The Helios Faraday Rotation (FR) Experiment, a passive radio science investigation requiring no on-board hardware other than the existing spacecraft radio subsystem, was designed to study the dynamic and quiescent structure of the magnetic fields and electron density in the solar corona. Measurements of coronal Faraday rotation were derived from the linearly polarized S-band downlink carrier signal, which probed otherwise inaccessible regions of the corona in the radial range from 2 to 15 solar radii during the regularly recurring solar conjunctions. More than 1250 hours of Helios FR data were recorded over the duration of the Helios 1 (1974-84) and Helios 2 (1976-80) missions. The time scales of FR variations provide information on various physical phenomena: (a) slowly-varying rise and fall associated with the changing ray path offset, combined with the rotation of the quasi-static corona; (b) ubiquitous random oscillations with higher fluctuation amplitude at smaller solar offset distances, probably caused by coronal Alfven waves; (c) occasional nearly discontinous jumps in the polarization angle, most likely caused by transient events such as coronal mass ejections (CMEs). The Helios FR data, aspects of which have been reported in more than forty publications to date, have now been systematically collected in a data archive for public dissemination. A brief review of the main results of the Helios FR Experiment are presented, together with some suggestions for possible use of the archive for continued solar wind research.

The two Helios spacecraft underwent regular solar occultations during their extended missions from Dec 1974-Feb 1986 (Helios 1) and Jan 1976-Mar 1980 (Helios 2) thereby providing many opportunities for radio propagation experiments in the solar corona. On certain rare occasions over the course of these investigations, Faraday rotation measurements of the linearly polarized Helios signals could be recorded simultaneously at two widely-spaced ground stations. Many of these two-station measurement intervals display clear evidence of wave-like structures with quasi-periods of the order of a few minutes to a few hours. These structures are attributed to coronal Alfven waves. The radial propagation direction and velocity of these waves are estimated from a cross-correlation analysis of the data between the two stations. The majority of the waves appear to propagate away from the Sun, but about 30 percent of the cases indicate a propagation direction toward the Sun.

view Abstract Citations (55) References (9) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS 15 GHz Space VLBI Observations Using an Antenna on a TDRSS Satellite Linfield, R. P. ; Levy, G. S. ; Edwards, C. D. ; Ulvestad, J. S. ; Ottenhoff, C. H. ; Hirabayashi, H. ; Morimoto, M. ; Inoue, M. ; Jauncey, D. L. ; Reynolds, J. ; Nishimura, T. ; Hayashi, T. ; Takano, T. ; Yamada, T. ; Barrett, J. W. ; Conner, S. R. ; Heflin, M. B. ; Lehar, J. ; Burke, B. F. ; Roberts, D. H. ; Whitney, A. R. ; Cappallo, R. J. ; Rogers, A. E. E. ; Pospieszalski, M. W. ; Dinardo, S. J. ; Skjerve, L. J. ; Stavert, L. R. ; Maher, M. J. Abstract A 4.9 m diameter antenna in earth orbit, part of the Tracking and Data Relay Satellite System, was previously used for space VLBI observations at 2.3 GHz. It has now been used successfully with two ground antennas for VLBI measurements at 15 GHz. Although the sensitivity (gain/system temperature) of the orbiting antenna was a factor of 6 poorer at 15 GHz than at 2.3 GHz, 11 out of 22 extragalactic sources were detected on space- ground baselines, up to a maximum baseline of 1.63 Earth diameters. The results suggest that the distribution of source visibilities at 15 and 2.3 GHz are similar for these baseline lengths. Model-dependent brightness temperatures of 1-2.5 x 10^12^ K were derived for six sources. This suggests that sources with brightness temperatures in this range are as common at 15 GHz as at 2.3 GHz. The coherence on space-ground baselines (This includes effects due to the phase transfer from the ground to the satellite and the reconstruction of the spacecraft orbit) was 0.76 for 340 s integrations. A small number of VLBI data at 2.3 GHz were also obtained. Publication: The Astrophysical Journal Pub Date: July 1990 DOI: 10.1086/168992 Bibcode: 1990ApJ...358..350L Keywords: Radio Galaxies; Spacecraft Antennas; Tdr Satellites; Very Long Base Interferometry; Block Diagrams; Brightness Temperature; Coherence Coefficient; Data Reduction; Field Of View; Astronomy; INSTRUMENTS; INTERFEROMETRY; RADIO SOURCES: GALAXIES full text sources ADS | data products NED (22) SIMBAD (14)

view Abstract Citations (28) References (10) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS VLBI Using a Telescope in Earth Orbit. I. The Observations Levy, G. S. ; Linfield, R. P. ; Edwards, C. D. ; Ulvestad, J. S. ; Jordan, J. F., Jr. ; Dinardo, S. J. ; Christensen, C. S. ; Preston, R. A. ; Skjerve, L. J. ; Stavert, L. R. ; Burke, B. F. ; Whitney, A. R. ; Cappallo, R. J. ; Rogers, A. E. E. ; Blaney, K. B. ; Maher, M. J. ; Ottenhoff, C. H. ; Jauncey, D. L. ; Peters, W. L. ; Reynolds, J. ; Nishimura, T. ; Hayashi, T. ; Takano, T. ; Yamada, T. ; Hirabayashi, H. ; Morimoto, M. ; Inoue, M. ; Shiomi, T. ; Kawaguchi, N. ; Kunimori, H. ; Tokumaru, M. ; Takahashi, F. Abstract A VLBI experiment has been conducted at a frequency of 2.3 GHz, using an antenna in Earth orbit. The observational procedure for the orbiting antenna, part of the Tracking and Data Relay Satellite System, is described. Nonstandard VLBI procedures were necessary to obtain adequate phase stability and to correlate the data. Several technical concepts necessary for a dedicated space VLBI mission were successfully tested. Publication: The Astrophysical Journal Pub Date: January 1989 DOI: 10.1086/167080 Bibcode: 1989ApJ...336.1098L Keywords: Earth Orbits; Interferometry; Radio Astronomy; Telescopes; Very Long Base Interferometry; Angular Resolution; Satellite Antennas; Synchronous Satellites; Spacecraft Instrumentation; INSTRUMENTS; INTERFEROMETRY full text sources ADS | Related Materials (1) Part 2: 1989ApJ...336.1105L

At the 37th IAF Congress [Levy et al., Acta Astronautica 15, 481 (1987)] we presented the initial results of a demonstration using a TDRSS spacecraft antenna as the orbiting observatory in a very long baseline array. This demonstration established the feasibility of the OVLBI technique. In the beginning of 1987, additional OVLBI observations were made at a radio frequency of 2.3 GHz. Of the 24 sources observed, 23 were detected. These data have been analyzed and the results are presented. After the successful completion of the 2.3 GHz observations, it was decided to attempt to use the 15 GHz capability of the TDRSS for a higher resolution demonstration. Simultaneous 2.3 and 15 GHz observations were successfully conducted in February and March 1988. The radiometric sensitivity of TDRSS at 15 GHz was very much less than at 2.3 GHz. Of the 23 sources observed, 11 were detected.

view Abstract Citations (69) References (18) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS VLBI Using a Telescope in Earth Orbit. II. Brightness Temperatures Exceeding the Inverse Compton Limit Linfield, R. P. ; Levy, G. S. ; Ulvestad, J. S. ; Edwards, C. D. ; Dinardo, S. J. ; Stavert, L. R. ; Ottenhoff, C. H. ; Whitney, A. R. ; Cappallo, R. J. ; Rogers, A. E. E. ; Hirabayashi, H. ; Morimoto, M. ; Inoue, M. ; Jauncey, D. L. ; Nishimura, T. Abstract VLBI observations at 2.3 GHz were conducted using an antenna of the Tracking and Data Relay Satellite System (TDRSS), in geosynchronous orbit, and two ground stations. Twenty-three of 24 sources were detected, on baselines as long as 2.15 Earth diameters. These baseline lengths gave the interferometer much better sensitivity to high brightness temperatures than any Earth-based observations. Brightness temperatures of 1-4 times the 10^12^ K inverse Compton limit were measured for 10 sources, suggesting bulk relativistic motion in these sources. Coherence values of approximately 85% for integration times of 360 s were obtained. Publication: The Astrophysical Journal Pub Date: January 1989 DOI: 10.1086/167081 Bibcode: 1989ApJ...336.1105L Keywords: Brightness Temperature; Spaceborne Astronomy; Tdr Satellites; Very Long Base Interferometry; Calibrating; Compton Effect; Earth Orbits; Satellite Antennas; Spaceborne Telescopes; Spacecraft Instrumentation; INSTRUMENTS; INTERFEROMETRY full text sources ADS | data products NED (24) SIMBAD (23) Related Materials (1) Part 1: 1989ApJ...336.1098L

The Voyager 2 encounter with the Neptune system included radio science investigations of the masses and densities of Neptune and Triton, the low-order gravitational harmonics of Neptune, the vertical structures of the atmospheres and ionospheres of Neptune and Triton, the composition of the atmosphere of Neptune, and characteristics of ring material. Demanding experimental requirements were met successfully, and study of the large store of collected data has begun. The initial search of the data revealed no detectable effects of ring material with optical depth tau [unknown] 0.01. Preliminary representative results include the following: 1.0243 x 10(26) and 2.141 x 10(22) kilograms for the masses of Neptune and Triton; 1640 and 2054 kilograms per cubic meter for their respective densities; 1355 +/- 7 kilometers, provisionally, for the radius of Triton; and J(2) = 3411 +/- 10(x 10(-6)) and J(4) = -26(+12)(-20)(x10(-6)) for Neptune's gravity field (J>(2) and J(4) are harmonic coefficients of the gravity field). The equatorial and polar radii of Neptune are 24,764 +/- 20 and 24,340 +/- 30 kllometers, respectively, at the 10(5)-pascal (1 bar) pressure level. Neptune's atmosphere was probed to a pressure level of about 5 x 10(5) pascals, and effects of a methane cloud region and probable ammonia absorption below the cloud are evident in the data. Results for the mixing ratios of helium and ammonia are still being investigated; the methane abundance below the clouds is at least 1 percent by volume. Derived temperature-pressure profiles to 1.2 x 10(5) pascals and 78 kelvins (K) show a lapse rate corresponding to "frozen" equilibrium of the para- and ortho-hydrogen states. Neptune's ionosphere exhibits an extended topside at a temperature of 950 +/- 160 K if H(+) is the dominant ion, and narrow ionization layers of the type previously seen at the other three giant planets. Triton has a dense ionosphere with a peak electron concentration of 46 x 10(9) per cubic meter at an altitude of 340 kilometers measured during occultation egress. Its topside plasma temperature is about 80 +/- 16 K if N(2)(+) is the principal ion. The tenuous neutral atmosphere of Triton produced distinct signatures in the occultation data; however, the accuracy of the measurements is limited by uncertainties in the frequency of the spacecraft reference oscillator. Preliminary values for the surface pressure of 1.6 +/- 0.3 pascals and an equivalent isothermal temperature of 48 +/- 5 K are suggested, on the assumption that molecular nitrogen dominates the atmosphere. The radio data may be showing the effects of a thermal inversion near the surface; this and other evidence imply that the Triton atmosphere is controlled by vapor-pressure equilibrium with surface ices, at a temperature of 38 K and a methane mixing ratio of about 10(-4).

An antenna in geostationary orbit was used for VLBI observations at 2.3 GHz, in combination with ground antennas in Australia and Japan. 23 of the 25 observed sources were detected on orbiter-ground baselines, with baseline lengths as large as 2.15 earth diameters. Brightness temperatures between 1012 K and 4 × 1012 K were measured for 10 sources.

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.

A desire for increased angular resolution at microwave frequencies has led to the development of radio telescopes with very large effective apertures. Very long baseline interferometry (VLBI) has made it possible to synthesize telescopes with effective dimensions of a large fraction of an Earth diameter. By using a satellite-borne radio telescope as part of a VLBI array, the dimensions of the Earth cease to be a limitation. The use of a satellite VLBI telescope puts stringent requirements on the communication links between the spacecraft and the ground. A demonstration was performed to show that the orbiting VLBI (OVLBI) concept is feasible. The Tracking and Data Relay Satellite System (TDRSS) was used as the orbiting element of the VLBI demonstration. Stability tests were made before the observations to determine the suitability of the TDRSS for OVLBI use. The first successful OVLBI observations were performed using the 64-m antenna observatories of NASA's Deep Space Network in Tidbinbilla, Australia, and of the Institute for Space and Astronautical Science in Usuda, Japan in conjunction with the TDRSS. Data from three quasars were successfully correlated at the Haystack Observatory in Westford, Mass.; the results were used to deduce the system performance.

Over the past two decades, radar and radio observations of planets and spacecraft have been made by stations of the Deep Space Network (DSN), which by the unprecedented nature of their accuracy have produced the most accurate tests of general relativity available. We review the history of the instrumentation and data analysis of the first spacecraft test, the three percent determination of the effect of solar gravity on radio signals between DSN stations and the two spacecraft, Mariner 6 and Mariner 7 (Anderson et al., 1975), as well as later more accurate tests using the Mariner 9 spacecraft anchored to Mars (Reasenberg and Shapiro, 1977; Anderson et al., 1978) and the Viking orbiters and landers (Shapiro et al., 1977; Hellings, 1985). We also review tests of the metric nature of gravity using radar, optical observations, and radio astrometry of the planets (Anderson et al., 1978; Reasenberg, 1985; Hellings, 1985) and the limits placed on the variability of the gravitational constant G (Hellings et al., 1983). Finally, we discuss the prospects for improved accuracy through ongoing upgrades of DSN instrumentation and show the results of covariance analyses for a possible future NASA mission to the Sun (Solar Probe) in the mid 1990's (Mease et al., 1984) and the next NASA mission to Mars, the Mars Observer mission planned for launch in late 1990.

Very long baseline interferometry (VLBI) has been developed to the point where angular resolution at any given wavelength is limited by the dimensions of the earth. This limitation can be removed by placing a VLBI radio telescope in orbit. A demonstration of the feasibility of this approach was arranged. The Tracking and Data Relay Satellite System was used as an orbiting observatory in conjunction with the NASA Deep Space Network 64-m telescope in Tidbinbilla, Australia, and the Institute for Space and Astronautical Science 64-m antenna in Usuda, Japan. Interferometric fringes were successfully obtained from three quasars. The longest projected baseline was 1.4 earth diameters.

An orbiting spacecraft and ground observatories have been used to obtain interferometric observations of cosmic radio sources. The Tracking and Data Relay Satellite System (TDRSS) was used as the orbiting observatory in conjunction with two 64- meter radio telescopes at ground observatories, one in Australia and one in Japan. The quasars 1730-130 (NRAO 530), 1510-089, and 1741-038 were observed at a frequency of 2.3 gigahertz, and a maximum projected baseline of 1.4 earth diameters was achieved. All quasar observations for which valid data were acquired resulted in detected fringes. Many of the techniques proposed for a dedicated very long baseline interferometry observatory in space were used successfully in this experiment.

Voyager 2 radio occultation measurements of the Uranian atmosphere were obtained between 2 and 7 degrees south latitude. Initial atmospheric temperature profiles extend from pressures of 10 to 900 millibars over a height range of about 100 kilometers. Comparison of radio and infrared results yields mole fractions near the tropopause of 0.85 and 0.15 +/- 0.05 for molecular hydrogen and helium, respectively, if no other components are present; for this composition the tropopause is at about 52 kelvins and 110 millibars. Distinctive features in the signal intensity measurements for pressures above 900 millibars strongly favor model atmospheres that include a cloud deck of methane ice. Modeling of the intensity measurements for the cloud region and below indicates that the cloud base is near 1,300 millibars and 81 kelvins and yields an initial methane mole fraction of about 0.02 for the deep atmosphere. Scintillations in signal intensity indicate small-scale stucture throughout the stratosphere and upper troposphere. As judged from data obtained during occultation ingress, the ionosphere consists of a multilayer structure that includes two distinct layers at 2,000 and 3,500 kilometers above the 100-millibar level and an extended topside that may reach altitudes of 10,000 kilometers or more. Occultation measurements of the nine previously known rings at wavelengths of 3.6 and 13 centimeters show characteristic values of optical depth between about 0.8 and 8; the maxim value occurs in the outer region of the in ring, near its periapsis. Forward-scattered signals from this ring have properties that differ from those of any of Saturn's rings, and they are inconsistent with a discrete scattering object or local (three-dimensional) assemblies of orbiting objects. These signals suggest a new kdnd of planetary ring feature characterized by highly ordered cylindrical substructures of radial scale on the order of meters and azimuthal scale of kilometers or more. From radio data alone the mass of the Uranian system is GM(sys) = 5,794,547- 60 cubic kilometers per square second; from a combination of radio and optical navigation data the mass of Uranus alone is GM(u) = 5,793,939+/- 60 cubic kilometers per square second. From all available Voyager data, induding imaging radii, the mean uncompressed density of the five major satellites is 1.40+/- 0.07 grams per cubic centimeter; this value is consistent with a solar mix of material and apparently rules out a cometary origin of the satellites.