Theoretical studies of the mid-latitude ionosphere

Following a brief description of the atmosphere and ionosphere in Chapter I we describe how the equations of continuity and momentum for 0+, H+, He+, 0++ are derived from the formulations of St-Maurice and Schunk(1977) and Quegan et al.(1981) in Chapter II. In Chapter III we investigate the nature of the downward flow of protons in a collapsing post-sunset ionosphere. We derive an analytical form for the limiting temperature, we also note the importance of the polarization field term and concluded that the flow will remain subsonic for realistic conditions. The time-dependent behaviour of He+ under sunspot minimum conditions is investigated in Chapter IV. This is achieved by numerical solution of the 0+, H+ and,He+ continuity and momentum equations, treating He+ as a minor ion with 0+ , H+ as major ions. We found that He+ flows upwards during the day-time and downwards during the nighttime. He+ flux tube content reached a maximum on the 8th day of the integration period and started to decreasing. This is due to the large amount of H+ present at the late stages of the integration period which makes He+ unable to diffuse through the H+ layer away from the loss region. In Chapter V we investigate the behaviour of 0++ using sunspot maximum parameters. Although our results support the findings of Geis and Young (1981) that the large amounts of 0++ at the equator are caused mainly by thermal diffusion, the model used by Geis and Young overemphesizes the effect of thermal diffusion. The importance of 0++ - 0+ collision frequency is also noted. In Chapter VI we extend the work of Chapter IV, presenting a comparative study of H and He at sunspot minimum and sunspot maximum.In this last Chapter all three ions, O+ ,H+ and He+ , are treated theoretically as major ions and we concentrate mainly on light ion contents and fluxes. The results of this Chapter indicate that by assuming He+ as a minor ion we under-estimate He+ and over-estimate. H+. Some interesting features concerning the day to day behaviour of the light ion fluxes arise. In particular the day-time H+ fluxes decrease from day to day in contrast to the work of Murphy et al.(1976). In appendix.A we derive some analytical forms for the optical depth so that the models can include a realistic description of photoionization.

Determination of the hemispheric character (i.e. southern or northern latitudinal position) of equatorial ionosonde stations as deduced from long-term recordings of the minimum height of the F-region (h ' F)

The virtual (or minimum) height of the F-region (h'F), recorded over a number of solar cycles for I I equatorial and mid-latitude ionosonde stations, was used to deduce the hemispheric (i.e. southern or northern hemisphere) character of equatorial stations. The semi-annual median monthly height (h'F) variations consist of two components: major local summer maximum and winter sub-maximum (about 5 percent of the summer maximum). This hemispheric pattern was most consistently observed for equatorial stations (within 5degrees of the geomagnetic equator) in a period centred on the local midnight (21-03 LT) but was also present, to a lesser extent, at mid-latitude stations and at other time intervals. It is evident that the physical parameter h'F defines the hemispheric character of an equatorial station which has different (sometimes opposite) geographic and geomagnetic latitudes. There is a sharp transition in the latitudinal character of the stations on both sides of the equator leading to hypothesis that the equal maxima in h'F in December and June solstices are observed at a near-equator position labelled as ionosonde deduced equator (IDE). Such a signature was observed for an American equatorial (both geographic and geomagnetic) station Talara (Peru) which is an experimental support of the hypothesis. The IDE can be another useful parameter characterising the equatorial ionosphere. This finding reveals a new application of the standard ionosonde data in defining the geophysical character of equatorial stations, being an important contribution to space climatology. (C) 2002 Elsevier Science Ltd. All rights reserved.

In search of the ultimate ionosonde deduced equator

A further progress has been made in defining the ionosonde deduced equator (IDE) which characterises a latitudinal transition from the northern to southern hemisphere. It is now possible to define the global IDE location as the locus of the average position between geographic and geomagnetic equators. A more complete insight to the phenomenon of the third equator (i.e. after geographic and geomagnetic equators) was made possible due to availability of ionospheric height (h'F) data from three stations positioned close to the IDE in the American and the far-east sectors. The IDE ionospheric signature (or E-type signature), detected at these stations, consists of bi-annual h'F height increases. This signature however is not consistently observed during solar cycle and at times, particularly at sunspot minimum, a weak hemispheric signature is observed (i.e. the northern or southern hemisphere signature). In general, the height increase at the IDE are considerably smaller (by a factor of 4) than at other equatorial locations, indicating that the ionosphere at the IDE location becomes less disturbed. It is suggested that the equatorial longitudinal regions which can be associated with more consistent E-type signature are located in the central Pacific and at the east coast of America, close to the intersection points of the geographic and geomagnetic equators. (C) 2003 Elsevier Ltd. All rights reserved.

Исследование диффузионных процессов в ионосфере

30-40 წლების ჩეპმენის, ჰალბარტის, ფერაროს ნაშრომებიდან დაწყებული და ბოლო გამოკვლევებით დამთავრებული წარმოდგენილ მიმოხილვით ნაშრომში გაანალიზებული და განხილულია გადატანითი პროცესებისა და ელექტრონ-იონური გაზის სიმაღლის მიხედვით განაწილების ანალიზური კვლევების შედეგები ქვედა და ზედა შუაგანედურ იონოსფეროში. დღეისთვის ძნელია რომელიმე ნაშრომის დასახელება, სადაც გაანალიზებულია იონოსფეროს F-ფენის ყველა ძირითადი თვისება, დინამიკური რეჟიმი და სხვა მოვლენა, რომელიც იქ მიმდინარეობს. ნაშრომში შედარებით სრულადაა წარმოდგენილი სხვადასხვა ქვეყნის თეორეტიკოსის წვლილი იონოსფერული პლაზმის F-ფენის ამბიპოლარულ დიფუზიაში. ავტორების მიერ მიღებული ძირითადი თეორიული შედეგი დადასტურებულია ბოლო წლებში ჩატარებული ექსპერიმენტული გამოკვლევებით.

«Геомагнитная аномалия» широтного распределения электронной концентрации F2 слоя ионосферы

მსოფლიო ქსელის იონოსფერული სადგურების კრიტიკული სიხშირეების - fmin f0E, f0El, f0F2, დედამიწის ეკვატორთან ახლომდებარე გეომაგნიტური ობსერვატორიების (რომლებიც მოიცავენ ჩრდილოეთისა და სამხრეთის განედებს j≈±300-ს) დედამიწის მაგნიტური ველის X მდგენელისა და დედამიწის ახლომდებარე კოსმოსურ სივრცეში განხორციელებული ექსპერიმენტების კვლევის შედეგები, ერთობრივი კომპლექსური ანალიზი.

К вопросу затухания медленных мгд волн нижней ионосферы

გამოკვლეულია იონოსფეროს E-ფენის სუსტად იონიზირებულ პლაზმაში გავრცელებული მცირე ტალღური შეშფოთებების შესაბამისი დისპერსიული თანაფარდობა. გათვალისწინებულია ჩვეულებრივი და მაგნიტური სიბლანტის დისიპაციური ეფექტი, რაც იწვევს ალვენის განივი ტალღის ტიპის ნელი მჰდ ტალღების სიხშირული სპექტრის ცვლილებას. იონოსფეროს მოცემული არის მჰდ პარამეტრების ცვლილების მახასიათებელი ინტერვალებისათვის დადგენილია ტალღური რიცხვის ის სიდიდე გეომაგნიტური ველის ვერტიკალური კომპონენტის მიმართულებით, რომლის ზემოთაც მოხდება დროში შეშფოთებების მაქსიმალური მილევა.

Diseño e implementación de un receptor multifrecuencia basado en muestreo paso banda aplicado al sistema Galileo

Este trabajo tiene como objetivo diseñar e implementar un receptor multifrecuencia requerido para aplicaciones Galileo centradas a realizar correcciones de errores y estudios de la ionosfera. Estas características obligan a buscar alternativas respecto los receptores superheterodinos convencionales dado que para éstos los retardos de propagación entre las diferentes bandas de interés son inaceptables. Por ello, se presenta un receptor basado en la técnica de muestreo paso banda, que permite trasladar el espectro mediante el conversor ADC a través de un aliasing intencionado, eliminando así los retardos de propagación entre bandas de interés, dado que todas se albergan en un mismo canal. En este trabajo nos hemos centrado únicamente en las etapas críticas del receptor presentado, siendo éstas la etapa de filtrado y conversión digital. La etapa de filtrado requerirá filtros muy selectivos, ya que el ruido existente fuera de banda se solapará a nuestra banda de interés, degradando la SNR del sistema a medida que tenga más potencia. Esta etapa se ha realizado mediante una estructura duplexora conjuntamente con dos filtros de líneas acopladas. La etapa de conversión se ha realizado fabricando el layout de un conversor comercial, del cual se ha validado el correcto funcionamiento para la aplicación requerida.

Ionospheric tomography using GNSS reflections

In this paper, we report a preliminary analysis of the impact of Global Navigation Satellite System Reflections (GNSS-R) data on ionospheric monitoring over the oceans. The focus will be on a single polar Low Earth Orbiter (LEO) mission exploiting GNSS-R as well as Navigation (GNSS-N) and Occultation (GNSS-O) total electron content (TEC) measurements. In order to assess impact of the data, we have simulated GNSS-R/O/N TEC data as would be measured from the LEO and from International Geodesic Service (IGS) ground stations, with an electron density (ED) field generated using a climatic ionospheric model. We have also developed a new tomographic approach inspired by the physics of the hydrogen atom and used it to effectively retrieve the ED field from the simulated TEC data near the orbital plane. The tomographic inversion results demonstrate the significant impact of GNSS-R: three-dimensional ionospheric ED fields are retrieved over the oceans quite accurately, even as, in the spirit of this initial study, the simulation and inversion approaches avoided intensive computation and sophisticated algorithmic elements (such as spatio-temporal smoothing). We conclude that GNSS-R data over the oceans can contribute significantly to a Global/GNSS Ionospheric Observation System (GIOS). Index Terms Global Navigation Satellite System (GNSS), Global Navigation Satellite System Reflections (GNSS-R), ionosphere, Low Earth Orbiter (LEO), tomography.

Inverting ionospheric D-region electron density pro-file from riometer measurement

The aim of this work is to invert the ionospheric electron density profile from Riometer (Relative Ionospheric opacity meter) measurement. The newly Riometer instrument KAIRA (Kilpisjärvi Atmospheric Imaging Receiver Array) is used to measure the cosmic HF radio noise absorption that taking place in the D-region ionosphere between 50 to 90 km. In order to invert the electron density profile synthetic data is used to feed the unknown parameter Neq using spline height method, which works by taking electron density profile at different altitude. Moreover, smoothing prior method also used to sample from the posterior distribution by truncating the prior covariance matrix. The smoothing profile approach makes the problem easier to find the posterior using MCMC (Markov Chain Monte Carlo) method.

Propagation of Microwaves in the Troposphere with Potential Application to GPS based Navigation and Meteorology with emphasis on Indian region

Global Positioning System (GPS), with its high integrity, continuous availability and reliability, revolutionized the navigation system based on radio ranging. With four or more GPS satellites in view, a GPS receiver can find its location anywhere over the globe with accuracy of few meters. High accuracy - within centimeters, or even millimeters is achievable by correcting the GPS signal with external augmentation system. The use of satellite for critical application like navigation has become a reality through the development of these augmentation systems (like W AAS, SDCM, and EGNOS, etc.) with a primary objective of providing essential integrity information needed for navigation service in their respective regions. Apart from these, many countries have initiated developing space-based regional augmentation systems like GAGAN and IRNSS of India, MSAS and QZSS of Japan, COMPASS of China, etc. In future, these regional systems will operate simultaneously and emerge as a Global Navigation Satellite System or GNSS to support a broad range of activities in the global navigation sector.Among different types of error sources in the GPS precise positioning, the propagation delay due to the atmospheric refraction is a limiting factor on the achievable accuracy using this system. The WADGPS, aimed for accurate positioning over a large area though broadcasts different errors involved in GPS ranging including ionosphere and troposphere errors, due to the large temporal and spatial variations in different atmospheric parameters especially in lower atmosphere (troposphere), the use of these broadcasted tropospheric corrections are not sufficiently accurate. This necessitated the estimation of tropospheric error based on realistic values of tropospheric refractivity. Presently available methodologies for the estimation of tropospheric delay are mostly based on the atmospheric data and GPS measurements from the mid-latitude regions, where the atmospheric conditions are significantly different from that over the tropics. No such attempts were made over the tropics. In a practical approach when the measured atmospheric parameters are not available analytical models evolved using data from mid-latitudes for this purpose alone can be used. The major drawback of these existing models is that it neglects the seasonal variation of the atmospheric parameters at stations near the equator. At tropics the model underestimates the delay in quite a few occasions. In this context, the present study is afirst and major step towards the development of models for tropospheric delay over the Indian region which is a prime requisite for future space based navigation program (GAGAN and IRNSS). Apart from the models based on the measured surface parameters, a region specific model which does not require any measured atmospheric parameter as input, but depends on latitude and day of the year was developed for the tropical region with emphasis on Indian sector.Large variability of atmospheric water vapor content in short spatial and/or temporal scales makes its measurement rather involved and expensive. A local network of GPS receivers is an effective tool for water vapor remote sensing over the land. This recently developed technique proves to be an effective tool for measuring PW. The potential of using GPS to estimate water vapor in the atmosphere at all-weather condition and with high temporal resolution is attempted. This will be useful for retrieving columnar water vapor from ground based GPS data. A good network of GPS could be a major source of water vapor information for Numerical Weather Prediction models and could act as surrogate to the data gap in microwave remote sensing for water vapor over land.

Drastic variation in the surface boundary layer parameters over Cochin during the annular solar eclipse: Analysis using sonic anemometer data

Atmospheric surface boundary layer parameters vary anomalously in response to the occurrence of annular solar eclipse on 15th January 2010 over Cochin. It was the longest annular solar eclipse occurred over South India with high intensity. As it occurred during the noon hours, it is considered to be much more significant because of its effects in all the regions of atmosphere including ionosphere. Since the insolation is the main driving factor responsible for the anomalous changes occurred in the surface layer due to annular solar eclipse, occurred on 15th January 2010, that played very important role in understanding dynamics of the atmosphere during the eclipse period because of its coincidence with the noon time. The Sonic anemometer is able to give data of zonal, meridional and vertical wind as well as the air temperature at a temporal resolution of 1 s. Different surface boundary layer parameters and turbulent fluxes were computed by the application of eddy correlation technique using the high resolution station data. The surface boundary layer parameters that are computed using the sonic anemometer data during the period are momentum flux, sensible heat flux, turbulent kinetic energy, frictional velocity (u*), variance of temperature, variances of u, v and w wind. In order to compare the results, a control run has been done using the data of previous day as well as next day. It is noted that over the specified time period of annular solar eclipse, all the above stated surface boundary layer parameters vary anomalously when compared with the control run. From the observations we could note that momentum flux was 0.1 Nm 2 instead of the mean value 0.2 Nm-2 when there was eclipse. Sensible heat flux anomalously decreases to 50 Nm 2 instead of the mean value 200 Nm 2 at the time of solar eclipse. The turbulent kinetic energy decreases to 0.2 m2s 2 from the mean value 1 m2s 2. The frictional velocity value decreases to 0.05 ms 1 instead of the mean value 0.2 ms 1. The present study aimed at understanding the dynamics of surface layer in response to the annular solar eclipse over a tropical coastal station, occurred during the noon hours. Key words: annular solar eclipse, surface boundary layer, sonic anemometer

An observing system simulation experiment for climate monitoring with GNSS radio occulation data: setup and testbed study

The long-term stability, high accuracy, all-weather capability, high vertical resolution, and global coverage of Global Navigation Satellite System (GNSS) radio occultation (RO) suggests it as a promising tool for global monitoring of atmospheric temperature change. With the aim to investigate and quantify how well a GNSS RO observing system is able to detect climate trends, we are currently performing an (climate) observing system simulation experiment over the 25-year period 2001 to 2025, which involves quasi-realistic modeling of the neutral atmosphere and the ionosphere. We carried out two climate simulations with the general circulation model MAECHAM5 (Middle Atmosphere European Centre/Hamburg Model Version 5) of the MPI-M Hamburg, covering the period 2001–2025: One control run with natural variability only and one run also including anthropogenic forcings due to greenhouse gases, sulfate aerosols, and tropospheric ozone. On the basis of this, we perform quasi-realistic simulations of RO observables for a small GNSS receiver constellation (six satellites), state-of-the-art data processing for atmospheric profiles retrieval, and a statistical analysis of temperature trends in both the “observed” climatology and the “true” climatology. Here we describe the setup of the experiment and results from a test bed study conducted to obtain a basic set of realistic estimates of observational errors (instrument- and retrieval processing-related errors) and sampling errors (due to spatial-temporal undersampling). The test bed results, obtained for a typical summer season and compared to the climatic 2001–2025 trends from the MAECHAM5 simulation including anthropogenic forcing, were found encouraging for performing the full 25-year experiment. They indicated that observational and sampling errors (both contributing about 0.2 K) are consistent with recent estimates of these errors from real RO data and that they should be sufficiently small for monitoring expected temperature trends in the global atmosphere over the next 10 to 20 years in most regions of the upper troposphere and lower stratosphere (UTLS). Inspection of the MAECHAM5 trends in different RO-accessible atmospheric parameters (microwave refractivity and pressure/geopotential height in addition to temperature) indicates complementary climate change sensitivity in different regions of the UTLS so that optimized climate monitoring shall combine information from all climatic key variables retrievable from GNSS RO data.

An overview of Earth's global electric circuit and atmospheric conductivity

The Earth’s global atmospheric electric circuit depends on the upper and lower atmospheric boundaries formed by the ionosphere and the planetary surface. Thunderstorms and electrified rain clouds drive a DC current (∼1 kA) around the circuit, with the current carried by molecular cluster ions; lightning phenomena drive the AC global circuit. The Earth’s near-surface conductivity ranges from 10−7 S m−1 (for poorly conducting rocks) to 10−2 S m−1 (for clay or wet limestone), with a mean value of 3.2 S m−1 for the ocean. Air conductivity inside a thundercloud, and in fair weather regions, depends on location (especially geomagnetic latitude), aerosol pollution and height, and varies from ∼10−14 S m−1 just above the surface to 10−7 S m−1 in the ionosphere at ∼80 km altitude. Ionospheric conductivity is a tensor quantity due to the geomagnetic field, and is determined by parameters such as electron density and electron–neutral particle collision frequency. In the current source regions, point discharge (coronal) currents play an important role below electrified clouds; the solar wind-magnetosphere dynamo and the unipolar dynamo due to the terrestrial rotating dipole moment also apply atmospheric potential differences. Detailed measurements made near the Earth’s surface show that Ohm’s law relates the vertical electric field and current density to air conductivity. Stratospheric balloon measurements launched from Antarctica confirm that the downward current density is ∼1 pA m−2 under fair weather conditions. Fortuitously, a Solar Energetic Particle (SEP) event arrived at Earth during one such balloon flight, changing the observed atmospheric conductivity and electric fields markedly. Recent modelling considers lightning discharge effects on the ionosphere���s electric potential (∼+250 kV with respect to the Earth’s surface) and hence on the fair weather potential gradient (typically ∼130 V m−1 close to the Earth’s surface. We conclude that cloud-to-ground (CG) lightning discharges make only a small contribution to the ionospheric potential, and that sprites (namely, upward lightning above energetic thunderstorms) only affect the global circuit in a miniscule way. We also investigate the effects of mesoscale convective systems on the global circuit.