Determination of Debye Temperatures and Lamb-Mössbauer Factors for LnFeO3 Orthoferrite Perovskites (Ln = La, Nd, Sm, Eu, Gd)

Lanthanide orthoferrites have wide-ranging industrial uses including solar, catalytic and electronic applications. Here a series of lanthanide orthoferrite perovskites, LnFeO3 (Ln = La; Nd; Sm; Eu; Gd), prepared through a standard stoichiometric wet ball milling route using oxide precursors, has been studied. Characterisation through X-ray diffraction and X-ray fluorescence confirmed the synthesis of phase-pure or near-pure LnFeO3 compounds. 57Fe Mössbauer spectroscopy was performed over a temperature range of 10 K to 293 K to observe hyperfine structure and to enable calculation of the recoil-free fraction and Debye temperature (θD) of each orthoferrite. Debye temperatures (Ln = La 474 K; Nd 459 K; Sm 457 K; Eu 452 K; Gd 473 K) and recoil-free fractions (Ln = La 0.827; Nd 0.817; Sm 0.816; Eu 0.812; Gd 0.826) were approximated through minimising the difference in the temperature dependent experimental Centre Shift (CS) and theoretical Isomer Shift (IS), by allowing the Debye temperature and Isomer Shift values to vary. This method of minimising the difference between theoretical and actual values yields Debye temperatures consistent with results from other studies determined through thermal analysis methods. This displays the ability of variable-temperature Mössbauer spectroscopy to approximate Debye temperatures and recoil-free fractions, whilst observing temperature induced transitions over the temperature range observed. X-ray diffraction and Rietveld refinement show an inverse relationship between FeO6 octahedral volume and approximated Debye temperatures. Raman spectroscopy show an increase in the band positions attributed to soft modes of Ag symmetry, Ag(3) and Ag(5) from La to GdFeO3 corresponding to octahedral rotations and tilts in the [010] and [101] planes respectively

Comparison of ionospheric radio occultation CHAMP data with IRI 2000

Abstract. GPS radio occultation measurements on board low Earth orbiting satellites can provide vertical electron density profiles of the ionosphere from satellite orbit heights down to the bottomside. Ionospheric radio occultation (IRO) measurements carried out onboard the German CHAMP satellite mission since 11 April 2001 were used to derive vertical electron density profiles (EDPs) on a routine basis. About 150 vertical electron density profiles may be retrieved per day thus providing a huge data basis for testing and developing ionospheric models. Although the validation of the EDP retrievals is not yet completed, the paper addresses a systematic comparison of about 78 000 electron density pro- files derived from CHAMP IRO data with the International Reference Ionosphere (IRI 2001). The results are discussed for quite different geophysical conditions, e.g. as a function of latitude, local time and geomagnetic activity. The comparison of IRO data with corresponding IRI data indicates that IRI generally overestimates the upper part of the ionosphere whereas it underestimates the lower part of the ionosphere under high solar activity conditions. In a first order correction this systematic deviation could be compensated by introducing a height dependence correction factor in IRI profiling

Sounding the Ionosphere by GPS Measurements on CHAMP

Ground based GPS measurements are used successfully since several years to monitor the vertical Total Electron Content (TEC) in an operational manner. The installation of GPS receivers on board of LEO (Low Earth Orbiting) satellites such as CHAMP offers new opportunities of ionospheric remote sensing. Beside the GPS radio occultation measurements the German small satellite CHAMP tracks up to 8 GPS satellites simultaneously for precise orbit determination using a dedicated zenith looking antenna. These 0.1 Hz sampled navigation measurements are permanently performed and provide valuable information on the ionization state of the upper ionosphere and plasmasphere between CHAMP and GPS altitude on global scale. After preprocessing and calibration, link related TEC measurements are derived from the GPS navigation observations on board CHAMP. The three dimensional electron density distribution is reconstructed by assimilating the TEC measurements of a full CHAMP revolution into the ionospheric/plasmaspheric model PIM. For this purpose the discretization of PIM on a global voxel structure is necessary. The assimilation method is based on an iterative algorithm which adapts the initial model assumption to the TEC measurements by applying multiplicative modifications. In this talk reconstruction results are presented for selected assimilation examples by means of two dimensional slices along the respective CHAMP orbit plane. The assimilation technique will be described briefly.We present validation results using electron density measurements from the Langmuir Probe on board CHAMP, from incoherent scatter radars and from ionosondes. Potentials and limitations of the reconstruction technique will be addressed

Ionosphere sounding by ground and space based GNSS measurements

The world-wide use of Global Navigation Satellite Systems (GNSS) such as GPS and GLONASS offer 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. Whereas ground based measurements provide a good horizontal resolution of the total ionization, space based GNSS measurements provide global information on the vertical structure of the ionosphere and on the 3D- electron distribution of the topside ionosphere/plasmasphere. The talk addresses the capabilities of global GPS networks, such as generating global and regional maps of the total electron content (TEC) for research and studying the ionospheric impact on GNSS applications from the International GPS Service (IGS) network. During ionospheric storms TEC is highly variable and indicates a close correlation with geomagnetic activity indices and even with geo-stationary satellite particle flux data. The Ionospheric Radio Occultation (IRO) technique has the big advantage to unify profiling through the entire ionosphere with global coverage. Additionally, the GPS navigation measurements onboard LEO's may be used to reconstruct the electron density distribution of the topside ionosphere and plasmasphere with high resolution. Both techniques are considered when presenting results obtained so far from GPS measurements onboard CHAMP. Since 11 April 2001 more than 20000 vertical electron density profiles have been retrieved from the IRO data. Due to the rather low orbit of the CHAMP satellite (h < 450 km) the classical Abel transform algorithm is problematic to apply. To overcome this problem, a model assisted retrieval technique has been developed. Validation studies reveal RMS deviations of the F2 layer parameters f0F2 and hmF2 of about 1 MHz and 45 km, respectively. Entire IRO derived electron density profiles are compared with corresponding profiles derived from vertical sounding measurements obtained at some European ionosonde stations and incoherent scatter facilities during selected campaigns. The topside electron density reconstruction has been made by assimilating calibrated TEC data into a global ionospheric model (PIM) providing a 3D- electron density reconstruction near the CHAMP orbit. The results indicate that ground and space based GNSS measurements have a very high potential for a permanent and high resolution monitoring. In the future, this potential will be fully utilized considering the enhanced GNSS signal availability by Galileo from 2008 onward and the international activities in preparing multi-satellite missions

About the potential of GPS radio occultation measurements for exploring the ionosphere

The GPS radio occultation technique onboard LEO satellites such as CHAMP is a rather simple and relatively inexpensive tool for profiling the electron density of the ionosphere from satellite orbit heights down to the bottomside. The paper addresses the capabilities of the ionospheric radio occultation (IRO) technique for monitoring the global ionosphere on a routine basis to derive value added data products and to study particular ionospheric processes such as perturbations. The model assisted retrieval technique, operational data processing and the validation of vertical electron density profiles are discussed. These profiles may not only be used to validate unknown models, they provide also a good data basis for developing new models of ionospheric key parameters such as the critical frequency foF2, the peak height hmF2 and the scale height H. Such models are helpful to improve retrieval procedures and tomographic reconstruction techniques. Due to the operational data processing capabilities the data products may contribute to space weather monitoring of the ionospher