Norbert Jakowski

The Potential of Low-Frequency SAR Systems for Mapping Ionospheric TEC Distributions

Ionospheric propagation effects have a significant impact on the signal properties of low-frequency synthetic aperture radar (SAR) systems. Range delay, interferometric phase bias, range defocusing, and Faraday rotation are the most prominent ones. All the effects are a function of the so-called total electron content (TEC). Methods based on two-frequency global positioning system observations allow measuring TEC in the ionosphere with coarse spatial resolution only. In this letter, the potential of broadband L-band SAR systems for ionospheric TEC mapping is studied. As a basis, the dispersive nature of the ionosphere and its effects on broadband microwave radiation are theoretically derived and analyzed. It is shown that phase advance and group delay can be measured by interferometric and correlation techniques, respectively. The achievable accuracy suffices in mapping small-scale ionospheric TEC disturbances. A differential TEC estimator that separates ionospheric from tropospheric contributions is proposed

A new method for reconstruction of the vertical electron density distribution in the upper ionosphere and plasmasphere

Ground-based ionosphere sounding measurements alone are incapable of reliably modeling the topside electron density distribution above the F layer peak density height. Such information can be derived from Global Positioning System (GPS)-based total electron content (TEC) measurements. A novel technique is presented for retrieving the electron density height profile from three types of measurements: ionosonde (foF2, foE, M3000F2, hmf2), TEC (GPS-based), and O+-H+ ion transition level. The method employs new formulae based on Chapman, sech-squared, and exponential ionosphere profilers to construct a system of equations, the solution of which system provides the unknown ion scale heights, sufficient to construct a unique electron density profile at the site of measurements. All formulae are based on the assumption of diffusive equilibrium with constant scale height for each ion species. The presented technique is most suitable for middle- and high-geomagnetic latitudes and possible applications include: development, evaluation, and improvement of theoretical and empirical ionospheric models, development of similar reconstruction methods utilizing low-earth-orbiting satellite measurements of TEC, operational reconstruction of the electron density on a real-time basis, etc.

Total electron content of the ionosphere during thegeomagnetic storm on 10 January 1997

Abstract Measurements at GPS ground stations of the International GPS Service (IGS) havebeen used to derive the total electron content (TEC) of the ionosphere over Europe and overthree North American stations for the 6–11 January 1997 storm event. The derived TEC dataindicate large deviations from the average behaviour especially at high latitudes on thenight-side/early morning longitude sector. The high-latitude perturbation causes a well-pronounced positive phase on the day-sidesector over Europe. Both meridional winds as well as transient electric fields are assumed to contribute to thesignature of the ionospheric perturbation propagating from high to low latitudes. Theobservations indicate a subsequent enhanced plasma loss which is probably due to theequatorward expansion of storm induced composition changes.

Geomagnetic Storm Effects on the Topside Ionosphere and Plasmasphere: A Compact Tutorial and New Results

The coupled ionosphere–thermosphere–plasmasphere system is very complex. The study of its interrelationships during geomagnetically disturbed conditions is an especially challenging task.Significant progress has been achieved during the last few years in developing comprehensive theoretical models to describe its global behaviour.Moreover, more simple, specialized numerical modelling of some specialaspects of storm behaviour and/or regional models have contributedto the progress in this field.This paper summarizes recent developments in upper ionosphereand plasmasphere storm studies and modelling.From an observational point of view the upper ionosphere/plasmasphereregion is well reflected in radio beacon measurements providing the totalelectron content (TEC). The development of space-based radio navigation systems such as GPS offersnew opportunities to derive TEC on both regional and global scale.Combining TEC with ionosonde data enables the variability of the shape of the electron density distribution during storms to be studied.We present some examples of co-ordinated investigation,made during the CEDAR storm study intervals.

GPS/GLONASS-based TEC measurements as a contributor for space weather forecast

Space weather monitoring and forecast require a permanent monitoring of the ionospheric state on global scale. The world-wide use of global navigation satellite systems such as GPS and GLONASS o7ers the unique chance for a permanent monitoring of the total ionization (total electron content—TEC) of the global ionosphere=plasmasphere up to about 20000 km height. In this study we turn our attention to TEC variations over the European area. Using the data of more than 15 GPS stations of the GPS tracking network of the International GPS Service (IGS), a horizontal resolution in the order of 500 km is achieved, the standard time resolution is 10 min. The total ionization of the ionosphere reacts very sensitive to solar radiation changes. As correlation studies with the solar radio =ux index F10.7 have shown, the ionospheric response over the European area is delayed by about 1–3 days depending on geophysical conditions. Consequently, the turn o7=on of the solar radiation during the solar eclipse on August 11, 1999 was seen as a signi@cant reduction of TEC following the obscuration function with a delay of up to 40 min. Ground-based GPS measurements can e7ectively be used for detecting large-scale horizontal structures and their motion (up to 30 s time resolution) during perturbation processes (see http://www.kn.nz.dlr.de/). These capabilities are demonstrated by analyzing individual storms of January 10, 1997 and of April 6, 2000. For the latter also TEC maps of the Northern polar cap down to 50 ◦ N were computed. These polar maps indicate strong ionization enhancements around the geomagnetic pole in the evening hours. Furthermore, simultaneous high rate sampled GPS and GLONASS data are presented that demonstrate the impact of perturbation-induced small-scale irregularities in the ionosphere on satellite signals in operational communication and navigation systems. c

Understanding space weather to shield society : A global road map for 2015-2025 commissioned by COSPAR and ILWS

There is a growing appreciation that the environmental conditions that we call space weather impact the technological infrastructure that powers the coupled economies around the world. With that co ...

Radio occultation data analysis by the radioholographic method

The radioholographic method is briefly described and tested by using data of 4 radio occultation events observed by the GPS/MET experiment on 9 February 1997. The central point of the radioholographic method (Pavelyev, 1998) is the generation of a radiohologram along the LEO satellite trajectory which allows the calculation of angular spectra of the received GPS radio wave field at the LEO satellite. These spectra are promising in view of detection, analysis and reduction of multipath/diffraction effects, study of atmospheric irregularities and estimation of bending angle error. Initial analysis of angular spectra calculated by the multiple signal classification (MUSIC) method gives evidence that considerable multibeam propagation occurs at ray perigee heights below 20 km and at heights around 80–120 km for the 4 GPS/MET occultation events. Temperature profiles obtained by our analysis (radioholographic method, Abel inversion) are compared with those of the traditional retrieval by the UCAR GPS/MET team (bending angle from slope of phase front, Abel inversion). In 3 of 4 cases we found good agreement (standard deviation σT∼1.5°K between both retrievals at heights 0–30 km).

Comparative testing of four ionospheric models driven with GPS measurements

In the context of the European Space Agency/European Space Operations Centre funded Study A¢Â�Â�GNSS Contribution to Next Generation Global Ionospheric Monitoring,A¢Â�Â� four ionospheric models based on GNSS data (the Electron Density Assimilative Model, EDAM; the Ionosphere Monitoring Facility, IONMON v2; the Tomographic Ionosphere model, TOMION; and the Neustrelitz TEC Models, NTCM) have been run using a controlled set of input data. Each model output has been tested against differential slant TEC (dSTEC) truth data for high (May 2002) and low (December 2006) sunspot periods. Three of the models (EDAM, TOMION, and NTCM) produce dSTEC standard deviation results that are broadly consistent with each other and with standard deviation spreads of ~1 TECu for December 2006 and ~1.5 TECu for May 2002. The lowest reported standard deviation across all models and all stations was 0.99 TECu (EDAM, TLSE station for December 2006 night). However, the model with the best overall dSTEC performance was TOMION which has the lowest standard deviation in 28 out of 52 test cases (13 stations, two test periods, day and night). This is probably related to the interpolation techniques used in TOMION exploiting the spatial stationarity of vertical TEC error decorrelation.

GPS-based TEC observations in comparison with IRI95 and the European TEC model NTCM2

GPS-derived TEC data are compared with IRI95 and the European TEC model NTCM2. It is evident that IRI95 does not include the plasmaspheric content. Monthly medians differ by about (1 to 4) 1016m−2 which fits quite well to earlier estimations of the plasmaspheric content under low solar activity conditions.

November 2004 space weather events: Real‐time observations and forecasts

Space weather events with their solar origin and their distribution through the heliosphere affect the whole magnetosphere-ionosphere-Earth system. Their real-time monitoring and forecasting are important for science and technology. Here we discuss one of the largest space weather events of Solar Cycle 23, in November 2004, which was also one of the most difficult periods to forecast. Nine halo coronal mass ejections (CMEs), interacting on their way through the interplanetary medium and forming two geoeffective interplanetary structures, exemplify the complexity of the event. Real-time and quasi-real-time observations of the ground geomagnetic field show rapid and extensive expansion of the auroral oval to 55° in geomagnetic latitude accompanied by great variability of the ionosphere. Geomagnetically induced currents (GICs) seen in ground networks, such as power grids and pipelines, were significant during the event, although no problems were reported. Forecasts of the CME propagation, global and local ground geomagnetic activity, and ionospheric parameters, issued by several regional warning centers, revealed certain deficiencies in predictions of the interplanetary characteristics of the CME, size of the geomagnetic disturbances, and complexity of the ionospheric variations produced by this event. This paper is a collective report based on the materials presented at the splinter session on November 2004 events during the first European Space Weather Week.