Nighttime fluctuations (scintillations) in Faraday rotation angle of VHF signals from geostationary satellites

Faraday rotation angle records of VHP (137.35 MHz) signals from ATS-3 received at São José dos Campos (Magnetic dip-23.7°) exhibit nighttime fluctuations during equinoctial and summer months. These fluctuations have periodicities ranging from several seconds to about an hour. We investigate the relationship between these fluctuations and the F-layer irregularities.     

Electron content measurements of equatorial anomalous ionosphere using the geostationary satellite signals

Measurements of electron content (N_T) near the crest of the equatorial ionosphere anomaly in South America have been made and analysed to investigate N_T variations with solar hour, solar rotation and geomagnetic storms. The annual mean of diurnal ratio, defined as the ratio of the maximum to the minimum electron content of the day is found to be 5.0. Anomalous increases in night time electron content are observed with maxima around 2100 LMT and 2300 LMT during summer and equinoctial months. These increases are found to be linked with vertical motion of the F-layer. Spatial resonance in equatorial F-layer plasma appears to be the possible cause of these increases.     

The quasi-stationary coronal magnetic field and electron density as determined from a Faraday rotation experiment

Pioneer VI was launched into a circumsolar orbit on December 16, 1965, and was occulted by the sun in the latter half of November, 1968. During the occultation period, the 2292-MHz S-band telemetry carrier underwent Faraday rotation due to the interaction of this signal with the plasma and magnetic field in the solar corona. The NASA/JPL 210-ft diameter antenna of the Deep Space Network near Barstow, California, was used for the measurement. The antenna feed was modified for automatic polarization tracking for this experiment. The measurement results are interpreted with a theoretical model of the solar corona. This model consists of a modified Allen-Baumbach electron density and a coronal magnetic field calculated both from Mount Wilson magnetograph observations using a source surface model and field extrapolations from the Explorer 33 satellite magnetometer. The observations and the calculated rotation show general agreement with respect to magnitude, sense, and timing, suggesting the source-surface model and field extrapolations from 1 AU are a valid technique to obtain the magnetic field in the corona from 4 to 12 solar radii. Variations present can easily be ascribed to density enhancements known to be present in the corona. Longitudinal variations of the density in the corona cannot be obtained from coronagraph observations, and thus a purely radial variation was assumed. An improved fit to the Faraday rotation data is obtained with an equatorial electron density $$N = 10^8 \left( {\frac{{6000}}{{R^{10} }} + \frac{{0.002}}{{R^2 }}} \right)...{\text{ cm}}^{{\text{ - 3}}} {\text{ (4 < }}R < 12){\text{ }}...$$ where R is in solar radii. The work of W. V. T. Rusch and J. E. Ohlson was supported in part by research sponsored by the Joint Services Electronics Program through the Air Force Office of Scientific Research under Grant AF-AFOSR 69-1622A at the University of Southern California. The work done by K. H. Schatten was in part supported by the National Academy of Science on a National Research Council postdoctoral fellowship. The work of J. M. Wilcox was supported in part by the Office of Naval Research under Contract Nonr 3656(26), by the National Aeronautics and Space Administration under Grant NGR 05-003-230, and by the National Science Foundation under Grant GA-1319 at the University of California at Berkeley.     

The Faraday rotation of sunspots

In this paper the numerical solutions of the Unno-Beckers's equations for the magneto-sensitive line Fei 5250.216 are used to demonstrate the importance and role of Faraday rotation in sunspot magnetic fields and to study the influence of this effect on the measurements of the azimuth of the transverse field. We propose a method to determine the intrinsic direction of the transverse field with the observed azimuthal angle of the plane of linear polarization.     

Mapping monthly quantiles of ionospheric electron content and foF2

Monthly quantiles of ionization parameters are important information for system planning and to assess one aspect of ionospheric variability. For ionosonde parameters the data base is probably large enough to allow mapping of deciles. For electron content the situation is not so good: at least for Europe the data base only allows to construct maps for the upper and and the lower monthly quartiles.We present mapping procedures and maps for monthly quartiles of electron content and of foF2 based on experiences with mapping procedures for monthly medians. It turned out that for the lower quartiles it is necessary to to use more coefficients for the quartile maps than for the median maps.We discuss possibilities to gain decile information for electron content from foF2. For a few selected months and locations deciles can be gained from the Faraday effect on the signals of geostationary satellites. These selected cases are used to check on procedures to use foF2 experience to construct electron content deciles.     

Theoretical studies of storm effects in ionospheric total electron content

Investigations have been made of the effects produced by thermospheric winds, composition changes and magnetospheric electric fields on the ionospheric F-layer during disturbed conditions. The results of the computations suggest that a combination of realistic temperature and electric field changes would explain fairly satisfactorily the observed changes in total electron content.     

A PIM-aided kalman filter for gps tomography of the ionospheric electron content

We develop the formalism to perform PIM-based stochastic tomography of the ionospheric electron content with a Kalman filter, in which the inversion problem associated with four-dimensional ionospheric stochastic tomography is regularized. For consistency, GPS data is used to select dynamically the best PIM parameters, in a 3DVAR fashion. We demonstrate the ingestion of (10S and GPS/MEI) GPS data into a parameterized ionospheric model, in order to select the set of parameters that minimize a suitable cost functional. The resulting PIM-fitted model is compared to direct 3D voxel tomography. We demonstrate the value of this method analyzing IGS and GPS/M ET GPS data, and present our results in terms of a 4D model of the ionospheric electronic density.     

Inversions of satellite to satellite electron content: Simulation studies with NeUoG-plas

The signals of Global Navigation Satellites have found a large number of uses in atmospheric and ionospheric research. Reception of the signals from a satellite in a Low Earth Orbit (LEO) leads regularly to occultation of the signals by the surface of the Earth. Before an occultation the signals traverse the ionosphere with rays with decreasing height of their perigees. Satellite electron content observed prior to ‘setting’ occultations or after `rising' occultations can be used as input data for inversion. The inversion procedure gives horizontally averaged height profiles of electron density.Assessment studies are needed to find out under which conditions the profiles from inversions are representative for ‘true’ electron density profiles above the Earth occultation point.A great number of such studies have been carried out using the ionosphere/plasmasphere model NeUOG-plas for forward and backward modelling. Different transmitter-receiver scenarios have been investigated.We describe the assessment procedure and report on results showing the most interesting cases and statistics.     

Observations of ionospheric electron content at medium latitude geomagnetically conjugate stations

Electron content data recorded at Cold Bay, Alaska and Invercargill, New Zealand have been used to study the geomagnetic conjugate effects. The correlation on day-to-day variations at these two stations is found to be negative at extreme magnetic quiet and it then increases to positive values with magnetic activities. When disturbed magnetically, the correlation is + 0.73. Evidence is presented to show that, on the first day following the sudden commencement, the afternoon rise and the subsequent pre-sunset fall in electron content is controlled geomagnetically.     

High-frequency Faraday rotation observations of the solar corona

This thesis, presented on January 31, 2007 under the supervision of Professor Christopher T. Russell, discusses the solar coronal magnetic field observations that can be obtained using the phenomenon of Faraday rotation. It was defended in the Department of Earth and Space Sciences at the University of California, Los Angeles (595 Charles E. Young, Dr. East, Los Angeles, CA 90095). A resume can be found at http://acs-consulting.com/.     

Seamless maps of major elements of the Moon: Results from high-resolution geostationary satellite

Major elements such as Fe,Ti,Mg,Al,Ca and Si play very important roles in understanding the origin and evolution of the Moon.Previous maps of these major elements derived from orbital data are based on mosaic images or low-resolution gamma-ray data.The hue variations and gaps among orbital boundaries in the mosaic images are not conducive to geological studies.This paper aims to produce seamless and homogenous distribution maps of major elements using the single-exposure image of the whole lunar disk obtained by China’s high-resolution geostationary satellite,Gaofen-4,with a spatial resolution of500 m.The elemental contents of soil samples returned by Apollo and Luna missions are regarded as ground truth,and are correlated with the reflectance of the sampling sites extracted from Gaofen-4 data.The final distribution maps of these major oxides are generated with the statistical regression model.With these products,the average contents and proportions of the major elements for maria and highlands were estimated and compared.The results showed that Si O2 and Ti O2 have the highest and lowest fractions in mare and highland areas,respectively.Moreover,the relative concentrations of these elements could serve as indicators of geologic processes,e.g.,the obviously asymmetric distributions of Al2 O3,Ca O and Si O2 around Tycho crater may suggest that Tycho crater was formed by an oblique impact from the southwest direction.     

A Faraday rotation template for the Galactic sky

Using a set of compilations of measurements for extragalactic radio sources, we construct all-sky maps of the Faraday rotation produced by the Galactic magnetic field. In order to generate the maps, we treat the radio source positions as a kind of 'mask' and construct combinations of spherical harmonic modes that are orthogonal on the masked sky. As long as relatively small multipoles are used, the resulting maps are quite stable to changes in the selection criteria for the sources, and show clearly the structure of the local Galactic magnetic field. We also suggest the use of these maps as templates for cosmic microwave background (CMB) foreground analysis, illustrating the idea with a cross-correlation analysis between the Wilkinson Microwave Anisotropy Probe ( WMAP ) data and our maps. We find a significant cross-correlation, indicating the presence of a significant residual contamination.     

Faraday rotation and the turbulent structure of the Galaxy

We extend Jokipii and Lerche's analysis of the turbulent structure of our Galaxy by means of a study of the rotation measure of extragalactic sources. Like them we use a simple, statistically homogeneous and isotropic disc model of the Galaxy and assume that the magnetic field has both an average component and a fluctuating one. We assume that the electron density is proportional to some power of the magnetic field (N _eB ⁿ with 1n2). Using the rotation measure data on 242 extragalactic sources given by Vallée and Kronberg we consider both an exponential and a Gaussian two-point correlation function for the (Gaussian) fluctuating component of the magnetic field with a correlation lengthL. We find reasonable agreement between theory and observations for an average magnetic field of about 3 G, a fluctuating magnetic field component with an amplitude of about 2.6G, an average electron density of about 0.03 cm^–3, a fluctuating density component of about 0.05 cm^–3, and a correlation length of about 300 pc.     

Determination of linear polarization and faraday rotation of pulsar signals from spectral intensity modulation

Most of the known pulsars are sources of highly linearly polarized radiation. Faraday rotation in the intervening medium rotates the plane of the linear polarization as the signals propagate through the medium. The Rotation Measure (RM), which quantifies the amount of such rotation as a function of wavelength, is useful in studying the properties of the medium and in recovering the intrinsic polarization characteristics of the pulsar signal. Conventional methods for polarization measurements use telescopes equipped with dual orthogonally polarized feeds that allow estimation of all 4 Stokes parameters. Some telescopes (such as the Ooty Radio Telescope) that offer high sensitivity for pulsar observations may however be receptive to only a single linear polarization. In such a case, the apparent spectral intensity modulation, resulting from differential Faraday rotation of the linearly polarized signal component within the observing bandwidth, can be exploited to estimate the RM as well as to study the linear polarization properties of the source. In this paper, we present two improved procedures by which these observables can be estimated reliably from the intensity modulation over large bandwidths, particularly at low radio frequencies. We also highlight some other applications where such measurements and procedures would be useful.     

Faraday rotation in the VLBI core of BL Lacertae

External Faraday rotation has been detected in both the core and the parsec-scale jet of BL Lac in a four-frequency very long baseline interferometry (VLBI) experiment. This unexpected result indicates the presence of significant amounts of thermal gas close to the nucleus of this object. The rotation measure (RM) in the jet components is constant, and differs from the currently accepted Galactic RM, indicating that this value (−205 rad m⁻²) is not applicable to the components in the parsec-scale jet. The similarity of the RM in these jet components leads us to suspect that the jet RM is caused by a foreground screen in our Galaxy, although we cannot rule out a combination of Galactic RM and RM local to the jet. If the jet RM is due solely to the Galaxy, this would mean that the currently accepted value of the foreground RM (−205 rad m⁻² ) is not correct, either because the value changed between 1982 and 1997, or because the assumption of no intrinsic source rotation was incorrect, as it was at our later epoch of observation. Our observations suggest a value of     .
After correcting for the foreground RM, the core value is −427 rad m⁻², which is unexpected since, owing to the weakness of their line emission, BL Lac objects are often assumed to be depleted in gas. The core RM appears to be variable, probably because of the presence of at least two polarized components close to the core the relative contributions of which vary with time.     

The mean coronal magnetic field determined from HELIOS Faraday rotation measurements

Coronal Faraday rotation of the linearly polarized carrier signals of the HELIOS spacecraft was recorded during the regularly occurring solar occultations over almost a complete solar cycle from 1975 to 1984. These measurements are used to determine the average strength and radial variation of the coronal magnetic field at solar minimum at solar distances from 3–10 solar radii, i.e., the range over which the complex fields at the coronal base are transformed into the interplanetary spiral. The mean coronal magnetic field in 1975–1976 was found to decrease with radial distance according to r ^?α, where α = 2.7 ± 0.2. The mean field magnitude was 1.0 ± 0.5 × 10 ^?5 tesla at a nominal solar distance of 5 solar radii. Possibly higher magnetic field strengths were indicated at solar maximum, but a lack of data prevented a statistical determination of the mean coronal field during this epoch.     

The mean coronal magnetic field determined from HELIOS Faraday rotation measurements

Coronal Faraday rotation of the linearly polarized carrier signals of the HELIOS spacecraft was recorded during the regularly occurring solar occultations over almost a complete solar cycle from 1975 to 1984. These measurements are used to determine the average strength and radial variation of the coronal magnetic field at solar minimum at solar distances from 3–10 solar radii, i.e., the range over which the complex fields at the coronal base are transformed into the interplanetary spiral. The mean coronal magnetic field in 1975–1976 was found to decrease with radial distance according to r ^–, where = 2.7 ± 0.2. The mean field magnitude was 1.0 ± 0.5 × 10 ^–5 tesla at a nominal solar distance of 5 solar radii. Possibly higher magnetic field strengths were indicated at solar maximum, but a lack of data prevented a statistical determination of the mean coronal field during this epoch.     

Ionospheric scale height from the refraction of satellite signals

Accurate observations of the elevation angle of arrival of 20 MHz signals from the polar orbiting satellite Beacon-B for a 20 month period have provided transmission ionograms which may be reduced to give H_p, the scale height at the peak of the ionosphere. Noon seasonal averages of H_p are 1.35 (in winter) to 1.55 (in summer) times greater than the scale height obtained from bottom-side ionograms. A comparison of scale height at the peak with routine measurements of total content and peak electron density indicates that the O⁺/H⁺ transition level is above 1000 km during the day but comes down to about 630 km on winter nights. A predawn peak in the overall scale height (∝ total content/peak density) is caused by a lowering of the layer to a region of increased recombination and is magnified in winter by low O⁺/H⁺ transition levels. After sunrise in winter and equinoxes the overall scale height is less than the scale height at the peak, implying an outwards flux of ionisation which lasts for about three hours. The summer evening increase in ƒ₀F2 requires both a cooling and a raising of the layer for its occurrence.     

A detection of the Faraday rotation by the solar vector magnetograph

The Faraday rotation in the sunspot atmosphere is statistically detected by examining directions of the linear polarization obtained with the vector magnetograph of the Okayama Astrophysical Observatory. It is very effective near the spectral line center and the azimuth of the linear polarization deviates greatly from the magnetic field azimuth. In the case of the iron line, 5250 Å, the magnetic field azimuth will be obtained with an accuracy better than 15°, if observed in the line wing from 27 to 80 mÅ relative to the line center.