, and aN index respectively) were used to compare the time variations of the ionospheric with the solar and geomagnetic activity data. Periodogram, complex demodulation, auto- and cross-correlation analysis have been used. It was found that 18-day quasi-periodic oscillation exists and persists in the temporal variations of the ionospheric parameters under study with high level of correlation and mean period of 18–19 days. The time variation of the amplitude of the 18-day quasi-periodic oscillation in the ionosphere seems to be modulated by the long-term solar cycle variations. Such oscillations exist in some solar and geomagnetic parameters and in the planetary wave activity of the middle atmosphere. The high similarities in the amplitude modulation, long-term amplitude variation, period range between the oscillation of investigated parameters and the global activity of oscillation suggests a possible solar influence on the 18-day quasi-periodic oscillation in the ionosphere.

An improved phenomenological model of ionospheric density

A global representation of the large scale variations of ionospheric electron density with the annual, diurnal and solar activity cycles is described. The phenomenological model, requiring minimal computer storage space, is constructed from monthly-averaged hourly ionospheric sounding data from some 50 stations during the epoch 1957–1970, as provided by World Data Center (A). The model, describing noontime F2-layer critical frequencies to better than 2 MHz average maximum error and to better than 0.5 MHz root mean squared error, is particularly suitable for applications in space communication and ionosphere-atmosphere coupling studies. Magnetic dip angle and north-south asymmetry effects are incorporated in the model. We show that a ‘secular’ variation of the ionospheric density exists in the epoch 1960–1971. The basis of comparison with data is broadened to include the 1958–1968 chronologies of 18 stations in addition to the original samplings of 50 stations. In addition, a modelling of the polar ionospheres is included.

A phenomenological model of global ionospheric electron density in the E-, F1- and F2-regions

A global phenomenological model of the large scale variations of ionospheric electron density with the annual, diurnal and solar activity cycles has been constructed from monthly-averaged hourly ionospheric sounding data from some 50 stations spanning the years 1957–1970 as provided by World Data Center (A), Boulder, Colorado. The model, summarizing the data in simple empirical formulae and thus requiring little computer storage, is specifically designed for global thermospheric and ionospheric dynamical calculations such as ion drag, Joule heating and atmospheric dynamo effects which require electron or ion densities as essential inputs. Extensive comparison of the model with data and with other models is discussed.

Ionospheric structure near the dayside boundary of closed field lines

Variations of ionospheric electron density at altitudes of ≳600 km in the vicinity of the boundary of closed field lines are described. Ionospheric densities are determined from the Isis 1 swept frequency sounder data; the locations of the boundary are determined from latitude profiles of counting rates of electrons (E > 20 kev). Ionospheric densities at all altitudes decrease around the boundary and are low in the polar ionosphere, usually during geomagnetic storms. A trough in ionospheric density at altitudes of ≲1000 km, centered on the boundary field lines, is often observed and is always associated with a peak of density at altitudes of ≳1400 km centered on the same field lines. When there are no systematic changes below 1000 km near the boundary, the density above 1400 km either changes randomly or increases at and poleward of the boundary.